CONODONT BIOSTRATIGRAPHY OF THE BARNETT SHALE IN THE LLANO UPLIFT REGION, TEXAS

The type locality of the Chappel Limestone southeast of San Saba, Texas contains a long section of Barnett Shale with both its lower and upper contacts exposed. The top of the underlying Chappel Limestone has solution vugs, red internal sediment and a well-developed oxidized zone indicating a disconformable contact with the Barnett. The lower Barnett Shale contains Gnathodus texanus, Gn. bilineatus (sensu latu) and Cavusgnathus altus suggesting an upper Meramecian to earliest Chesterian age. The upper Barnett Shale contains Gnathodus bilineatus, Lochriea commutata, with a Cravenoceras ammonoid assemblage indicative of a late Chesterian age. The top of the “Barnett Shale” is glauconitic with a high gamma ray reading and contains a Morrowan age conodont assemblage of Idiognathoides sinuatus and Neognathodus symmetricus.

In a near surface core west of San Saba, the White's Crossing Coquina Member of the Chappel Limestone yields an upper Osagean fauna of Gnathodus texanus and Gn. bulbosus. The overlying lower Barnett Shale contains Gnathodus texanus and Gnathodus ”praebilineatus” suggestive of a Meramecian age. The Chesterian part of the Barnett and Morrowan part of the Marble Falls Limestone are missing, cut out by an erosional truncation surface as evidenced by Atokan Marble Falls with Idiognathodus “gibbus” resting directly on top of a paleosol in the upper Barnett Shale that contains Gnathodus texanus. It is clear that the Barnett Shale in the Llano Uplift was influenced by Mississippian and Pennsylvanian syndepositional tectonics.