Skip to Main Content

*
retired

Abstract

The San Luis Basin encompasses the largest structural and hydrologic basin of the Rio Grande rift. On this field trip, we will examine the timing of transition of the San Luis Basin from hydrologically closed, aggrading subbasins to a continuous fluvial system that eroded the basin, formed the Rio Grande gorge, and ultimately, integrated the Rio Grande from Colorado to the Gulf of Mexico. Waning Pleistocene neotectonic activity and onset of major glacial episodes, in particular Marine Isotope Stages 11–2 (~420–14 ka), induced basin fill, spillover, and erosion of the southern San Luis Basin. The combined use of new geologic mapping, fluvial geomorphology, reinterpreted surficial geology of the Taos Plateau, pedogenic relative dating studies, 3He surface exposure dating of basalts, and U-series dating of pedogenic carbonate supports a sequence of events wherein pluvial Lake Alamosa in the northern San Luis Basin overflowed, and began to drain to the south across the closed Sunshine Valley–Costilla Plain region ≤400 ka. By ~200 ka, erosion had cut through topographic highs at Ute Mountain and the Red River fault zone, and began deep-canyon incision across the southern San Luis Basin. Previous studies indicate that prior to 200 ka, the present Rio Grande terminated into a large bolson complex in the vicinity of El Paso, Texas, and systematic, headward erosional processes had subtly integrated discontinuously connected basins along the eastern flank of the Rio Grande rift and southern Rocky Mountains. We propose that the integration of the entire San Luis Basin into the Rio Grande drainage system (~400–200 ka) was the critical event in the formation of the modern Rio Grande, integrating hinterland basins of the Rio Grande rift from El Paso, Texas, north to the San Luis Basin with the Gulf of Mexico. This event dramatically affected basins southeast of El Paso, Texas, across the Chisos Mountains and southeastern Basin and Range province, including the Rio Conchos watershed and much of the Chihuahuan Desert, inducing broad regional landscape incision and exhumation.

Introduction

The San Luis Basin forms the headwaters of the Rio Grande (RG) and is the largest structural and hydrologie basin of the Rio Grande rift and watershed (Figs. 1, 2, and 3). This field trip will present new field and analytical data to constrain the timing of transition from closed, aggrading subbasins to a coalesced fluvial system exhuming the basin, forming the RG gorge, and ultimately, integrating the Rio Grande from Colorado to the Gulf of Mexico. We discuss the neotectonic and climatic influences leading to basin-fill-and-spillover effects on the fluvial geomor-phology and reinterpreted surficial geology of the Taos Plateau. The combined result of pedology, 3He surface exposure dating, and U-series ages on pedogenic carbonate support a sequence of events whereby the northern San Luis Basin filled, overflowed, and began to drain to the south across the Taos Plateau <400 ka, initiating incision across the Red River fault zone and other topographically high surfaces. By ~200 ka, erosion across the Red River fault zone had occurred and deep-canyon incision had started to the south across the Taos Plateau.

Figure 1.

Basins of the Rio Grande and age of integration into an axial Rio Grande fluvial system (after Connell et al., 2005). Ages of basin integration from previous works are as follows: Lower Rio Grande-Rio Conchas-Pecos River (Hawley et al., 1976; Galloway et al., 2011), Middle Rio Grande basin (Dethier et al., 1988), Albuquerque basin (Cole et al., 2007; Connell et al., 2005), Espa-nola Basin (Konning et al., 2011), and San Luis Basin (Machette et al., 2007, 2013; Ruleman et al., 2007, 2013). NSLB—northern San Luis Basin; SSLB—southern San Luis Basin; CO—Colorado; NM—New Mexico; TX—Texas.

Figure 1.

Basins of the Rio Grande and age of integration into an axial Rio Grande fluvial system (after Connell et al., 2005). Ages of basin integration from previous works are as follows: Lower Rio Grande-Rio Conchas-Pecos River (Hawley et al., 1976; Galloway et al., 2011), Middle Rio Grande basin (Dethier et al., 1988), Albuquerque basin (Cole et al., 2007; Connell et al., 2005), Espa-nola Basin (Konning et al., 2011), and San Luis Basin (Machette et al., 2007, 2013; Ruleman et al., 2007, 2013). NSLB—northern San Luis Basin; SSLB—southern San Luis Basin; CO—Colorado; NM—New Mexico; TX—Texas.

Figure 2.

Physiographic map of the San Luis Basin generated from 10 m digital elevation data. The San Luis Hills form the main barrier between the northern and southern basins. Extent of middle Pleistocene Lake Alamosa shown in northern San Luis Basin. Sangre de Cristo fault system shown as northern, central, and southern fault zones. Field-trip stop locations are indicated by yellow stars.

Figure 2.

Physiographic map of the San Luis Basin generated from 10 m digital elevation data. The San Luis Hills form the main barrier between the northern and southern basins. Extent of middle Pleistocene Lake Alamosa shown in northern San Luis Basin. Sangre de Cristo fault system shown as northern, central, and southern fault zones. Field-trip stop locations are indicated by yellow stars.

Figure 3.

Geologic map of the southern San Luis Basin showing cross sections described in text, field-trip stop locations, and 3He surface exposure ages at each location. Marine oxygen isotope stage chart (modified from Lisiecki and Raymo, 2005) provided for convenience. Tables 2 and 3 modified from Ruleman et al. (2013) to show 3He and U-series chronologic constraints on regional correlation of surficial deposits. CO— Colorado; NM—New Mexico; TX—Texas.

Figure 3.

Geologic map of the southern San Luis Basin showing cross sections described in text, field-trip stop locations, and 3He surface exposure ages at each location. Marine oxygen isotope stage chart (modified from Lisiecki and Raymo, 2005) provided for convenience. Tables 2 and 3 modified from Ruleman et al. (2013) to show 3He and U-series chronologic constraints on regional correlation of surficial deposits. CO— Colorado; NM—New Mexico; TX—Texas.

Previous work indicates that the ancestral Rio Grande terminated into a large bolson complex in the vicinity of El Paso, Texas, and northern Mexico (Hawley et al., 1976). Furthermore, no significant sediment supply from the hinterland of the Rio Grande rift to the Gulf of Mexico occurred from the Miocene to the late middle Pleistocene (Galloway et al., 2011). We propose that the drainage integration of the entire San Luis Basin at ~200 ka into the more southerly rift basins formed the modern Rio Grande, integrating the southerly basins into an axial river flowing to the Gulf of Mexico. Basins to the south all demonstrate a major regional incision episode occurring <640-130 ka (Connell et al., 2005; Cole et al., 2007; Konning et al., 2011) (Fig. 1).

This guidebook describes a two-day field trip across the northern and southern San Luis Basins (Figs. 2 and 3). The first day examines and describes evidence supporting the presence and overflow history of Pleistocene Lake Alamosa in the northern San Luis Basin. The second day examines the Pliocene to Pleistocene geomorphic evolution of the southern San Luis Basin and Taos Plateau, and the temporal and spatial evolution of Rio Grande incision history, including timing of formation of the spectacular Rio Grande gorge.

The integration of the upper (Colorado) and lower (New Mexico) reaches of the Rio Grande expanded the river’s drainage basin by nearly 18,000 km2, and added recharge areas in the high-altitude, glaciated San Juan Mountains and northern Sangre de Cristo Mountains (Machette et al., 2013) (Fig. 2). This large increase in mountainous drainage influenced the river’s dynamics downstream in New Mexico, through down-cutting and lowering of local base levels in the southern part of the San Luis Valley and central and lower reaches of the Rio Grande, ultimately to sea level. Figure 1 shows the timing of each basin’s integration along the Rio Grande corridor, of which all show a dramatic change in basin incision occurring during this time interval.

Important geologic background noted in the geologic map text of Thompson et al. (2015) includes the following: The Rio Grande has headwaters in the San Juan Mountains of Colorado and ultimately discharges into the Gulf of Mexico 3000 km downstream (Fig. 1). Alluvial floodplains and associated deposits of the Rio Grande and east-draining tributaries, La Jara Creek and Conejos River, occupy the north-central and northwestern part of the map area. Alluvial deposits of west-draining Rio Grande tributaries, Culebra and Costilla Creeks, bound the Costilla Plain in the south-central part of the map area. The San Luis Hills, a northeast-trending series of flat-topped mesas and hills, dominate the landscape in the central and southwestern part of the map and preserve fault-bound Neogene basin surfaces and deposits. The Precambrian-cored Sangre de Cristo Mountains rise to an elevation of nearly 4300 m asl (above sea level), almost 2000 m above the valley floor, in the eastern part of the map area. In total, the map area contains deposits that record surficial, tectonic, sedimentary, volcanic, magmatic, and metamorphic processes over the past 1.7 b.y.

Three important sources of information pertaining to the stops of Day 1 are the Friends of the Pleistocene guidebook (Machette et al., 2007), dating results published in Machette et al. (2013), and the Alamosa quadrangle published by the USGS (Thompson et al., 2015).

Day 1—Lake Alamosa and the Northern San Luis Basin in Colorado

The field trip starts in Alamosa, Colorado, and traverses the San Luis Basin and San Luis Hills in the southern part of the state (Fig. 2). The first three stops relate primarily to deposits of Lake Alamosa, a Pliocene to middle Pleistocene lake that occupied the northern San Luis Basin. By mid-day, we will have lunch atop a basalt-mantled horst of Proterozoic basement rock in the historic town of San Luis, established in 1858. From there, we travel south along the Rio Grande in the basin axis to Ute Mountain (northernmost New Mexico). We will end the day at the northern section of the Rio Grande gorge to discuss 3He exposure ages constraining the initiation of deep-gorge incision, and topographic relationships related to the temporal and spatial fluvial integration of the San Luis Basin. We will then traverse the basin south along the southern Sangre de Cristo fault zone, crossing middle Pleistocene geomorphic environments that predate this basin integration interval (~420-200 ka). Our first day ends at Taos, New Mexico, where we stay overnight before starting Day 2.

Lake Alamosa and the Northern San Luis Basin

From Pliocene to middle Pleistocene time, a large lake occupied most of the northern San Luis Basin of southern Colorado, with its highest shorelines and sediments above 2300 m (7550 ft) elevation. This ancient lake accumulated sediments (as seen at Stop 1) of the Alamosa Formation (Siebenthal, 1910), for which the lake was named (Fig. 2). The existence of this lake was first postulated in 1822 and subsequently supported by identification of lake sediments in well logs (Siebenthal, 1910). At its maximum extent of nearly 4000 km2, it was one of the largest high-altitude lakes in North America, geomorphically comparable but larger than Lake Texcoco in the Valley of Mexico. Lake Alamosa probably persisted for hundreds of thousands of years, expanding and contracting and progressively filling the valley with sediment. The highest shoreline spits of the lake (as seen at Stop 2) formed during the lake’s maximum level and appear to be coeval with the lake’s overtopping of a low sill in the San Luis Hills (as seen at Stop 3), facilitating erosion of a deep gorge through Oligocene volcanic rocks in the San Luis Hills and spillover into closed drainage basins to the south. As the lake drained over a period of ~200 k.y., spanning two glacial periods (Marine Isotope Stages [MIS] 10 and 8), more than 100 km3 (>81 χ 106 acre-ft) of water coursed toward poorly integrated drainages to the south, ultimately developing into the modern drainage of the Rio Grande (described on Day 2). Critical to this interpretation is the discovery of ancient shoreline deposits, including spits, barrier bars, and lagoon deposits nestled among bays and in backwater positions on the northern margin of the San Luis Hills, southeast of Alamosa, Colorado, some of which we will observe at Stop 2.

Alluvial and lacustrine sediment nearly filled the basin prior to the lake’s overtopping, which occurred ca. <400 ka. Machette et al. (2007) previously estimated a time of ~440 ka for this event. However, new 3He surface-exposure ages and a recalibration of the 439 ± 6 ka date (Machette et al., 2013) from basalt boulders on shoreline spit deposits at the lake’s high shoreline elevation show an age range of 388.4 ± 13.8 ka to 273.7 ± 9.5 ka and average 289.1 ± 9.9 at 2330-2340 m (7645-7676 ft) asl (Table 1). Overtopping of the lake’s hydrologic sill was probably driven by high lake levels associated with deglaciation of global cooling cycles in the MIS record of Lisiecki and Raymo (2005) (Fig. 3). Based on the 3He exposure ages for landforms associated with Lake Alamosa’s highstand and exhumation of the basin, we tentatively place initial spillover during MIS 11 (424-374 ka) (Lisiecki and Raymo, 2005), the interglacial cycle following one of the largest Pleistocene global cooling cycles recorded in the marine record. All 3He surface exposure ages on lacustrine landforms and sequential lower elevation erosional surfaces are MIS 11 (<~420 ka) and younger. Hydrologic modeling of stream inflow during full-glacial-maximum conditions suggests that Lake Alamosa could fill at modern precipitation amounts if the mean annual temperature were just 5 °C (10 °F) cooler, or could fill at modern temperatures with 1.5χ current mean annual precipitation (Machette et al., 2013). Thus, during pluvial epochs, the lake would rise to successively higher levels owing to sedimentation; ultimately during MIS 12, the lake overflowed and spilled to the south. These new chronologic constraints indicate that filling and draining of Lake Alamosa occurred during deglaciation of MIS 12 (~<420 ka), and MIS 11-correlated interglacial 424-374 ka (Lisiecki and Raymo, 2005).

Sample Information and 3He Cosmogenic Surface Exposure Ages For Basalt Samples from the Northern Rio Grande Rift

Table 1.
Sample Information and 3He Cosmogenic Surface Exposure Ages For Basalt Samples from the Northern Rio Grande Rift

U-Th concentrations, U-series isotopic compositions, and calculated 230Th/U ages and initial 234U/238U activity ratios for subsamples of clast rinds from Guadalupe gravel pit, Taos County, New Mexico.

Table 4.
U-Th concentrations, U-series isotopic compositions, and calculated 230Th/U ages and initial 234U/238U activity ratios for subsamples of clast rinds from Guadalupe gravel pit, Taos County, New Mexico.

The discussions for Stops 1, 2, and 3 have been abstracted from two lengthy papers by Machette and others. The first is a detailed field guidebook for the 2007 Rocky Mountain Section Friends of the Pleistocene Field Trip—Quaternary Geology of the San Luis Basin of Colorado and New Mexico, September 7-9, 2007 (Machette et al., 2007). This is available online as a full color publication (http://pubs.usgs.gov/of/2007/1193/). The second summary paper focused on lake evolution and preliminary 3He dating results on Lake Alamosa’s shoreline deposits (Machette et al., 2013).

A third source of basic geologic data for the lake and surrounding basin can be found in the Geologic Map of the Alamosa 30′ × 60′ quadrangle, South-Central Colorado (Thompson et al., 2015). Most of the reconnaissance mapping is based on 1:25,000 or 1:50,000 scale detailed mapping previously published by the USGS. The Alamosa 30′ × 60′ quadrangle covers the central San Luis Basin of southern Colorado, which is bisected by the Rio Grande and encompasses the stops identified in Day 1 of this guide.

Stop 1—Bachus Borrow Pit

Coordinates: N 37.4458°, W 105.9169°, WGS84

Alamosa West 7.5′ quadrangle N., near center of Bachus pit Various locations around perimeter of Bachus pit

Elevation: About 2298 m (7540ft) asl at base of pit; ~2305 m (7562 ft) asl at top of pit

Driving directions: From Alamosa take U.S. Hwy. 285 south ~4.5 km (2.8 mi) and head west on County Rd. 10S. At 3.3 km (2.1 mi) turn right onto Rd. S 106, proceed ~1.6 km (1 mi) north and turn right into quarry entrance.

Landownership: The Bachus pit is owned by Dan Russell of Alamosa (Fig. 4). Dan earned a B.S. in geology from Adams State College in the 1970s, and currently owns and operates a land-surveying company in Alamosa. Visitors to this stop must obtain the owner’s permission prior to entering the pit and abide by any and all required safety requirements. We acknowledge Dan’s continuing support of geologic research in the San Luis Valley and topical studies of Lake Alamosa.

Figure 4.

Bachus pit owner sign and contact information. Photo by M.M. Machette.

Figure 4.

Bachus pit owner sign and contact information. Photo by M.M. Machette.

Introduction and Geology of the Pit

The Bachus pit is located on an east-trending topographic ridge that extends from west of the Alamosa 1:100,000 scale sheet (longitude 106° W) almost to Hwy. 285 (Alamosa to Antonito). The pit supplies ungraded and graded sand and gravel to local contractors for use as backfill. At an altitude of ~2298-2305 m (7540-7562 ft) asl, it is 30-25 m (100-120) ft below the threshold (overflow) elevation of Lake Alamosa in the San Luis Hills (Figs. 2, 3). Thus, during the highest stand of Lake Alamosa, the Bachus pit was under ~30-35 m of water off the southwestern coast of the lake. It is one of the few places in the paleolake basin where lake deposits are well exposed; elsewhere, post-lake surficial deposits, high water tables, and strong efflorescence of inter-bedded gypsum deposits typically obscure exposures of lacustrine sediment. Although the ridge containing the pit is only ~6 m (20 ft) above the surrounding landscape, its elevated position is well above the local water table, keeping this and other similar ridges in the area (Fig. 5) dry.

Figure 5.

Surficial geologic map of Stop 1. Units listed as follows (Thompson et al., 2015): Qai/QTla—middle Pleistocene gravel overlying Pliocene to Pleistocene lacustrine deposits of Lake Alamosa; Qay—late Pleistocene alluvium; Qa—late Pleistocene to Holocene alluvium; Qed—late Pleistocene to Holocene eolian deposits; Qfp—Holocene floodplain and overbank deposits; Qaa—active alluvium.

Figure 5.

Surficial geologic map of Stop 1. Units listed as follows (Thompson et al., 2015): Qai/QTla—middle Pleistocene gravel overlying Pliocene to Pleistocene lacustrine deposits of Lake Alamosa; Qay—late Pleistocene alluvium; Qa—late Pleistocene to Holocene alluvium; Qed—late Pleistocene to Holocene eolian deposits; Qfp—Holocene floodplain and overbank deposits; Qaa—active alluvium.

The ridge is relatively planar and has a gentle east slope of less than 0.002 (2 m/km [10 ft/mi]). It is composed of intermediate-age (map unit Qai, middle Pleistocene) alluvium, 2-3 m thick, that rests unconformably on lacustrine sediment of the Alamosa Formation (map unit QTla). Adjacent to the ridge are slightly lower valleys floored by latest Pleistocene (Qay?) to Holocene alluvium (Fig. 5) and Holocene eolian sand.

Excavated in 2005, exposures within the pit show evidence of transgression and regression of Lake Alamosa, followed by incision of alluvial channels graded to lower, post-lake highstand base levels, as the basin was exhumed and elevation of the active streams dropped. Deposits in the pit are well-graded sand and pebble gravel limited to 2-3 cm diameter, owing to the long transport distances (~15-20 km) from source rocks on the eastern margin of the San Juan Mountains. Coarse gravel is so rare in this flat basin that it must be hauled from pits in coarser-grained alluvial fans located ~15 km to the southwest. Although the pit walls change with continued mining, the flat-floored base of the pit rests on fine-grained, water-saturated silts and clays deposited by Lake Alamosa prior to its last transgression and subsequent overflow (~2298 m [7540 ft] elevation).

The sand and gravel making up the walls of the pit represent transgressive and regressive nearshore and low-energy beach deposits (Figs. 6 and 7). Between these deposits is sandy silt that is finely laminated and contorted by pressure loading from the overlying sedimentary package. The transgressive deposits were not exposed in the west wall of the pit but were visible in 2005 on the north and east walls (exposures depend on activity in the pit and regrading of the pit walls). The nearshore sandy gravels typically are heavily stained by iron and manganese oxides, which in some locations are abundant enough to cement beds of gravel. The transgressive deposits are overlain by laminated sandy silt representing a deeper water phase (unit 3, Figs. 6, 7). The silt is overlain by sandy pebble gravel that represents regressive near-shore deposits related to drawdown of the lake (unit 2, Figs. 6, 7). These deposits contain rip-up clasts of lake-bottom mud (derived from unit 3, Fig. 8), further indicating that they represent lake regression. Altogether, the entire cycle of lake sediment (units 2-4, Fig. 6) is only ~3 m thick.

Figure 6.

Middle Pleistocene stratigraphic section exposed in the Ba-chus pit. Modified from Machette et al. (2007).

Figure 6.

Middle Pleistocene stratigraphic section exposed in the Ba-chus pit. Modified from Machette et al. (2007).

Figure 7.

Photo of Bachus pit exposure with division of middle Pleistocene units. (A) Section showing a regressive lake sequence capped by a progradational, late middle Pleistocene gravel. (B) Convoluted and contorted lacustrine silts and clays capped by a regressive lakeshore gravel facies. Tape measure is 2 m in height, with white and red increments of 20 cm. Modified from Machette et al. (2007).

Figure 7.

Photo of Bachus pit exposure with division of middle Pleistocene units. (A) Section showing a regressive lake sequence capped by a progradational, late middle Pleistocene gravel. (B) Convoluted and contorted lacustrine silts and clays capped by a regressive lakeshore gravel facies. Tape measure is 2 m in height, with white and red increments of 20 cm. Modified from Machette et al. (2007).

Figure 8.

Photo of rip-up clasts of gravel of underlying desiccated lacustrine clays within a sandy pebble gravel. Knife is ~20 cm long.

Figure 8.

Photo of rip-up clasts of gravel of underlying desiccated lacustrine clays within a sandy pebble gravel. Knife is ~20 cm long.

Above the lacustrine sediment are several meters of late middle Pleistocene alluvial deposits (unit Qai, Fig. 5) and loess (unit 1, Fig. 7) deposited in response to falling base level as the lake emptied through the San Luis Hills. After the water level in Lake Alamosa dropped, the lake floor was probably mantled by thin and discontinuous regressive nearshore deposits and deeper-water fine-grained deposits, most of which were eroded and reworked in alluvial channels that coursed across the newly exposed lake bottom. Wind erosion of the lacustrine sediment resulted in deposition of a 0.5-1-m-thick layer of loess (silt and sand) that caps the alluvium. Relict soils formed on the loess and alluvium show discontinuous Bt and Bk? horizons suggestive of middle Pleistocene age.

Summary

The sediments exposed in the Bachus pit represent the last deep-water cycle of Lake Alamosa, which we believe culminated in overflow at <~360 ka. This package may be thinner than those deposited during older cycles of the lake; the entire regressive phase of the sediment package is related to lowering of the lake as its waters flowed over and cut through the bedrock threshold in the Fairy Hills (see Stop 3). If the lake had receded by evaporation, we would expect to see a thicker, coarsening-upward section of sediment between units 3a and 2 (Fig. 7) as the shoreline approached this site from the west and conditions changed from deep, quiet water to shallow, nearshore conditions, rather than an abrupt unconformity between these units.

As the lake dropped, so did base level for the tributary streams emanating from the eastern part of the San Juan Mountains and heading on the exposed lake floor. The largest of these streams, La Jara Creek and the Conejos River, flowed northeast and east toward the Rio Grande’s outlet from the Alamosa subba-sin. Stream channels that cut into the exposed lake-bottom sediment became preferentially filled with alluvium (unit Qai, Fig. 9) as the region was transformed from an exposed lake bottom to a gentle piedmont slope. However, the slightly more resistant nature of the alluvium-filled channels led to topographic reversals through time (i.e., inverted topography), forming the elongate ridges preserved today.

Figure 9.

Cross-sectional relationships of surficial deposits at Stop 1. Units listed as follows: QTla—Pliocene to Pleistocene lacustrine deposits of paleolake Alamosa; Qai—middle Pleistocene alluvium; Qes—middle to late Pleistocene eolian sediment; Qay—late Pleistocene alluvium; and Qa—Holocene alluvium.

Figure 9.

Cross-sectional relationships of surficial deposits at Stop 1. Units listed as follows: QTla—Pliocene to Pleistocene lacustrine deposits of paleolake Alamosa; Qai—middle Pleistocene alluvium; Qes—middle to late Pleistocene eolian sediment; Qay—late Pleistocene alluvium; and Qa—Holocene alluvium.

