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Abstract

Enjoying geology does not necessarily mean strenuous hikes. This day-long, easygoing field trip is literally out of a car with various stops along the northern Colorado Front Range from north of Boulder to the Wyoming state line, with the longest hike being ~1.6 km (1 mi) round trip. We will examine and discuss stratigraphic units and geologic features from the Precambrian to the Quaternary, including the Great Unconformity. Participants will see inclined strata and differential weathering; gravel-topped mesas and inverted topography; vegetation that relies on ancient volcanic ash; evidence of the Cretaceous Seaway; a fracking simulation in the Niobrara Formation; a fault-interrupted S-shaped plunging fold trace; quarried slabs from ancient sand dunes; and impacts of the September 2013 Front Range flood. We will explore pegmatite emplacement and mineralization in the crystalline interior of the Rocky Mountains and in the Colorado Mineral Belt. Continuing north, we will see dinosaur bones in the Jurassic Morrison Formation; a mysterious snaggle-toothed rock wall; evidence of the 1976 Big Thompson flash flood; and Paleozoic strata with calcite spar, and microfossils. In conclusion, we will observe: block faulting of the northern Colorado terrain; the Virginia Dale ring dike and its famous magma mixing outcrop; and a deeply weathered, freely accessible diamond pipe recently discovered by Metropolitan State University of Denver students and faculty. This trip is excellent for anyone who would like to sample the amazing diversity of northern Colorado’s geology, including scientists, students, educators, and rock hounds with opportunities to collect some nice specimens for teaching or personal enjoyment.

Introduction

Objective

The goal of this trip is to sample easily accessible sites that demonstrate the geologic diversity present along the Front Range of Colorado, including rock units of igneous, sedimentary, and metamorphic origin; rocks and minerals of different ages; sedimentary and structural features; economically significant ore and industrial minerals; and evidence of geological processes such as uplift, weathering, erosion, glaciation, and flooding. The trip is appropriate for participants with any level of geologic experience and expertise, from students to guests to practicing professionals. The field-trip leaders are experienced in hydrogeology, petroleum exploration, mineral exploration, and high-resolution stratigraphy.

Most of the field trip parallels the eastern flank of the Front Range from north of Boulder, Colorado, to the Wyoming state line. Stop 1 and Stops 6 through 9 focus on sedimentary strata of Paleozoic or Mesozoic age. At Stop 1, Cenozoic unconsolidated and poorly consolidated deposits are also especially well developed. Stops 2 through 5 focus on latest Mesozoic to early Cenozoic (Laramide) igneous intrusions, and Stops 3 and 4 highlight Laramide hydrothermal mineralization. Stop 1 and Stops 6 through 9 exhibit a variety of Laramide fold and fault structures. The importance of differential weathering and steeply dipping strata in producing Front Range topography is especially apparent at Stop 1 and Stops 6 through 9. Stops 10 and 11 focus on a Precambrian ring-dike complex and associated kimberlite pipes of Precambrian and Devonian age.

Much of the geology will be visible directly from the vehicles. At some locations, we will take short hikes less than 1.6 km (1 mi) in length. Trails are easy to moderate, but the high altitude of the region may affect participants. Weather changes very quickly along the Front Range, so participants should wear layers of clothing and be prepared for wind and precipitation. Extremes of late September weather range from dry heat to thunderstorms to snow. Sunscreen is needed year-round. Long pants and sturdy shoes with low heels and good tread, such as hiking boots, are highly recommended. Open-toed shoes and footwear with higher heels are not appropriate. Hazards include high altitude, irregular terrain, cacti, yucca, weather conditions, and the possibility of encountering rattlesnakes and other wildlife. Restroom facilities are limited and are not available at most of the field stops. A water refill station is available at our lunch stop. If weather, road, or other conditions lead to delays on our route, the trip leaders reserve the right to bypass one or more stops in order to complete the trip on time.

Stratigraphic Overview and Geology

Figure 1 provides an overview of the lithology and strati-graphic variations encountered during this field excursion. The included map and stratigraphic section, showing geologic changes from south to north, can be used as a ready reference for trip participants.

Figure 1.

Stratigraphy and geologic map within the general field-trip area—idealized stratigraphic section in combination with a geologic map showing changes in lithostratigraphy along the field-trip route. Stops 1, 5, 6, 7, 8, and 9 are indicated by circled number inserts on the map. Field stops 2, 3, 4, 10, and 11 are outside the map boundary. Symbols for geologic units used and associated lithologic descriptions are summarized in Table 1. Adapted from Matthews (2004), Google Earth (2013), and U.S. Geological Survey Cogeol.kml (Geologic Units of Colorado) file (Stoeser et al., 2005).

Figure 1.

Stratigraphy and geologic map within the general field-trip area—idealized stratigraphic section in combination with a geologic map showing changes in lithostratigraphy along the field-trip route. Stops 1, 5, 6, 7, 8, and 9 are indicated by circled number inserts on the map. Field stops 2, 3, 4, 10, and 11 are outside the map boundary. Symbols for geologic units used and associated lithologic descriptions are summarized in Table 1. Adapted from Matthews (2004), Google Earth (2013), and U.S. Geological Survey Cogeol.kml (Geologic Units of Colorado) file (Stoeser et al., 2005).

The mountains immediately west and northwest of Denver are part of Colorado’s Front Range. Exposed in the core of the Front Range are Precambrian igneous and metamorphic rocks. The metamorphic rocks and some of the Precambrian granites are ca. 1.7 billion yr old, but there are also a set of granitic intrusions ca. 1.4 Ga and pegmatite dikes slightly more than 1.0 Ga. The 1.7-b.y.-old metamorphic and igneous rocks formed when plate tectonic motion caused Colorado to collide with older parts of the North American craton just to the north in Wyoming (Sims and Stein, 2003).

During the early part of the Paleozoic Era, from the Cambrian through the Mississippian Periods, a series of shallow epeiric seas transgressed and regressed across the region, leaving an incomplete depositional record of thin sandstone, shale, and carbonate strata. Episodes of epeirogenic upwarping and erosion removed the lower Paleozoic strata and created a major unconformity. Today, Pennsylvanian strata ca. 300 m.y. old lie directly on Precambrian rocks that are mainly 1.7 and 1.4 b.y. old. The surface along which there is no rock record for such a long time is called the Great Unconformity. We cross this unconformity several times during our field excursion. Reconstructing the geologic events that occurred during the interval represented by the Great Unconformity was difficult, and volcanic kimberlite intrusions, like the one we visit at Stop 11, played a surprising role in understanding what this area was like during the early Paleozoic.

Toward the end of the Paleozoic Era, the tectonic setting in this region changed drastically. Globally, the final assembly of the supercontinent Pangaea was taking place, and mountain ranges were being built in many parts of the world. Colorado was no exception; the Ancestral Rocky Mountains, otherwise known as Frontrangia, and other mountainous regions developed in Colorado. An abundance of erosional debris from the highlands resulted in the deposition of conglomerate, sandstone, and mudstone strata of Pennsylvanian and Permian age. A relatively arid climate at the time of deposition created an abundance of red beds among the alluvial and eolian deposits (Walker, 1967). At times, shallow fingers of sea crept into the area along the flank of the highlands; limestone and gypsum beds are increasingly common as we travel northward, because our route takes us farther away from the former front of the Ancestral Rocky Mountains (Maughan, 1980).

Erosion continued to lower the Ancestral Rocky Mountains. During the Triassic Period, red mudrocks, very finegrained sandstone, and thin evaporite beds were deposited in fluvial to shallow marine environments. By the Late Triassic, Pangaea had begun to break apart, and the Gulf of Mexico and the Atlantic Ocean were beginning to develop. By the Jurassic Period, the climate became less arid, and the area of today’s Front Range was dominated by lowlands with low-energy rivers, floodplains, and lakes. Fluvial environments continued into the early part of the Cretaceous Period, but major changes were coming to the region. By the end of the Early Cretaceous, an arm of marine water from the Arctic region to the north would connect with an arm of warm water from the opening Gulf of Mexico, creating the Cretaceous Western Interior Seaway. A series of transgressions and regressions caused the position of the shorelines to move back and forth, but for much of the Cretaceous Period, the Front Range area was below sea level, and relatively thick deposits of sediment developed. At different times, sediment was delivered from both elevated highlands to the west and from gentler-gradient exposed craton to the east. When clastic sources were cut off, carbonate deposition occurred. At times, significant amounts of volcanic ash arrived from volcanic areas to the west. Strata of Cretaceous age comprise the majority of the preserved sedimentary record along the Front Range, and they provide a variety of geologic resources such as limestone, claystone, silica sand, coal, oil, and gas.

Toward the end of the Cretaceous Period, the Cretaceous sea retreated from the area for the final time. From the latest Cretaceous into the middle of the Paleogene Period, a sharp increase in tectonic activity accompanied the Laramide orogeny and the development of the Laramide Rocky Mountains. Structural features such as the downwarped Denver Basin and Laramide folds and faults developed during this time. Intrusion of mafic to intermediate igneous dikes and sills occurred along the Front Range, and explosive volcanism occurred in southern and south-central Colorado. Felsic intrusions and hydrothermal fluids resulted in mineralization in the Colorado Mineral Belt. Erosional surfaces were beveled across the highlands. Abundant erosional debris filled intermontane basins. Gradually, wetter climatic conditions and renewed uplift resulted in the development of through-flowing rivers that carried the load of clastic material out to the mountain front and deposited blankets of debris on top of eroded sedimentary bedrock. In the Quaternary Period, meltwaters from mountain glaciers incised the gravel cover and resulted in a series of stream terraces along significant drainages and flat, gravel-topped mesas like those we observe at Stop 1.

Today, Precambrian igneous and metamorphic rocks are exposed in the mountains of the Front Range. Younger sedimentary strata, primarily of Paleozoic and Mesozoic age, are tilted up against the Precambrian rocks and dip steeply eastward into the Denver Basin. The Precambrian basement lies nearly 4300 m (14,000 ft) below the ground surface near the eastern flank of the Front Range (Tweto, 1980), but is exposed at the summit of Longs Peak at an elevation of 4346 m (14,259 ft), demonstrating structural relief in excess of 8600 m (28,200 ft).

Previous Work

A few of the many geologists who have contributed to the current understanding of geologic conditions and processes in the area of our field trip are mentioned here because their work was among the foundational research in the area, or because it has made an especially significant contribution. Among the early descriptions of geological units and features along the northern Front Range is the work of Emmons et al. (1896); Fenneman (1905); Butlers (1913); Reeside (1923); Lee (1927); Lovering (1929); Ver Wiebe (1930); Brainerd et al. (1933); Van Tuyl and Lovering (1935); Heaton (1939); Brown (1943); Fischer (1946); Wahlstrom (1947); and Thompson (1949).

Among the research that investigates tectonics or structural geologic settings, events, and processes is (Boos and Boos, 1957); Curtis (1958); LeMasurier (1970); Hoblitt and Larson (1975); Tweto (1975); Matthews and Sherman (1976); Matthews and Work (1978); Warner (1980); and Matthews (2004).

