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

OFR 2004-1003 Home 

Introduction

Geologic Setting

Data Acquisition and Processing

Sidescan Imagery

Rocky and Gravelly Areas
Light and Dark Area
Sedimentary Furrows
Arcuate Bands of Low Backscatter
Trawl Marks
Surficial Sediments
Sedimentary Environments

GIS Data Catalog

References

Figures List

Acknowledgments

Contacts

Disclaimer

README

 SIDESCAN SONAR IMAGERY

 The sidescan sonar mosaic of the sea floor off Branford, Connecticut NOAA survey H11043, portrays an acoustic image of the sea floor which, when combined with subbottom and bathymetric data, can be used to interpret the surficial geology. Distinctive acoustic patterns revealed on the mosaic include: (1) complex patches of high and low backscatter with individual high-backscatter targets (objects), (2) areas of relatively high backscatter (light tones), (3) areas of relatively low backscatter (dark tones), (4) alternating lines of high and low backscatter (sedimentary furrows), and (5) arcuate bands of low backscatter. Boundaries between patterns are commonly gradational; backscatter is not uniform throughout these areas. Water column phenomena such as boat wakes and turbulence around obstructions on the bottom, and manmade features such trawl marks and dredge-spoil disposal mounds are also present on the image.

 Rocky and Gravelly Areas

The complex patches of high and low backscatter with individual high-backscatter targets represent rocky, bouldery areas. Rocky patches, which correspond to Townshend Ledge, Branford Reef and several smaller unnamed features, occur in the northern part of the image, and contain boulders that exceed 3 m in diameter. Aprons of relatively high backscatter, representing sandy and gravelly sediment, surround the rocky areas. Together the areas trend to the northeast toward another rocky patch, Crooked S Shoal, located off the image. Although underlying bedrock highs may control the location of these rocky areas, subbottom profiles (Line 20, Line 22) show that instead they are cored by glacial till (Needell and others, 1987; Poppe and others, 2002a) and suggest that surficial processes here winnowed away the finer fractions of the glacial drift, leaving boulders to form the lag deposits now capping Townshend Ledge, Branford Reef, and the smaller unnamed features. The dimensions, composition, position, and east-northeast trend of these features favor the interpretation by Flint and Gebert (1976) that they are seaward extensions of the Madison moraine.

Isolated spots of high backscatter, not associated with water-column disturbances, are scattered along the northern part of the study area. These spots represent individual boulders, probably glacial erratics, and small patches of bouldery bottom. Depressions adjacent to the boulders caused by scour and linear accumulations of sediment in the current shadows of the boulders attest to the strong oscillatory nature of the tidal currents.

 Light and Dark Areas

Areas characterized by relatively high backscatter, which occur adjacent to deposits of boulders and gravel on Townshend Ledge and Branford Reef, primarily coincide with exposures of glaciodeltaic deposits (Stone and others, 1998) and with sediments that have been winnowed from the crests of the bathymetric highs. The high backscatter tends to be produced by coarser-grained sediments, typically gravelly sand and sand.

Patches and spots of high backscatter in the southwestern part of the image are related to the New Haven Dumping Ground. The high backscatter results from a combination of relatively coarse-grained sediments in dredge spoils or in materials used to cap the spoils, and the angle of incidence of the sonar against the sides of disposal mounds (Morris and others, 1996). Halos of different tones surround some of the disposal piles and may represent the effect of density surges caused by the impact of dredged materials on the bottom or they may represent the subsequent slower sedimentation of the diffuse plume of residual finer-grained spoil material (Schubel and others, 1979).

