Coastal and Marine Geology Program
Woods Hole Science Center

Skip titleU.S. Geological Survey Open-File Report 2005-1018
Surficial Geologic Interpretation and Sidescan Sonar Imagery of the Sea Floor in West-Central Long Island Sound

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OFR 2005-1018 Home


Geologic Setting

Bathymetry Discussion

Data Acquisition and Processing

Sidescan Imagery

GIS Data Catalog


Figures List






Clam trawler BIVALVE at sea in Long Island Sound.

Clam trawler BIVALVE at sea in Long Island Sound.

The Long Island Sound estuary, which is 182 km long and up to 32 km wide, is located south of Connecticut and north of Long Island, New York. It is bordered to the east by Block Island Sound and to the west by the East River. The sea floor covered by NOAA survey H11044 lies to the south of the Milford and Woodmont quadrangles of the Western Highlands in Connecticut (Flint, 1965; Flint, 1968). The bedrock beneath the study area is presumably similar to that lying to the north in the Milford quadrangle, which consists of Early and Middle Paleozoic schists, metavolcanic and metasedimentary rocks (Flint, 1968). Within parts of the study area, these rocks are also presumably overlain by a seaward extension of the Hartford Basin, which contains red arkosic sandstones and intrusive igneous rocks of Triassic and Jurassic age (Flint, 1965; Flint, 1968; Rodgers, 1985), and erosional remnants of coastal plain strata of Cretaceous age (Lewis and Needell 1987; Needell and others, 1987).

Map showing location of NOAA survey H11044

Map showing location of NOAA survey H11044.

Onshore, the bedrock is overlain by glacial drift of late-Wisconsinan age (Flint, 1965; Flint, 1968). By 19 ka, the Wisconsinan glacier had formed the Ronkonkoma terminal moraine and was about to begin its northerly retreat from the Harbor Hill-Roanoke Point-Charlestown moraine (Lewis and DiGiacomo-Cohen, 2000). Further ice sheet retreat allowed melt water to dam north of the Harbor Hill-Roanoke Point-Charlestown moraine, forming glacial Lake Connecticut. Varved lacustrine sediments were deposited throughout much of LIS, and deltas started to form along the northern coast once the glacier receded from the lake. At this time, erosion of the spillway located at the eastern end of the Sound had started and lake levels began dropping until the lake was completely drained by 15.5 ka (Lewis and DiGiacomo-Cohen, 2000). The glaciolacustrine deposits of glacial Lake Connecticut and the underlying glacial drift are truncated by a regional unconformity that is a composite product of the subaerial exposure, which occurred after the lake drained, and the subsequent marine transgression (Lewis and Stone, 1991). Marine deposits, found in quiet-water areas throughout the LIS basin, overlie the unconformity and earlier deposits, and record deposition during the postglacial Holocene eustatic sea level rise.

Our interpretations of H11044 expand on two regional, but less detailed, studies conducted by Poppe and others (2000a) and Knebel and Poppe (2000). Poppe and others, (2000a) mapped the surficial sediments throughout LIS using bottom samples, photography and sidescan-sonar imagery. Adding bathymetric, geologic, and bottom current data, Knebel and Poppe (2000) mapped four categories of sedimentary environments: erosion or nondeposition, coarse-grained bedload transport, sediment sorting and reworking, and fine-grained deposition. We also draw upon the work of Signell and others (2000), who made model simulations of tide-, density-, wave-, and wind-driven bottom currents in LIS and related them to the Sound’s sedimentary environments.

Modern bathymetry within the survey H11044 area is influenced by the geological framework and modern current regime. Most of the study area sea floor slopes gently to the southeast or south; however, more complex bathymetry consisting of shoals, lesser bathymetric highs, and troughs are present in the western, southern, and northeastern parts of the study area. Water depths in the study area range from 3 m near the Connecticut coastline to over 50 m in troughs.

Lobster boat in Long Island Sound.

Lobster boat in Long Island Sound.

Two shoal complexes, Stratford Shoal Middle Ground and a less-pronounced unnamed shoal south of Lordship, Connecticut, make up the west-central and northwestern parts of the study area, respectively. Stratford Shoal rises over 20 m from the surrounding sea floor. The shoal off Lordship and northern parts of Stratford Shoal are composed of glaciolacustrine deltaic sediments, while southern parts of Stratford Shoal are probably composed of coastal-plain deposits (Lewis and DiGiacomo-Cohen, 2000). The shoals are separated by an east-west trending axial trough, which is the deepest feature in the study area, with water depths reaching 52 m. The northeastern corner of the mosaic contains a northwest-southeast trending elongate bathymetric high, which is about 2-6 m in height, 1.5 km wide, and extends for a length of 11 km within the study area. This bathymetric ridge is composed of ice-proximal glaciolacustrine fan deposits (Lewis and Stone, 1991). The southeastern corner of the study area contains two isolated bathymetric highs, probably cored by coastal plain deposits (Lewis and DiGiacomo-Cohen, 2000), rising about 5 m above the surrounding sea floor. An east-west trending trough reaching depths of 45 m cuts between the two highs. The south-central section of the study area contains a small, elongate basin with an east-west trend and a slightly steeper gradient.