U-PB GEOCHRONOLOGY AND CHRONOSTRATIGRAPHY OF BENTONITE BEDS WITHIN THE EAGLE FORD SHALE

The Eagle Ford Shale and equivalent Boquillas Formation (late Cretaceous) contain abundant volcanic ash beds of varying thickness. These bentonite beds represent a unique facies that displays a range of sedimentary structures, bed continuity, and diagenetic alteration. They are prominent not only in West Texas outcrops, but also in the subsurface of South Texas where hydrocarbon production is occurring. These bentonite beds have the potential to be used for stratigraphic correlation, for understanding early diagenesis, and most importantly for obtaining high-resolution geochronology, which in turn can be used for defining depositional rates and chronostratigraphy.

Initial sampling of bentonite beds was conducted at outcrops along U.S. 90 west of Del Rio, Texas, subsurface in Atascosa and Karnes County, and at a construction site in Austin. Bed thicknesses range from 0.1- 33 cm and were collected throughout the entirety of the Eagle Ford succession. Mineral separation yielded abundant non-detrital zircons of a single population used for U-Pb dating. Dating was conducted using two methods; ICP-MS at The University of Texas, Austin and ID-TIMS in conjunction with the Berkeley Geochronology Center.

Ages obtained from fourteen bentonites demonstrated significant variation in regards to period of active Eagle Ford facies deposition. Deposition in the Austin study location was determined to be from Early Cenomanian through Middle Turonian, while subsurface deposition occurred from Middle Cenomanian ending at the Turonian-Coniacian boundary. Outcrop deposition was the most prolonged of the study areas, with deposition occurring from Early Cenomanian through late Coniacian. Results indicate lithostratigraphic correlations from outcrop to the subsurface are unreliable, and deposition of Eagle Ford facies was prolonged in areas more proximal to the restricted waters of the Western Interior Seaway.