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*
e-mail: mfoster@usbr.gov

Abstract

During the Quaternary, large deep-seated landslides were initiated along the eastern flank of the Colorado Front Range, and rivers cut and deposited large strath terraces along the western High Plains. These are the most extensive and prominent geomorphic features in the landscape. On this field trip, we will explore the Quaternary evolution of these Front Range features, in addition to viewing the smaller erosion scars and deposits associated with a 1000-yr precipitation event in 2013. We begin the trip near Golden, Colorado, where we will view the most extensive Quaternary strath terrace (Rocky Flats) preserved in the Denver Basin. We then head to Boulder, Colorado, to view the contrast between recent debris flows and deep-seated Quaternary landslides. Near Lefthand Creek, north of Boulder, we will view a suite of strath terraces and discuss the cosmogenic radionuclide dates that indicate both rapid incision and a new version of the terraces ages. Throughout the day, we will focus on the geomorphic work done by rare events, as well as discuss numeric and relative dating of Quaternary terraces and landslides.

Purpose

Kirk Bryan was a renowned geologist and physical geographer in the early twentieth century. His research was wide reaching and greatly advanced the study of arid regions, fluvial processes, soils, climate responses, and archaeology (see Whittlesey, 1951, for a short biography). To honor the contributions of Kirk Bryan, the Quaternary Geology and Geomorphology Division of the Geological Society of America (GSA) sponsors a field trip and presents an award to the author(s) of a publication advancing Quaternary geology at the GSA Annual Meeting.

The 2016 Kirk Bryan field trip is a day trip focusing on Quaternary geology and recent geomorphic events along the Colorado Front Range and western High Plains near Boulder, Colorado. This trip encourages interaction between participants and researchers, and endeavors to generate discussion about how these surfaces have evolved over the Quaternary and the rates at which they evolve. We will discuss the ages of Quaternary strath terraces; processes controlling the cutting and deposition of terraces; the contrast between deep-seated Quaternary landslides and smaller, shallow, historic debris flows; and the hillslope processes controlling the morphology of hogbacks. Where possible, we use numeric and relative dating techniques to support our interpretations, as well as numerical models. We will also witness the geomorphic evidence of a large precipitation event in September 2013 that resulted in debris flows and high rates of sediment transport on several local streams. We will visit four locations (Fig. 1): (1) Rocky Flats, a Quaternary strath terrace cut and deposited by Coal Creek, south of Boulder, Colorado; (2) the Lee Hill trailhead, where we will hike an easy-to-moderate ~2.1 mi trail to view Quaternary landslides and recent debris flows; (3) the Eagle terrace, a Quaternary strath terrace north of Boulder that also provides a view of both hogbacks along the Front Range and dated terraces along Lefthand Creek; and (4) Buckingham Park in Lefthand Creek, where we will view the modern channel, 2013 flood deposits, and a fluvial fill terrace remnant on the right canyon wall.

Figure 1.

Overview of field-trip stops. GSA: Colorado Convention Center, Denver, 39.742310° N, 104.997139° W. 1: Rocky Flats, 39.891097° N, 105.230732° W. 2: Lee Hill trailhead, 40.062596° N, 105.288785° W. 3: Eagle terrace, 40.078024° N, 105.276570° W. 4: Buckingham Park, 40.113751° N, 105.308345° W.

Figure 1.

Overview of field-trip stops. GSA: Colorado Convention Center, Denver, 39.742310° N, 104.997139° W. 1: Rocky Flats, 39.891097° N, 105.230732° W. 2: Lee Hill trailhead, 40.062596° N, 105.288785° W. 3: Eagle terrace, 40.078024° N, 105.276570° W. 4: Buckingham Park, 40.113751° N, 105.308345° W.

Regional Setting

In the late Mesozoic, the present-day location of the Colorado Rocky Mountains and western High Plains sat near or below sea level. During this time, thick sedimentary packages associated with the Cretaceous Interior Seaway were deposited; in the Boulder area, this deposit includes the Pierre Shale, which overlies a variety of sedimentary rocks deposited between ca. 300 and 65 Ma (Fig. 2). With the local onset of the Laramide orogeny, ca. 65 Ma, compression associated with flat slab subduction of the Farallon plate resulted in crustal shortening between ca. 65 and 40 Ma (e.g., Dickinson and Snyder, 1978; Bird, 1998). As the mountains rose, simultaneous erosion of the Mesozoic sedimentary rock cover exposed the Precambrian crystalline core of this and other Laramide ranges in Wyoming, Utah, and South Dakota. Outcrops of the remnants of the overlying sedimentary strata flank the Colorado Front Range, forming prominent east-dipping hogbacks and the well-known Flatirons near Boulder, Colorado.

Figure 2.

Cross section of generalized geology and topography near Boulder, Colorado. West to east distance is ~4 mi (~6.5 km) and relief is ~3000 ft (~1 km) (after Birkeland et al., 2004).

Figure 2.

Cross section of generalized geology and topography near Boulder, Colorado. West to east distance is ~4 mi (~6.5 km) and relief is ~3000 ft (~1 km) (after Birkeland et al., 2004).

Figure 3.

(A) Longitudinal profiles of major streams along the Colorado Front Range. Knickpoint locations were chosen to avoid lithologic contacts (Maureen Berlin, 2010, written commun.). Crystalline rock contact and extent of past glaciers are noted for each drainage. Dashed lines are paleolongitudinal projections from knickpoints across the western High Plains. The North Forks of Boulder Creek (BC) and St. Vrain Creeks (SV) are also shown. Ralston Creek (RC) is not included in A because its confluence with these other rivers is much farther downstream. (B-E) Individual stream profiles and paleo-profile projections. Dated terrace surfaces are also shown. Figure after Foster (2016).

Figure 3.

(A) Longitudinal profiles of major streams along the Colorado Front Range. Knickpoint locations were chosen to avoid lithologic contacts (Maureen Berlin, 2010, written commun.). Crystalline rock contact and extent of past glaciers are noted for each drainage. Dashed lines are paleolongitudinal projections from knickpoints across the western High Plains. The North Forks of Boulder Creek (BC) and St. Vrain Creeks (SV) are also shown. Ralston Creek (RC) is not included in A because its confluence with these other rivers is much farther downstream. (B-E) Individual stream profiles and paleo-profile projections. Dated terrace surfaces are also shown. Figure after Foster (2016).

Figure 4.

Major streams along the Colorado Front Range exhibiting large knickpoints. Stream profiles for selected creeks are plotted in Figure 3, along with prominent dated terraces. Dating sites on terraces are shown with yellow stars. Sample sites correspond to sample sites for basin averaged denudation rates reported in Foster (2016). Figure after Foster (2016). BC—Boulder Creek; CC—Coal Creek; LHC—Lefthand Creek; RC—Ralston Creek; SV—St. Vrain Creek; TM—Table Mountain.

Figure 4.

Major streams along the Colorado Front Range exhibiting large knickpoints. Stream profiles for selected creeks are plotted in Figure 3, along with prominent dated terraces. Dating sites on terraces are shown with yellow stars. Sample sites correspond to sample sites for basin averaged denudation rates reported in Foster (2016). Figure after Foster (2016). BC—Boulder Creek; CC—Coal Creek; LHC—Lefthand Creek; RC—Ralston Creek; SV—St. Vrain Creek; TM—Table Mountain.

It has been proposed that by the mid-Cenozoic, the Rocky Mountains and adjacent High Plains rose an additional kilometer in a broad pattern, possibly due to de-densification of the lower crust as a result of hydration derived from dewatering of the Farallon slab (Jones et al., 2015). Recent paleoaltimetric studies suggest that modern elevations in the southwestern United States were not achieved until the Neogene (Cather et al., 2012). The Neogene also marked a shift from deposition to exhumation in the Denver basin and many similar basins bounding the Laramide Ranges. Exhumation of the western edge of the High Plains likely also triggered a propagating wave of fluvial incision, expressed as knickpoints, into the crystalline core of the Front Range (Anderson et al., 2006, 2012). The switch to basin exhumation has been temporally correlated with aridification (Cather et al., 2012); however, whether climate change or tectonic tilting initiated this Neogene pulse of incision is debated (e.g., McMillan et al., 2002; Wobus et al., 2010; Cather et al., 2012).

Geomorphology

Quaternary Geomorphology

The edge of the Colorado Front Range is marked by narrow canyons and steep sub-tributary basins incised into the Precam-brian bedrock, associated with the passage of knickpoints along the mainstem channels (Anderson et al., 2006, 2012; Dethier et al., 2014). Prominent hogbacks line the eastern edge of the Front Range; the Dakota Group underlies the easternmost hogback and Lyons and Fountain Formations underlie the western hogback. A series of large Quaternary earthflows, translational and rotational rockslides, and slump deposits flank the Dakota Ridge; large blocks of the Dakota Sandstone litter landslide deposits. Translational and rotational rockslides can be deep seated and may have led to overturned and folded beds within the Dakota Group (Braddock and Eicher, 1962; Birkeland et al., 2004).

Along the western High Plains, adjacent to the Colorado Front Range, a series of gravel-capped strath terraces record the late Cenozoic exhumation of the Denver Basin. Strath terraces form as rivers incise to a particular level, then stabilize and cut laterally into the bedrock. When the river is able to vertically incise again, it abandons the surface, leaving a terrace with a thin mantle of alluvium (e.g., Bull, 1990). The alluvium on these strath terraces consists of cobbles derived from the crystalline core, which are far more resistant to erosion than the underlying soft Cretaceous shale bedrock into which the straths are cut; crystalline cobbles are the tools that do the work of incision, both laterally into the stream banks and vertically into the underlying bedrock. It is thought that during times of high sediment supply, rivers favor lateral planation into the easily erodible valley walls, resulting in very wide straths. In times of low sediment supply, when the bed is poorly armored, rivers are able to vertically incise (e.g., Bull, 1990; Madole, 1991; Hancock and Anderson, 2002; Wobus et al., 2010; Seizilles et al., 2014). Given that sediment supply from source basins is likely governed by climate and climate change, the occupation and abandonment of the gravel-capped strath terraces lining the Front Range has been linked to climate cycles, as in other regions (e.g., Reheis et al., 1991; Dühnforth et al., 2012).

The western High Plains have experienced -200–400 m of erosion since basin exhumation began in the Neogene, based on the downstream projections of paleolongitudinal profiles from knickpoints (Foster et al., 2015a; Fig. 3). The early erosional record is not preserved, as the highest terrace surfaces lie well below projections of paleolongitudinal profiles (Anderson et al.,

2006, 2012; Leonard and Langford, 1994; Leonard, 2002). Preserved strath terraces in the Denver Basin have traditionally been grouped into approximately six units (Table 1), each thought to represent a fairly consistent elevation during exhumation driven by base-level fall of the South Platte River (Hunt, 1954; Scott, 1960; Scott, 1975). In addition to elevation above the modern stream, researchers in the area have used relative dating to group terraces into units, based on the soil development in the alluvial parent material on straths (Hunt, 1954; Malde, 1955; Scott, 1963; Van Horn, 1957; Machette et al., 1976a, 1976b; Machette, 1977; Van Horn, 1976). Common soil features and properties used to estimate relative age include carbonate morphology, clay accumulation, rubification, as well as a host of laboratory analyses (see Birkeland, 1999; Schaetzl and Anderson, 2005; and many others for detailed description of these techniques). Numeric dates used to assign ages to units are sparse, and in some cases are imported from terraces located outside of the South Platte River system (Table 1).

