Stratigraphy of Upper Silurian Cayugan Evaporites

ABSTRACT The stratigraphy of the Cayugan series strongly contrasts classical terminology of the basin margins with subsurface relationships of the basin centers. Both lithofacies and paleontological facies are sharply defined and narrowly restricted so that the few outcrops in the basin margin carbonate facies serve poorly to define a proper framework for Cayugan stratigraphy. Basinal rocks are exposed only in New York state and in the Appalachian fold-belt in Pennsylvania, Maryland, and Virginia where the rocks are largely atypical of the bulk of the Cayugan series. A review of the evolution of Cayugan stratigraphic terminology, and the nature and distribution of the faunas demonstrates the provincial nature of knowledge concerning correlation among the regions of classical study. A different stratigraphic terminology is used in New York, Pennsylvania-Maryland, Ohio, Indiana, Wisconsin, and Michigan-Ontario. In most regions the terminology of the basin margins is inappropriate for the basin centers. Moreover, in the faunas reported, out of a total of 243 species, 199 (82 per cent) are reported from only one locality, and 225 (93 per cent) are reported from one or two localities. To solve some of these problems a regional framework of sedimentation is established so that there can be a standard of reference within which stratigraphic classification and correlation over the entire Cayugan depositional basin can be made. The framework of sedimentation establishes general stratigraphic relations between disconnected outcrops, outcrop and subsurface, and subsurface units within and between basins. It is prescribed from regional lithofacies analyses of the distribution of evaporite rock types, limestone-dolomite relations, terrigenous detrital debris, and thicknesses of rock units. The sedimentary framework for Cayugan rocks involves the development and growth of Niagaran reef platforms around the Michigan and Ohio basins which prevented free interchange of oceanic brine between the open sea and the basin, and which formed the ultimate control on subsequent evaporite deposition. In the eastern part of the Appalachian basin evaporites were deposited in New York and northern Pennsylvania while open marine limestone and shale were being deposited in southern Pennsylvania and Maryland in the Wills Creek-Tonoloway sea. Oceanic brine flow between the southern open ocean and the northern evaporite basin was restricted by the distal end of the Bloomsburg-Vernon delta complex, deposited by westward flowing streams carrying red terrigenous debris onto the eastern shelf of the Appalachian basin.