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Environmental and Sequence Stratigraphic Control on Microbialite Morphology and Microfacies in a Marine Mixed Carbonate Siliciclastic System, Upper Cambrian Point Peak Formation, Llano River and Mill Creek, Mason County, Central Texas


The Upper Cambrian microbiallites exposed near Mason serve as an excellent analogue to study reservoir characteristics of hydrocarbon rich microbial systems off the coast of Brazil. Detailed sections, spectral gamma ray logs and petrographic analysis were used to interpret sequence stratigraphy and environments of the microbial bearing strata. Alternating siliciclastic and carbonate facies and high-frequency oscillations in gamma ray depict high-frequency cyclic fluctuations in sea level and two additional lower-frequency oscillations that define depositional sequences. Siliciclastics were delivered into shoreline and shallow-marine environments during regression. Increasing carbonate proportions represent transgression. Detrital sand is reworked with skeletal and intraclastic carbonate in high-energy tidal channels and storm deposits. Glauconitic siltstone accumulated in lower energy intertidal and shallow subtidal environments. Flaser-bedded heterolithic facies with reversing ripple forms, tidal bundles and desiccation cracks is interpreted to represent the intertidal zone on tidal flats. Biostromes composed of amalgamated low thrombolitic microbial mounds are associated with the heterolitihc facies and thus are interpreted to be intertidal. Higher relief isolated “bread loaf mounds” in the lower Point Peak and larger microbial buildups and “reef complexes” in the upper Point Peak are associated with carbonate skeletal and oolitic grainstone and burrowed glauconitic siltstones interpreted to represent somewhat deeper subtidal environments and greater accommodation during transgressive phase of depositional sequences. However, the close association of intertidal facies indicates that relatively shallow conditions prevailed on the vast ramp even during transgression. Both the biostromes and the breadloaf mounds are composed of a thrombolitic and micro-digitate columns and calcimicrobial framework. Siliciclastics and glauconite are incorporated in the microbialite biostromes and breadloaf mounds but not within the larger buildups and indicating that the microbialites thrive even in the presence of siliciclastics, but that with sufficient relief siliciclastics may be transported on the seafloor adjacent to but not overlapping the top of actively growing mounds. Thick buildups and “reef complexes” of the upper Point Peak are associated with the thickest skeletal and oolitic grainstone and maximum transgressive phase in the sequence stratigraphy.