Table 1.

Quaternary Terrace Units

UnitHeight (m)Generalized soil profileCarbonate morphologyAge
Nussbaum135Typically not well preservedLate Pliocene to early Pleistocene, based on correlation with the Broadwater Formation in Nebraska and Blanco Formation in Texas
Rocky Flats105A/Bt/Btk/K/BkStage IV1.35 Ma based on soil development and paleomagnetics; 400 ka— 2 Ma based on cosmogenic radionuclide dating of type locality
Verdos75A/Bt/Btk/K/BkStage III—IV640 ka, based on presence of Lava Creek B ash at type locality and several other locations
Slocum30A/Bt/Btk/K/BkStage III240 ka, based on U-series dating of a bison horn and geomorphic position of type locality near Cañon City, Colorado
Louviers21, variableA/Bt/BkStage II—IIICorrelated with Bull Lake glaciation (MIS* 6)
Broadway12, variableA/Bw or A/Bt/BkStage ICorrelated with Pinedale glaciation (MIS 2) based on geomorphic position and presence of fossil mammals and mollusks
UnitHeight (m)Generalized soil profileCarbonate morphologyAge
Nussbaum135Typically not well preservedLate Pliocene to early Pleistocene, based on correlation with the Broadwater Formation in Nebraska and Blanco Formation in Texas
Rocky Flats105A/Bt/Btk/K/BkStage IV1.35 Ma based on soil development and paleomagnetics; 400 ka— 2 Ma based on cosmogenic radionuclide dating of type locality
Verdos75A/Bt/Btk/K/BkStage III—IV640 ka, based on presence of Lava Creek B ash at type locality and several other locations
Slocum30A/Bt/Btk/K/BkStage III240 ka, based on U-series dating of a bison horn and geomorphic position of type locality near Cañon City, Colorado
Louviers21, variableA/Bt/BkStage II—IIICorrelated with Bull Lake glaciation (MIS* 6)
Broadway12, variableA/Bw or A/Bt/BkStage ICorrelated with Pinedale glaciation (MIS 2) based on geomorphic position and presence of fossil mammals and mollusks
*

MIS—Marine Isotope Stage.

Note: Table is based on work by Hunt (1954); Scott (1960, 1975); Scott and Lindvall (1970); Szabo (1980); Izett and Wilcox (1982); Madole (1991); Birkeland et al. (1996); and Riihimaki et al. (2006).

In Situ 10Be Data For Landslide Boulders

Table 2.
In Situ 10Be Data For Landslide Boulders
Sample nameρ
(g/cm3)
Thickness or sample depth
(cm)
Ps
(atoms/ g-qtz/yr)*
(atoms/
g-qtz/yr)*
Quartz
(g)
9Be carrier
(g)
Carrier (ppm)10Be/9Be
(10—15)§
.10Be
(M.atoms/
g-qtz)#
WLS12.651.8516.760.3217.02960.6972577 ± 3209.5 ± 10.40.325 ± 0.017
WLS22.651.8516.790.3219.26820.7022577 ± 3538.6 ± 18.70.752 ± 0.027
WLS32.651.8516.540.3215.30940.5868577 ± 3739.2 ± 25.31.087 ± 0.038
WLSA2.651.8516.550.3222.68130.4766389 ± 2765.8 ± 25.80.416 ± 0.014
WLSB2.651.8516.410.3228.31820.5192389 ± 21021.4 ± 42.50.484 ± 0.020
Sample nameρ
(g/cm3)
Thickness or sample depth
(cm)
Ps
(atoms/ g-qtz/yr)*
(atoms/
g-qtz/yr)*
Quartz
(g)
9Be carrier
(g)
Carrier (ppm)10Be/9Be
(10—15)§
.10Be
(M.atoms/
g-qtz)#
WLS12.651.8516.760.3217.02960.6972577 ± 3209.5 ± 10.40.325 ± 0.017
WLS22.651.8516.790.3219.26820.7022577 ± 3538.6 ± 18.70.752 ± 0.027
WLS32.651.8516.540.3215.30940.5868577 ± 3739.2 ± 25.31.087 ± 0.038
WLSA2.651.8516.550.3222.68130.4766389 ± 2765.8 ± 25.80.416 ± 0.014
WLSB2.651.8516.410.3228.31820.5192389 ± 21021.4 ± 42.50.484 ± 0.020
*

Production rates were obtained from the CRONUS calculator (http://hess.ess.washington.edu/; Balco et al., 2008), based on the constant production rate model (Lal, 1991; Stone, 2000). Shielding values are 0.999.

t10Be/9Be ratios for process blanks for different carriers are as follows: two process blanks with an average ratio of 3.8 ± 0.7 x 10—15 (577 ppm carrier); three process blanks with an average ratio of 4.6 ± 0.8 x 10—15 (389 ppm carrier).

§

Ratio with 1σ uncertainty, as reported by Purdue Rare Isotope Measurement Laboratory.

#

10Be concentrations include propagated uncertainty from accelerator mass spectrometry measurement, error introduced by the blank, and uncertainty in 9Be carrier concentration. Error in mass measurements, associated with the scale, was not propagated through calculations; mass is reported with the appropriate number of significant digits.

More recent investigation of terrace ages using cosmogenic radionuclides (CRNs), either with two nuclides or in conjunction with other dating methods, reveals a more complicated and longer-term evolution of terrace surfaces (Riihimaki et al., 2006; Dühnforth et al., 2012; Foster et al., 2015a). The highest and widest strath terraces (mapped as Rocky Flats or Verdos units; Table 1) along Ralston, Lefthand, and Coal Creeks all indicate long periods of fluvial occupation (100s of k.y.) and relatively recent fluvial abandonment (see Fig. 4 for location). These data collectively indicate the need for caution when correlating and assigning “ages” to terraces, as the age of deposition and age of fluvial abandonment can differ greatly. This recent work also points to the need for caution in using elevations above the river as anything but a local constraint on relative ages of surfaces.

In Situ 10Be Exposure Ages For Landslide Boulders

Table 3.
In Situ 10Be Exposure Ages For Landslide Boulders
SampleLatitudeLongitudeElevationAgeInternalExternal
name(decimal ° W)(decimal ° N)(m)(ka)uncertainty
(ka)*
uncertainty
(ka)*
WLS140.0502105.2968178119.100.981.93
WLS240.0497105.2969178444.411.594.21
WLS340.0495105.2959176265.542.326.24
WLSA40.0503105.2959176228.921.232.81
WLSB40.0497105.2952175029.171.242.84
SampleLatitudeLongitudeElevationAgeInternalExternal
name(decimal ° W)(decimal ° N)(m)(ka)uncertainty
(ka)*
uncertainty
(ka)*
WLS140.0502105.2968178119.100.981.93
WLS240.0497105.2969178444.411.594.21
WLS340.0495105.2959176265.542.326.24
WLSA40.0503105.2959176228.921.232.81
WLSB40.0497105.2952175029.171.242.84
*

Internal and external 1σ uncertainty, as reported by the CRONUS exposure age calculator (http://hess.ess.washington.edu/; Balco et al., 2008). Internal uncertainty includes the accelerator mass spectrometry error, and error introduced by the blank and carrier. External uncertainty includes uncertainty in the production rate.

Recent Deposits Associated with the September 2013 Precipitation Event

On this trip, we will visit recent deposits that are associated with an extreme precipitation event that delivered a year’s worth of precipitation in a few days along the Front Range of the Rocky Mountains, from New Mexico to Wyoming, in September 2013. The highest precipitation intensities were recorded in Boulder, Colorado, and were equivalent to a 1000 yr rainstorm (Gochis et al., 2015). The duration of the storm was also anomalously long in the Boulder area, with consistently high rainfall from 9-13 September. Many creeks and streams had high sediment transport rates and efficiently evacuated sediment from the mountains; however, most mountain streams draining the high precipitation core of the storm in the Front Range typically remained within the banks of the main channel with only a small portion of flow over the narrow floodplains (Moody, 2016). Of the >1100 debris flows that were triggered along the Colorado Front Range associated with the September storm (Coe et al., 2014), -120 debris flows occurred in the Boulder area alone (Anderson et al., 2015).

Debris flows occurred along the Dakota hogback and on small drainages in the canyons. While the direct role of slope aspect in the initiation of debris flows is debated, it apparently played a larger role than the history of wildfire in the region (Ebel et al., 2015). Debris flows were concentrated on south-facing slopes (Coe et al., 2014), which became fully saturated during the storm (Ebel et al., 2015). More recent work suggests that the aspect differences are best explained as reflecting forest cover, south-facing slopes being less forested than north-facing slopes (Rengers et al., 2016). Landslides and debris flows on broad slopes bounding the Dakota hogback were mostly shallow, leaving thin landslide scars and only small deposits (Coe et al., 2014; Anderson et al., 2015). Low-order tributaries to the large canyon creeks, steepened by knickpoint propagation (i.e., Anderson et al., 2012), were also prone to debris flow initiation. In these tributaries, debris flows scoured channels to bedrock, removing the -1 m of colluvial cover (Fig. 5). As stream flows at the time of these debris flows were adequate to transport the sediment delivered to them, the sediment was delivered to the plains (Anderson et al., 2015). In the Boulder area, the volume of sediment evacuated from debris-flow-dominated tributaries was roughly equivalent to several hundred to a thousand years of hillslope weathering products, suggesting that a large portion of the sediment transport from mountain channels to the plains can be performed by rare debris flows (Anderson et al., 2015).

Figure 5.

Looking up a debris flow chute on a Boulder Creek tributary, following the September 2013 flood. Photo credit: R.S. Anderson.

Figure 5.

Looking up a debris flow chute on a Boulder Creek tributary, following the September 2013 flood. Photo credit: R.S. Anderson.

Cosmogenic Radionuclide Methods and Background

Where available, we have employed the concentrations of CRNs 10Be and 26Al to aid in our interpretation of landform ages and lowering rates. CRN constraints of the ages of Quaternary terraces are summarized from Riihimaki et al. (2006), Dühnforth et al. (2012), and Foster (2016); methods are described therein. 10Be-based dates on Quaternary landslide boulders were prepared following the methods described in Foster and Anderson (2016).

On this field trip, we will discuss CRN age estimates from both surface exposure dating and from depth profiles into alluvium. Surface exposure dating estimates the amount of time fluvial cobbles and boulders have been exposed on the surface of terraces. In the absence of significant shielding (from loess deposition at the surface, for example), surface exposure ages are maximum age estimates because this method does not account for inheritance of CRNs. Inheritance is the concentration of CRNs that the sediment has at the time of deposition on a surface. CRN age estimates for alluvium on terraces were also calculated by using several samples collected across a depth profile in soil pits. For depth profiles in terrace alluvium, χ2 statistical models were used to estimate both the inheritance of CRNs as well as the accumulation of CRNs at the terrace site. The concentration of CRNs accumulated in alluvium, while on the terrace surface, has an exponential shape that approaches the inherited concentration at depth; χ2 statistical models are able to constrain the amount of time alluvium has been exposed to CRN production at the sampling site.

