| Guadalupian siliciclastic rocks and bioclastic limestones crop out near Shafter, Presidio County, Texas. The lithology and stratigraphy of these rocks show them to be basin margin/platform edge strata representing a eustatic cycle during which lowstand siliciclastics were twice succeeded by prograding highstand foresets. Highstand carbonates and basinal sandstones and shales lie beneath lowstand wedge systems tract sandstones of the Wordian upper Ross Mine formation that themselves are overlain by siliciclastic sandstones and shales and carbonate mudstones of a transgressive systems tract. These strata are overlain in turn by a second highstand systems tract, the foresets of the Capitanian Mina Grande formation. The bioclasts of the wackestones and packstones composing these foresets indicate a normal marine environment. The stereographic rotation of these tracts to their original orientation reveals that the Mina Grande foresets prograded toward the west from a carbonate platform located to the immediate east. Drill cores from east and west of the study area suggest the same orientation: to the west, dark basinal shales and mudstones are succeeded by basin in-filling evaporites, while to the east lie platform edge carbonate mudstones (possible mud mounds) indicative of a normal shallow marine setting. |
