ULTIMATE DEMISE OF LARGE UPPER CAMBRIAN MICROBIAL REEFS (MASON COUNTY, CENTRAL TEXAS)

The robust growth of Upper Cambrian microbial reefs (Upper Point Peak Member, Wilberns Formation), exposed in outcrops along the Llano/James Rivers and Mill Creek in Central Texas, occurred in equatorial shallow seas, in close proximity to landmasses, and as part of the southwestern Great American Carbonate Bank. Variable siliciclastic input into these shallow seas played a critical role in controlling the reef 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.

Geochemical/mineralogical investigations focus on a uniquely accessible outcrop along the James River of mixed carbonate-siliciclastic sediments that accumulated between two individual reefs, referred to as “Rose” and “Andrea”. The lower half of the 2 m-thick section is contemporaneous to the upper microbial growth phases 2/3, whereas the upper half clearly buries the top of the reefs. CaCO₃ content was measured along two vertical transects, one in the middle of the inter-reef area and the other on the reef flank of Andrea. CaCO₃ content values display an overall increasing upward trend from 55 to 90 % in the lower half of the inter-reef sedimentary section, which is capped by an ooid-rich bioclastic grainstone bed. This trend clearly records a systematic lowering and even shutdown of the siliciclastic influx during the end of Phase 2. The capping growth phase 3 in both reefs clearly overlies the ooid-rich bioclastic bed. The sediment, immediately overlying this bioclastic bed and clearly onlapping the adjacent reef capping phase 3, displays the lowest CaCO₃ content values (16 %) of the entire inter-reef section and initiates the final burial of the reefs.

Based on these observations, the inter-reef sedimentation apparently ceased during Phase 3 and increased accommodation allowed reefs to develop significant 2-3 meters high synoptic relief and form a characteristic well-develop densely cemented microbial thrombolytic rind. Maximum siliciclastic flux (a mixture of fine quartz, microcline, and montmorillonite making 84 % of the bulk sediment) occurred at the very end of the microbial reef growth. Sudden turbidity increase in the water column is interpreted to have caused the ultimate demise of the reefs.