It is also important to understand how 26Al and 10Be are used to indicate a history of sediment burial. 26Al is produced at a ratio of 6.78 to 10Be (Balco et al., 2008), and decays more quickly than 10Be. The 26Al/10Be ratio will decrease as a sample approaches secular equilibrium, but ratios below 5 indicate the samples must have been buried at a depth that allowed decay to dominate.

Stop 1: Rocky Flats (39.891097° N, 105.230732° W)

Activity

We will stop on the Rocky Flats surface to discuss the soil data and numeric dating on the Rocky Flats, formed by Coal Creek, and on terraces formed by the adjacent Ralston Creek. From this location, we can view the morphology of the terraces and the crystalline cobbles that litter the surface.

Background

Rocky Flats is the largest and most prominent terrace preserved in the Denver-Boulder area (Fig. 1). It stretches over 7 km east to west, at its widest is -7 km from north to south, and is -1790-1980 m in elevation. This is the type locality for the Rocky Flats Alluvium described by Scott (1960; see Table 1). Alluvial deposits on the surface are typically <5 m, but can be up to 30 m in buried paleochannels (Shroba and Carrara, 1996; Knepper, 2005).

Rocky Flats was formed by Coal Creek, which drains a relatively small drainage basin (44 km2) that does not extend to the glaciated crest of the Front Range (Fig. 4). The large continuous portion of Rocky Flats has a slight southeasterly gradient, which may indicate that Coal Creek was captured to move the stream to its current alignment north of the terrace. At its eastern edge, Rocky Flats is up to 100 m above the modern Coal Creek channel; however, this height declines to the west, as the Rocky Flats surface approaches the modern Coal Creek channel profile (Fig. 3; Riihimaki et al., 2006).

CRN Data on Rocky Flats

Riihimaki et al. (2006) sampled four sites for CRN dating on the Rocky Flats surface, including surface clasts and two depth profiles, between 1.4 and 3.7 km from the mountain front; their data are summarized here.

The western depth profile exposed at least one paleosol, and low 26Al/10Be ratios indicate that this profile experienced periods of burial. Using numerical models to explore scenarios of erosion and deposition, the best-fitting ages for the lower and upper gravels are 930 ± 140 ka and 550 ± 80 ka, respectively. In addition, model results indicate a 3 m package of gravels must have been stripped from the lower gravels prior to deposition of the upper gravels. Modeling of the upper gravel package suggests it was 3 m thick at the time of deposition and subsequently was stripped by 1 m, possibly involving removal of intermittent loess. Therefore, this site is interpreted as having experienced periods of burial, erosion, and redeposition by fluvial processes, along with more recent erosion of loess capping the terrace surface.

The eastern profile is less well constrained because 26Al data were problematic. The package of gravels at the eastern site (Fig. 4) appears to represent fairly continuous deposition, as the site lacks paleosols or evidence of disconformities. Very low 10Be concentrations at the base of the profile suggest deposition earlier than 1 Ma. Using χ2 models to interpret a single age for the deposit suggests an age of 2.1 ± 0.3 Ma with recent surface stripping of 1.7 m. Surface erosion could have been due to mining operations or due to erosion of a loess cap.

Like the depth profiles, surface clasts also yielded a range of ages, between 400 ka and 1 Ma, and also indicate average burial depths of <0.6 m. Therefore, the oldest deposit dated on Rocky Flats is 2 Ma (depth-profile data on eastern gravel pit) and the youngest date is 400 ka (exposure dating of surface clasts, located on the west edge of the surface). Progressive abandonment of this terrace from east to west, over several hundred thousand years, is consistent with a model in which waves of incision propagate upstream from the Denver Basin into the Front Range. The slow and progressive abandonment of this surface likely reflects the low stream power of Coal Creek, which would result in slower propagation rates for knickpoints initiated by dropping base level along the South Platte River. In addition, the Rocky Flats surface has very thick gravels that must be eroded prior to incising the underlying Cretaceous shale bedrock and abandoning the strath surface. Creeks incising into terraces with thinner alluvial cover may be able to erode down to the bedrock more quickly, leading to terrace abandonment within a shorter period of time.

Soils on Rocky Flats

The soil at the type Rocky Flats Alluvium is no longer exposed, but its study has a long history, which we will review. Rocky Flats soils are polygenetic, and consist of clayey-skeletal, montmorillonitic mesic Aridic Paleustolls (Price and Amen, 1984). The alluvial parent material is a very cobbly and gravelly loamy alluvium that is well drained. The mollic epipe-don is 25-50 cm thick, which overlies thick argillic horizons (>1 m), as well as petrocalcic horizons in some places. Weathering of the alluvium extends to depths greater than 4 m (Riihimaki et al., 2006).

The soil at the type locality was described at an abandoned gravel pit -5 km east of Colorado Highway 93, on the south side of Highway 72 (Fig. 6; see also Machette et al., 1976a, 1976b). The soil consists of a 13-cm-thick A horizon, over a 36-cm-thick Bt horizon. This in turn overlies a 1-m-thick carbonate enriched K horizon (referred to as Bkm in NRCS [Natural Resources Conservation Service] nomenclature); at its top this is cemented enough to be stage IV, whereas the lower part is less cemented and stage III. Although other horizons are at greater depth, the horizons described above are the most important. The Bt is 10R 3.5/6, 69% clay, with many thick clay films. The K horizon has stage IV morphology and a maximum carbonate content of 94%. The K horizon overlies at least one paleosol. In summary, the soils on Rocky Flats are very red in color (10R), with high clay content, and cemented carbonate horizons—all features that take a long period of time to develop in a soil profile. The pedogenic color, clay content, and carbonate content suggest that this soil is as well developed as any in the western United States, and maybe beyond (Birkeland, 1999).

Figure 6.

Type locality of the upper soil described on the Rocky Flats Alluvium, abandoned gravel pit. Photo credit: Peter Birkeland.

Figure 6.

Type locality of the upper soil described on the Rocky Flats Alluvium, abandoned gravel pit. Photo credit: Peter Birkeland.

The Rocky Flats has high carbonate content, but soils on many nearby terraces do not. This is because Front Range terraces are close to the line separating calcic soils (to the east) from noncalcic soils (to the west). The line demarcating the two soil types is called the pedocal-pedalfer boundary, or “Malde line,” honoring Hal Malde who worked in this area (i.e., Malde, 1955; Machette, 1985). Previous workers have recognized variability in the occurrence of pedogenic carbonates near this boundary (i.e., Machette et al., 1976a; Price and Amen, 1984; Birkeland et al., 1996). The location of the Malde line fluctuates with moisture availability due to climate, with wetter periods leaching out calcium carbonates and drier periods enhancing carbonate accumulation. The position of the Malde line has no doubt moved laterally with Quaternary climate cycles, thus affecting both carbonate accumulation and depletion of a soil near the line. Therefore, it is difficult to interpret relative age correlations across terraces using the carbonate morphology (see Machette, 1985).

Before cosmogenic dating, University of Colorado students estimated an age for the Rocky Flats Alluvium using soil data from terraces formed by Ralston Creek. Mabee (1978) used the oxidation of magnetite to hematite in a soil-terrace chronosequence. The alteration is progressive with age, and at the time the only numeric age datum to calibrate the mineral alteration was the presence of Lava Creek B ash (640 ka; Van Horn, 1976; Izett and Wilcox, 1982) in alluvium on the Pioneer terrace (see Fig. 4 for terrace location). Using a linear plot of percent oxidation versus time, and comparing it with similar alteration data for the Rocky Flats Alluvium, resulted in the first age estimate of 0.9 Ma for Rocky Flats. As most weathering phenomena are defined by a logarithmic function, the latter age should be viewed as a minimum.

Paleomagnetic and soil data were also used to estimate the age difference (and therefore time) between the Rocky Flats Alluvium and the alluvium (mapped as Verdos) on the Pioneer terrace, which was cut and deposited upon by Ralston Creek; we drove across the Pioneer terrace before our final climb in elevation onto the Rocky Flats surface (see Fig. 3). Patterson et al. (1984) found reversed remnant magnetism in the soils of a high terrace formed by Ralston Creek, which along with quantitative soil data (D.C. Miller, 1988; written commun.), pointed to an estimated age for the Rocky Flats Alluvium of 1.4-1.6 Ma. However, it should be noted that this age estimate for Rocky Flats assumes a synchronous history of incision along Ralston and Coal Creeks; CRN data along Lefthand Creek indicate that this assumption may not always hold true for creeks draining the Colorado Front Range.

In fact, numeric dating reveals that estimating a singular “age” for the Pioneer terrace is not straightforward. The Pioneer terrace has traditionally been assumed to be 640 ka, consistent with the presence of Lava Creek B ash within the alluvium (Izett and Wilcox, 1982; see also Verdos Alluvium; Table 1). However, 10Be depth profiles indicate that the terrace was still occupied at roughly 175 ka (Dühnforth et al., 2012). Therefore, the combination of CRN dating and tephra dating suggests that this terrace also had a long period of fluvial occupation (several hundred thousand years), with more recent fluvial abandonment.

Key Points at Stop 1

  • Flats experienced a long period of fluvial occupation, marked by periods of erosion and re-deposition.

  • data on Rocky Flats indicate a progressive abandonment of this surface, from east to west. The oldest CRN ages on Rocky Flats are ca. 2 Ma. The youngest CRN date on Rocky Flats suggests fluvial occupation as recently as 400 ka.

  • Pioneer terrace, formed by Ralston Creek, also experienced a long period of fluvial occupation (100s of k.y. and hence many glacial cycles).

Stop 2: Lee Hill Trailhead Parking Lot (40.062596° N, 105.288785° W)

Activity

From the Lee Hill trailhead parking lot (point A, Fig. 7), we will begin a 2.1 mi hike. This hike covers the same ground as Stops 1-6 on the Birkeland et al. (2004) eco-geo hike field trip, which details Quaternary landsliding in the area. We will view the recent landslide scars and debris flows as well as fluvial deposition along Fourmile Canyon Creek, all associated with the 2013 storm. We will also point out and discuss a shallow earthflow that was initiated in May 2015 on the slopes above Wonderland Lake (40.050178° N, 105.289291° W) in response to high amounts of spring precipitation. We will return north from Wonderland Lake to end our hike at the Foothills Community Park, where we will have lunch at point D (Fig. 7).

Figure 7.

Quaternary landslides, cos-mogenic radionuclide sample sites, and trail for our hike from Lee Hill trail-head to Wonderland Lake, and then to the Foothills Community Park (Stop 2). Modified from Birkeland et al. (2004). CRN—cosmogenic radionuclide.

Figure 7.

Quaternary landslides, cos-mogenic radionuclide sample sites, and trail for our hike from Lee Hill trail-head to Wonderland Lake, and then to the Foothills Community Park (Stop 2). Modified from Birkeland et al. (2004). CRN—cosmogenic radionuclide.

Background

Quaternary Landslides

Landslides north of Lee Hill Drive. In the Boulder area, Quaternary landslides typically appear to be younger than -130 ka, based on soils and landslide morphology (Birkeland et al., 2004), although one buried landslide north of Lee Hill Drive suggests an older age. These earthflows were best observed in fresh roadcuts during the early 2000s (see photographs from the 2000s in Fig. 8). A red soil has formed on the younger landslide (Fig. 8A); this younger landslide buries an older Quaternary landslide deposit with a prominent red soil that Birkeland estimates is ca. 0.5 Ma, based on soil properties (Fig. 8B). Parts of the upper portion of the older slide, including the red soil, are highly distorted and are incorporated into the base of the younger slide. This suggests that the younger Quaternary landslide likely occurred during exceptionally wet conditions.

Figure 8.

Photographs depicting younger Quaternary landslide deposited over an older Quaternary landslide with a prominent red soil. Photo credit: Peter Birkeland.

Figure 8.

Photographs depicting younger Quaternary landslide deposited over an older Quaternary landslide with a prominent red soil. Photo credit: Peter Birkeland.

Quaternary landslides between points B and C (Fig.7). Over this segment of the hike, we will observe a series of Quaternary landslides, earthflows, transitional and rotational rockslides, and slump deposits (Fig. 9). These landslides were mapped by Birkeland et al. (2004), and some earthflows are differentiated into older and younger units based on landslide morphology and cross-cutting relations (Fig. 7). As many landslide deposits overlie Quaternary alluvium, the landslides postdate deposition of the alluvium (mapped as Slocum; Birkeland et al., 2004; see Table 1). We note that at other sites, it is clear that the landslide lobes directly mantle shale bedrock. Based on morphology, Birkeland estimates these Quaternary landslides are younger than 130 ka. Five boulders on landslide surfaces above Wonderland Lake were sampled for 10Be (Fig. 7). Exposure ages ranged from 19 to 66 ka (Tables 2 and 3). These preliminary ages do not support the relative age designations based on morphology, likely because the samples from large boulders on the surface contain an inherited CRN concentration. Many additional CRN samples would be needed to discern such a relation.

Figure 9.

Annotated aerial photograph showing Quaternary landslides along the hike at Stop 2. Photo credit: R.S. Anderson. LGM—last glacial maximum.

Figure 9.

Annotated aerial photograph showing Quaternary landslides along the hike at Stop 2. Photo credit: R.S. Anderson. LGM—last glacial maximum.

The depth and scale of these large Quaternary landslides are unmatched in the historical record. Historical landslides have been limited to shallow debris flows. While we do not know the mechanism for the large, deep-seated Quaternary failures, we surmise that they require sustained wet conditions. The preliminary 10Be sampling we report suggests that these occurred during wet climate conditions of the last glacial cycle.

Debris Flows along the Mountain Front Unleashed during the 2013 Storm

On our hike from point B (40.062111° N, 105.294372° W) to point C (40.049127° N, 105.292553° W), we will also see a few of the scars and deposits from several debris flows launched in the midst of the 2013 storm. The debris flow legacy of the storm is well discussed in work of Coe et al. (2014), who provide an overview of the >1000 failures that occurred in that extraordinary precipitation event, and in Anderson et al. (2015), who detail the >100 debris flows that occurred in both crystalline rock and sedimentary rocks in this sector of the Front Range. The debris flows we will visit occurred on the east flank of the Dakota hogback (Fig. 2). More than a dozen failures occurred along this section of the hogback in the 2013 storm. Each of these failures initiated close to the top of the ridge, within a few 10s of m downslope of the Dakota Group sandstone outcrop. Failures on the steep grassy slopes created scars roughly 1 m high, and 10-20 m wide, on slopes close to 30°. The evolution of the failed material varied from site to site, each of which we visited within a week of the failures. Most of the failures, especially those on nearly planar slopes, evolved into thin muddy slurries that traveled 10s to a few hundred m from their sources. Visits to these sites immediately after the storm revealed that the paths of these flows were marked by grass that had been overrun, bent downhill, and coated with a thin veneer of fine-grained material. The paths are dotted with sandstone blocks (Fig. 10). These flows failed to bulk up into thicker debris flows that were commonly seen in the crystalline rocks that line the major creeks draining the range (Anderson et al., 2015). Instead, these flows simply scattered the debris from the initial failure site downhill. Most of these failures and shallow debris flow paths have since become much more difficult to see due to the regrowth of grasses that were not ripped up, and the beginning of re-vegetation of the failures and deposits.

Figure 10.

Thin debris flow dotted with sandstone blocks. Located on slope above our hike at Stop 2. Photo credit: R.S. Anderson.

Figure 10.

Thin debris flow dotted with sandstone blocks. Located on slope above our hike at Stop 2. Photo credit: R.S. Anderson.

In only a few instances did these flows become erosive, and bulk up substantially as they traveled downhill. To the south of this field-trip stop, one of these flows made its way into a house at the base of the hillslope. We will visit the toe of one of these longer flows, which traveled several hundred meters downhill from its source, leaving a debris path that includes scattered blocks, levees, and a debris flow lobe at its base. The source, again near the top of the ridge, is in this case complex, involving the amalgamation of at least three failures. The flow gathered in a preexisting swale, and entrained significant volumes of new material from along its path. It crossed a steep knickpoint in the channel,

where both abrasion and plucking of rock occurred. Below this, the flow pinches and expands as it passes through the more confined swale before debouching onto the flat plain below, where it deposited much of its debris. This is now a thin lobe that is rapidly re-vegetating.

Key Points at Stop 2

  • Morphology indicates that most Quaternary landslides are younger than -130 ka; the buried red soil suggests that the older landslide on Lee Hill is likely older. 10Be dates on similarly large Quaternary landslides along this sector of the hogback indicate ages between 19 and 66 ka.

  • The majority of small debris flows from the 2013 storm in this area initiated close to the contact between shale and sandstone. While those on planar slopes produced minor and thin debris flows, those in swales evolved to longer flows.

  • Large, deep-seated Quaternary landslides shape this landscape. Historical landslides and debris flows are orders of magnitude smaller, leaving thin scars and deposits that will likely be obscured within years to decades.

Stop 3: Eagle Strath Terrace

Activity

We will take a short -0.5 mi hike from the vehicle drop-off location (40.076301° N, 105.2777612° W) to the top of the surface (40.078024° N, 105.276570° W). The exposures on the way to this surface allow us to view the steeply dipping bedding of the underlying Pierre shale, as well as the overlying fluvial gravels deposited on the bedrock strath. Gravels capping this surface were clearly sourced from a drainage that taps into the crystalline core of the range, as they contain granitics and metamorphic rocks characteristic of the core of the range; Four Mile Canyon Creek is one likely source for the igneous and metamorphic rocks deposited here. The Eagle strath terrace provides a good view of the Lefthand Creek terraces to the north and the hogbacks along this sector of the eastern edge of the Front Range. At this stop we will present soil and CRN data for the Lefthand Creek strath terraces, as well as numerical model results exploring how hogbacks evolve through time.

Background

Lefthand Creek Strath Terraces

North of the Eagle terrace we can view a series of strath terraces that record the incision history of Lefthand Creek (Fig. 11), which drains a 144 km2 basin (Fig. 4). Haystack Mountain is a tiny remnant of the oldest terrace, now eroded into a pyramidal hill (Fig. 11). The second highest surface is the prominent Table Mountain surface, with an extent of -2100 m χ 3000 m. Table Mountain and two adjacent lower terraces (the Middle and Low terraces, Fig. 11) have been the focus of recent dating and soil studies by Dühnforth et al. (2012), Rindfleisch et al. (2013), and Foster et al. (2015a).

Figure 11.

Terraces formed by Lefthand Creek (LHC), numbered from oldest (1, Haystack Mountain, HM) to youngest (5), and modern channels (6). Unrelated Quaternary terraces to the north and south are mapped as dark gray. Figure after Foster (2016). CRN/ OSL—cosmogenic radionuclide/optically stimulated luminescence.

Figure 11.

Terraces formed by Lefthand Creek (LHC), numbered from oldest (1, Haystack Mountain, HM) to youngest (5), and modern channels (6). Unrelated Quaternary terraces to the north and south are mapped as dark gray. Figure after Foster (2016). CRN/ OSL—cosmogenic radionuclide/optically stimulated luminescence.

Previous age estimates for these three terraces suggested they were all older than marine isotope stage 6 (MIS 6). Table Mountain is 50-70 m above the modern stream; based on soil development and elevation, alluvium on Table Mountain has been mapped as either Rocky Flats (Cole and Braddock, 2009) or Verdos Alluvium (Madole, 1963), placing the age at 640 ka to over 1 Ma (Table 1). The Middle terrace is -20 m above the modern stream. Cole and Braddock (2009) mapped the Middle terrace as consisting of Slocum Alluvium, whereas Madole (1963) mapped it as its own unit; however, age estimates for both were mid-Pleistocene. The Low terrace is 8-12 m above the modern stream and has been mapped as late Pleistocene in age (Madole, 1963; Cole and Braddock, 2009).

The first CRN dating study on the Lefthand Creek terraces, using profiles of both in situ and meteoric 10Be, indicated that Table Mountain may have been fluvially occupied as recently as -100 ka (Dühnforth et al., 2012), an order of magnitude younger than previously thought. Results from the Dühnforth et al. (2012) study were surprising because they appeared to contradict the well-developed soils on Table Mountain. Subsequent studies by Foster et al. (2015a) and Rindfleisch et al. (2013) aimed to reconcile the results from CRN dating on Table Mountain with relative dating methods using soil properties. These studies combined numeric age constraints with soil descriptions on a series of terraces that were formed by the same source stream (Lefthand Creek). This approach is unique in the region, as most numeric dating studies along the Colorado Front Range have focused on isolated terraces formed by different fluvial systems, and do not incorporate relative dating using soil properties. Studying terraces formed by the same creek should enable better constraint on the timing of fluvial occupation and abandonment from terrace to terrace, allowing a better comparison for the soils on each terrace, since the source basin for the alluvium is the same.

CRN Data

CRN data on Table Mountain from Dühnforth et al. (2012) and Foster et al. (2015a) are in close agreement. Six surface clasts on Table Mountain yielded 10Be exposure ages between 96 and 220 ka (Dühnforth et al., 2012). Foster et al. (2015a) combined new 26Al and 10Be measurements with 10Be measurements by Dühnforth et al. (2012) into a single χ2 model, which resulted in an age estimate of 93 ± 3 ka (note that this small reported error is associated with the model fit only). This age estimate supported the original 10Be age estimate of 95 ± 912 ka (Dühnforth et al., 2012). However, and importantly, the 26Al/10Be ratios are typically below 5; as this is well below the production ratio of 6.78, it suggests that samples within the Table Mountain alluvium were previously buried for a significant period of time.

