MAPPING AND A NEW DATE IN BASALTS IN THE SOUTH FORK OF THE CROOKED RIVER REGION OF OREGON ILLUMINATES THE REGIONAL RELATIONSHIPS BETWEEN THE MASCALL FORMATION AND DIFFERENT PHASES OF THE COLUMBIA RIVER BASALTS

Detailed mapping of six 7.5 minute USGS 1:24,000 quadrangles in the South Fork of the Crooked River area in central Oregon in conjunction with the University of Oregon Field Camp allows us to construct the geologic history of the area during the eruption of the Columbia River Basalt Group (CRBG). We are interested in this time period because rapid climate and faunal changes, volcanism, and tectonics dramatically changed the landscape and ecology of the region. After the extrusion of the local CRBG (mapped as Picture Gorge Basalts) the Mascall Formation was deposited, consisting of fluvial and lacustrine sediments with variable soil development and interbedded tuffs. A new zircon date from an interbedded tuff near the base of what has been mapped as the Picture Gorge Basalts, a member of the CRBG, is 16.549 ± 0.007 Ma, ~0.5 Ma older than the widely accepted age of the type Picture Gorge Basalts. This new age, in conjunction with ages from two tuffs in the overlying Hawk Rim Mascall requires that the extrusion and folding of the Picture Gorge Basalts and the deposition of the lowest Mascall occurred in a maximum of 100,000 years. The Mascall Formation thickens in synclines and fills in steep paleotopography consistent with rapid deformation and deposition. This new date for the CRBG in this area suggests that these flows and tuffs are older than the type Picture Gorge Basalts that are dated at 16.0 Ma. Likely, we have a completely different eruptive event within the CRBG associated with dikes found in the Camp Creek area. Although it is possible that the 16.0 Ma date for the type Picture Gorge is inaccurate and diagnostic fossils from the two areas come from different portions of the section. Currently we are re-dating the type Picture Gorge Basalts tuff to distinguish if we have completely different eruptive events. We speculate that blockage of drainages by volcanism and extremely rapid tectonics created accommodation space that allowed the deposition of the Mascall Formation.