Abstract: Relationship Between Fluvial Facies and Paleoclimate of Paleocene-Lower Eocene Rocks and the Timing of Laramide Uplifts in the Southern Piceance Basin, Western Colorado

Ronald C. Johnson, Romeo M. Flores, Douglas J. Nichols

Paleocene and lower Eocene rocks in the southern Piceance Basin record climatic changes and shifts in sediment sources controlled by tectonism. The middle(?) and upper Paleocene Atwell Gulch Member of the Wasatch Formation unconformably overlies Cretaceous rocks throughout the study area. Although the member thins toward the Uncompahgre uplift to the southwest, the White River uplift to the east, and the Douglas Creek arch to the west, it was probably derived from an andesitic volcanic source to the southeast. In the southeastern part of the basin, the Atwell Gulch Member contains about 50% conglomeratic, lenticular, multiscoured and multistorey sandstone containing andesitic pebbles that was deposited by northerly flowing braided streams. In the southwest part of the basin, the Atwel Gulch consists of smectitic floodplain mudstone and sparse channel sandstone that were deposited by northwest-trending braided and meandering streams. A shift from variegated paleosoil development in the lower part of the Atwell Gulch Member to coal deposition in the upper part indicates a change from short-wet-seasonal to humid climate conditions.

A period of northeastward regional tilting, which probably marks the rise of the Uncompahgre uplift to the southwest, followed deposition of the Atwell Gulch Member. Northeast-trending paleovalleys were cut and later filled by gas-productive fluvial sandstone of the overlying Molina Member of the Wasatch Formation (uppermost Paleocene? to lower Eocene). These sandstones later spread across the entire southern part of the basin, the Douglas Creek arch, and the White River uplift, indicating that the uplifts were not topographically elevated at this time. The lack of volcanic detritus in the sandstones indicates that the rise of the Uncompahgre cut off the southerly source of volcanics. The source of the sand was apparently the eroding sedimentary cover of the Uncompahgre. Finally, the resence of variegated paleosols interbedded with the sandstone record a return to a seasonal wet climate.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994