Stop 2—Ancient Spits of Lake Alamosa on Saddleback Mountain

Coordinates: N 37.25893°, W 105.84224°, WGS84

About 3 mi (4.8 km) east of Sanford, Colorado; 1.5 mi northeast of bridge over Rio Costilla

Pikes Stockade 7.5′ quadrangle

Elevation: About 2309 m (7575 ft) asl (base of slope at turnoff from paved road)

Original stop B-3 from Machette et al. (2007)

Driving directions: From Hwy. 285, head south ~19.7 km (12.2 mi) to the south side of La Jara, Colorado, and turn left onto Co Rd. W/CO-136 and proceed ~4.8 km (3 mi) to Sanford, Colorado. Merge south onto 1st St. and proceed ~1.5 km (0.9 mi) and turn left onto 2nd St. Proceed ~6.1 km (3.8 mi) to the turnoff road to the Saddleback Mountain spit deposits.

Landownership: This stop is on federal land managed by the Bureau of Land Management (BLM). Prior permission is not currently (2016) required to access this stop; please drive only on existing dirt roads.

Introduction and Local Geology

At Stop 2, we examine shoreline deposits preserved as spits on the southern side of Saddleback Mountain, an erosional remnant of Oligocene basaltic and basaltic andesite lava flows of the San Luis Hills. The spits at Stop 2 were formed by longshore (southwest) transport of eroded bedrock blocks and subsequent deposition of sandy cobble to boulder gravels on the leeside of Saddleback Mountain (Figs. 1012). The tops of both spits are planar and gently east sloping, which reflects deposition into deepening water in back bays to the south and southeast of Saddleback Mountain.

Figure 10.

Bedrock, alluvial deposits, and lacustrine deposits of Lake Alamosa. Map units: Qaa—active alluvium; Qa— Holocene alluvium; Qfp—floodplain alluvium; Qay—younger (late Pleistocene) alluvium; Qla—lacustrine deposits (post-Lake Alamosa); Qlag (gravelly) and Qlam (lagoonal) nearshore deposits of Lake Alamosa (middle Pleistocene part of QTla); QTla—Alamosa Formation (undivided, fine-grained deposits here); Th—Hinsdale Formation (26 Ma). Geology modified from Thompson and Machette (1989).

Figure 10.

Bedrock, alluvial deposits, and lacustrine deposits of Lake Alamosa. Map units: Qaa—active alluvium; Qa— Holocene alluvium; Qfp—floodplain alluvium; Qay—younger (late Pleistocene) alluvium; Qla—lacustrine deposits (post-Lake Alamosa); Qlag (gravelly) and Qlam (lagoonal) nearshore deposits of Lake Alamosa (middle Pleistocene part of QTla); QTla—Alamosa Formation (undivided, fine-grained deposits here); Th—Hinsdale Formation (26 Ma). Geology modified from Thompson and Machette (1989).

Figure 11.

Oblique aerial photograph of the south side of Saddleback Mountain, location of Stop 2. Unit Th—Hinsdale Formation (upper Oligocene andesitic basalt). View to the northeast (photo by M.M. Machette).

Figure 11.

Oblique aerial photograph of the south side of Saddleback Mountain, location of Stop 2. Unit Th—Hinsdale Formation (upper Oligocene andesitic basalt). View to the northeast (photo by M.M. Machette).

Figure 12.

Schematic cross sections of surficial and bedrock geology at Saddleback Mountain, modified from Machette et al. (2007). (A) Stacked spits (Qlag1 is older than Qlag2). (B) Superposed spits (Qlag1 is younger than Qlag2). Map units: Qlam—lagoonal deposits of Lake Alamosa; Qlag—gravel (spit) deposits of Lake Alamosa (Qlag1, lower spit; Qlag2, upper spit); QTal—Alamosa Formation, undivided; Th—Hinsdale Formation (upper Oligocene andesitic basalt). View to the west. Based on geologic mapping of Thompson and Machette (1989) and Thompson et al. (2015).

Figure 12.

Schematic cross sections of surficial and bedrock geology at Saddleback Mountain, modified from Machette et al. (2007). (A) Stacked spits (Qlag1 is older than Qlag2). (B) Superposed spits (Qlag1 is younger than Qlag2). Map units: Qlam—lagoonal deposits of Lake Alamosa; Qlag—gravel (spit) deposits of Lake Alamosa (Qlag1, lower spit; Qlag2, upper spit); QTal—Alamosa Formation, undivided; Th—Hinsdale Formation (upper Oligocene andesitic basalt). View to the west. Based on geologic mapping of Thompson and Machette (1989) and Thompson et al. (2015).

Saddleback Mountain, the elongate bedrock ridge north of this site, is formed by subhorizontal volcanic flows of the upper Oligocene to Miocene Hinsdale Formation (Figs. 10 and 11). These flows are mildly alkaline olivine basalts with as much as 20% phenocrysts of olivine and clinopyroxene. In the San Luis Hills, they have been dated at ca. 26 Ma (see Thompson and Machette, 1989; Thompson et al., 1991, 2015). At Lake Alamosa’s highest stand (~2335 m [7660 ft] asl), Saddleback Mountain was an island in the southern part of the lake. At this elevation, the lake extended westward to the eastern margin of the San Juan Mountains (near Monte Vista) and northward, nearly to Saguache, a distance of ~55 mi (88 km). Thus, the lake had a long fetch to the northwest, and during large storms waves eroded the northern face of Saddleback Mountain and other bedrock hills to the east of this stop.

Young gravelly alluvium (units Qa, Qaa, Qfp, and Qay; Fig. 10) that surrounds the northern and western flanks of Saddleback Mountain was deposited by the Rio Conejos, which eroded into a preexisting platform of fine-grained lacustrine sediment of the Alamosa Formation (unit QTla, Fig. 10). However, to the east and south of Saddleback Mountain, both coarse- and fine-grained deposits of Lake Alamosa are preserved to the leeward (south) of the mountain. The fine-grained deposits are poorly exposed and highly eroded, whereas the coarse-grained deposits are preserved in gravelly spits on the south side of Saddleback Mountain and as gravel-capped, bedrock-cored saddles between hills (unit Qlag; note: from here Rd. V passes through such a saddle). Younger lacustrine deposits (unit Qla, Fig. 10), which are probably reworked from sediment of the Alamosa Formation, typically occupy playas in topographic depressions, such as the one south of Saddleback Mountain.

Shoreline Deposits of Lake Alamosa

The two bouldery spits record two elevations of shoreline stability: an upper spit at 2340 m (7660 ft) and a lower spit at 2330 m (7640 ft) (Fig. 12). At localities where shoreline deposits were preserved elsewhere, typically only one spit, a shoreline or barrier bar, is recognized. Figure 12 shows two scenarios for the depositional/geomorphic relationships between the upper and lower spits. Figure 12A represents an aggradational rising-lake level scenario for deposition of the two spits. Figure 12B indicates construction and deposition of the upper spit during highstand and the lower spit forming as the lake is draining. Based on decreasing carbonate percentages from the upper spit to the lower spit, discussed below, we interpret the geomorphic relationships between the two spits to indicate a highstand-regressional draining episode.

Lower Spit

The lower spit has a well-developed calcic soil (Fig. 13) that is morphologically identical to the soil on the upper spit (Fig. 11), but it is not as well developed in terms of thickness and total carbonate. In New Mexico to the south, similar calcic soils are formed on middle Pleistocene deposits. Considering the site’s semiarid climate and high elevation (2329 m [7640 ft] asl), this soil is particularly well developed in a noncalcareous sandy basalt-rich gravel.

Figure 13.

Soil profile with textural and CaCO3 analyses of the lower spit at Saddleback Mountain.

Figure 13.

Soil profile with textural and CaCO3 analyses of the lower spit at Saddleback Mountain.

The elevation in this part of the basin ranges from ~2300-2400 m (7550-8000 ft) asl, so it is surprising to find abundant, thick calcic soils on the older landscapes. However, the climatic conditions are conducive to their formation now and apparently in the past also. Modern climatic conditions in Alamosa, ~15 mi (24 km) to the north of this stop, are cold (41 °F [5 °C] mean annual temperature) and relatively dry (180 mm [~7 in.] mean annual precipitation) on average for the year (see www.wrcc.dri.edu). These conditions are well within the normal limits of pedo-cal formation (see Machette, 1985). Rainfall increases slightly across the basin with distance from the San Juan Mountains, which creates a strong rain shadow in the San Luis Valley. Glacial climates must have been considerably colder and probably drier than present, thus preventing massive leaching of calcium carbonate from the soil.

The lower spit soil is formed in basaltic gravel that has no easily weatherable source of Ca2+. The entire Bk horizon is ~95 cm thick and has a maximum stage III morphology with a maximum of 50% CaCO3 in the <2 mm portion of the Bk2 horizon (Fig. 13). Texturally, the parent material for this soil is a sandy silt, with ~25% <2 mm matrix in a coarse cobble to boulder gravel. Textural analyses of the soil show a dominance of sand in the profile, but the silty component (as much as 41%) may be lacustrine in origin or be deeply translocated dust. Chittick analyses of the lowest Bk subhorizon show 3.8% CaCO3 in the <2 mm fraction; thus, we estimated that the parent material has only 1.0% CaCO3 in the <2 mm fraction (Machette et al., 2007).

At lower elevations of ~1370-1675 m (4500-5500 ft) in New Mexico to the south, morphologically similar soils are middle Pleistocene in age (Machette, 1985; Machette et al., 1997). Thus, the relict calcic soil on the lower spit is particularly well developed considering it is in a semiarid climate and at a high elevation (2329 m [7640 ft] asl), where leaching is more likely during wet/cold climates. Ruleman et al. (2013) provide a regional correlation of relative age of deposits and pedogenic carbonate development (Fig. 3, Table 1).

We interpret the stratigraphic relations between the lower and upper spits to reflect a multistage drawdown of the lake (rather than a single drop) subsequent to the breaching of the lake through the Fairy Hills (Stop 3). This is based largely on the relative degree of soil carbonate observed in each deposit. Exposures of preserved deposits are insufficient to resolve the stratigraphic relationships in the field.

Upper Spit

As with the lower spit, this soil is formed in basaltic gravel (Fig. 14) and thus has no easily weatherable source of Ca2+. The entire Bk horizon is morphologically identical to the soil on the lower spit, but it contains ~30% more calcium carbonate (on a total profile basis). The Bk horizon (undivided) is ~40 cm thicker than the lower spit’s carbonate soil at ~150 cm thick, and has a maximum stage III morphology and ~64% CaCO3 in the <2 mm portion of the Bk2 horizon. The parent material for this soil is gravelly sandy silt that contains ~25% <2 mm matrix in a coarse cobble to boulder gravel. Textural analyses of the soil (Fig. 14) show a dominance of silt (as much as 65%) in the <2 mm fraction; thus, much of the silt may be lacustrine in origin or deeply translocated eolian dust. The lowest Bk subhorizon has an estimated 4% CaCO3 in the <2 mm fraction; thus, we assumed that the parent material has only 1.0% CaCO3 in the <2 mm fraction (these assumptions are based on the soil on the lower spit).

Figure 14.

Soil profile with textural and CaCO3 analyses of the upper spit at Saddleback Mountain.

Figure 14.

Soil profile with textural and CaCO3 analyses of the upper spit at Saddleback Mountain.

Soil Carbonate Accumulation

We have calculated calcium carbonate accumulation rates at this site for comparison with rates at other sites in New Mexico, mainly as a relative dating tool. One can approximate the total amount of CaCO3 in a soil profile from the following parameters for each horizon (Machette et al., 2007): thickness, percent <2 mm fraction, percent CaCO3 in the <2 mm fraction, and bulk density of the <2 mm fraction. For a more complete discussion of this method, see Machette (1978, 1985) and Machette et al. (1997). The only fraction of pedogenic CaCO3 not accounted for in the following discussion are the thick pendants (rinds) that have accumulated on the base of some clasts. We suspect that this component might add as much as 10% to the calculations. The two soils that we analyzed have quite different amounts of pedogenic CaCO3: the lower spit has ~26.6 g of CaCO3 per cm2 column through the soil, whereas the upper spit has ~35.0 g (30% more). Using 30 ± 5 g as an average value, and a duration of soil formation of 390 ka yields a long-term accumulation rate of ~0.077 g/cm2/1000 yr. The upper spit contains the most carbonate; using its 35 g content yields a rate of ~0.090 g/cm2/1000 yr. By comparison, soils that Machette (1985) considered to be ca. 0.5 Ma in New Mexico had much higher calculated accumulation rates of 0.2-0.4 g/cm2/1000 yr. However, new dating control for the deposits that host these soils shows that they may be as much as five times older than considered in 1985, reducing the New Mexico accumulation rates to only 0.04-0.08 g/cm2/1000 yr. Thus, the maximum rate of 0.090 g/cm2/1000 yr calculated for this stop suggests that calcic soils in the San Luis Basin may have accumulated as fast as those in Albuquerque. Ruleman et al. (2013) provided a regional correlation of relative carbonate stage morphology with relative-age deposits associated with the MIS record (Fig. 3, Table 2).

Dating the Upper Spit

The upper spit has a large number of basalt boulders at the surface but most of them have been heavily weathered. The largest unweathered appearing boulders on this upper spit are located east of the soil pit (Fig. 10). Cosmogenic surface exposure dating of the boulders should reflect the length of time that they have been exposed to cosmic radiation and thus provide a minimum time for the overflow and draining of Lake Alamosa. Approximately 220 m east of the soil pit, Machette et al. (2007) reported a 3He exposure age of 439 ± 6 ka (sample PS-MM05-72He, Fig. 11). We have recalibrated that with contemporary production rates (Goehring et al., 2010; Borchers et al., 2015) 384.7 ± 13.6 ka and provide two additional 3He exposure ages of 349.0 ± 12.1 ka and 289.1 ± 9.9 ka (Table 1). Owing to possible inherited 3He or recent spalling, the ages can be younger or older than the time of deposition. The mean average of these ages and standard deviation for age uncertainty is 341.0 ± 11.9 ka.

Using the 3He surface exposure ages of 384.7 ± 13.6, 349.0 ± 12.1, and 289.1 ± 9.9 ka, we conclude that the upper spit at Saddleback Mountain was deposited in a high-energy shoreline geomorphic setting during the lake-draining interval of ~100 k.y., beginning after ~400 ka (MIS 11, ~424 ka-374 ka) (Fig. 3; Table 3; Lisiecki and Raymo, 2005). Being located proximal to the outlet/dam, the spits could be constructed and reworked during the entire drainage interval as glacial-interglacial episodes associated with MIS 10-7 (374-243 ka) occurred. These ~390-290 ka age estimates for the boulders sampled at Saddleback Mountain should reflect the time since these boulders were exposed to cosmic radiation, and thus they provide a minimum time for the overflow and draining of Lake Alamosa.

Stop 3—Outlet of Lake Alamosa in San Luis Hills

Coordinates: N 37.21755°, W 105.74721°, WGS84

Overlook, east of BLM Rd. 5003, west side of Rio Grande,

~1.5 mi southeast of La Sauses cemetery

Mesito Reservoir 7.5′ quadrangle

Elevation: 2319 m (7610 ft) asl (overlooking river)

Original stop B-4 from Machette et al. (2007)

Driving directions: From the Saddleback Mountain turnoff, go back southwest ~0.7 km (0.4 mi) and turn left onto Rd. V. Proceed to the southeast ~4.4 km (2.7 mi) and turn left onto Rd. U. Follow Rd. U ~4.6 km (2.9 km) and follow main gravel road to the La Sauses cemetery. At the cemetery, proceed south on the main dirt road along the Rio Grande south for ~2.2 km (1.4 mi) and turn left on to the road leading to the Rio Grande ~0.5 km (0.3 mi) to the east.

Landownership: Bureau of Land Management

Introduction

This scenic overview of the Rio Grande is located near the overflow point of Lake Alamosa (Figs. 3 and 15). The gap between the Fairy Hills and Brownie Hills to the south of here is the lowest point in the San Luis Hills, and thus it represents the first opportunity for a rising Lake Alamosa to overtop a hydro-logic sill. Overflow of the lake through the Fairy Hills cut a deep, narrow gorge and allowed the lake waters to drain southward across the Costilla Plain and northern Taos Plateau, eventually joining the Red River and Rio Grande, west of Questa, New Mexico. In addition, Stop 3 is near the point where Jacob Fowler described the first account of Lake Alamosa—in 1811-1812 (from Siebenthal, 1910, p. 112-114):

Figure 15.

Geologic map of Stop 3. Units listed as follows: Tc—Oligocene Conejos Formation; Td—dacite sills; QTla—Pliocene to middle Pleistocene Alamosa Formation; Qlag—middle Pleistocene outflow lag gravel deposits; Qai—late middle Pleistocene alluvium; Qay—late Pleistocene alluvium; Qa— Holocene alluvium. Faults shown by thick red lines. Modified from Thompson et al. (2015).

Figure 15.

Geologic map of Stop 3. Units listed as follows: Tc—Oligocene Conejos Formation; Td—dacite sills; QTla—Pliocene to middle Pleistocene Alamosa Formation; Qlag—middle Pleistocene outflow lag gravel deposits; Qai—late middle Pleistocene alluvium; Qay—late Pleistocene alluvium; Qa— Holocene alluvium. Faults shown by thick red lines. Modified from Thompson et al. (2015).

I Have no doubt but the River from the Head of those Rocks up for about one Hundred miles Has once been a lake of about from forty to fifty miles Wide and about two Hundred feet deep—and that the running and dashing of the Watter Has Woren a Way the Rocks So as to form the present Chanel.

We suspect that Jacob Fowler made this observation from the San Luis Hills (probably in the Fairy Hills, above the river’s gorge near here), looking north into the upper San Luis Valley. Fowler accurately estimated the size, depth, and overflow history of the lake—186 years ago.

Almost a half century later, F.V. Hayden led a team of scientists through the San Luis Valley. In his report dated 1877, geologist F.M. Endlich (1877) reports the following about the valley (our additions are in brackets):

Judging from the evident deflection of rivers, the failure of mountain-streams to carry speci-mens of the rock through which they pass into the valley for any distance,. . . and the cañoned outlet of the Rio Grande, I have come to the conclusion that at one time San Luis Valley was covered by two large lakes, the northern and the southern [there is no evidence for a southern lake on the Costilla Plain]. These I have named. . . Coronado’s Lakes. Of these the former covered about 1,400 square miles [3626 km2]; the latter 300 square miles [777 km2]. I have alluded to the cañon near station 105, cut through the trachyte [Conejos Formation]. It is about three miles [5 km] in length, and its general direction is perfectly straight. In case that narrow passage [gorge through the Fairy- Brownie Hills], which I assume to have been opened by seismic force, should be closed today, the result would be an accumulation of water in the northern end of the San Luis Valley, the formation of a lake. This lake would reach a certain depth of water, consequently increase in area until the slight rise southwest of Fort Garland would be overcome, and it would flow over into the southern region [Costilla Plain].. . . It may seem curious that no heavy deposits of alkali or old “shore-lines” mark the presence of these ancient lakes. If, however, the assumption that the Grande found a sudden egress through the deep fissure produced by a volcanic earthquake is true, there is no reason why the waters should not have flown off by far too rapidly to permit of the formation of either. . . (in Hayden’s 1875 Ninth Annual Report, dated 1877, p. 147)

Not surprisingly, many of the details reported by the Hayden team have proved wrong, such as the presence of a southern lake, eastern flow path for the Rio Grande, and lack of shoreline (related) features, but Hayden accurately suspected that fault-weakened rocks may have led to quick and easy erosion of the canyon through the Fairy-Brownie Hills. It seems unlikely that an earthquake caused a fissure and sudden draining of the lake, but rather that the discharge was the result of a rising lake that overtopped a natural bedrock sill during pluvial conditions.

Local Geology

On the route to Stop 3, we drive across a bedrock-cored saddle (Fig. 15). This saddle is covered with small rounded pebbles of reworked volcanic rock from the adjacent slopes (unit Qlag, Fig. 15). These gravels represent low-energy, beach (shoreline) deposits of Lake Alamosa at its highest stand (~2335 m [7660 ft] asl). A soil pit excavated nearby exposed sandy pebble to small cobble gravel that was deposited just offshore (west of) the highest shoreline. Further west, gullies expose fine-grained sand, silt, and marl that represent near-shore deposits (Alamosa Formation) of Lake Alamosa.

Upper Oligocene intrusive and extrusive rocks of the Conejos Formation (Fig. 15) dominate the bedrock geology in the San Luis Hills (Thompson and Machette, 1989; Thompson et al., 2015). These hills are the surface expression of a large northeast-southwest-trending horst that divides the sedimentary fill of San Luis Basin into subbasins. To the north of the hills, the Alamosa subbasin is primarily an east-tilted half graben, although seismic reflection and geophysical data show internal complications (such as two grabens and an intervening horst). To the southeast of the San Luis Hills, the Costilla subbasin extends into New Mexico as far south as Questa. The Costilla subbasin contains the Culebra graben on the east (Fort Garland south to Sanchez Reservoir) and San Pedro Mesa (a Precambrian-cored Miocene horst) and the Costilla Plain on the west (Fig. 3).

The lower part of the Conejos Formation is exposed throughout the Fairy and Brownie Hills. Most of the bedrock is 29 to 30 Ma andesitic to porphyritic dacite flows, but dacite dikes with north to north-northeast trends are common (Fig. 15). Although unmapped by Thompson and Machette (1989), Burroughs (1972, 1978) mentioned a major north-south-trending Neogene fault (the Lasauses fault) that parallels the course of the Rio Grande. There is little local stratigraphic evidence for this fault (Conejos against Conejos). However, extensive subparallel dikes and hydrothermally altered rock in this area support extensive deformation and weakening of the bedrock (Conejos Formation) in the eventual outlet area for Lake Alamosa. Thompson et al. (2015) subsequently identified this zone marked by extensive alteration and deformation as the Fair Hills fault zone. This is supported by down-to-east displacement of Hinsdale Basalt across the Fairy Hills along a transect through Flat Top Mesa (directly to the west) and the Brownie Hills to the east.

Before an outlet for the lake was carved through the San Luis Hills, the hills were a formidable barrier to southward-flowing streams. The rocks of the San Luis horst extend from near Antonito on the southwest to 8 km (~5 mi) southwest of Blanca on the northeast (Fig. 3). In essence, these hills blocked the entire south margin of the Alamosa subbasin. In addition, eruption of tholeiitic flood basalts (Servilleta Basalt) from 4.8 to 3.7 Ma also blocked possible southward drainage (or drainages) from the San Luis Basin. These flows extended north from the Taos Plateau at least to the areas of La Jara on the west and Fort Garland on the east. Thus, by middle Pliocene time (ca. 3.5 Ma), drainage southward from the Alamosa subbasin was blocked across the entire width of the San Luis Valley (Machette et al., 2013). Although it is undated, we assume that the lacustrine sediment at the base of the Alamosa Formation has a maximum age of 3.5 Ma.

From Stop 3, one would have looked northwest across Lake Alamosa ~48 km (30 mi) to see the Sangre de Cristo Mountains and Blanca Peak, which at 4372 m (14,345 ft) asl, is the highest point between Pikes Peak and the San Juan Mountains. At its highest elevation of 2335 m (7660 ft) asl, Lake Alamosa would have occupied most of the San Luis Valley north of here. As such, the lake would have been 105 km (65 mi) from north to south and as much as 48 km (30 mi) from west to east (Fig. 3). The 2335 m (7660 ft) contour extends west to the Monte Vista National Wildlife Refuge and beneath Monte Vista (town), north to a point several miles southwest of Saguache and south of Mineral Hot Springs in the north end of the valley and then south along the east margin of the valley. The contour is coincident with several natural springs that emerge from the western base of the Great Sand Dunes.

Sedimentation in the closed Alamosa subbasin led to aggradation of the basin floor, although Pliocene to Pleistocene movement on the Sangre de Cristo fault system probably continued to keep the eastern side of the basin somewhat lower. As with classic, one-sided grabens (such as Death Valley), fans on the passive (west) side of the San Luis Valley are very large, whereas the east-side fans tend to be more compact owing to downdropping and continued burial.

By middle Pleistocene time, Lake Alamosa had grown laterally to such an extent that it occupied most of the Alamosa subbasin. The 3He-exposure dates from basalt in the upper spit at Stop 2 suggest that the lake reached its highest (topographic) level around 340 ka (average of all 3He ages from the upper spit at Saddleback Mountain) during MIS 10 (374-337 ka, Lisiecki and Raymo, 2005). At a water level of 2335 m (7660 ft asl), Lake Alamosa would have surrounded many of the isolated hills in this area and formed an embayment into the Fairy-Brownie Hills, as shown in Figure 2.

