INTER-HERM GRAINSTONE ACCUMULATION IN AN UPPER CAMBRIAN MICROBIAL REEF COMPLEX AND ITS SUBSEQUENT SEDIMENT BURIAL (JAMES RIVER, MASON COUNTY, TEXAS)

In the past eight years, research on microbialites in industry as in academia has gained interest due to large discoveries of hydrocarbon in pre-salt microbial reservoirs offshore Brazil and NASA focus on finding potential life outside Earth. Upper Cambrian microbial reef complexes, within the Wilberns Formation, spectacularly crop out on cliff exposures along Mill Creek and the Llano/James Rivers on recently accessible private ranches. Stream bed extensive pavement exposures also provide 3-D detailed views of these microbial reef complexes. Among the James River reef complex, well-preserved 3-8 m high relief microbial herms are separated by inter-reef bioclastic grainstone layers intercalating with thin beds of detrital fine silt that form large dunes characterized by curved asymmetrical ripples and bounded by well-developed moats along some sides of the herms. The final microbial growth in the reef complexes is expressed as a capping phase with clearly defined rinds, onlapped by a silt-rich unit evolving into a series of skeletal packstone layers devoid of any microbial occurrence.

The juxtaposition of individual herms and beds of bioclastic grainstones along the James River resemble surprisingly well the environmental growth conditions of 2-3 meter tall modern Holocene living subtidal microbial thrombolytic reefs surrounded by moving ooid sand dunes on the Great Bahama Bank margin. The similarities between the Holocene subtidal microbial herms and the ones we study in the Upper Cambrian Wilberns Formation will provide meaningful comparisons for understanding the interactions between microbial hermal growth and their inter-reef sediments, specifically the incorporation of grainstones into the microbial growth as well as a better understanding of the history of microbial reefs and their relation to carbonate sand and silt flow and accumulation.

The Rice/Trinity Industry Microbial Research Consortium is funded by Chevron, ConocoPhillips, Shell, and Statoil.