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Clast lithology, population, provenance, and U-Pb geochronology: What can the late Eocene Castle Rock Conglomerate tell us about the history of the Colorado Piedmont and Front Range?

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Matthew L. Morgan at Colorado Geological Survey
Matthew L. Morgan
  • Colorado Geological Survey
S.M. Keller
S.M. Keller
S.M. Keller
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Wayne Premo at United States Geological Survey
Wayne Premo
Daniel Miggins at Oregon State University
Daniel Miggins
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CLAST LITHOLOGY, POPULATION, PROVENANCE, AND U-PB
GEOCHRONOLOGY: WHAT CAN THE LATE EOCENE CASTLE ROCK
CONGLOMERATE TELL US ABOUT THE HISTORY OF THE
COLORADO PIEDMONT AND FRONT RANGE?
MORGAN, Matthew L., Colorado Geological Survey, Colorado School of Mines, Golden,
CO 80401, matt.morgan@state.co.us, KELLER, Stephen M., Wheat Ridge, CO 80033,
PREMO, Wayne R., USGS,MS 963, Denver Federal Center, Denver, CO 80225, MIGGINS,
Daniel P., College of Ocean and Atmospheric Sciences, Oregon State University, Corvallis,
OR 97331, and MOSCATI, Richard J., U.S. Geological Survey, Box 25046, MS963, Denver
Federal Center, Denver, CO 80225
The Castle Rock Conglomerate is a late Eocene conglomeratic fluvial deposit that caps
buttes and mesas within east-central Colorado. Its main paleochannel occupied a south-
southeast-trending valley that now extends ~65 km from its northernmost exposures near
the town of Castle Rock. Newly documented east to northeast-flowing tributary
paleochannels originated west and southwest of the main paleochannel. As part of our
paleocurrent study, we surveyed ~11,000 clasts at 24 locations, which revealed that the
coarse-grained parts of the unit are composed of pebble- to boulder-sized clasts of (in order
of decreasing population): granite, Wall Mountain Tuff, quartz, blue-gray quartzite, other
quartzites, and probable Lower Paleozoic sedimentary rocks. The concentrations of granitic
clasts are slightly greater in the main paleochannel versus the tributaries. Conversely, the
Wall Mountain Tuff is consistently more prominent in the tributaries. The concentration of
these clasts is in agreement with the paleo-distribution and preserved outcrops of Wall
Mountain Tuff in the tributary source areas. The blue-gray quartzite, suggested by previous
workers to originate solely from Coal Creek Canyon south of Boulder, is ubiquitous in the
main paleochannel and present at several locations in the tributaries; this suggests
additional source areas for the blue-gray quartzite. 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 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 Front
Range. Absent from our 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 not yet exposed during the late Eocene. This study indicates that large quantities of
granitic and volcanic material existed along the Front Range; this material likely buried the
Mesozoic section along the range front and left part of the Paleozoic section (mainly
Fountain Fm.) exposed. Exhumation of the Mesozoic rocks occurred sometime after the
deposition of the Castle Rock Conglomerate.
Abstract ID#: 225084
Password: 771944
Meeting: 2013 GSA Annual Meeting in Denver: 125th Anniversary of GSA (27-30 October
2013)
Session Type: Topical/Theme
Selection: T196. Ancient Floodplains and Rivers: Unraveling the Mysteries of Colorado’s
Conglomerates
Title: CLAST LITHOLOGY, POPULATION, PROVENANCE, AND U-PB
GEOCHRONOLOGY: WHAT CAN THE LATE EOCENE CASTLE ROCK
CONGLOMERATE TELL US ABOUT THE HISTORY OF THE COLORADO PIEDMONT
AND FRONT RANGE?
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Opal Cement in the Eocene Castle Rock Conglomerate, Central Colorado
Article
Full-text available
  • Feb 2024
The Castle Rock Conglomerate is one of Colorado's most iconic, youngest, and coarsest grained rock units. It is also one of the hardest sedimentary rocks in Colorado and forms prominent buttes in the southwestern Denver Basin. Yet the reasons for its induration and resistance to weathering have not previously been investigated. Sedimen-tologic observations paired with sedimentary petrology indicate that much of the unit is comprised of a planar-bedded to cross-bedded, mostly poorly sorted, angular to subrounded assemblage of quartz, K-feldspar, quartzite, and unusually large volcanic rock fragments along with some plagioclase and mica flakes. The largest volcanic rock fragments are up to ~2 m in size and composed of the immediately subjacent Wall Mountain Tuff of late Eocene age. Sedimentary rock fragments and well-rounded quartz grains are rare. Together these features suggest a diverse and relatively proximal provenance for the unit. Pervasive opaline cement coats most grains, and locally exhibits pendant features typical of vadose precipitation. These opal cements formed prior to any grain compaction and indicate early silica precipitation at shallow burial depths. Where the primary pores were not completely cemented by the opal, most were later filled with length-fast chalcedony cement. We hypothesize that cementation of the conglomerate began soon after deposition as weathering of the Wall Mountain Tuff and weathering of clasts of the tuff within the conglomerate, yielded ground water super-saturated with silica. These fluids initially catalyzed precipitation of common opal (hydrous amorphous silica) and later fostered precipitation of length-fast chalcedony. Together, these cements created a silica-cemented "con-crete" much more resistant to weathering than any carbonate-cemented sandstone, and much harder than man-made calcite-cemented concrete found in many sidewalks and roadways.
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Provenance of the upper Eocene Castle Rock Conglomerate, south Denver Basin, Colorado, U.S.A.
Article
  • Jun 2018
The Castle Rock Conglomerate contains distinctive clasts from the Colorado Front Range, and when combined with detrital zircon ages, the unit can be subdivided into two lithofacies. Precambrian quartzites and stretched-pebble conglomerates from Coal Creek Canyon (to the northwest of the Castle Rock Conglomerate outcrop belt) and detrital zircons from Precambrian and Tertiary igneous rocks identify a northern provenance with detritus derived from tens of kilometers northwest of Denver, Colorado. A second source, composed of mainly granite from the Pikes Peak batholith, lies in the southern Front Range west of the Castle Rock Conglomerate outcrop belt. Both the north and west lithofacies can be mapped in the Castle Rock Conglomerate outcrop belt by using the presence (north) and absence (west) of Coal Creek Canyon quartzite clasts. This distinction is confirmed by detrital zircon ages. The north lithofacies dominates the present-day, northernmost outcrops, but dilution and interbedding with west lithofacies increase as the southeast-flowing basin axial paleodrainage meets piedmont tributaries that carried Pikes Peak batholith detritus from the west and southwest. The basin axial drainage transported coarse conglomerate southward about 120 km during Castle Rock Conglomerate deposition (36.7-34.0 Ma). The Precambrian quartzite exposed in Coal Creek Canyon is interpreted to be an important point source that can be useful in provenance studies of sediments shed from the Colorado Front Range. Additionally, detrital zircons from Laramide-age igneous rocks show potential for improved stratigraphic resolution in Paleogene strata of the Denver Basin.
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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
Conference Paper
  • Sep 2016
The Castle Rock Conglomerate (Tcr) 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 Tcr 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 Tcr 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 Tcr 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.
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