The central and southern parts of the study area are characterized by relatively low backscatter. These areas coincide with accumulations of fine-grained Holocene marine sediment. Subbottom profiles (Needell and others, 1987; Poppe and others, 2002a) show that the deposits of fine-grained sediment thicken southward away from the surface expression of the Madison moraine and overlie high-backscatter glaciodeltaic sediments. Maximum thickness of the Holocene marine sediment occurs along the east-central edge of the study area and is about 16 m; thickness of this unit along the southern edge of the study area is about 10 m. Some of the small isolated spots and patches of higher backscatter that occur within the low-backscatter areas are artifacts of small-scale bathymetric changes that affect the angle of incidence of the sidescan sonar or the result of water column turbulence.

 Sedimentary Furrows

The low backscatter area in the southern part of the Branford survey is also characterized by numerous linear depressions interpreted to be sedimentary furrows. The furrows, which trend east-northeast and are concentrated in water depths greater than 18 m, appear as thin slightly sinuous paired lines of high and low backscatter in the sidescan sonar mosaic. The furrows have a patchy irregularly spaced distribution; mean distances between them are about 20 m. The furrows average over 200 m long, but range from about 30 m to over 1.3 km in length. Much of the pronounced "wavy" appearance of the furrows in the southernmost part of the Branford survey is an artifact of the irregular ship's tracks.

Most of the sedimentary furrows appear to gradually taper out at both ends, although the ends of some furrows show a "tuning fork" joining pattern. The junctions open predominantly toward the east, but some also open toward the west. A few of the sedimentary furrows cross, start at, or end at dredge-spoil mounds, but most are not associated with the mounds or with any other identifiable bottom features.

Bottom video and high-resolution seismic-reflection profiles collected as part of an earlier study (Poppe and others, 2001) reveal that the sedimentary furrows in north-central Long Island Sound are shallow (about 0.4 m deep), relatively narrow (about 9 m wide), broadly V-shaped, symmetrical, rounded linear depressions with gently sloping walls, similar to the Type-2 troughs of Flood (1983). A current-swept appearance characterizes the bottom video collected within the furrows. In this regard, scour around coarser grains, sediment accumulations in the current shadows of obstacles, longitudinal ripples, saltating nutclam (Nucula spp.) shells, and downstream deflections in the orientation of attached megafauna (hydrozoans and anemones) are common. Also commonly recorded in the video were the resuspension of sediment and nutclam shells (Weiss, 1995)by the burrowing and feeding activities of the benthic megafauna (e.g. crabs); and small clouds of sediment generated by the impacts of saltating nutclam shells. These shells are tiny (about 6-mm long), thin walled, light (each valve weighs less than 0.05 g), and once resuspended apparently have a hydraulic equivalence that allows them to be transported by the weak bottom currents. Burrows (constructed by shrimp, clams, mud crabs, and lobsters), animal tracks, burrowing anemones, worm tubes, hydrozoans, and amphipod communities are present in the heavily bioturbated bottom throughout the eastern province.

Although the deeper waters of north-central Long Island Sound are long-term depositional areas characterized by fine-grained, cohesive sediments (Poppe and others, 2000a) and relatively weak bottom currents (Signell and others, 2000; Knebel and Poppe, 2000), the presence of sedimentary furrows indicates localized sediment erosion or transport and environments that have recurring, directionally stable, and occasionally strong currents (Dyer, 1970; Lonsdale and others, 1973; Hollister and others, 1974; Flood, 1983). The lack of abrupt lithologic transitions, the faint appearance of the associated longitudinal ripples, and the abundance of tracks made by bottom-dwelling animals suggest that the processes that created these furrows are slow or only intermittently active. In any case, the furrows are not relict. Relict features would have been either obliterated by bioturbation or by the relatively high postglacial sedimentation rates within the study area (Lewis and DiGiacomo-Cohen, 2000).