Overflow and Erosion

When Lake Alamosa rose to 2335 m asl (7660 ft), water could spill south over the low divide between the Fairy Hills and the Brownie Hills (Figs. 3, 15). There may have been several pathways to the south, one of which is now abandoned but intact at the top of the hill to the west of us (Fig. 15, sill at 2332 m [7650 ft] asl). This sill has no gravel on it now, probably owing to scouring. The southward-flowing water must have found a softer avenue (along the river’s present course) and cut a deep gorge through the Tertiary volcanic rock (Fig. 15). We don’t know how deeply or how quickly the outlet gorge through the San Luis Hills was cut, but there are several lines of evidence to suggest it was cut rapidly and incised deeply.

First, in most cases the preserved barrier bars, lagoons, and spits are at restricted elevation ranges, typically 2329-2337 m (7640-7670 ft) asl (Machette et al., 2007). The highest lacustrine landforms and features are located in the southern part of the basin (Fig. 2), where storm surge, coupled with wave action, generated by the lake’s long fetch, may have formed higher than normal shorelines. Where multiple shoreline features are preserved (for example, the two spits at Stop 2), they are constructional features that are typically related to lake transgression rather than regression. Also, the Alamosa subbasin lacks the repetitious shorelines characteristic of slow drawdown, such as between the Provo and Gilbert shorelines of latest Pleistocene Lake Bonneville that Machette had previously mapped in Utah Valley, Utah (Machette, 1988).

Second, the isolated topographic ridges associated with intermediate-age (unit Qai) alluvial channels (Stop 1) are graded to base levels that are relatively low (2295-2298 m [7530-7540 ft] asl), perhaps only 9-12 m (30-40 ft) above the present channel of the Rio Grande. If these channels formed primarily in response to lake lowering, then the canyon of the Rio Grande must have been cut deeply as a result of overtopping, perhaps excavating as much as 45 m of the total 55 m of relief between the Lake Ala-mosa overflow and present river level.

Interestingly, we have not found large-scale, outburst or abnormal flood deposits along the Rio Grande to the south, thus ruling out rapid, catastrophic draining. There is one feature that might be suggestive of catastrophic overflow from Lake Alamosa. Where the Rio Grande leaves the San Luis Hills and enters the Costilla Plain, the intermediate-age (post-Lake Ala-mosa) terraces of the river (units Qam1 and Qam2 of Thompson and Machette, 1989) become extremely wide. In map pattern, these terraces suggest that the Rio Grande excavated widely and deeply into older alluvium (unit Qao) and the underlying Santa Fe Group sediment (unit QTsf) before flow was constricted to the present gorge area and cutting into Pliocene Servilleta Basalt of the Taos Plateau.

Path of the Rio Grande

Prior to overflow of Lake Alamosa, the Rio Grande probably had its headwaters in the present Rio Chama, and southern flanks of the basaltic tablelands buttressing the southern margin of the southern San Luis Basin and Picuris Mountains. Wells et al. (1987) suggested that the Rio Grande was sourced by the Red River prior to complete basin integration. This would indicate that the southern San Luis Basin, south of the Red River fault zone, and the Taos Plateau were already integrated with basins to the south, contrary to the interpretations presented herein. Streams draining from the Sangre de Cristo Mountains from Questa north to San Luis drained out into Sunshine Valley and the Costilla Plain, both of which were largely closed or poorly drained basins. As drainage of Lake Alamosa occurred, the outflowing water eroded through subbasin bounding structures (i.e., Ute Mountain, Red River fault zone, and Los Cordovas fault zone) first before systematic headward erosion could ensue. The position of the Gorge fault zone is where the overflow waters began to incise into along the axis of the present Rio Grande gorge. Previous studies have suggested a systematic northward/ headward erosion and stream capture (Wells et al., 1987) of the Rio Grande into the southern San Luis Basin. However, these new chronologic constraints indicate that the entire San Luis Basin was not integrated into the Rio Grande until this draining event beginning at ~360 ka.

The modern Rio Grande (north of Questa) probably has been in the same channel since Lake Alamosa overflowed in middle Pleistocene time (~360 ka), owing to progressive incision through the Servilleta Basalt. Upon exiting the Fairy Hills, the overflow entered the lower, southward-flowing part of Culebra Creek (Fig. 3). From here, the overflow took a southerly route following the lowest elevation area between the southwest-sloping Costilla Plain and the southeast-sloping piedmonts of the San Luis Hills. Santa Fe Group basin-fill sediment containing abundant Precambrian clasts extends across the entire Costilla Plain and west of the present Rio Grande, indicating that the Santa Fe sediment and the unconformable cap of older gravel (unit Qao1, Fig. 3) that forms the Costilla Plain predate overflow of the lake (Fig. 3). At the south end of the Costilla Plain (near the New Mexico-Colorado border), the overflowing lake water excavated basin-fill sediment and exhumed some previously buried Servilleta Basalt. The overflow continued to the south, generally taking a course along the east margin of the east-dipping Servilleta Basalt and Gorge fault zone (Fig. 3). Finally, the overflow coursed between Cerro Chiflo and Guadalupe Mountain (near Cerro, New Mexico) and began excavating the canyon across the Red River fault zone.

Stop 4—Stations of the Cross, San Pedro Mesa, San Luis, Colorado

Coordinates: N 37.2018°, W 105.4295°, WGS84

Overlook of the Costilla Plain from atop San Pedro Mesa

Mesito Reservoir 7.5′ quadrangle

Elevation: 2478 m (8130ft) asl

Driving directions: From the intersection of Rd. U and Rd. 28, proceed north ~7.5 km (4.7 mi) and turn right onto Rd. Z. Proceed ~12.6 km (7.8 mi) to Rd. 11. Follow Rd. 11 merging into Rd. X 17.2 km (10.7 mi) to Hwy. 159 and turn south into the town of San Luis and Stop 4.

Introduction and Local Geology

This stop is located on the footwall of the San Pedro horst block, bound on both the east and west sides by active Quaternary faults (Thompson et al., 2015). The spatial pattern and magnitude of offset of these and other regional Quaternary faults through time provide evidence of the effect of neotectonics of the San Luis Basin on the drainage integration of the Rio Grande. The horst block is cored with Proterozoic rocks (ca. 1.4-1.8 Ga), unconform-ably buried by Oligocene to Pliocene volcanic and sedimentary rocks. To the east across the Sanchez graben, the Culebra Range rises to greater than 4270 m 14,000 ft asl in elevation, uplift that is accommodated by the central Sangre de Cristo fault zone (Figs. 2 and 16). The morphology of the range front is strikingly different from the linear, precipitous range fronts to the north and south along the northern Sangre de Cristo fault zone and southern Sangre de Cristo fault zone, respectively. The eastern margin of San Pedro Mesa is bound by the northern end of the southern Sangre de Cristo fault zone, displaying long-term slip rates >0.1 mm/yr, but lacking convincing evidence for a late Pleistocene event (Rule-man and Machette, 2007; Ruleman et al., 2007, 2013).

Figure 16.

Sangre de Cristo fault system and fault zone boundaries (modified from Menges, 1990; Ruleman and Machette, 2007; Ruleman et al., 2013). Sangre de Cristo fault zone— SDCFZ. Fault zone boundaries shown as: A—northern SDCFZ; B—central SDCFZ; and C—southern SDCFZ.

Figure 16.

Sangre de Cristo fault system and fault zone boundaries (modified from Menges, 1990; Ruleman and Machette, 2007; Ruleman et al., 2013). Sangre de Cristo fault zone— SDCFZ. Fault zone boundaries shown as: A—northern SDCFZ; B—central SDCFZ; and C—southern SDCFZ.

Figure 17.

(A) Photo of fluvially scoured basalt at Stop 5. (B) Fluvially scoured surface and rounded basalt clasts of Servilleta Basalt. (C) Scour pit in highly vesicular gas vent tubes. (D) View to the north from Stop 5 across the northern Costilla Plain showing the very subtle topographic relief between pre-gorge gravels. (E) Cross section A-A (Fig. 3) showing position of Stop 5 in relation to the middle Pleistocene drainage path of Lake Alamosa and 3He surface exposure ages along the gorge rim. asl— above sea level. Labels as follows: RPT—Rio Pueblo de Taos; LP—La Junta Point; RG—Rio Grande; RRFZ—Red River fault zone; NSLB—northern San Luis Basin. Tsb—Servilleta Basalt; Tov—Tertiary older volcanics; Tsf—Santa Fe Group older; QTsf—Santa Fe Group. (F) Maximum basin aggradation level at the close of MIS 12 (~420 ka). (G) Initial dissection of Qao1 surface during the first phase of Lake Alamosa drainage and basin integration culminating ~MIS 8 (~243 ka) (photo by C.A. Ruleman). All marine oxygen isotope stages (MIS) ages from Lisiecki and Raymo (2005).

Figure 17.

(A) Photo of fluvially scoured basalt at Stop 5. (B) Fluvially scoured surface and rounded basalt clasts of Servilleta Basalt. (C) Scour pit in highly vesicular gas vent tubes. (D) View to the north from Stop 5 across the northern Costilla Plain showing the very subtle topographic relief between pre-gorge gravels. (E) Cross section A-A (Fig. 3) showing position of Stop 5 in relation to the middle Pleistocene drainage path of Lake Alamosa and 3He surface exposure ages along the gorge rim. asl— above sea level. Labels as follows: RPT—Rio Pueblo de Taos; LP—La Junta Point; RG—Rio Grande; RRFZ—Red River fault zone; NSLB—northern San Luis Basin. Tsb—Servilleta Basalt; Tov—Tertiary older volcanics; Tsf—Santa Fe Group older; QTsf—Santa Fe Group. (F) Maximum basin aggradation level at the close of MIS 12 (~420 ka). (G) Initial dissection of Qao1 surface during the first phase of Lake Alamosa drainage and basin integration culminating ~MIS 8 (~243 ka) (photo by C.A. Ruleman). All marine oxygen isotope stages (MIS) ages from Lisiecki and Raymo (2005).

Figure 18.

(A) View looking north from the southern margin of the Taos Plateau showing the extremely flat basaltic tableland of the southern San Luis Basin that Lake Alamosa flowed across initiating ~<400 ka (photo by R.A. Thompson). (B) Cross section A-A’ showing position of Stop 6 in relation to the middle Pleistocene drainage path of Lake Alamosa and 3He surface exposure ages along the gorge rim. (C) Maximum basin aggradation level at the close of MIS 12 (~420 ka). (D) Initial dissection of Qao1 surface during the first phase of Lake Alamosa drainage and basin integration culminating ~MIS 8 (~243 ka). See Figure 17 for geologic unit descriptions.

Figure 18.

(A) View looking north from the southern margin of the Taos Plateau showing the extremely flat basaltic tableland of the southern San Luis Basin that Lake Alamosa flowed across initiating ~<400 ka (photo by R.A. Thompson). (B) Cross section A-A’ showing position of Stop 6 in relation to the middle Pleistocene drainage path of Lake Alamosa and 3He surface exposure ages along the gorge rim. (C) Maximum basin aggradation level at the close of MIS 12 (~420 ka). (D) Initial dissection of Qao1 surface during the first phase of Lake Alamosa drainage and basin integration culminating ~MIS 8 (~243 ka). See Figure 17 for geologic unit descriptions.

Figure 19.

(A) View looking west across the Rio Grande gorge from Qao2 surface to Stop 7. Exposure ages and fluvially scoured strath surfaces are shown (photo by C.A. Ruleman). (B) Cross section B-B’ (showing topographic relationships between La Junta Point (LP)/confluence of the Rio Grande (RG) and Red River (RR) and the pre-gorge outflow alluvial plain to the south. CM—Cebolla Mesa; unit Ty is Servilleta Basalt and related rocks, Pliocene to Pleistocene (<5 Ma). (C) Cross section C-C (Fig. 3) showing structural and depositional/ erosional relationships at Arroyo Hondo. (D) Cross section D-D (Fig. 3) showing 3He surface exposure ages in relation to canyon incision history.

Figure 19.

(A) View looking west across the Rio Grande gorge from Qao2 surface to Stop 7. Exposure ages and fluvially scoured strath surfaces are shown (photo by C.A. Ruleman). (B) Cross section B-B’ (showing topographic relationships between La Junta Point (LP)/confluence of the Rio Grande (RG) and Red River (RR) and the pre-gorge outflow alluvial plain to the south. CM—Cebolla Mesa; unit Ty is Servilleta Basalt and related rocks, Pliocene to Pleistocene (<5 Ma). (C) Cross section C-C (Fig. 3) showing structural and depositional/ erosional relationships at Arroyo Hondo. (D) Cross section D-D (Fig. 3) showing 3He surface exposure ages in relation to canyon incision history.

To the southwest lies the vast Costilla Plain, bound by the Gorge fault zone (Fig. 3) and San Luis Hills. The southern margin of the San Luis Hills marks the northern boundary of the southern San Luis Basin and Sunshine Valley-Costilla Plain subbasin (Ruleman et al., 2013). This broad, gently westward-sloping, alluvial plain is cut by the Mesita fault, which shows evidence for repeated late Pleistocene displacements (Fig. 3). The youngest volcanic activity in the entire basin emanates from this fault zone at Mesita cone, a ca. 1 Ma cinder cone that was partially buried by the alluvial plain. Subsequent subsidence along the southern Sangre de Cristo fault zone and associated intra-basin fault zones allowed alluvium of unit Qao1 to gently prograde westward around the shoulders of the edifice, but not completely bury it (Thompson et al., 2007). Recent mining of the cone for scoria has removed most of its geomorphic signature.

Heading west from San Luis, Colorado, and crossing through San Pedro Mesa (horst), the broad vast Costilla Plain has pro-graded westward from sources in the Sangre de Cristo Mountains (Figs. 2 and 3). Due west, the southern margin of the San Luis Hills can be seen as the distal margin of this Pliocene to middle Pleistocene closed basin. The basin is bounded on the west by the Gorge fault zone, on the north by the San Luis Hills horst, and on the south by the Red River fault zone (Fig. 3) (Ruleman et al., 2007, 2013). Multi-generational landslides, of possible seismo-genic origin, flank the western margin of San Pedro Mesa. The middle of the basin is cut by the active Sunshine Valley-Mesita fault zone, displaying at least one late Pleistocene (<30 ka) event (Ruleman and Machette, 2007; Ruleman et al., 2013). Pliocene volcanic edifices, such as Ute Mountain and Guadalupe Mountain, are generally aligned subparallel and proximal to intra-basin fault zones, with the youngest center being Mesita cone (ca. 1 Ma), a spatter/cinder cone erupted through the Santa Fe Group, comprised of relatively fine-grained, lower-energy, basin-fill deposits. Note the linear, precipitous range front of the Latir Peaks section of the Sangre de Cristo Mountains to the south. This range front is marked by juvenile, steep, faceted spurs, and older, steep alluvial fans with discontinuous fault scarps. Geo-morphic, paleoseismic, structural, and geophysical studies have determined that the juvenile appearance of the range front is a relict of >200 k.y. of accelerated tectonic activity rates (Ruleman and Machette, 2007; Ruleman et al., 2013). As middle to late Pleistocene tectonic activity waned, steeper fanheads have been incised and younger alluvium (<200 ka) has prograded out into the basin (Ruleman et al., 2013).

The highest aggradational/basin-fill level and its underlying deposit (Qao1) are marked by a regional unconformity comprised of fine-grained basin-fill deposits, with a dominant eolian component, which locally contain the Lava Creek B tephra (~640 ka) in the uppermost section, truncated and capped by a coarse-grained gravel. This datum provides a minimum age for the Santa Fe Group and maximum age for overlying coarser gravel deposits in the San Luis Basin (Ruleman et al., 2007, 2013). We interpret the marked transition from a lower-energy to higher-energy dep-ositional environment as the onset of regional North American Pleistocene glacial episodes associated with MIS 16 (~676-624 ka) and younger glacial episodes (Lisiecki and Raymo, 2005).

Temporal-Spatial Neotectonic Activity of the Sangre de Cristo Fault System

Paleoseismic evidence and relative tectonic geomorphology of the region indicate that activity along the southern Sangre de Cristo fault zone was an order of magnitude greater (>0.3 mm/ yr) prior to ~130 ka (Ruleman et al., 2013). This prior period of accelerated basin subsidence allowed for the Sunshine Valley-Costilla Plain subbasin to aggrade, and to remain hydrologically separated from the Taos Plateau by the Red River fault zone. With the onset of Pleistocene glacial episodes in the region (>620 ka) (MIS 16) (Ruleman et al., 2013), basin sedimentation rates may have equated or exceeded tectonic subsidence rates and the basin have begun to fill. By the end of MIS 12 (~420 ka), sediment accumulated in the northern San Luis Basin enough to raise base level, and thus caused Lake Alamosa to spill over through the San Luis Hills during the next lake cycle.

The temporal-spatial migration of tectonic activity can be easily demonstrated by the tectonic geomorphology at this location, as well as elsewhere within the Rio Grande rift (Machette, 1998). To the south, the linear, precipitous range front of the Latir Peaks, bound by the southern Sangre de Cristo fault zone, displays a waning late Pleistocene paleoseismic activity, but rangefront morphology is a relict of previous periods of greater tectonic activity. To the east of Stop 4 lies the highly embayed, dissected, and eroded range front of the Culebra Range bounded by the central Sangre de Cristo fault zone displaying <40 ka displacement. We propose that this represents a temporal-spatial migration of tectonic activity along the Sangre de Cristo fault system. The three fault zones can behave differently in order of magnitude of slip rate (>0.1 versus <0.1 mm/yr) on the 100 k.y. seismic cycle, but generally maintain a regional extensional rate of >0.1 mm/yr. This explains why the older, eroded morphology of the central Sangre de Cristo fault zone has the most recent seismic activity, and the linear, precipitous range front of the southern Sangre de Cristo fault zone demonstrates a hiatus in paleoseismicity (Rule-man and Machette, 2007; Ruleman et al., 2013).

Stop 5—Northwest Flank of Ute Mountain

Coordinates: N 36.9601°, W 105.7181°, WGS84

Rio Grande gorge overlook on northwest flank of Ute Mountain

and proximal to confluence of Costilla Creek

Ute Mountain, NM 7.5′ quadrangle

Elevation: 2298 m (7540 ft) asl (overlooking river)

Driving directions: From Hwy. 159 head south approximately to Rd. B north of Costilla, New Mexico. Turn right and proceed ~2.5 km (1.6 mi) and turn left and proceed another 1.0 km (0.6 mi) south and merge right onto Rd. A. Proceed ~8.9 km (5.5 mi) to the entrance to the Ute Mountain management area. Follow the main management road ~10.5 km (6.5 mi) to the parking area.

Landownership: Bureau of Land Management

Introduction and Local Geology

Heading west from San Luis, Colorado, and crossing through San Pedro Mesa, the broad vast Costilla Plain spans westward, prograding from the Sangre de Cristo Mountains (Figs. 2 and 3). Due west, the southern margin of the San Luis Hills can be seen at the distal margin of this Pliocene to middle Pleistocene closed basin. The basin is bound on the west by the Gorge fault zone, on the north by the San Luis Hills horst, and on the south by the Red River fault zone (Fig. 3) (Ruleman et al., 2007, 2013). Multi-generational landslides, of probable seismogenic origin, line the western flank of San Pedro Mesa. The basin is centrally cut by the active Sunshine Valley-Mesita fault zone, displaying at least one late Pleistocene (<30 ka) event (Ruleman and Machette, 2007; Ruleman et al., 2013).

Ute Mountain is a ca. 3.9 Ma dacitic volcano built along the western margin of the Sunshine Valley-Costilla Plain (Thompson et al., 2014a). It is strikingly aligned, as are other volcanic centers, with the Gorge and Mesita fault zone. Ute Mountain is the first topographic high along the path of the Rio Grande across the southern San Luis Basin between the San Luis Hills and the southern edge of the Taos Plateau at Pilar, New Mexico. Prior to drainage of Lake Alamosa and dissection/incision of the closed southern San Luis Basin, Qao1 gravels had prograded across the basin to the maximum aggradation level. At this location on the northwest flank of Ute Mountain, 3He exposure ages reveal initial southward-driven erosion as Lake Alamosa began to drain across this first topographic impediment within this basin of little topographic relief.

Geomorphic Setting, Timing of Canyon Incision, and Canyon Incision Rates

At this location, gravels sourced from alluvial and debris flow fans on the flanks of Ute Mountain aggraded and prograded across the present position of the gorge prior to gorge incision (Ruleman et al., 2007, 2013). 3He exposure ages average ~250 ka (Table 1) at this location and are interpreted as the time at which pre-gorge gravels were ultimately eroded, exhuming the underlying basalt, and fluvially scouring the basaltic surface as canyon incision and basin exhumation proceeded. This indicates these pre-gorge gravels must be >250 ka in age. Note the abundant rounded basalt clasts on the scoured basalt (Fig. 17B). Scour pits have formed on the basaltic rim surface, where pebbles have abraded into highly vesicular gas tubes within the more massive basaltic flows (Fig. 17C). These original volcanic emplacement structures are beautifully preserved in the polygonal crack pattern radiating around the brittle disruption feature occurring on the surface of the chilled lava flow. Thus, these surfaces show little to no erosion of the original volcanic surface prior to deposition of closed-basin gravels, and subsequent exhumation and erosion during this basin integration interval. This indicates the surface exposure ages should provide accurate constraint on the timing of gorge incision at this elevation level. Based on the elevation difference between the basaltic rim and active Rio Grande, the average incision rate at this stop over the past 250 ka was between 0.2 and 0.4 mm/yr.

As Lake Alamosa reached its highstand and begin to drain to the south across the relatively flat Costilla Plain (Figs. 17D and 17E), preexisting, maximum aggradation-level gravels (Qao1) were eroded and reworked to the south (Figs. 17F and 17G). The age of this exhumed surface (~250 ka) is younger than 3He exposure ages calculated along the gorge rim on the southwestern flank of Ute Mountain (~274 ka, Stop 14), the oldest 3He exposure ages recorded along the gorge rim and across the previously closed Sunshine Valley-Costilla Plain. We interpret this spatial and temporal younging relationship of exposure ages from south to north across Ute Mountain to be the result of initial southward/ downstream erosion across the closed Sunshine Valley-Costilla Plain, beginning coeval to Lake Alamosa’s highstand (~389 ka). By ~275 ka, outflow waters had reworked preexisting gravels in the vicinity of the gorge and eroded through the southern basin-bounding Red River fault zone to establish regional base level at this elevation (2298 m [7540 ft] asl). Once the southward-driven erosion through the major topographic and structural boundary of the Red River fault zone was attained, systematic headward erosion began to be established across the basin.

Day 2—Southern San Luis Basin and the Rio Grande Gorge

We begin Day 2 by visiting the Rio Grande gorge where Highway 64 crosses. We will traverse north along the Rio Grande corridor and visit locations where the ages and timing of Rio Grande gorge formation have been quantified and placed into the regional geomorphic sequence. Locations where preexisting topography existed (e.g., Ute Mountain and the Red River fault zone) have deeper canyons than preexisting structural depressions where gravels aggraded into the basin prior to drainage of Lake Alamosa, exhumation of maximum aggradation surfaces, and formation of the Rio Grande gorge. We will examine 3He surface exposure dating and supportive U-series dating techniques on adjacent basin-fill gravels. Ultimately, we will see the profound effects of a large body of water draining across a relatively flat basin and integration of the San Luis Basin with central and southern Rio Grande rift basins with preexisting substantially lower base levels.

Stop 6—Rio Grande Gorge High Bridge, Highway 64

Coordinates: N 36.4764°, W 105.7356°, WGS84

Highway 64 Rio Grande High Bridge

Los Cordovas, NM 7.5 quadrangle

Elevation: 2115 m (6940ft) asl (overlooking river)

Driving directions: Out of Taos, New Mexico, follow Hwy. 64 to the Rio Grande gorge bridge and overlook area.

