ABSTRACT

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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