Chronostratigraphic and Sedimentological Interpretation of the Barnett Shale (Mississippian) of the Llano Uplift and Fort Worth Basin, Texas

The base of the Barnett Shale in outcrop and in subsurface of the Fort Worth Basin is lower Chesterian (upper Visean of global usage) in age based upon the co-occurrence of Gnathodus texanus, Cavusgnathus unicornis, and Lochriea commutata. The basal Barnett Shale rests unconformably upon the Chappel Limestone that is middle Osagean in age (anchorali-latus Zone). The intra-Mississippian unconformity separating the Barnett Shale from the underlying Chappel Limestone is highly significant and represents a hiatus that comprises the upper Osagean, Meramecian and basal Chesterian stages. On outcrop, this is denoted by a red paleosol developed on the underlying Chappel Limestone. In the Fort Worth Basin (FWB) locally massive pyrite layer marks the disconformity. The significance of this regional unconformity is paramount for the understanding of the tectonic evolution and basin-filling pattern of the FWB. Currently published sedimentological models of the Barnett Shale suggest a deep basinal interpretation for the unit. However, we interpret the Barnett Shale of the FWB as a cyclical, dysoxic to anoxic shelfal, but not basinal mudrock. This mudrock contains pelagic taxa (conodonts, radiolarians, ammonoids) along with episodic occurrences of opportunistic benthic biota (Bivalves, brachiopods). Much of the Barnett Shale consists of dark, organic-rich, mudrock that is barren of benthic taxa. The highly restricted and toxic nature of the Barnett Shale is consistent with geochemical evidence recently presented by Rowe and others (2008). The upper part of the lower Barnett Shale of the northernmost FWB laterally grades into Mississippian-aged platform carbonates that comprise bioturbated packstones, encrinites, and carbonate mounds capped by oolitic grainstones. A high density of closely spaced samples reveals that these shallow-water carbonates contain only a very rare occurrence of a single species of conodont (Cavusgnathus unicornis). This species gives a Chesterian age as the youngest possible date for the top of the lower Barnett Shale.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California