OF-16-01 New Paleocurrent Measurements, Clast Population Data, and Age Dates in the Late Eocene Castle Rock Conglomerate, East-Central Colorado

This report is the result of a multi-year field study by Stephen M. Keller (P. Geo, PG), and Matthew L. Morgan (CGS Deputy Director). Digital PDF download. OF-16-01D

This new publication is a substantial addition to knowledge of the Eocene Castle Rock Conglomerate which, since the 19th century, has been of great interest both to geologists and to the general public. The Castle Rock Conglomerate (CRC) is a late Eocene fluvial deposit flanking the east side of the Colorado Front Range and lying within the Colorado Piedmont. It is the youngest preserved sedimentary rock unit in the Denver Basin. It may reach about 70 m (230 ft) in thickness, is nearly flat lying, and is discontinuous, capping both high-relief mesas and gentle hills. The unit occurs in a southeast-trending swath 63 km (39 mi) in length and about 3 to 10 km (2 to 6 mi) in width, extending from north of Castle Rock to southeast of Elbert. The conglomerate has an arkosic, coarse sand and granule matrix with abundant pebble- to boulder-sized clasts that decrease in frequency upward.

The report is one outcome of a long-term CGS research project examining ancient current (paleocurrent) directions in the stream system that deposited the Castle Rock Conglomerate. It provides the first comprehensive paleocurrent map of the unit as well as the first quantitative description of the formation’s gravel content.

It also presents a detailed paleocurrent analysis indicating the existence of northeast-flowing tributaries to the main southeast-flowing Castle Rock Conglomerate stream system. In addition to refining the paleogeography of the Castle Rock Conglomerate, the report augments an understanding of the hydrogeologic framework of the Colorado Piedmont and the depositional and stratigraphic setting of the High Plains aquifer.