Abstract: Lithofacies Mapping of Pennsylvanian Sequences in the Paradox Evaporite Basin of Utah, Colorado, New Mexico and Arizona

Donald L. Rasmussen

Each of the thirty-five evaporite sequences identified in the Pennsylvanian Paradox Member of the Hermosa Formation have predictable cyclic lithofacies variations. Using the regionally-correlative bounding transgressive flooding surfaces (sharp boundary with overlying clastics) and the wide-spread internal carbonates, each sequence can be recognized and studied beyond the defined limits of its respective evaporite depocenter to adjacent platform or shelf areas where deposition was dominated by carbonates and/or clastics. Depositional trends, lithofacies distribution or changes in distribution, and carbonate and clastic reservoirs for each sequence can be often directly related to tectonic trends and sometimes used to indicate hydrocarbon exploration trends.

Lowstand, transgressive, and highstand system tracts can be identified, each often separated with sharp lithologic boundaries. Lowstand systems tracts are often characterized by deposition of evaporites with interbedded thin dolomites and organic-rich shales, and interbedded or overlying coarser clastics. Evaporite trends usually parallel the Uncompaghre uplift and may thicken into local topographic lows and between carbonate build-ups (forming traps). Transgressive systems tracts have a basal flooding surface often marked with very organic-rich shales and mudstones (source-rocks). Interbedded clastics are coarser and more numerous toward the Uncompaghre upland and may be important gas reservoirs. Highstand system tracts contain the bulk of the carbonates in the distal shelf and deepe basin but contain significant interbedded coarse clastics proximal to the Uncompaghre upland. Important oil and gas reservoirs occur in algal mounds and dolomitization trends within these carbonates.

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