Sidescan Sonar Imagery and Surficial Geologic Interpretation
of the Sea Floor off Branford, Connecticut

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Introduction

Geologic Setting

  Bathymetry

Data Acquisition and Processing

Sidescan Imagery

GIS Data Catalog

References

Figures List

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 GEOLOGIC SETTING

Long Island Sound is 182 km long and as much as 32 km wide. It is bordered on the north by the rocky shoreline of Connecticut, on the east by Block Island Sound, on the south by the eroding sandy bluffs of Long Island, and on the west by the East River. Long Island Sound not only contains large commercial port facilities, such as New Haven harbor, but also supports a large fishing industry. Finfish, lobster, and shellfish, primarily  farmed eastern oysters (Crassostrea virginica) and hard clams or quahogs (Mercenaria mercenaria), thrive under conditions that include a stable shallow-water habitat, appropriate nutrients, suitable salinities, and favorable tidal currents.

The study area, NOAA survey H11043, covers about 41.1 km² off the central Connecticut shoreline, imaging an area seaward of the Branford quadrangle (Flint, 1964). This quadrangle   is underlain by rocks of Proterozoic to Paleozoic age which presumably extend offshore and underlie the study area. The principal onshore equivalent of these rocks north of the western part of the study area is the Lighthouse Gneiss. This rock is a pink or gray-to-red, medium-grained, generally well-foliated granitic gneiss of Proterozoic age that underlies most of the onshore area west of Branford Harbor and is exposed on Kelsey Island and at Mansfield Point (Flint, 1964; Rodgers, 1985). North and east of Branford Harbor Proterozoic intrusive rocks of the Branford Gneiss and Stony Creek Granite Gneiss and stratified rocks of the Waterford Group crop out. The Branford Gneiss is gray to white and well foliated; the Stony Creek Granite Gneiss is red to pink and medium to very coarse grained; and the rocks of the Waterford Group are light to dark and medium grained (Rodgers, 1985).

The bedrock across much of south-central and southeastern Connecticut is unconformably overlain by two tills, one of pre-Wisconsinan age, the Lake Chamberlain till, and one of late Wisconsinan age, the Hamden till (Flint, 1965; Schafer and Hartshorn, 1965; Flint and Gebert, 1976; Stone and others, 1992). The younger till forms a thin (0-5 m), discontinuous mantle over nearly all of the higher land across the Branford Quadrangle. This till is exposed onshore around Mansfield Point and Indian Neck (Flint, 1964; Stone and others, 1992). Although both tills have been identified in the Mount Carmel quadrangle located to the northwest (Flint, 1962), the older till has not been found in the Branford quadrangle and, if present, probably lies buried in some of the deeper valleys (Flint, 1964). The northward retreat of the late Wisconsinan ice sheet is marked in southeastern Connecticut by a succession of minor recessional moraines (including the Madison, Hammonasset-Ledyard, Old Saybrook, and Mystic moraines. The Madison moraine, which trends east northeast, is exposed near Hogshead and Chipman Points in the adjoining Guilford quadrangle and presumably extends offshore across the northern part of the study area (Flint, 1971, Flint and Gebert, 1976; Stone and others, 1998).

Stratified drift, deposited in contact with the melting ice near the glacier margin, blankets the till and bedrock over parts of the Branford quadrangle (Flint, 1964). Stratified drift deposited by glacial streams as outwash, is present onshore as part of the Farm River valley train (Flint, 1964; Stone and others, 1992) and offshore as valley-fill deposits (Stone and others, 1998). Deltaic sediments and proximal and distal lacustrine fan facies deposited in glacial Lake Connecticut overlie both bedrock and glacial drift within the study area and extend about 9.5 km south of Mansfield Point (Lewis and Stone, 1991; Stone and others, 1992; Stone and others, 1998). This lake, which occupied most of the Long Island Sound basin, was formed when the ice front began to recede from the Harbor Hill-Roanoke Point-Charlestown moraine position and meltwater was impounded in the expanding, long, narrow basin between the moraine and the retreating ice to the north (Stone and Borns, 1986).

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, which occurred after glacial Lake Connecticut drained, and before the  marine transgression, which took place after 15 ka (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).

The Central Long Island Sound Disposal Site, which is partially located within the study area and about 10.4 km south of Mansfield Point, covers about 6.7 km² in north-central Long Island Sound. This area and three other open-water disposal sites within the Sound (Eatons Neck, Bridgeport, and New London) historically have received the vast majority of the spoils dredged from the borders of the Sound. For example, during 1954 and 1956 more than 4 million cubic meters were dumped at the New Haven site (Schubel and others, 1979). During the period October 1993 through January 1994 the U.S. Army Corps of Engineers dredged approximately 642,000 cubic meters of sediments from the navigational channel in New Haven Harbor, which were then disposed at the New Haven Dumping Ground (Bohlen and others, 1996). The New Haven Dumping Ground continues to be one of the most active disposal sites in New England because its proximity to major commercial and recreational port facilities minimizes disposal costs (Boyd and others, 1972) and because it is characterized by relatively weak bottom current is thought to be located in a depositional sedimentary environment (Morris and others, 1996).

Tidal and wind-driven currents have extensively reworked both the glacial and postglacial deposits south of Branford 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 locally exceed 20 cm/s at 1 to 2 m above the bottom across the study area (Gordon and others, 1972; Signell and others, 2000; Knebel and Poppe, 2000).