SEDIMENTOLOGY AND DEPOSITIONAL ENVIRONMENTS OF CYCLIC MICROBIAL BEARING STRATA OF THE CAMBRIAN POINT PEAK MEMBER OF THE WILBERNS FM., EXPOSED ALONG MILL CREEK, THE LLANO RIVER, AND JAMES RIVER, MASON COUNTY, TX

Several sections were described and correlated using spectral gamma ray logs. Facies were used to interpret environments and sequence stratigraphy in order to evaluate the factors that led to the origin and growth of large, reservoir scale, microbial reef complexes.

Facies are comprised of alternating mixed carbonate-siliciclastics. Siliciclastics are interpreted to have been delivered into shoreline and shallow marine environments during regression. Sandstone is interpreted to be reworked in a high-energy, shallow-subtidal environments. Heterolithic siltstone is interpreted to represent a low-energy intertidal environment based on the presence of lamination, ripples with reversing current indicators and mud cracks. Tidal bundles indicate fluctuation between tides and slack tide. The skeletal grainstone-packstone represents a lower energy, open-marine subtidal environment that experienced periods of high energy during storms. Skeletal grains are typically filled with carbonate mud indicating initial low-energy deposition followed by reworking. Flat pebble conglomerates are interpreted to represent storm events on the basis of channels, scoured bases and rouded clasts. Oolite grainstone formed in a high energy, shallow-subtidal environment based on cross-bedding and asymmetrical megaripples. Azimuths of ripples indicate northeast-southwest directed tidal currents across a shoreline bordering the Llano Uplift. Facies associations indicate that the microbial biostrome and mounds represent shallow-subtidal to emergent tidal flat environments.

Alternating siliciclasic and carbonate-rich beds and high-frequency oscillations in gamma ray depict multiple frequencies of cyclic fluctuations in sea level. Siliciclastics are incorporated in microbialite and intermound facies demonstrating that it is possible for microbial reefs to thrive in the presence of significant siliciclastic flux. However, the thick microbial reef complexes (~ 10 m thick, 100s of m across) are developed within a significant carbonate shift with the thickest skeletal and oolitic grainstones in the succession. Thus, large potential microbial reservoirs were developed during transgression. Carbon isotope and elemental data were used to tie to the global carbon curve and further interpret sea level signal.