SWEZEY, CHRISTOPHER¹ and CHARLOTTE SULLIVAN^2
1BP Exploration, Houston, TX
²Southwestern Energy, Houston, TX

Abstract: Outcrop Sequence Stratigraphy of the Late Cretaceous (Campanian) Anacacho Limestone, Texas, U.S.A.

The youngest Cretaceous shallow water carbonate outcrop in Texas is the Late Cretaceous (Campanian) Anacacho Limestone, which is exposed in a 15 km wide east-west trending zone parallel to Highway 90 between San Antonio and Del Rio. The limestone unconformably overlies the Austin Chalk, and onlaps a structural arch centered around the town of Uvalde. Asphalt is present within Anacacho outcrops along the margins of this arch and oil occurs in the subsurface. On the east side of this arch, between Uvalde and San Antonio, good exposures of the Anacacho are found along Blanco Creek, Seco Creek, and Hondo Creek. At Seco Creek, the Anacacho is overlain by the Corsicana Marl, but at the other two locations the Anacacho outcrops are not in stratigraphic contact with younger strata. At all three of these locations, the base of the Anacacho outcrop consists of cross-stratified carbonate grainstones, interpreted as dune deposits that developed in shallow water depths above wave base. The nature of the strata overlying these grainstones varies from location to location (Blanco Creek: volcaniclastics; Seco Creek and Hondo Creek: carbonate mudstones and wackestones), but all of the immediately overlying strata are interpreted as deeper water deposits that developed below wave base. This change from shallow water deposits to deeper water deposits denotes a sequence boundary that resulted from tectonic subsidence and/or sea level rise.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas