WILLIAMS, MARK R., Colorado School of Mines, Golden, CO

ABSTRACT: Base-Level Conditions Resulting in the Preservation of Highly Aggradational Eolian Sandstones in a Late Pennsylvanian, Mixed Carbonate-Siliciclastic Depositional System, Western Paradox Basin, Utah

The facies variations and stacking geometry of a single, reservoir scale sequence within the Upper Hermosa Formation of Dark Canyon were examined by tracing the geometry of bed bounding surfaces on a dip oriented, photo-mosaic calibrated to outcrop. Open marine, cherty carbonate mudstones containing a thin glauconitic shale bed overlie an erosional marine flooding surface. Succeeding carbonate beds shoal vertically, with increasing sand content and a restricted faunal assemblage dominated by silicified pelecypods. These beds exhibit toplap with progradational geometry. They are capped by thin caliches and sand filled fissures providing evidence for exposure of the carbonate ramp and the development of a surface of sediment bypass. A sharp, regionally traceable contact separates these andy carbonates from a thick bed of steeply cross-stratified eolian sandstone. This unit grades down-dip and vertically into marine sandstone indicating that eolian deposition was terminated here by a water table rising at a greater rate than eolian sediment supply. Overlying, laterally discontinuous, landward stepping colian sandstones are interbedded with fine grained playa deposits containing mud cracks and rhizocretions which provide evidence for periodic subaerial exposure and decreased sediment accommodation. These sandstones are erosionally overlain by marine carbonates of the succeeding sequence of sedimentation.

These stacking geometries and facies successions indicate that falling base-level conditions prevailed between deposition of the glauconitic shale bed and the uppermost caliche cap, with rising base-level conditions prevailing through the remainder of the sequence. The position of the thick eolian sandstone bed immediately above the exposed carbonates ramp indicates that the potential for accumulation and preservation of aggradational eolian deposits is greatest during base-level rise and the consequent landward shift in sediment accommodation.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.