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

With discovery of oil fields in the pre-salt offshore Brazil, interest in microbialites has intensified. Upper Cambrian microbial reefs exposed on private ranches along the Llano River and Mill Creek promise to serve as powerful analogues. Sections beneath the microbial reefs are mixed carbonate siliciclastic facies including glauconitic siltstone, hetrolitiic facies, skeletal and oolitic packstone and grainstone, flat pebble conglomerate, microbial mounds and biostromes.

The siltstone and heterolithic facies are interpreted to represent low-energy intertidal environments based on the presence of ripples with reversing currents, tidal bundles and mud cracks. The skeletal and oolitic grainstone is interpreted to represent an open-marine high-energy environment based on stenohaline fauna, cross-bedding, and mega-ripples. Skeletal grains are filled with carbonate mud indicating initial low-energy deposition followed by reworking. Bioturbated packstone beds indicate a lower energy subtidal environment. Flat pebble conglomerates are interpreted to represent storms on the basis of channels, scoured bases and rounded clasts. The microbiallites are interpreted to represent shallow subtidal to intertidal environments with stenohaline conditions based on alternating grainy and muddy sediments between mounds and stenohaline fauna.

The gamma ray curve depicts cyclic fluctuations in sea level and siliciclastic influx with multiple orders of oscillation. Small microbial mounds and biostromes occur within the carbonate-siliciclastic part of the section and siliciclastic sediment is trapped within microbiallites indicating they formed in the presence of siliciclastic flux.

The switch to large microbial reef complexes is coincident with a widespread skeletal-oolitic grainstone 1.6 m thick and a decrease in siliciclastics. The grainstones represent a large ramp crest shoal. The reefs nucleated on a hardground and transgressive flat pebble conglomerate near the middle of the grainstone. Grainstone dips toward the reefs indicating accelerated currents forming a moat. The base of the grainstone bed as well as the intertonguing reef flank beds are deformed downward by compaction. Flank beds include heterolithic, oolites, and skeletal grainstone indicating continuing shallow-marine conditions.