Eastern Section Meeting

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Origin and Importance of Shell Beds in the Logana Member of the Lexington Limestone, (Katian, Ordovician) on the South Flank of the Seebree Through, Cincinnati, Ohio


Siliciclastics are transported into basins, but carbonates may accumulate in place or be transported. Thin shell beds within the Utica have been interpreted as allochthonous deposits. However, petrographic studies in the up-ramp equivalent Logana Member in Cincinnati reveal organic-rich calcareous shales with thin shell beds, but no evidence of significant shell transport. These thin shell pavements are dominated by horizontally-oriented unbroken shells, some of which are articulated and either spar-filled or filled with the matrix-like muds or calcisiltites.

The growth of shell beds in place in un-bioturbated organic-rich muds suggests that the overlying water column was, at least occasionally, sufficiently oxygenated and sediment free for long enough to allow colonization by a single species of brachiopod. Some intervals contain a progression of limestones ranging from low-diversity, whole-fossil packstones to high-diversity highly-comminuted phosphatic grainstones. These shell beds probably developed in place over longer periods of non-deposition, which fostered diverse ecological communities and the accumulation of diagenetic phosphate derived from the oxidative decay of organic matter in the sediment.

This depositional model is similar to one proposed for the overlying Kope Formation where thin beds can be traced for long distances. If Logana shell beds are similarly extensive and continuous as the “episodic sediment starvation model” suggests, it could have implications for stratigraphic correlation, fracturing mechanics, and fluid transport.