POSSIBLE EVIDENCE OF “PARACONFORMITY” BETWEEN THE MANHATTAN SCHIST AND THE INWOOD MARBLE, NEW YORK CITY

At the very northwestern tip of Manhattan Island, in the shadow of the Henry Hudson Bridge, one can observe, at times of low tide, by the shore of the ‘Spuyten Duyvil’, an apparently conformable contact between the outcrop of the Inwood Marble (Mg-calcite rich, commonly dolomitic, often exhibiting saccharoidal texture and insoluble silicates such as tremolite, chlorite, and muscovite) and an overlying schist. It is dominantly muscovite-biotite (phlogopite) schist, garnetiferous in places. This overlying schist has been presumed by many geoscientists working in the New York City area to be the Manhattan Schist. This, however, is problematic because the age of the Manhattan Schist is late Proterozoic to Cambrian and the Inwood Marble, considered to be a member of the Stockbridge Formation as mapped in New England, is Cambrian to Ordovician in age (Bedrock Geological Map of Connecticut, Rodgers, 1985). This implies that either the overlying schist or the underlying marble have been misidentified as to age which is unlikely, or that subsequent tectonic processes, such as over thrusting during the Taconic Orogeny, have juxtaposed the older Manhattan Schist over the Inwood Marble. The authors suggest that the appearance of conformity or paraconformity is the result of subsequent deformation of the fault surface during the Acadian Orogeny.