MICROBIAL GROWTH PROCESSES AND FABRIC DEVELOPMENT IN UPPER CAMBRIAN MICROBIAL BUILDUPS; MASON COUNTY, CENTRAL TEXAS

A detailed study establishing an analog to pre-salt microbial deposits offshore Brazil focuses on Upper Cambrian microbial buildups in Mason County, central Texas.

A 3 phase growth model was constructed to understand the buildup geometries and their relationships with coeval interbuildup strata. Systematic coring allowed a 3D understanding of the buildup’s microbial fabrics and internal structures.

Nucleating on sheets of flat-pebbles, Phase 1 is a colonizing microbial phase defined by a distinct outer margin, occurring coeval to deposition of interbuildup oolitic bioclastic grainstones and onlapping mixed silts. The Phase 1 interior exhibits amalgamated microbial heads with poorly preserved internal structures enveloped by thin thrombolitic rinds, while the Phase 1 exterior is a thick thrombolitic rind. The overlying Phase 2 consists of vertically aggrading and laterally expanding stromatolitic columns, exhibiting thin thrombolitic rinds, interacting with interbuildup bioclastic grainstones. Phase 3 is a well-defined thrombolitic phase crowning Phase 2, and onlapped by mixed silts.

These growth fabrics, studied in cores and thin sections, highlight two dominant microbial growth processes and provide compositional insight to the growth phases. Thrombolitic fabrics evidence direct precipitation of calcium carbonate induced by the microbial colonies. Thrombolitic cores and thin sections dominantly consist of original microbial micrite with microbial clots and fossil fragments within an extremely tight framework that hindered diagenetic effects. Cavities not infilled by microbial calcite exhibit dolomitized sediments. Stromatolitic fabrics studied in core and thin sections show alternating laminations of calcium carbonate, directly precipitated by the microbial colonies, and amalgamated grainstones. Within thin sections, we observe ferroan dolomite replacement, and subsequent oxidation, of amalgamated grains. Heavy diagenesis of Phase 2 was facilitated due to a higher porosity-permeability and an interfingering nature with coeval sediments, visible in core and outcrop.

A diverse microbial diversity, including Epiphyton, Renalcis, Girvanella, and Tarthinia, is dominantly restricted to the thrombolitic fabrics with few distinguishable stromatolitic microbe species.