MIDDLE PERMIAN DEPOSTIONAL SETTING OF THE EASTERN MARFA BASIN, SHAFTER, TEXAS

Shafter, Texas, is located in the southwestern Permian Basin of West Texas. Middle Permian (Guadalupian) siliciclastic rocks and bioclastic limestones crop out around the Shafter silver mine owned by Silver Standard Resources. The Permian limestones of the Ross Mine and Mina Grande formations of the Shafter area are contemporaneous with the deposits of the intracratonic sea of the Delaware Basin and, thus, with the Capitan Reef in the Guadalupe Mountains and its southern equivalent exposed in the Glass Mountains. The precise relation of the Shafter rocks to a second sea to the south and west, that of the Marfa Basin, remains problematic.

Preliminary study of subsurface cores from the mining operation suggest that the Mina Grande formation represents bioclastic-rich debris prograded westward from a carbonate platform on the eastern edge of the Marfa Basin. The upper part of the Mina Grande may intergrade with post-Capitanian beds equivalent to the Tessey Formation of the Glass Mountains. Selected cores are being examined in detail to determine the age and nature of the turbidites, debris flows, displaced blocks and their relation to the Middle and Late Permian history of the rest of the Permian Basin.