Precambrian rocks and structures and their relationship to features of other ages were investigated by Hedge et al. (1967); Peterman and Hedge (1968); Peterman et al. (1968); Tweto (1977); Hedge et al. (1986); and Sims and Stein (2003). Pro-terozoic ring dikes were studied by Eggler (1968) and Vasek and Kolker (1999).

Research on Lower Paleozoic strata, processes, and structures includes Berg (1960); Rothrock (1960); Chronic et al. (1965, 1969); Baars and Campbell (1968); Baars (1972); Craig (1972); De Voto (1980a); and Ross and Tweto (1980). Early work on the pre-upper Pennsylvanian Great Unconformity includes Maher (1950) and MacLachlan and Kleinkopf (1969).

Investigations of Pennsylvanian or Permian strata and related processes and structures were conducted by Mallory (1958, 1960, 1972); Taylor (1958); Hubert (1960); Maughan and Wilson (1960); Hoyt (1963); Howard (1966); De Voto (1972, 1980b); and Rascoe and Baars (1972).

Studies on Triassic, Permo-Triassic, or Triassic-Jurassic depositional systems and controls were carried out by Pipirin-gos (1957); Oriel and Craig (1960); Pipiringos et al. (1969); MacLachlan (1972); Pipiringos and O’Sullivan (1976); and Maughan (1980). Work focused on Jurassic systems includes Imlay (1952); Jackson (1979); and Peterson (1972, 1994). Berman et al. (1980) conducted research on Jurassic and Cretaceous systems.

Among the researchers whose work is significant to Cretaceous systems within the field-trip area are Cobban and Reeside (1952); Waagé (1955, 1959, 1961); Weimer (1960); Haun (1963); MacKenzie (1963); Scott and Cobban (1965); Kauffman (1969, 1977); McGookey et al. (1972); Barlow and Kauffman (1985); Cobban et al. (1994); and Kauffman et al. (2007).

Post-Cretaceous, pre-Quaternary rocks, and processes relevant to the northern Front Range were investigated by Scott (1960, 1975); Epis and Chapin (1975); Izett (1975); Soister (1978); Epis et al. (1980); and Mutschler et al. (1987).

Studies on Laramide mineralization in the area of the field trip include Lovering and Goddard (1950); Tweto and Sims (1963); Tweto (1968); Kelly and Goddard (1969); Bryant et al. (1975); Worl et al. (1976); Romberger (1980); Cappa (1998); and Wallace (2010).

Relevant Quaternary research includes work in the northern Front Range by Richmond (1965); Madole (1969); Madole and Shroba (1979); and Meierding and Birkeland (1980).

Sources of geologic mapping for quadrangles throughout the field-trip region are given at the beginning of the traveling directions to each field stop.

Geologic Units

See Table 1 for the most common geologic units encountered during the extent of the field trip:

Geologic Symbols and Descriptions of Lithologic Units

Table 1.
Geologic Symbols and Descriptions of Lithologic Units
Abbrev.Description
Sedimentary units
TgvTertiary unconsolidated boulder-rich gravel on old erosion surfaces in Front Range
KpCretaceous Pierre Shale:
Olive-gray and dark-gray shale and silty shale, organic-rich in part; numerous bentonite beds; five sandstone intervals in middle part of unit
KnbCretaceous Niobrara and Benton Formations, undivided:
Calcareous shale, chalk, and limestone, and gray non-calcareous shale and silty shale
KJdsCretaceous-Jurassic Dakota, Morrison, and Sundance Formations, undivided:
Nonmarine, marginal marine, and shallow marine sandstone, shale, mudstone, and minor limestone and claystone
TR PllyTriassic-Permian Lykins Formation and Lyons Sandstone, undivided (from Lyons, Colorado, southward):
Red mudrock and siltstone, with beds of gypsum and limestone superjacent to quartzose cross-bedded sandstone
TR PjoTriassic-Permian Jelm, Lykins, Lyons, and Owl Canyon Formations, undivided (from Lyons, Colorado, northward):
Red siltstone, shale, and sandstone, with minor limestone
P|PifPermian-Pennsylvanian Ingleside and Fountain Formations, undivided (from Lyons, Colorado, northward):
Limestone and calcareous sandstone superjacent to arkosic sandstone, mudstone, and conglomerate
P|PfPermian-Pennsylvanian Fountain Formation (from Lyons, Colorado, southward):
Arkosic sandstone, mudstone, and conglomerate
Igneous and metamorphic units
TKiTertiary-Cretaceous Laramide intrusive igneous rocks: mainly intermediate to felsic, some mafic
YgMesoproterozoic 1400 m.y. age group:
Granitic rocks, including Silver Plume and Sherman granites
XgPaleoproterozoic 1700 m.y. age group:
Granitic rocks, including Boulder Creek Granodiorite
XbPaleoproterozoic 1700 m.y. age group:
Principally biotite gneiss, schist, and migmatite derived from sedimentary rocks
XfhPaleoproterozoic 1700 m.y. age group:
Principally felsic and hornblende gneiss, separate or interlayered, derived from volcanic rocks
Virginia Dale ring-dike units
Ylc~1390 m.y.: Log Cabin Granite; fine-grained to aplitic, abundant muscovite
Yscr~1410 m.y.: Cap Rock Quartz Monzonite-Sherman Granite facies; light-gray to pink, porphyritic, biotitic
XYd~1410 to 1700 m.y.: Mafic rocks, including aplitic diorite and minor hornblende gabbro, andesite, and dacite
Abbrev.Description
Sedimentary units
TgvTertiary unconsolidated boulder-rich gravel on old erosion surfaces in Front Range
KpCretaceous Pierre Shale:
Olive-gray and dark-gray shale and silty shale, organic-rich in part; numerous bentonite beds; five sandstone intervals in middle part of unit
KnbCretaceous Niobrara and Benton Formations, undivided:
Calcareous shale, chalk, and limestone, and gray non-calcareous shale and silty shale
KJdsCretaceous-Jurassic Dakota, Morrison, and Sundance Formations, undivided:
Nonmarine, marginal marine, and shallow marine sandstone, shale, mudstone, and minor limestone and claystone
TR PllyTriassic-Permian Lykins Formation and Lyons Sandstone, undivided (from Lyons, Colorado, southward):
Red mudrock and siltstone, with beds of gypsum and limestone superjacent to quartzose cross-bedded sandstone
TR PjoTriassic-Permian Jelm, Lykins, Lyons, and Owl Canyon Formations, undivided (from Lyons, Colorado, northward):
Red siltstone, shale, and sandstone, with minor limestone
P|PifPermian-Pennsylvanian Ingleside and Fountain Formations, undivided (from Lyons, Colorado, northward):
Limestone and calcareous sandstone superjacent to arkosic sandstone, mudstone, and conglomerate
P|PfPermian-Pennsylvanian Fountain Formation (from Lyons, Colorado, southward):
Arkosic sandstone, mudstone, and conglomerate
Igneous and metamorphic units
TKiTertiary-Cretaceous Laramide intrusive igneous rocks: mainly intermediate to felsic, some mafic
YgMesoproterozoic 1400 m.y. age group:
Granitic rocks, including Silver Plume and Sherman granites
XgPaleoproterozoic 1700 m.y. age group:
Granitic rocks, including Boulder Creek Granodiorite
XbPaleoproterozoic 1700 m.y. age group:
Principally biotite gneiss, schist, and migmatite derived from sedimentary rocks
XfhPaleoproterozoic 1700 m.y. age group:
Principally felsic and hornblende gneiss, separate or interlayered, derived from volcanic rocks
Virginia Dale ring-dike units
Ylc~1390 m.y.: Log Cabin Granite; fine-grained to aplitic, abundant muscovite
Yscr~1410 m.y.: Cap Rock Quartz Monzonite-Sherman Granite facies; light-gray to pink, porphyritic, biotitic
XYd~1410 to 1700 m.y.: Mafic rocks, including aplitic diorite and minor hornblende gabbro, andesite, and dacite

Field Stops

Stop 1: Neva Road and U.S. 36, North of Boulder, Colorado (N40°06.34752’, W105°16.84206) Drive time: 50 min; stop duration: 30 min

Traveling to Stop 1

At the Colorado Convention Center in Denver, Colorado (700 14th Street, Denver, Colorado 80202), we begin our trip on a broad terrace capped with Quaternary Broadway Alluvium. As we travel north and west on I-25 and U.S. 36 toward Boulder, Colorado, we pass backward in time down the stratigraphic section, first over the Paleogene and Cretaceous Denver Formation and then through the Cretaceous Arapahoe Formation, coal-bearing Laramie Formation, and Fox Hills Sandstone. As we descend into the Boulder Valley, we see the steeply dipping red rock slabs of the Flatirons, composed of resistant conglomeratic arkose of the Permian and Pennsylvanian Fountain Formation. In Boulder, many of the buildings on the University of Colorado campus are faced with Permian Lyons Sandstone, a quartzose, silica-cemented sandstone predominantly of eolian origin (Walker and Harms, 1972). Flagstone is quarried from the Lyons Sandstone from just north of Boulder to north of Fort Collins, Colorado, and is a trademark for many buildings in the region. We follow U.S. 36 north out of Boulder on Upper Cretaceous Pierre Shale and cross onto the Niobrara Formation as we approach Neva Road and our first stop.

Geologic Features

Reference maps: U.S. Geological Survey geologic maps: Boulder quadrangle (Wrucke and Wilson, 1967); Niwot quadrangle (Trimble, 1975); Hygiene quadrangle (Madole et al., 1998); Lyons quadrangle (Braddock et al., 1988b).

Six-Mile Fold. Six-Mile Fold Open Space (Boulder County Parks and Open Space) lies immediately west of U.S. 36 at Neva Road. The name is appropriate because an anticline-syncline couplet crops out here, and the location is six miles north of the City of Boulder’s municipal complex. The trace of the folds is well expressed due to differential weathering of three Cretaceous lithostratigraphic units. The Fort Hays Limestone Member of the Niobrara Formation forms a low persistent ridge between two less resistant units: the overlying Smoky Hill Shale Member of the Niobrara Formation and the underlying Benton Formation. All of the Cretaceous strata we observe in the folds were deposited in the Western Interior Seaway (Kauffman, 1977; Cobban et al., 1994), which at its greatest extent stretched more than 1600 km (1000 mi) from west to east and connected arctic waters from the north with warm waters from the Gulf of Mexico to the south (Fig. 2).

Figure 2.

Map of Late Cretaceous Western Interior epeiric seaway (85 Ma), western hemisphere projection with position of Colorado indicated; globe insert 90 Ma. Map modified from R. Blakey © 2013 and globe © 2006 by Colorado Plateau Geosystems, used with permission.

Figure 2.

Map of Late Cretaceous Western Interior epeiric seaway (85 Ma), western hemisphere projection with position of Colorado indicated; globe insert 90 Ma. Map modified from R. Blakey © 2013 and globe © 2006 by Colorado Plateau Geosystems, used with permission.

