Shell and Grain Layers in the Barnett Shale; Event Deposition Or in Situ Accumulations?

Robert G. Loucks and Stephen C. Ruppel
Bureau of Economic Geology, The University of Texas at Austin, Austin, TX

The Barnett Shale in the Fort Worth Basin (FWB) was deposited in a deeper water depositional setting under predominately anoxic conditions. The siliceous mudstone that composes the majority of the stratigraphic section contains thin shell and grain layers. These layers have been attributed to two contrasting depositional processes: (1) event beds deposited by gravity flows and (2) in situ accumulations. Basinal setting, biota composition, grain types, textures and fabrics all suggest that these shell and grain layers were deposited by gravity-flow processes. Evidence for event deposition include (1) grain composition that suggests mechanical mixture of grain types (phosphatic coated grains and filibranch mollusk), (2) sorting of grains attributed to turbidity and debris flow processes, (3) absence of biota in living position, and (4) basal scours. Although sediments have been highly compacted during burial distorting some of the evidence, carbonate concretions, which formed within a few meters of the sediment surface, reveal the uncompacted state of the shell and grain layers. Shell-layer textures and fabrics demonstrate that shells are generally disarticulated and suspended within a peloidal matrix. In the northern FWB no bioturbation is associated with these shell accumulations, whereas in the southern FWB bioturbation is locally apparent at the base of shell accumulations. Burrows may be the product of “doomed pioneers”, organisms that briefly populated bottom sediment following short-lived oxygenation of the sediment-water interface during event deposition. The interpretation of these deposits as gravity-flow events is consistent with the depositional setting of Barnett mudstones in the FWB, which has been documented to have been deeper water, anoxic, and hostile to biota.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas