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USGS Open-File Report 2005-1162, Sidescan-Sonar Imagery and Surficial Geologic Interpretation of the Sea Floor off Bridgeport, Connecticut

Geologic Setting

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Figure 1. Map showing location of the Bridgeport, Connecticut study area (red polygon).
Figure 1. Map showing location of the Bridgeport, Connecticut, study area. Click on figure for larger image.

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 New York metropolitan area. 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 NOAA survey H11045 covers about 190.3 km² off the west-central Connecticut shoreline, imaging an area seaward of the Bridgeport quadrangle (fig. 1) (Crowley, 1968). Schists, gneisses and phyllites of the Connecticut River valley synclinorium (Middle to Early Paleozoic Iapetos terrane) constitute the majority of outcrops along the north shore of LIS (Lewis and DiGiacomo-Cohen, 2000). The Bridgeport quadrangle is underlain by rocks of Paleozoic to Middle Mesozoic in age, present in the volcanic belt of the Western Highlands Province (Crowley, 1968). The schists, metavolcanics and metasedimentary rock making up the bedrock presumably extend offshore and are partially overlain by rocks of the Hartford Basin (Flint, 1968). The Orange Formation is present northeast of Bridgeport Harbor and includes the Oronoque schist; gray to silver, medium to fine-grained schist and the Wepawaug Schist; medium to dark gray, medium to fine grained schist or phyllite (Rodgers, 1985). The Bridgeport syncline is west of Bridgeport Harbor and may extend beneath the northwest corner of NOAA survey H11045; east of the Bridgeport Quandrangle, the majority of the bedrock underlying New Haven is the reddish poorly sorted New Haven Arkose (Rodgers, 1985). Presumably, these or similar formations extend offshore beneath the study area. Depth and thickness of these units varies greatly beneath the study area.

The bedrock across much of southern 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. However, no deposits associated with end moraines have been reported in this study area. 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) (fig. 1).

Deltaic sediments and proximal and distal lacustrine fan facies deposited in glacial Lake Connecticut overlie both bedrock and glacial drift within the study area (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 (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 land west and east of Bridgeport Harbor has been artificially emplaced. East of the harbor, salt marsh deposits overlying fines are present in a protected inlet with low wave energy. Sand and gravel characterize the beach deposits, west of Point No Point. The coastal surficial geology of the Bridgeport quadrangle is characterized by post glacial deposits composed of stacked coarser deposits overlying finer deposits. The sand varies in thickness; the fines are thinly bedded. This sequence represents distal glacioldeltaic deposits overlying glacial lake bottom sediment (Stone and others, 1992).

Tidal and wind-driven currents have extensively reworked both the glacial and postglacial deposits south of Bridgeport 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).

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