Controls On Microbial And Oolitic Carbonate Sedimentation And Stratigraphic Cyclicity Within A Mixed Carbonate-Siliciclastic System: Upper Cambrian Wilberns Formation, Llano Uplift, Mason County, Texas, Usa

The upper Cambrian Wilberns Formation in central Texas records deposition on a low-gradient shelf within a mixed carbonate-siliciclastic tidal-flat system that changes offshore to subtidal shelf and open-marine oolitic skeletal shoals with large microbial mounds. Siliciclastic sediment is interpreted to have been delivered to the tidal flat by aeolian processes because of the narrow range in grain size and paucity of clay. Tidal influence is dominant as evidenced by reversing currents and desiccation on the tidal flat, and megaripples with reversing current indicators in offshore shoals. Intraclastic conglomerates were deposited in broad channels on the tidal flats during storm surges. Microbialite deposition is interpreted to be controlled by accommodation favouring amalgamated thin biostromes developed in the tidal flat vs. larger mounds with greater synoptic relief in the offshore, and current energy resulting in preferential elongation of offshore mounds in a NE-SW orientation. Intertidal mounds and biostromes grew in the presence of significant siliciclastic flux and trapped it within their structure, whereas offshore large buildups incorporated little siliciclastic component. Oolite and skeletal grainstone formed in tide agitated shoals associated with large subtidal microbial mounds. Storms extensively recycled and redistributed skeletal and oolitic sands from the offshore shoals across the shelf as thin sand sheets. Spatial mixing of siliciclastic and carbonate sediment occurred across the tidal flat and shelf. Low-frequency and intermediate-frequency stratigraphic cycles were driven by shifts in the shoreline and changes in rate of siliciclastic flux in response to relative sea-level fluctuation. Random facies stacking and the lack of metre-scale cyclicity are interpreted to reflect stratigraphic incompleteness and an episodic signal introduced by storms.