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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

Interpretation

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Interpretation

The multibeam bathymetric data from survey H11255 (fig. 4) show distinctive patterns in the morphology of the sea floor that can be correlated with the effects of modern oceanographic and sedimentary processes and with anthropogenic activities. These correlations have been used to construct the interpretive map presented in figure 8.

Sedimentary Furrows

The sea floor within the large elongate depression near the southwestern corner of the survey area is characterized by numerous linear depressions interpreted to be sedimentary furrows (fig. 8 and fig. 9). The sedimentary furrows are erosional bedforms, and appear as thin, straight to very slightly sinuous pairs of dark and light lines that trend east-northeast in the hill-shaded bathymetry. This trend is oblique to that of the large depression, but parallel to that of the semi-enclosed depressions described earlier.

The furrows near the axis of the large elongate depression are closely spaced and average over 1.5 km in length, but range to at least 2.3 km in length. Although sedimentary furrows of greater length may be present in this part of the Sound, many extend out of the mapped area and could not be measured. Most of the furrows are shallow, broadly V-shaped, and symmetrical in cross section (fig. 10). Average width of furrows within the axis of the depression is slightly less than 10 m; average furrow relief is slightly less than 0.5 m. However, the largest furrows are about 15 m wide and exceed 0.7 m deep. No evidence of control by subsurface structure was evident in the high-resolution seismic-reflection profiles (fig. 5, fig. 6, and fig. 7). The furrows on the sides of the elongate depression are much less well developed. They have a patchy irregularly spaced distribution and are much shorter, averaging about 0.4 km long. As the furrows become smaller, they become shallower, grade into the surrounding sea floor, and can no longer be resolved.

Most of the sedimentary furrows appear to gradually taper out at both ends, but the ends of some furrows show a "tuning fork" joining pattern. Although the junctions open more commonly toward the east (fig. 9), many also open toward the west. This orientation of the "tuning fork" joining patterns suggests a net westward sediment transport (Dyer, 1970; Flood, 1983). However, because adjacent furrow junctions do commonly 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. None of the furrows within the elongate depression are associated with any other identifiable bottom feature.

Bottom video from north-central Long Island Sound (Poppe and others, 2001; Poppe and others, 2002b; Poppe and others, 2004) reveals that the sedimentary furrows there are rounded linear depressions with gently sloping walls similar to the Type-2 troughs of Flood (1983). A current-swept appearance characterizes the bottom within the furrows. Scour around coarser grains, sediment accumulations in the current shadows of obstacles, saltating nutclam (Nucula spp.) shells (fig. 11), and downstream deflections in the orientation of attached megafauna (hydrozoans and anemones) were observed in the bottom video. Also commonly recorded in the video were the resuspension of sediment and nutclam shells 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. Their shells are tiny (about 6-mm long), thin walled, light (each valve weighs less than 0.05 g), and once resuspended are apparently of a hydraulic equivalence that enables transportation by weak bottom currents. Faint, low (<2 cm high) longitudinal ripples, which trend east-northeast, are common in the north-central Sound, but, along with nutclam shells, the ripples appear to be concentrated within sedimentary furrows. 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 north-central Sound characterized by sedimentary furrows (Poppe and others, 2002b; Poppe and others, 2004).

Mapping of surficial sediment distribution has shown that the deeper waters (>20 m) of the east-central Sound are characterized by fine-grained, cohesive sediments (Poppe and others, 2000a). A comparison of these sediment data with the reported distribution of sedimentary furrows (Poppe and others, 2002b) shows that the furrows are limited to areas where clayey silt is the dominant sediment type (fig. 12). A similar comparison of the reported distribution of sedimentary furrows with mapped sedimentary environments (Knebel and Poppe, 2000) shows that these features are limited to areas of the sea floor characterized by long-term deposition (fig. 13 and fig. 14). Furrows are absent in areas characterized by even slightly coarser sediments (e.g. silty sand or sand-silt-clay) or in areas characterized by sedimentary environments with slightly greater energy levels (e.g. sorting or reworking; fig. 15).

Although the deeper (>20 m) waters of east-central Long Island Sound are long-term depositional areas characterized by muddy sediments and relatively weak bottom currents (Signell and others, 2000; Knebel and Poppe, 2000; Poppe and others, 2000a), our data and the data of earlier mapping studies (Poppe and others, 2001; Poppe and others, 2002b; Poppe and others, 2004) reveal the localized presence of sedimentary furrows and longitudinal ripples. These features indicate sediment erosion or transport, and they typically form in 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 in the furrows reported from the north-central Sound suggest that the processes that created the furrows are slow or only intermittently active. We believe that the furrows in the southeastern Sound are similar and are not relict. Relict features would have been either obliterated by bioturbation or by postglacial marine sedimentation (Lewis and DiGiacomo-Cohen, 2000).

Previous work in east-central Long Island Sound (Signell and others, 2000) indicates that (1) resuspension is the major mechanism of bottom sediment transport; (2) the principal factor controlling resuspension is the speed of the tidal currents, episodically enhanced (over a period of several days) by wind and wave-driven currents; and (3) once suspended, sediment does not entirely settle out between tidal cycles. Furthermore, bottom-video data suggest that benthic biological activity, rather than currents, is responsible for most of the sediment resuspension (Poppe and others, 2002b).

