The Cobleskill and Akron Members of the Rondout Formation: Late Silurian Carbonate Shelf Sedimentation in the Appalachian Basin, New York State

Abstract

ABSTRACT The outcrop of the Upper Silurian Cobleskill and Akron Members of the Rondout Formation of New York State extends from Buffalo to eastern Schoharie County and is continuous except where the units have been removed by pre-Onondaga erosion. Strata included in the Cobleskill, Akron, and Chrysler Members of the Rondout Formation, underlying Williamsville Member of the Bertie Formation, and underlying Brayman Shale include several distinct carbonate facies which have been identified by field characteristics, thin section petrography, insoluble residue analysis, and X-ray diffraction. Subfacies of the Cobleskill include subtidal biomicrites with stromatoporoid biostromes and intertidal fossiliferous micrites, whereas the Akron contains dolomitized analogues of these limestones. The overlyin Chrysler is composed of supratidal, laminated, thinly bedded, finely crystalline dolostone. Careful lateral tracing of lithofacies, analysis of vertical lithofacies sequences, and study of key beds within the Cobleskill and Akron have resulted in recognition of the lateral equivalence of these two members. Limestones of the Cobleskill and dolostones of the Akron contain strata deposited synchronously in similar environments. Lithologic variance resulted from different intensities of dolomitization. Lateral gradation of lower Akron strata into upper Williamsville strata suggests a close stratigraphic relationship that is ignored in present stratigraphic nomenclature. The Cobleskill and Akron Members are part of a transgressive-regressive carbonate sequence which overlies older evaporitic and dolomitic sediments of the Salina Group. Subtidal facies represent deposition under normal marine conditions which changed northward and westward into restricted environments of the intertidal and supratidal zones. Dolomitization and evaporite precipitation dominated sedimentation in these near-shore environments. Late Silurian sedimentation ended in a regional regression, resulting in seaward progradation of the intertidal and supratidal facies over subtidal carbonates of the basal Cobleskill-Akron. Portions of the subtidal facies were dolomitized locally by the super-position of these facies. Natural gas from the Akron Member of Erie County accounts for approximately one percent of New York State annual production. Production comes from a vuggy zone directly beneath the pre-Onondaga disconformity. This zone is interpreted as the result of dissolution of rugose corals from low intertidal storm deposits during dolomitization.