Abstract: Transgressive Sandstone/Carbonate Sequence--Hosston/Sligo Formations of South Texas

D. G. Bebout

The oldest recognized Lower Cretaceous of South Texas consists of a sandstone/carbonate couplet, the Hosston (sandstone) and Sligo (dolomite and limestone) Formations. The Hosston comprises the updip part of the section and crops out in Central Texas as the Sycamore Sandstone. Downdip, the Sligo limestone and dolomite comprise the entire section. Both the Hosston and Sligo lie unconformably on Paleozoic basement.

Several wells cored the entire Hosston/Sligo sequence; a cross section using data from these wells shows a transgressive vertical sequence. The basal quarter to third of the Hosston/Sligo consists of laminated sandstone, dolomite, and nodular anhydrite, typical of arid tidal flats. The middle part of the section is dolomite, with lesser amounts of limestone; fabrics indicate that conditions alternated between supratidal and subtidal environments. Finally, the upper 50-75 ft (15-23 m) of the Sligo is predominantly limestone and includes both interbedded grainstone and wackestone deposited in intertidal and subtidal environments. The contact of the top of the Sligo carbonate rocks with the overlying Pine Island Shale is gradational in most wells studied; however, locally a few wells sho a sharp disconformable contact. This same sequence from tidal-flat to subtidal environments described vertically also is present laterally from updip to downdip, culminating in subtidal rudist banks and bars of the Sligo shelf edge.

Thus, the Hosston/Sligo sequence does not show the normal progradational or shallowing-upward cycle so common in many carbonate models throughout the world; rather, it is a deepening-upward sequence capped by an offshore shale (Pine Island Shale). Lack of an erosional surface or of evidence of diagenetic fabrics which result from supratidal and subaerial conditions at the top of the Sligo support this interpretation.

Porosity is not concentrated at the top of the carbonate unit as is common in the regressive or progradational model. In the Sligo, porosity zones are thin and scattered downdip and thicker and more continuous updip. Porosity formation and preservation are complex and are related to both depositional and diagenetic processes.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA