5: Overview of the Eocene Castle Rock Conglomerate, east-central Colorado: Remapping the fluvial system, and implications for the history of the Colorado Piedmont and Front Range
Published:September 07, 2016
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Stephen M. Keller, Matthew L. Morgan, 2016. "Overview of the Eocene Castle Rock Conglomerate, east-central Colorado: Remapping the fluvial system, and implications for the history of the Colorado Piedmont and Front Range", Unfolding the Geology of the West, Stephen M. Keller, Matthew L. Morgan
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The Castle Rock Conglomerate is a late Eocene fluvial deposit flanking the east side of the Colorado Front Range and lying within the Colorado Piedmont. It occurs as a northwest-southeast–trending swath ~63 km in length and between 3 and 10 km in width, and is ~70 m in thickness. The conglomerate consists of a matrix of arkosic coarse sand and granules along with pebble- to boulder-sized clasts that vary in abundance. Locally, the upper portion of the conglomerate is well exposed in cliffs and ledges and also in flat outcrops along drainages. Large to very large-scale cross- bedding, of both the planar and trough types, is characteristic of the unit. Clasts are dominantly Front Range granitic rocks and Wall Mountain Tuff, and boulders of the latter can exceed 0.5 m in diameter. Minor quantities of quartzite and vein quartz, and rare sedimentary clasts, also are present. The large sizes of the bedforms and clasts indicate deep water and high velocity during deposition. A recent, extensive paleocurrent study of the upper part of the conglomerate has produced a new map of the fluvial system, indicating a main, southeast-trending paleochannel with two major northeast-trending tributaries. This field trip will present an overview of the Castle Rock Conglomerate lithology, bedforms, and associated geologic units; ideas about its deposition; and its bearing on uplift along the Front Range and in the southwest Denver Basin. Planned stops will be in Douglas and Elbert Counties, south of Denver, and include Castlewood Canyon State Park, Prairie Canyon Ranch open space, private farm and ranch properties, and an inactive quarry. Several of the stops provide excellent vistas of the Front Range, majestic Pikes Peak, and also of the Colorado Piedmont.
Both geologists and the general public find the late Eocene Castle Rock Conglomerate (Tcr) of absorbing interest. The unit is especially well exposed in cliffs, ledges, and flat outcrops at Castlewood Canyon State Park (Stop 2). These exposures allow scientists and others to view large portions of the formation, appreciate its characteristics, and understand its fluvial depo-sitional environment. It is easy for visitors to the park to make the connection between large clasts found in the conglomerate, some being >0.5 m in diameter, to the movement of cobbles and boulders by deep and swift currents. Although not as intuitive, relating the large fluvial bedforms in the Tcr to the concept of migrating subaqueous dunes formed during ancient flood events is fairly straightforward. It would not be hyperbolic to say that deposition of some Tcr horizons was dramatic: stream depth of several meters, stream velocity at 6–10 m/s (13–22 mph), and transport of large quantities of cobbles and boulders. Because much of the Tcr was deposited under high-energy conditions, fossils are rare, but titanothere bones and fossil logs do occur in the unit and enhance its interest. The high-energy depositional environment of the Tcr is vividly portrayed by a painting in the Ancient Denvers exhibit at the Denver Museum of Nature and Science. The painting is included in the companion book to the exhibit, Ancient Denvers (Johnson et al., 2002), and we will view two illustrations from this book during the field trip.
During the six geological stops on the trip, we will visit four areas of Tcr and two areas of formations associated with it: the Dawson Arkose (Tda) and Wall Mountain Tuff (Twm). Concentrating on the Tcr, we will observe up close its lithology and bedforms, and briefly discuss its areal extent, its depositional environment, and its relation to Front Range uplift and Denver Basin formation. We will view the Tcr where it occurs in impressive mesas and canyons (some with attractive vistas) and also in gentler terrain, and the settings will range from residential developments to parks to ranches.
The Castle Rock Conglomerate is a late Eocene fluvial deposit flanking the east side of the Colorado Front Range and lying within the Colorado Piedmont. It may reach ~70 m in thickness (Morse, 1985), is nearly flat-lying, and is discontinuous. It caps both high-relief mesas and gentle hills and also occurs in flat outcrops along the sides of drainages. The unit lies in a northwest-to-southeast-trending swath ~63 km in length and varying between 3–10 km in width. It extends from near Rueter-Hess Reservoir, ~8 km southwest of Parker, to the buttes at Fremont Fort, ~15 km southeast of Elbert. Eroded blocks of Tcr are found at Paint Mines Interpretive Park, ~13 km southeast of Fremont Fort and ~2 km southeast of Calhan. The width of the unit increases southeastward. Figure 1 is a shaded relief map of the extent of the Tcr as mapped by the Colorado Geological Survey and also shows the locations of field-trip Stops 1 through 8.
The following summary of the geologic setting in the region of the Tcr is summarized from Thorson (2011). The Tcr occurs within the southwestern part of the Denver Basin, which lies east of the Front Range and extends to eastern Colorado, southeastern Wyoming, and southwestern Nebraska. The majority of deposition in the Denver Basin occurred during Late Cretaceous, Paleocene, and Eocene time, and was synorogenic with the start of the Laramide orogeny at ca. 70 Ma. East-flowing braided stream systems carried clastic sediments from the uplift area (west) into the basin (east), and in the Castle Rock area deposited the Denver Formation (TKd) and the Dawson Arkose (Tda). The TKd consists of sandy claystone and clayey sandstone, with beds of fine- to medium-grained, feldspathic, cross-bedded sandstone; 120–150 m of the TKd is exposed in the Castle Rock area. The overlying Tda consists mainly of thick-bedded to massive, cross-bedded arkoses; pebbly arkoses and arkosic pebble to cobble conglomerate; beds of fine- to medium-grained, feldspathic, cross-bedded sandstone; and local beds of sandy claystone. The Tda is ~220 m thick in the Palmer Divide area (~30 km south of Castle Rock), and thins northeastward to between 45 and 60 m thick in the Parker and Highlands Ranch area (~20 km north of Castle Rock). Renewed uplift, probably during middle Eocene time, caused fluvial deposition of the Larkspur Conglomerate (Tlc) in valleys cut into the Tda paleotopography. In the late Eocene, a volcanic eruption ~100 km to the west propelled ignimbrite of the Twm eastward to the Castle Rock area and draped it on the surface of the Tda and Tlc. The age of the Twm is ~36.7 m.y. (McIn-tosh and Chapin, 1994). Near the end of the Eocene, the Tcr was deposited mostly by southeast flow in a large paleovalley incised into the Tda, Tlc, and Twm. The Tcr incorporated numerous clasts and slump blocks of Twm, and also some rip-up clasts of Tda.
Description of Castle Rock Conglomerate
The unit that would eventually be formally named the Castle Rock Conglomerate was first mapped as part of the Ferdinand Hayden (1874) Survey where it was included in the upper part of the Monument Creek Group. Richardson (1915) suggested the name “Castle Rock Conglomerate” for its type locality on the prominent butte north of the town of Castle Rock. He described it as a cross-bedded, conglomeratic arkose of fluvial origin, derived from the uplifted Front Range to the west, and mapped its extent from Sedalia to Elbert (Thorson, 2011).
