Integrated stratigraphy and petrology of the Silurian Wink formation (Wenlock-Ludlow) Howard County, Texas

The Cobra-Frazier #1 well in Howard County penetrated the Silurian Wristen Group and Fusselman Formation, and a core with sixty feet of strata was recovered, the majority of which belongs to the Wink Formation of the Wristen Group. Petrologic, petrographic, and geochemical studies allow interpretation of the depositional and diagenetic history of the Wink Formation in the eastern margin of the Permian Basin region.

The Wink of the Cobra-Frazier #1 consists of argillaceous skeletal wackestones and mudstones that are dominated by a deep water fauna dominated by arthropods. Conodont faunas indicate that the age of the Wink ranges from the early Sheinwoodian (Wenlock), into the Gorstian (Ludlow) and perhaps as young as the early Ludfordian. The Wink Formation was deposited when the exposed Fusselman platform was flooded in the early Sheinwoodian. Deposition continued in a carbonate ramp setting until differential subsidence produced the platform and basin fades of the Fasken and Frame formations.

During the hiatus that separated the Thirtyone Formation from the Woodford Formation, the Wink was karsted, evidenced by the presence of crackle, mosaic, and chaotic breccia fabrics related to paleocave formation and collapse. The karst-related porosity was subsequently filled by fine-grained argillaceous carbonate sediments. After the karsting event, the Wink strata underwent to three dolomitization events, two diagenetic, and one epithermal event related to the Ouachita Orogeny. The two diagenetic dolomitization events did not affect the entire Wink equally, with increased dolomitization in the lower portion of the core. In the lower Wink, this created a dolostone with intercrystalline, vuggy, and fracture porosity. Subsequent precipitation of baroque dolomite, from the epithermal event, and calcite cements destroyed much of the porosity throughout the core.

Analysis of carbon and oxygen stable isotopes produced curves that were used to test models of oceanic and environmental cyclicity. When compared to oceanic and environmental models, the data was inconclusive, neither clearly supporting nor refuting any of the models. Two positive excursions associated with the Mulde Event and near the Wenlock-Ludlow Boundary are present in both the carbon and oxygen curves. These excursions have carbon §-values increased by slightly more than l%o, and the oxygen 5-values increases of more than 2%o. The excursions allow correlations between the Wink and strata in Oklahoma, Nevada, and across many parts of Europe, particularly the Baltic region.