NEW PALEOCURRENT MEASUREMENTS IN THE LATE EOCENE CASTLE ROCK CONGLOMERATE, EAST-CENTRAL COLORADO: REMAPPING THE FLUVIAL SYSTEM

The late Eocene Castle Rock Conglomerate adjoins the east side of the Colorado Front Range and lies within the Colorado Piedmont. It may reach 40 m or more in thickness, is nearly flat-lying, and is discontinuous, capping both high-relief mesas and gentle hills. The unit occurs in a south-southeast-trending swath 65 km in length and 4 km to 10 km in width, extending from north of the town of Castle Rock to southeast of the town of Elbert. The conglomerate has an arkosic coarse sand and granule matrix with abundant pebble to boulder-sized clasts that decrease in frequency upward. Large trough cross beds are common in the fairly well exposed upper portion of the unit, and in this interval a comprehensive survey of paleocurrent directions from trough cross beds yielded 2,897 measurements from flat ledge and pavement exposures. At each trough we measured the axis azimuth (paleocurrent direction) and several other features. Length-azimuth rose diagrams and large-scale digital topographic maps allowed the paleocurrent data to be areally and stratigraphically discretized into 411 local paleocurrent directions. The result is a paleocurrent map of the south-southeast-flowing main paleochannel, and the recognition of two hitherto not described, east to northeast-flowing tributary paleochannnels that originated west and southwest of the main paleochannel. Both are approximately 12 km in length and 1 km to 2 km in width, and both display inverted topography along their upper reaches. In the area of Castlewood Canyon State Park the northern tributary widened northeastward to become an alluvial fan, which now is partially overlain by deposits of the main paleochannel. The southern tributary joined the main paleochannel in an area east of Running Creek and west of Elbert. Twenty-four clast surveys (10,804 clasts) of the Castle Rock Conglomerate were performed and each survey corresponded to a single trough. We recorded lithology, maximum dimension, and roundness of all clasts > 2 cm. Consolidated histograms of the 15 clast surveys in the main paleochannel versus the nine in the tributaries indicate some marked differences between the two populations. Histograms of local pairs and groups of main paleochannel surveys versus tributary surveys indicate significant differences, especially in clast lithology and size distribution.