Abstract

The Eagle Ford crops out at two localities just northeast of San Antonio, Texas; one along Cibolo Creek approximately three quarters of a mile south of Selma and one along a feeder stream on the Olympia Hills Golf Course in Universal City. The Eagle Ford–Austin Chalk contact is very clearly exposed and exhibits features that have not been observed at other exposures of this contact in the San Antonio area. Both outcrop bases generally consist of three feet of alternating six-inch beds of grey silty marlstone with bentonitic clay capped by a two-inch crystalline limestone bed with abundant pyrite nodules. This bed is overlain by approximately eighteen inches of organic-rich shale with 4% total organic carbon (Velko et al., 2019) that contains abundant white phosphate ooids. Nannofossil data from samples collected at the Olympia Hills locality confirm this bed to be at or very close to the Cenomanian-Turonian boundary with the overlying Turonian being extremely condensed (B. Loucks and J. Pospichal, 2023, personal communication). The boundary above is disconformable with the overlying Austin Chalk which consists of highly burrowed marl that contains phosphatic clasts and skeletal debris. The Eagle Ford–Austin Chalk contact at these localities more closely resembles the descriptions of this contact to the north from Travis County (Adkins, 1949). Phosphatic ooids have also been described in the subsurface of Grimes County in the sub-Clarksville sands near the overlying Austin Chalk contact (Barton, 1982). The presence of ooids provides evidence for a widespread sea-level lowstand; however, the proximal platform location of the outcrops allowed the merging of the subsequent transgression to be preserved as a condensed bed. The high total organic carbon content of this bed where mature could support hydrocarbon migration into the overlying Austin Chalk even in proximal platform settings where the total Eagle Ford is thin.