Thorson (2011, p. 44) describes the Tcr as follows:
The Castle Rock Conglomerate is a pebble, cobble, and boulder arkosic conglomerate composed predominantly of sub-round to round fragments of pink and gray granite, pink feldspar, and vein quartz, with subordinate amounts of gneissic metamorphic rocks, quartzite, red sandstone, welded tuff, and chert in a coarse to very coarse sand matrix of quartz and feldspar grains. The distinguishing characteristic of this unit is the presence of angular to subangular pebble- to boulder-sized clasts of gray, brownish-gray, maroon, or lavender-gray welded tuff, which have been eroded from deposits of the Wall Mountain Tuff. Outcrops of the Tcr usually are very strongly cross-bedded.
On the basis of 24 clast surveys in the upper Tcr (Keller and Morgan, 2013, 2016), the majority of clasts >2 cm are granit-ics (~60%), followed by Twm (~30%), and then by vein quartz and quartzite (each ~5%). In these clast surveys, sedimentary rocks are uncommon, and most of them are red-brown sandstone superficially resembling the Pennsylvanian-Permian Fountain Formation, which is exposed in the hogback along the Front Range (Evanoff, 2007; Keller and Morgan, 2013, 2016). The Tcr is younger than the Twm, as evidenced by Twm clasts in the Tcr, and the tuff has been dated at ~36.7 m.y. (McIntosh and Chapin, 1994). Titanothere bones in the Tcr indicate that it is older than the end of the Eocene (K.R. Johnson, Denver Museum of Nature and Science, 2002, written commun., in Thorson, 2011). Two rounded volcanic clasts of probable dacitic composition were collected from the base of the main paleochannel in Castlewood Canyon State Park. U-Pb SHRIMP-RG (sensitive high-resolution ion microprobe-reverse geometry) zircon ages of these clasts range from 46 to 56 Ma. Potential source areas for these volcanic clasts lie along a northeast trend between Leadville and Boulder. All of the clast lithologies found in the unit are derived from the southern Rocky Mountains (Morgan et al., 2013).
Morse (1985) performed the most comprehensive study of the Tcr to date. He recognized three fluvial facies in the unit: (1) planar cross-bed facies, (2) trough cross-bed facies, and (3) massive gravel bed facies. These are evident in Morse’s four measured sections (Fig. 2, section locations shown on Fig. 1.) Large-scale planar cross-beds of the first facies are the dominant feature in the four sections, making up ~85% of measured thickness. The planar foreset beds have alternating beds of coarser and finer grain sizes. The cross-bed sets are 0.3 to >3 m thick, are composed of moderately sorted coarse sand and gravel, can have horizontal or broadly curving lower contacts (scour surfaces), and basal contacts either can be tangential or lie at a sharp angle. Observed at the ground surface, planar cross-beds can extend laterally up to 20 m. The basal portion of each planar cross-bed set contains the largest clasts (pebble size) (Morse, 1985). Trough cross-beds of the second facies make up ~10% of the measured sections. The trough cross-beds occur in sets from 0.25 to 1 m thick and with co-sets 0.3–1.8 m thick. The troughs are arcuate in plan view, forming rib and furrow structures commonly 3–5 m wide. Lower contacts are tangential, eroding the underlying beds. According to Morse (1985), average grain size in this facies is 5–15 mm, and clasts of large pebble size or greater are absent, as are Twm blocks. Massive gravel beds of the third facies make up 5% of the four measured sections; they occur at the base of the Tcr and also higher in the unit. These massive beds consist mostly of pebbles and cobbles with rare boulders, are moderately sorted, are clast-supported, and occur as broadly lenticular bodies 0.3–2.5 m in thickness. The clasts are well rounded except for angular Twm clasts, and the matrix consists of coarse sand, granules, and siliceous cement. Basal contacts of these beds are irregular but upper contacts are smooth; clasts include angular Twm blocks; and planar cross-bedding may occur in their upper portions (Morse, 1985).
The Tcr paleocurrent study of Keller and Morgan (2013, 2016) is comprehensive in areal coverage, but is confined to the upper and most accessible portion of the conglomerate. In the upper Tcr, trough cross-bedding facies is found to be dominant and planar cross-bedding rare, but the latter is expressed prominently where it does occur. Furthermore, in the upper Tcr, planar cross-bed sediment was found to be notably finer (coarse sand to pebbles <2 cm) than that of trough cross-bedding, and cobbles were found to be locally abundant in the troughs. Gravel in trough cross-beds was found to range mainly from 2 to 7 cm, but a small proportion of the gravel was as large as 20 cm.
Deposition of Castle Rock Conglomerate
Morse (1985) describes fining-upward sequences in his Tcr measured sections (Fig. 2). These sequences, from bottom to top, contain massive gravel beds, pebble conglomerate, coarse sandstone, and clayey siltstone or mudstone. Reineck and Singh (1973) note that coarse fining-upward sequences consisting of trough cross-bedded gravel, trough cross-bedded sand, and laminated mud can occur in gravelly braided streams. The coarse sediment is deposited in active bars and grain size decreases as the area aggrades and stream discharge decreases.
Morse (1985) attributes the Tcr planar cross-beds to migration of possible transverse bars or sand waves, probably equivalent to the straight-crested dunes described in Nichols (2009). Lateral channel accretion is also cited as a possible method of formation, but is noted to be rare in the conglomerate. Morse (1985) states that planar cross bedding is rare in the upper Tcr. Although they are uncommon, both straight-crested dunes (with foreset beds inclined in the dominant flow direction as indicated by surrounding trough cross-beds) and possible lateral-accretion cross-bedding (with foreset beds inclined transverse to dominant flow) occur in the upper Tcr (Keller and Morgan, 2016). Morse (1985) attributes the Tcr trough cross-beds to migrating dunes, presumably meaning sinuous or isolated dunes (Nichols, 2009) as opposed to straight-crested dunes. According to Morse (1985), the troughs are relatively uncommon and consist of finer-grained sediments than in the planar cross-beds. They occur toward the tops of fining-upward sequences in the Tcr, and suggest deposition during waning flood conditions (Morse, 1985). In the upper Tcr, however, Keller and Morgan (2016) found trough cross-bedding to be dominant, and planar cross-beds to be finer-grained than trough cross-beds. Cross-stratified beds of granules, pebbles, or cobbles, formed by migrating gravel bars (straight-crested dunes, or sinuous or isolated dunes), are characteristic of a braided river system (Nichols, 2009).
According to Morse (1985), the Tcr was deposited by a moderately deep (3–5 m), low-sinuosity braided stream, which was confined by valley sides and occasionally inundated by high-energy floods. Based upon analysis of clast size, Morse (1985) estimates flow velocities at 6–10 m/s (13–22 mph). These velocities are similar to flows of 4–5 m/s measured during the 2013 Colorado Front Range flooding (Yochum and Moore, 2013), and flows of 6–7 m/s measured during the 1965 Colorado River Basin flooding (Matthai, 1969).
Previous investigators have presented interpretations of Tcr deposition on the basis of paleocurrents and clast lithologies, and also the paleotopography upon which the unit was deposited. Desborough et al. (1970) investigated the Russellville gold placers south of Franktown. This gold is of unique fineness and is believed to have weathered from the Tcr; the primary source of the gold is not known. They propose that the dominant Tcr paleoflow was northeast, based upon 149 paleocurrent measurements at 14 localities, and also upon the absence of gold of similar fineness in the Clear Creek watershed to the northwest. Their paleocurrent map indicates that their 14 groups of paleocurrents have widely divergent resultant directions, but also that there is a pattern: southeast of Cherry Creek the trends are east to south-southwest, and northeast of the creek the trends are mostly east-northeast to north-northwest. It is possible that these two groupings of locations represent different Tcr horizons with different paleoflow directions, a concept that will be amplified below.
Morse (1985) proposes that the Tcr stream system was an ancestral course of the South Platte River, emerging from the Front Range at a location between Denver and Castle Rock, and then flowing southeast, occupying a wide, southeast-trending paleovalley. Later subsidence of the Denver Basin initiated an episode of stream piracy, diverting the stream system northeastward away from its old orientation and forming the present drainage pattern of the South Platte River.
Evanoff (2007) concurs with Morse (1985) that the Tcr was deposited primarily by northwest-to-southeast flow in a large, southeast-trending Castle Rock paleovalley, cut below the surface of the Twm and down into the Tda, and also states that paleo-current measurements from the largest Tcr cross-bed sets support the southeast flow direction. Evanoff (2007), however, proposes a different place of origin for the Tcr stream system than does Morse. Bluish-gray quartzite and stretched-pebble conglomerate clasts in the Tcr indicate a Front Range origin from similar lithologies in Coal Creek Canyon, ~50 km northwest of the northernmost occurrence of Tcr. (We note also that this quartzite location is on trend with a hypothetical northwestward extension of the swath of present-day Tcr occurrences.) Coal Creek Canyon quartzite clasts as large as 2 m in diameter, found in the northernmost Tcr exposures, indicate very large discharges in the ancient Tcr stream system. The Tcr is one of a series of latest Eocene conglomerates reaching from central Wyoming to South Park, Colorado. The presence of large blocks in these conglomerates suggests that a series of large storms may have transported the blocks for great distances (Evanoff, 2007).
Thorson (2011) states that Tcr sediments filled paleovalleys of an erosion surface on the Tda and Twm. He observes that in many areas the upper surfaces of Tcr deposits are gently sloping (nearly flat), and that they appear to be original bedding surfaces. The elevations of these surfaces are used to reconstruct a paleo-valley system in the southeast corner of Douglas County; the system slopes mainly northeast and (in a few places) north.
Koch (2013a) proposes that the Tcr can be subdivided into a northern facies, containing quartzite and quartzite conglomerate from Coal Creek Canyon, and a western facies derived mainly from the Pikes Peak Granite. The two facies also are distinguished based upon preliminary laser ablation age analysis of Tcr sandstone. The northern facies has zircon ages of 1.7–1.4 Ga and the western facies has dominant zircon ages of 1.0 Ga. Koch (2013a) states that the two facies are interstrati-fied along a paleo-Cherry Creek corridor, postulated as part of the late Eocene axial drainage of the Denver Basin. This corridor concept is similar to that of the Tcr paleovalley of Morse (1985) and Evanoff (2007). According to Koch (2013a), the northern Tcr facies has a consistent south-southeast paleocurrent direction. The western facies has northerly flow and also east-southeasterly flow, the latter direction being attributed to the filling of north-trending paleovalleys and the subsequent spilling of sediment eastward into the south-southeast-flowing axial drainage. Koch (2013b) uses petrographic comparison of the in-place quartzite of Coal Creek Canyon with quartzite clasts in the Tcr to support the former as the source of the latter. An andalusite and/or sillimanite high-grade metamorphic facies is found to occur in both quartzites, and Tcr quartzite clasts containing andalusite and/or sillimanite reportedly are found as far southeast as Paint Mines Interpretive Park, south of Calhan and ~130 km southeast of the quartzite occurrence at Coal Creek Canyon.
Keller and Morgan (2013, 2016) have performed an extensive paleocurrent study of the Tcr. The study is limited to the upper Tcr but covers almost the entire area of the conglomerate as mapped by the Colorado Geological Survey. The study database consists of ~3000 paleocurrent direction measurements (nearly all from trough cross-beds), and also 24 surveys of gravel-fraction lithology, size, and roundness (~11,000 clasts >2 cm in maximum dimension). Using U.S. Geological Survey (USGS) digital elevation data together with commercial contouring and plotting software, all the paleocurrent measurements are grouped areally and stratigraphically as 411 locations. Each location consists of a continuous group of measurements lying in the same horizon and sharing a general paleocurrent direction. Presented at a small scale, these locations constitute a comprehensive paleocurrent map of the upper Tcr, and a simplified version of this map appears as Figure 3. The locations also are used to construct cross sections that show the Tcr as having correlatable horizons, whose general paleocurrent directions are markedly juxtaposed (e.g., southeast-flowing overlying northeast-flowing). Relative abundances of upper Tcr gravel-fraction lithologies are shown in Figure 4.
The locations also are used to construct cross sections showing the Tcr to have groups of correlatable outcrops, in which groups the general paleocurrent directions are markedly different. For example, at Castlewood Canyon State Park, a horizon of outcrops with southeast-flowing paleocurrents consistently overlies a correlatable horizon of outcrops with northeast-flowing paleocurrents. We term this southeast trend the main paleochannel belt, corresponding to the large paleovalley of Morse (1985) and Evanoff (2007), and possibly to the paleo-Cherry Creek corridor of Koch (2013a). The main paleochannel belt extends from Rueter-Hess Reservoir near Parker southeast to the buttes of Fremont Fort ~15 km southeast of Elbert. (Tcr paleocurrents were measured at the Fremont Fort buttes area, which has not yet been mapped by the Colorado Geological Survey.) The width of the main paleochannel belt increases to the southeast, from 3 km at its northwest end to 10 km at its southeast end. In the lower, downstream reaches of the belt, the mapped conglomerate areas become narrower and more linear, and the paleocurrent directions appear to be more variable. This suggests that the main paleochannel belt may have transitioned to a network of distributary streams (Keller and Morgan, 2016).
Approximately one-third of paleocurrent azimuths in the upper Tcr range mostly from north to east-northeast, and there are prominent occurrences of horizons having a generally northeast trend (Keller and Morgan, 2013, 2016). These horizons are interpreted as mostly belonging to two major northeast-flowing tributaries to the southeast-flowing main paleochannel belt (Fig. 3). In the area of Castlewood Canyon State Park, the JA Ranch paleochannel (northern tributary) widened northeastward from a narrow paleochannel to become an alluvial fan, now partially overlain by deposits of the main paleochannel belt. In contrast, the Bucks Mountain trend (southern tributary) appears to have been a wide swath of subparallel streams, which joined the main paleochannel belt in an area west of Elbert, and also is overlain by main paleochannel belt deposits. A possible third tributary system may have existed north of Castle Rock, as suggested by a small number of measurements in three isolated areas: Castle Rock butte, Cherokee Mountain, and the mesa west of Rueter-Hess Reservoir (Fig. 3). The number of measurements is small and the area is large, and therefore different interpretations are possible here. Clast surveys of the main paleochannel versus the tributary JA Ranch paleochannel have notable differences in lithology, clast size, and rounding. The tributary has a markedly greater content of Twm and a moderately lower content of granitics than the main paleochannel. Average clast size is 0.6 cm greater in the tributary; the proportion of 6 cm to 18 cm clasts is moderately greater; and the proportion of 2 cm to 3 cm clasts is smaller. There is a moderately greater proportion of angular clasts in the tributary, and a markedly smaller proportion of rounded clasts than in the main paleochannel (Keller and Morgan, 2016).
Field observations indicate that a large southeast-trending paleovalley was eroded into the Tda and Twm, and that a southeast-flowing fluvial system, possibly consisting of braided channels, filled the paleovalley with Tcr deposits (Morse, 1985; Evanoff, 2007; Keller and Morgan, 2013, 2016; Koch, 2013a). The Tcr fluvial system may have emerged from the Front Range near Coal Creek Canyon, between the sites of Boulder and Golden. Tcr between Coal Creek Canyon and Parker would have been removed later by the South Platte River and its tributaries. The main paleochannel belt of Keller and Morgan (2013, 2016) and the northern facies of Koch (2013a) correspond to the large paleovalley of Morse (1985) and Evanoff (2007). The western facies of Koch (2013a) may correspond to the tributaries of Keller and Morgan (2013, 2016).
The “quartzite line” of Koch (2013b) implies that the bluegray quartzite of Coal Creek Canyon is not found except in the southeast-flowing northern facies (or main paleochannel belt) of the Tcr. The clast surveys of Keller and Morgan (2013, 2016), however, record blue-gray quartzite in four of the nine surveys performed in trough cross-bedded exposures of the northeast-flowing JA Ranch paleochannel tributary. During summer 2015, many Tcr ledges in the JA Ranch paleochannel at Castlewood Canyon State Park were re-examined for bluegray quartzite clasts, and 54 samples were collected among 17 locations. However, no blue-gray quartzite was found in situ at Hunt Mountain in the well-exposed upper reach of the JA Ranch paleochannel (Fig. 1) (Keller, 2015, personal observ.). Despite the presence of blue-gray quartzite in the lower portion of this tributary, there is no evidence as yet linking this quartzite to sources other than Coal Creek Canyon. The blue-gray quartzite clasts in the JA Ranch paleochannel may have been originally part of an underlying main paleochannel belt horizon, and may have been entrained by subsequent flow in the tributary stream.
After deposition of the Tcr during the late Eocene, there were periods of erosion and deposition in the southwest Denver Basin. The Tcr is not overlain by any younger Tertiary units, although it is possible that the latter could have been deposited upon and subsequently eroded from the Tcr. The youngest sediments in the area are unconsolidated Quaternary sands and gravels found in paleochannels, alluvial plains, and higher erosion surfaces (Thorson, 2011).
As stated earlier, sedimentary clasts are rare in the upper Tcr. The majority of these are red-brown sandstone, resembling the Pennsylvanian-Permian Fountain Formation exposed in the Front Range hogback (Evanoff, 2007; Keller and Morgan, 2016). Absent from the upper Tcr clast surveys is the suite of Mesozoic sedimentary rocks now exposed along the mountain front, suggesting that the hogbacks of these rocks visible today were either not exposed during the late Eocene, or were not present in the Tcr source area. Gravel-fraction lithologies in the Tcr indicate that large quantities of granitic and volcanic material existed along the Front Range during the time of Tcr deposition. This arkosic and volcanic material may have buried the Paleozoic and Meso-zoic sections along the range front. Later downcutting by the Tcr stream system may have exposed the Paleozoic section, prior to exhumation of the Mesozoic rocks sometime after the deposition of the Tcr (Morgan et al., 2013).
Evanoff (2007, p. 26) supports the concept of a large, southeast-flowing paleovalley in which the Tcr was deposited by a southeast-flowing stream system. He notes, however, that the Tcr currently is 300 m higher at its southeast end than at its northwest end, and proposes that this is the result of “significant northward tilting” of the southern Denver Basin during the Neogene. Thus, the Tcr may have been tectonically reversed to a northwest dip from its original position on a southeast-sloping paleovalley.
Thorson (2011) estimates that the present slope of “the main Castle Rock Conglomerate paleochannels” is ~40 ft/mi (~8 m/km) to the northwest. Our own estimate is as follows. Between the mesa west of Rueter-Hess Reservoir and the buttes at Fremont Fort, a distance of 63 km, the difference in elevation of the Tcr surface is ~240 m (from USGS topographic maps), with the southeast end being higher. Making the (questionable) assumption that the Tcr surface at both the northwest and southeast ends of the Tcr swath is the same stratigraphic position, a rough estimate of the dip is 0.3° (4.5 m/km) northwest. Even if, however, different and widely spaced horizons constitute the Tcr surface at either end, the estimate will vary but the unit still will be found to tilt slightly northwest, assuming the 70 m thickness of Morse (1985). This is in the opposite direction from the dominant southeast trend of paleocurrents measured in the unit.
Logistics and Hazards of the Field Trip
This field trip does not require off-trail hiking or clambering. There are no steep slopes on our walks, but we will ascend and descend along gentle gaps in low-relief ledges, so please exercise care. At Daniels Park (Stop 1), we will cross a county road that has limited visibility, so we should cross carefully as a group there. At Castlewood Canyon State Park (Stop 2), the Brown property (Stop 4), and the Krupa property (Stop 6), we will briefly depart as far as 400 m from a trail, road, or residence, but we will remain in open country and seldom will be out of sight of where we departed. Rattlesnakes are occasionally encountered at Castlewood Canyon State Park (Stop 2), so keep an eye on the ground there. At Prairie Canyon Ranch (Stop 3), we will pass through a 0.7 m gap between a fence post and an outcrop face; the fence post has barbed wire wrapped around it, but the wire can easily be avoided. At the inactive “cut-stone” quarry (Stop 7), please remain on the quarry floor and do not clamber among the tuff blocks or go up to the face. The blocks may be unstable and their edges can be very sharp. Snakes may also be present here. There will be a first aid kit in the lead vehicle.
Prairie Canyon Ranch (Stop 3) can be visited by groups and individuals, but a permit from Douglas County Division of Open Space and Natural Resources is necessary. Stops 4 and 6 are on properties owned by Irene and Phil Brown, and Kathy and John Krupa, respectively, and permission from the landowners is required for access. Stop 7 is on land adjoining the Mencenberg quarry, and permission from Schmidt Aggregates is required for access.
We will have a brief lunch break at Prairie Canyon Ranch (Stop 3). There are rest rooms at Daniels Park (Stop 1) and the Castlewood Canyon State Park visitor center (Stop 2), and a portable toilet at Prairie Canyon Ranch (Stop 3). HI-TEST store (Stop 5) and King Soopers (Stop 8) specifically are rest-room stops.
Stop 1: Daniels Park (Dawson Arkose—Tda) Park entrance coordinates: N 39.472964°, W 104.917158° (WGS84)
From the main entrance to the Colorado Convention Center (south side of 14th St., between Champa and Welton Sts.), drive east on 14th St. past Welton St. and turn right (south) at Glenarm Pl. (next street after Welton St.). Drive two blocks on Glenarm Pl. and turn right (west) on Colfax Ave. Take Colfax Ave. for two blocks and turn right (north) on Speer Blvd. Take Speer Blvd. north for ~1.6 km (1 mi) to the southbound entrance to Interstate 25 (I-25), and get on I-25 south. Take I-25 south for ~27 km (~17 mi) and exit west on C-470. Take C-470 west for ~14 km (~9 mi) and exit south on Colorado Hwy. 85 (Santa Fe Dr.). Head south on Hwy. 85 for ~21 km (13 mi), look for the sign indicating a left turn for Daniels Park, and after the sign turn left (north) on Daniels Park Rd. (Douglas County Rd. [CR 29]). Drive north on CR 29 for ~5 km (3 mi), and turn left (west) at the Daniels Park entrance. Take the park road for 0.4 km (0.2 mi) to the rest-room area on the left, and park in the lot there for a brief rest-room stop (if necessary). Turn left (north) out of the lot and drive for ~200 m (~700 ft) to the second small parking area on the left (west), park vehicle, and walk west to the adjacent Tda sandstone rim overlook.
The purpose of Stop 1 is to discuss the Tda and its role as constituting most of the paleotopography upon which the Tcr was deposited. From the overlook, we can view a prominent mesa of Tda sandstone to the northwest. To the west, we are looking across the Colorado Piedmont. In the distance is the Front Range hogback composed of uplifted Paleozoic and Mesozoic strata, and behind the hogback is the crystalline Front Range. As discussed earlier, the Tda was deposited by stream systems flowing eastward off the rising mountains during the Laramide orogeny. Following a period of erosion of the Tda, the Twm ignimbrite was deposited upon Tda paleotopography, followed by deposition of the Tcr by a southeast-flowing braided river in a large, southeast-trending Tda paleovalley.
From the overlook, drive back along the park road until near-ing the stop sign at Douglas CR 29. Slow down well ahead of the stop sign, and pull all the way off the road into the grassy area ahead of the sign. Walk past the stop sign, turn right, then walk for ~120 m (~400 ft) south on CR 29 until facing a prominent roadcut of Tda on the opposite side of the road, then carefully cross the road.
This exposure consists of a cross-bedded, coarse-grained, sand to granule, arkosic sandstone with an erosion surface at its base (Fig. 5). Overlying this unit is a mudstone or siltstone layer containing an orange-mottled zone interpreted as a paleosol. At the top of the roadcut is a coarse-grained arkosic sandstone. The sandstones are fluvial in origin, and the mudstone or silt-stone may be one of the massive structureless beds interpreted as mudflows (Thorson, 2011). Tda sandstone locally can resemble some of the finer-grained layers of the Tcr in which pebbles and cobbles are absent.
Stop 2: Castlewood Canyon State Park (Castle Rock Conglomerate—Tcr) Park entrance coordinates: N 39.3234°, W 104.734769° (WGS84)
At the entrance to Daniels Park, turn right (south) on to Douglas CR 29 and drive for ~5 km (~3 mi) back to Hwy. 85. Turn left (south) and follow Hwy. 85 for ~5 km (~3 mi) to Founders Pkwy. (Colorado Hwy. 86), a major intersection. Turn left (east) at the signal light and follow Founders Pkwy./Hwy. 86 for ~8 km (~5 mi) to another signal light bearing a small green sign pointing left to continue on Hwy. 86. Turn left (east) here and follow Hwy. 86 for ~8 km (~5 mi) to Franktown and the signallight junction with Colorado Hwy. 83. Turn right (south) on Hwy. 83 and drive for ~8 km (~5 mi) to the entrance to Castlewood Canyon State Park, which is on the right-hand (west) side of the highway. Turn in here, follow the entrance road for ~0.8 km (0.5 mi), then turn right (north) into the parking lot for Bridge Canyon Overlook. Walk the Bridge Canyon Overlook trail for ~90 m (300 ft) toward the overlook structure. Before reaching the structure, go east around the end of the rail and walk to the ledge adjacent to the structure.
The purpose of Stop 2 is to outline the stratigraphy and bedforms of the Tcr. While we are presenting, we will pass around a copy of Ancient Denvers (Johnson et al., 2002), open to the illustration of Tcr flooding. Across Cherry Creek from the overlook is a steep exposure of the upper portion of the Tcr, ~20 m in thickness, and here it is possible to view the trough cross-bedding characteristic of that portion. Note the jutting outcrop with a weathered hole in it. On the ledge behind and to the right of the outcrop, between the trees above and a less resistant layer below, are two layered, concave-upward features (Fig. 6). Each is roughly 4–5 m wide and ~2 m in height, and both belong to a set of northeast-trending trough cross-beds. We are looking approximately down-paleocurrent and along the axes of these troughs. In examining trough cross-bedding, the paleocurrent direction cannot readily be discerned from vertical outcrop faces. In the present example, however, the northeast paleocurrent direction was measured from trough exposures in the roughly flat-lying outcrop along the ledge; open ends of the troughs face northeast and the trough axes lie southwest-northeast. (A reliable method of measuring paleocurrent direction is to record the trough axis azimuth while facing the open end of the trough.) Looking down and to the right, in the exposure lying between the valley floor and the highway, we can view a lower horizon also displaying trough cross-bedding (Fig. 7). At this location, the cross-beds are characteristically truncated at the top and have gently curved, tangential contacts at the base. We are looking northeast and roughly transverse to paleocurrent flow, and the flow direction is southeast. Shifting our attention up-section, we can see a relatively thin horizon of nearly flat-lying planar beds occurring at the top of the ledge exposure and just left of the bridge (Fig. 8); the beds appear to dip very gently northward. These might be either lower-phase or upper-phase flat beds, according to the bedform stability diagram of Nichols (2009). Occurrences like this are rare in the upper Tcr.
Before leaving the overlook structure, note that it is built of blocks of Twm, which once was a well-utilized building material in the Denver area; the Twm will be discussed in more detail at Stop 7. Twm clasts are an important constituent of the gravel fraction of the Tcr, and they range from sub-rounded pebbles to angular boulders >0.5 m in diameter. Very large Twm fragments (as great as 1 χ 2 m in plan view) possibly are slump blocks from paleochannel banks. Walking back along the trail to the parking lot, and pausing just beyond the left-hand (east) railing, are two tuff boulders in the flat exposure of conglomerate. A little further along, and also on the left-hand side of the trail, there is a prominent cobble of the blue-gray quartzite discussed earlier; this quartzite is believed to have been transported from Coal Creek Canyon south of Boulder.
Arriving back at the parking lot, turn right and walk for ~60 m to the start of the Canyon View Nature Trail. Walk north on the trail for ~60 m, and then walk to the right (east) away from the trail. We now are in a horizon of north-northeast-trending trough cross-beds, which may correlate with the northeast-trending horizon we viewed at the overlook. This is a good place to see the lithology and bedforms of the Tcr in plan view. Fortunately, the Tcr has many flat-lying and gently sloping occurrences like this, making possible the efficient collection and mapping of large numbers of paleocurrent measurements, as well as collection of clast data.
After arriving back at the parking lot, drive out of the lot to the intersection and turn right. Drive north for ~0.3 km (~0.2 mi) to a “T” with another park road. Turn right and drive north for ~0.3 km (~0.2 mi) to the west end of the parking lot next to the picnic pavilions. From the west end of the lot, walk west on the Lake Gulch Trail for ~0.5 km (~0.3 mi). After descending the trail’s staircase, proceed for ~300 m (~1000 ft) further west to a bluff of Tcr outcrop with a spur at its western end, adjacent to the trail. Here we can view a well-exposed horizon of the main paleochannel and its southeast to south-southeast-trending trough cross-beds, which are observable in three dimensions (Fig. 9). The outcrop at the spur suggests a fining-upward sequence as presented by Morse (1985) and discussed earlier. There is a massive, cobbly deposit at its base overlain by finer, bedded sediments (Fig. 10); note the red-brown sandstone boulder resembling the Pennsylvanian-Permian Fountain Formation.
As presented earlier, Morse (1985) used Tcr clast size to estimate stream velocity. An alternative approach suggested by Abbott (2015, personal commun.) is to begin by using the method of Paola and Mohrig (1996), in which median grain size and bank-full channel depth are measured to estimate paleoslope of the channel bed. This method cannot be used for the flood deposits of the Tcr (i.e., strata containing bedforms), but potentially can be used on fining-upward sequences like the outcrop at the spur. Using this method, it is quite likely that the channel paleodepth will be underestimated, owing to the fact that the channel top will typically be removed when the subsequent channel migrates over it. Underestimating the channel depth results in an overestima-tion of paleoslope. However, this effect is countered by the fact that it is equally likely that the channel’s median grain size will be underestimated. The estimated paleoslope can be used in the Manning equation to estimate paleovelocity, and this is a possible avenue of continued research in the Tcr.
Walk back east along the trail to the top of the staircase, then leave the trail and follow the base of the Tcr ledge southeast for ~50 m (~170 ft). The ledge is a trough cross-bedded horizon of east-northeast to east-southeast paleocurrents probably belonging to the JA Ranch paleochannel, one of the two major northeast-flowing tributaries to the southeast-flowing main paleochannel. From the ledge, walk for ~50 m (~170 ft) northeast to rejoin the Lake Gulch Trail, turn right (east), then follow the trail ~380 m (~1200 ft) back to the picnic pavilions parking lot. On the way out, we can stop at the Castlewood Canyon State Park visitor center for the rest room if necessary.
Stop 3: Prairie Canyon Ranch (Castle Rock Conglomerate—Tcr) Ranch arch and gate coordinates: N 39.294747°, W 104.735972° (WGS84)
At the Castlewood Canyon State Park entrance, turn right (south) on to Colorado Hwy. 83, drive for ~3 km (~2 mi) to the Prairie Canyon Ranch arch and gate on the left (east) side of highway, and turn into the entrance. There is no road sign for the ranch, so make sure not to miss the arch. Take the ranch road ~2 km (~1 mi) northeast to the farmhouse and the old garage and park between them. Prairie Canyon Ranch has a portable toilet next to the old garage.
The purpose of Stop 3 is to examine a well-exposed vertical face of Tcr. We will observe the lithology of the cobble and boulder fraction and also the trough cross-bedding. About 1.3 km (~0.8 mi) from the gate along the ranch road there is a ridge on the left (west) with Twm outcrop. We will stop briefly here. Note that this Twm, although older than the bluffs of Tcr in the valley in front of us, is higher in elevation. This is because Tcr streams incised both Twm and the underlying Tda.
From the parking area, walk west behind the old garage for ~45 m (~150 ft) to the gravel trail, then head northwest on the trail for ~120 m (~400 ft) until arriving at a fenced area, behind which is an old cave door. Trough cross-bedding is prominent in the ledge here, and tangential lower contacts of the beds are evident (Fig. 11). The paleocurrent in this horizon appears to be approximately southeast, part of the southeast-flowing main paleochannel belt; southeast paleocurrent measurements are dominant on the ranch property. A portion of the alluvial fan of the northeast-flowing JA Ranch paleochannel tributary, however, is believed to extend this far south, as evidenced by a few groups of northeast measurements (Keller and Morgan, 2016).
A little west of the cave door is a cave window, where there is a pocket of what may be massive gravel bed facies of Morse (1985). Walk for ~90 m (~300 ft) past the two stone walls and head northwest along the base of the bluff. There are rare dacite clasts in the face just north of the second stone wall. Note continuation of the apparently southeast-trending trough cross-bedding in the ledge, and also the angular to subrounded Twm boulders. Pass between the fence post and the bluff, being careful of the barbed wire wrapped around the fence post. Northwest and down slope, across the clearing, is a large dish-shaped feature in the Tcr face; this is trough cross-bedding viewed parallel to the paleocur-rent. Based upon the attitudes of the beds on the right-hand side of the trough, a generally southward paleocurrent direction (i.e., toward the viewer) appears indicated. Two other, smaller troughs also are expressed as dish shapes, occurring to the right of the large trough.
Follow the bluff northwest for ~25 m (~80 ft), between the clump of trees and the outcrop face, and you will arrive standing on a resistant Tcr shelf with Cherry Creek adjacent on the left (west). This horizon is very coarse and has an unusual abundance of rounded blue-gray quartzite cobbles. (The importance of the quartzite to concepts of Tcr deposition was presented earlier.) Contrasting the coarseness of this horizon with the finer-grained ledge at the cave door, it is possible that we have walked down through a fining-upward sequence as described by Morse (1985).
Cherry Creek is the stream we crossed on our way in, just before reaching the farmhouse. On 5 April 2016, the stream was flowing vigorously. Immediately south of the crossing, and under ~20 cm of water, there was a bar of coarse sand and granules. Small straight-crested and sinuous dunes on the bar were actively migrating north with the current. They were ~1 m long (transverse to flow), ~10 cm wide, ~5 cm high, and spaced ~0.5 m apart. They were reminiscent of the Tcr bedforms on the bluff we have just visited, even though they were at least an order of magnitude smaller.
We will have a brief lunch break back at the farmhouse before leaving for Stop 4.
Stop 4: Brown Property (Castle Rock Conglomerate—Tcr) Brown property entrance coordinates: N 39.226175°, W 104.606958° (WGS84)
Take the ranch road back southwest to the Prairie Canyon Ranch arch and gate, turn left (south) on Colorado Hwy. 83, and drive for ~10 km (~6 mi) to Gillian Ave. (Douglas CR 78). Turn left (east) and follow CR 78 for ~2 km (~1 mi) to East Cherry Creek Rd. (Douglas CR 81). Turn right (south) on CR 81 and proceed for ~1 km (~0.5 mi) to Steele Ave. (Elbert CR 98). Turn left (east) on CR 98 and drive for ~6 km (~4 mi) to Elbert CR 13, turn left (north) on CR 13, and drive for ~1 km (0.5 mi) to the Brown property. The Brown home (light green) and outbuildings are the first buildings along the east side of CR 13. Park on the gravel at the east side of the residence.
The purpose of Stop 4 is to observe different Tcr horizons having markedly different paleocurrent directions, indicating that the southeast-flowing main paleochannel belt was fed by northeast-flowing tributaries; here the postulated tributary is the Bucks Mountain trend (discussed earlier). The Brown property lies in that area of the upper Tcr where, according to the interpretation of Keller and Morgan (2013, 2016), the second major Tcr tributary, the northeast-flowing Bucks Mountain trend, joined the southeast-flowing main paleochannel belt. The Bucks Mountain trend differs in character from the first major tributary, the JA Ranch paleochannel. The latter is a single paleovalley leading to an alluvial fan, while the former (where we are now) may be a system of subparallel northeast-flowing channels, as suggested by the trend’s overall width (transverse to paleoflow direction) of ~20 km.
