An Exposed Evaporite Weld and Related Deformational Structures, Edwards Plateau, I-10 Corridor, Kerrville-Junction-Sonora area, Texas

J. J. Willis, B. E. Lock, K. C. Cornell, and D. A. Ruberg
Energy Institute, University of Louisiana-Lafayette

As 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 and Sonora, though the “bulk” stratigraphic package remains nearly horizontal, roadcuts reveal intense brecciation and deformation, including local overturning. We interpret the causal mechanism 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 mi along I-10. The basal contact is sharp, marked by horizontal, undisturbed strata below, and intense brecciation above. Original bedding is unrecognizable for several ft above the contact. Proceeding upsection, large blocks of strata several ft 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.

AAPG Search and Discovery Article #90901©2001 GCAGS, Annual Meeting, Shreveport, Louisiana