Field studies, extensive shallow coring, thin section petrography, and hyperspectral scanning are used to characterize a series of Upper Cambrian (Wilberns Formation) subtidal microbial reefs outcropping in Mason County, central Texas. Results document details of microbial textures that were responsible for growth of the buildups, thereby providing an understanding of the environmental conditions that influenced varied stages in their evolution. Thrombolitic fabrics of dominantly microbial micrite with fossil fragments evidence carbonate precipitation induced by microbial colonies that produced a tight framework hindering diagenesis. Stromatolitic fabrics show alternating laminations of microbially precipitated calcite and amalgamated trapped grainstones exhibiting ferroan dolomite replacement and subsequent oxidation of these grains. Initial characterization of the microbial reefs identified successive growth stages, repeatedly identifiable across several separate outcrops, from which a three-phase growth model was formulated. Within the context of the three phases, the core-based geologic characterization of the buildups and coeval sediments refines the growth model into a more robust depositional model. Nucleating on lenses of flat pebbles, an initial “colonizing” Phase 1 growth results in 3-4 m high buildups defined by their distinct and early-cemented outer margins of a thick thrombolitic rind. Buildup interiors exhibit amalgamated microbial heads with poorly preserved internal structure enveloped by cm-thin thrombolitic rinds. The buildups grew in high-energy conditions, but without interacting with coeval interbuildup oolitic-bioclastic grainstones. Phase 1 terminated with onlapped terrigenous and calcareous silts (~35% CaCO3). The overlying Phase 2 growth produced buildups up to 8 m thick, characterized by a mutual interaction with interbuildup high-energy oolitic-bioclastic grainstones and packstones and generally lacking an external rind.
Internally, Phase 2 growth consists of vertically aggrading and laterally expanding stromatolitic columns, each exhibiting cm-thick thrombolitic rinds directly interacting with intercolumn bioclastic grainstones. Phase 3 of growth produces a well-defined thrombolitic 2-3 m thick rind atop Phase 2, and in some instances on top of the interbuildup grainstones.