PALEOGEOGRAPHY OF THE WALL MOUNTAIN TUFF IN THE CASTLE ROCK AREA OF COLORADO

New mapping and evaluation of the Wall Mtn Tuff (Twm) west of Castle Rock show ”in place” rhyolite capping terraces that lie 150 ft below previously mapped Twm. The lower position of the in-place rhyolite within one quarter mile of the upper mesa rhyolite strongly suggests previously unrecognized paleotopography ,namely, a broad river valley formed prior to the arrival of the ash flow. Because the valley was wide (3-4 mi) and long (6-8 mi) it was not filled by the ash flow, which cascaded into and across the low, depositing a layer of rhyolite on the valley surface. The steeper slope and rugosity of the valley surface gives the rhyolite layer a steeper dip near the valley margin and a more undulating surface than the upper mesa. Because the ash flow was near its termination, 90 mi from its source, the paleogeography present in the Castle Rock area influenced the ultimate distribution of the Twm, deflecting it down the stream valleys and around paleotopographic highs such as Daniels Park and Diamond Ridge, where Twm is absent. Recognition of this paleotopography sets the stage for subsequent erosional and depositional events that help explain the newly found strata deposited in the time interval between the Twm and Castle Rock Conglomerate. Reconstructions of the valley, which drained to the northwest, also give more realistic estimates of the depth of subsequent erosion into the Twm and underlying strata.