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Upper Cambrian Microbial Reefs and Inboard/Outboard Facies Associations, Central Texas


Following the discovery of large hydrocarbon microbial reservoirs in pre-salt strata of offshore Brazil and Angola, research on microbial-dominated carbonate systems has intensified. Extensive Upper Cambrian microbial reefs are observed on the shallow shelf surrounding Laurentia, including the Point Peak Member of the Wilberns Formation in Mason and surrounding counties of Central Texas. The Point Peak Member is divided by characteristic brachiopod-rich beds, referred to as the Plectrotrophia zone (Plectrotrophia bridgei and species of Billingsella), into the Lower and Upper Point Peak. The Plectrotrophia zone is found in most of the outcrops across the 2500 km2 of the shallow shelf and thus represents a good biomarker bed and considered as a possible timeline. Over most of the shallow shelf, the microbial section occurs only in the lower Point Peak, whereas microbial intervals are observed both below and above the Plectrotrophia zone in its western part, including in the detailed study area that focuses on outcrops exposed along the Llano and James Rivers, and Mill Creek. Lower Point Peak outcrops consist of a series of microbial biostromes and bioherms (<1 m in height), separated by ooid-rich skeletal beds with bidirectional ripples. The buildups are overlain by heterolithic facies, glauconitic siltstones, and detrital sands indicating very shallow subtidal to intertidal environments and considered to be part of the inboard shelf. Further to the east, the presence of larger microbial buildups (up to 30 m high) indicates the wide extent of the microbial facies belt corresponding to the outboard shelf. Upper Point Peak outcrops in the study area have spectacular 3D exposures of numerous larger microbial bioherms (10-14 m high), and inter-reef thick skeletal and oolitic grainstone indicating subtidal high-energy environments. These buildups are interpreted to be growing within the outboard shelf. The lateral succession of depositional environments from inboard to outboard across the shelf is observed to be stacked vertically in the continuous sections of the study area. The facies associations can be summarized as: Inboard – siliciclastic sands, heterolithic facies, glauconitic siltstones, carbonate ooid-rich skeletal grainstone, low relief (<1 m) microbial bioherms and biostromes; and Outboard – high-relief (10-14 m) microbial bioherms with inter-reef ooid and skeletal grainstones and thin mixed carbonate clastic silty beds.