Sandstone and Conglomerate Shoreline Deposits in Triassic-Jurassic Newark and Hartford Basins of Newark Supergroup: ABSTRACT

Abstract

Three types of sandstones and conglomerates in the Newark and Hartford basins of the lower Mesozoic Newark Supergroup are similar to Holocene and to presently forming deltaic and wave-sorted shoreline deposits in modern, block-faulted, closed basins in the western United States. (1) Sandstones and siltstones with internal, sequences dominated by climbing ripple cross-lamination that comprise 2 to 4-m thick, low-angle, inclined foresets. The foreset beds intertongue with silty shale at their toes. Stacks of these foresets define coarsening-upward sequences that are interpreted as small, Gilbert-like delta fronts stacked on the topset plain due to fluctuations in lake level. (2) Sheetlike, decimeter-scale, muddy sandstone beds. These are composed of g aded, deceleration-of-flow sequences similar to those described above but contain mudstone partings with large polygonal cracks and soft-sediment deformation structures. Fining-upward sequences of these beds are interpreted as broad, flat delta fronts formed by flash-flooding streams intersecting an expanding shallow lake. (3) Cobble and pebble conglomerates with well-sorted sand or granule matrix. These rocks are typically associated with well-sorted, medium to coarse-grained sandstones with planar, horizontal lamination or oscillatory ripple cross-lamination. These beds are interpreted as wave-sorted alluvial fan-toe deposits. The Gilbert-like deltaic sandstones and the sheetlike muddy sandstones in the Newark and Hartford basins generally have low porosity and, therefore, would be poor reservoir rocks. The wave-sorted conglomerates, which are limited to the faulted basin margins, are potentially excellent reservoir rocks, in many places occurring directly in contact with black shales. Thick shoreline sandstones and conglomerates deposited in deeper lakes may occur in the Newark and Hartford basins, but they were probably restricted to the basin margins and may have been removed by subsequent faulting or erosion, or possibly they have not yet been recognized due to the scarcity of large outcrop exposures. End_of_Article - Last_Page 1448------------