Date of Award
Thesis campus only
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 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 prompts a detailed outcrop study of the facies architecture, diagenesis, and porosity/permeability distributions of microbialites as potential analogues for pre-salt microbial carbonate reservoirs.
In parallel with the larger project, I undertook 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. The constraints on depositional environments have implications for understanding the environments of the larger reef complexes.
I refined environmental interpretations using detailed description and assessment of sedimentologic logs, evaluated of stratigraphic cyclicity and spectral gamma ray signature and created a detailed petrography of thin sections and polished slabs. My 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 mega-ripples 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 flaser-bedded siltstones with mud draped ripples and rare desiccation cracks indicating shallow subtidal and tidal flat conditions.
Mercado, Lauren, "Constraints on the depositional environments of cyclic, microbialite-bearing carbonate-siliciclastic strata; Cambrian Wilberns Formation, Mason County, Texas" (2014). Geosciences Student Honors Theses. 11.
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