To explore scenarios for the time and depth of sediment burial on Table Mountain, Foster et al. (2015a) created a forward numerical model to explore scenarios for sediment burial and erosion, using the χ2 model to constrain the time for fluvial abandonment. While this model does not yield a unique solution for the measured CRN concentrations and 26Al/10Be ratios, it constrains plausible scenarios for burial time and depth of burial. For this model, Foster et al. (2015a) assume that at the time of deposition, the 26Al/10Be ratios in alluvium were at or close to the 26Al/10Be production ratio of 6.78, meaning that they were not buried in the source basin prior to deposition at Table Mountain. This is likely a fair assumption as there is minimal flood-plain storage on Lefthand Creek. Although there is evidence that Front Range canyons infill and evacuate sediment, this occurs on timescales that should not significantly depress the 26Al/10Be ratio by burial within the stream system (Schildgen et al., 2002). Further, the soil cover on Front Range hillslopes is replaced on a timescale of -20 k.y. (Foster et al., 2015b), meaning that sediment produced from Front Range hillslopes and entering stream systems will have 26Al/10Be ratios close to the 26Al/10Be production ratio of 6.78.

The best result from the forward model suggests that the original deposit on Table Mountain was roughly 3 m thicker than it is at present, was deposited on the surface at ca. 1 Ma, and that the stripping of this -3 m of material occurred over a relatively short period of time around 100 ka, rather than progressive stripping over 10s of k.y. Foster et al. (2015a) conclude that Table Mountain was occupied for several 100s of k.y., and was abandoned at or soon after the last major interglacial (MIS 5). The time of abandonment is also constrained by dating on the Middle terrace surface, the strath cut by Lefthand Creek after the Table Mountain surface was abandoned.

The CRN depth profile on the Middle terrace yields a model age of 96 ± 4 ka, which overlaps with the age estimate for Table Mountain (Foster et al., 2015a). The Middle terrace is lower in elevation and must be younger as they were both formed by Lefthand Creek. The vertical bedrock incision from Table Mountain to the Middle terrace likely occurred rapidly through the soft shale bedrock. Geographically, the Middle terrace is a narrow surface that was incised into the north side of Table Mountain (Fig. 11). The Middle terrace was likely preserved in its narrow state because Lefthand Creek was subsequently pirated to the south side of Table Mountain (Dethier et al., 2003; Foster et al., 2015a).

Foster et al. (2016) hypothesize that their χ2 model age for the Table Mountain surface is slightly too young, likely because it does not account for the presence of intermittent loess on the surface. Coating the surface with loess would depress CRN production rates, resulting in younger estimates of age. If just over a meter of loess was applied to the surface during glacial periods (-25 k.y. duration), the model age would be shifted older by -20 k.y. While loess is not currently present on the Table Mountain surface, it is present on the Middle terrace (CRN production rates on the Middle terrace were corrected to account for the present loess cap, see Foster et al., 2015a). In addition, intermittent loess has been interpreted to exist on Rocky Flats and other similar strath terraces (e.g., Riihimaki et al., 2006; see also soils discussion below). Given the possibility that loess intermittently covers this surface, Foster et al. (2015a) suggest that the abandonment of Table Mountain occurred perhaps 20 ka prior to the occupation of the Middle terrace, perhaps coincident with the last major interglacial.

The Low terrace was formed after Lefthand Creek was captured, causing the abandonment of the Middle terrace. The Low terrace consists of a stack of two alluvial packages that differ in their texture and 10Be inheritance. χ2 models used to fit the age and inherited concentration of 10Be indicate a twofold drop in the inherited concentration of 10Be below 117 cm depth, indicating that the Low terrace gravels are composed of two layers of alluvium deposited at separate times. Thermal-transfer optically stimulated luminescence (TT-OSL) indicates the lower deposit was emplaced at -65 ka (for details and application of this method, see Duller and Wintle, 2012). A 10Be depth profile in the upper deposit yields a χ2 model age of 40 ± 5 ka. Based on these data, the Low terrace was fluvially occupied for 20 k.y. or longer, with at least two periods of deposition at this site. A buried A horizon was not observed at the top of the lower alluvium, suggesting that the lower alluvium was stripped prior to deposition of the upper alluvium.

Synopsis of dating results. Following the long occupation and subsequent abandonment of the Table Mountain surface around the last interglacial (MIS 5e), up to four lower levels of terraces have been cut and abandoned by Lefthand Creek (Fig. 11): (1) the Middle terrace, dated to -96 ka, (2) the lower terrace occupied between -65 and 40 ka, and (3) a lower set of terraces that have not been dated (see Fig. 11). The Table Mountain terrace remained fluvially occupied for 100s of k.y. However, since the last interglacial (MIS 5e, -125 ka), numerous terraces have been cut into the underlying shale, deposited upon, and subsequently abandoned. While climate likely modulates sediment supply from the core of the Front Range to the edge of the High Plains, and thus must influence terrace formation, the occupation and abandonment dates of these surfaces are difficult to correlate with global climate.

The above age estimates differ from the ages assigned to the alluvial units originally outlined by Scott (1960; see Table 1 also). However, these data do fit with more recent studies supporting a conceptual model in which Front Range rivers experience a complex and basin-specific history during which long periods of aggradation and lateral planation are punctuated by brief episodes of rapid incision (Riihimaki et al., 2006; Dühn-forth et al., 2012). We therefore advise caution when correlating terraces from one to another stream system. We argue that despite many similarities between Front Range streams (e.g., climate and base level control from the South Platte River), other aspects differ between streams: (1) glaciated/non-glaciated headwaters; (2) basin-averaged denudation rates that can vary by a factor of 2 for Front Range streams (Foster, 2016); (3) rain- versus snow-melt-dominated stream flows; (4) drainage area and hence stream power; (5) underlying bedrock; and especially (6) any history of stream piracy that will certainly be basin specific. Other workers have also recognized the problematic nature of regional correlations, which in the past have been largely based on the elevation of terraces above streams channels (Madole, 1991; Dethier et al., 2003). We instead advocate the use of local and relative basin-specific stratigraphy, and determination of individual surface chronologies using compatible numeric dating methods. Finally, our work has also demonstrated the power of using both 10Be and 26Al profiles on at least the oldest of the terraces. The burial history of the Table Mountain surface only came into focus with the profiles of 26Al/10Be ratios.

Soils

The soils on the Lefthand Creek strath terraces, like Rocky Flats, are polygenetic. They have experienced at least one glacial episode, millennia of bioturbation and vegetative change, and likely periods of eolian deposition and subsequent erosion since fluvial abandonment. Disentangling the signature of these events and the events overprinting them is no simple task. The soils described here are largely consistent with soil components mapped by Moreland and Moreland (2008); however, our description shows a much coarser parent material on the Low terrace than is mapped. In Rindfleisch et al. (2013), Table Mountain soils and the Middle terrace soils are classified as loamy-skeletal Aridic Argiustoll, and as a fine-loamy, Aridic Argiustoll, respectively. The Low terrace soil is classified as a fine-loamy, Aridic Paleustalf (see Fig. 12 for detailed soil descriptions).

Figure 12.

Field descriptions of soils at the Low terrace, Middle terrace, and Table Mountain. Figure modified from Rindfleisch et al. (2013).

Figure 12.

Field descriptions of soils at the Low terrace, Middle terrace, and Table Mountain. Figure modified from Rindfleisch et al. (2013).

The parent material for soils on the Lefthand Creek terraces is primarily gravelly to stony sandy alluvium. The Middle terrace soil has silty surface horizons (0-26 cm), consistent with loess; the other sites lack a loess cap. The soils formed in alluvium on both Table Mountain and the Middle terrace each represent one period of deposition on their respective surfaces. The Low terrace, however, exhibits an increase from <25% to >50% rock fragments (gravels/cobbles/stones) below 117 cm depth. A definitive buried soil was not identified at this stratigraphic break (2Bt3-3Bt1b), but the increase in rock fragments may suggest a change in the energy of the fluvial system depositing the material. Following results from CRN analysis, which showed a sharp offset in the 10Be inheritance at this depth, we suggest that this stratigraphic break represents two distinct periods of fluvial deposition. It is possible that modern soil forming processes have overprinted the pedological features that were originally present in the lower soil, obscuring some features (eluvial horizons) and enhancing others (zones of illuviation).

Generally, soil color follows the expected age relations within the study area. The Table Mountain soils are consistently redder (generally 5YR 4/6 or 4/8) than either the Middle or Low terrace soils. The Middle terrace tends toward yellower hues (10YR), particularly in the silty upper parts of the profile where the carbonate accumulation is greatest, with redder (7.5YR) colors below this zone. The Low terrace has dominantly 7.5YR colors. As pedogenic carbonate was only observed at the Middle terrace site (stage this soil property could not be used to assign relative age correlation; as discussed in our presentation of the Rocky Flats surface, the presence of pedogenic carbonate near the Front Range is variable.

At our study sites, clay accumulation and argillic horizon thicknesses also do not strictly adhere to expected trends of increasing clay and greater thickness with greater age. For example, the argillic horizon on Table Mountain has a weighted average of 26% clay with a clay maximum of 33%, and is 96 cm thick, whereas the Middle terrace soil has an argillic horizon with a weighted average of 33% clay (clay maximum of 42%) and a thickness of 39 cm. The argillic horizon of the Low terrace soil has a weighted average of 23% clay (clay max of 38%) and was 136 cm thick. The open architecture presented by the sandy and gravelly alluvium on the Table Mountain and Low terraces allows water to move readily through the profile and redistribute clay over >1 m depth. In contrast, at the Middle terrace, the matric difference between the finer-grained “cap” of loess over coarser-grained sandy and gravelly alluvium has focused clay accumulation over a much narrower band that has a higher clay content. These matric differences have also focused carbonate precipitation and may serve to prohibit removal of carbonates below this depth, as wetting fronts hang at the boundary between the finer and coarser material.

Eluvial horizons (A) for the pedons seem thinner (9-29 cm) than would be expected given the amount and depth of clay illu-viation (>2 m). While some of this can be accounted for by pedo-genic strain (the expansion of an illuvial layer due to inputs of material from above), it also suggests that some amount of surface erosion has occurred, diminishing the thickness of the eluvial horizon(s). Evidence supporting this hypothesis, however, has been difficult to establish because: (1) there is no surface expression of erosion, and (2) the soil profiles do not show conclusive evidence of erosion, with the exception of the presence of argillic horizons very near the surface and a coarser, buried soil on the Low terrace (117+ cm; horizon 3Bt1b and below), which was likely stripped before emplacement of the upper alluvial parent material. We propose that the presence of intermittent loess on terrace surfaces may help explain the depth of clay illuviation and high percentage of clays in the soil profiles.

There is little doubt that loess has contributed to soil development in the region (Reheis, 1980); the presence, or perhaps preservation, of loess and loess-derived soils greatly increases within -3 km to the east of these study sites. Incorporation of loess into soil profiles could result in soils appearing to be more developed; see Reheis et al. (1989) for an example of enhanced soil development due to dust deposition in the northern Mojave Desert.

Gravels are common on the surfaces of most terraces sampled, particularly on the Table Mountain surface. Gravels on the Middle terrace most likely are the result of biological activities since the upper parts of this soil formed in loess. In the case of the Table Mountain surface, where no loess cap currently exists, one possible scenario to account for the extensive clay illuviation is that this surface was at one time mantled with loess, and the loess was subsequently eroded, leaving an erosional lag of gravels at the surface and partially removing the eluvial horizon (Fig. 13). This possibility was also suggested by Dühnforth et al. (2012) and Foster et al. (2015a). This hypothesis supports the interpretation of Foster et al. (2015a) on the Table Mountain surface; it would result in: (1) younger calculated CRN abandonment ages at Table Mountain, due to lower CRN production rates beneath a loess cap, while it was present at the site, and (2) enhanced translocation of fines and consequently soil development. While this scenario is plausible, it is difficult to prove because the surface has virtually no expression of modern or past erosion.