Lithostratigraphic Units

The Benton Formation is non-resistant and very poorly exposed at Six-Mile Fold. It consists predominantly of non-calcareous silty shale and silty claystone, with thin interbeds of clayey siltstone, a few thin beds of clayey limestone, and numerous thin bentonite beds. At the top of the Benton Formation, just below the base of the Fort Hays Limestone Member of the Niobrara Formation, ~0.6-09 m (2-3 ft) of sandy to silty bioturbated calcareous mudrock and clayey siltstone may be equivalent to the Codell Sandstone, which is present in both northern and southern Colorado. However, these beds are poorly exposed at Six-Mile Fold, and the Codell Sandstone is not mapped as a separate unit in the area (Wrucke and Wilson, 1967; Braddock et al. 1988b). The Benton Formation is ~150 m (495 ft) thick at this location (Wrucke and Wilson, 1967).

The Fort Hays Limestone Member of the Niobrara Formation overlies the Benton Formation. It forms the low ridge that delineates the trace of Six-Mile Fold. The Fort Hays Member consists of thin to thick beds of light-gray, micritic, pelagic limestone separated by poorly exposed thin beds of dark-gray calcareous shale. Fractures in the limestones are commonly filled with macrocrystalline calcite. The limestones are highly bioturbated. They commonly contain fossilized, heavily ribbed inoceramid clam shells, some with fossilized oyster shells on their outer surfaces, and abundant inoceramid shell fragments. Weathered surfaces of limestone beds in the Fort Hays are light yellowish-gray. The Fort Hays Limestone Member is ~5.5-6.5 m (15-18 ft) thick in the area (Wrucke and Wilson, 1967).

The Smoky Hill Shale Member of the Niobrara Formation overlies the Fort Hays Limestone Member. The non-resistant Smoky Hill forms sparsely vegetated low hills and gullies. It consists predominantly of dark-gray, fossiliferous, very fissile shale interbedded with thin gray fossiliferous limestone beds. The lower third of the Smoky Hill Shale Member is less calcareous and is non-fossiliferous. The Smoky Hill also contains numerous bentonite beds. As a whole, the Smoky Hill weathers to light gray, but the uppermost beds weather to a characteristic orange-brown throughout the region (Braddock et al., 1988b). The Smoky Hill Shale Member is ~113 m (370 ft) thick in the area (Wrucke and Wilson, 1967).

Platyceramus inoceramid clams and Pseudoperna congesta oysters are particularly common in calcareous beds of the Smoky Hill Member. The Platyceramus clams have very broad, thin valves up to 1 m (3 ft) in axial length. They are abundant in beds that reflect deposition in soft to soupy substrates with moderately dysoxic bottom conditions, low benthic biodiversity, and water depths of 37-47 m (120-155 ft) (Kauffman et al., 2007). More than 80 giant Platyceramus fossils are preserved, most apparently in their flat-lying benthic life positions, on a single bedding plane in a limestone quarry ~11 km (7 mi) north of Six-Mile Fold (Kauffman et al., 2007).

Bentonite is the term regionally applied to expansive clay layers and the soils derived from them. These predominantly smectitic clays (Kile, 2002) form diagenetically from airborne volcanic ash that settled to the bottom of the Cretaceous sea. Late Cretaceous volcanic eruptions associated with the Sevier orogeny to the west (Armstrong, 1968) were responsible for the volcanic ash. The expansive clays swell when exposed to water and shrink when they are dry. They are widely distributed across the Denver metropolitan area in most Cretaceous formations and cause damage to highways, foundations, and other structures (Hart, 1974; Abbott and Noe, 2016). Bentonites serve a valuable geologic purpose in that each bentonite layer records a geologically instantaneous volcanic ash fall. Bentonite layers are widely used as chronostratigraphic markers in Cretaceous strata of the Western Interior: biostratigraphic data from ammonites and inoceramid clams have been integrated with relative and numerical dating from bentonites to establish a highresolution chronostratigraphic framework for strata deposited in the Cretaceous Western Interior Seaway (Scott and Cobban, 1965; Cobban et al., 2006). Due to its ability to hydrate and swell in the presence of water, its colloidal properties, and its propensity for forming viscous and thixotropic fluids, bentonite has a wide range of commercial uses. A few of these are as: a sealant, lubricant, and viscosity agent in drilling muds; a sealant and purifier in landfill liners; a binder for pelletizing powders, from comminuted iron ore, to livestock feed, to cosmetics, to pharmaceuticals; a suspension agent in paints; and an absorbent agent for cat litter.

Cyclicity controlled by Milankovitchtype orbital variations may be reflected in both lithostratigraphic members of the Nio-brara Formation, based on the presence and regional persistence of rhythmic alternations of bedding thickness and of lithology (Kauffman et al., 2007; Locklair and Sageman, 2008). In both the Fort Hays and Smoky Hill Members, shale or marlstone beds representing wetter climatic intervals alternate with limestone beds representing drier climatic intervals, and the shale-limestone couplets are bundled into larger-scale groups (Pratt, 1984; Barlow and Kauffman, 1985; Laferriere et al., 1987). The rhythmic depositional units observed in the Niobrara Formation may be controlled by Milankovitchtype cyclicity (Barlow and Kauffman, 1985; Laferriere et al., 1987; Kauffman et al., 2007; Locklair and Sageman, 2008), but tectonic and eustatic controls on siliciclastic sediment supply and accommodation space could have altered the cyclicity of Cretaceous sedimentation, as well as the preservation of the Cretaceous depositional record (Armstrong, 1968; Kauffman, 1977; Barlow and Kauffman, 1985; Laferriere et al., 1987).

Vegetation is closely linked to geology at Six-Mile Fold Open Space. On the eroded surface of steeply dipping beds in the Smoky Hill Member immediately east of U.S. 36, a limited range of native plants, such as grasses and penstemon, grow in thin vegetated stripes separated by bands of barren dark shale. Each stripe of vegetation is growing along an upturned bentonite bed. Also growing on the Smoky Hill shale is Bell’s twinpod (Physaria bellii Mulligan), a yellow-flowering mustard endemic to Colorado, whose only populations are in these dark shale and limestone beds along the Front Range from just south of Denver to the Wyoming border. Although Bell’s twinpod hybridizes with more common Physaria species, the greatest threat to its survival is loss of habitat due to development and limestone mining (Kothera et al., 2007).

Resources locally produced from the Niobrara Formation include limestone, oil, and natural gas. The Niobrara Formation is the target of most of the directional drilling and hydraulic fracturing for oil and natural gas that is occurring in the Denver Basin, as seen along I-25 north of Denver in the Wattenberg Field. Limestone from the Fort Hays Member is quarried for cement several miles north of Six-Mile Fold in the area east of Lyons, Colorado (Colorado Division of Reclamation Mining and Safety, n.d.).

Looking east from Six-Mile Fold. Past the orange-brown ridge of weathered uppermost Smoky Hill shales are lower, flatter plains underlain by the marine Upper Cretaceous Pierre Shale, which is ~2438 m (8000 ft) thick at this location (Scott and Cobban, 1965). Quarries in the Pierre Shale near Boulder produce shale that is heated and expanded to form lightweight aggregate (Schwochow et al., 1974).

The flat-topped mesas visible to the east, south, and north are pediment surfaces capped by Quaternary alluvium brought from the Rocky Mountains and subsequently terraced to successively lower elevations by downcutting streams. The small, isolated, cone-shaped peak just to the east is Haystack Mountain. It is an erosional remnant of a high terrace capped by gravel and boulders of the Pleistocene Rocky Flats Alluvium (Trimble, 1975). Haystack Mountain is an example of inverted topography; a stream channel that was once topographically low is now preserved as a topographic high due to the resistance to weathering of its bouldery alluvium.

In the distance to the southeast is Valmont Butte, which appears as a dark vertical wall in front of the tall smokestacks of the soon-to-be-abandoned Valmont Power Plant, a former coal-fired electrical generating facility. Valmont Butte is a high-potassium basaltic (shoshonite) dike of early Paleocene age (Trimble, 1975; Mutschler et al., 1987; Larson, 2004) that intrudes Upper Cretaceous Pierre Shale (Trimble, 1975). Along the margins of the intrusion, the Pierre Shale has been altered by contact metamor-phism to hard, blocky, dark hornfels; as a consequence, Valmont Butte is the easternmost outcrop in Boulder County with igneous, sedimentary, and metamorphic rock exposed together in situ (EchoHawk, 2009). Valmont Butte has been the site of a variety of industrial uses, including quarrying its rock for aggregate, but it also has a long history as a spiritually significant site for people from vastly different cultures (EchoHawk, 2009). In the same time frame that the mafic intrusion at Valmont Butte was emplaced, calc-alkalic felsic sills were emplaced in sedimentary strata along the eastern margin of the foothills between Boulder and Lyons, Colorado (Larson, 2004).

Looking west from Six-Mile Fold. From atop the ridge of the Fort Hays Limestone Member, the higher hogback immediately to the west is supported by the Lytle Formation, the basal unit of the Lower to Upper Cretaceous Dakota Group. The Lytle Formation in the area is typically resistant, well-cemented sandstone and conglomerate. The strata dip steeply east toward our view from the Fort Hays ridge, and the three members of the South Platte Formation of the Dakota Group are expressed in the ridges and swales on the eastern flank of the Dakota hogback. In ascending stratigraphic sequence above the Lytle Formation are the Plainview Sandstone Member, Skull Creek Shale Member, and Muddy Sandstone Member of the South Platte Formation. In the area, the Dakota Group is ~84 m (275 ft) thick (Wrucke and Wilson, 1967). The Dakota Group records the approach and arrival of the Cretaceous seaway in the area, and it includes terrestrial features such as dinosaur tracks and fossilized plants; marginal marine features such as flat-topped, straight-crested, symmetrical ripples from tidal flats; and marine features such as shallow marine trace fossils. The Muddy Sandstone Member, also known as the J sand, has been a significant petroleum producer in the Denver Basin. The South Platte Formation contains kaolinitic refractory clays (Waagé, 1961; Kile, 2002). These have been mined north of Golden, Colorado, between Denver and Boulder, for use in the manufacture of high-temperature ceramics and fire bricks.

Looking to the south, the Flatirons are visible leaning against the flank of the Front Range at the south end of Boulder. To the north-northeast, Rabbit Mountain anticline folds the eastern flank of the Front Range (Madole et al., 1998). We will pass by Rabbit Mountain as we travel between Stops 5 and 6.

Stop 2: Granitic Pegmatite-Lefthand Canyon Drive (N40°06.50202’, W105°20.02146) Drive time: 15 min; stop duration: 15 min

Traveling to Stop 2

Reference maps: U.S. Geological Survey geologic maps: Boulder quadrangle (Wrucke and Wilson, 1967); Lyons quadrangle (Braddock et al., 1988b).

