THE CASTLE ROCK CONGLOMERATE

The Castle Rock Conglomerate is the youngest preserved sedimentary rock unit in the Denver Basin. It is a thick-bedded, arkosic conglomerate with large blocks of granite, Fountain Formation conglomerate, quartzite, and Wall Mountain Tuff. The conglomerates are interbedded with rare beds of tuffaceous brown mudstone that contains mammal bones. At the thickest outcrops of the Castle Rock, such as at Castlewood Canyon, bedding includes huge crossbed sets of normally graded, coarse conglomerate, capped by thinner, but wide trough crossbed sets in granular conglomerate. The age of the Castle Rock Conglomerate is latest Eocene, as indicated by included fragments of the 36.7 Ma Wall Mountain Tuff and the bones of large Chadronian brontotheres that became extinct 35.5 Ma. The Castle Rock Conglomerate was deposited primarily in a large, southeast-trending paleovalley that cut below the surface of the Wall Mountain Tuff. Paleocurrent data derived from the largest crossbed sets and provenance of the gravel-clasts indicate northwest- to southeast-flow of the river that cut the Castle Rock paleovalley. Clasts of bluish-gray quartzite and stretch-pebble metaconglomerates in the Castle Rock indicate Front Range sources from Coal Creek Canyon, 75 km northwest of the northern-most outcrops of the Castle Rock. These northern-most exposures contain blocks of Coal Creek quartzite as much as 2 m in diameter, indicating transport in flows with very large discharges. The Castle Rock Conglomerate is one of a series of latest Eocene conglomerates containing huge blocks that occur from central Wyoming to the South Park region. These contemporaneous conglomerates with large blocks suggest the widespread occurrence of multiple large storms that allowed transport of large blocks many kilometers downstream from the source areas. Finally, the modern southeastern Castle Rock outcrops (originally downstream) are 300 m higher in elevation than the northwestern outcrops (originally upstream), indicating significant northward tilting of the southern Denver Basin during the Neogene.