Lithostratigraphy and Depositional Environment of the Eden Shale (Ordovician) in the Tri-State Area of Indiana, Kentucky, and Ohio

Abstract

The Eden Shale (Ordovician) of the tri-state area of Indiana, Kentucky, and Ohio consists mainly of blue-gray clay shale alternating with thin beds of sparry and micritic limestone differing significantly from the underlying Lexington Limestone (uppermost beds predominantly sparry limestone) and from the overlying Dillsboro Formation (lowermost beds predominantly micritic limestone). Lexington lithologies reflect deposition in agitated water on a carbonate shoal; Eden deposition took place in adjacent areas of deeper, quieter water; Dillsboro sediments represent shallower conditions than those of the Eden but less agitated than those of the Lexington. In-place accumulation of skeletal debris and lime mud on minor elevated areas of the sea floor and subsequent burial by terrigenous mud resulted in a basic pattern of alternating beds of shale and biomicrudite in the Eden. Periodic storms that blanketed the inner neritic mud bottom with thin layers of winnowed coarse skeletal sand most likely produced the sparry limestones. Although small-scale limestone-shale alternations do not represent cyclic sedimentation, large-scale sedimentary cycles in the Ordovician Cincinnati Group apparently resulted from major oscillations of sea level.