--> Distinct Growth Phases of an Upper Cambrian Microbial Reef Complex; Depositional Environment Indicators (James River, Mason County, Texas)

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Distinct Growth Phases of an Upper Cambrian Microbial Reef Complex; Depositional Environment Indicators (James River, Mason County, Texas)

Abstract

Interest in microbial deposits has increased following discoveries of hydrocarbon reservoirs in pre-salt deposits of offshore Brazil and Angola. In an effort to establish an analog to these reservoirs, a Rice-Trinity Microbial Research Consortium* was established to study a 10-15 meter thick Upper Cambrian microbial reef unit (Point Peak Member of the Wilberns Formation) covering as much as 25 km2 of southern Mason County, Texas. Drone based photogrammetry and systematic core drilling of a reef complex within this unit reveal three successive microbial shallow marine growth phases. Phase 1 of microbial thrombolitic growth initiated on low-relief sheets of flat rip-up clasts interpreted as a transgressive lag and occurred contemporaneous to deposition of bioclastic grainstones. Sufficient accommodation allowed the bioherms to develop appreciable synoptic relief; continued cementation and lithification developed the bioherm margin into a well-defined, purely microbial rind. A shallowing event, marked by siltstones onlapping the uppermost Phase 1 rind, ends Phase 1 growth. The microbial bioherms grew contemporaneously in Phase 2 to deposition of inter-reef bioclastic grainstones using Phase 1 as a substrate for growth. Growth was characterized by juxtaposed columnar stromatolites. Incremental pauses in sea level rise periodically constrained vertical growth and induced lateral expansion beyond the margins of Phase 1 growth. These expansions, coupled with low synoptic relief of the bioherm, allowed interaction of the reef margin with inter-reef sediment, creating an interfingering nature between boundstone and surrounding bioclastic grainstones and inhibiting the formation of a well-defined bioherm rind. Termination of phase 2 microbial growth resulted from a deepening event and is marked by the end of deposition of bioclastic grainstone. Phase 3 of microbial growth, characterized by a series of juxtaposed, well-defined bioherms, acts as a “capping phase” for the full bioherm unit. Increased accommodation allowed the bioherms to develop significant synoptic relief, once again permitting the formation of a well-defined microbial rind. The demise of the microbial reef complex is marked by siltstones onlapping the bioherms. *Representing a collaboration between Rice and Trinity Universities with funding and interaction from Chevron, ConocoPhillips, Shell, and Statoil.