Walk for ~30 m (~100 ft) on the vehicle track leading south from the residence, go through the gate, and then head east for ~90 m (~300 ft) to a large, linear, northeast-trending band of Tcr outcrop. North to northeast-trending trough cross-bedding is visible as we walk northeast along this outcrop for ~55 m (~180 ft), to where it ends at a drainage. Walking north along this exposure, we view a prominent exposure of north-trending trough cross-beds overlain by east-trending planar cross-beds (Fig. 12), suggesting that the planar beds may have accreted from sediment spilling over to the right-hand side (looking down-paleocurrent) of a bar of migrating dunes. The planar cross-beds are finer-grained than the trough cross-beds, indicating lower energy during deposition of the former. Walk southeast along the drainage for ~30 m (~100 ft) to an outcrop of east-trending planar cross-beds incised by north-trending trough cross-beds. Continue for ~100 m (~300 ft), keeping the drainage on your left (east), to where the utility line crosses the stream. At the utility pole next to the stream, there is an outcrop of north-trending planar cross-beds, possibly formed by straight-crested dunes in the northeast-flowing channels of the Bucks Mountain trend (Fig. 13). Follow the utility line southeast for ~140 m (~460 ft) to arrive at a flat outcrop of east-southeast to southeast-trending cross-bedding. As interpreted by Keller and Morgan (2013, 2016), we have walked upward from a Bucks Mountain trend horizon, having paleocurrent azimuths ranging from 334° to 78°, to an overlying main paleochannel belt horizon, having paleocurrents ranging from 75° to 188°.
Figure 14 is a diagram from Nichols (2009) illustrating straight-crested dunes and sinuous or isolated (linguoid or lunate) dunes, both of which appear to occur at this stop. Figure 15 (from Nichols, 2009) is a bedform stability diagram showing bedform type according to sediment grain size and flow velocity. Dunes tend to occur in coarser sediments and at greater flow velocities. Figure 16 is a large-scale paleocurrent map of the ranch adjoining the Brown property to the southeast, and depicts the same Tcr horizons we have just observed. The map shows how clusters of Tcr paleocurrent measurements can be grouped both areally and stratigraphically, as presented earlier.
Here, standing in the main paleochannel belt horizon of the upper Tcr, we are ~16 km (10 mi) southeast and down-paleo-current from the equivalent horizon at Castlewood Canyon State Park. Moving down-paleocurrent, there are distinct changes in gravel characteristics within the main paleochannel belt (Keller and Morgan, 2016). Mean clast size decreases, roundness increases, relative tuff content decreases, and relative granitic content increases. The lithologic changes may be due to the increasing distance downstream from the JA Ranch paleochannel tributary, which at Castlewood Canyon State Park may have supplied a large amount of tuff clasts to the main paleochannel belt. The increase in roundness can be similarly attributed, because the JA Ranch paleochannel contains a large proportion of angular tuff clasts. Angularity of tuff clasts would decrease downstream in the main paleochannel, and average roundness of the entire clast population would correspondingly increase.
Stop 5: HI-TEST Store (Elbert, Colorado) (Rest Rooms) HI-TEST store coordinates: N 39.2238°, W 104.536547° (WGS84)
From the Brown property, turn left (south) on to Elbert CR 13 and drive for ~1 km (~0.8 mi) to Steele Ave. (Elbert CR 98), then turn left (east). Take CR 98 to CR 21, turn right (south), and take CR 21 for ~0.4 km (~0.2 mi) to another leg of CR 98. Turn left (east) on CR 98 and follow it for ~4 km (~2 mi) to Elbert. At its end, CR 98 curves to the north (a school will be on the left here), then curves south to join a north-south gravel street in Elbert. After driving south for ~0.2 km (~0.1 mi) on this street, turn left on the east-heading gravel street, then after one block turn left (north) on Elbert Rd. (paved). Drive north on Elbert Rd. for ~0.3 km (~0.2 mi) and the HI-TEST store will be on the left (west).
Stop 6: Krupa Property (Castle Rock Conglomerate—Tcr) Krupa property entrance coordinates: N 39.257347°, W 104.606981° (WGS84)
From the HI-TEST store, drive for ~0.3 km (~0.2 mi) south on Elbert Rd., turn right (west) at the west-heading gravel street, then after one block turn right (north) on the north-south gravel street leading to Elbert CR 98. Go back up the hill, pass by the school, then drive west on CR 98 for ~4 km (~0.2 mi) to CR 21. Turn right (north) on CR 21 and go ~3 km (~2 mi) to CR 106. Turn left (west) on CR 106 and drive for ~3 km (~2 mi) to CR 13. Turn right (north) on CR 13, proceed for ~1 km (~0.5 mi) to the Krupa property on the left (red farm buildings), and turn in here. After ~130 m (~450 ft) on the entrance road, take the left fork and drive for ~0.4 km (~0.2 mi) southwest to a large new outbuilding (orange roof) and park there.
The purpose of Stop 6 is to again observe different Tcr horizons having markedly contrasting paleocurrent directions, indicating that the southeast-flowing main paleochannel belt was fed by northeast-flowing tributaries. Here the postulated tributary is the Bucks Mountain trend, which is located ~20 km southeast of the JA Ranch paleochannel tributary (the latter at Stop 2, Castlewood Canyon State Park). Approximately 60 m (~200 ft) southeast of the outbuilding there is a Tcr ledge several meters in relief and overlooking a stream. Before reaching the ledge, you will traverse an area of flat Tcr outcrop. Here there are four large troughs (meters in scale) that resemble sets of nested parabolas, with their open sides facing north and their axes trending north-south. This horizon is believed to be part of the northeast-flowing Bucks Mountain trend (Keller and Morgan, 2016). Across the short distance southeast from here to the ledge, we descend directly into a horizon of southeast-trending trough cross-bedding that is exposed in both plan view and within the dissected ledge. It is believed to be a horizon of the southeast-flowing main paleochannel belt, and the difference in paleocurrent directions between the two horizons is ~150°. The southeast-trending troughs are several times smaller than the north-trending ones, suggesting that depth and flow velocity were lesser (at this particular location) in the main paleochannel than in the tributary. In the upper Tcr, this far downstream in the main paleochannel belt, the sediment is finer than further upstream, and the diverging paleocurrent pattern begins to suggest a distributary stream system (Keller and Morgan, 2016). Just west of the spur of ledge we just examined, descend a short distance southeast, following a gentle gully and keeping the spur on your left. This location provides the relatively uncommon opportunity to readily view the same set of Tcr bedforms from the side, transverse to paleoflow, and from the top, parallel to paleoflow. The vertical face of the spur allows us a side view of trough cross-bedding in the main paleochannel belt; we are looking northeast and transverse to the flow direction, which is southeast (to the right) (Fig. 17). Ascending back up the gully to above the ledge, walk west for ~90 m (~300 ft) to a flat exposure containing a large southeast-trending trough. Many of the paleo-current measurements in the recent paleocurrent study (Keller and Morgan, 2013, 2016) were collected from occurrences like this one—not an excellent example, but one with enough definition to allow a paleocurrent measurement. On the way back to the outbuilding, we ascend back into the Bucks Mountain trend horizon, observing two large north-trending troughs.