Leaving Stop 1, we immediately cross the Fort Hays Limestone Member at a drainage gap, and then the low Fort Hays ridge continues parallel to our path on the right (east) as we travel northward along U.S. 36 for 2.7 km (1.7 mi). Turning west on Lefthand Canyon Drive, we traverse the stratigraphic section back in time through the Cretaceous Dakota Group, Upper Jurassic Morrison Formation, Lower Jurassic Canyon Springs Member of the Sundance Formation, Lower Triassic and Upper Permian Lykins Formation, Permian Lyons Sandstone, and Permian and Pennsylvanian Fountain Formation before we cross into Precambrian crystalline basement rocks. Beginning as soon as we turn onto Lefthand Canyon Drive and continuing throughout this leg of our journey, evidence of the great 2013 Colorado Front Range flood is visible at many locations as we travel along Lefthand Creek.

The Dakota hogback is the first large ridge we cut through as we travel west on Lefthand Canyon Drive. The non-marine Upper Jurassic Morrison Formation, known across the U.S. Western Interior region for its multi-colored mudrocks and dinosaur bones, lies in the slope on the west side of the hogback. Here, the Morrison Formation is ~76-97 m (250-320 ft) thick (Wrucke and Wilson, 1967). It records deposition in low-relief topography dotted by lakes and crossed by sluggish meandering streams (Oriel and Craig, 1960) in a warm, humid climate. One lake, now represented by a limestone bed, may have stretched from south of Denver into Boulder County (Peterson and Turner, 1998). Uplifts in western Utah were source areas for the sediment in the Morrison Formation (Peterson, 1994).

Stratigraphically just below the Morrison Formation is a thin, very light-gray, fine- to very fine-grained sandstone, the Middle Jurassic Canyon Springs Member of the Sundance Formation. In older literature, this sandstone was assigned to the Entrada Formation, but later research indicates that the sand source that fed the Canyon Springs deposits was connected to a younger set of marine and eolian environments that produced the Sundance Formation to the north (Pipiringos and O’Sullivan, 1976; Berman et al., 1980). The Canyon Springs Member thins southward from Wyoming and pinches out ~3 km (2 mi) south of Six-Mile Fold Open Space. The Canyon Springs sandstone was once known informally as the Dr. Bond sandstone (Chronic, 1957b) after a local physician who tried to quarry it for building stone; it proved to be too friable.

The strike valley west of the Dakota hogback is underlain by the Upper Permian and Lower Triassic Lykins Formation, which is 152-195 m (500-639 ft) thick in the area (Wrucke and Wilson, 1967). The Lykins Formation is predominantly orange- to brick-red mudrock and very fine-grained sandstone; at least two regionally persistent limestone beds lie within the Permian portion of the formation. One of these, the Upper Permian Forelle Limestone Member, is particularly well known for its lateral persistence and for the crinkled nature of its laminae. These are stro-matolitic forms produced by cyanobacteria (blue-green algae) that created calcite and trapped siliciclastic grains. Deposition in the Lykins Formation took place in tropical low-lying sabkhas, tidal flats, mudflats, tidal streams, and coastal settings along the margin of a shallow sea. The most profound mass extinction in Earth’s history occurred at the close of the Permian Period, during the time spanned by the Lykins Formation. During deposition of the upper portion of the Lykins Formation, the supercontinent of Pangaea was beginning to rift apart. The Lykins Formation takes its name from Lykins Gulch, located ~3.2 km (2 mi) north of our location on Lefthand Creek.

West of the Lykins strike valley, Lefthand Canyon Drive cuts through a second major ridge built from the resistant beds of the Lower Permian Lyons Sandstone, named for the town of Lyons, Colorado, a few kilometers to the north. The Lyons Sandstone in the area of our field trip is known for its large-scale, steeply dipping cross-bedding; its resistant nature; and the manner in which it splits into flat slabs along its bedding planes. The last two of these characteristics make it a sought-after dimension stone. Although in some areas to the south, particularly near the town of Morrison in Jefferson County, the Lyons Sandstone has fluvial characteristics (Weimer and Erickson, 1976), in Boulder County and adjacent areas to the north, it is predominantly eolian (Walker and Harms, 1972). High-angle cross-bedding with tangential toes, sedimentary features interpreted as sand avalanches, animal tracks, and raindrop impressions are among the properties that identify the Lyons Sandstone as eolian (Walker and Harms, 1972.) Dendrites, diagenetic precipitates of manganese oxide that branch in fern-like patterns along bedding planes, are an additional feature of the Lyons Sandstone. In this area, the Lyons Sandstone is ~76 m (250 ft) thick (Wrucke and Wilson, 1967).

West of the Lyons Sandstone ridge, Lefthand Creek runs along a weakly cemented zone in the Permian and Pennsylva-nian Fountain Formation. The Fountain Formation here is ~274-290 m (900-950 ft) thick (Wrucke and Wilson, 1967). It consists predominantly of purple-red, red, and gray arkosic conglomerate, sandstone, and mudrock deposited in braided alluvial systems along the eastern flank of the Ancestral Rocky Mountains, which rose during the assemblage of the supercontinent Pangaea ca. 315-300 Ma (Weimer and Le Roy, 1987). Some of the red color of the Fountain Formation is due to pink potassium feldspars derived from its granitic source areas, but the majority is due to iron oxide, present primarily in the matrix clays of the sedimentary rocks. The iron oxides developed authigenically from the decomposition of iron-bearing silicate minerals derived from Precambrian source rocks (Walker, 1967).

Just past Buckingham Park, which is a City of Boulder Open Space and Mountain Parks property, we encounter Colorado’s Great Unconformity, where the 300-m.y.-old Fountain Formation lies nonconformably on top of 1.7-b.y.-old schists and gneisses of the Idaho Springs Formation. While this conjunction of metamorphics and the Fountain conglomerates is not as well exposed here as it is farther south in Boulder on Flagstaff Mountain or at Red Rocks Park, differences in morphology and weathering characteristics between the two units are readily apparent. As we continue into the mountains, the Precambrian crystalline rocks are occasionally dissected by quartz veins. Our second stop will be exactly 7.2 km (4.5 mi) after our turn onto Lefthand Canyon Drive.

Geologic Features

Reference map: U.S. Geological Survey geologic map: Boulder quadrangle (Wrucke and Wilson, 1967).

Occasionally the previously observed prominent quartz veins have pegmatitic characteristics. A pegmatite is a very coarse-grained igneous system, usually of granitic composition, with distinct crystals of several centimeters in size. Next to common rock-forming minerals, such as quartz, feldspar, and micas, which are often mined as industrial source materials, pegmatites can also be major sources of gems (e.g., fine tourmalines, Pala District, California) and ore materials (e.g., spodumene, a Li-bearing pyroxene, Black Hills, South Dakota) (London and Kon-tak, 2012). The size of individual crystals can be staggering. Shigley and Kampf (1984) give examples of 14-m-long spodumene crystals from South Dakota, an 18-m-long beryl from Madagascar, and a 300-kg, gem-quality topaz from Minas Gerais, Brazil. The reason for such abnormal sizes spawned many debates over the past century. The idea of aqueous separation from a magma melt with highly reduced viscosity allowing for easier and larger growth of minerals was the prevailing hypothesis (London and Morgan, 2012). However, new evidence suggests a flux composed of high Cs and F concentrations located between the growing minerals and the bulk melt. In laboratory experiments, this combination, in conjunction with cooling, produces larger than normal crystal growth (London and Morgan, 2012).

An outcrop to the north of Stop 2 shows a 1-2-m-wide granitic pegmatite dike invading Precambrian gneisses and schists (Fig. 3). Crystallization follows the discontinuous branch of Bowen’s reaction series with small, <1-cm biotites occurring at the fringe of the vein, indicating possible contact metasomatism. Orthoclase crystals from 3 to 30 cm in size interfingered with muscovite booklets of up to 15 cm in diameter occupy the outside of the vein toward the host rock. The central 10-40 cm of the vein shows massive white quartz, the last product to crystallize according to Bowen’s reaction series. Specimens may be collected from the dike.

Figure 3.

Idealized representation of a granitic pegmatite vein cutting through metamorphic host rock as found at Stop 2. Mineral growth process follows Bowen’s reaction series as indicated and can be observed in the field.

Figure 3.

Idealized representation of a granitic pegmatite vein cutting through metamorphic host rock as found at Stop 2. Mineral growth process follows Bowen’s reaction series as indicated and can be observed in the field.

Just 5 m to the west of the pegmatite dike is a 3-m-wide yellowish to yellowish-brown discolored vertical segment that exhibits highly decomposed, friable metamorphic crystalline rock. This discolored band is the first indication of the much younger Paleogene hydrothermal mineralization that produced the Colorado Mineral Belt, a predominantly SW-NE-trending system that contains most of the state’s mineralization. Early miners used host rock discoloration as an indicator during their hunt for precious commodities. Tunneling into these systems often produced no minerals of value and the project was abandoned after only a few meters, leaving the historic mining districts dotted with blind adits and small tailings piles. However, occasionally economically feasible deposits were discovered and larger scale mining operations ensued. Stop 3 will show a much larger-scale hydrothermal alteration.

Stop 3: Hydrothermal Mineralization—James Canyon Drive/Mills Street (N40°06.50910’, W105°22.34442) Drive time: 7 min; stop duration: 10 min

Traveling to Stop 3

The drive to Stop 3 is 4.3 km (2.7 mi) long and continues to show exposures of Paleoproterozoic biotitic gneisses, schists, and occasional migmatites. After approximately 1 km (0.6 mi), we stay to the right to continue on James Canyon Drive toward Jamestown. About halfway to Stop 3, the crystalline lithology changes to Mesoproterozoic granitic rocks.

Geologic Features

Reference map: U.S. Geological Survey geologic map: Boulder quadrangle (Wrucke and Wilson, 1967).

During this short stop, we will investigate hydrothermal mineralization associated with the Paleogene induction of minerals within the Colorado Mineral Belt. Hot mineralized fluids inundated fractured rocks and deposited a variety of minerals. Types of mineralization within the Jamestown area of the Colorado Mineral Belt include: fluorite deposits, lead-silver mineralization, telluride veins, and pyritic gold veins (Lovering and Goddard, 1950; Kelly and Goddard, 1969). At Stop 3, finely disseminated pyrite crystals of up to 1 mm can be observed within a 1-2-m- (3-6-ft-) wide set of vertical fractures. As previously explained, the typical yellow-red discoloration of Fe mineralization observed in this outcrop is a telltale clue used by ancient miners to find possible ore deposits.

Small fractures in the rock are often outlined by a light-gray, 3-mm-wide alteration rim. These stains are relics of sulfuric acid etching that forms as a decompositional byproduct during the chemical weathering of pyrite. This strong acid is the main contributor to Colorado’s acid mine drainage problem and the pH of surface waters can be very low. In 2000, Metropolitan State University of Denver students measured a pH value of 2.6 in one of the surface water puddles in the vicinity of Jamestown, Colorado, during a field segment of their introductory environmental science course.

Stop 4: Burlington Fluorite Mine—James Canyon Drive/Overland Road and CR87 (N40°07.71534’, W105°24.03828’) Drive time: 7 min; stop duration: 35 min

Traveling to Stop 4

Reference maps: U.S. Geological Survey geologic maps: Boulder quadrangle (Wrucke and Wilson, 1967); Gold Hill quadrangle (Gable, 1980).