Figure 13.

Conceptual figure showing possible erosion of an eolian cap. At time T1, the cap is present. Following erosion, the argillic horizon is now at the surface at time T2. Figure modified from Rindfleish et al. (2013).

Figure 13.

Conceptual figure showing possible erosion of an eolian cap. At time T1, the cap is present. Following erosion, the argillic horizon is now at the surface at time T2. Figure modified from Rindfleish et al. (2013).

Reconciling Numeric Dating with Soil Properties

One of the original goals of the research by Foster et al. (2015a) and Rindfleisch et al. (2013) was to reconcile the results from past CRN dating with relative dating methods that use soil properties. Relative age estimates for these terraces based on the thickness of argillic horizons, soil color, and carbonate morphology indicate longer durations of pedogenesis (see Madole, 1963) than the recent fluvial abandonment ages interpreted from numeric dating. On Table Mountain, this may be due to soil development in the thick alluvial package prior to fluvial stripping. Similarly deep (many meters) weathering has been observed in the Rocky Flats Alluvium (Riihimaki et al., 2006). On the Middle terrace, Madole (1963) suggested that the majority of gravels at the Middle terrace were cannibalized from the adjacent Table Mountain terrace during abandonment of that terrace. Alluvium deposited on the Middle terrace, sourced from the nearby Table Mountain, would have been quite weathered at the time of deposition. Cannibalized gravels from Table Mountain should not affect the CRN age interpretation, as the model age is interpreted from the exponential profile of CRNs accumulated after deposition; inherited concentrations of CRNs are not included in the model age estimate.

The presence of intermittent loess caps, now eroded, would account for the presence of an argillic horizon at the surface on Table Mountain, and allow for a slightly older age estimate for fluvial abandonment. However, the soils on Table Mountain and the Middle terrace appear to show more of a difference in soil development than is accounted for by these numeric age estimates. In summary, while numeric dating provides age constraint for terraces in the region, complete reconciliation of interpretations based upon soil development and upon CRN is still a work in progress.

The Evolution of Hogbacks

An iconic feature of the Front Range landscape, the Dakota Ridge is a prominent hogback defining the western edge of the High Plains (Fig. 14). The Dakota Ridge is the topographic manifestation of eastward-dipping layers of resistant largely sandstone Dakota Group rocks that are sandwiched between fine-grained Pierre Shale of the Cretaceous Western Interior Seaway stratigraphically above, and the fine-grained Morrison Formation below. Outcrops of Dakota sandstone near Morrison, Colorado, contain exemplary trace fossils of dinosaur footprints and well-preserved coastal sand ripples formed along the coast of the Western Interior Seaway (Waggé, 1955). The Morrison Formation, famous for its wealth of dinosaur fossils and the Bone Wars of the late nineteenth century (see Jaffe, 2000), is composed of mudstone, siltstone, and isolated lenses of sandstone representative of Jurassic floodplains and channels (Turner and Peterson, 2004). The depositional settings of these units give rise to differential weathering rates for the rock units, resulting in hogbacks that define the high local relief along the Front Range.

Figure 14.

Aerial photograph of Dakota Ridge hogbacks in Boulder, Colorado. Photo credit: R.S. Anderson.

Figure 14.

Aerial photograph of Dakota Ridge hogbacks in Boulder, Colorado. Photo credit: R.S. Anderson.

Figure 15.

(A) Photograph of hogback profile in Morrison, Colorado, shows concave hillslope on the west-facing side. Photo credit: R.C. Glade. (B) 1-m-resolution LiDAR profiles of hogbacks in Boulder, Colorado, show concave slopes. (C) Photograph shows block-strewn hillslope adjoining a hogback in Fort Collins, Colorado. Photo credit: R.C. Glade. Figure by R.C. Glade.

Figure 15.

(A) Photograph of hogback profile in Morrison, Colorado, shows concave hillslope on the west-facing side. Photo credit: R.C. Glade. (B) 1-m-resolution LiDAR profiles of hogbacks in Boulder, Colorado, show concave slopes. (C) Photograph shows block-strewn hillslope adjoining a hogback in Fort Collins, Colorado. Photo credit: R.C. Glade. Figure by R.C. Glade.

Classic hillslope theory can explain convex hilltops developed in homogeneous lithology (e.g., Gilbert, 1909; Dietrich et al., 2003) and linear slopes governed by landsliding processes (e.g., Roering et al., 1999, 2001). The influence of heterogeneous lithology on convex hillslope evolution has also recently been explored (Johnstone and Hilley, 2015). However, current theoretical and numerical models cannot explain concave slopes; nor do models address how geologic structure and the presence of bedrock blocks derived from resistant units influence hillslope features developed in layered rocks. This phenomenon is well represented by hogbacks, but also includes escarpments and the topographic signature of volcanic dikes.

Hogback morphology observed in the field sheds light on the geomorphic evolution of these features. East-facing slopes of the hogbacks are relatively linear with thin colluvial cover, mirroring the uplifted bedding plane of the Dakota sandstone from which the Pierre Shale has been largely removed. In contrast, west-facing slopes exhibit steep linear-to-concave hillslopes developed on the Morrison Formation (Figs. 15A and 15B), which are sprinkled with boulder-sized blocks derived from the overlying resistant Dakota sandstone (Fig. 15C). Blocks are not found outboard of the base of the hillslope, where the topography transitions to a relatively flat surface (Fig. 15C). These observations suggest that the otherwise easily weathered and eroded Morrison Formation is armored by blocks of Dakota sandstone, allowing steep slopes to persist as the cliffed edge of the hogback migrates downdip, to the east and downward (Glade et al., 2016).

To better understand the evolution of such landscapes, weathering and transport interactions between blocks and underlying bedrock must be honored (Anderson, 2014) (Fig. 16). The presence of blocks on the slope stalls sediment transport. They serve as colluvial dams: colluvium builds up behind the blocks, and is pulled away from the slope immediately downslope of the block. Slow sporadic downslope motion of blocks on the hillslope is promoted by the evolution of a hole into which blocks can either slide or roll. Sandstone blocks may also reduce weathering of the underlying soft rock as has been observed in granitic landscapes (Granger et al., 2001). Further, the blocks protect the Dakota caprock from being undermined, stalling the rate of cliff retreat when compared to a system with no blocks (Glade et al., 2016). Slow downslope motion of blocks ultimately allows the next block-release event from the caprock. The resistant blocks do weather slowly over time. As block size decreases, the armoring effects decrease. The base of the steep hillslope is defined by the disappearance of the blocks. The result is a landscape that can become quasi-steady in form, and translates both blocks and form downdip (Glade et al., 2016).

Figure 16.

Conceptual model of the important components for understanding hogback evolution. Figure by R. Glade.

Figure 16.

Conceptual model of the important components for understanding hogback evolution. Figure by R. Glade.

The details of the landscapes that emerge during exhumation of layered rocks depend upon the dip of the rock package, the relative weathering rates of the rocks involved, and the history of base level governed by the adjacent streams. For example, the Flatirons just west of Boulder, Colorado, dip more steeply than the Dakota Ridge, and are composed of Fountain Formation that is locally less jointed, and more resistant than the Dakota Group. The steeper dip has allowed the overlying rock to be eroded entirely, leaving the resistant dip slope exposed for the enjoyment of the local climbers.

At other locations along the Front Range, relict armored slopes, or “talus flatirons,” have been observed (e.g., Gutiérrez et al., 2015). Here and elsewhere in the world, these are interpreted to have formed in landscapes that are subjected to periods of rapid incision of nearby streams, likely reflecting swings in climate, that serve to disconnect such relict slopes from the escarpment from which the blocks were calved. While we do not observe talus flatirons on the Dakota Ridge, they can be found in the Colorado Piedmont where resistant limestone overlies the highly erodible Dawson arkose (Gutiérrez et al., 2015).

Finally, in areas where rapid incision occurs in close proximity to a hogback, or to other hillslopes from which blocks may be released, blocks may choke the channel and stall further incision until they are abraded or moved downstream (Shobe et al., 2016). In areas that experience periods of low incision, blocks may persist on the hillslopes for long periods of time until they weather away. This interplay between base level and armoring of the Dakota Ridge hogbacks is a topic of ongoing research.

Key Points at Stop 3

  • As at Rocky Flats to the south, the occupation history of the Lefthand terrace sequence implies long periods of fluvial occupation, generating remarkably wide strath terrace surfaces; here the occupation of the highest terrace (Table Mountain) was of the order of 1 m.y., and its abandonment occurred at roughly 100-120 ka.

  • Use of both 10Be and 26Al profiles on the Table Mountain surface has shed light on the long period of fluvial occupation, and allows us to reconcile the relative dating using soils, and the numeric dating using cosmo-genic radionuclides.

  • Age estimates based on soil development alone would suggest older ages for alluvium on terrace surfaces. Complete reconciliation of interpretations based upon soil development and upon CRN is still a work in progress.

  • The high local relief and the boulder-scattered ramps characteristic of hogbacks imply that the boulders both damp the rate at which the underlying shale weathers, and stall the creep of the soil.

Stop 4: Buckingham Park (40.113751° N, 105.308345° W)

Activity

We will stop at Buckingham Park, located on Lefthand Canyon Drive, -3.2 km to the west from where the Front Range meets the western High Plains. Here we will be able to observe a thin layer of boulders, cobbles, and sands deposited on the floodplain and within the channel during the 2013 storm. In addition, we can also see sections of the creek that scoured to bedrock near Buckingham Park. At this stop, we will present model-derived estimates of the mass of bed load transported through Left Hand Creek during the 2013 flood, and we will discuss the significance of those estimates in the context of short- and long-term erosion rates.

In addition to pondering the recent geomorphology, we will walk over to the right canyon wall (across the road from Buckingham Park) to observe an older fill terrace. The older terrace includes highly grussified cobbles and therefore appears to be quite old. Based on soil development, Birkeland (personal observ.) would estimate that this terrace predates the last glacial maximum. Attempts to date this terrace deposit using OSL were not successful, possibly due to recent turbation or mixing with colluvial sediment from above.

Background

Peak Discharge on Lefthand Creek and the Work Done by the 2013 Flood

Bed load transport rates were estimated using a series of hydraulic relations, calibrated to observed flows, to model changes in discharge, shear stress, and channel width over the duration of the 2013 flood. Measurements of stream-bed sediment were also taken to characterize thresholds for bed material entrainment. Bed load transport rates were calculated at daily time steps, using a transport function developed by Parker and Klingeman (1982),

 

formula
(1)

where W* is a dimensionless transport rate, and φ is the ratio between the available shear stress and the critical shear stress, τ/τ.