Landownership: Department of Transportation and Bureau of Land Management

Introduction and Local Geology

The Hwy. 64 High Bridge is located at the southern margin of the structural depression created between the Red River fault zone and the Los Cordovas fault zone, uplifting the basaltic bench to the south (Fig. 3). At the well-visited Hwy. 64 High Bridge, the Rio Grande gorge cuts through Pliocene Servilleta Basalt and is ~152 m (500 ft) deep (elevation 6950-6450 ft). Just to the south, the gorge deepens to a maximum depth of ~240 m (~800 ft). Looking to the north, the relatively flat alluvial plain slopes gently upward north to the southern flanks of the Red River fault zone and Cebolla Mesa. Guadalupe Mountain and Cerro Chiflo are on the northern horizon, buttressing the Rio Grande gorge (Figs. 18A and 18B). This alluvial plain is comprised of post-Lake Alamosa highstand (~389 ka) outflow deposits of units Qao2 and Qao3, deposited in this depression by the collective contributions of the Red River and Rio Grande following coeval erosion through the Red River fault zone.

Geomorphic Setting, Timing of Canyon Incision, and Canyon Incision Rates

The elevation of the gorge rim here is correlative with the elevation of the highest and oldest 3He surface exposure age on the Taos Plateau (~250 ka) between the Red River and Los Cordovas fault zones (sample RG-10-9). If we project the ~250 ka age and correlative surface for gorge incision initiation at Stop 7 to the elevation of the High Bridge, canyon incision rates are ~152 m/250 k.y., or 0.6 mm/yr. The deepening canyon to the south across the structural and topographic high Los Cordovas fault zone reaches a maximum depth of 240 m (800 ft), equivalent to that at La Junta Point within both the Rio Grande and Red River gorges. With Lake Alamosa attaining a maximum level and initiating spillover and downstream basin erosion ~360 ka (average age), initial erosion of the basin-bounding structural and topographic highs began, resulting in the greater canyon depth across these previously formed topographically higher regions. As protracted basin integration progressed, subsequent inset relationships of units Qao1 and Qao2 began to be defined during MIS 8, culminating ~243 ka (Lisiecki and Raymo, 2005) (Figs. 18C and 18D).

Quantified incision rates are correlative with those measured along the entire Rio Grande gorge for this ~400-200 ka interval at the High Bridge. Maximum canyon depth is ~240 m (~800 ft) at structural boundaries and resulting topographic bedrock highs (i.e., Red River and Los Cordovas fault zones). With Lake Ala-mosa drainage and subsequent basin erosion initiating <400 ka, 240 m/400 k.y. equals a post-400 ka incision rate of 0.6 mm/yr, further supported by post-250 ka, 220 ka, and 180 ka incision rates of 0.6 mm/yr.

Stop 7—West Side Gorge Rim above Dunn Bridge, Rio Grande Gorge

Coordinates: N 36.53665°, W 105.71055°, WGS 84

West side gorge rim, Rio Grande-Arroyo Hondo Confluence

Arroyo Hondo, NM 7.5 quadrangle

Elevation: 2095 m (6870 ft) asl (overlooking river)

Driving directions: From Stop 6, proceed west on Hwy. 64 ~10.4 km (6.5 mi) and turn right onto Montoso Rd. Follow Mon-toso Rd. ~7.6 km (4.7 mi) to the Rio Grande gorge rim at the Dunn Bridge overlook parking area.

Landownership: Bureau of Land Management

Introduction and Local Geology

At this location along the Rio Grande gorge, there are three basaltic strath terraces that formed as the Rio Grande incised into the previously deposited spillover/outflow alluvial plain (Fig. 19A). This stop is within an intrabasin structural depression occurring between the Red River and Los Cordovas fault zones (Fig. 19B). The alluvial gravels capping this alluvial plain are the result of reworking and redeposition of closed-basin gravels deposited in a vast bajada complex emanating from the Sangre de Cristo Mountains. Prior to drainage of Lake Alamosa, this position in the landscape was occupied by the distal margins of the fan complex aggrading westward above Servilleta Basalt and the present position of the RG gorge, and grading westward to the basalt plain capped by loess. Based on the timing of Lake Ala-mosa’s highstand (~360 ka) and 3He surface exposure ages on the underlying basalt surface (<250 ka), we place deposition of gravel unit Qao2 in the time interval of ~389-250 ka, and relate it to Lake Alamosa outflow waters eroding through the Red River fault zone, reworking >400 ka, closed-basin fan gravels from the toe of Cebolla Mesa (Qao1?), and redepositing them as a wide alluvial plain in the depression between Guadalupe Mountain and the southern edge of the Taos Plateau (Fig. 19B). The outflow then reentered the RG gorge south of the High Bridge at the relative high topographic barrier of the Los Cordovas fault zone. Exposures of the Qao2 outflow gravel stratigraphy indicate a south-southwestward-migrating meander belt.

Geomorphic Setting, Timing of Canyon Incision, and Canyon Incision Rates

The highest exhumed and scoured basalt flow underlying unit Qao2 has an average 3He surface exposure age of 247.7 ± 8.6 ka (sample RG-10-9, Table 1, Fig. 19A, ~248 ka in Figs. 19B, 19D, and 20A). The elevation and corresponding 3He exposure age on this highest exhumed basalt flow are correlative to those on the northwest flank of Ute Mountain, indicating that erosion through the Ute Mountain and Red River fault zone topographic highs had been established, a new regional base level could be established at the elevation of this stop, and canyon incision and headward erosion to the north could resume (Fig. 18B).

As drainage of paleolake Alamosa progressed, canyon cutting continued, and regional base level dropped. The next inset basaltic strath has an age of 223.6 ± 7.8 ka (Figs. 19A and 20A). At the confluence of the Rio Grande and Arroyo Hondo, post-250 ka incision rates along the Rio Grande gorge average 0.6 mm/yr. The elevations of the strath terraces corresponding to the 247.7 ± 8.6 ka and 223.6 ± 7.8 ka ages also grade to terrace treads on major landslide deposits upstream within the gorge at La Junta Point and the confluence of the Red River (Fig. 3). We interpret this time interval of ~250-220 ka to be when enough incision into the gorge had occurred to undercut Servilleta Basalt flows and induce mass-wasting events in the gorge at La Junta point, which were subsequently modified by fluvial and slope processes, but preserved relict terrace treads. Exposure ages along the western RG gorge rim at the confluence of Arroyo Hondo average 172.0 ± 6.1 ka (Figs. 20B and 20C), and an age of 187.6 ± 6.5 ka (sample DAN-01; Table 1, Figs. 3 and 19D) on exhumed, fluvially scoured basalt on the western margin of the outflow alluvial plain indicate that abandonment of deposition on this plain, and exhumation of the uppermost Servilleta Basalt, was established by this time. We infer a conservative ~200 ka age for deep gorge incision initiation at Arroyo Hondo (Fig. 20D), and migration and constriction of the fluvial channel to the gorge location occurred during the time interval of ~225-170 ka.

Following inception of the axial Rio Grande gorge at the confluence of Arroyo Hondo, headward erosion up the tributary canyon of Arroyo Hondo progressed at a slower rate due to the steep mountain-piedmont slope created by the Sangre de Cristo fault system. A 3He exposure age along the north canyon rim of Arroyo Hondo is 96.6 ± 3.4 ka (Figs. 3, 19D, and 20E; Table 1, sample RG-10-13). Canyon incision rates calculated from this age and depth are ~0.7-0.8 mm/yr. Based on these chronologic and geomorphic constraints, exhumation of the scalloped basin east of the Arroyo Hondo tributary gorge occurred <100 ka (Fig. 20F), at a similar incision rate to that observed within the RG gorge since ~200 ka.

Stop 8—La Junta Point, Confluence of the Rio Grande and Red River

Coordinates: N 36.6563°, W 105.6864°, WGS84 Bureau of Land Management Wild and Scenic Rivers Recreation Area

Guadalupe Mountain, NM 7.5 quadrangle Elevation: 2265 m (7430 ft) asl (overlooking river)

Driving directions: From Stop 7, follow the Dunn Bridge Rd. switchback down and across the Rio Grande and Camino Del Medio Rd. ~6 km (3.7 mi) back out to Hwy. 522. Proceed north ~27 km (~17 mi) through Questa, New Mexico, to the turnoff to the BLM Wild and Scenic Rivers/Rio Grande Del Norte recreation area. Follow the main road out to La Junta Point overlook.

Introduction and Local Geology

The section of the gorge proximal to the confluence of the Rio Grande and Red River marks a dramatic change in the canyon depth (Figs. 3, 21A, and 21B). This position is located on the footwall of the Red River fault zone, elevated above the previously closed-basin base level to the north (Fig. 21C). Previous work (Wells et al., 1987; Pazzaglia and Wells, 1990) suggested that the headwaters of the Rio Grande migrated into the southern San Luis Basin, and were sourced up the Red River prior to capture and headward erosion across the Red River fault zone and into the Sunshine Valley-Costilla Plain closed basin (~<640-300 ka). The canyon depths at La Junta Point of both the Rio Grande and Red River are equivalent to canyon depths south of the High Bridge across the Los Cordovas fault zone, ~240 m [800 ft] (Fig. 21D). We correlate the timing of initial formation of the stretch of the canyons near La Junta Point to be coeval to erosion of the Los Cordovas topographic high and the canyon south of the High Bridge, resulting in the comparable depths and <400 ka incision rates.

Geomorphic Setting, Timing of Canyon Incision, and Canyon Incision Rates

La Junta Point is on the southward, down-dip, topographic slope created by the Red River fault zone to the north (Fig. 21C) and Kelson et al. (2008). Topographic profiles show this location to be generally equal in elevation to the hanging wall of the Red River fault zone at the southern margin of Sunshine Valley (~2274 m [7460 ft] asl). This mesa on the southwestern flank of Guadalupe Mountain between the Rio Grande and Red River is characterized by Servilleta Basalt with a lag gravel along the rim of both gorges capped by loess with stage III-IV+ pedogenic carbonate development. As drainage of Lake Alamosa initiated ~<400 ka, outflow began to flow across the Sunshine Valley-Costilla Plain, eroding through the major structural and fluvial barrier at the Red River fault zone (Fig. 21E). Erosion through the uplifted basalt transitioned into deposition of a lag gravel on the gorge rim at the same elevation as the bottom of Sunshine Valley prior to initiation of deep canyon incision.

Upstream from the Red River gorge within the Questa region, the active river channel is inset ~110 m (~360 ft) into the highest/maximum aggradation level defined by gravels of the Sunshine Valley (units Qao2 and Qao3) to the north (Figs. 21E and 21G). The depth of ~110 m seen at Questa is similar to that of the inset deep gorge canyon within the wider, compound, deeper RG gorge seen at La Junta Point (Fig. 21D). The middle Pleistocene gravel, marking maximum aggradation in the Red River drainage, caps a thick sequence of middle Pleistocene Santa Fe Group (>MIS 16 ~620 ka) (Lisiecki and Raymo, 2005; Ruleman et al., 2013) aggraded within the Questa graben (Fig. 21F). This highest surface is composed of the correlative gravel unit Qao3 exposed north of Guadalupe Mountain at Stop 10 (constrained by U-series ages of ~<200 ka) (Fig. 3). This indicates that the 110 m of gorge incision observed in the Red River gorge is <200 ka in age. This is similar to the age of deep canyon incision of the RG gorge identified by the inset terrace exposure ages at Arroyo Hondo to the south (Stop 7; Fig. 20); these terraces grade to similar terraces on landslide deposits in the RG gorge adjacent to La Junta Point. The similar 110 m depth in both the inner RG and Red River gorges, and similar ages, suggests coeval evolution of these canyons at a similar incision rate of ~0.6 mm/yr. Figure 21D demonstrates the sequential coeval incision history of the Red River in cross-sectional view.

Figure 20.

(A) View from sample location RG-10-9, the highest basalt strath surface exposed at Arroyo Hondo (photo by C.A. Ruleman). (B) Photo of fluvially scoured surface at Stop 7 and sample location RG-10-8. (C) Scour pits in vesicular gas tubes and preservation of chilled, polygonal crack pattern formed on the surface of Pliocene basalt flows. (D) View south from Stop 7 showing the Rio Grande gorge and 3He exposure age constraints on timing of canyon incision at Arroyo Hondo. (E) View west from sample location RG-10-13, Arroyo Hondo gorge rim. (F) Schematic geomorphic setting of final basin integration phase. See Figure 3 for geologic units and color scheme.

Figure 20.

(A) View from sample location RG-10-9, the highest basalt strath surface exposed at Arroyo Hondo (photo by C.A. Ruleman). (B) Photo of fluvially scoured surface at Stop 7 and sample location RG-10-8. (C) Scour pits in vesicular gas tubes and preservation of chilled, polygonal crack pattern formed on the surface of Pliocene basalt flows. (D) View south from Stop 7 showing the Rio Grande gorge and 3He exposure age constraints on timing of canyon incision at Arroyo Hondo. (E) View west from sample location RG-10-13, Arroyo Hondo gorge rim. (F) Schematic geomorphic setting of final basin integration phase. See Figure 3 for geologic units and color scheme.

Figure 21.

Stop 8. (A) View from La Junta Point of the confluence of the Rio Grande and Red River (photo by Cal Ruleman). (B) View looking north along the gorge from La Junta Point showing geomorphic and geochronologic constraints on gorge incision history. (C) View westward from the Questa, New Mexico, cemetery looking toward the Red River gorge. (D) Cross section D-D showing coeval gorge incision of the Rio Grande and Red River. (E) Cross section E-E showing relationships between southward-directed outflow waters eroding through the Red River fault zone and depositing down on La Junta Point and Cebolla Mesa. (F) Cross section F-F showing relationships of the Red River incision history and Red River fault zone-Guadalupe Mountain. (G) Cross section G-G (Fig. 3) showing relationships of maximum aggradation level and incision history into the Questa subbasin. Units in cross sections as follows: Qao3—alluvium, late middle Pleistocene (MIS 6-7, ~130-243 ka); Qao2—alluvium, middle Pleistocene (MIS 8-11, ~243-400 ka); QTsf—Santa Fe Group (>640 ka); Tsb— Servilleta Basalt and associated rocks (ca. 5-2 Ma); Tov—volcanics undifferentiated, Oligocene to Pliocene (ca. 34-5 Ma), locally includes high-relief Pliocene volcanic centers, Texas, Oligocene to Proterozoic rocks undifferentiated. Marine oxygen isotope stage (MIS) ages from Lisiecki and Raymo (2005).

Figure 21.

Stop 8. (A) View from La Junta Point of the confluence of the Rio Grande and Red River (photo by Cal Ruleman). (B) View looking north along the gorge from La Junta Point showing geomorphic and geochronologic constraints on gorge incision history. (C) View westward from the Questa, New Mexico, cemetery looking toward the Red River gorge. (D) Cross section D-D showing coeval gorge incision of the Rio Grande and Red River. (E) Cross section E-E showing relationships between southward-directed outflow waters eroding through the Red River fault zone and depositing down on La Junta Point and Cebolla Mesa. (F) Cross section F-F showing relationships of the Red River incision history and Red River fault zone-Guadalupe Mountain. (G) Cross section G-G (Fig. 3) showing relationships of maximum aggradation level and incision history into the Questa subbasin. Units in cross sections as follows: Qao3—alluvium, late middle Pleistocene (MIS 6-7, ~130-243 ka); Qao2—alluvium, middle Pleistocene (MIS 8-11, ~243-400 ka); QTsf—Santa Fe Group (>640 ka); Tsb— Servilleta Basalt and associated rocks (ca. 5-2 Ma); Tov—volcanics undifferentiated, Oligocene to Pliocene (ca. 34-5 Ma), locally includes high-relief Pliocene volcanic centers, Texas, Oligocene to Proterozoic rocks undifferentiated. Marine oxygen isotope stage (MIS) ages from Lisiecki and Raymo (2005).

Figure 22.

Stop 9. (A) Photo of basaltic surface dated ~475 ka on the footwall of the Red River fault (photo by C.A. Rule-man). (B) Cross section A-A (Fig. 3) showing geomorphic relationships between outflow discharge and topographic position. Labels as follows: RPT—Rio Pueblo de Taos; LP—La Junta Point; RG—Rio Grande; RRFZ—Red River fault zone; NSLB—northern San Luis Basin. Tsb—Servilleta Basalt; Tov—Tertiary older volcanics; Tsf—Santa Fe Group older; QTsf—Santa Fe Group. (C) Schematic geologic map of the geomorphic environment prior to drainage of Lake Alamosa. Unit Qao1 represents the maximum aggradation level within the southern San Luis Basin. Lake Alamosa highstand age and closed basin exposed bedrock ages on the footwall of the Red River fault zone are shown.

Figure 22.

Stop 9. (A) Photo of basaltic surface dated ~475 ka on the footwall of the Red River fault (photo by C.A. Rule-man). (B) Cross section A-A (Fig. 3) showing geomorphic relationships between outflow discharge and topographic position. Labels as follows: RPT—Rio Pueblo de Taos; LP—La Junta Point; RG—Rio Grande; RRFZ—Red River fault zone; NSLB—northern San Luis Basin. Tsb—Servilleta Basalt; Tov—Tertiary older volcanics; Tsf—Santa Fe Group older; QTsf—Santa Fe Group. (C) Schematic geologic map of the geomorphic environment prior to drainage of Lake Alamosa. Unit Qao1 represents the maximum aggradation level within the southern San Luis Basin. Lake Alamosa highstand age and closed basin exposed bedrock ages on the footwall of the Red River fault zone are shown.

Figure 23.

Stop 10. (A) Photo of exposure at the gravel quarry on the northwestern flank of Guadalupe Mountain (photo by C.A. Ruleman). Qai—middle Pleistocene alluvium. (B) Cross-section images of carbonate rinds on clasts sampled and analyzed for U-series dating from the coarse gravel sourced from the Red River-Cabresto Creek fluvial system. Sampled areas are enclosed by red polygons. (C) U-Th plot and ages of samples analyzed. Samples from the Guadalupe Mountain pit are shown by red error ellipses, 2σ errors are indicated by ellipse size. Samples indicated by horizontal gray arrows have substantial unsupported Th, indicating open-system behavior, and are deemed unreliable. (D) Geomorphic setting and 3He exposure ages along the gorge rim during the final gorge formation phase, culminating MIS 6 (~130 ka).

Figure 23.

Stop 10. (A) Photo of exposure at the gravel quarry on the northwestern flank of Guadalupe Mountain (photo by C.A. Ruleman). Qai—middle Pleistocene alluvium. (B) Cross-section images of carbonate rinds on clasts sampled and analyzed for U-series dating from the coarse gravel sourced from the Red River-Cabresto Creek fluvial system. Sampled areas are enclosed by red polygons. (C) U-Th plot and ages of samples analyzed. Samples from the Guadalupe Mountain pit are shown by red error ellipses, 2σ errors are indicated by ellipse size. Samples indicated by horizontal gray arrows have substantial unsupported Th, indicating open-system behavior, and are deemed unreliable. (D) Geomorphic setting and 3He exposure ages along the gorge rim during the final gorge formation phase, culminating MIS 6 (~130 ka).

Figure 24.

Stop 11. Lone Tree sample location RG-10. (A) View north of fluvially scoured surface with Dan Miggins (Oregon State University) and Gary Landis (USGS, retired) (photo by C.A. Ruleman). (B) View looking south at sample location RG-10-3A (photo by C.A. Ruleman). (C) Cross section A-A (Fig. 3) showing geomorphic relationships between outflow discharge and topographic position. Labels as follows: RPT—Rio Pueblo de Taos; LP—La Junta Point; RG—Rio Grande; RRFZ—Red River fault zone; NSLB—northern San Luis Basin. Tsb—Servilleta Basalt; Tov—Tertiary older volcanics; Tsf—Santa Fe Group older; QTsf—Santa Fe Group. (D) Geomorphic setting and 3He exposure ages along the gorge rim during the final gorge formation phase, culminating at MIS 6 (~130 ka).

Figure 24.

Stop 11. Lone Tree sample location RG-10. (A) View north of fluvially scoured surface with Dan Miggins (Oregon State University) and Gary Landis (USGS, retired) (photo by C.A. Ruleman). (B) View looking south at sample location RG-10-3A (photo by C.A. Ruleman). (C) Cross section A-A (Fig. 3) showing geomorphic relationships between outflow discharge and topographic position. Labels as follows: RPT—Rio Pueblo de Taos; LP—La Junta Point; RG—Rio Grande; RRFZ—Red River fault zone; NSLB—northern San Luis Basin. Tsb—Servilleta Basalt; Tov—Tertiary older volcanics; Tsf—Santa Fe Group older; QTsf—Santa Fe Group. (D) Geomorphic setting and 3He exposure ages along the gorge rim during the final gorge formation phase, culminating at MIS 6 (~130 ka).

Figure 25.

Stop 12. Incised meander sample location RG-10-4. (A) Photo of fluvially scoured basalt at Stop 12 (photo by C.A. Ruleman). (B) Photo of sample RG-10-4 and scour pits (photo by C.A. Ruleman). (C) View south of gorge from sample location RG-10-4 (photo by C.A. Ruleman). (D) Cross section A-A’ (Fig. 3) showing geomorphic relationships between outflow discharge and topographic position. Labels as follows: RPT— Rio Pueblo de Taos; LP—La Junta Point; RG—Rio Grande; RRFZ—Red River fault zone; NSLB—northern San Luis Basin. Tsb—Servilleta Basalt; Tov—Tertiary older volcanics; Tsf— Santa Fe Group older; QTsf—Santa Fe Group. (E) Geomorphic setting and 3He exposure ages along the gorge rim during the final gorge formation phase, culminating at MIS 6 (~130 ka).

Figure 25.

Stop 12. Incised meander sample location RG-10-4. (A) Photo of fluvially scoured basalt at Stop 12 (photo by C.A. Ruleman). (B) Photo of sample RG-10-4 and scour pits (photo by C.A. Ruleman). (C) View south of gorge from sample location RG-10-4 (photo by C.A. Ruleman). (D) Cross section A-A’ (Fig. 3) showing geomorphic relationships between outflow discharge and topographic position. Labels as follows: RPT— Rio Pueblo de Taos; LP—La Junta Point; RG—Rio Grande; RRFZ—Red River fault zone; NSLB—northern San Luis Basin. Tsb—Servilleta Basalt; Tov—Tertiary older volcanics; Tsf— Santa Fe Group older; QTsf—Santa Fe Group. (E) Geomorphic setting and 3He exposure ages along the gorge rim during the final gorge formation phase, culminating at MIS 6 (~130 ka).

Figure 26.

Stop 13. Inset fluvially scoured straths, central Sunshine Valley. (A) Oblique aerial view from Google Earth of Stop 13 and 3He surface exposure ages. (B) Cross section A-A (Fig. 3) showing geomor-phic relationships between outflow discharge and topographic position. Labels as follows: RPT—Rio Pueblo de Taos; LP—La Junta Point; RG—Rio Grande; RRFZ—Red River fault zone; NSLB—northern San Luis Basin. Tsb—Servilleta Basalt; Tov—Tertiary older volcanics; Tsf—Santa Fe Group older; QTsf—Santa Fe Group. (C) Geomorphic setting and 3He exposure ages along the gorge rim during the final gorge formation phase, culminating at MIS 6 (~130 ka).

Figure 26.

Stop 13. Inset fluvially scoured straths, central Sunshine Valley. (A) Oblique aerial view from Google Earth of Stop 13 and 3He surface exposure ages. (B) Cross section A-A (Fig. 3) showing geomor-phic relationships between outflow discharge and topographic position. Labels as follows: RPT—Rio Pueblo de Taos; LP—La Junta Point; RG—Rio Grande; RRFZ—Red River fault zone; NSLB—northern San Luis Basin. Tsb—Servilleta Basalt; Tov—Tertiary older volcanics; Tsf—Santa Fe Group older; QTsf—Santa Fe Group. (C) Geomorphic setting and 3He exposure ages along the gorge rim during the final gorge formation phase, culminating at MIS 6 (~130 ka).

Figure 27.

Stop 14. Southwestern flank of Ute Mountain. (A) Oblique aerial view from Google Earth of Stop 14 and corresponding 3He surface exposure ages on fluvially scoured Servilleta Basalt. (B) Photo of fluvially scoured surface at Stop 14 (photo by C.A. Ruleman). (C) Cross section A-A (Fig. 3) showing geomorphic relationships between outflow discharge and topographic position. Labels as follows: RPT—Rio Pueblo de Taos; LP—La Junta Point; RG—Rio Grande; RRFZ—Red River fault zone; NSLB—northern San Luis Basin. Tsb—Servilleta Basalt; Tov—Tertiary older vol-canics; Tsf—Santa Fe Group older; QTsf—Santa Fe Group. (D) Geomorphic setting and 3He exposure ages along the gorge rim following completion of gorge formation and backfilling of <MIS 6 alluvium into tributary drainages.

