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Woods Hole Coastal and Marine Science Center

Geologic Interpretation and Multibeam Bathymetry of the Sea Floor in Southeastern Long Island Sound

Geologic Setting

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Geologic Setting
Figure 1. Map showing location of NOAA survey H11255 (light blue cross-hatched polygon).
Figure 1. Map showing location of NOAA survey H11255 (light blue cross-hatched polygon). Click on figure for larger image.

Long Island Sound, a major estuary along the east coast of the United States, is an industrial and recreational resource for the over 8 million people that live within its watershed (Knebel and others, 2000; fig. 1).

The surficial geology of the Sound is a result of glacial and eustatic processes (Schafer and Hartshorn, 1965; Flint and Gebert, 1976; Stone and Borns, 1986; Lewis and Needell, 1987; Needell and others, 1987; Lewis and Stone, 1991; Stone and others, 1992; Lewis and DiGiacomo-Cohen, 2000; Stone and others, 2005). The late Wisconsinan Laurentide Ice Sheet scoured the surface, deposited drift, and produced a recessional moraine across northern Long Island and a succession of minor recessional moraines in Connecticut. Glacial Lake Connecticut, which occupied most of the Long Island Sound basin, was forming by 19 ka when meltwater was impounded in the expanding basin between the recessional moraine on Long Island and the retreating ice. Deltaic and varved sediments deposited in this lake variously overlie the glacial drift throughout much of the basin. Erosion of a spillway at its eastern end drained glacial Lake Connecticut by 15.5 ka, and subaerially exposed the lakebed until the marine transgression that began about 15 ka. The glaciolacustrine deposits of glacial Lake Connecticut and the underlying glacial drift are truncated by a regional unconformity. This unconformity is a composite product of the subaerial exposure and marine transgression (Needell and others, 1987; Lewis and Stone, 1991). Marine deposits, which occur in quiet-water areas throughout the Long Island Sound basin, overlie the unconformity and earlier deposits, and they record deposition during the postglacial Holocene eustatic rise of sea level (Lewis and DiGiacomo-Cohen, 2000). Since about 13.5 ka, the glaciolacustrine and marine deltaic deposits in the eastern Sound have been eroded, selectively sorted, and transported westward (Knebel and Poppe, 2000; Poppe and others, 2000a; Lewis and DiGiacomo-Cohen, 2000).

Circulation within Long Island Sound is tidally dominated, and is stronger in constricted areas where large volumes of water must pass through relatively narrow openings, and weaker in broad deeper basins (Koppleman and others, 1976; Signell and others, 2000). However, wind-driven and wave-produced currents are relatively important in shallow, nearshore areas, especially during aperiodic storms when strong winds blow the length of the Sound. Minor nontidal estuarine circulation affects residual bottom currents throughout the year (Gordon and Philbeam, 1975; Signell and others, 2000).

The sea floor in the easternmost Sound, which is connected to Block Island Sound through the Race and Fishers Island Sound, is extremely irregular owing to strong tidal currents that have scoured the bottom (Knebel and Poppe, 2000; Lewis and DiGiacomo-Cohen, 2000; Poppe and others, 2000a). In addition, the area between the easternmost Sound and its central basin contains large west-southwest-trending tidal ridges and channels which are mantled by sand ribbons and sand waves (Lewis and Needell, 1987; Fenster and others, 1990). These features have developed on the eastern part of the remnant of the -40-m postglacial marine delta that separates the erosional and nondepositional environments of the eastern sound from the depositional environments prevalent in the central basin (Knebel and Poppe, 2000).

The topography in the central and western basins is characterized by broad areas of relatively smooth sea floor, separated by the Stratford Shoal Middleground complex (fig. 1; Knebel and others, 1999; Knebel and Poppe, 2000). This shoal complex is a coastal plain outlier capped by glacial drift (Lewis and Stone, 1991; Twichell and others, 1998) that lies between the central and western basins. Locally, small knolls, ridges, and bathymetric lows interrupt the otherwise smooth sea floor in the central basin.

Tidal and wind-driven currents have extensively reworked both the glacial and postglacial deposits in the eastern Sound and continue to influence the sedimentary processes and surficial sediment distributions in the study area (Lewis and Stone, 1991; Lewis and DiGiacomo-Cohen, 2000). Tidal currents alone can locally exceed 20 cm/s at 1 m above the bottom across the eastern half of survey H11255, but drop to less than 15 cm/s over the western part of the study area (Signell and others, 2000).

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This page last modified on Thursday, 14-Jun-2007 10:53:35 EDT