At this site, the 2013 flood left very distinct high-water marks far above normal flood levels (Fig. 17) (a “normal” flood is defined here as the discharge exceeded 1% of the time, Q001, comparable to flows that occur -4 d/yr, on average). The high-water marks were surveyed in conjunction with channel cross sections in spring 2014, and these data were used with discharge measurements taken at several flow levels to calibrate a 1-D hydraulic model (HEC-RAS). The results of our flow modeling indicate that the 2013 flood reached a peak discharge of 225 m3/s at this site. A separate study estimated that the peak discharge was somewhere between 149 and 261 m3/s, depending on the method used (Moody, 2016). These estimates suggest that at this site (Fig. 18), the 2013 flood was 15-25x times the mean annual flood (10 m3/s), and 5-8x the highest recorded peak on Lefthand Creek.

Figure 17.

Cross sections of Lefthand Creek at Buckingham Park showing water levels corresponding to the 2013 flood, and a typical peak flow in May 2015. Figure by J. Pitlick.

Figure 17.

Cross sections of Lefthand Creek at Buckingham Park showing water levels corresponding to the 2013 flood, and a typical peak flow in May 2015. Figure by J. Pitlick.

Figure 18.

High-water estimates along Lefthand Creek, near Buckingham Park, downstream from the confluence with James Creek. Figure after Moody (2016, his figure 14).

Figure 18.

High-water estimates along Lefthand Creek, near Buckingham Park, downstream from the confluence with James Creek. Figure after Moody (2016, his figure 14).

The total mass of bed load carried by the flood was calculated at daily time steps using Equation 1, and a daily time series of shear stress derived from the hydrograph shown in Figure 19. This hydrograph was developed with the aid of data from nearby gauging stations that remained in operation during the flood. Hydraulic relations for mean velocity, shear stress, and wetted perimeter were formulated as functions of discharge, and were then used with Equation 1 to compute bed load transport rates. Transport rates were calculated for all flows (d) exceeding the threshold for motion (dashed line in Fig. 19), then summed to obtain the total mass of bed material carried by the flood. We estimate that, between 10 September and 31 October 2013, Lefthand Creek carried -45,000 tonnes of bed load, and almost half of that in the 2 d encompassing the peak flow.

Figure 19.

Hydrograph of the 2013 flood developed with daily discharge data from nearby gauges that remained in operation during the flood. Figure by J. Pitlick.

Figure 19.

Hydrograph of the 2013 flood developed with daily discharge data from nearby gauges that remained in operation during the flood. Figure by J. Pitlick.

To place these estimates of transport in a longer-term context, we completed a second set of calculations to determine the annual bed load sediment flux of Lefthand Creek. The approach used here follows the magnitude-frequency analysis described by Wolman and Miller (1960). A regional relation for flow frequency was constructed using data from eight gauging stations along the Colorado Front Range (Fig. 20). Flows were grouped into a series of 27 discharge classes, the bed load sediment flux was computed for each discharge class, and the fluxes were weighted by the frequency of discharge in each class to produce a magnitude-frequency curve. Integration of the area under that curve yields the average annual sediment flux. From this analysis, we estimate that the annual bed load sediment flux of Lefthand Creek is 460 tonnes/yr, which equates to a specific sediment yield of 4.0 tonnes/km2/yr and a basin-wide erosion rate of 0.0015 mm/yr = 0.15 cm/k.y. The estimated average annual bed load sediment flux is roughly 1/100 of the flux accomplished by the 2013 storm hydrograph; thus our results imply that, at this site, the 2013 flood carried the equivalent of 100 years’ worth of bed load. This result is similar to the finding that debris flows evacuated the equivalent of several hundreds to a thousand years’ worth of weathering products from colluvial hollows and small sub-tributary channels flushed during the 2013 storm (Anderson et al., 2015).

Figure 20.

Normalized flow duration curves for eight gauging stations along the Colorado Front Range (LHand—Lefthand Creek; LThomp— Little Thompson River; NFBT—North Fork Big Thompson River; NSVR—North St. Vrain River; SBOCR—South Boulder Creek). The curves for each site were formed by normalizing the daily discharges by the flow exceeded 1% of the time, then determining the percentage of time flows were equaled or exceeded. Figure by J. Pitlick.

Figure 20.

Normalized flow duration curves for eight gauging stations along the Colorado Front Range (LHand—Lefthand Creek; LThomp— Little Thompson River; NFBT—North Fork Big Thompson River; NSVR—North St. Vrain River; SBOCR—South Boulder Creek). The curves for each site were formed by normalizing the daily discharges by the flow exceeded 1% of the time, then determining the percentage of time flows were equaled or exceeded. Figure by J. Pitlick.

Key Points at Stop 4

  • Bed load transport during the flood was equivalent to 100 years of bed load transport accomplished during base flow conditions.

  • A large portion of sediment transport and geomorphic work is conducted by rare, high sediment transport events.

Concluding Remarks

  • Numeric dating on Rocky Flats (Coal Creek), Pioneer terrace (Ralston Creek), and Table Mountain indicate long periods of fluvial occupation (100s of k.y.).

  • Terrace occupations and abandonment do not appear to occur simultaneously across drainage basins.

  • Different climatic conditions, much wetter than present climate, were likely required to initiate large, deep-seated landslides.

  • Modern landslides and debris flows produce shallow scars and deposits that are orders of magnitude smaller than prehistoric Quaternary landslides.

  • The interaction of large, armoring blocks with underlying bedrock shapes hillslope evolution, and can explain the persistence of landforms, such as the iconic hogbacks viewed along the Colorado Front Range.

  • The majority of geomorphic work is done during rare events, such as the September 2013 storm.

Acknowledgments

We would like to thank Suzanne Anderson, Billy Armstrong, Dave Dethier, and Eric Winchell for assistance with this manuscript and planning of the field trip. We would also like to thank our reviewers, Francis Rengers and Nicole West, for helping to improve the manuscript. This field trip was sponsored by the Boulder Creek Critical Zone Observatory, the Quaternary Geology and Geomorphology Division of the Geological Society of America, and the Colorado Scientific Society.

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Figures & Tables

Figure 1.

Overview of field-trip stops. GSA: Colorado Convention Center, Denver, 39.742310° N, 104.997139° W. 1: Rocky Flats, 39.891097° N, 105.230732° W. 2: Lee Hill trailhead, 40.062596° N, 105.288785° W. 3: Eagle terrace, 40.078024° N, 105.276570° W. 4: Buckingham Park, 40.113751° N, 105.308345° W.

Figure 1.

Overview of field-trip stops. GSA: Colorado Convention Center, Denver, 39.742310° N, 104.997139° W. 1: Rocky Flats, 39.891097° N, 105.230732° W. 2: Lee Hill trailhead, 40.062596° N, 105.288785° W. 3: Eagle terrace, 40.078024° N, 105.276570° W. 4: Buckingham Park, 40.113751° N, 105.308345° W.

Figure 2.

Cross section of generalized geology and topography near Boulder, Colorado. West to east distance is ~4 mi (~6.5 km) and relief is ~3000 ft (~1 km) (after Birkeland et al., 2004).

Figure 2.

Cross section of generalized geology and topography near Boulder, Colorado. West to east distance is ~4 mi (~6.5 km) and relief is ~3000 ft (~1 km) (after Birkeland et al., 2004).

Figure 3.

(A) Longitudinal profiles of major streams along the Colorado Front Range. Knickpoint locations were chosen to avoid lithologic contacts (Maureen Berlin, 2010, written commun.). Crystalline rock contact and extent of past glaciers are noted for each drainage. Dashed lines are paleolongitudinal projections from knickpoints across the western High Plains. The North Forks of Boulder Creek (BC) and St. Vrain Creeks (SV) are also shown. Ralston Creek (RC) is not included in A because its confluence with these other rivers is much farther downstream. (B-E) Individual stream profiles and paleo-profile projections. Dated terrace surfaces are also shown. Figure after Foster (2016).

Figure 3.

(A) Longitudinal profiles of major streams along the Colorado Front Range. Knickpoint locations were chosen to avoid lithologic contacts (Maureen Berlin, 2010, written commun.). Crystalline rock contact and extent of past glaciers are noted for each drainage. Dashed lines are paleolongitudinal projections from knickpoints across the western High Plains. The North Forks of Boulder Creek (BC) and St. Vrain Creeks (SV) are also shown. Ralston Creek (RC) is not included in A because its confluence with these other rivers is much farther downstream. (B-E) Individual stream profiles and paleo-profile projections. Dated terrace surfaces are also shown. Figure after Foster (2016).

Figure 4.

Major streams along the Colorado Front Range exhibiting large knickpoints. Stream profiles for selected creeks are plotted in Figure 3, along with prominent dated terraces. Dating sites on terraces are shown with yellow stars. Sample sites correspond to sample sites for basin averaged denudation rates reported in Foster (2016). Figure after Foster (2016). BC—Boulder Creek; CC—Coal Creek; LHC—Lefthand Creek; RC—Ralston Creek; SV—St. Vrain Creek; TM—Table Mountain.

Figure 4.

Major streams along the Colorado Front Range exhibiting large knickpoints. Stream profiles for selected creeks are plotted in Figure 3, along with prominent dated terraces. Dating sites on terraces are shown with yellow stars. Sample sites correspond to sample sites for basin averaged denudation rates reported in Foster (2016). Figure after Foster (2016). BC—Boulder Creek; CC—Coal Creek; LHC—Lefthand Creek; RC—Ralston Creek; SV—St. Vrain Creek; TM—Table Mountain.

Figure 5.

Looking up a debris flow chute on a Boulder Creek tributary, following the September 2013 flood. Photo credit: R.S. Anderson.

Figure 5.

Looking up a debris flow chute on a Boulder Creek tributary, following the September 2013 flood. Photo credit: R.S. Anderson.

Figure 6.

Type locality of the upper soil described on the Rocky Flats Alluvium, abandoned gravel pit. Photo credit: Peter Birkeland.

Figure 6.

Type locality of the upper soil described on the Rocky Flats Alluvium, abandoned gravel pit. Photo credit: Peter Birkeland.

Figure 7.

Quaternary landslides, cos-mogenic radionuclide sample sites, and trail for our hike from Lee Hill trail-head to Wonderland Lake, and then to the Foothills Community Park (Stop 2). Modified from Birkeland et al. (2004). CRN—cosmogenic radionuclide.

Figure 7.

Quaternary landslides, cos-mogenic radionuclide sample sites, and trail for our hike from Lee Hill trail-head to Wonderland Lake, and then to the Foothills Community Park (Stop 2). Modified from Birkeland et al. (2004). CRN—cosmogenic radionuclide.

Figure 8.

Photographs depicting younger Quaternary landslide deposited over an older Quaternary landslide with a prominent red soil. Photo credit: Peter Birkeland.

Figure 8.

Photographs depicting younger Quaternary landslide deposited over an older Quaternary landslide with a prominent red soil. Photo credit: Peter Birkeland.

Figure 9.

Annotated aerial photograph showing Quaternary landslides along the hike at Stop 2. Photo credit: R.S. Anderson. LGM—last glacial maximum.

Figure 9.

Annotated aerial photograph showing Quaternary landslides along the hike at Stop 2. Photo credit: R.S. Anderson. LGM—last glacial maximum.

Figure 10.