Figure 27.

Stop 14. Southwestern flank of Ute Mountain. (A) Oblique aerial view from Google Earth of Stop 14 and corresponding 3He surface exposure ages on fluvially scoured Servilleta Basalt. (B) Photo of fluvially scoured surface at Stop 14 (photo by C.A. Ruleman). (C) Cross section A-A (Fig. 3) showing geomorphic relationships between outflow discharge and topographic position. Labels as follows: RPT—Rio Pueblo de Taos; LP—La Junta Point; RG—Rio Grande; RRFZ—Red River fault zone; NSLB—northern San Luis Basin. Tsb—Servilleta Basalt; Tov—Tertiary older vol-canics; Tsf—Santa Fe Group older; QTsf—Santa Fe Group. (D) Geomorphic setting and 3He exposure ages along the gorge rim following completion of gorge formation and backfilling of <MIS 6 alluvium into tributary drainages.

Figure 28.

Stop 15. One-lane bridge site. (A) Photo of the one-lane bridge and depth of the Rio Grande gorge at this location (photo by C.A. Ruleman). (B) View looking north from Stop 15 towards the San Luis Hills and northern Sangre de Cristo Mountains, showing the basin of extremely low topographic relief (photo by C.A. Ruleman). (C) Photo of fluvial scoured basaltic surface and sample RG-10-11 (photo by C.A. Ruleman). (D) Cross section A-A (Fig. 3) showing geomorphic relationships between outflow discharge and topographic position. Labels as follows: RPT—Rio Pueblo de Taos; LP—La Junta Point; RG—Rio Grande; RRFZ—Red River fault zone; NSLB—northern San Luis Basin. Tsb—Servilleta Basalt; Tov—Tertiary older volcanics; Tsf—Santa Fe Group older; QTsf—Santa Fe Group. (E) Geomorphic setting and 3He exposure ages along the gorge rim following completion of gorge formation and back-filling of <MIS 6 alluvium into tributary drainages.

Figure 28.

Stop 15. One-lane bridge site. (A) Photo of the one-lane bridge and depth of the Rio Grande gorge at this location (photo by C.A. Ruleman). (B) View looking north from Stop 15 towards the San Luis Hills and northern Sangre de Cristo Mountains, showing the basin of extremely low topographic relief (photo by C.A. Ruleman). (C) Photo of fluvial scoured basaltic surface and sample RG-10-11 (photo by C.A. Ruleman). (D) Cross section A-A (Fig. 3) showing geomorphic relationships between outflow discharge and topographic position. Labels as follows: RPT—Rio Pueblo de Taos; LP—La Junta Point; RG—Rio Grande; RRFZ—Red River fault zone; NSLB—northern San Luis Basin. Tsb—Servilleta Basalt; Tov—Tertiary older volcanics; Tsf—Santa Fe Group older; QTsf—Santa Fe Group. (E) Geomorphic setting and 3He exposure ages along the gorge rim following completion of gorge formation and back-filling of <MIS 6 alluvium into tributary drainages.

Stop 9—Pay Station

Coordinates: N 36.6991°, W 105.7015°, WGS84 Registration Station, BLM Wild and Scenic Rivers recreation area

Guadalupe Mountain, NM 7.5 quadrangle

Elevation: 2295 m (7530ft) asl (overlooking Rio Grande)

Driving directions: From Stop 8, proceed back to the BLM Wild and Scenic Rivers Entrance and Fee Station.

Introduction and Local Geology

At this location along the gorge rim, we are on the elevated footwall of the Red River fault zone (Figs. 22A and 22B). Topographic profiles reveal that this location is above the ~400 ka outflow path that carved the gorge and deposited lag gravels on the mesa top at La Junta Point (Figs. 3 and 22B). Across the gorge, buried Oligocene volcanic rocks associated with the Latir volcanic locus of the Southern Rocky Mountain volcanic field are exposed, indicating the amount of post-Oligocene extension and associated basin subsidence and burial that has occurred. This volcanic edifice is not directly associated with any present volcanic landform observed at the surface, and reflects earlier phases of volcanism followed by substantial tectonic subsidence and burial within the basin.

Geomorphic Setting, Timing of Canyon Incision, and Canyon Incision Rates

Along the gorge rim, we find fluvially scoured Servilleta Basalt (Fig. 22A), but the surface appears to be more weathered, cracked, and friable on tabular planes, relative to other locations where the exposure ages are >100 k.y. younger. To the east, we find a relatively freshly preserved surface of Servilleta Basalt mantled by a thick loess cap of variable thickness. Where exposed, stage IV pedogenic carbonate morphology is commonly observed within the loess cap. 3He exposure ages here on the exhumed Servilleta Basalt average 474.2 ± 16.4 ka (Table 1), predating the ages calculated for middle Pleistocene Lake Ala-mosa’s highstand and overflow ~360 ka (Fig. 22C).

The evidence for fluvial erosion on the basaltic surface is abundant at this location; however, this erosional event does not appear to be related to the final San Luis Basin integration event into the Rio Grande, as it is too high in elevation. The fluvial erosion could instead have been a product of early to middle Pleistocene episodic flash floods on the flanks of Guadalupe Mountain and Cerro Chiflo. Or, it could possibly be related to the most recent event on the Red River fault zone during accelerated regional middle Pleistocene tectonic activity rates. Prior to uplift of the Red River footwall during this event, Sunshine Valley could have drained across this region >~470 ka, creating the fluvial erosion and scour pits. In this scenario, paleoseismic activity on the Red River fault zone impeded drainage and closed the basin to the north until approximately ~100 k.y. later, when basin sedimentation rates finally overrode tectonic subsidence rates and Lake Alamosa began to drain, providing the stream power in the Sunshine Valley to erode through the uplifted topography of the Red River footwall.

Stop 10—Northwest Flank of Guadalupe Mountain, Borrow Pit

Coordinates: N 36.7691°, W 105.6384°, WGS84

Gravel quarry on the northwestern flank of Guadalupe Mountain

Guadalupe Mountain, NM 7.5 quadrangle

Elevation: 2290 m (7515 ft) asl

Driving directions: From Stop 9, follow the main road out of the recreation area ~10.4 km (6.5 mi) and turn left onto Rd. B048, on the east side of the cattle guard. Proceed north ~0.2 km (0.1 mi) and turn right into the quarry.

Landownership: Bureau of Land Management

Introduction and Local Geology

On the northern flank of Guadalupe Mountain and southern margin of Sunshine Valley, a gravel pit exposes gravels sourced from the Red River-Cabresto Creek drainage, now flowing south toward the RG confluence at La Junta Point, and separated by an ~120-m-high drainage divide at Questa, New Mexico (Fig. 3) (Ruleman et al., 2007, 2013; Thompson et al., 2014b). An aggrading package of loess and accompanying buried soils overlies the gravels at this location (Fig. 23A). Based on the regionally calibrated degree of soil development (Table 2), and geomorphic relations in the Sunshine Valley-Costilla Plain region, Ruleman et al. (2007) determined that the region began to incise and dissect <640 ka (MIS 16), and most likely Rio Grande canyon formation across the once-closed Sunshine Valley-Costilla Plain subbasin began by ~420 ka (MIS 12) (Ruleman et al., 2013).

We present new U-series chronologic data on the soil carbonate (Table 4) to provide constraints on timing of basin dissection/ incision and correlation of the temporo-spatial, protracted fluvial integration of this basin, and ultimately the San Luis Basin.

Geomorphic Setting, Timing of Canyon Incision, and Canyon Incision Rates

The exposure shows a dramatic change in the depositional system, with deposits of a coarse-grained high-energy fluvial system disconformably overlain by a low-energy eolian deposit. This section shows two paleosols in the loess cap and significant carbonate rinds on clasts within the alluvial gravels. U-series analysis for the soil carbonate sampled from carbonate rinds within the stage III+-IV horizon overlying the coarse-grained gravel yields ages of less than 200 ka (<MIS 7, ~243-191 ka) (Figs. 23B and 23C). We interpret the two overlying loess packages and accompanying soils to represent abandonment of this higher elevation fluvial surface, landscape incision, and mantling of aggrading eolian material during MIS 6-5 (~191-130 ka) and 4-2 (~80-14 ka) (Fig. 23D).

Stop 11—Lone Tree Site, Southern Sunshine Valley

Coordinates: N 36.7967°, W 105.6841°, WGS84 Southern Sunshine Valley, east side Rio Grande gorge rim Guadalupe Mountain, NM 7.5 quadrangle Elevation: 2262 m (7420 ft) asl (overlooking river)

Driving directions: From Stop 10, follow the main Cerro Rd. heading west to the gorge rim and follow the gorge rim’s unmaintained road north ~7.2 km (4.5 mi).

Landownership: Bureau of Land Management

Introduction and Local Geology

This site is chosen for 3He exposure sampling and lies just below and down-profile from the gravel quarry exposure seen at Stop 10 (Fig. 3). Here, fluvially scoured Servilleta Basalt lies at the lowest elevation of the gorge rim within Sunshine Valley between Guadalupe and Ute Mountains (Figs. 24A, 24B, and 24C) and Thompson et al. (2014b). To the north, exposure of the east-dipping Gorge fault zone crops out along the western basin-bounding fault of Sunshine Valley. East of the rim lie units Qao2 and Qao3, composed of medium- to fine-grained pebble gravels deposited as the middle Pleistocene (>200 ka?) distal fan facies that prograded westward into the basin and across the present position of the gorge.

Geomorphic Setting, Timing of Canyon Incision, and Canyon Incision Rates

A 3He exposure age of 141.8 ± 4.9 ka for the scoured fluvial surface (Fig. 24D), indicating incision and abandonment of the surface, is younger than the maximum U-series ages on the gravels sourced from the Red River exposed at Stop 10. The correlation of ages at these two sites permits the interpretation that the previously connected Sunshine Valley-Red River fluvial system was disconnected by regional deep canyon incision beginning ~200 ka. During the time interval of ~200-130 ka, the Red River abandoned deposition to the north into the closed Sunshine Valley and incised into its present gorge flowing southwest into the RG gorge at La Junta Point.

Coeval Rio Grande-Red River gorge development progressed and <200 ka incision rates are equivalent (~0.6 mm/yr) at this location. This site elevation is also along the same paleoriver profile of the lower surface sampled along the gorge rim upstream at Stop 13 (147.0 ± 9.5 ka) (Fig. 3). This provides insight to the effect of southward-directed outflow erosion establishing new base levels downstream, followed by headward erosion to that base level, and then subsequent downstream canyon cutting to a lower base level. Following the initiation of deep gorge incision at ~200-250 ka, subsequent headward erosion propagates northward until another episode of hydrologic increase occurs (i.e., interglacial deglaciation), causing additional canyon erosion and establishment of a new base level.

Stop 12—Incised Meander/Latir Creek Confluence, Central Sunshine Valley

Coordinates: N 36.8111°, W 105.6955°, WGS84 Central Sunshine Valley, east side Rio Grande gorge Guadalupe Mountain, NM 7.5 quadrangle Elevation: 2277 m (7470ft) asl (top of Qao2 gravel)

Driving directions: From Stop 11, continue north ~6.4 km (4 mi) on the unmaintained gorge rim road to the east around the east-facing fault line scarp on Servilleta Basalt, and up and around to the top of the incised meander in the Rio Grande gorge.

Landownership: Bureau of Land Management

Introduction and Local Geology

This site is located on the footwall of the Gorge fault zone, creating the western structural boundary of the Sunshine Valley-Costilla Plain subbasin (Ruleman et al., 2013) (Fig. 3). The eastfacing basalt fault line scarp crops out just to the east of this site. A medium- to fine-grained silty sandy pebble gravel with stage III-III+ pedogenic carbonate development overlies and onlaps the scoured basalt surface along the gorge rim. Ruleman et al. (2007, 2013) mapped this gravel as Qao3, correlated to deposition during MIS 12 (~470-420 ka). This gravel represents the last pulse of middle Pleistocene deposition prior to canyon incision initiation across the basin. Our 3He surface exposure ages have aided in placing this gravel and erosion surface instead during the interglacial of MIS 7 (~243-191 ka).

Geomorphic Setting, Timing of Canyon Incision, and Canyon Incision Rates

A sample of the fluvially scoured basalt along the gorge rim yielded a 3He exposure age of 203.1 ± 7.0 ka (Figs. 25A, 25B, and 25C). The elevation of this surface projects to a profile elevation above Stop 11 (dated to 142 ka), and through the Red River fault zone to an elevation between the middle and lower basalt strath surfaces dated at 223.6 ± 7.8 ka and 172.0 ± 6.1 ka, respectively, at the John Dunn Bridge near Arroyo Hondo (Stop 7). This site is also correlative in elevation along stream profile to the higher basalt strath surface at Stop 13 to the north, dated at 211.0 ± 13.7 ka (Figs. 25D and 25E). These topographic and geochronologic constraints further support a coeval post-224 ka incision of the RG gorge through both Sunshine Valley and the Taos Plateau. Calculated post-200 ka canyon incision rates at this location are ~0.6 mm/yr, similar to those observed in previous stops.

Stop 13—Inset Strath Terraces on Servilleta Basalt, Central-Northern Sunshine Valley

Coordinates: N 36.8395°, W 105.6928°, WGS84 North-central Sunshine Valley, east side Rio Grande gorge rim Guadalupe Mountain, NM 7.5 quadrangle Elevation: 2271 m (7450ft) asl (top of overlying gravel unit Qao3)

Driving directions: From Stop 12, follow the gorge rim road ~3.5 km (2.2 mi) north to the upper strath surface. Landownership: Bureau of Land Management

Introduction and Local Geology

At this location along the Rio Grande gorge rim, two inset strath surfaces on fluvially scoured Servilleta Basalt provide further 3He chronologic constraints on the timing of canyon incision initiation and resulting incision rates (Fig. 26A). The upper and lower surfaces have a 3He exposure age of 210.9 ± 13.7 ka and 147.0 ± 9.5 ka, respectively.

Geomorphic Setting, Timing of Canyon Incision, and Canyon Incision Rates

The ages of the upper and lower surfaces at Stop 13 correlate in age and elevation to sampled surfaces along the gorge rim at Stops 12 and 11, respectively. Figure 26B shows the topographic relationships of 3He exposure ages across the southern San Luis Basin. Ages of basin exhumation and incision at this location indicate that the Rio Grande gorge was fully captured and integrated across the Red River fault zone and Ute Mountain by the close of MIS 6 (~130 ka). Enough erosion through the San Luis Hills had occurred that most of Lake Alamosa had drained and become a series of localized playas in closed depressions north of the San Luis Hills. This is also the time interval at which lake drainage had emptied the central portion of the northern San Luis Basin and allowed eolian saltation processes to begin construction of the Great Sand Dunes (Madole et al., 2008, 2013). The upper and lower surfaces yielded 3He exposure ages of 210.9 ± 13.7 ka and 147.0 ± 9.5 ka, respectively. Figure 26C shows the generalized geomorphic setting and associated 3He surface exposure ages associated with basin integration during the time interval of MIS 7-6 (~243-130 ka).

Incision rates at this location for the time interval ~210 ka-present average 0.2-0.3 mm/yr. Erosion rates into the gorge apparently increased as the Sunshine Valley-Costilla Plain was gradually integrated with the Taos Plateau and Rio Grande corridor to the south. Incision from the oldest, highest strath at this location (~210 ka) into the lower strath on the gorge rim (~147 ka) is ~10 m for an incision rate of 0.2 mm/yr for this interval. Following protracted basin integration and establishment of a through-going, axial river system from the San Luis Hills to Pilar, incision rates could increase along the Rio Grande gorge. The exposure age of 147.0 ± 9.5 ka on the gorge rim is ~40 m above the Rio Grande, yielding an incision rate of 0.3 mm/yr.

Stop 14—Southwestern Flank of Ute Mountain

Coordinates: N 36.8729°, W 105.7050°, WGS84 Registration Station, Wild and Scenic Rivers recreation area Guadalupe Mountain, NM 7.5 quadrangle Elevation: 2283 m (7490ft) asl (overlooking Rio Grande)

Driving directions: From Stop 13, continue north on the gorge rim road ~5.5 km (3.4 mi) and turn left onto the narrow road leading to the gorge ~1.0 km (0.6 mi) to the west.

Landownership: Bureau of Land Management

Introduction and Local Geology

The oldest exposure ages related to the northern San Luis Basin integration event are along the southwest shoulder of Ute Mountain (Figs. 27A and 27B). Ruleman et al. (2007, 2013) established the pre-gorge progradational relationships of alluvium sourced from Ute Mountain and the Sangre de Cristo Mountains, establishing a maximum basin integration timing of <400 ka (based on soil morphology and relative relationships). With the addition of 3He surface exposure and U-series pedogenic carbonate chronology, we demonstrate that canyon incision across the previously closed Sunshine Valley-Costilla Plain subbasin initiated during MIS 7 (~243-191 ka). Cross-sectional relationships along the axis of the Rio Grande show this region to be a bedrock topographic high and thus first to be exhumed and eroded following overflow of Lake Alamosa (Fig. 27C). As episodically changing base levels were established downstream, headward erosion could be induced upstream, carving the RG gorge progressively farther toward the outlet through the San Luis Hills. This is demonstrated in the south-to-north/headward-younging 3He exposure ages recorded at this location, and at the accompanying Stop 5 site on the northwest flank of Ute Mountain (Fig. 27D).

Geomorphic Setting, Timing of Canyon Incision, and Canyon Incision Rates

At the northern margin of Sunshine Valley, the Rio Grande wraps around and has eroded into the relatively softer volcanic rocks of the western shoulder of Ute Mountain. Eight samples along the gorge rim at this latitude average ~277 ka. This age precedes complete erosion through the Red River fault zone and incision into the Taos Plateau, initiating ~250 ka (see Stop 7). Prior to this time, overflowing water from Lake Alamosa and drainage from the Red River drainage were discharging into the closed basin of Sunshine Valley. By ~250 ka, the RG gorge had eroded through the western shoulder of Ute Mountain and migrated to the northwestern shoulder of Ute Mountain (Stop 5). This is also coeval to the time interval at which outflow waters had eroded through the Red River fault zone to an elevation along profile of the Taos Plateau west of Arroyo Hondo and Stop 7. Once these two topographic barriers (i.e., Ute Mountain and the Red River fault zone) were eroded through to a base-level equivalent to that of the Taos Plateau (~300-250 ka), regional deep canyon incision could proceed and produce the 3He exposure ages recorded along the gorge rim <250 ka.

Stop 15—One-Lane/Stateline Bridge

Coordinates: N 37.0781°, W 105.7561°, WGS84

One-Lane/Stateline Bridge, western Costilla Plain, northernmost Rio Grande gorge exposure

Guadalupe Mountain, NM 7.5 quadrangle

Elevation: 2274 m (7460 ft) asl (overlooking river)

Driving directions: Approximately 13.7 km north of Costilla, New Mexico, or 17.5 km south of San Luis, Colorado, on Hwy. 159, turn west onto Rd. H and proceed through Mesita, Colorado, and past Mesita cone to the north ~11.3 km (7.0 mi). Turn south and proceed 2.2 km (1.4 mi) to Rd. 7 and head west ~10.7 km (6.6 mi) to the Rio Grande and one-lane bridge.

Landownership: Bureau of Land Management

Introduction and Local Geology

Located at the northernmost exposure of the Rio Grande gorge, Stop 15 provides an opportunity to view the pre-basin integration geomorphology, and the sequential geomorphic stages of development to the present (Figs. 3 and 28A). To the east and west of this stop, the highest alluvial surfaces represent the pre-drainage position of the middle Pleistocene Lake Alamosa, pre-gorge, highest Costilla Plain aggradational surface. As the northern San Luis Basin filled and began to spill over, the ero-sional base level was above the present elevation of this stop and extended across the higher western shoulder of Ute Mountain to the south. As protracted basin integration was established by ~200 ka, systematic headward erosion could begin up the gorge across Ute Mountain. Base level did not drop to the present elevation and expose the basalt here until ~140 ka.

Geomorphic Setting, Timing of Canyon Incision, and Canyon Incision Rates

At this location, an average 3He surface exposure age of 146.3 ± 5.0 ka on fluvially scoured basalt provides insight into the last phase of basin integration during this time interval. As at other locations to the south along the gorge rim, exposure ages show a relationship between the MIS record and geomorphic processes. To the north, unit Qay (<130 ka) is mapped onlap-ping this exhumed and fluvially scoured basalt surface (Fig. 3). We interpret this age and geomorphic relationship to indicate that canyon incision and headward migration proceeded during glaciation associated with MIS 6 (~190-130 ka), and onlap of late middle to late Pleistocene (<130 ka) alluvium resulted as sediment was flushed from the north during deglaciation.

Summary and Conclusions

Integration of the San Luis Basin into the Rio Grande watershed occurred between ~400-130 ka. Gradual drainage of Lake Alamosa provided a dramatic increase in hydraulic fluxes that contributed to erosion of preexisting bedrock topographic highs across the southern San Luis Basin at Ute Mountain, the Red River fault zone, and the uplifted basalt south of Arroyo Hondo. Reorganization of the RG fluvial system resulted in confinement of channels to the approximate location of the present gorge and deep canyon incision across the entire San Luis Basin. By ~200 ka, incision within the upper gorge had undercut older sediments below Servilleta lava flows, inducing mass-wasting events within the gorge. Integration of the San Luis Basin into the RG watershed created substantial increases in hydraulic fluxes and dramatically influenced the entire Rio Grande drainage basin and its discharge into the Gulf of Mexico.

The geomorphic evolution model presented here provides a framework for future studies of the San Luis Basin, the axial Rio Grande, and tributary corridors. Estimates of post-400 ka rates of incision along the Rio Grande corridor are >0.2 mm/yr compared to substantially lower pre-400 ka values, which reflected lower-energy environments and aggradation in more isolated tectonic basins. We conclude that the addition of the San Luis Basin to the Rio Grande watershed provided a driver for connecting the hinterland of the intermountain West to the Gulf of Mexico.

South of the San Luis Basin, post-640 ka incision rates along the entire Rio Grande increased an order of magnitude (~0.03-0.4-0.6 mm/yr) in the middle Pleistocene (Connell et al., 2005), and notably terminated in a large bolson complex proximal to El Paso, Texas, until the late middle Pleistocene (Haw-ley et al., 1976, Fig. 1). No significant sediment supply from the hinterland of the Rio Grande rift to the Gulf of Mexico occurred from the Miocene to the late middle Pleistocene (Galloway et al., 2011). Proximal to Las Cruces, New Mexico (Fig. 1), erosion rates have been ~0.4 mm/yr on basalt and erosional surfaces formed prior to 350 ka, 350-240 ka, 240-130 ka, and 130-80 ka (Dethier et al., 1988). Within the Albuquerque (Cole et al., 2007) and Espanola (Konning et al., 2011) basins (Fig. 1), during the time interval <640-130 ka, incision rates increased an order of magnitude, from ~0.02 mm/yr to ~0.3-0.5 mm/yr. All of these previously documented rates of RG incision and timing of basin integration south of the San Luis Basin are in accordance with the time interval at which the San Luis Basin was integrated into the RG corridor. Thus, we provide an integrated model for the integration and exhumation of the San Luis Basin and its effect on establishing the modern Rio Grande.

Field-Trip Summary

Based on this unified approach to a local and regional geo-morphic syntheses, we constrain the integration of the San Luis Basin into the Rio Grande watershed to be between ~400-200 ka. As Lake Alamosa drained, southward-directed discharge eroded the Taos Plateau, preexisting bedrock topographic highs at Ute Mountain and the Red River fault zone, and the uplifted basalt south of Arroyo Hondo. At ~350-200 ka, erosion of the upper, broad gorge at the confluence of the Rio Grande and Red River and below Arroyo Hondo occurred, while initially depositing and then dissecting the alluvial plain between the two.

By 200 ka, the Rio Grande was confined to the approximate location of the present gorge, and deep canyon incision began to take place from the San Luis Hills to Pilar across the entire San Luis Basin. Incision within the upper gorge had progressed to begin undercutting processes inducing mass-wasting events within the gorge. Lake Alamosa had decreased in size enough to allow erosion and saltation processes to begin forming the Great Sand Dunes (Madole et al., 2008, 2013). The Rio Grande headwaters were now within the current location of the San Juan and Sangre de Cristo Mountains. Cooperative efforts of downstream watersheds and the influx of discharge from the San Luis Basin had coalesced and dramatically influenced the Rio Grande’s path to the Gulf of Mexico.