Previous work near the New Haven Dumping Ground (Gordon and others, 1972; Signell and others, 2000; Poppe and others, 2001) indicates that (1) resuspension is the major mechanism of bottom sediment transport; (2) once suspended, sediment does not entirely settle out between tidal cycles, and (3) benthic biological activity, rather than currents, is responsible for most of the sediment resuspension. While it is true that the currents in the north-central Sound are usually not strong enough to initiate erosion, once sediments are resuspended by biological activity, the currents can transport them. The "tuning fork" joining patterns of the furrows, which usually open toward the east, indicate net westward sediment transport (Dyer, 1970; Flood, 1983). However, because adjacent furrow junctions do occasionally open in opposite directions, these joining patterns also suggest that the tidal regime is important to furrow formation and that the furrows can form when water flows in either direction. Studies by Flood (1981, 1983) also show that coarse-grained sediments are also important for the initiation and development of furrows in muddy sediments. Coarse sediments that are available within the study area include nutclam shells and sand associated with the dredge spoils.

Given the geometry of the basin and conditions in the Branford study area, we can offer two possible mechanisms that could produce and maintain the sedimentary furrows in north-central Long Island Sound. In the first mechanism, adapted from Flood (1983), benthic biologic activity resuspends nutclam shells. Then, secondary helical-flow patterns produced by the tidal currents, which develop just above the sea floor, align the nutclam shell debris mobilized by biological activity into convergent flow zones (Hollister and others, 1974; Mclean, 1981). Furrow development is initiated due to enhanced erosion within the elongate shell beds caused by abrasion (related primarily to sediment resuspension from the impacts of saltating shells) and from current scour around individual shells. The furrows lengthen as the concentrated shells move downstream in the bottom currents.

Alternately, in a mechanism adapted from McLean (1981), depressions form in the turbulent wakes of current flow around dredge-spoil disposal mounds. Easily transported non-cohesive grains (coarse silt and very fine sand) eroded from the disposal mounds subsequently are made available to abrade and lengthen such depressions into furrows in the muddy seabed and cuspate high-backscatter lobes do extend off both the eastern and western sides of disposal mounds associated with furrows. These lobes are evidence for the reworking of high-backscatter materials from the disposal mounds by near-bottom currents and for the transport of these materials into the adjacent furrows. However, it is important to note that most of the sedimentary furrows in the north-central Sound are not associated with disposal mounds or any other identifiable obstacles.

In summary, the elongate geometry and regional bathymetric contours of Long Island Sound combine to constrain the tidal and storm currents and cause dominantly east-west flow directions. These conditions, in turn, produce the helical flow patterns that are conducive to the development of erosional furrows (Poppe and others, 2002b). Through resuspension due to biological activity and the subsequent development of sedimentary furrows and longitudinal ripples, fine-grained cohesive sediment can be remobilized, and, at least episodically, be made available for transport farther westward into the estuary (Poppe and others, 2002b).

 Arcuate Bands of Low Backscatter

Arcuate bands of low backscatter occur in the fine-grained sediment of the southwestern part of the study area. These crescent-shaped features, which are of unknown origin, are concave westward and have long axes that are generally perpendicular to the regional bathymetry. The bands average 20-35 m in width at their widest points and commonly taper toward each end. The bands, which do not appear to be related to the sedimentary furrows or to the dredge spoils, have little or no relief and average 250-300 m in length.

If these features were primarily related to fine-grained down-slope transport pathways, then we would expect only their southern ends to be bent toward the west, reflecting the directional effects of net bottom-sediment transport. Inasmuch as both the northern and southern ends are deflected westward, however, these features may represent bands of fine-grained sediment moving westward in a manner similar to that of barchan dunes.  Current models for the southern part of the Branford study are do show net westward bottom transport (Signell and others, 2000).

 Trawl Marks

Shallow curvilinear depressions, interpreted to represent trawl marks associated with commercial fishing operations, occur throughout much of the study area, but are most conspicuous across the northern part. Whether this association is because trawl marks are more visible in the coarser sediments present there or whether it is because these sediments are a preferred biologic habitat is unknown.