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 are able to transport sediment. Studies by Flood (1981, 1983) 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 H11255 survey area include nutclam shells (fig. 11).

We do not completely understand the processes that produce or maintain the sedimentary furrows in southeastern Long Island Sound. However, given the geometry of the basin and conditions in the H11255 survey area, we can offer the following mechanism, adapted from Flood (1983). Initially, 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 (fig. 16; Hollister and others, 1974). 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.

Based on where sedimentary furrows have been observed and on the constraints imposed by sediment grain size and the need for directionally stable currents, we have interpreted the distribution of these bedforms in the east-central Sound (fig. 17). The eastward extent of the furrow distributions is limited by coarser sediments and commensurately higher energy sedimentary environments (Poppe and others, 2000a; Knebel and Poppe, 2000). Westward extent in the north-central Sound is limited by an elongate bathymetric high interpreted to be a glaciodeltaic deposit (Lewis and DiGiacomo-Cohen, 2000; McMullen and others, 2005). In the south-central Sound, a constricted area in the axial depression between Stratford Shoal Middle Ground and Long Island results in coarser sediments and higher energy sedimentary environments limiting the western extent of the furrows observed in the present survey area. Furthermore, the absence of these features within NOAA survey H11044 (McMullen and others, 2005) and the presence of the Stratford Shoal Middle Ground bathymetric high, which would disrupt the bi-directional tidal flow, suggests that the northern and southern areas where furrows have been reported do not connect in the central part of this basin.

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 and turbulent flow patterns that are conducive to the development of erosional furrows. 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.

Anthropogenic Artifacts

Several anthropogenic artifacts are discernable in the bathymetry. These artifacts include a shipwreck, numerous anchor scars, and a pipeline or cable (fig. 8). A previously uncharted wreck found in the north-central part of the survey area (near 41°5.39'N and 72°42.65'W) exceeds 20 m in length, and lies in about 27 m of water. The presence and asymmetry of a faint scour mark around the wreck suggests that bottom currents in this area are at least occasionally strong enough to transport bottom sediments and that net transport is toward the west.

Scars caused by anchors dragging across the sea floor are represented by narrow curvilinear depressions that range up to 0.8 km in length (fig. 8). Sediment piled at the end of a scar indicate where the anchor finally caught hold in the bottom. The scars, which are concentrated in the southeastern part of the survey, probably owe their occurrence within this area to ships seeking shelter in the lee of Roanoke Point during storms, to the proximity of an offshore petroleum terminal located about 3.4 km east of Roanoke Point (Poppe and others, 1999a), and because this area is shallower than the rest of the study area.

A narrow (about 10 m wide), shallow (<1 m) depression extends from the western edge of the survey area to its northern edge, trending about 70°E (fig. 8). We interpret this feature to be the surface expression of a buried pipeline or cable. The feature becomes noticeably less pronounced about the last 1 km before it exits the survey area along its northern edge, suggesting that currents there are stronger and that sediment is being transported into the depression.

Click on figures for larger images.

Figure 4. Digital terrain model (DTM).
Figure 4. Digital terrain model (DTM) from survey H11255.


Figure 5. Subbottom profile from line 17.
Figure 5. Subbottom profile from line 17.


Figure 6. Subbottom profile from line 15 in southeastern Long Island Sound.
Figure 6. Subbottom profile from line 15.


Figure 7. 11 Subbottom profile from line 11.
Figure 7. 11 Subbottom profile from line 11.


Figure 8. Interpretation of the bathymetry from NOAA survey H11255.
Figure 8. Interpretation of the bathymetry from NOAA survey H11255.


Figure 9. Detailed view of sedimentary furrows from survey H11255
Figure 9. Detailed view of sedimentary furrows from survey H11255


Figure 10. Perspective view of an idealized sedimentary furrow.
Figure 10. Perspective view of an idealized sedimentary furrow.


Figure 11.Image of Nucula spp.
Figure 11. Image of Nucula spp.


 Map showing the distribution of surficial sediments in east-central Long Island Sound.
Figure 12. Map showing the distribution of surficial sediments in east-central Long Island Sound.


Figure 13. Map from showing the distribution of sedimentary environments in east-central Long Island Sound.
Figure 13. Map showing the distribution of sedimentary environments in east-central Long Island Sound.


Example of a sedimentary environment characterized by deposition.
Figure 14. Example of a sedimentary environment characterized by deposition.


Example of a sedimentary environment characterized by sorting or reworking.
Figure 15. Example of a sedimentary environment characterized by sorting or reworking.


Schematic of a possible mechanism  for the formation of sedimentary furrows.
Figure 16. Schematic of a possible mechanism for the formation of sedimentary furrows.


Map showing the distribution of sedimentary furrows in east-central Long Island Sound.
Figure 17. Map showing the distribution of sedimentary furrows in east-central Long Island Sound.


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