Based on ages of Rio Grande rift basin integration to the south (~<640-130 ka) (Hawley et al., 1976; Connell et al., 2005; Cole et al., 2007; Konning et al., 2011; Galloway et al., 2011), we propose that the addition of the San Luis Basin to the Rio Grande watershed (~400-200 ka) ultimately completed the integration of the hinterland of the Rio Grande rift and southern Rocky Mountains to the Gulf of Mexico (Fig. 1). Within the Espanola Basin, Dethier et al. (1988) determined canyon incision to have occurred at intervals >350 ka, 350-240 ka, 240-130 ka, and 130-80 ka with incision rates averaging 0.4 mm/yr. We interpret canyon incision within the Espanola Basin to integration of the San Luis Basin.

In conclusion, this model provides a testable chronologic framework for future studies pertaining to the geomorphic evolution of the San Luis Basin, as well as the axial Rio Grande and its tributary corridors. All post-400 ka rates of incision along the Rio Grande corridor are 0.2-0.8 mm/yr, and the prior geo-morphic environment was distinctly lower energy and generally aggrading during a more tectonically active period. The onset of major middle Pleistocene glaciations for this region, and waning tectonic activity rates along the Sangre de Cristo fault system, allowed for sedimentation rates to exceed tectonic subsidence rates and thus for basin fill-up, spillover, and southward-driven, basin integration processes to begin.

Dedication and Acknowledgments

This field guide is dedicated to the extraordinary memory and lasting presence of Aron Rael, who provided much deeper insight into the spiritual and cultural richness of the region centered on the southern San Luis Basin and Rio Grande gorge. His companionship and knowledge of the area provided invaluable access and assistance to data collection and his observations have been assimilated in this study. His presence and renown in the region will continue to inspire and motivate others to explore the vast archaeological and geological curiosities. We will carry forth your never-ending thirst for understanding the natural settings that surround us all.

We would also like to give our deep gratitude to John Bailey, Josef Leon, Marty Passaglia, and Daniel Leon and the Bureau of Land Management (BLM). John permitted access to and provided enthusiasm for our investigations over the years as they pertain to groundwater modeling, basin structure, and seismic hazards. Josef, Marty, and Daniel provided hospitality, invaluable knowledge, and support with access to the remote landscape of this study. We are indebted to the well-kept facilities made available at the BLM Questa Wild and Scenic Rivers recreation area and all of the friendly, energetic staff over the years.

Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

References Cited

Benson
,
L.
Madole
,
R.
Phillips
,
W.
Landis
,
G.
Thomas
,
T.
Kubic
,
P.
,
2004
,
The probable importance of snow and sediment shielding on cosmogenic ages of north-central Colorado Pinedale and pre-Pinedale moraines
:
Quaternary Science Reviews
 , v.
23
, p.
193
206
, doi: 10.1016/j.quascirev.2003.07.002.
Benson
,
L.
Madole
,
R.
Landis
,
G.
Gosse
,
J.
,
2005
,
New data for late Pleistocene alpine glaciation from southwestern Colorado
:
Quaternary Science Reviews
 , v.
24
, p.
49
65
, doi: 10.1016/j.quascirev.2004.07.018.
Borchers
,
B.
Marrero
,
S.
Balco
,
G.
Caffee
,
M.
Goehring
,
B.
Lifton
,
N.
,
2015
,
Geological calibration of spallation production rates in the CRONUS-Earth project: Quaternary Geochronology
 , doi: 10.1016/j.quageo.2015.01.009.
Burroughs
,
R.L.
,
1972
,
Geology of the San Luis Hills, south-central Colorado [Ph.D. diss.]
 :
Albuquerque
,
University of New Mexico
,
139
p.
Burroughs
,
R.L.
,
1978
, Alamosa to Antonito, Colorado (p. 33-36, in Northern Rift Guide 2, Alamosa, Colorado—Santa Fe, New Mexico), in
Hawley
,
J.W.
, ed.,
Guidebook to Rio Grande Rift in New Mexico and Colorado
 :
New Mexico Bureau of Mines and Mineral Resources Circular
103
,
241
p.
Cole
,
J.C.
Mahan
,
S.A.
Stone
,
B.D.
Shroba
,
R.R.
,
2007
,
Ages of Quaternary Rio Grande terrace-fill deposits, Albuquerque area, New Mexico
:
New Mexico Geology
 , v.
29
, no.
4
, p.
122
132
.
Connell
,
S.D.
Hawley
,
J.W.
Love
,
D.W.
,
2005
, Late Cenozoic drainage development in the southeastern Basin and Range of New Mexico, south-easternmost Arizona, and western Texas, in
Morgan
,
L.S.G.
Zeigler
,
K.E.
, eds.,
New Mexico’s Ice Ages
 :
New Mexico Museum of Natural History and Science Bulletin
no.
28
, p.
125
149
.
Dethier
,
D.P.
Harrington
,
C.D.
Aldrich
,
M.J.
,
1988
,
Late Cenozoic rates of erosion in the western Espanola basin, New Mexico
:
Evidence from geologic dating of erosion surfaces: Geological Society of America Bulletin
 , v.
100
, p.
928
937
, doi: 10.1130/0016-7606(1988)100<0928:LCROEI>2.3.CO;2.
Endlich
,
F.M.
,
1877
,
Part I—Geology, Chap. II—San Luis Valley, in Hayden, F.V., Ninth Annual Report of the United States Geological and Geographical Survey of the Territories, Embracing Colorado and Parts of Adjacent Territories
:
Being a Report of Progress of the Exploration for the Year 1875
 :
Washington, D.C.
,
Government Printing Office
, p.
140
149
and plate XVII.
Galloway
,
W.E.
Whiteaker
,
T.L.
Ganey-Curry
,
P.
,
2011
,
History of Ceno-zoic North American drainage basin evolution
,
sediment yield, and accumulation in the Gulf of Mexico basin: Geosphere
 , v.
7
, no.
4
, p.
938
973
, doi: 10.1130/GES00647.1.
Goehring
,
B.M.
Kurz
,
M.D.
Balco
,
G.
Schaefer
,
J.M.
,
2010
,
A reevaluation of in situ cosmogenic
3He
production rates
:
Quaternary
 , v.
5
, p.
410
418
.
Hawley
,
J.W.
Bachman
,
G.O.
Manley
,
K.
,
1976
, Quaternary stratigraphy in the Basin and Range and Great Plains Provinces, New Mexico and western Texas, in
Maheny
,
W.C.
, ed.,
Quaternary Stratigraphy of North America
 :
Strouds
-burg, Pennsylvania, Dowden
,
Hutchinson and Ross
, p.
235
274
.
Kelson
,
K.I.
Thompson
,
R.A.
Bauer
,
P.W.
,
2008
,
Geologic Map of the Guadalupe Mountain 7.5 Minute Quadrangle, Taos County, New Mexico
:
New Mexico Bureau of Geology and Mineral Resources Open-File Geologic Map 168
 , scale 1:24, 000.
Konning
,
D.J.
Newell
,
D.L.
Sarna-Wojcicki
,
A.
Dunbar
,
N.
Karlstrom
,
K.
Salem
,
A.
Crossey
,
L.
,
2011
,
Terrace stratigraphy, ages, and incision rates along the Rio Ojo Caliente, north-central New Mexico
,
in
 
New Mexico Geological Society Guidebook, 62nd Field Conference, Geology of the Tusas Mountains-Ojo Caliente
 :
Socorro, New Mexico Geological Society
, p.
281
300
.
Lifton
,
N.
Sato
,
T.
Dunai
,
T.J.
,
2014
,
Scaling in situ cosmogenic nuclide production rates using analytical approximations to atmospheric cosmic-ray fluxes
:
Earth and Planetary Science Letters
 , v.
386
, p.
149
160
.
Lisiecki
,
L.E.
Raymo
,
M.E.
,
2005
,
A Pliocene-Pleistocene stack of 57 globally distributed benthic records
:
Paleoceanography
 , v.
20
, PA1003, doi: 10.1029/2004PA001071.
Machette
,
M.N.
,
1978
,
Dating Quaternary faults in the southwestern United States using buried calcic paleosols
:
U.S. Geological Survey Journal of Research
 , v.
6
, no.
3
, p.
369
381
.
Machette
,
M.N.
,
1985
, Calcic soils of the southwestern United States, in
Weide
,
D.L.
, ed.,
Soils and Quaternary Geology of the Southwestern United States
 :
Geological Society of America Special Paper 203
, p.
1
21
, doi:10.1130/SPE203-p1.
Machette
,
M.N.
, ed.,
1988
, In the Footsteps of G.K. Gilbert: Lake Bonneville and Neotectonics of the Eastern Basin and Range Province:
Field Trip Guidebook for Field Trip 2, Geological Society of America Annual Meeting
 :
Utah Geological and Mineral Survey Miscellaneous Publication 88-1
, p.
111
116
.
Machette
,
M.N.
,
1998
, Contrasts between short- and long-term records of seis-micity in the Rio Grande rift—Important implications for seismic-hazards analysis in areas of slow extension, in
Lund
,
W.R.
, ed.,
Proceedings, Western States Seismic Policy Council (WSSPC) Basin and Range Province Seismic-Hazards Summit
 :
Utah Geological Survey Miscellaneous Publication 98-2
, p.
84
95
.
Machette
,
M.N.
Long
,
T.
Bachman
,
G.O.
Timbel
,
H.R.
,
1997
,
Laboratory Data for Calcic Soils in Central New Mexico—Background Information for Mapping Quarternary Deposits in the Albuquerque Basin
:
New Mexico Bureau of Mines and Mineral Resources Circular 2-5
 ,
63
p. (Supersedes U.S. Geological Survey/Open-File Report 96-722, 60 p.)
Machette
,
M.N.
,
Coates
,
M.M.
Johnson
,
M.L.
, eds.,
2007
,
2007 Rocky Mountain Section Friends of the Pleistocene Field Trip—Quaternary Geology of the San Luis Basin of Colorado and New Mexico, September 7-9, 2007
 :
U.S. Geological Survey Open-File Report 2007-1193
,
197
p.; online at http://pubs.usgs.gov/of/2007/1193/.
Machette
,
M.N.
Thompson
,
R.A.
Drenth
,
B.J.
,
2008
, Geologic Map of the San Luis Quadrangle, Costilla County,
Colorado
:
U.S. Geological Survey Scientific Investigations Map 2963
 , scale 1:24, 000, 1 sheet; online at http://pubs.usgs.gov/sim/2963/.
Machette
,
M.N.
Thompson
,
R.A.
Marchetti
,
D.W.
Smith
,
R.S.U.
,
2013
, Evolution of ancient Lake Alamosa and integration of the Rio Grande during the Pliocene and Pleistocene, in
Hudson
,
M.R.
Grauch
,
V.J.S.
, eds.,
New Perspectives on Rio Grande Rift Basins: From Tectonics to Groundwater
 :
Geological Society of America Special Paper 494
, p.
1
20
, doi:10.1130/2013.2494(01).
Madole
,
R.F.
,
1986
, Lake Devlin and Pinedale glacial history, Front Range,
Colorado
:
Quaternary Research
, v.
25
, p.
43
54
, doi: 10.1016/0033-5894(86)90042-6.
Madole
,
R.F.
Romig
,
J.H.
Aleinikoff
,
J.N.
Vansistine
,
D.P.
Yacob
,
E.Y.
,
2008
, On the origin and age of the Great Sand Dunes,
Colorado
:
Geomorphology
, v.
99
, p.
99
119
.
Madole
,
R.F.
Mahan
,
S.A.
Romig
,
J.H.
Havens
,
J.C.
,
2013
,
Constraints on the age of the Great Sand Dunes, Colorado, from subsurface stratigraphy and OSL dates
:
Quaternary Research
 , v.
3
, p.
435
446
, doi: 10.1016/j.yqres.2013.09.009.
Mccalpin
,
J.P.
,
1981
,
Quaternary Geology and Neotectonics of the West Flank of the Northern Sangre de Cristo Mountains, South-Central Colorado [Ph.D. thesis]
:
Golden
, Colorado School of Mines,
287
p.
Mccalpin
,
J.P.
,
1982
,
Quaternary Geology and Neotectonics of the West Flank of the Northern Sangre de Cristo Mountains
,
South-Central Colorado: Colorado School of Mines Quarterly
 , v.
77
, no.
3
,
97
p.
Menges
,
C.M.
,
1990
, Late Quaternary fault scarps, mountain-front landforms, and Pliocene-Quaternary segmentation on the range-bounding fault zone, Sangre de Cristo Mountains, New Mexico, in
Krinitzsky
,
E.L.
Slemmons
,
D.B.
, eds.,
Neotectonics in Earthquake Evaluation
 :
Geological Society of America Reviews in Engineering Geology
, v. VIII, p.
131
156
, doi:10.1130/REG8-p131.
Nelson
,
A.R.
Millington
,
A.C.
Andrews
,
J.T.
Nichols
,
H.
,
1979
,
Radiocarbon-dated upper Pleistocene glacial sequence, Fraser Valley, Colorado Front Range
:
Geology
 , v.
7
, p.
410
414
, doi:10.1130/0091-7613(1979)7<410:RUPGSF>2.0.CO;2.
Pazzaglia
,
F.J.
Wells
,
S.G.
,
1990
, Quaternary stratigraphy, soils and geo-morphology of the northern Rio Grande Rift, in
Bauer
,
P.W.
Lucas
,
S.G.
Mawer
,
C.K.
Mcintosh
,
W.C.
, eds.,
Tectonic Development of the Southern Sangre de Cristo Mountains, New Mexico
 :
New Mexico Geological Society Guidebook 41
, p.
423
430
.
Pierce
,
K.L.
,
2004
, Pleistocene glaciation of the Rocky Mountains, in
Gillespie
,
A.R.
Porter
,
S.C.
Atwater
,
B.F.
, eds.,
The Quaternary Period in the United States
 :
Amsterdam
,
Elsevier
, p.
63
76
.
Ruleman
,
C.
Machette
,
M.N.
,
2007
, Chapter J—An overview of the Sangre de Cristo fault system and new insights to interactions between Quaternary faults in the northern Rio Grande rift, in
Machette
,
M.N.
Coates
,
M.M.
Johnson
,
M.L.
, eds.,
2007 Rocky Mountain Section Friends of the Pleistocene Field Trip—Quaternary Geology of the San Luis Basin of Colorado and New Mexico, September 7-9, 2007
 :
U.S. Geological Survey Open-File Report 2007-1193
, p.
187
197
; online at http://pubs.usgs.gov/of/2007/1193/.
Ruleman
,
C.
Shroba
,
R.
Thompson
,
R.
,
2007
, Field-trip Day 3, Quaternary geology of Sunshine Valley and associated neotectonics along the Latir Peaks section of the southern Sangre de Cristo fault zone, in
Machette
,
M.N.
Coates
,
M.M.
Johnson
,
M.L.
, eds.,
2007 Rocky Mountain Section of the Friends of the Pleistocene Field Trip—Quaternary Geology of the San Luis Basin of Colorado and New Mexico
 :
U.S. Geological Survey Open-File Report 20071193
, p.
111
133
; online at http://pubs.usgs.gov/of/2007/1193/.
Ruleman
,
C.A.
Thompson
,
R.A.
Shroba
,
R.R.
Anderson
,
M.
Drenth
,
B.J.
Rotzien
,
J.
Lyon
,
J.
,
2013
, Late Miocene-Pleistocene evolution of a Rio Grande rift subbasin, Sunshine Valley-Costilla Plain, San Luis Basin, New Mexico and Colorado, in
Hudson
,
M.R.
Grauch
,
V.J.S.
, eds.,
New Perspectives on Rio Grande Rift Basins: From Tectonics to Ground-water
 :
Geological Society of America Special Paper 494
, p.
47
73
, doi:10.1130/2013.2494(03).
Schildgen
,
T.F.
Dethier
,
D.P.
,
2000
, Fire and ice—Using isotopic dating techniques to infer the geomorphic history of Middle Boulder Creek,
Colorado
:
Geological Society of America Abstracts with Programs
,
v
 .
32
, no.
7
, p.
A18
.
Schildgen
,
T.
Dethier
,
D.P.
Bierman
,
P.
Caffee
,
M.
,
2002
, 26
Al and
10
Be dating of late Pleistocene and Holocene fill terraces—A record of fluvial deposition and incision, Colorado Front Range
:
Earth Surface Process and Landforms
 , v.
27
, p.
773
787
.
Scholz
,
C.H.
,
1990
, The Mechanics of Earthquakes and Faulting:
Cambridge, UK
,
Cambridge University Press
,
439
p.
Shackleton
,
N.J.
Opdyke
,
N.D.
,
1973
,
Oxygen isotope and paleomagnetic stratigraphy of equatorial Pacific core V28-238
:
Oxygen isotope temperatures and ice volumes on a 10
  5
year and 106 year scale: Quaternary Research
 , v.
3
, p.
39
55
, doi: 10.1016/0033-5894(73)90052-5.
Shackleton
,
N.J.
Opdyke
,
N.D.
,
1976
, Oxygen isotope and paleomagnetic stratigraphy of Pacific core V28-239, late Pliocene to latest Pleistocene, in
Cline
,
R.M.
Hays
,
J.D.
, eds.,
Investigation of Late Quaternary Paleoceanography and Paleoclimatology
 :
Geological Society of America Memoir 145
, p.
449
464
.
Sharp
,
W.D.
Ludwig
,
K.R.
Chadwick
,
O.A.
Amundson
,
R.
Glaser
,
L.L.
,
2003
, Dating fluvial terraces by 230Th/U on pedogenic carbonate, Wind River basin,
Wyoming
:
Quaternary Research
, v.
59
, p.
139
150
, doi: 10.1016/S0033-5894(03)00003-6.
Shroba
,
R.R.
Thompson
,
R.A.
Minor
,
S.A.
Grauch
,
V.J.S.
Brandt
,
T.R.
,
2005
,
Geologic Map of the Agua Fria Quadrangle, Santa Fe County, New Mexico
:
U.S. Geological Survey Scientific Investigations Map 2896
 ,
22
p., 1 plate, scale 1:24, 000.
Siebenthal
,
C.E.
,
1910
, Geology and Water Resources of the San Luis Valley,
Colorado
:
U.S. Geological Survey Water Supply Paper 240
,
128
p.
Thompson
,
R.A.
Machette
,
M.N.
,
1989
,
Geologic Map of the San Luis Hills Area, Conejos and Costilla Counties
 ,
Colorado
:
U.S. Geological Survey Miscellaneous Investigation
Series Map I-1906, scale 1:50, 000.
Thompson
,
R.A.
Johnson
,
C.M.
Mehnert
,
H.H.
,
1991
, Oligocene basaltic volcanism of the northern Rio Grande Rift: San Luis Hills,
Colorado
:
Journal of Geophysical Research
, v.
96
, p.
13
, 577-13, 592, doi: 10.1029/91JB00068.
Thompson
,
R.A.
Machette
,
M.N.
Drenth
,
B.J.
,
2007
, Geologic Map of San Pedro Mesa and Surrounding Area, Costilla County,
Colorado
:
U.S. Geological Survey Open-File Report 2007-1074
, scale 1:24, 000, 1 sheet, http://pubs.usgs.gov/ofr/2007/1074.
Thompson
,
R.A.
Turner
,
K.J.
Shroba
,
R.R.
Cosca
,
M.A.
Ruleman
,
C.A.
Lee
,
J.P.
Brandt
,
T.R.
,
2014a
, Geologic Map of the Ute Mountain 7.5 Quadrangle, Taos County, New Mexico, and Conejos and Costilla Counties,
Colorado
:
U.S. Geological Survey Scientific Investigations Map 3284
 , scale 1:24, 000, http://dx.doi.org/10.3133/sim3284.
Thompson
,
R.A.
Turner
,
K.J.
Shroba
,
R.R.
Cosca
,
M.A.
Ruleman
,
C.A.
Lee
,
J.P.
Brandt
,
T.R.
,
2014b
,
Geologic Map of the Sunshine 7.5 Quadrangle
 ,
Taos County, New Mexico
:
U.S. Geological Survey Scientific Investigations Map 3283
, scale 1:24, 000, http://dx.doi.org/10.3133/sim3283.
Thompson
,
R.A.
Shroba
,
R.R.
Machette
,
M.N.
Fridrich
,
C.J.
Brandt
,
T.R.
Cosca
,
M.A.
,
2015
,
Geologic Map of the Alamosa 30’ X 60’ Quadrangle, South-Central Colorado
 :
U.S. Geological Survey Scientific Investigations Map 3342
,
23
p., scale 1:100, 000, http://dx.doi.org/10.3133/sim3342. (Supersedes Open-File Report 2005-1392 and Open-File Report 2008-1124.)
Wells
,
S.G.
Kelson
,
K.I.
Menges
,
C.M.
,
1987
, Quaternary evolution of fluvial systems in the northern Rio Grande rift, New Mexico and Colorado: Implications for entrenchment and integration of drainage systems, in
Menges
,
C.
, ed.,
Quaternary Tectonics, Landforms Evolution, Soil Chronologies, and Glacial Deposits—Northern Rio Grande Rift of New Mexico
 :
Field Trip Guidebook; Friends of the Pleistocene, Rocky Mountain Cell, Oct. 8-11, 1987: Albuquerque
,
University of New Mexico Department of Geology
, p.
55
69
.

Figures & Tables

Figure 1.

Basins of the Rio Grande and age of integration into an axial Rio Grande fluvial system (after Connell et al., 2005). Ages of basin integration from previous works are as follows: Lower Rio Grande-Rio Conchas-Pecos River (Hawley et al., 1976; Galloway et al., 2011), Middle Rio Grande basin (Dethier et al., 1988), Albuquerque basin (Cole et al., 2007; Connell et al., 2005), Espa-nola Basin (Konning et al., 2011), and San Luis Basin (Machette et al., 2007, 2013; Ruleman et al., 2007, 2013). NSLB—northern San Luis Basin; SSLB—southern San Luis Basin; CO—Colorado; NM—New Mexico; TX—Texas.

Figure 1.

Basins of the Rio Grande and age of integration into an axial Rio Grande fluvial system (after Connell et al., 2005). Ages of basin integration from previous works are as follows: Lower Rio Grande-Rio Conchas-Pecos River (Hawley et al., 1976; Galloway et al., 2011), Middle Rio Grande basin (Dethier et al., 1988), Albuquerque basin (Cole et al., 2007; Connell et al., 2005), Espa-nola Basin (Konning et al., 2011), and San Luis Basin (Machette et al., 2007, 2013; Ruleman et al., 2007, 2013). NSLB—northern San Luis Basin; SSLB—southern San Luis Basin; CO—Colorado; NM—New Mexico; TX—Texas.

Figure 2.

Physiographic map of the San Luis Basin generated from 10 m digital elevation data. The San Luis Hills form the main barrier between the northern and southern basins. Extent of middle Pleistocene Lake Alamosa shown in northern San Luis Basin. Sangre de Cristo fault system shown as northern, central, and southern fault zones. Field-trip stop locations are indicated by yellow stars.

Figure 2.

Physiographic map of the San Luis Basin generated from 10 m digital elevation data. The San Luis Hills form the main barrier between the northern and southern basins. Extent of middle Pleistocene Lake Alamosa shown in northern San Luis Basin. Sangre de Cristo fault system shown as northern, central, and southern fault zones. Field-trip stop locations are indicated by yellow stars.

Figure 3.

Geologic map of the southern San Luis Basin showing cross sections described in text, field-trip stop locations, and 3He surface exposure ages at each location. Marine oxygen isotope stage chart (modified from Lisiecki and Raymo, 2005) provided for convenience. Tables 2 and 3 modified from Ruleman et al. (2013) to show 3He and U-series chronologic constraints on regional correlation of surficial deposits. CO— Colorado; NM—New Mexico; TX—Texas.

Figure 3.

Geologic map of the southern San Luis Basin showing cross sections described in text, field-trip stop locations, and 3He surface exposure ages at each location. Marine oxygen isotope stage chart (modified from Lisiecki and Raymo, 2005) provided for convenience. Tables 2 and 3 modified from Ruleman et al. (2013) to show 3He and U-series chronologic constraints on regional correlation of surficial deposits. CO— Colorado; NM—New Mexico; TX—Texas.

