ABSTRACT

As Interstate Highway 10 (I-10) transects the Edwards Plateau northwest of San Antonio, Texas, its roadcuts augment natural outcrops allowing detailed examination of stratigraphic and structural relations. Overall, strata are nearly horizontal, with hills capped by Cretaceous Edwards Group carbonates and younger strata, and exposures of older Cretaceous clastics and locally Paleozoic strata in deep river valleys. Between Kerrville, Junction, and Sonora, though the "bulk" stratigraphic package remains nearly horizontal, roadcuts reveal intense brecciation and deformation, including local overturning. The causal mechanism is generally interpreted to be gypsum dissolution and resultant welding of Edwards Group strata.

A distinct zone of brecciation and related deformation occurs at the same stratigraphic position for over 60 miles along I-10. The basal contact is sharp, marked by horizontal, undisturbed strata below, and intense brecciation above. Original bedding is unrecognizable for several feet above the contact. Proceeding upsection, large blocks of strata several feet across occur in chaotic arrangement. Bedding becomes progressively more coherent upsection, but is still intensely folded and faulted. Deformation decreases upsection, until overlying strata appear undisturbed, nearly horizontal. Structural details of withdrawal synclines provide outcrop analogues to similar structures in the Gulf of Mexico salt basin.

A nearby quarry and well control indicate presence of gypsum layers within Edwards Group strata. No gypsum occurs in I-10 roadcuts, but macroscopic and petrographic evidence indicates its former presence. The occurrence of peat deposits and caliche over structural lows, and the breccia's commonly open framework favor relatively recent (Quaternary) gypsum dissolution, although some dissolution may have occurred much earlier.