ABSTRACT: Upper Strawn (Desmoinesian) Carbonate and Clastic Depositional Environments, Southeastern King County, Texas

Todd H. Boring

The Pennsylvanian upper Strawn Group of southeastern King County, Texas, provides a unique setting to study interactions between coeval carbonate and clastic deposition during the Desmoinesian. One of the most perplexing problems is the relationship of massive Pennsylvanian platform carbonates to shallow-water terrigenous clastic sediments.

Within the study area, carbonate facies were deposited along the northern edge of the Knox-Baylor trough on the Spur platform, and terrigenous clastics were carried toward the Midland basin through the Knox-Baylor trough. Based on the analysis of subsurface cores, five carbonate lithofacies and four clastic lithofacies were recognized in southeastern King County, Texas. The distribution and geometry of these lithofacies are related to variations in the rate of subsidence in the Knox-Baylor trough, Pennsylvanian tectonics, deltaic progradation, avulsion, and compaction.

The platform carbonates within the northern region of southeastern King County record environments within the carbonate platform complex, including middle platform, outer platform, algal mound, and platform margin. The typical carbonate cycle consists of the following facies, from bottom to top: (1) algal bioclastic wackestones, (2) crinoidal wackestones, (3) algal bioclastic packstones-grainstones/fusulinid crinoidal packstones-grainstones, and (4) crinoidal bryozoan wackestones/shales.

During the Desmoinesian, clastic sediment derived from the Wichita and Arbuckle mountains filled the Knox-Baylor trough. The quartzarenitic sandstones within the southern region of southeastern King County occur in a variety of complex depositional geometries, including distributary-bar fingers, lobate deltas, and offshore bars. Cores of these sandstones represent mainly the uppermost portion of the various sandstone bodies. A typical core consists of the following facies, from bottom to top: (1) cross-bedded sandstones, (2) intercalated sandstones and shales, and (3) mudstones/shales.

The upper Strawn Group provides an attractive area for exploration geology. Both carbonates and clastics provide excellent reservoirs from a depth of approximately 5000-6000 ft. Total production within the area is over 100 million bbl of oil since the early 1940s. Multiple pay zones within a 600-ft interval also provide an added incentive for exploration. Areas within and around the Knox-Baylor trough deserve a detailed study due to these relatively shallow, unexplored, multiple pay zones.

AAPG Search and Discovery Article #90096©1990 AAPG Southwest Section Meeting, Wichita Falls, Texas, March 11-13, 1990