--> Abstract: Cool-Water Origin of the Thirtyone Formation (Lower Devonian), Tobosa Basin, Subsurface of West Texas, by Gregory P. Wahlman and Ian W. Moxon; #90039 (2005)

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Cool-Water Origin of the Thirtyone Formation (Lower Devonian), Tobosa Basin, Subsurface of West Texas

Gregory P. Wahlman1 and Ian W. Moxon2
1 BP America, Houston, TX
2 BP America Production Co, Houston, TX

The Thirtyone Formation (Lower Devonian), subsurface west Texas, consists of up to 1000 ft. of chert, cherty limestone, and limestone that display many characteristics of cool-water carbonate depositional systems. The basal part of the formation consists of dark-gray to white, laminated to burrowed cherts deposited in deepwater basinal settings. The lower transgressive basinal facies grades gradually upwards into a coarsening-upward succession of very fine- to coarse-grained cherty packstone and grainstone deposited in a broad deepwater ramp slope setting. Slope turbidite channel facies and basinal fan complexes are recognizable. Ichnofossils are abundant and often well-preserved in the grain-rich cherty carbonates. Essentially all bioclasts are disarticulated and sand-sized, but are neither worn nor encrusted. Sponge spicules are the dominant bioclast in the lowermost chert-rich part of the section. Throughout the overlying thicker succession of limestone and cherty limestone, crinoid ossicles and fenestrate bryozoan fragments are overwhelmingly dominant, with associated sparse ostracod valves and brachiopod shell fragments. Significantly, the Early Devonian Tobosa Basin was located at 40-45 degrees South paleolatitude along the southwest paleocontinental margin, and counterclockwise circulation of adjacent open-ocean currents would have brought colder seawater from higher paleolatitudes to upwell along the seaward basin margin. Therefore, the interpretation of a cool-water depositional setting for the Thirtyone formation is supported by: paleogeographic location, the abundance of chert in the system, the occurrence of only low-Mg calcite bioclasts throughout the limestone section, the absence of any warm-water indicators (e.g., calcareous algae, corals, coated grains, etc.), and the relatively low carbonate mud content through most of the section.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005