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?

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.