EROSIONAL AND DEPOSITIONAL EVENTS IN THE TIME INTERVAL BETWEEN THE WALL MOUNTAIN TUFF AND THE CASTLE ROCK CONGLOMERATE (CASTLE ROCK AREA, COLORADO)

Newly discovered outcrops on Cherokee Ranch, near Sedalia have added insight into the events that occurred after deposition of the Wall Mountain Tuff (Twm) and before deposition of the Castle Rock Conglomerate (Tcr). A 200-ft section of strata show evidence of a deep erosion cycle following the Twm event that cut 120-ft canyons into the rhyolite and underlying Dawson (D2 sequence). The canyons were then filled with a mixture of current-bedded sandstone and gravel and huge blocks (up to 12 ft diameter) of Twm, which had collapsed into the channel fill from undercut canyon walls. The breccia-like canyon fill was then buried by a fining-upward sequence of mainly sandstone with floating pebbles of Twm. This depositional cycle refilled the valley to above the level of the Twm, to an elevation of about 6600 ft at Cherokee Mountain. This cut-and-fill sequence is located preferentially in broad paleogeographic lows that predated the Twm. Therefore, the deposit is largely removed by erosion but well preserved in a few outcrops capped by Tcr. Although most mappers would designate this deposit as Tcr because it contains Twm clasts, the entire 200 ft of section directly underlies 50 ft of Castle Rock Conglomerate capping Cherokee Mtn. Two to three million years or more could have passed after Twm deposition and before the Castle Rock Conglomerate arrived from the north and west. Although the new sequence contains clasts of Twm, it is distinctly different from the Tcr in clast composition, texture, cementation, and current directions.