Cyclic Stratigraphy and Conodont Biostratigraphy of the Barnett Shale from the Type Locality of the Chappel Limestone and near-Surface Cores from the Llano Uplift

Boardman, D. R.¹, J. Puckette², B. Coffey³, E. Gerding², P. Kamman⁴, N. Aryal², M. Singh², A. Rihn¹ (1) Oklahoma State University, Stillwater, OK (2) Oklahoma State University, (3) Devon Energy, Oklahoma City, OK (4) Devon Energy, Oklahoma City,

A complete stratigraphic section of the Barnett Shale is exposed in a roadcut and adjoining shale quarry 2.5 miles southeast of San Saba, Texas on Texas county road 1031. The basal contact of the Barnett Shale is unconformable based on both lithic and faunal analysis. The top of the underlying Chappel Limestone has solution vugs, and a well developed oxidized zone. The basal Barnett Shale contains Gnathodus texanus, Gn. bilineatus (sensu latu) and Cavusgnathus suggesting the basal Barnett Shale is most likely upper Meramecian to earliest Chesterian. Therefore, the uppermost Osagean and majority or all of the Meramecan is apparently missing at this section. A carbonate near the top of the Barnett Shale contains an abundant conodont fauna with Gnathodus bilineatus dominant, with Cravenoceras ammonoids indicative of a late Chesterian age. The top of the Barnett Shale is glauconitic with limestone nodules and abundant caliche It appears superficially to be gradational with the Marble Falls Limestone. However, two conodont zones are apparently missing from the basal Pennsylvanian Morrowan Marble Falls Limestone suggesting subaerial exposure.

Shallow cores from both east and west of San Saba illustrate additional section present at the base of the Barnett Shale (White's Crossing coquina) which is Meramecian in age. It seems apparent that there was a proto-Bend Arch at least passively active during deposition of the Barnett Shale in the vicinity of San Saba, Texas. These cores also have evidence of subaerial unconformity at the top of the Barnett Shale and just below the Marble Falls Limestone.