Surface Textures of Detrital Quartz Silt, Northeastern United States Continental Shelf
D. Prusak, P. Leschak, J. Mazzullo
The coarse silt fraction of the surficial sediment on the northeastern U.S. continental shelf contains three distinct quartz-grain morphologic types, reflecting three general sources for this sediment. Type I grains are rounded and spherical, and their surfaces are covered by textures produced by moderate to intense chemical weathering in soils. This grain type is commonly found on the shelf between New England and Cape Hatteras, and is especially abundant near shelf exposures of pre-Pleistocene strata. It is concluded that this grain type was derived from unconsolidated, multicyclic Cretaceous and Tertiary strata of the Mid-Atlantic coastal plain. Type II grains are elongate and very angular, and their surfaces are covered by a variety of breakage textures, including con hoidal, steplike, and arc-shaped fractures and breakage blocks. Such grains are abundant on those parts of the shelf that were glaciated during the late Wisconsinan and in the mouths of rivers that drained glaciated terrane. They are presumed to have been derived from glacial and periglacial deposits. Type III grains are characterized by igneous, metamorphic, and sedimentary quartz grains with moderately to well-formed euhedral and subhedral crystals. This grain type is abundant in modern estuarine deposits as well as in upper Pleistocene fluvial and deltaic deposits on the shelf, and is thought to be derived from the crystalline and sedimentary rocks of the Appalachian Mountains.
It is commonly stated that quartz silt grains are created by the breakage of quartz sand and gravel clasts during transport. If this hypothesis is true, then breakage surface textures should be relatively abundant on all types of silt grains. In this study, however, such textures were only observed in abundance on grains transported by glaciers (type II). The type I and III grains, which were transported through fluvial and littoral environments, display no evidence for a fracture origin. Rather, their morphologies reflect their source and the type of weathering that occurred there. Therefore, it is concluded that nonglacial coarse quartz silt grains in this area are the stable residual products of weathering at the source.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.