NATURE OF THE OVERLYING STRATA, OR "COVER" OF AN UPPER CAMBRIAN MICROBIAL REEF COMPLEX (MASON COUNTY, CENTRAL TEXAS): POTENTIAL CLUES TO THE MICROBIAL DEMISE

Pre-salt hydrocarbon reservoir exploration in offshore Brazil has promoted great interest in microbial carbonates and a search for relevant analogs. The well preserved Upper Cambrian microbial reef complex in Mason County (Central Texas) is considered an important analogue for many aspects of the microbial reservoirs. A three-phase growth model was developed for the reef complex by integrating the overall buildup geometries and their relationships with coeval interbuildup strata. The ultimate demise of the reef complex of the Upper Point Peak Member (Wilberns Formation) is linked to a sudden influx of siliciclastics which onlaps and partially buries the buildups and intervening strata. Gradually this onlapping unit becomes fully carbonate and completely covers the buildups. This layered carbonate deposit or “Cover”, is a set of layers within the San Saba Member of the Wilberns Formation. This study focuses on sedimentary characteristics of the Cover to understand its depositional environment and the reasons why the microbial reefs were terminated.

By field observation and photogrammetry, the general architecture of the Cover and its relationship with the underlying microbial reef complex were defined in the northeast and southwest corners of a key outcrop area - the James River Pavement and adjacent cliff. In the northeast corner, the Cover overlies growth Phase 1 of a buildup, whereas in the southwest area it overlies Phases 2 and 3 of another. Based on analyses of a series of oriented short cores (7.5 cm or 15 cm in diameter and up to 50 cm in length) and thin sections, bedding organization, lithofacies, and the nature of non-skeletal and skeletal grains were characterized. The transition between the irregular upper surface of the microbial reef and the onlapping/downlapping Cover was recovered in several cores, and is a poorly sorted skeletal-rich rudstone with large irregular intraclasts, evolving into a series of grainstone and packstone fining-upward beds with often-erosive bases. Well-preserved sponges accumulated in the troughs of low-relief bedforms. These observations point to a high-energy depositional regime for the Cover, albeit with energy waning with time. Gastropods, which are rare in the microbial reef complex, become more common within the Cover that may explain the lack of microbial development.