Sedimentary facies and tectonic implications of Lower Mesozoic alluvial-fan conglomerates of the Newark Basin, northeastern United States

Abstract

Upper Triassic to Lower Jurassic conglomeratic sequences of the Passaic and succeeding formations along the northwest-bounding fault of the Newark Basin in eastern Pennsylvania, New Jersey and New York, have six relatively distinct lithofacies that record continental basin filling under arid to semi-arid conditions. Matrix-supported conglomerates represent debris flows: clast-supported conglomerates are hyperconcentrated steamflood deposits. Coarse pebbly sandstones represent a braided-stream channel facies, and medium-to-fine sandstones are interpreted as a sheetflood facies, deposits normally succeeded by thin mudstones of a waning-flood facies. Associated with such mudstones at Milford, New Jersey is a sixth, pedogenic facies: caliches that document pauses in deposition. Areal lithologic variations and analyses of vertical changes, including Markov analyses of facies transitions, document intermittent, cyclical debris flows and streamfloods on alluvial fans prograding into the basin. The intermittency is best accounted for by episodic, localized tectonic rejuvenation of fan provenance at the northwest bounding fault of the Newark Basin during the late Triassic to early Jurassic, the final phase of basin filling. Penecontemporaneity of tectonism and fan deposition is corroborated by deformation of some conglomerate clasts which transmitted tectonic stresses to each other before their enclosing matrices were completely lithified. Some pebbles and cobbles are shattered at points of contact, and clasts of relatively soft lithologies became pocked while grinding against harder fragments during post-depositional tectonic episodes.