Stratigraphy and Paleoecology of the Richmond Group in Southeastern Indiana

Abstract

The Richmond Group (Upper Ordovician) in southeastern Indiana is subdivided into rock-stratigraphic units (formations and members) and biostratigraphic units (assemblage zones). The formations and members established by earlier workers on the basis of faunal zones are either redefined as mappable rock units or discarded. A newly proposed formation replaces the lower three “formations,” Arnheim, Waynesville, and Liberty, which are based on paleontologic criteria and are lithologically indistinguishable. The new unit consists of thin layers of fossiliferous and barren limestone alternating with layers of calcareous shale. Two barren shale beds in conjunction with silicified fossil and pyritic limestone horizons are used to subdivide the new unit for a quantitative study of its lithologic variation. The Whitewater Formation above is redefined and subdivided into five rock units on the basis of lithology: (1) a lower unnamed member of massive earthy limestone, (2) a black carbonaceous shale bed, (3) the lower Saluda Member, which is a northward thinning wedge of mud-cracked slabby limestone with layers of massive dolomitic limestone at the base and top, (4) the upper Saluda Member, lithologically similar to the lower wedge but separated from it by a layer of massive dolomitic limestone, and (5) an upper unnamed member of massive earthy limestone. Four faunal assemblage zones are recognized in the newly proposed unit: (1) Resserella meeki, (2) Leptaena richmondensis, (3) Sowerbyella rugosus, and (4) Strophomena planumbona. The assemblage zones become thin and pinch out to the south, and their boundaries cross lithologic marker horizons within the newly proposed rock unit. The Homotrypa wortheni and Tetradium minus assemblage zones within the Whitewater Formation are split into upper and lower parts by the barren Saluda Member. The numbers of individuals in each species are counted in small unit areas and are statistically analyzed to determine the distribution of species within the assemblage zones and the geographic extent of the zones. The following interpretation of depositional environments is based on the distribution of the rock types and faunal assemblage zones. The lower unit of the Richmond Group was deposited in a shallow, inner sub-littoral, marine environment. The Whitewater Formation was formed during the regression and transgression of a shoal area with the lower and upper unnamed members representing the submerged mud flats, and the Saluda Member, the exposed mud flats.