Depositional Significance of Siliciclastic Component of the Lower Cretaceous Pearsall Cow Creek Limestone Member in the Western Maverick Basin, South Texas

Robert G. Loucks¹ and David C. Hull²
¹Bureau of Economic Geology, Jackson School of Geosciences, University of Texas at Austin, University Station, Box X, Austin, Texas 78713–8924
²Chesapeake Energy Corporation, 6100 N. Western Ave., Oklahoma City, Oklahoma 73118

The updip Cow Creek Member of the Pearsall Formation in the South Texas area is dominated by an extensive carbonate-shoal complex composed of echinoid-mollusk lime packstone and grainstone with minor boundstone development. In contrast to this broad carbonate-shoal development is an area of higher siliciclastic input in western Maverick County on the west side of the Maverick Basin. The carbonate-rich sandstones are composed predominantly of fine- to medium-grained quartz sand (quartzarenite) and fossils (oysters, other mollusks, serpulid worm tubes, and echinoid fragments). Much of the calcareous sandstone is bioturbated, but several intervals display well-developed crossbedding. Interbedded with the sandstone are carbonatedominated units with various mixtures of siliciclastic sand. In a cored section in the Dilley Ritchie #1 well, the upper Cow Creek third-order sequence is 108 ft thick. It contains seven high-frequency cycles, with the lower cycle being a possible lowstand-system incised channel fill. The transgressive part of each cycle is carbonate rich, whereas the progradational part is generally siliciclastic rich, except for the youngest cycle, which is a skeletal/ooid-coated-grain lime grainstone. The Pine Island Shale/lower Cow Creek Members below the upper Cow Creek Member are predominantly terrigenous mudrocks grading up into sandy lime packstones and wackestones. The transgressive lower section of the lower Bexar Shale Member, above the upper Cow Creek Member, is composed of argillaceous lime mudstone, wackestones, and packstones. The Pine Island Shale Member was deposited during the oceanic anoxic event (OAE) 1–A, the updip Cow Creek Member was deposited during a third-order sequence in an incised-valley to shoreface system, and the lower Bexar Shale Member was deposited in a transgressive system leading into a regional oceanic anoxic event.

AAPG Search and Discovery Article #90158©2012 GCAGS and GC-SEPM 6nd Annual Convention, Austin, Texas, 21-24 October 2012