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.