Another short drive of 3.7 km (2.3 mi) to our next destination traverses the same 1.4-b.y.-old granitic rocks previously experienced. Our trip will extend through historic Jamestown, Colorado, founded in 1883 after a gold discovery at nearby Jim Creek brought settlers to the area. Jamestown was especially hard hit during the 2013 floods with relic evidence present everywhere. In Jamestown, we continue straight on James Canyon Drive, which becomes Overland Road west of Jamestown.

Jamestown sits within Cretaceous to Paleogene intrusive rocks, mainly felsic to intermediate in composition, and associated with the Laramide orogeny. At the contact of the younger intrusives and the older granites we have reached our destination, the Burlington fluorite mine. The mine site is approximately 300 m (985 ft) south of the intersection of Overland Road and CR 87.

Geologic Features

Reference map: U.S. Geological Survey geologic map: Gold Hill quadrangle (Gable, 1980).

The Burlington fluorite mine in Jamestown, Colorado, operated from 1942 to 1972. It produced ~700,000 tons of fluorite ore as flux for iron smelting and was the largest fluorite producer in Colorado (Cowart and Levin, 2004). Figure 4 illustrates the mineralization process. About 65 Ma, a hornblende granodiorite invaded the Precambrian metamorphic rocks, causing some fracturing and brecciation of the overlying lithologies, but little to no mineralization. This changed during the early Eocene (5654 Ma), when an alkali granite quartz monzonite invaded the same area, refracturing the same lithologies and introducing large amounts of mineral-laden hydrothermal fluids, which produced fluorite-bearing silicified zones within the monzonite stock (Worl et al., 1976). Some previously established brecciated zones show subdued fluorite mineralization. Wallace (2010) gives the fluorite mineralization temperature in the Jamestown district as 250° to 375 °C and saturated salinities of the hydrothermal brines between 26-50%. Erosion to the present level exposed the mineralized zone.

Figure 4.

Stylized graphic showing the mineralization process within the Jamestown District, Colorado, as seen at Stop 4. Adapted from anonymous author (n.d.).

Figure 4.

Stylized graphic showing the mineralization process within the Jamestown District, Colorado, as seen at Stop 4. Adapted from anonymous author (n.d.).

The Overland Road west of Jamestown is built right on the old mine tailings of the Burlington Mine and nice fluorite specimens can be collected right at the side of the road within these tailings. The fluorite mineral decomposes rather quickly when exposed to the elements. Rich ore specimens are often dark black on the outside. By cracking these rocks with a hammer, an often gemmy interior with blue to purple, translucent to transparent crystals is revealed. Occasional cubic pyrite of up to 2 cm and similar galena cubes can be found embedded in the fluorite. To collect samples from abandoned mine sites, obtain permission from the mining claim owners before visiting the site.

On the southwest side of Overland Road ~300 m (985 ft) south of our stop, relics of a collapsed mine shaft can be observed. Water percolating through the Burlington Mine glory hole above becomes acidic and depleted in oxygen and will mobilize iron. As these Fe-rich waters slowly exit the collapsed mine shaft structure, oxidation occurs immediately and results in the in situ formation of small hematite terraces a few cm high and up to 30 cm wide. Iron oxide will also precipitate on the surface of small puddles, creating an iridescent oil-like film on the water.

Stop 5: Dacite Columnar Jointing—Hwy. 7/S St. Vrain Drive (N40°12.06798’, W105°18.26886) Drive time: 40 min; stop duration: 10 min

Traveling to Stop 5

Reference maps: U.S. Geological Survey geologic maps: Gold Hill quadrangle (Gable, 1980); Estes Park quadrangle (Cole and Braddock, 2009); Lyons quadrangle (Braddock et al., 1988b).

We continue westward on Overland Road for 8.5 km (5.3 mi) to Hwy. 72 W, traversing the Silver Plume Granite (Yg, ~1400 m.y.) and Boulder Creek Granodiorite (Xg, ~1700 m.y.) Turning north on Hwy. 72 W, we follow 9 km (5.6 mi) along one of the renowned scenic routes in the Colorado Rockies, the famous Peak-to-Peak Scenic Byway, thrilling travelers since 1918. To the west, the glacial geomorphology of the Rocky Mountains comes into view. Cirques, arêtes, and moraines of various types can be seen on the eastern slope of the high mountains along the Continental Divide. A small segment of the road is built on Quaternary glacial drift (Qd) deposits. As we turn east onto Hwy. 7, we continue through the crystalline interior, predominately Mesopro-terozoic granites, toward Lyons, Colorado, where we will reach Stop 5 after 17.7 km (11.5 mi).

Geologic Features

Reference map: U.S. Geological Survey geologic map: Lyons quadrangle (Braddock et al., 1988b).

The outcrop of Paleocene dacite exhibits columnar jointing. The light-gray fine-grained porphyritic dacite intrudes sedimentary Fountain Formation at this site (Braddock et al., 1988a). The age of the intrusion calculated from zircon fission tracks is 62.2 ± 3 Ma (Hoblitt and Larson, 1975). This is the same age as sills that intrude the Permian and Triassic Lykins Formation at Boulder County Heil Valley Ranch Open Space ~5.5 km (3.5 mi) to the south. The dacite dikes and sills weather into rounded, greenish-gray blocks. Dacite at this location was quarried for aggregate, although it is referred to as an andesite quarry in earlier reports (e.g., Schwochow et al., 1974). Boulder County Open Space now owns the quarry site; a bridge connecting the quarry road to Hwy. 7 was washed away during the 2013 Front Range flood.

Stop 6: Carter Lake Syncline Waypoint—S. County Road 31 (N 40°20.78783’, W 105°12.52667’) Drive time: 35 min; stop duration: 5 min

Traveling to Stop 6

Reference maps: U.S. Geological Survey geologic maps: Lyons quadrangle (Braddock et al., 1988b); Hygiene quadrangle (Madole et al., 1998); Carter Lake Reservoir quadrangle (Braddock et al., 1988c).

We continue our travel along Hwy. 7 toward the historic town of Lyons, Colorado, following the South St. Vrain River for 4.3 km (2.7 mi). More evidence of the 2013 flooding along the Front Range is apparent along the river. In this short segment, we traverse the Late Paleozoic units of the Fountain and Lyons Formations. The silica-cemented, highly durable Lyons Sandstone is quarried as ornamental and building material in this vicinity, and slabs of flagstone are visible at several stone yards as we pass through town. Turning eastward in Lyons we follow CO Hwy. 66 for 8.7 km (5.4 mi) to N 75th St., crossing the Permian, Triassic, Jurassic, and Lower Cretaceous lithostratigraphic units before we leave Lyons. About 1.6 km (1 mi) east of town, the asymmetrical anticlinal fold of Rabbit Mountain is visible to the north (Boulder County Parks and Open Space). The resistant hogbacks formed by the Cretaceous Dakota Group beautifully outline this structure. Rabbit Mountain is one of a series of en echelon folds (Cole and Braddock, 2009) that we will see as we travel to Carter Lake (Stop 6) and Devil’s Backbone (Stop 7). Turning north on N 75th St. near Hygiene, Colorado, the steep east flank of the anticline can be seen toward the west as we travel 3.2 km (2 mi) toward a sharp eastward turn onto Woodland Road. During this short stretch, we are traveling over Upper Cretaceous Pierre Shale. Woodland Road will take us in an easterly 1.4 km (0.9 mi) jog to N 83rd St, where we continue our journey northward for 3.4 km (2.1 mi), still within the Pierre Shale. At the border between Boulder and Larimer Counties, the road changes to S County Road 23E as we continue northward on Pierre Shale for another 5.5 km (3.4 mi), including a 0.6 km (0.4 mi) eastward jog, where the road becomes N County Road 23E. We continue for 0.8 km (0.5 mi) north on N County Road 23E, then turn west on W County Road 8E toward Carter Lake for 5.6 km (3.5 mi), crossing successively older Mesozoic units to the Jurassic Morrison Formation. We turn north on S County Road 31 for 3.7 km (2.3 mi) and reach Stop 6 for a short structural and geologic overview.

As we progress along our route to the north, we encounter Permian lithostratigraphic units between the Fountain Formation and the Lyons Sandstone that were not present closer to Boulder. This is because the direction of our traverse is at an angle to the strike of Permian depositional facies so that, as we continue farther north, we encounter Permian strata deposited farther and farther from the front of the Ancestral Rockies. The Permian Ingleside Formation, for example, lies stratigraphically above the Fountain Formation. The Ingleside thickens from a feather edge north of Lefthand Canyon to 61 m (200 ft) at Carter Lake (Brad-dock et al., 1988c) and to 79 m (260 ft) at its type section 4.8 km (3 mi) south of Stop 9 at Owl Canyon (Maughan and Wilson, 1960.) In addition, north of Lefthand Canyon, the Ingleside is fluvial, at Lyons it is fluvial and eolian, at Carter Lake it is coastal to shallow marine, and at Owl Canyon it is shallow marine to deeper marine (Maughan, 1980).

Geologic Features

Reference map: U.S. Geological Survey geologic map: Carter Lake Reservoir quadrangle (Braddock et al., 1988c).

The Colorado-Big Thompson project annually diverts more than 250,000 acre-feet of water from Colorado’s western slope to the drier and more populated Front Range for municipal, industrial, and agricultural use (Northern Colorado Water Conservancy District, n.d., Colorado-Big Thompson Project). Water enters the 21 km (13.1 mi) Alva B. Adams Tunnel at Grand Lake; flows by gravity beneath the Continental Divide to the Estes Park area; and enters a series of pipelines, canals, tunnels, and reservoirs, including Carter Lake reservoir, that transport it down the Big Thompson drainage and distribute it to Front Range communities (Northern Colorado Water Conservancy District, n.d., ColoradoBig Thompson Project).

Three dams control the flow out of Carter Lake reservoir; the largest is a 65-m- (214-ft-) high earth-fill dam (U.S. Bureau of Reclamation, n.d.) across a natural drainage gap in the Dakota hogback on the east side of the reservoir (Braddock et al., 1988c). The abutments for this dam are in Dakota Group sandstone and shale, and the foundation is on Morrison Formation shale and limestone and Sundance Formation sandstone, all of which dip ~15° downstream (U.S. Bureau of Reclamation, n.d.).

The second dam, located less than 1.6 km (<1 mile) north of the first, lies across a folded and faulted offset in the trend of the Dakota hogback on the steeply southwest-dipping fold limb shared by the Carter Lake anticline to the east and the Carter Lake syncline to the west (Braddock et al., 1988c). The west dam abutment is on Morrison Formation dipping 45°SW, and the east abutment is on Lykins Formation dipping 70°SW; the dam foundation rests on steeply dipping strata in the Lykins, Sundance, and Morrison Formations (U.S. Bureau of Reclamation, n.d.). Some beds within both the Morrison and Lykins Formations are susceptible to dissolution, including limestone beds in both formations and gypsum beds up to 15 m (50 ft) thick that occur in the Lykins Formation along the Front Range (White, 2012).

The third dam, at the northeast end of the lake, abuts and is founded on Lyons Sandstone (U.S. Bureau of Reclamation, n.d.).

Small quarries lie in the Lyons, Dakota, and Owl Canyon Formations at various points around the lake (Schwochow et al., 1974; Braddock et al., 1988c; Colorado Division of Reclamation Mining and Safety, n.d.).

Stop 7: Devil’s Backbone Open Space (N 40°24.69817’, W 105°09.14100) Drive time: 18 min; stop duration: 50 min

Traveling to Stop 7

Reference maps: U.S. Geological Survey geologic maps: Carter Lake Reservoir quadrangle (Braddock et al., 1988c); Masonville quadrangle (Braddock et al., 1970).

We continue northward on S County Road 31 for 4.2 km (2.6 mi), traversing at first the Triassic to Permian red siltstones, shales, and sandstones of the Jelm, Lykins, Lyons, and Ingleside Formations and continuing into the Permian to Pennsylvanian arkosic conglomerates of the Fountain Formation. Here the road makes a right or easterly turn and changes into W County Road 18 E/Pole Hill Road, which we will follow for 3.2 additional km (2 mi). As we travel back into the Triassic to Permian strata, we see the nose of an anticlinal fold when looking toward the south. Traveling within these lithologies, we turn north onto S County Road 29, which we follow for 3.4 km (2.1 mi) to U.S. 34. Turning east, the next 5 km (3.1 mi) on U.S. 34 will take us up the stratigraphic section into the Cretaceous Benton and Niobrara Formations. It is noteworthy that we cross a fault line ~0.2 km (0.13 mi) after the turn. This predominately normal fault with the footwall to the northeast has an apparent right lateral strike-slip component of roughly 1 km (0.62 mi) of offset. From U.S. 34, we turn north on Hidden Valley Drive and travel 0.5 km (0.3 mi) to the Larimer County Devil’s Backbone Open Space parking lot. Here we will stop for lunch and geologic sightseeing.

Geologic Features

Reference map: U.S. Geological Survey geologic map: Mason-ville quadrangle (Braddock et al., 1970).

This 50 min stop includes our sack lunch break. Drinking water and maintained vault toilets are available.

Devil’s Backbone is one of the prominent and well-visited landmarks within Larimer County, Colorado. We will take a leisurely stroll along the 1.3 km (0.8 mi) Morrison Loop, which doubles as a geologic nature trail to the south of the parking lot. At the highest point of the loop, the lithology of Devil’s Backbone can be investigated. The resistant hogback consists of silica-cemented, well-rounded Cretaceous Dakota conglomerate and comprises the steeply westward-dipping limb (~80°) of an asymmetrical anticline. The impressive structure reaches heights of 67 m (220 ft) above the valley floor. Looking due east from the visited outcrop, at a distance of 780 m (2560 ft) the eastern flank of the Dakota conglomerate dips a gentle 15° to the east. Scanning the landscape toward the north, the extent of the anticline can be seen as illustrated in the 3D adapted Google Earth rendering of Figure 5. The Mariana Butte Golf Course is nestled in the nose of this plunging anticline.

Figure 5.

3D topographic rendering of Devil’s Backbone and the superimposed eroded lithology of an asymmetrical anticline as found at Stop 7. View is to the north. Adapted from Google Earth (2013).

Figure 5.

3D topographic rendering of Devil’s Backbone and the superimposed eroded lithology of an asymmetrical anticline as found at Stop 7. View is to the north. Adapted from Google Earth (2013).

A 7-m-wide and 40-m-long trench just to the north of the trail loop exposes the underlying Morrison Formation. This historic relic at Devil’s Backbone Open Space bears witness to the dinosaur excavation expeditions in the late nineteenth century that eventually led to the so-called Bone Wars, which we discuss at our next stop.

On our way back to the parking lot, we come across the remnants of an old plaster factory. In 1880, Alfred Wild, a Loveland native and industrialist, discovered high-grade gypsum while digging a ditch in the vicinity. After some experimentation with the newly discovered resource, Wild established the first plaster factory in Colorado and became known as the plaster king, making millions in today’s dollars. The factory was in operation until 1965 when it was destroyed by flooding.

Wild also mined rich clay deposits at Devil’s Backbone, which were used to fire quality bricks. The clay pit is easily visible to the west of the parking lot. Some remaining inventory of Wild’s brick operations was used to pave Open Space trail rest stops with this historic brick flooring.

Stop 8: Dinosaur Bones, Morrison Formation— U.S. 287 Roadcut, N. Fort Collins, Colorado (N 40°40.38050’, W 105°11.35533) Drive time: 35 min; stop duration: 20 min

Traveling to Stop 8

Reference maps: U.S. Geological Survey geologic maps: Mason-ville quadrangle (Braddock et al., 1970); Horsetooth Reservoir quadrangle (Braddock et al., 1989a); Laporte quadrangle (Braddock et al., 1988a).

As we leave Devil’s Backbone Open Space, we turn east on U.S. 34 and travel for 3.4 km (2.1 mi) into Loveland, Colorado, to Wilson Avenue. During this short trip, we traverse an asymmetrical anticline with a gently dipping (~30°) east limb and turn onto Wilson Avenue within the lower unit of the Cretaceous Pierre Shale. Continuing in this shale unit, we follow Wilson Avenue northward for 5 km (3.1 mi). Here the road changes to Taft Hill Road, which we will follow in the same direction and within the same geologic strata for another 19.3 km (12 mi), criss-crossing a few Quaternary deposits. To the west, majestic views of hogbacks of the Dakota Group and the Fountain Formation with the crystalline Rockies rising beyond are visible at several locations. A short 0.6 km (0.4 mi) jog to the east along bypass U.S. 287B/CR 54G gets us to the major U.S. Hwy. 287, which we follow northwest for 10.8 km (6.7 mi) to our destination. Here, we cross back into the Cretaceous Benton and Niobrara Formations through an area of anticlines and syn-clines in the Cretaceous Dakota Group; Jurassic Morrison and Sundance Formations; and Triassic and Permian Jelm, Lykins, Lyons, and Ingleside Formations (Braddock et al., 1988a). The quarry east of U.S. 287 as we leave Fort Collins, Colorado, is in the Niobrara Formation (Braddock and Connor, 1988). It is the old Ideal Cement Plant, which opened in 1927 and used the dry process for cement production in order to save water; the plant ceased operations in 2003, under the ownership of Holcim, Inc. (Workman, n.d.). Field Stop 8 is a roadcut within the Jurassic Morrison Formation.

Geologic Features

Reference map: U.S. Geological Survey geologic map: Laporte quadrangle (Braddock et al., 1988b).

The Upper Jurassic Morrison Formation became famous through dinosaurian fossil discoveries in 1877 by Arthur Lakes near Morrison, Colorado. An interesting historic side note is an escalation of the famous fossil feud or the Bone Wars the same year between Edward Drinker Cope (18401897) and Othniel Charles Marsh (1831-1899) in Colorado. According to Noel (2015), Lakes contacted Marsh and informed him of dinosaur bone finds within the Morrison Formation near Cañon City, Colorado. When Marsh did not respond, Lakes turned his attention to Cope, who answered quickly, drawing attention to the discoveries. Marsh realized that he had neglected a scientific bonanza and immediately followed Cope to Colorado, mounting his own expedition. This was the inception of the famous dinosaur rush to Colorado and intensified digging for prized vertebrate specimens. Cope and Marsh actively sabotaged each other’s digs and expeditions as both were trying to out-do each other for scientific discoveries (Wallace, 1999).

The Morrison Formation has ever since been a prime location for discoveries of dinosaurs, and bones or bone fragments are much more prolific than one might anticipate. This outcrop contains multi-colored blocky-weathering claystone and siltstone, lenses of gray micritic limestone, and fine- to medium-grained sandstone with dinosaur bone fragments. The Morrison Formation here is 98 m (320 ft) thick (Brad-dock and Connor, 1988).

Stop 9: Owl Canyon Roadcut, Ingleside Formation— U.S. 287 and Owl Canyon Road (N 40°45.77900’, W 105°10.87100’) Drive time: 7 min; stop duration: 15 min

Traveling to Stop 9

Reference maps: U.S. Geological Survey geologic maps: Laporte quadrangle (Braddock et al., 1988a); Livermore quadrangle (Braddock et al., 1988d).

Leaving Stop 8 we continue northward on U.S. 287 within the red, fine-grained lithologies of the Triassic to Permian Lykins Formation for a short 11-km (6.9-mi) drive to the Owl Canyon roadcut. While driving along the Lykins strike valley, the tall cliff to the east is capped by eastward-dipping strata of the resistant Dakota Group. Next to the highway on the west side is a much smaller hogback supported by a prominent limestone bed in the Lykins Formation. The more distant and taller hogbacks to the west are the interbedded limestones and sandstones of the Permian Ingleside Formation. The valley we drive up is the proposed site of the 170,000 acre-foot Glade Reservoir, part of a water-storage project called the Northern Integrated Supply Project. A map of the proposed realignment of U.S. 287 has been drafted (Northern Colorado Water Conservancy District, n.d., U.S. 287 Realignment-Glade Reservoir).

Geologic Features

Reference map: U.S. Geological Survey geologic map: Liver-more quadrangle (Braddock et al., 1988d).

As we reach our destination, U.S. 287 curves to the west until it intersects east-dipping Ingleside strata. The Ingleside Formation at this location contains marine fusulinids of Early Permian age (Maughan and Wilson, 1960), oolitic sandstone (Chronic, 1957a), and carbonate beds with solution breccia, brachiopods, and cubic impressions made by salt crystals (Maughan, 1980). Quarries in the area around Owl Canyon produce limestone from the Ingleside Formation (Schwochow et al., 1974; Braddock et al., 1988d). The limestone produced from the Colorado Lien Company quarry is pure enough to be used as animal feed supplements and to help make Coors beer bottles.

Stratigraphically above the Ingleside Formation is the Permian Owl Canyon Formation, referred to in older literature as the Satanka Formation (e.g., Lee, 1927). The Owl Canyon consists of interbedded sandstone, limestone, limey sandstone, and shale, mostly ranging in color from gray to light pink to red (Lee, 1927). A limestone bed ~20 m (65 ft) below the top of the formation contains abundant solution cavities, many of them partially filled with dogtooth spar calcite crystals. The Owl Canyon Formation is ~61 m (200 ft) thick at the type section (Chronic, 1957a), but is as much as 107 m (350 ft) thick elsewhere in the area (Braddock et al., 1988d). The Owl Canyon thins to the south and is not present as far south as Lyons, Colorado.

The Owl Canyon is overlain by the Permian Lyons Sandstone, which consists of cross-bedded quartzose sandstone and is ~70 m (230 ft) thick (Chronic, 1957a). The Lyons Sandstone is stratigraphically overlain by the Permian and Triassic Lykins Formation, which is 265 m (870 ft) thick (Chronic, 1957a). The Lykins Formation is quarried for gypsum (used for cement) and limestone in the region (Colorado Division of Reclamation Mining Safety, n.d.; Schwochow et al., 1974).

The Lykins Formation is overlain by the Triassic Jelm Formation, which is in turn overlain by the Canyon Springs Member of the Sundance Formation. The Jelm Formation consists of cross-bedded, hard red sandstone, and the Canyon Springs Member consists of massive to cross-bedded, gray to pink sandstone; both units are dominated by rounded, well-sorted, frosted quartz sand grains probably of eolian origin (Chronic, 1957a). The Jelm Formation is 21 m (70 ft) thick, and the Jurassic Canyon Springs Member of the Sundance Formation is 15 m (50 ft) thick (Chronic, 1957a). Both units thin southward; the Jelm pinches out south of Lyons, Colorado (Chronic, 1957b), and the Canyon Springs pinches out ~3.2 km (2 mi) south of Six-Mile Fold Open Space (Stop 1).

Stop 10: Ring-Dike Complex—U.S. 287, Virginia Dale, Colorado (N 40°56.748’, W 105°20.596) Drive time: 20 min; stop duration: 20 min

Traveling to Stop 10

Reference maps: U.S. Geological Survey geologic maps: Liver-more Mountain quadrangle (Braddock and Connor, 1988); Virginia Dale quadrangle (Braddock et al., 1989b).

Our route to the famous Virginia Dale ring-dike complex is highlighted in Figure 6. We follow U.S. 287 northwestward toward the Colorado-Wyoming border for 26.2 km (16.3 mi) to the vicinity of the small community of Virginia Dale, Colorado. The first 8 km (5 mi) of this journey are within the Permian Ingleside and Permian to Pennsylvanian Fountain Formations, but rounded outcrops of granitic rock and occasional metamor-phic crystalline rock become more prevalent as we travel northward. Eventually, we cross into Paleoproterozoic felsic and hornblendic, interlayered gneisses, and then progress into Meso-proterozoic granitic rocks. Just before we reach our destination, the famous Virginia Dale ring-dike complex, we again encounter the Fountain Formation.

Figure 6.

Geologic map for Stops 9, 10, and 11, as indicated on map by respective numbers. Stops 10 and 11 are within the Virginia Dale ring-dike complex. See Table 1 for explanation of geologic unit symbols. Compiled from Google Earth (2013), U.S. Geological Survey Cogeol.kml (Geologic Units of Colorado) file (Stoeser et al., 2005), and Cappa (1998).

Figure 6.

Geologic map for Stops 9, 10, and 11, as indicated on map by respective numbers. Stops 10 and 11 are within the Virginia Dale ring-dike complex. See Table 1 for explanation of geologic unit symbols. Compiled from Google Earth (2013), U.S. Geological Survey Cogeol.kml (Geologic Units of Colorado) file (Stoeser et al., 2005), and Cappa (1998).

Geologic Features

Reference map: U.S. Geological Survey geologic map: Virginia Dale quadrangle (Braddock et al., 1989b); see also http://ngmdb.usgs.gov/Prodesc/proddesc_1140.htm.

The Virginia Dale ring-dike complex is a unique structure, measuring ~15 km in diameter. Coexisting magmas of mafic and felsic origin have created a hybrid mixing pattern that can be easily seen in the U.S. 287 roadcut and adjacent quarry. The 1.4 Ga ring dike invaded metamorphic rocks that originated ca. 1.8 Ga as a result of plate subduction to the south of the Wyoming Craton (Vasek and Kolker, 1999). Ring dikes have steeply dipping sides and are formed when segregated magma chambers experience a roof collapse of the host lithology as illustrated in Figure 7.

Figure 7.

Schematic diagram of ring-dike formation showing early fracture patterns during emplacement (A) and resulting magma chamber roof collapse with dike formation (B). Present erosion surface indicative of visible ring-dike geology as seen at Stop 10. Modified after Billings (1972).

Figure 7.

Schematic diagram of ring-dike formation showing early fracture patterns during emplacement (A) and resulting magma chamber roof collapse with dike formation (B). Present erosion surface indicative of visible ring-dike geology as seen at Stop 10. Modified after Billings (1972).

The Virginia Dale ring dike is more or less concentric, with the Log Cabin Granite Stock (Ylc) at its center. Eggler (1968) describes the Log Cabin Granite as a fine-grained granite with abundant muscovite and multiple aplitic dikes, most likely being younger than the ring-dike complex itself. The central bulk of the ring structure is composed of two members of the Sherman Granite, the Cap Rock Quartz Monzonite (Yscr) and the further outlying Trail Creek Granite, which is in sharp contact with any surrounding rock (Eggler, 1968). The Cap Rock Quartz Monzo-nite is further subdivided into the Inner and Outer Cap Rock with the latter having smaller phenocrysts (Eggler, 1968). Of interest is the area of our field stop at the southern end of the complex, where a mafic igneous rock in the form of an aplitic diorite separates the Outer quartz monzonite and the Trail Creek Granite. Here, the Outer Cap Rock invades a mafic system and generates hybrid igneous rocks with varying granodioritic tendencies as described by Eggler (1968). The mixing and hybridization is very prominent, as the mafic host was near its solidus, creating a blocky, distinct appearance (Fig. 8). The resulting rock patterns are very decorative and can easily be collected at this road stop.

Figure 8.

Photo showing the intermingling of felsic and mafic magmas at Stop 10 within the famous Virginia Dale ring-dike complex at a roadside quarry on the west side of U.S. 287 ~1.6 km (1 mi) south of Virginia Dale, Colorado.

Figure 8.

Photo showing the intermingling of felsic and mafic magmas at Stop 10 within the famous Virginia Dale ring-dike complex at a roadside quarry on the west side of U.S. 287 ~1.6 km (1 mi) south of Virginia Dale, Colorado.

Stop 11: Kimberlite—CR 45E and Moen Ranch Road, Virginia Dale, Colorado (N 40°58.03600’, W 105°23.06033) Drive time: 7 min; stop duration: 15 min

Traveling to Stop 11

Reference maps: U.S. Geological Survey geologic maps: Virginia Dale quadrangle (Braddock et al., 1989b); Cherokee Park quadrangle (Eggler and Braddock, 1988).

The short, last leg of our field excursion proceeds northward along U.S. 287 for 4.8 km (3.0 mi) within the ring-dike complex where we turn west onto CR 45E. After traveling another 1.0 km (0.6 mi) southwest, we reach our final stop, the vicinity of a recently discovered, deeply weathered kimberlite.

Geologic Features

Reference map: U.S. Geological Survey geologic map: Cherokee Park quadrangle (Eggler and Braddock, 1988).

Kimberlites can be classified as the predominant ore rock for diamonds. These ultramafic igneous lithologies originate in the mantle and are carrot shaped in cross section. Their surface exposure can be limited to only a few meters in diameter, tapering downward to smaller diameters toward their point of origin (Fig. 9). Because kimberlites are composed of ultramafic mantle minerals, where phenocrysts of olivine, chromium diopside, pyrope garnets, ilmenite, and phlogopite are disseminated in a fine-grained matrix of serpentine, carbonates, and mantle perovskite (McCallum and Mabarak, 1976), they weather very rapidly when exposed to surface conditions. Thus they become very elusive lithologies and are difficult to discover even for well-trained geologists. In the early 1960s, before they were recognized as kimberlite pipes, the diatremes in this area were first described as unusual features with brecciated, fossil-bearing rock (Chronic and Ferris, 1961). By the mid-1960s, the unusual fossil-bearing structures had been recognized as diatremes (Chronic et al., 1965). Sedimentary strata overlying the Precambrian basement rocks were fragmented during emplacement of the diatremes, and some blocks of the sedimentary strata sank into the kimberlite pipes. Some of these sedimentary xenoliths contain Ordovician and Silurian fossils (Chronic and Ferris, 1961; Chronic et al., 1965) and Cambrian fossils (Ross and Tweto, 1980). The discovery of the fossiliferous blocks changed prevailing views of early Paleozoic paleogeography, because they provided the first evidence that strata of those ages had once existed in the region; the nearest preserved Ordovician strata are more than 160 km (100 mi) to the south, and the nearest preserved Silurian strata are more than 480 km (300 mi) to the north (Chronic et al., 1969).

Figure 9.

Cross-sectional schematic diagram through a kimberlite pipe in the Wyoming-Colorado State Line Kimberlite District. Modified after McCallum and Mabarak (1976).

Figure 9.

Cross-sectional schematic diagram through a kimberlite pipe in the Wyoming-Colorado State Line Kimberlite District. Modified after McCallum and Mabarak (1976).

Diamonds were first identified from the State Line kim-berlites in 1975, when a sand grain-sized diamond scratched a grinding plate being used by a U.S. Geological Survey technician while making a thin section from a garnet peridotite nodule (Colorado Geological Survey, 1999). Following that discovery, it was determined that nearly all of the kimberlite bodies in the State Line district are diamondiferous, but not all contain commercial grade deposits (McCallum and Waldman, 1991). Two kimberlite pipes in the Kelsey Lake kimberlite swarm just south of the Wyoming border were mined between the mid-1990s and 2001. More than 15,000 carats of diamonds were recovered, including a nearly flawless octahedron of 14.2 carats and a yellow diamond of 28.3 carats (Colorado Geological Survey, 1999); Coopersmith et al., 2003). The operations ceased when the mining interests of the owners changed, and reclamation on the mining site was completed in 2003 (Coopersmith et al., 2003).

Kimberlites tend to occur in swarms, and the famous Colorado-Wyoming State Line district has yielded dozens of kimberlite bodies. Occurrence of these ultramafic lithologies appears to follow a northerly trend from Green Mountain near Boulder, Colorado, to the north through the area of Virginia Dale, Colorado, to the Iron Mountain complex in Wyoming. While the majority of the kimberlite pipes in Colorado and Wyoming are believed to be of Devonian age, newer geochronological data suggest ages for some of the pipes of 500-800 Ma; all of the known dates for pipe emplacement fall within periods of relative tectonic inactivity within the area (Lester et al., 2001).

Kimberlites are elusive lithologies and are not easily detected. Weathering can corrode the upper 10 m of a kimberlite into a soft soil cover. Only a slight depression and an unusual soil mineralogy may point to a hidden kimberlite below. Prospecting for kimberlites usually entails mapping the occurrence of KIMs (kimberlite indicator minerals) in the soil, such as resilient red pyrope garnets, hoping that their presence and quantification may point to the discovery of a new pipe. The new kimberlite at Stop 11 was discovered in 2012 by Dr. Uwe Kackstaetter and his Metropolitan State University of Denver mineralogy students by looking at some unusual soil in the area. Both physical testing and normative calculation of the soil geochemistry using SEDMIN, a Microsoft Excel™ spreadsheet for calculating finegrained sedimentary rock mineralogy from bulk geochemical analysis (Kackstaetter, 2014), showed unusually high amounts of clinochlore in the soil material, divergent from the felsic igneous lithologies of the area. Additional verification of the soil material through XRD (X-ray diffraction), optical microscopy, and SEM analytical procedures using TIMA (Tescan Industrial Mineral Analyzer), produced mounting evidence for a weathered zone of a new kimberlite pipe within the State Line district.

