The upper Trinity Group is predominantly a carbonate system with minor clastic couplets that were deposited during the Middle Cretaceous in the south Llano Uplift region. The upper Trinity Group was deposited on a southward dipping platform in the Kimble County area. Stratigraphic units of the upper Trinity Group are the Hensel Formation, determined to be supratidal claystones, the Glen Rose Formation, which are mudstones that were deposited in a carbonate lagoon, and the Walnut and Fort Terrett formations, which are wackestones to packstones interpreted to have been deposited on a shallow carbonate shelf.
Nine stratigraphic sections were measured along Interstate 10 and U.S Highway 377 in Kimble County, Texas to analyze the lithostratigraphic, sequence stratigraphic, petrographic, and paleotologic deposition. Three sequences were determined based on disconformites. The Upper Hensel Formation contact with the Lower Glen Rose Formation is based on the uppermost red bed of the Hensel Formation, forming Sequence 1. Sequence 2 begins at the mudstones atop of the Hensel Formation and end at the burrowed mudstone unit, this
sequence represents the Glen Rose Formation. Sequence 3 extends from claystones atop of the burrowed limestone of the Glen Rose Formation and terminates at the disconformable contact with the Fort Terrett Formation, representing the Walnut Formation. Petrographic evidence indicates that marine diagenesis is prevalent. Common bivalves in the area were Ceratostreon texanum, which are index fossils for the Walnut Formation and provided substantial evidence for the placement of the Walnut Formation in Kimble County, Texas.
Wilkerson, Austin C., "PETROGRAPHIC AND STRATIGRAPHIC ANALYSIS ALONG THE LOWER CRETACEOUS STRATA, IN KIMBLE COUNTY TEXAS" (2018). Student Publications. 1.
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