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Ultimate Demise of Upper Cambrian Microbial Reefs Through Increasing Water Column Turbidity (Mason County, Central Texas)


Upper Cambrian 10-14m-thick microbial reefs, occurring along the Llano/James Rivers and Mill Creek in Central Texas, serve as good 3D analogues for microbial reservoirs. Periodic siliciclastic influx from nearby landmasses played an important role in microbial reef growth from an initial transgressive “colonizing” Phase 1, a “vertical aggrading/lateral expanding” Phase 2, and a well-defined “capping” Phase 3. The reef complex is onlapped by inter reef sediments and buried by a distinct unit, together referred to as the cover. The cover sediments consist of mixed siliciclastic silts grading into pure carbonates. They record variations in paleo-environmental conditions during late stages of reef growth and contains clues as to what stressed the reef growth and eventually killed them. The general architecture of cover and its relationship with the underlying microbial complex were defined through field observations and photogrammetry data. The study integrates core facies analyses, CaCO3 content values with thin section observations. The cover sediments directly overlie an ooid-rich bioclastic grainstone bed, interpreted to represent shallow subtidal conditions with eventual deepening at the end of Phase 2. The deeper subtidal setting allowed Phase 3 reefs to develop 2-3 m high synoptic relief and densely cemented thrombolytic rinds. The Phase 3 reefs clearly overlie the ooid-rich bioclastic packstone bed, and lack coeval inter reef sediment accumulation. The sediments, immediately overlying this ooid-rich bioclastic bed and clearly onlapping the adjacent Phase 3 reef, display the lowest CaCO3 content values (16 %) of the entire inter-reef section and initiate the burial of the reefs by sediments grading from mixed siliciclastics into pure carbonates. The contact between reef and overlying pure carbonates was recovered in several cores. Those sediments consist of poorly sorted skeletal-rich rudstone with large irregular intraclasts, evolving into a series of grainstone and packstone fining-upward beds with often-erosive bases. Above observations indicate a sudden increase in siliciclastic influx at the end of Phase 3 reef growth which led to prolonged turbidity increase in the water column, thus inhibiting cyanobacteria's photosysnthetic ability and ultimate leading to the reef demise.