Thin debris flow dotted with sandstone blocks. Located on slope above our hike at Stop 2. Photo credit: R.S. Anderson.

Figure 10.

Thin debris flow dotted with sandstone blocks. Located on slope above our hike at Stop 2. Photo credit: R.S. Anderson.

Figure 11.

Terraces formed by Lefthand Creek (LHC), numbered from oldest (1, Haystack Mountain, HM) to youngest (5), and modern channels (6). Unrelated Quaternary terraces to the north and south are mapped as dark gray. Figure after Foster (2016). CRN/ OSL—cosmogenic radionuclide/optically stimulated luminescence.

Figure 11.

Terraces formed by Lefthand Creek (LHC), numbered from oldest (1, Haystack Mountain, HM) to youngest (5), and modern channels (6). Unrelated Quaternary terraces to the north and south are mapped as dark gray. Figure after Foster (2016). CRN/ OSL—cosmogenic radionuclide/optically stimulated luminescence.

Figure 12.

Field descriptions of soils at the Low terrace, Middle terrace, and Table Mountain. Figure modified from Rindfleisch et al. (2013).

Figure 12.

Field descriptions of soils at the Low terrace, Middle terrace, and Table Mountain. Figure modified from Rindfleisch et al. (2013).

Figure 13.

Conceptual figure showing possible erosion of an eolian cap. At time T1, the cap is present. Following erosion, the argillic horizon is now at the surface at time T2. Figure modified from Rindfleish et al. (2013).

Figure 13.

Conceptual figure showing possible erosion of an eolian cap. At time T1, the cap is present. Following erosion, the argillic horizon is now at the surface at time T2. Figure modified from Rindfleish et al. (2013).

Figure 14.

Aerial photograph of Dakota Ridge hogbacks in Boulder, Colorado. Photo credit: R.S. Anderson.

Figure 14.

Aerial photograph of Dakota Ridge hogbacks in Boulder, Colorado. Photo credit: R.S. Anderson.

Figure 15.

(A) Photograph of hogback profile in Morrison, Colorado, shows concave hillslope on the west-facing side. Photo credit: R.C. Glade. (B) 1-m-resolution LiDAR profiles of hogbacks in Boulder, Colorado, show concave slopes. (C) Photograph shows block-strewn hillslope adjoining a hogback in Fort Collins, Colorado. Photo credit: R.C. Glade. Figure by R.C. Glade.

Figure 15.

(A) Photograph of hogback profile in Morrison, Colorado, shows concave hillslope on the west-facing side. Photo credit: R.C. Glade. (B) 1-m-resolution LiDAR profiles of hogbacks in Boulder, Colorado, show concave slopes. (C) Photograph shows block-strewn hillslope adjoining a hogback in Fort Collins, Colorado. Photo credit: R.C. Glade. Figure by R.C. Glade.

Figure 16.

Conceptual model of the important components for understanding hogback evolution. Figure by R. Glade.

Figure 16.

Conceptual model of the important components for understanding hogback evolution. Figure by R. Glade.

Figure 17.

Cross sections of Lefthand Creek at Buckingham Park showing water levels corresponding to the 2013 flood, and a typical peak flow in May 2015. Figure by J. Pitlick.

Figure 17.

Cross sections of Lefthand Creek at Buckingham Park showing water levels corresponding to the 2013 flood, and a typical peak flow in May 2015. Figure by J. Pitlick.

Figure 18.

High-water estimates along Lefthand Creek, near Buckingham Park, downstream from the confluence with James Creek. Figure after Moody (2016, his figure 14).

Figure 18.

High-water estimates along Lefthand Creek, near Buckingham Park, downstream from the confluence with James Creek. Figure after Moody (2016, his figure 14).

Figure 19.

Hydrograph of the 2013 flood developed with daily discharge data from nearby gauges that remained in operation during the flood. Figure by J. Pitlick.

Figure 19.

Hydrograph of the 2013 flood developed with daily discharge data from nearby gauges that remained in operation during the flood. Figure by J. Pitlick.

Figure 20.

Normalized flow duration curves for eight gauging stations along the Colorado Front Range (LHand—Lefthand Creek; LThomp— Little Thompson River; NFBT—North Fork Big Thompson River; NSVR—North St. Vrain River; SBOCR—South Boulder Creek). The curves for each site were formed by normalizing the daily discharges by the flow exceeded 1% of the time, then determining the percentage of time flows were equaled or exceeded. Figure by J. Pitlick.

Figure 20.

Normalized flow duration curves for eight gauging stations along the Colorado Front Range (LHand—Lefthand Creek; LThomp— Little Thompson River; NFBT—North Fork Big Thompson River; NSVR—North St. Vrain River; SBOCR—South Boulder Creek). The curves for each site were formed by normalizing the daily discharges by the flow exceeded 1% of the time, then determining the percentage of time flows were equaled or exceeded. Figure by J. Pitlick.

Table 1.

Quaternary Terrace Units

UnitHeight (m)Generalized soil profileCarbonate morphologyAge
Nussbaum135Typically not well preservedLate Pliocene to early Pleistocene, based on correlation with the Broadwater Formation in Nebraska and Blanco Formation in Texas
Rocky Flats105A/Bt/Btk/K/BkStage IV1.35 Ma based on soil development and paleomagnetics; 400 ka— 2 Ma based on cosmogenic radionuclide dating of type locality
Verdos75A/Bt/Btk/K/BkStage III—IV640 ka, based on presence of Lava Creek B ash at type locality and several other locations
Slocum30A/Bt/Btk/K/BkStage III240 ka, based on U-series dating of a bison horn and geomorphic position of type locality near Cañon City, Colorado
Louviers21, variableA/Bt/BkStage II—IIICorrelated with Bull Lake glaciation (MIS* 6)
Broadway12, variableA/Bw or A/Bt/BkStage ICorrelated with Pinedale glaciation (MIS 2) based on geomorphic position and presence of fossil mammals and mollusks
UnitHeight (m)Generalized soil profileCarbonate morphologyAge
Nussbaum135Typically not well preservedLate Pliocene to early Pleistocene, based on correlation with the Broadwater Formation in Nebraska and Blanco Formation in Texas
Rocky Flats105A/Bt/Btk/K/BkStage IV1.35 Ma based on soil development and paleomagnetics; 400 ka— 2 Ma based on cosmogenic radionuclide dating of type locality
Verdos75A/Bt/Btk/K/BkStage III—IV640 ka, based on presence of Lava Creek B ash at type locality and several other locations
Slocum30A/Bt/Btk/K/BkStage III240 ka, based on U-series dating of a bison horn and geomorphic position of type locality near Cañon City, Colorado
Louviers21, variableA/Bt/BkStage II—IIICorrelated with Bull Lake glaciation (MIS* 6)
Broadway12, variableA/Bw or A/Bt/BkStage ICorrelated with Pinedale glaciation (MIS 2) based on geomorphic position and presence of fossil mammals and mollusks
*

MIS—Marine Isotope Stage.

Note: Table is based on work by Hunt (1954); Scott (1960, 1975); Scott and Lindvall (1970); Szabo (1980); Izett and Wilcox (1982); Madole (1991); Birkeland et al. (1996); and Riihimaki et al. (2006).

In Situ 10Be Data For Landslide Boulders

Table 2.
In Situ 10Be Data For Landslide Boulders
Sample nameρ
(g/cm3)
Thickness or sample depth
(cm)
Ps
(atoms/ g-qtz/yr)*
(atoms/
g-qtz/yr)*
Quartz
(g)
9Be carrier
(g)
Carrier (ppm)10Be/9Be
(10—15)§
.10Be
(M.atoms/
g-qtz)#
WLS12.651.8516.760.3217.02960.6972577 ± 3209.5 ± 10.40.325 ± 0.017
WLS22.651.8516.790.3219.26820.7022577 ± 3538.6 ± 18.70.752 ± 0.027
WLS32.651.8516.540.3215.30940.5868577 ± 3739.2 ± 25.31.087 ± 0.038
WLSA2.651.8516.550.3222.68130.4766389 ± 2765.8 ± 25.80.416 ± 0.014
WLSB2.651.8516.410.3228.31820.5192389 ± 21021.4 ± 42.50.484 ± 0.020
Sample nameρ
(g/cm3)
Thickness or sample depth
(cm)
Ps
(atoms/ g-qtz/yr)*
(atoms/
g-qtz/yr)*
Quartz
(g)
9Be carrier
(g)
Carrier (ppm)10Be/9Be
(10—15)§
.10Be
(M.atoms/
g-qtz)#
WLS12.651.8516.760.3217.02960.6972577 ± 3209.5 ± 10.40.325 ± 0.017
WLS22.651.8516.790.3219.26820.7022577 ± 3538.6 ± 18.70.752 ± 0.027
WLS32.651.8516.540.3215.30940.5868577 ± 3739.2 ± 25.31.087 ± 0.038
WLSA2.651.8516.550.3222.68130.4766389 ± 2765.8 ± 25.80.416 ± 0.014
WLSB2.651.8516.410.3228.31820.5192389 ± 21021.4 ± 42.50.484 ± 0.020
*

Production rates were obtained from the CRONUS calculator (http://hess.ess.washington.edu/; Balco et al., 2008), based on the constant production rate model (Lal, 1991; Stone, 2000). Shielding values are 0.999.

t10Be/9Be ratios for process blanks for different carriers are as follows: two process blanks with an average ratio of 3.8 ± 0.7 x 10—15 (577 ppm carrier); three process blanks with an average ratio of 4.6 ± 0.8 x 10—15 (389 ppm carrier).

§

Ratio with 1σ uncertainty, as reported by Purdue Rare Isotope Measurement Laboratory.

#

10Be concentrations include propagated uncertainty from accelerator mass spectrometry measurement, error introduced by the blank, and uncertainty in 9Be carrier concentration. Error in mass measurements, associated with the scale, was not propagated through calculations; mass is reported with the appropriate number of significant digits.

In Situ 10Be Exposure Ages For Landslide Boulders

Table 3.
In Situ 10Be Exposure Ages For Landslide Boulders
SampleLatitudeLongitudeElevationAgeInternalExternal
name(decimal ° W)(decimal ° N)(m)(ka)uncertainty
(ka)*
uncertainty
(ka)*
WLS140.0502105.2968178119.100.981.93
WLS240.0497105.2969178444.411.594.21
WLS340.0495105.2959176265.542.326.24
WLSA40.0503105.2959176228.921.232.81
WLSB40.0497105.2952175029.171.242.84
SampleLatitudeLongitudeElevationAgeInternalExternal
name(decimal ° W)(decimal ° N)(m)(ka)uncertainty
(ka)*
uncertainty
(ka)*
WLS140.0502105.2968178119.100.981.93
WLS240.0497105.2969178444.411.594.21
WLS340.0495105.2959176265.542.326.24
WLSA40.0503105.2959176228.921.232.81
WLSB40.0497105.2952175029.171.242.84
*

Internal and external 1σ uncertainty, as reported by the CRONUS exposure age calculator (http://hess.ess.washington.edu/; Balco et al., 2008). Internal uncertainty includes the accelerator mass spectrometry error, and error introduced by the blank and carrier. External uncertainty includes uncertainty in the production rate.

Contents

GeoRef

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