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Growth Phases in the Upper Cambrian Wilberns Microbial Reef Evolution (Mason County, Texas)

Abstract

Research on Upper Cambrian microbial reefs, cropping out in Mason County (TX), has been recently initiated to better understand recent giant oil field discoveries in microbial carbonates offshore Brazil. Spectacular newly accessible outcrops of microbial reefs were mapped along Mill Creek, James and Llano Rivers, and numerous intervening gullies. The microbial reef complex forms a10-30 m-thick, continuous stratigraphic unit that covers as much as 25 km2. This unit clearly contrasts with the underlying, shallow subtidal, mostly siliciclastic unit which contains thinner less extensive microbial structures. The initial growth phase of the complex consists of low relief oblong, 3-5 m high/10-30 m in size, individual reefs. The periphery and top of each individual reef consists of a homogenous 1 m-thick rind enclosing numerous thrombolytic heads. The inter-reef sediments consist of a 1-2 m-thick, thinly bedded, grainstone unit that systematically dips towards the center of each individual reef. An onlapping clay-rich unit separates the first and the second microbial growth phases. The second phase is often directly rooted on top of the low relief reefs, then expends beyond their clearly defined periphery. During the second growth phase, the 5-10 m thick internal edifice of the often dolomitized microbial reef is characterized by a series of rectangular box structures, defined by radially oriented, stacked microbial columns each separated by their own thin homogeneous micrite rind. Sub-parallel and roughly horizontal erosion resistant 5-10 cm-thick layers may correspond to time of limited microbial growth. The preferentially eroded box center is interpreted as spurts of columnar microbial growth and may mimic heterogeneity in primary porosity. The second microbial growth phase is directly connected through a series a small wedge shaped, often undulating microbial beds thinning and changing facies into two to three, 1-2 m thick, grainstone sub-parallel beds linking each individual reef. The third and last growth microbial phase consists of a well-defined, 1- 2 m thick rind, clearly capping each reef in the complex. A clay-rich unit onlaps the microbial cap, prior to the deposition of a thick cover of thinly bedded, highly bioturbated grainstone rich in macrofossils that marks the end of the microbial reef complex. The Mason Microbial Research Consortium is funded by Chevron, ConocoPhillips, Shell, and Statoil.