INTEGRATED SURFACE AND SUBSURFACE GEOLOGICAL MAPPING OF THE VALLEY OF THE MISSIONS, SAN ANTONIO, TEXAS

In the 1700s, five missions, a presidio and a Spanish civil settlement were established along the San Antonio River and its tributaries, forming the heart of today's city. To understand their setting and their use of local resources, as well as to inform their conservation and the overall development of the metropolitan area, the quadrangles including the Valley of the Missions are being mapped at 1:24,000 scale. Most of the area is underlain by Upper Cretaceous lime mudstone, Upper Cretaceous and Paleocene mudstone and Paleocene sand and sandstone, mantled by numerous Quaternary terraces and alluvium, so outcrops are scarce to nonexistent. Concurrent subsurface mapping using abundant well control drilled to the Edwards Aquifer is critical in determining fault locations and inferring the outcrop or subcrop of bedrock units.

Significant results to date: A multistrand zone of faulting underlies the Mission Concepcion area, where a spring deposit (tufa) was quarried to build two missions. The faulting may have allowed ascending warm waters to pass through the thick Upper Cretaceous mudstone. The transition from clay to sandy soils is mappable, and appears to correspond to the thin Poth sandstones and related siltstone recognized in the subsurface.