PALEOECOLOGY AND DEPOSITIONAL ENVIRONMENT OF THE UPPER ORDOVICIAN (KATIAN) MARTINSBURG FORMATION ON THE SHAWANGUNK RIDGE

Unusually fossiliferous Martinsburg Formation (Katian/Edenian Stage) strata are exposed in a small shale quarry along the east side of Shawangunk Ridge on the property of Mohonk Mountain House in New Paltz, New York. The rocks are mostly dark gray shales and siltstones interbedded with fine grained graywacke beds, occasional prominent pyrite layers and disseminated sphalerite, chalcopyrite and galena. Carbonaceous material occurs mostly as fine-grained patches throughout the matrix. The studied section is tectonized, with shiny quartz slickensided surfaces. However, the strata also display relatively well-preserved fossils and sedimentary structures including planar and hummocky cross-laminated strata, graded beds and ripple marks, indicating storm deposited sand layers. Crinoid stems, some slightly disarticulated, and free columnals occur on different bedding surfaces indicating a possible change in current regime Scattered linear to sinusoidal horizontal burrow structures ranging in diameter from .5-3 cm are found on the silty beds. Some of the burrows are infilled with coarse quartz grains. The faunal constituents include brachiopods, crinoids, bivalves, cephalopods, including Kionoceras and the coiled nautiloid Trocholites, trilobites (Cryptolithus), ostracodes, and unidentified burrowers. The brachiopods are represented by a low diversity assemblage of Cincinnetina and Sowerbyella. The fauna can be classified into distinct trophic groups: (1) high-level suspension feeders (crinoids); (2) low-level suspension feeders (brachiopods, bivalves); (3) scavengers and sediment surface grazers (ostracodes, trilobites); (4) burrowers; (5) rare nektonic predators. This partitioning of feeding niches probably led to a reduced competitive trophic structure and therefore increased community stability. Sedimentological indicators suggest a moderate depth offshore mud bottom environment episodically affected by deep storm waves and currents. Overall, this assemblage is remarkably similar to coeval offshore Edenian deposits in the Cincinnati Arch despite its proximity to the Taconic front.