Structure and Stratigraphy of the Upper Silurian – Lower Devonian Rocks Exposed In the Rosendale, N. Y. Area, An Update

The rock units outcropping in the area of the Town of Rosendale, Ulster County, NY include Upper Silurian to Lower Devonian sedimentary rocks lying unconformably on Ordovician black shales locally called Hudson River Shale, but believed to correlate to the Martinsburg Formation. Just below the unconformity, the authors have observed a fossiliferous, calcareous facies. The Upper Silurian argillaceous Whiteport and Rosendale dolomitic limestones were extensively mined as a source of natural cement during the 19th and early 20th centuries. Together with the Cobbleskill limestone they constitute a group called the “Cement Series” (RCS) and carry distinctive field characteristics in terms of faunal assemblages, sedimentary structures, and mineralogical composition. Lithological correlation of these carbonate rocks is based on their general distribution, structural attributes (distinctly oriented joint and fracture patterns), associated sedimentary structures and fossil assemblages. The Cobbleskill limestone is a key stratigraphic marker in the field and is very conspicuous for its well-preserved Halysites fossils. Carbonate rocks in the RCS are consistent with shallow-water shelf to supratidal depositional setting with intermittent influx of fine-grained clastics and lime-mud. The RCS and associated litho-stratigraphic units are deformed in a series of tight north-northeast-south-southwest trending highly asymmetrical and, in places, overturned folds, plunging to the north-northeast, steeper dipping limbs to the west. These units, deformed during the Acadian Orogeny, are intensely folded and thrust faulted in the Rosendale area, causing multiple repetitions of the outcrop of these formations. These repetitions increase the effective thickness of the cement-forming rock, and this provided a greater economic potential for excavation. The authors present evidence that the concentration of intense deformation in the vicinity of Rosendale, NY is related to the Shawangunk Conglomerate, which thins from south to north and ends in the study area, acting as a massif against which the rocks were deformed.