While the discovery of a new kimberlite among the over 100 known ones at the Colorado-Wyoming border may not be significant, this new pipe is within a public roadcut and therefore easily accessible. Unfortunately, the majority of known pipes are inaccessible to the public, evermore denying access even for educational purposes. This new discovery has the advantage that it is within the public roadway easement and can be reached by anyone. Thus, awareness of the discovery should protect this site for use by educators.

Acknowledgments

The authors are grateful for the constructive and timely reviews from David Abbott, William Hoyt, and Thomas Van Arsdale that served to greatly improve this paper; errors and omissions that remain are solely ours. We gratefully acknowledge permission from Ronald Blakey to use his © 2006 and © 2013 reconstructions of North American Cretaceous paleogeography. We thank Boulder County Parks and Open Space and Larimer County Parks and Open Lands for access to selected open space sites. We greatly appreciate the assistance of April Leo, GSA managing editor for books, for patiently and efficiently answering our multitude of questions in the process of compiling this paper.

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Figures & Tables

Figure 1.

Stratigraphy and geologic map within the general field-trip area—idealized stratigraphic section in combination with a geologic map showing changes in lithostratigraphy along the field-trip route. Stops 1, 5, 6, 7, 8, and 9 are indicated by circled number inserts on the map. Field stops 2, 3, 4, 10, and 11 are outside the map boundary. Symbols for geologic units used and associated lithologic descriptions are summarized in Table 1. Adapted from Matthews (2004), Google Earth (2013), and U.S. Geological Survey Cogeol.kml (Geologic Units of Colorado) file (Stoeser et al., 2005).

Figure 1.

Stratigraphy and geologic map within the general field-trip area—idealized stratigraphic section in combination with a geologic map showing changes in lithostratigraphy along the field-trip route. Stops 1, 5, 6, 7, 8, and 9 are indicated by circled number inserts on the map. Field stops 2, 3, 4, 10, and 11 are outside the map boundary. Symbols for geologic units used and associated lithologic descriptions are summarized in Table 1. Adapted from Matthews (2004), Google Earth (2013), and U.S. Geological Survey Cogeol.kml (Geologic Units of Colorado) file (Stoeser et al., 2005).

Figure 2.

Map of Late Cretaceous Western Interior epeiric seaway (85 Ma), western hemisphere projection with position of Colorado indicated; globe insert 90 Ma. Map modified from R. Blakey © 2013 and globe © 2006 by Colorado Plateau Geosystems, used with permission.

Figure 2.

Map of Late Cretaceous Western Interior epeiric seaway (85 Ma), western hemisphere projection with position of Colorado indicated; globe insert 90 Ma. Map modified from R. Blakey © 2013 and globe © 2006 by Colorado Plateau Geosystems, used with permission.

Figure 3.

Idealized representation of a granitic pegmatite vein cutting through metamorphic host rock as found at Stop 2. Mineral growth process follows Bowen’s reaction series as indicated and can be observed in the field.

Figure 3.

Idealized representation of a granitic pegmatite vein cutting through metamorphic host rock as found at Stop 2. Mineral growth process follows Bowen’s reaction series as indicated and can be observed in the field.

Figure 4.

Stylized graphic showing the mineralization process within the Jamestown District, Colorado, as seen at Stop 4. Adapted from anonymous author (n.d.).

Figure 4.

Stylized graphic showing the mineralization process within the Jamestown District, Colorado, as seen at Stop 4. Adapted from anonymous author (n.d.).

Figure 5.

3D topographic rendering of Devil’s Backbone and the superimposed eroded lithology of an asymmetrical anticline as found at Stop 7. View is to the north. Adapted from Google Earth (2013).

Figure 5.

3D topographic rendering of Devil’s Backbone and the superimposed eroded lithology of an asymmetrical anticline as found at Stop 7. View is to the north. Adapted from Google Earth (2013).

Figure 6.

Geologic map for Stops 9, 10, and 11, as indicated on map by respective numbers. Stops 10 and 11 are within the Virginia Dale ring-dike complex. See Table 1 for explanation of geologic unit symbols. Compiled from Google Earth (2013), U.S. Geological Survey Cogeol.kml (Geologic Units of Colorado) file (Stoeser et al., 2005), and Cappa (1998).

Figure 6.

Geologic map for Stops 9, 10, and 11, as indicated on map by respective numbers. Stops 10 and 11 are within the Virginia Dale ring-dike complex. See Table 1 for explanation of geologic unit symbols. Compiled from Google Earth (2013), U.S. Geological Survey Cogeol.kml (Geologic Units of Colorado) file (Stoeser et al., 2005), and Cappa (1998).

Figure 7.

Schematic diagram of ring-dike formation showing early fracture patterns during emplacement (A) and resulting magma chamber roof collapse with dike formation (B). Present erosion surface indicative of visible ring-dike geology as seen at Stop 10. Modified after Billings (1972).

Figure 7.

Schematic diagram of ring-dike formation showing early fracture patterns during emplacement (A) and resulting magma chamber roof collapse with dike formation (B). Present erosion surface indicative of visible ring-dike geology as seen at Stop 10. Modified after Billings (1972).

Figure 8.

Photo showing the intermingling of felsic and mafic magmas at Stop 10 within the famous Virginia Dale ring-dike complex at a roadside quarry on the west side of U.S. 287 ~1.6 km (1 mi) south of Virginia Dale, Colorado.

Figure 8.

Photo showing the intermingling of felsic and mafic magmas at Stop 10 within the famous Virginia Dale ring-dike complex at a roadside quarry on the west side of U.S. 287 ~1.6 km (1 mi) south of Virginia Dale, Colorado.

Figure 9.

Cross-sectional schematic diagram through a kimberlite pipe in the Wyoming-Colorado State Line Kimberlite District. Modified after McCallum and Mabarak (1976).

Figure 9.

Cross-sectional schematic diagram through a kimberlite pipe in the Wyoming-Colorado State Line Kimberlite District. Modified after McCallum and Mabarak (1976).

Geologic Symbols and Descriptions of Lithologic Units

Table 1.
Geologic Symbols and Descriptions of Lithologic Units
Abbrev.Description
Sedimentary units
TgvTertiary unconsolidated boulder-rich gravel on old erosion surfaces in Front Range
KpCretaceous Pierre Shale:
Olive-gray and dark-gray shale and silty shale, organic-rich in part; numerous bentonite beds; five sandstone intervals in middle part of unit
KnbCretaceous Niobrara and Benton Formations, undivided:
Calcareous shale, chalk, and limestone, and gray non-calcareous shale and silty shale
KJdsCretaceous-Jurassic Dakota, Morrison, and Sundance Formations, undivided:
Nonmarine, marginal marine, and shallow marine sandstone, shale, mudstone, and minor limestone and claystone
TR PllyTriassic-Permian Lykins Formation and Lyons Sandstone, undivided (from Lyons, Colorado, southward):
Red mudrock and siltstone, with beds of gypsum and limestone superjacent to quartzose cross-bedded sandstone
TR PjoTriassic-Permian Jelm, Lykins, Lyons, and Owl Canyon Formations, undivided (from Lyons, Colorado, northward):
Red siltstone, shale, and sandstone, with minor limestone
P|PifPermian-Pennsylvanian Ingleside and Fountain Formations, undivided (from Lyons, Colorado, northward):
Limestone and calcareous sandstone superjacent to arkosic sandstone, mudstone, and conglomerate
P|PfPermian-Pennsylvanian Fountain Formation (from Lyons, Colorado, southward):
Arkosic sandstone, mudstone, and conglomerate
Igneous and metamorphic units
TKiTertiary-Cretaceous Laramide intrusive igneous rocks: mainly intermediate to felsic, some mafic
YgMesoproterozoic 1400 m.y. age group:
Granitic rocks, including Silver Plume and Sherman granites
XgPaleoproterozoic 1700 m.y. age group:
Granitic rocks, including Boulder Creek Granodiorite
XbPaleoproterozoic 1700 m.y. age group:
Principally biotite gneiss, schist, and migmatite derived from sedimentary rocks
XfhPaleoproterozoic 1700 m.y. age group:
Principally felsic and hornblende gneiss, separate or interlayered, derived from volcanic rocks
Virginia Dale ring-dike units
Ylc~1390 m.y.: Log Cabin Granite; fine-grained to aplitic, abundant muscovite
Yscr~1410 m.y.: Cap Rock Quartz Monzonite-Sherman Granite facies; light-gray to pink, porphyritic, biotitic
XYd~1410 to 1700 m.y.: Mafic rocks, including aplitic diorite and minor hornblende gabbro, andesite, and dacite
Abbrev.Description
Sedimentary units
TgvTertiary unconsolidated boulder-rich gravel on old erosion surfaces in Front Range
KpCretaceous Pierre Shale:
Olive-gray and dark-gray shale and silty shale, organic-rich in part; numerous bentonite beds; five sandstone intervals in middle part of unit
KnbCretaceous Niobrara and Benton Formations, undivided:
Calcareous shale, chalk, and limestone, and gray non-calcareous shale and silty shale
KJdsCretaceous-Jurassic Dakota, Morrison, and Sundance Formations, undivided:
Nonmarine, marginal marine, and shallow marine sandstone, shale, mudstone, and minor limestone and claystone
TR PllyTriassic-Permian Lykins Formation and Lyons Sandstone, undivided (from Lyons, Colorado, southward):
Red mudrock and siltstone, with beds of gypsum and limestone superjacent to quartzose cross-bedded sandstone
TR PjoTriassic-Permian Jelm, Lykins, Lyons, and Owl Canyon Formations, undivided (from Lyons, Colorado, northward):
Red siltstone, shale, and sandstone, with minor limestone
P|PifPermian-Pennsylvanian Ingleside and Fountain Formations, undivided (from Lyons, Colorado, northward):
Limestone and calcareous sandstone superjacent to arkosic sandstone, mudstone, and conglomerate
P|PfPermian-Pennsylvanian Fountain Formation (from Lyons, Colorado, southward):
Arkosic sandstone, mudstone, and conglomerate
Igneous and metamorphic units
TKiTertiary-Cretaceous Laramide intrusive igneous rocks: mainly intermediate to felsic, some mafic
YgMesoproterozoic 1400 m.y. age group:
Granitic rocks, including Silver Plume and Sherman granites
XgPaleoproterozoic 1700 m.y. age group:
Granitic rocks, including Boulder Creek Granodiorite
XbPaleoproterozoic 1700 m.y. age group:
Principally biotite gneiss, schist, and migmatite derived from sedimentary rocks
XfhPaleoproterozoic 1700 m.y. age group:
Principally felsic and hornblende gneiss, separate or interlayered, derived from volcanic rocks
Virginia Dale ring-dike units
Ylc~1390 m.y.: Log Cabin Granite; fine-grained to aplitic, abundant muscovite
Yscr~1410 m.y.: Cap Rock Quartz Monzonite-Sherman Granite facies; light-gray to pink, porphyritic, biotitic
XYd~1410 to 1700 m.y.: Mafic rocks, including aplitic diorite and minor hornblende gabbro, andesite, and dacite

Contents

GeoRef

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