Abstract

Silurian reefs in the Great Lakes area exhibit virtually any shape and size and represent as many as six partly collective generations. An improved understanding of their geometry, including their internal structure and their spatial distribution within the overall body of Silurian rocks, permits new interpretations and applications ranging in scope from local conditions of growth to a broad stratigraphic and paleogeographic setting from which ideas on Silurian tectonic and sedimentational histories must proceed. These reefs reflect in their geometry both restricted and normal-marine environments that alternately influenced sedimentation, reef and nonreef, throughout most of Silurian time. Terrigenous clastic and clastic-free sedimentation was as much a sequentially ordered process as it was geographically differentiated. Clastics influenced development of reef microfacies but were only incidental with respect to inception or continuation of growth, which seems to have had no preference between fine muddy substrate and coarse carbonate-sand substrate. Most Silurian reefs do not have flat bottoms; rather, bottom cross sections are broadly V shaped and have laterally digitate boundaries that are gradational with the substrate. Accentuation of this shape by substrate depression and (or) flexuring is common. Additional variation among such V configurations is related to rates of area subsidence and accumulation of interreef sediments. Thus, the large reefs that were situated in the slowly subsiding interbasin platform area, where interreef sediments accumulated nearly as rapidly as the reefs grew, emphasize these V-shaped bottom cross sections. These reefs generally experienced some postreef erosion and so tend to have flattened tops and to exhibit modest closures in subsurface locales. The platform-situated reefs contrast with basin-edge pinnacle reefs that grew relatively rapidly and high above the seafloor in response to more rapid subsidence and relatively slow interreef sedimentation. Also, they generally were longer lived than the pinnacle reefs, some probably persisting until the end of Silurian time. Reefs that aborted soon after their inception because of environments that became increasingly restricted tend to have rounded tops that are gradational with micritic carbonate covers. Many larger reefs reached their great sizes through coalescence of several small initial growth centers. Some, if situated on the interbasin platform, exhibit cross-sectional shapes that reflect the cyclicity of carbonate-evaporite deposition in the Michigan and Appalachian basins. In areal view most discrete reef masses are somewhat elliptical and, especially if eroded, exhibit conspicuous biolithic zonation that is obviously repetitive in some examples. Predictive reef modeling (shape, internal structure, energy relations, spatial distribution) has several uses, including: interpretation of physical parameters influencing growth, verification of many known reefs as true organic-framework reefs, correlation of sedimentational events in geographically separated areas, and forecasting and planning development of stone and hydrocarbon resources.

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