Figure 4.

Bachus pit owner sign and contact information. Photo by M.M. Machette.

Figure 4.

Bachus pit owner sign and contact information. Photo by M.M. Machette.

Figure 5.

Surficial geologic map of Stop 1. Units listed as follows (Thompson et al., 2015): Qai/QTla—middle Pleistocene gravel overlying Pliocene to Pleistocene lacustrine deposits of Lake Alamosa; Qay—late Pleistocene alluvium; Qa—late Pleistocene to Holocene alluvium; Qed—late Pleistocene to Holocene eolian deposits; Qfp—Holocene floodplain and overbank deposits; Qaa—active alluvium.

Figure 5.

Surficial geologic map of Stop 1. Units listed as follows (Thompson et al., 2015): Qai/QTla—middle Pleistocene gravel overlying Pliocene to Pleistocene lacustrine deposits of Lake Alamosa; Qay—late Pleistocene alluvium; Qa—late Pleistocene to Holocene alluvium; Qed—late Pleistocene to Holocene eolian deposits; Qfp—Holocene floodplain and overbank deposits; Qaa—active alluvium.

Figure 6.

Middle Pleistocene stratigraphic section exposed in the Ba-chus pit. Modified from Machette et al. (2007).

Figure 6.

Middle Pleistocene stratigraphic section exposed in the Ba-chus pit. Modified from Machette et al. (2007).

Figure 7.

Photo of Bachus pit exposure with division of middle Pleistocene units. (A) Section showing a regressive lake sequence capped by a progradational, late middle Pleistocene gravel. (B) Convoluted and contorted lacustrine silts and clays capped by a regressive lakeshore gravel facies. Tape measure is 2 m in height, with white and red increments of 20 cm. Modified from Machette et al. (2007).

Figure 7.

Photo of Bachus pit exposure with division of middle Pleistocene units. (A) Section showing a regressive lake sequence capped by a progradational, late middle Pleistocene gravel. (B) Convoluted and contorted lacustrine silts and clays capped by a regressive lakeshore gravel facies. Tape measure is 2 m in height, with white and red increments of 20 cm. Modified from Machette et al. (2007).

Figure 8.

Photo of rip-up clasts of gravel of underlying desiccated lacustrine clays within a sandy pebble gravel. Knife is ~20 cm long.

Figure 8.

Photo of rip-up clasts of gravel of underlying desiccated lacustrine clays within a sandy pebble gravel. Knife is ~20 cm long.

Figure 9.

Cross-sectional relationships of surficial deposits at Stop 1. Units listed as follows: QTla—Pliocene to Pleistocene lacustrine deposits of paleolake Alamosa; Qai—middle Pleistocene alluvium; Qes—middle to late Pleistocene eolian sediment; Qay—late Pleistocene alluvium; and Qa—Holocene alluvium.

Figure 9.

Cross-sectional relationships of surficial deposits at Stop 1. Units listed as follows: QTla—Pliocene to Pleistocene lacustrine deposits of paleolake Alamosa; Qai—middle Pleistocene alluvium; Qes—middle to late Pleistocene eolian sediment; Qay—late Pleistocene alluvium; and Qa—Holocene alluvium.

Figure 10.

Bedrock, alluvial deposits, and lacustrine deposits of Lake Alamosa. Map units: Qaa—active alluvium; Qa— Holocene alluvium; Qfp—floodplain alluvium; Qay—younger (late Pleistocene) alluvium; Qla—lacustrine deposits (post-Lake Alamosa); Qlag (gravelly) and Qlam (lagoonal) nearshore deposits of Lake Alamosa (middle Pleistocene part of QTla); QTla—Alamosa Formation (undivided, fine-grained deposits here); Th—Hinsdale Formation (26 Ma). Geology modified from Thompson and Machette (1989).

Figure 10.

Bedrock, alluvial deposits, and lacustrine deposits of Lake Alamosa. Map units: Qaa—active alluvium; Qa— Holocene alluvium; Qfp—floodplain alluvium; Qay—younger (late Pleistocene) alluvium; Qla—lacustrine deposits (post-Lake Alamosa); Qlag (gravelly) and Qlam (lagoonal) nearshore deposits of Lake Alamosa (middle Pleistocene part of QTla); QTla—Alamosa Formation (undivided, fine-grained deposits here); Th—Hinsdale Formation (26 Ma). Geology modified from Thompson and Machette (1989).

Figure 11.

Oblique aerial photograph of the south side of Saddleback Mountain, location of Stop 2. Unit Th—Hinsdale Formation (upper Oligocene andesitic basalt). View to the northeast (photo by M.M. Machette).

Figure 11.

Oblique aerial photograph of the south side of Saddleback Mountain, location of Stop 2. Unit Th—Hinsdale Formation (upper Oligocene andesitic basalt). View to the northeast (photo by M.M. Machette).

Figure 12.

Schematic cross sections of surficial and bedrock geology at Saddleback Mountain, modified from Machette et al. (2007). (A) Stacked spits (Qlag1 is older than Qlag2). (B) Superposed spits (Qlag1 is younger than Qlag2). Map units: Qlam—lagoonal deposits of Lake Alamosa; Qlag—gravel (spit) deposits of Lake Alamosa (Qlag1, lower spit; Qlag2, upper spit); QTal—Alamosa Formation, undivided; Th—Hinsdale Formation (upper Oligocene andesitic basalt). View to the west. Based on geologic mapping of Thompson and Machette (1989) and Thompson et al. (2015).

Figure 12.

Schematic cross sections of surficial and bedrock geology at Saddleback Mountain, modified from Machette et al. (2007). (A) Stacked spits (Qlag1 is older than Qlag2). (B) Superposed spits (Qlag1 is younger than Qlag2). Map units: Qlam—lagoonal deposits of Lake Alamosa; Qlag—gravel (spit) deposits of Lake Alamosa (Qlag1, lower spit; Qlag2, upper spit); QTal—Alamosa Formation, undivided; Th—Hinsdale Formation (upper Oligocene andesitic basalt). View to the west. Based on geologic mapping of Thompson and Machette (1989) and Thompson et al. (2015).

Figure 13.

Soil profile with textural and CaCO3 analyses of the lower spit at Saddleback Mountain.

Figure 13.

Soil profile with textural and CaCO3 analyses of the lower spit at Saddleback Mountain.

Figure 14.

Soil profile with textural and CaCO3 analyses of the upper spit at Saddleback Mountain.

Figure 14.

Soil profile with textural and CaCO3 analyses of the upper spit at Saddleback Mountain.

Figure 15.

Geologic map of Stop 3. Units listed as follows: Tc—Oligocene Conejos Formation; Td—dacite sills; QTla—Pliocene to middle Pleistocene Alamosa Formation; Qlag—middle Pleistocene outflow lag gravel deposits; Qai—late middle Pleistocene alluvium; Qay—late Pleistocene alluvium; Qa— Holocene alluvium. Faults shown by thick red lines. Modified from Thompson et al. (2015).

Figure 15.

Geologic map of Stop 3. Units listed as follows: Tc—Oligocene Conejos Formation; Td—dacite sills; QTla—Pliocene to middle Pleistocene Alamosa Formation; Qlag—middle Pleistocene outflow lag gravel deposits; Qai—late middle Pleistocene alluvium; Qay—late Pleistocene alluvium; Qa— Holocene alluvium. Faults shown by thick red lines. Modified from Thompson et al. (2015).

Figure 16.

Sangre de Cristo fault system and fault zone boundaries (modified from Menges, 1990; Ruleman and Machette, 2007; Ruleman et al., 2013). Sangre de Cristo fault zone— SDCFZ. Fault zone boundaries shown as: A—northern SDCFZ; B—central SDCFZ; and C—southern SDCFZ.

Figure 16.

Sangre de Cristo fault system and fault zone boundaries (modified from Menges, 1990; Ruleman and Machette, 2007; Ruleman et al., 2013). Sangre de Cristo fault zone— SDCFZ. Fault zone boundaries shown as: A—northern SDCFZ; B—central SDCFZ; and C—southern SDCFZ.

Figure 17.

(A) Photo of fluvially scoured basalt at Stop 5. (B) Fluvially scoured surface and rounded basalt clasts of Servilleta Basalt. (C) Scour pit in highly vesicular gas vent tubes. (D) View to the north from Stop 5 across the northern Costilla Plain showing the very subtle topographic relief between pre-gorge gravels. (E) Cross section A-A (Fig. 3) showing position of Stop 5 in relation to the middle Pleistocene drainage path of Lake Alamosa and 3He surface exposure ages along the gorge rim. asl— above sea level. Labels as follows: RPT—Rio Pueblo de Taos; LP—La Junta Point; RG—Rio Grande; RRFZ—Red River fault zone; NSLB—northern San Luis Basin. Tsb—Servilleta Basalt; Tov—Tertiary older volcanics; Tsf—Santa Fe Group older; QTsf—Santa Fe Group. (F) Maximum basin aggradation level at the close of MIS 12 (~420 ka). (G) Initial dissection of Qao1 surface during the first phase of Lake Alamosa drainage and basin integration culminating ~MIS 8 (~243 ka) (photo by C.A. Ruleman). All marine oxygen isotope stages (MIS) ages from Lisiecki and Raymo (2005).

Figure 17.

(A) Photo of fluvially scoured basalt at Stop 5. (B) Fluvially scoured surface and rounded basalt clasts of Servilleta Basalt. (C) Scour pit in highly vesicular gas vent tubes. (D) View to the north from Stop 5 across the northern Costilla Plain showing the very subtle topographic relief between pre-gorge gravels. (E) Cross section A-A (Fig. 3) showing position of Stop 5 in relation to the middle Pleistocene drainage path of Lake Alamosa and 3He surface exposure ages along the gorge rim. asl— above sea level. Labels as follows: RPT—Rio Pueblo de Taos; LP—La Junta Point; RG—Rio Grande; RRFZ—Red River fault zone; NSLB—northern San Luis Basin. Tsb—Servilleta Basalt; Tov—Tertiary older volcanics; Tsf—Santa Fe Group older; QTsf—Santa Fe Group. (F) Maximum basin aggradation level at the close of MIS 12 (~420 ka). (G) Initial dissection of Qao1 surface during the first phase of Lake Alamosa drainage and basin integration culminating ~MIS 8 (~243 ka) (photo by C.A. Ruleman). All marine oxygen isotope stages (MIS) ages from Lisiecki and Raymo (2005).

Figure 18.

(A) View looking north from the southern margin of the Taos Plateau showing the extremely flat basaltic tableland of the southern San Luis Basin that Lake Alamosa flowed across initiating ~<400 ka (photo by R.A. Thompson). (B) Cross section A-A’ showing position of Stop 6 in relation to the middle Pleistocene drainage path of Lake Alamosa and 3He surface exposure ages along the gorge rim. (C) Maximum basin aggradation level at the close of MIS 12 (~420 ka). (D) Initial dissection of Qao1 surface during the first phase of Lake Alamosa drainage and basin integration culminating ~MIS 8 (~243 ka). See Figure 17 for geologic unit descriptions.

Figure 18.

(A) View looking north from the southern margin of the Taos Plateau showing the extremely flat basaltic tableland of the southern San Luis Basin that Lake Alamosa flowed across initiating ~<400 ka (photo by R.A. Thompson). (B) Cross section A-A’ showing position of Stop 6 in relation to the middle Pleistocene drainage path of Lake Alamosa and 3He surface exposure ages along the gorge rim. (C) Maximum basin aggradation level at the close of MIS 12 (~420 ka). (D) Initial dissection of Qao1 surface during the first phase of Lake Alamosa drainage and basin integration culminating ~MIS 8 (~243 ka). See Figure 17 for geologic unit descriptions.

Figure 19.

(A) View looking west across the Rio Grande gorge from Qao2 surface to Stop 7. Exposure ages and fluvially scoured strath surfaces are shown (photo by C.A. Ruleman). (B) Cross section B-B’ (showing topographic relationships between La Junta Point (LP)/confluence of the Rio Grande (RG) and Red River (RR) and the pre-gorge outflow alluvial plain to the south. CM—Cebolla Mesa; unit Ty is Servilleta Basalt and related rocks, Pliocene to Pleistocene (<5 Ma). (C) Cross section C-C (Fig. 3) showing structural and depositional/ erosional relationships at Arroyo Hondo. (D) Cross section D-D (Fig. 3) showing 3He surface exposure ages in relation to canyon incision history.

Figure 19.

(A) View looking west across the Rio Grande gorge from Qao2 surface to Stop 7. Exposure ages and fluvially scoured strath surfaces are shown (photo by C.A. Ruleman). (B) Cross section B-B’ (showing topographic relationships between La Junta Point (LP)/confluence of the Rio Grande (RG) and Red River (RR) and the pre-gorge outflow alluvial plain to the south. CM—Cebolla Mesa; unit Ty is Servilleta Basalt and related rocks, Pliocene to Pleistocene (<5 Ma). (C) Cross section C-C (Fig. 3) showing structural and depositional/ erosional relationships at Arroyo Hondo. (D) Cross section D-D (Fig. 3) showing 3He surface exposure ages in relation to canyon incision history.

Figure 20.

(A) View from sample location RG-10-9, the highest basalt strath surface exposed at Arroyo Hondo (photo by C.A. Ruleman). (B) Photo of fluvially scoured surface at Stop 7 and sample location RG-10-8. (C) Scour pits in vesicular gas tubes and preservation of chilled, polygonal crack pattern formed on the surface of Pliocene basalt flows. (D) View south from Stop 7 showing the Rio Grande gorge and 3He exposure age constraints on timing of canyon incision at Arroyo Hondo. (E) View west from sample location RG-10-13, Arroyo Hondo gorge rim. (F) Schematic geomorphic setting of final basin integration phase. See Figure 3 for geologic units and color scheme.

Figure 20.

(A) View from sample location RG-10-9, the highest basalt strath surface exposed at Arroyo Hondo (photo by C.A. Ruleman). (B) Photo of fluvially scoured surface at Stop 7 and sample location RG-10-8. (C) Scour pits in vesicular gas tubes and preservation of chilled, polygonal crack pattern formed on the surface of Pliocene basalt flows. (D) View south from Stop 7 showing the Rio Grande gorge and 3He exposure age constraints on timing of canyon incision at Arroyo Hondo. (E) View west from sample location RG-10-13, Arroyo Hondo gorge rim. (F) Schematic geomorphic setting of final basin integration phase. See Figure 3 for geologic units and color scheme.

Figure 21.

Stop 8. (A) View from La Junta Point of the confluence of the Rio Grande and Red River (photo by Cal Ruleman). (B) View looking north along the gorge from La Junta Point showing geomorphic and geochronologic constraints on gorge incision history. (C) View westward from the Questa, New Mexico, cemetery looking toward the Red River gorge. (D) Cross section D-D showing coeval gorge incision of the Rio Grande and Red River. (E) Cross section E-E showing relationships between southward-directed outflow waters eroding through the Red River fault zone and depositing down on La Junta Point and Cebolla Mesa. (F) Cross section F-F showing relationships of the Red River incision history and Red River fault zone-Guadalupe Mountain. (G) Cross section G-G (Fig. 3) showing relationships of maximum aggradation level and incision history into the Questa subbasin. Units in cross sections as follows: Qao3—alluvium, late middle Pleistocene (MIS 6-7, ~130-243 ka); Qao2—alluvium, middle Pleistocene (MIS 8-11, ~243-400 ka); QTsf—Santa Fe Group (>640 ka); Tsb— Servilleta Basalt and associated rocks (ca. 5-2 Ma); Tov—volcanics undifferentiated, Oligocene to Pliocene (ca. 34-5 Ma), locally includes high-relief Pliocene volcanic centers, Texas, Oligocene to Proterozoic rocks undifferentiated. Marine oxygen isotope stage (MIS) ages from Lisiecki and Raymo (2005).

Figure 21.

Stop 8. (A) View from La Junta Point of the confluence of the Rio Grande and Red River (photo by Cal Ruleman). (B) View looking north along the gorge from La Junta Point showing geomorphic and geochronologic constraints on gorge incision history. (C) View westward from the Questa, New Mexico, cemetery looking toward the Red River gorge. (D) Cross section D-D showing coeval gorge incision of the Rio Grande and Red River. (E) Cross section E-E showing relationships between southward-directed outflow waters eroding through the Red River fault zone and depositing down on La Junta Point and Cebolla Mesa. (F) Cross section F-F showing relationships of the Red River incision history and Red River fault zone-Guadalupe Mountain. (G) Cross section G-G (Fig. 3) showing relationships of maximum aggradation level and incision history into the Questa subbasin. Units in cross sections as follows: Qao3—alluvium, late middle Pleistocene (MIS 6-7, ~130-243 ka); Qao2—alluvium, middle Pleistocene (MIS 8-11, ~243-400 ka); QTsf—Santa Fe Group (>640 ka); Tsb— Servilleta Basalt and associated rocks (ca. 5-2 Ma); Tov—volcanics undifferentiated, Oligocene to Pliocene (ca. 34-5 Ma), locally includes high-relief Pliocene volcanic centers, Texas, Oligocene to Proterozoic rocks undifferentiated. Marine oxygen isotope stage (MIS) ages from Lisiecki and Raymo (2005).

Figure 22.

Stop 9. (A) Photo of basaltic surface dated ~475 ka on the footwall of the Red River fault (photo by C.A. Rule-man). (B) Cross section A-A (Fig. 3) showing geomorphic relationships between outflow discharge and topographic position. Labels as follows: RPT—Rio Pueblo de Taos; LP—La Junta Point; RG—Rio Grande; RRFZ—Red River fault zone; NSLB—northern San Luis Basin. Tsb—Servilleta Basalt; Tov—Tertiary older volcanics; Tsf—Santa Fe Group older; QTsf—Santa Fe Group. (C) Schematic geologic map of the geomorphic environment prior to drainage of Lake Alamosa. Unit Qao1 represents the maximum aggradation level within the southern San Luis Basin. Lake Alamosa highstand age and closed basin exposed bedrock ages on the footwall of the Red River fault zone are shown.

Figure 22.

Stop 9. (A) Photo of basaltic surface dated ~475 ka on the footwall of the Red River fault (photo by C.A. Rule-man). (B) Cross section A-A (Fig. 3) showing geomorphic relationships between outflow discharge and topographic position. Labels as follows: RPT—Rio Pueblo de Taos; LP—La Junta Point; RG—Rio Grande; RRFZ—Red River fault zone; NSLB—northern San Luis Basin. Tsb—Servilleta Basalt; Tov—Tertiary older volcanics; Tsf—Santa Fe Group older; QTsf—Santa Fe Group. (C) Schematic geologic map of the geomorphic environment prior to drainage of Lake Alamosa. Unit Qao1 represents the maximum aggradation level within the southern San Luis Basin. Lake Alamosa highstand age and closed basin exposed bedrock ages on the footwall of the Red River fault zone are shown.

Figure 23.

Stop 10. (A) Photo of exposure at the gravel quarry on the northwestern flank of Guadalupe Mountain (photo by C.A. Ruleman). Qai—middle Pleistocene alluvium. (B) Cross-section images of carbonate rinds on clasts sampled and analyzed for U-series dating from the coarse gravel sourced from the Red River-Cabresto Creek fluvial system. Sampled areas are enclosed by red polygons. (C) U-Th plot and ages of samples analyzed. Samples from the Guadalupe Mountain pit are shown by red error ellipses, 2σ errors are indicated by ellipse size. Samples indicated by horizontal gray arrows have substantial unsupported Th, indicating open-system behavior, and are deemed unreliable. (D) Geomorphic setting and 3He exposure ages along the gorge rim during the final gorge formation phase, culminating MIS 6 (~130 ka).

Figure 23.

Stop 10. (A) Photo of exposure at the gravel quarry on the northwestern flank of Guadalupe Mountain (photo by C.A. Ruleman). Qai—middle Pleistocene alluvium. (B) Cross-section images of carbonate rinds on clasts sampled and analyzed for U-series dating from the coarse gravel sourced from the Red River-Cabresto Creek fluvial system. Sampled areas are enclosed by red polygons. (C) U-Th plot and ages of samples analyzed. Samples from the Guadalupe Mountain pit are shown by red error ellipses, 2σ errors are indicated by ellipse size. Samples indicated by horizontal gray arrows have substantial unsupported Th, indicating open-system behavior, and are deemed unreliable. (D) Geomorphic setting and 3He exposure ages along the gorge rim during the final gorge formation phase, culminating MIS 6 (~130 ka).

Figure 24.

Stop 11. Lone Tree sample location RG-10. (A) View north of fluvially scoured surface with Dan Miggins (Oregon State University) and Gary Landis (USGS, retired) (photo by C.A. Ruleman). (B) View looking south at sample location RG-10-3A (photo by C.A. Ruleman). (C) Cross section A-A (Fig. 3) showing geomorphic relationships between outflow discharge and topographic position. Labels as follows: RPT—Rio Pueblo de Taos; LP—La Junta Point; RG—Rio Grande; RRFZ—Red River fault zone; NSLB—northern San Luis Basin. Tsb—Servilleta Basalt; Tov—Tertiary older volcanics; Tsf—Santa Fe Group older; QTsf—Santa Fe Group. (D) Geomorphic setting and 3He exposure ages along the gorge rim during the final gorge formation phase, culminating at MIS 6 (~130 ka).

Figure 24.

Stop 11. Lone Tree sample location RG-10. (A) View north of fluvially scoured surface with Dan Miggins (Oregon State University) and Gary Landis (USGS, retired) (photo by C.A. Ruleman). (B) View looking south at sample location RG-10-3A (photo by C.A. Ruleman). (C) Cross section A-A (Fig. 3) showing geomorphic relationships between outflow discharge and topographic position. Labels as follows: RPT—Rio Pueblo de Taos; LP—La Junta Point; RG—Rio Grande; RRFZ—Red River fault zone; NSLB—northern San Luis Basin. Tsb—Servilleta Basalt; Tov—Tertiary older volcanics; Tsf—Santa Fe Group older; QTsf—Santa Fe Group. (D) Geomorphic setting and 3He exposure ages along the gorge rim during the final gorge formation phase, culminating at MIS 6 (~130 ka).

Figure 25.

Stop 12. Incised meander sample location RG-10-4. (A) Photo of fluvially scoured basalt at Stop 12 (photo by C.A. Ruleman). (B) Photo of sample RG-10-4 and scour pits (photo by C.A. Ruleman). (C) View south of gorge from sample location RG-10-4 (photo by C.A. Ruleman). (D) Cross section A-A’ (Fig. 3) showing geomorphic relationships between outflow discharge and topographic position. Labels as follows: RPT— Rio Pueblo de Taos; LP—La Junta Point; RG—Rio Grande; RRFZ—Red River fault zone; NSLB—northern San Luis Basin. Tsb—Servilleta Basalt; Tov—Tertiary older volcanics; Tsf— Santa Fe Group older; QTsf—Santa Fe Group. (E) Geomorphic setting and 3He exposure ages along the gorge rim during the final gorge formation phase, culminating at MIS 6 (~130 ka).

Figure 25.

Stop 12. Incised meander sample location RG-10-4. (A) Photo of fluvially scoured basalt at Stop 12 (photo by C.A. Ruleman). (B) Photo of sample RG-10-4 and scour pits (photo by C.A. Ruleman). (C) View south of gorge from sample location RG-10-4 (photo by C.A. Ruleman). (D) Cross section A-A’ (Fig. 3) showing geomorphic relationships between outflow discharge and topographic position. Labels as follows: RPT— Rio Pueblo de Taos; LP—La Junta Point; RG—Rio Grande; RRFZ—Red River fault zone; NSLB—northern San Luis Basin. Tsb—Servilleta Basalt; Tov—Tertiary older volcanics; Tsf— Santa Fe Group older; QTsf—Santa Fe Group. (E) Geomorphic setting and 3He exposure ages along the gorge rim during the final gorge formation phase, culminating at MIS 6 (~130 ka).

Figure 26.

Stop 13. Inset fluvially scoured straths, central Sunshine Valley. (A) Oblique aerial view from Google Earth of Stop 13 and 3He surface exposure ages. (B) Cross section A-A (Fig. 3) showing geomor-phic relationships between outflow discharge and topographic position. Labels as follows: RPT—Rio Pueblo de Taos; LP—La Junta Point; RG—Rio Grande; RRFZ—Red River fault zone; NSLB—northern San Luis Basin. Tsb—Servilleta Basalt; Tov—Tertiary older volcanics; Tsf—Santa Fe Group older; QTsf—Santa Fe Group. (C) Geomorphic setting and 3He exposure ages along the gorge rim during the final gorge formation phase, culminating at MIS 6 (~130 ka).

