SAN MARCOS ARCH AND SABINE ARCH - FLEXURES RELATED TO MEXICAN LARAMIDE (HIDALGOAN) COMPRESSION

Texas contains several subtle but distinctive arches that were active during Cretaceous and Cenozoic time, that are likely to have been generated as by-products of Mexican tectonic activity.

The San Marcos Arch is a broad arch extending WNW-ESE through central Texas, with San Antonio high on its southern flank and Austin lower on its northern flank. It has a history of recurrent uplift generating unconformities from late Albian (pre-Georgetown) to Maastrichtian (pre-Escondido) and possibly into the Paleocene. The arch also subsided and received marine sediments through the Cretaceous, though some of them (Austin, Pecan Gap) show evidence of shallower-water deposition on the arch. The arch is best defined where Upper Cretaceous strata are preserved (Balcones trend and southeast) but probably extended westward across the Edwards Plateau, as shown by Rose's structure maps. It probably extends into a flexure across which the Georgetown-equivalent strata thicken markedly southward.

The San Marcos Arch behaves similarly to a forebulge, a zone of intermittent erosion and nondeposition landward of a foredeep basin. The geometry of the arch and the thickness of strata indicates that the foredeep lay to the southwest, in northeastern Mexico. However, it is unclear that load-generated basins were formed in northeastern Mexico in the late Albian; this needs more detailed study.

Cenozoic arches also formed in north and east Texas, and affect the present outcrop patterns. The Preston Anticline near Lake Texoma deforms Cretaceous sediments, probably on a reactivated Paleozoic fault trend. The age of this arch is unknown. The Sabine area has a complex earlier history, but a late episode of arching uplifted Wilcox strata (Paleocene-lower Eocene). The age of arching is not closely determined, but is suspected to be Middle Eocene, possibly forming a sediment source for Yegua deltas in southeast Texas. A Middle Eocene age is coincident with Laramide folding in northeastern Mexico and the Rio Grande borderlands. Far-field stress from the Mexican Laramide (Hidalgoan) orogen are likely candidates for creating these late arches, as previously suggested by Laubach.