STRATIGRAPHIC AND SEDIMENTOLOGIC INSIGHTS FROM SCHOHARIE, NY: COEYMANS FORMATION (LOWER LOCHKOVIAN, HELDERBERG GROUP)

Several large road cuts along I-88 near Schoharie, NY provide nearly continuous exposure of the limestones of the Helderberg Group (U. Sil. - L. Dev.). Detailed examination of the easternmost outcrop for an undergraduate class project in Sedimentary Geology has yielded new stratigraphic and sedimentologic insights into these well-studied strata.

The Coeymans Formation at I-88 is divisible into three distinct members. In ascending order these are: Dayville, Ravena, and Deansboro. Recognition of these three members contrasts with previous works which only recognized the Ravena Member in this area. The Dayville occurrence was documented previously by Ebert and Matteson (2003a, b). Detailed sedimentologic description has enabled recognition of distinctly different Ravena and Deansboro units at Schoharie. The Ravena (5.5m) is characterized by blue-gray, nodular to pod-like beds of fine to medium grained skeletal grainstone. Remnant trough cross-strata and coarse-filled cut and fill structures, suggest that much of this nodular bedding resulted from modification of these structures by extensive bioturbation. Gypidulid brachiopods and dm-scale colonies of favocitid corals occur sporadically in the Ravena, but are most common in the lower half.

The Deansboro Member (2.35 m exposed) displays thicker and more laterally continuous bedding than the Ravena. Weathering of reticulate partings of pyrite-enriched insoluble seams imparts a rusty color to the Deansboro. Deansboro grainstones are coarser than those of the Ravena and are characterized by greater fragmentation of bioclasts. Trilobite fragments are abundant, but the most obvious faunal elements are holdfasts of the callocystid rhombiferan Lepocrinites gebhardi.

Lithologic, sedimentologic and faunal data suggest deepening from Ravena into the Deansboro, accompanied by increased reworking of bioclasts and some degree of sediment starvation. The timing of this event may be revealed by a well-preserved assemblage of chitinozoans which have been recovered from both Coeymans members. Identification of the chitinozoans is in progress.