Petrography of the northernmost exposure of the Shawangunk Formation; Central Hudson Valley, New York

The scale of geologic structures within the Appalachian Mountains is largely controlled by the most mechanically rigid units within the deformed stratigraphic sequence. Perhaps the greatest contributor in structural scale Appalachians of SE New York is the Shawangunk Formation, a rigid, silica-cemented, white quartz pebble conglomerate. The Shawangunk Formation is the along-strike equivalent of the Tuscarora Formation, which similarly controls the Appalachian structural scale between Pennsylvania and Georgia. Because of its geologic significance, the Shawangunk Formation is currently the subject of studies examining its stratigraphy and how it was deformed during the tectonic events that created the Appalachians. To support these efforts, we conducted a petrographic analysis of samples from the northernmost exposures of the Shawangunk Formation. Interestingly, these rocks differ significantly from exposures further south. Specifically, the northernmost Shawangunk Fm is a silica-cemented, quartz pebble conglomerate. Fresh faces are white to dark gray and weathered surfaces are red-brown from iron oxide staining. Matrix is dark gray siltstone to coarse grained quartz sandstone with few iron oxide grains. Clasts are sub-angular to round, poorly sorted pebble quartz. Our analysis utilized photomicrographs of polished petrographic thin sections taken with a microscope outfitted with a high-resolution digital SLR camera. To provide adequate resolution, 40-50 individual photomicrographs were stitched together with Adobe Photoshop into large composite images. The composite photomicrographs were then overlain with a grid sized according to the largest grain in each sample. These grids were the n used to conduct a systematic inventory of the modal compositions of grains within the Shawangunk Formation