Some of the trawl marks have a looping appearance. These trawl marks probably owe their extreme curvature to (1) the side-mounted deployment of trawl gear that requires the fishing vessel to turn toward that side to minimize abrasion against the hull, and (2) the practice of trying to stay near an area once a concentration of hard clams has been discovered. The trawl marks can be differentiated from the furrows in that they are much fainter, are usually curved, and show no preferential orientation. Some of the curvilinear depressions, especially the more conspicuous ones, may represent anchor scars.

 Distribution of Surficial Sediments

Gravel, including boulders, occurs on the crests of Townshend Ledge, Branford Reef, and on isolated bathymetric highs that trend toward the east-northeast across the northern part of the study area. These coarse deposits are associated with outcrops of glacial drift, and are probably lag deposits resulting from the reworking of these deposits by the tidal current and waves. Tonal changes on the sidescan sonar mosaic suggest that small-scattered patches of sand and silty sand locally occur among the boulders. Aprons of gravelly sand, which are commonly observed armoring finer-grained underlying sediments in the bottom samples and video, locally surround the gravel areas. Sand is the dominant sediment textural class in the glaciodeltaic deposits that surround and  lie north of Townshend Ledge and within small patches around Branford Reef. The sand, which fines with increasing distance from Townshend Ledge and Branford Reef, is typically poorly sorted, coarse-skewed, and extremely leptokurtic.

Aprons of sand silt clay and, around Townshend Ledge, silty sand separate the coarser grained sediments from the muds. These sandy and muddy sediments are very poorly sorted, nearly symmetrical, and platykurtic. Clayey silt, occasionally grading to silt, is the dominant sediment type in the study area, covering almost the entire north-central, central, and southern parts. These muddy cohesive sediments are typically poorly to very poorly sorted, coarse skewed to nearly symmetrical, and platykurtic to mesokurtic.

Although presented here in much greater detail, the sediment distribution in the study area is similar to this part of the regional distribution reported by Poppe and others (2000a).

 Distribution of Sedimentary Environments

Three sedimentary environments are mapped within the study area on the basis of samples, tonal changes in the sidescan sonar mosaic, and bathymetry. These include environments characterized by erosion or nondeposition, sorting or reworking, and deposition. These environments represent the dominant long-term conditions and may not always reflect small-scale or intermittent processes. For example, erosional bedforms such as longitudinal ripples and sedimentary furrows may locally occur in areas characterized by deposition. As with the textural distributions, the contacts between these environments are inferred because the transitions between the various environments are gradational, and lateral changes are seldom abrupt.

Environments characterized by erosion or nondeposition, which reflect high-energy conditions (Knebel and Poppe, 2000; Poppe and others, 2000c), prevail in the relatively shallow waters around Townshend Ledge, Branford Reef, and a few isolated bathymetric highs across the northern part of the study area. Strong tidal currents and wind-driven waves prevent the deposition of Holocene marine sediments and erode the finer fraction from the sea-floor sediments, leaving exposed lag deposits of boulders and gravel. Scour features, which are typically present around isolated boulders and cobbles, are most conspicuous northeast of Townshend Ledge.

As water depth increases, the strength of storm and tidal currents at the sea floor decreases. Conditions favoring erosion or nondeposition are replaced by sedimentary environments characterized by sediment sorting or reworking. Faint current ripples, observed on the surface of grab samples of sandy muds and muddy sands, and sediment resuspension, observed in bottom video, reflect this constant sorting by tidal and storm currents. Hermit crabs, starfish, and shell debris are locally common in bottom video.

The central and southern parts of the study area are characterized predominantly by long-term deposition. In these areas fine-grained sediments accumulate in the deeper, lower energy environments protected from strong tidal and storm conditions. Amphipod and polychaete tubes, shrimp burrows, and snail and rock crab tracks are locally common in these muddy sediments.

Although presented here in greater detail, the distribution of sedimentary environments in the study area is similar to this part of the regional distribution reported by Knebel and Poppe (2000).