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Two hundred to three hundred feet of highly fossiliferous limestone, interbedded with abundant shale, are exposed in the Ohio Valley. The monotonous apparent uniformity of these Ordovician rocks (Cynthiana and Eden) has long discouraged lithostratigraphic work. Studies of the past several years have demonstrated that lithostratigraphic analysis is possible and useful.
Petrographic conclusions are based on standard methods. Lithostratigraphic methods of greatest effect are the computation of curves of clastic ratio, bedding index, and frequency of limestone types from detailed measured sections.
The "shales" include beds of shale, siltstone, and some mudstone. They consist largely of clay and fine and medium silt of illite, chlorite, quartz and pyrite, and about 15 per cent calcite. The fineness of these evenly layered muddy rocks and the variety of thicknesses of the beds indicate that the persistent sedimentary accumulation in the region was fine terrigenous detritus that was locally and intermittently interrupted by accumulation of biogenic carbonate debris.
Biogenic limestones in a limited variety of textures and structures occur in beds ranging from thin laminae to ledges about a foot thick. The silty limestones are laminated and cross-laminated. Pararipples are common in the coarse shell-fragment ledges and are thought to have been formed by surf and tidal currents augmented by waves. Submarine slump structures are conspicuous at several levels in the Cynthiana limestone, and occur typically in the fine-grained silty limestones. The principal diluent of calcite in the limestones is quartz silt, much of which is authigenic.
Several limestone texture-structure systems are repeated many times. These form the basis of a practical classification of the limestones which aids environmental interpretations. Limestone beds were formed of debris originating in local colonies of benthos on the sea floor, which appear to have been concentrated on low swells along the shoal forming the incipient Cincinnati arch. Such colonies were intermittently destroyed and the debris sorted into lenses of different texture. Continual alteration of the topography of the sea floor and the sites of colonies of benthos led to the interbedded limestones and shales preserved today.
For the future, attention to the pararipples and possible
facies relationships between the several types of limestone may contribute to a better understanding of the sedimentary environment.
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