LITHOFACIES WITHIN THE LATE EOCENE CASTLE ROCK CONGLOMERATE, CASTLE ROCK AREA, COLORADO

The Castle Rock Conglomerate (Tcr) can be subdivided into two mappable lithofacies, a western facies derived from mainly Pikes Peak Granite, and a northern facies that contains distinctive Precambrian quartzite and quartzite conglomerate from Coal Creek Canyon, northwest of Denver. Both facies are comprised of arkosic sandstone and conglomerate that contain clasts of Wall Mountain Tuff.

The two facies are interstratified along the paleo-Cherry Creek corridor, thought to be part of the southeast-flowing axial drainage for the Denver Basin during Late Eocene time. A “quartzite line” can be mapped that approximates the western edge of the northern facies.

The northern facies shows a consistent paleocurrent direction of SSE. The western facies is more complicated showing northerly flow and then ESE flow as paleovalleys filled spilling sediments eastward over paleo-interfluves into the SE-flowing axial drainage.

Flow regimes of both facies show an alternation between normal fluvial flow conditions and high volume-short duration “flood events”. The flood event strata are coarse conglomerate with spectacular trough cross strata. Some of the northern facies flood events carried boulder-sized clasts as far south as Calhan, 85 miles southeast of Coal Creek Canyon. The flood conglomerates are cliff formers, highly cemented with opal and chalcedony.

Preliminary laser ablation age analysis of Tcr sandstone shows zircon ages of 1.7-1.4 Ga in the northern facies. The western facies is dominated by zircon ages of 1.0 Ga.