FAR FIELD EFFECTS OF CORDILLERAN TECTONICS ON THE SAN MARCOS ARCH IN SOUTH AND CENTRAL TEXAS DURING THE UPPER CRETACEOUS

A regional stratigraphic study of the San Marcos Arch in Texas, based on correlation of hundreds of wells, multiple cores and outcrops, and nannofossil biostratigraphy, addresses questions about paleogeography of Western Interior Seaway and far field effects of Cordilleran deformation. The San Marcos Arch, as a subsurface extension of the Pre-Cambrian Llano Uplift, localized continental break-up in the Cambrian, collision in the Pennsylvanian, break-up in the Triassic, and differential sedimentation from Jurassic through Tertiary. The overall tectonic setting of the area is extensional, with strong subsidence toward the Gulf of Mexico. Movement on the San Marcos Arch mainly occurred updip of the Lower Cretaceous carbonate shelf edges. The Upper Cretaceous Eagle Ford Group (~90-97 My) is thin in central Texas, due to uplift and erosion, and is much thicker in south Texas. The overlying Austin Chalk (~81.5-90 My) consists of three unconformity-bound composite sequences, defined by nannofossil biostratigraphic hiatuses, coincident with transgressive glauconite lags and angular truncation of underlying section. The three sequences are extremely asymmetric across the San Marcos Arch. The lowest sequence (~87.5-90 My) is similar to the Eagle Ford Group, uplifted and eroded in central Texas and much thicker in south Texas. The middle sequence (~84.5-87.5 My) is very different; the axis of the arch is shifted westward, the section is thin in the west, mostly eroded over the arch and thickens eastward toward central Texas. The upper sequence (~81.5-84.5) is similar to the middle sequence. New paleogeographic maps show that water circulation in the Western Interior Seaway initially went through south Texas, but changed ~86 My to go through the East Texas Basin instead. Movement of the San Marcos Arch is interpreted as due to differential Cordilleran compressional stresses acting on a pre-existing structure at the southern edge of the North American continent, accompanied by eruption of many ultramafic phreatic tuff cones along zones of weakness. The East Texas/Louisiana/Arkansas Sabine Arch is a larger uplift than the San Marcos Arch that formed at nearly the same time, due to similar stresses. Thin-skinned compression from Mexico only reached Texas near the Rio Grande River, during the Eocene (~50 My).