Petrography of Algal Bioherms in Burnt Bluff Group (Silurian), Wisconsin

Abstract

The Burnt Bluff Group of Wisconsin consists of approximately 100 feet of completely dolomitized, generally thin-bedded non-fossiliferous strata, which, when analyzed petrographically, reveal that, despite thorough dolomitization, their original textures and components could be recognized, allowing subdivision of the rocks studied into distinct microfacies. More than 600 thin sections were studied from three locations (Nasbro, Sturgeon Bay, and Chilton) at which mounds or bioherms had been described in the previous literature. The indices of frequency and(or) clasticity were determined for algal fragments, algal pellets, algal balls, leperditids, other ostracods, oolites, lithic fragments, detrital quartz, and feldspar, and curves for these parameters were plotted. A crystallinity curve showing the maximum crystal diameters of important components was also compiled, and the presence of algal mats, stromatoporoids, corals, Stromatactis, brachiopods, and large and extremely well rounded (St. Peter type) quartz grains was recorded. The petrographic data, general appearance, and field characteristics of the samples were used in subdividing them into eight microfacies, all deposited in water probably not exceeding a few tens of feet in depth. Microfacies 1, a coarsely crystalline dolomite matrix enclosing leperditid carapaces, clay aggregates, and pyrite, is a dolomitized calcilutite which was deposited below wave base. Microfacies 2 represents a shallower environment and is transitional between microfacies 1 and 3, and consists of interlayered dolomitized calcilutite and calcisiltite. Microfacies 3 is a dolomitized calcisiltite, in some places fossiliferous or oolitic. Layers of dolomitized oolitic limestone with an original clear calcite cement make up microfacies 4, which developed in fairly shallow water. Micr facies 5 is a dolomitized algal fragment breccia-conglomerate which was deposited in an extremely shallow, agitated environment. Microfacies 6 forms algal bioherms and is laterally equivalent to microfacies 1, 2, and 3. Microfacies 7 is a dolomitized biocalcarenite to calcisiltite containing large quartz grains, rounded lithic debris, and leperditid fragments, and is interpreted as a lateral equivalent of microfacies 4. Microfacies 8 is represented by algal, stromatoporoidal or coral biostromes which are laterally equivalent to microfacies 5. The investigated bioherms are small (reaching 8 feet in height and 30 feet in width) and display simple forms. They developed initially on coarser layers within a calcilutite unit or on slight positive irregularities. In a normal marine environment (Sturgeon Bay), algal growth was slow and poorly developed, whereas in a penesaline environment (Nasbro), growth was rapid, and numerous, well developed bioherms formed. The simple shapes of the bioherms are an expression of the nature of the constructing algae and of the relatively quiet, non-subsiding environment in which they developed, and to which they did not contribute any debris. Their associated sedimentary sequences exhibit textural changes expressing a general shallowing. In all respects, the Burnt Bluff bioherms are entirely dif erent from the classical Niagaran reefs of the Great Lakes area. The principal textural effect of dolomitization has been an increase of the crystal size of the components of the rock with maximum effect upon the originally fine-grained part of the sediments.