AN INVESTIGATIVE STUDY INTO THE DEMISE OF THE UPPER CAMBRIAN MICROBIAL REEFS (MASON COUNTY, TEXAS)

Upper Cambrian microbial reefs (Upper Point Peak Member, Wilberns Formation), exposed in outcrops along the Llano/James Rivers and Mill Creek in Central Texas, represent an important part of the Phanerozoic reef history. These microbial reefs grew in shallow equatorial seas along the southwestern Laurentian margin in close proximity to landmasses. The periodic siliciclastic influx from nearby exposed land played an important role in their morphological evolution. A commonly-recognized three phase reef growth model consists of an initial “colonizing” Phase 1, a “vertical aggrading/lateral expanding” Phase 2, and a well-defined “capping” Phase 3. This study integrates carbonate content variation data with thin section data to investigate the demise of the large microbial buildups at the end of the Upper Cambrian. The initial analyses focus on a 3 m thick inter-reef sediment section between two individual microbial reefs. The lower half of the section was deposited contemporaneously to the upper portion of the microbial growth Phase 2, whereas the upper half buries the Phase 3 reefs. CaCO3 content was measured in great details along two vertical transects in the inter-reef area and on the flank of one reef. In the lower half of the section, which is capped by an ooid-rich bioclastic grainstone bed, CaCO3 content values increase upward from 55 to 90 %, showing a systematic lowering and even shutdown of the siliciclastic flux during the end of Phase 2. The capping growth Phase 3 clearly overlies the ooid-rich bioclastic packstone bed and lacks coeval inter reef sediment accumulation. The sediments, immediately overlying this bioclastic bed and onlapping the Phase 3 reef, display the lowest CaCO3 content values (16 %) of the entire inter-reef section and initiate the final burial of the reefs. These trends imply siliciclastic influx ceased during late Phase 2 growth as the depositional environment moved towards more subtidal settings with increased accommodation which allowed Phase 3 reefs to develop 2-3 m high synoptic reliefs. The sudden increase in siliciclastics at the end of Phase 3 reef growth points to increased turbidity in the water column, thus inhibiting cyanobacteria’s photosysnthetic ability and leading to reef demise. Relatively lower CaCO3 content values at the very end of Phase 3 strengthen our interpretation.