SEDIMENTOLOGY AND GEOCHEMICAL INVESTIGATIONS OF THE EXPOSED ORDOVICIAN-DEVONIAN SEDIMENTARY ROCKS, ROSENDALE, ULSTER COUNTY, NEW YORK

Many of the Ordovician-Devonian formations in Rosendale, Ulster County, NY are dolomitic to mixed dolomitic-calcitic limestone, shale, sandstone and conglomerate. Sporadic occurrences of chertiferous and phosphatic zones are known to occur within the Devonian units. Carbonate rocks are highly fossiliferous and fossils observed in the area include: Halysites (Silurian), Gypidula Coemanensis, and numerous Crinoids and Brachiopods (Lower Devonian). Other primary sedimentary features observed in the exposed formations include ripple marks, mud cracks, cross bedding, sole marks, and penecontemporaneous slumping (in the High Falls shale). Shallow water ichnofaunal assemblages also characterize several formations particularly the Binnewater. Depositional environments range from strictly continental to fluvio-deltaic to shallow marine with periodic incursions of the shallow transgressive sea. Stratigraphic contacts between formations are of sharp, inter-fingering and gradational nature. Several formations display a remarkable facies change laterally. Representative samples collected from several outcropping formations were investigated to establish geochemical fingerprints by using both major and trace elements. In terms of establishing a viable geochemical markers with respect to individual formations, several bulk oxides including SiO₂, Al₂O₃, CaO, MgO, Na₂O, K₂O, FeO, TiO₂ and individual elements such as Mn, Cu, Ti, Zr, Ni, Pb, Y, Rb, Ba, and Ce were found to be statistically significant for characterizing investigated formations and further studies involving a large sample-set are underway to expand this geochemical work for regional correlation purpose. Structurally all of the exposed formations particularly the Shawangunk conglomerate(Silurian), High Falls shale (Silurian), Binnewater sandstone (Silurian), and cement series (Devonian) were subjected to Acadian Orogenic events and displayed pronounced structural deformation and exhibited development of rock cleavages in shaley units, tight folding, intersecting joints, and faulting. Furthermore, tectonic overprinting on the depositional units is obvious as evidenced from the development of an angular unconformity between the Ordovician Hudson River shale and Silurian High Falls shale.