Abstract

Remapping at 1:48,000 scale of the north half of the San Antonio 1:100,000 quadrangle is being undertaken as a contribution to supporting urban development, conservation of historical and natural resources, and understanding the geologic and geomorphic history of the urban area. Mapping is essentially complete from the east side of San Antonio westward to Castroville.

The area includes exposed formations from Austin Limestone through Lower Wilcox Group sandstone and shale; however, most of the surface consists of very poorly exposed Upper Cretaceous mudstones and Quaternary alluvium, which conceal the underlying structures. Therefore, subsurface mapping on base of the Del Rio Clay, using the abundant water well and some oil well control in the region, has been used to interpret the pattern of Balcones faulting and extrapolate it to the surface. Mudstone contacts are projected based on thicknesses from logged wells. Such integrated surface-subsurface techniques should be applied to other poorly-exposed areas of Texas, wherever the subsurface information is sufficient.

The Upper Cretaceous section changes significantly across the area. The Campanian Pecan Gap marl is overlain westward by a thickening tongue of the Anacacho Limestone, which is a fine-grained micrite that is hydrocarbon-productive in a number of significant fields. Overlying Taylor mudstone is assigned to the Bergstrom Formation; the Bigfoot Unconformity truncates it at its top. Overlying shales in the east (Navarro-Kemp) pass westward into a varied unit of calcareous mudstone containing thin sandstones and limestones (Escondido Formation). These changes can be seen on a log cross-section through the area as well as surface exposures.

Paleogene strata of the Midway and lower Wilcox are exposed in the southern part of the area. Marginal marine sandstone and siltstone of the Poth member of the Midway are exposed, forming the northwest limit of sandy soils mapped as Wilcox.

Major Balcones faults can be traced across the mapped area, trending N60°E. Significant antithetic faults are present and form grabens and major upthrown fault blocks; irregular anticlinal structures are also present (most famously the Culebra Anticline).

A complex series of three or more terrace deposits form the dominant geomorphology of much of the area. The highest preserved deposits (generally called the ’Uvalde gravels’) form fairly narrow ridges across the landscape that are capped by calichified gravels. They are loosely correlative to present streams. A series of middle terraces occur at lower levels, including the ’Leon Creek Fan’ and fans sourced from the Culebra Anticline. Lower terraces are found near the major drainages, including the Applewhite terrace of Holocene age along the Medina River; still lower surfaces form part of the present floodplain.