Abstract

The Bonneterre Formation in the Viburnum Trend area can be divided into a fore reef, reef complex, back reef, and shelf facies. These facies are petrographic units representing different paleoenvironments whose characteristics were developed by sea level fluctuations and the distribution of Precambrian knobs and ridges in a shallow Cambrian sea.The fore reef is primarily an argillaceous lime mudstone composed of reef detritus and fecal pellets. The reef complex is dominantly a combination of boundstones and oolitic grainstones. The boundstones are composed of algal stromatolites, ooids, trilobite and echinoderm fragments, and Girvanella-rich pellets. The back reef is an argillaceous boundstone and wackestone composed of planar stromatolites and burrowed pelleted muds. The overlying offshore shelf facies is a grainstone to mudstone composed of ooids, trilobite and echinoderm fragments, pellets, oncolites, and reef detritus.A limestone-dolomite interface with the limestone on the seaward side is superimposed upon and cross-cuts the fore reef, the reef complex, and the offshore shelf facies. These cross-cutting relations support an epigenetic origin of the dolomite.A temporal sequence of carbonate diagenesis of the Bonneterre includes neomorphism, cementation, solution and pore filling, fracturing, and stylolitization of the original calcium carbonates. During stylolitization, grainstones and stylolites were selectively dolomitized. This was followed by dolomitization of pellets, algal balls, ooids, trilobites, echinoderms, and calcite cement. Later dedolomitization appears to be related to inter-crystalline clay zones. This sequence of diagenesis was apparently controlled by changing Mg (super ++) concentrations and gradually reduced salinities of the diagenetic waters.

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