CONSTRAINTS ON THE DEPOSITIONAL ENVIRONMENTS OF CYCLIC, MICROBIALITE BEARING CARBONATE- SILICICLASTIC STRATA; CAMBRIAN, WILBERNS FORMATION, MASON COUNTY, TX

Research into lacustrine microbialites has intensified in response to recent giant oil field discoveries in pre-salt offshore Brazil. Similarities between lacustrine and marine carbonates indicate that marine microbial carbonates may provide meaningful analogues for understanding the heterogeneity of pre-salt reservoirs. The large scale of recently accessible Upper Cambrian microbial mound and reef complexes within the Wilberns Formation in private ranches of Mason County prompted a detailed outcrop study of the facies architecture, diagenesis, and porosity/permeability distributions of microbialites as potential analogues for pre-salt microbial carbonate reservoirs.

We have undertaken a detailed sedimentologic and petrographic study of cyclic, microbialite bearing carbonate-siliciclastic strata that underlie the major microbial reef complexes in a spectacularly exposed stratigraphic section at Mill Creek. Our constraints on depositional environments have implications for understanding the environments of the larger reef complexes.

Our preliminary observations indicate that small microbial heads and biostromes as well as colonies of Eocrinoids initiated on grainstone and firm-ground carbonate substrates. High energy is demonstrated by oolitic and skeletal grainstones with ripples and megaripples and intraclastic (flat pebble) conglomerates. At the base of the section microbialite biostromes alternate with grainstones at a sub meter scale. Open-marine salinity is indicated by pelmatozoan, trilobite, and brachiopod skeletal grains. Carbonate grainstone and fine siliciclastic sandstone and siltstone commonly contain large amounts of glauconite. Occasionally fine sandstones and siltstones are extensively bioturbated indicating lower energy conditions and oxygenation. Sediment in “channels” between microbial heads ranges from siltstone to coarse grainstones indicating variability of energy levels. Meter scale cycles commonly begin with grainstone or flat pebble conglomerates followed by microbial biostromes and are capped with flasar-bedded siltstones with mud draped ripples and rare desiccation cracks indicating shallow-subtidal and tidal flat conditions. Spectral gamma ray logs reveal a strong cyclicity.