The Shawangunk Formation (Upper Ordovician(?) to Middle Silurian) in eastern Pennsylvania

Abstract

The Shawangunk Formation of Early and Middle Silurian age, and possibly Late Ordovician age, in eastern Pennsyl vania and northwestern New Jersey forms a thick clastic wedge of sediments derived from sourcelands to the southeast uplifted during the Taconic orogeny. The formation is divided into four newly defined members, from bottom to top: Weiders Member (coarse conglomerate and quartzose sandstone), Minsi Member (quartzose conglomeratic sandstone and minor argillite), Lizard Creek Member (complex sequence of quartzose sandstone, siltstone, shale, and a few red beds, with sparse fauna), and Tammany Member (quartzose conglom eratic sandstone and minor argillite). The Weiders Member pinches out to the east near Smith Gap. The Tammany Mem ber cannot be conveniently mapped west of Smith Gap because of interfingering with and replacement by beds of the Lizard Creek Member. The contact of the Shawangunk with the underlying Martinsburg Formation is an angular unconform ity. The boundary between the Shawangunk and overlying Bloomsburg Red Beds is irregular and transitional through about 130-700 feet of red, green, and gray rocks. Sedimentary features in the Shawangunk indicate that the sediments were deposited by streams and in a complex transitional marine continental environment, including tidal flats, barrier bars, estuaries, and lagoons.