Referring back to the earlier discussion of blue-gray quartz-ite clasts in the Tcr, no blue-gray quartzite has yet been found in the horizons of the northeast-flowing Bucks Mountain trend. This supports the hypothesis that the quartzite originated solely in Coal Creek Canyon south of Boulder.
Stop 7: “Cut-Stone” Quarry (Inactive) on Mencenburg Quarry Property (Wall Mountain Tuff—Twm) Mencenburg quarry entrance coordinates: N 39.320814°, W 104.812586° (WGS 84)
From the Krupa property entrance, turn right (south) on Elbert CR 13 and drive for ~1 km (~0.5 mi) to CR 106. Turn right (west) at CR 106 and drive for ~4 km (~2 mi) to CR 5. Turn left (south) on CR 5 and drive for ~4 km (~2 mi) to Steele Ave. (Elbert CR 98). Turn right (west) at CR 98 and drive for ~3 km (~2 mi) to East Cherry Creek Rd. Turn right (north) on to East Cherry Creek Rd. and drive for ~0.3 km (~0.2 mi) to Gillian Ave. (Douglas CR 78). Turn left (west) at CR 78 and drive for ~2 km (1 mi) to Colorado Hwy. 83. Turn right (north) at Hwy. 83 and drive for ~7 km (~4 mi) to Lake Gulch Rd. Turn left (west) at Lake Gulch Rd. and drive for ~9 km (~6 mi) to South Ridge Rd. Turn right (north) at South Ridge Rd. and drive for ~1 km (~0.5 mi) to the Mencenburg quarry entrance on the right (east) (there will be a sign here). Turn right (east) and follow the quarry road for ~1.5 km (~0.9 mi), past the gully to a dirt road on the left (north) (no sign here). Turn left (north), drive for ~0.3 km (~0.2 mi) to the inactive “cut-stone” quarry, and park there. Please stay on the quarry floor and avoid clambering on the tuff fragments; the piles may be unstable, and some fragments have sharp edges. Rattlesnakes have been reported here. This our last geological stop before we head back to Denver.
The purpose of Stop 7 is to observe the Twm in place and discuss its origin and relation to the Tcr. While we are presenting, we will again pass around a copy of Ancient Denvers (Johnson et al., 2002), open to the illustration of the Twm eruption. The Twm is a rhyolitic, moderately to densely welded tuff with a wide range of subdued colors: light-brown, lavender, pink, reddish-brown, and maroon. The groundmass is fine-grained with small phenocrysts of biotite and sanidine. The tuff is late Eocene in age and has been dated at 36.7 m.y. (McIntosh and Chapin, 1994). It is believed to be from an ignimbrite eruption in the Sawatch Range, ~100 km west of Castle Rock (Larsen and Lipman, this volume). The erupted ash flow contained enough heat to be welded as a viscous plastic, and to develop some flow structures before cooling. The Twm ranges in thickness between 2 and 16 m. On most outcrops, the tuff has a horizontal fracture pattern, with hackley plates 10–22 cm thick (Thorson, 2011), and this fracturing is evident in Figure 18. The tuff caps some higher buttes, and small rubble outcrops also occur at higher elevations (e.g., at Prairie Canyon Ranch, Stop 3). Twm occurrences seen in direct contact with Tcr are rare, although an exposure of Tcr directly overlying Twm was photographed by Morse (1985) at a location in Sellers Gulch, south of Castle Rock. The Tcr was deposited mostly on Tda paleotopography, upon which the Twm already had erupted, and thus (as we saw at Stop 3) the younger conglomerate can be found at lower elevations than the older tuff. As presented earlier, Twm clasts are characteristic of the Tcr. While viewing Twm here at the quarry, note that flow layering and lithic fragments occur in some of the blocks; there are lithic fragments in the block lying on the west side of the quarry entrance.
The Wall Mountain Tuff also is known as the Castle Rock Rhyolite, and historically was a widely used building stone in the Denver area. The material is attractive in its various colors, fractures conchoidally, is easy to shape, and is relatively light in weight (Simmons, 2013). St. Elizabeth of Hungary Church, located across Speer Blvd. from the Colorado Convention Center (the site of the 2016 GSA Annual Meeting), is a late-nineteenth-century Denver landmark built of this stone. The Twm/Castle Rock Rhyolite was first quarried in 1872 at a location south of Castle Rock. Other quarry operations began not long afterward, and the industry benefited from an extension of the rail line from Denver. Quarrying of the rhyolite for building stone ceased in 1906 owing to the prevalence of brick and concrete (Simmons, 2013). The Mencenburg quarry, located ~2 km (~1 mi) east of here, currently produces Twm/Castle Rock Rhyolite for use as aggregate. In modern times, a small amount of material from the “cut-stone” quarry was hand-carved for use as veneer, but this operation ended in 2008. As we leave the quarry, look left (east) to view Twm outcrop along the drainage.
Stop 8: King Soopers on Ridge Rd. (Rest Rooms) King Soopers coordinates: N 39.375828°, W 104.760856° (WGS84)
From the “cut-stone” quarry, drive south for ~0.3 km (~0.2 mi) to the quarry road, turn right (west) and drive for ~1.5 km (~0.9 mi) to the Mencenburg quarry gate. Turn right (north) on to Ridge Rd. and drive for ~7 km (~4 mi) to King Soopers, on the right (east) just before the signal light at the intersection with Colorado Hwy. 86.
Return to Colorado Convention Center (Denver)
From King Soopers, turn right (north) on to Ridge Rd., proceed north across Hwy. 86 (signal light), and take Founders Pkwy. for ~7 km (~4 mi) to the northbound entrance to I-25. Drive north on I-25 for ~43 km (~27 mi) back to Denver and take exit 211 for southbound Speer Blvd. Drive south on Speer Blvd. for ~1 km (~0.6 mi), turn left (north) on Blake St., and immediately turn right (east) on 14th St. Drive for five blocks on 14th St. Champa St. will be the fifth block, and the Colorado Convention Center will be on the right (south) immediately after Champa St.
The authors are grateful for the assistance received in preparing this Castle Rock Conglomerate field trip. Property access was graciously given by Don Opheim of Schmidt Aggregates, Castle Rock; property owners Irene and Phil Brown, and Kathy and John Krupa, all of Elbert County; and Mary Ann Monzani of Douglas County Open Space & Natural Resources. Manuscript reviewers were Vince Matthews of Leadville Geology, LLC and Lon Abbott of the University of Colorado at Boulder. Larry Scott of the Colorado Geological Survey prepared several of the figures.
Figures & Tables
Unfolding the Geology of the West
Prepared in conjunction with the 2016 GSA Annual Meeting in Denver, Colorado, this volume contains sixteen guides to field trips in this rich geologic region. The four “Great Surveys” of the late 1800s ventured west to explore and document the region’s unknown natural resources and collect valuable geologic information. Many of the field guides in this volume, aptly titled Unfolding the Geology of the West, will cover the same hallowed ground as the early geologic expeditions. Organized into four sections, this volume spans some of the major subdisciplines of geology: (1) stratigraphy, sedimentology, and paleontology; (2) structure and metamorphism; (3) Quaternary landscape evolution; and (4) engineering and environmental geology.