CYCLIC STRATIGRAPHY AND CONODONT BIOSTRATIGRAPHY OF THE BARNETT SHALE FROM THE TYPE LOCALITY OF THE CHAPPEL LIMESTONE

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 apparently 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 Meramecan. Therefore, the uppermost Osagean and lower part of the Meramecan is apparently missing at this section. The top of the Barnett Shale is glauconitic with limestone nodules and appears to be gradational with the Marble Falls Limestone. The Mississippian-Pennsylvanian contact appears to be conformable within the top 2 feet of the Barnett Shale. Therefore, it seems that at this locality the upper Meramecan and entire Chesterian stages are represented.

At this locality the Barnett Shale consists of 39.4 feet (12m) of gray to black locally phosphatic shales that contain thin interbedded carbonates most of which are lenticular. The entire stratigraphic section is fossiliferous with conodonts being the most abundant fossil. The carbonates contain phosphatic mollusks (bivalves, gastropods, ammonoids, bactritoids) and also locally abundant glauconite. Despite the generally monotonous appearance of this section a clear pattern of cyclic sedimentation can be delineated based. Several condensed sections within the Barnett Shale are identified and consist of glauconitic shales with abundant phosphatic pelloids and abundant condodonts. The ultimate goal of this research is to correlate the cycles with coeval cycles that are well developed in the Namurian of Great Britain.