Figure 26.

Stop 13. Inset fluvially scoured straths, central Sunshine Valley. (A) Oblique aerial view from Google Earth of Stop 13 and 3He surface exposure ages. (B) Cross section A-A (Fig. 3) showing geomor-phic relationships between outflow discharge and topographic position. Labels as follows: RPT—Rio Pueblo de Taos; LP—La Junta Point; RG—Rio Grande; RRFZ—Red River fault zone; NSLB—northern San Luis Basin. Tsb—Servilleta Basalt; Tov—Tertiary older volcanics; Tsf—Santa Fe Group older; QTsf—Santa Fe Group. (C) Geomorphic setting and 3He exposure ages along the gorge rim during the final gorge formation phase, culminating at MIS 6 (~130 ka).

Figure 27.

Stop 14. Southwestern flank of Ute Mountain. (A) Oblique aerial view from Google Earth of Stop 14 and corresponding 3He surface exposure ages on fluvially scoured Servilleta Basalt. (B) Photo of fluvially scoured surface at Stop 14 (photo by C.A. Ruleman). (C) Cross section A-A (Fig. 3) showing geomorphic relationships between outflow discharge and topographic position. Labels as follows: RPT—Rio Pueblo de Taos; LP—La Junta Point; RG—Rio Grande; RRFZ—Red River fault zone; NSLB—northern San Luis Basin. Tsb—Servilleta Basalt; Tov—Tertiary older vol-canics; Tsf—Santa Fe Group older; QTsf—Santa Fe Group. (D) Geomorphic setting and 3He exposure ages along the gorge rim following completion of gorge formation and backfilling of <MIS 6 alluvium into tributary drainages.

Figure 27.

Stop 14. Southwestern flank of Ute Mountain. (A) Oblique aerial view from Google Earth of Stop 14 and corresponding 3He surface exposure ages on fluvially scoured Servilleta Basalt. (B) Photo of fluvially scoured surface at Stop 14 (photo by C.A. Ruleman). (C) Cross section A-A (Fig. 3) showing geomorphic relationships between outflow discharge and topographic position. Labels as follows: RPT—Rio Pueblo de Taos; LP—La Junta Point; RG—Rio Grande; RRFZ—Red River fault zone; NSLB—northern San Luis Basin. Tsb—Servilleta Basalt; Tov—Tertiary older vol-canics; Tsf—Santa Fe Group older; QTsf—Santa Fe Group. (D) Geomorphic setting and 3He exposure ages along the gorge rim following completion of gorge formation and backfilling of <MIS 6 alluvium into tributary drainages.

Figure 28.

Stop 15. One-lane bridge site. (A) Photo of the one-lane bridge and depth of the Rio Grande gorge at this location (photo by C.A. Ruleman). (B) View looking north from Stop 15 towards the San Luis Hills and northern Sangre de Cristo Mountains, showing the basin of extremely low topographic relief (photo by C.A. Ruleman). (C) Photo of fluvial scoured basaltic surface and sample RG-10-11 (photo by C.A. Ruleman). (D) Cross section A-A (Fig. 3) showing geomorphic relationships between outflow discharge and topographic position. Labels as follows: RPT—Rio Pueblo de Taos; LP—La Junta Point; RG—Rio Grande; RRFZ—Red River fault zone; NSLB—northern San Luis Basin. Tsb—Servilleta Basalt; Tov—Tertiary older volcanics; Tsf—Santa Fe Group older; QTsf—Santa Fe Group. (E) Geomorphic setting and 3He exposure ages along the gorge rim following completion of gorge formation and back-filling of <MIS 6 alluvium into tributary drainages.

Figure 28.

Stop 15. One-lane bridge site. (A) Photo of the one-lane bridge and depth of the Rio Grande gorge at this location (photo by C.A. Ruleman). (B) View looking north from Stop 15 towards the San Luis Hills and northern Sangre de Cristo Mountains, showing the basin of extremely low topographic relief (photo by C.A. Ruleman). (C) Photo of fluvial scoured basaltic surface and sample RG-10-11 (photo by C.A. Ruleman). (D) Cross section A-A (Fig. 3) showing geomorphic relationships between outflow discharge and topographic position. Labels as follows: RPT—Rio Pueblo de Taos; LP—La Junta Point; RG—Rio Grande; RRFZ—Red River fault zone; NSLB—northern San Luis Basin. Tsb—Servilleta Basalt; Tov—Tertiary older volcanics; Tsf—Santa Fe Group older; QTsf—Santa Fe Group. (E) Geomorphic setting and 3He exposure ages along the gorge rim following completion of gorge formation and back-filling of <MIS 6 alluvium into tributary drainages.

Sample Information and 3He Cosmogenic Surface Exposure Ages For Basalt Samples from the Northern Rio Grande Rift

Table 1.
Sample Information and 3He Cosmogenic Surface Exposure Ages For Basalt Samples from the Northern Rio Grande Rift

U-Th concentrations, U-series isotopic compositions, and calculated 230Th/U ages and initial 234U/238U activity ratios for subsamples of clast rinds from Guadalupe gravel pit, Taos County, New Mexico.

Table 4.
U-Th concentrations, U-series isotopic compositions, and calculated 230Th/U ages and initial 234U/238U activity ratios for subsamples of clast rinds from Guadalupe gravel pit, Taos County, New Mexico.

Contents

GeoRef

References

References Cited

Benson
,
L.
Madole
,
R.
Phillips
,
W.
Landis
,
G.
Thomas
,
T.
Kubic
,
P.
,
2004
,
The probable importance of snow and sediment shielding on cosmogenic ages of north-central Colorado Pinedale and pre-Pinedale moraines
:
Quaternary Science Reviews
 , v.
23
, p.
193
206
, doi: 10.1016/j.quascirev.2003.07.002.
Benson
,
L.
Madole
,
R.
Landis
,
G.
Gosse
,
J.
,
2005
,
New data for late Pleistocene alpine glaciation from southwestern Colorado
:
Quaternary Science Reviews
 , v.
24
, p.
49
65
, doi: 10.1016/j.quascirev.2004.07.018.
Borchers
,
B.
Marrero
,
S.
Balco
,
G.
Caffee
,
M.
Goehring
,
B.
Lifton
,
N.
,
2015
,
Geological calibration of spallation production rates in the CRONUS-Earth project: Quaternary Geochronology
 , doi: 10.1016/j.quageo.2015.01.009.
Burroughs
,
R.L.
,
1972
,
Geology of the San Luis Hills, south-central Colorado [Ph.D. diss.]
 :
Albuquerque
,
University of New Mexico
,
139
p.
Burroughs
,
R.L.
,
1978
, Alamosa to Antonito, Colorado (p. 33-36, in Northern Rift Guide 2, Alamosa, Colorado—Santa Fe, New Mexico), in
Hawley
,
J.W.
, ed.,
Guidebook to Rio Grande Rift in New Mexico and Colorado
 :
New Mexico Bureau of Mines and Mineral Resources Circular
103
,
241
p.
Cole
,
J.C.
Mahan
,
S.A.
Stone
,
B.D.
Shroba
,
R.R.
,
2007
,
Ages of Quaternary Rio Grande terrace-fill deposits, Albuquerque area, New Mexico
:
New Mexico Geology
 , v.
29
, no.
4
, p.
122
132
.
Connell
,
S.D.
Hawley
,
J.W.
Love
,
D.W.
,
2005
, Late Cenozoic drainage development in the southeastern Basin and Range of New Mexico, south-easternmost Arizona, and western Texas, in
Morgan
,
L.S.G.
Zeigler
,
K.E.
, eds.,
New Mexico’s Ice Ages
 :
New Mexico Museum of Natural History and Science Bulletin
no.
28
, p.
125
149
.
Dethier
,
D.P.
Harrington
,
C.D.
Aldrich
,
M.J.
,
1988
,
Late Cenozoic rates of erosion in the western Espanola basin, New Mexico
:
Evidence from geologic dating of erosion surfaces: Geological Society of America Bulletin
 , v.
100
, p.
928
937
, doi: 10.1130/0016-7606(1988)100<0928:LCROEI>2.3.CO;2.
Endlich
,
F.M.
,
1877
,
Part I—Geology, Chap. II—San Luis Valley, in Hayden, F.V., Ninth Annual Report of the United States Geological and Geographical Survey of the Territories, Embracing Colorado and Parts of Adjacent Territories
:
Being a Report of Progress of the Exploration for the Year 1875
 :
Washington, D.C.
,
Government Printing Office
, p.
140
149
and plate XVII.
Galloway
,
W.E.
Whiteaker
,
T.L.
Ganey-Curry
,
P.
,
2011
,
History of Ceno-zoic North American drainage basin evolution
,
sediment yield, and accumulation in the Gulf of Mexico basin: Geosphere
 , v.
7
, no.
4
, p.
938
973
, doi: 10.1130/GES00647.1.
Goehring
,
B.M.
Kurz
,
M.D.
Balco
,
G.
Schaefer
,
J.M.
,
2010
,
A reevaluation of in situ cosmogenic
3He
production rates
:
Quaternary
 , v.
5
, p.
410
418
.
Hawley
,
J.W.
Bachman
,
G.O.
Manley
,
K.
,
1976
, Quaternary stratigraphy in the Basin and Range and Great Plains Provinces, New Mexico and western Texas, in
Maheny
,
W.C.
, ed.,
Quaternary Stratigraphy of North America
 :
Strouds
-burg, Pennsylvania, Dowden
,
Hutchinson and Ross
, p.
235
274
.
Kelson
,
K.I.
Thompson
,
R.A.
Bauer
,
P.W.
,
2008
,
Geologic Map of the Guadalupe Mountain 7.5 Minute Quadrangle, Taos County, New Mexico
:
New Mexico Bureau of Geology and Mineral Resources Open-File Geologic Map 168
 , scale 1:24, 000.
Konning
,
D.J.
Newell
,
D.L.
Sarna-Wojcicki
,
A.
Dunbar
,
N.
Karlstrom
,
K.
Salem
,
A.
Crossey
,
L.
,
2011
,
Terrace stratigraphy, ages, and incision rates along the Rio Ojo Caliente, north-central New Mexico
,
in
 
New Mexico Geological Society Guidebook, 62nd Field Conference, Geology of the Tusas Mountains-Ojo Caliente
 :
Socorro, New Mexico Geological Society
, p.
281
300
.
Lifton
,
N.
Sato
,
T.
Dunai
,
T.J.
,
2014
,
Scaling in situ cosmogenic nuclide production rates using analytical approximations to atmospheric cosmic-ray fluxes
:
Earth and Planetary Science Letters
 , v.
386
, p.
149
160
.
Lisiecki
,
L.E.
Raymo
,
M.E.
,
2005
,
A Pliocene-Pleistocene stack of 57 globally distributed benthic records
:
Paleoceanography
 , v.
20
, PA1003, doi: 10.1029/2004PA001071.
Machette
,
M.N.
,
1978
,
Dating Quaternary faults in the southwestern United States using buried calcic paleosols
:
U.S. Geological Survey Journal of Research
 , v.
6
, no.
3
, p.
369
381
.
Machette
,
M.N.
,
1985
, Calcic soils of the southwestern United States, in
Weide
,
D.L.
, ed.,
Soils and Quaternary Geology of the Southwestern United States
 :
Geological Society of America Special Paper 203
, p.
1
21
, doi:10.1130/SPE203-p1.
Machette
,
M.N.
, ed.,
1988
, In the Footsteps of G.K. Gilbert: Lake Bonneville and Neotectonics of the Eastern Basin and Range Province:
Field Trip Guidebook for Field Trip 2, Geological Society of America Annual Meeting
 :
Utah Geological and Mineral Survey Miscellaneous Publication 88-1
, p.
111
116
.
Machette
,
M.N.
,
1998
, Contrasts between short- and long-term records of seis-micity in the Rio Grande rift—Important implications for seismic-hazards analysis in areas of slow extension, in
Lund
,
W.R.
, ed.,
Proceedings, Western States Seismic Policy Council (WSSPC) Basin and Range Province Seismic-Hazards Summit
 :
Utah Geological Survey Miscellaneous Publication 98-2
, p.
84
95
.
Machette
,
M.N.
Long
,
T.
Bachman
,
G.O.
Timbel
,
H.R.
,
1997
,
Laboratory Data for Calcic Soils in Central New Mexico—Background Information for Mapping Quarternary Deposits in the Albuquerque Basin
:
New Mexico Bureau of Mines and Mineral Resources Circular 2-5
 ,
63
p. (Supersedes U.S. Geological Survey/Open-File Report 96-722, 60 p.)
Machette
,
M.N.
,
Coates
,
M.M.
Johnson
,
M.L.
, eds.,
2007
,
2007 Rocky Mountain Section Friends of the Pleistocene Field Trip—Quaternary Geology of the San Luis Basin of Colorado and New Mexico, September 7-9, 2007
 :
U.S. Geological Survey Open-File Report 2007-1193
,
197
p.; online at http://pubs.usgs.gov/of/2007/1193/.
Machette
,
M.N.
Thompson
,
R.A.
Drenth
,
B.J.
,
2008
, Geologic Map of the San Luis Quadrangle, Costilla County,
Colorado
:
U.S. Geological Survey Scientific Investigations Map 2963
 , scale 1:24, 000, 1 sheet; online at http://pubs.usgs.gov/sim/2963/.
Machette
,
M.N.
Thompson
,
R.A.
Marchetti
,
D.W.
Smith
,
R.S.U.
,
2013
, Evolution of ancient Lake Alamosa and integration of the Rio Grande during the Pliocene and Pleistocene, in
Hudson
,
M.R.
Grauch
,
V.J.S.
, eds.,
New Perspectives on Rio Grande Rift Basins: From Tectonics to Groundwater
 :
Geological Society of America Special Paper 494
, p.
1
20
, doi:10.1130/2013.2494(01).
Madole
,
R.F.
,
1986
, Lake Devlin and Pinedale glacial history, Front Range,
Colorado
:
Quaternary Research
, v.
25
, p.
43
54
, doi: 10.1016/0033-5894(86)90042-6.
Madole
,
R.F.
Romig
,
J.H.
Aleinikoff
,
J.N.
Vansistine
,
D.P.
Yacob
,
E.Y.
,
2008
, On the origin and age of the Great Sand Dunes,
Colorado
:
Geomorphology
, v.
99
, p.
99
119
.
Madole
,
R.F.
Mahan
,
S.A.
Romig
,
J.H.
Havens
,
J.C.
,
2013
,
Constraints on the age of the Great Sand Dunes, Colorado, from subsurface stratigraphy and OSL dates
:
Quaternary Research
 , v.
3
, p.
435
446
, doi: 10.1016/j.yqres.2013.09.009.
Mccalpin
,
J.P.
,
1981
,
Quaternary Geology and Neotectonics of the West Flank of the Northern Sangre de Cristo Mountains, South-Central Colorado [Ph.D. thesis]
:
Golden
, Colorado School of Mines,
287
p.
Mccalpin
,
J.P.
,
1982
,
Quaternary Geology and Neotectonics of the West Flank of the Northern Sangre de Cristo Mountains
,
South-Central Colorado: Colorado School of Mines Quarterly
 , v.
77
, no.
3
,
97
p.
Menges
,
C.M.
,
1990
, Late Quaternary fault scarps, mountain-front landforms, and Pliocene-Quaternary segmentation on the range-bounding fault zone, Sangre de Cristo Mountains, New Mexico, in
Krinitzsky
,
E.L.
Slemmons
,
D.B.
, eds.,
Neotectonics in Earthquake Evaluation
 :
Geological Society of America Reviews in Engineering Geology
, v. VIII, p.
131
156
, doi:10.1130/REG8-p131.
Nelson
,
A.R.
Millington
,
A.C.
Andrews
,
J.T.
Nichols
,
H.
,
1979
,
Radiocarbon-dated upper Pleistocene glacial sequence, Fraser Valley, Colorado Front Range
:
Geology
 , v.
7
, p.
410
414
, doi:10.1130/0091-7613(1979)7<410:RUPGSF>2.0.CO;2.
Pazzaglia
,
F.J.
Wells
,
S.G.
,
1990
, Quaternary stratigraphy, soils and geo-morphology of the northern Rio Grande Rift, in
Bauer
,
P.W.
Lucas
,
S.G.
Mawer
,
C.K.
Mcintosh
,
W.C.
, eds.,
Tectonic Development of the Southern Sangre de Cristo Mountains, New Mexico
 :
New Mexico Geological Society Guidebook 41
, p.
423
430
.
Pierce
,
K.L.
,
2004
, Pleistocene glaciation of the Rocky Mountains, in
Gillespie
,
A.R.
Porter
,
S.C.
Atwater
,
B.F.
, eds.,
The Quaternary Period in the United States
 :
Amsterdam
,
Elsevier
, p.
63
76
.
Ruleman
,
C.
Machette
,
M.N.
,
2007
, Chapter J—An overview of the Sangre de Cristo fault system and new insights to interactions between Quaternary faults in the northern Rio Grande rift, in
Machette
,
M.N.
Coates
,
M.M.
Johnson
,
M.L.
, eds.,
2007 Rocky Mountain Section Friends of the Pleistocene Field Trip—Quaternary Geology of the San Luis Basin of Colorado and New Mexico, September 7-9, 2007
 :
U.S. Geological Survey Open-File Report 2007-1193
, p.
187
197
; online at http://pubs.usgs.gov/of/2007/1193/.
Ruleman
,
C.
Shroba
,
R.
Thompson
,
R.
,
2007
, Field-trip Day 3, Quaternary geology of Sunshine Valley and associated neotectonics along the Latir Peaks section of the southern Sangre de Cristo fault zone, in
Machette
,
M.N.
Coates
,
M.M.
Johnson
,
M.L.
, eds.,
2007 Rocky Mountain Section of the Friends of the Pleistocene Field Trip—Quaternary Geology of the San Luis Basin of Colorado and New Mexico
 :
U.S. Geological Survey Open-File Report 20071193
, p.
111
133
; online at http://pubs.usgs.gov/of/2007/1193/.
Ruleman
,
C.A.
Thompson
,
R.A.
Shroba
,
R.R.
Anderson
,
M.
Drenth
,
B.J.
Rotzien
,
J.
Lyon
,
J.
,
2013
, Late Miocene-Pleistocene evolution of a Rio Grande rift subbasin, Sunshine Valley-Costilla Plain, San Luis Basin, New Mexico and Colorado, in
Hudson
,
M.R.
Grauch
,
V.J.S.
, eds.,
New Perspectives on Rio Grande Rift Basins: From Tectonics to Ground-water
 :
Geological Society of America Special Paper 494
, p.
47
73
, doi:10.1130/2013.2494(03).
Schildgen
,
T.F.
Dethier
,
D.P.
,
2000
, Fire and ice—Using isotopic dating techniques to infer the geomorphic history of Middle Boulder Creek,
Colorado
:
Geological Society of America Abstracts with Programs
,
v
 .
32
, no.
7
, p.
A18
.
Schildgen
,
T.
Dethier
,
D.P.
Bierman
,
P.
Caffee
,
M.
,
2002
, 26
Al and
10
Be dating of late Pleistocene and Holocene fill terraces—A record of fluvial deposition and incision, Colorado Front Range
:
Earth Surface Process and Landforms
 , v.
27
, p.
773
787
.
Scholz
,
C.H.
,
1990
, The Mechanics of Earthquakes and Faulting:
Cambridge, UK
,
Cambridge University Press
,
439
p.
Shackleton
,
N.J.
Opdyke
,
N.D.
,
1973
,
Oxygen isotope and paleomagnetic stratigraphy of equatorial Pacific core V28-238
:
Oxygen isotope temperatures and ice volumes on a 10
  5
year and 106 year scale: Quaternary Research
 , v.
3
, p.
39
55
, doi: 10.1016/0033-5894(73)90052-5.
Shackleton
,
N.J.
Opdyke
,
N.D.
,
1976
, Oxygen isotope and paleomagnetic stratigraphy of Pacific core V28-239, late Pliocene to latest Pleistocene, in
Cline
,
R.M.
Hays
,
J.D.
, eds.,
Investigation of Late Quaternary Paleoceanography and Paleoclimatology
 :
Geological Society of America Memoir 145
, p.
449
464
.
Sharp
,
W.D.
Ludwig
,
K.R.
Chadwick
,
O.A.
Amundson
,
R.
Glaser
,
L.L.
,
2003
, Dating fluvial terraces by 230Th/U on pedogenic carbonate, Wind River basin,
Wyoming
:
Quaternary Research
, v.
59
, p.
139
150
, doi: 10.1016/S0033-5894(03)00003-6.
Shroba
,
R.R.
Thompson
,
R.A.
Minor
,
S.A.
Grauch
,
V.J.S.
Brandt
,
T.R.
,
2005
,
Geologic Map of the Agua Fria Quadrangle, Santa Fe County, New Mexico
:
U.S. Geological Survey Scientific Investigations Map 2896
 ,
22
p., 1 plate, scale 1:24, 000.
Siebenthal
,
C.E.
,
1910
, Geology and Water Resources of the San Luis Valley,
Colorado
:
U.S. Geological Survey Water Supply Paper 240
,
128
p.
Thompson
,
R.A.
Machette
,
M.N.
,
1989
,
Geologic Map of the San Luis Hills Area, Conejos and Costilla Counties
 ,
Colorado
:
U.S. Geological Survey Miscellaneous Investigation
Series Map I-1906, scale 1:50, 000.
Thompson
,
R.A.
Johnson
,
C.M.
Mehnert
,
H.H.
,
1991
, Oligocene basaltic volcanism of the northern Rio Grande Rift: San Luis Hills,
Colorado
:
Journal of Geophysical Research
, v.
96
, p.
13
, 577-13, 592, doi: 10.1029/91JB00068.
Thompson
,
R.A.
Machette
,
M.N.
Drenth
,
B.J.
,
2007
, Geologic Map of San Pedro Mesa and Surrounding Area, Costilla County,
Colorado
:
U.S. Geological Survey Open-File Report 2007-1074
, scale 1:24, 000, 1 sheet, http://pubs.usgs.gov/ofr/2007/1074.
Thompson
,
R.A.
Turner
,
K.J.
Shroba
,
R.R.
Cosca
,
M.A.
Ruleman
,
C.A.
Lee
,
J.P.
Brandt
,
T.R.
,
2014a
, Geologic Map of the Ute Mountain 7.5 Quadrangle, Taos County, New Mexico, and Conejos and Costilla Counties,
Colorado
:
U.S. Geological Survey Scientific Investigations Map 3284
 , scale 1:24, 000, http://dx.doi.org/10.3133/sim3284.
Thompson
,
R.A.
Turner
,
K.J.
Shroba
,
R.R.
Cosca
,
M.A.
Ruleman
,
C.A.
Lee
,
J.P.
Brandt
,
T.R.
,
2014b
,
Geologic Map of the Sunshine 7.5 Quadrangle
 ,
Taos County, New Mexico
:
U.S. Geological Survey Scientific Investigations Map 3283
, scale 1:24, 000, http://dx.doi.org/10.3133/sim3283.
Thompson
,
R.A.
Shroba
,
R.R.
Machette
,
M.N.
Fridrich
,
C.J.
Brandt
,
T.R.
Cosca
,
M.A.
,
2015
,
Geologic Map of the Alamosa 30’ X 60’ Quadrangle, South-Central Colorado
 :
U.S. Geological Survey Scientific Investigations Map 3342
,
23
p., scale 1:100, 000, http://dx.doi.org/10.3133/sim3342. (Supersedes Open-File Report 2005-1392 and Open-File Report 2008-1124.)
Wells
,
S.G.
Kelson
,
K.I.
Menges
,
C.M.
,
1987
, Quaternary evolution of fluvial systems in the northern Rio Grande rift, New Mexico and Colorado: Implications for entrenchment and integration of drainage systems, in
Menges
,
C.
, ed.,
Quaternary Tectonics, Landforms Evolution, Soil Chronologies, and Glacial Deposits—Northern Rio Grande Rift of New Mexico
 :
Field Trip Guidebook; Friends of the Pleistocene, Rocky Mountain Cell, Oct. 8-11, 1987: Albuquerque
,
University of New Mexico Department of Geology
, p.
55
69
.

Related

Citing Books via

Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal