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U-Pb Geochronology and Chemostratigraphy 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 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 is being conducted using two methods; ICP-MS at The University of Texas, Austin and ID-TIMS in conjunction with the Berkeley Geochronology Center. Preliminary study of bentonites and adjacent facies in both outcrops and cores demonstrates that ash beds co-occur with limestones rich in planktonic foraminifera. This suggests that episodes of high foraminiferal production and sediment supply may have been driven by nutrient flux associated with volcanism. To further test this premise we are collecting major, minor, trace, and rare earth element data on the bentonites and adjacent deposits. These data can be used to identify variations in mineralogy and chemistry that help define the nature of the original ash fall sediments, and their interaction with ocean water and sea floor sediment.