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U.S. Geological Survey Open-File Report 2013–1060

Sea-Floor Geology and Topography Offshore in Northeastern Long Island Sound


Bathymetry

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Click on figures for larger images
Thumbnail image of figure 1 and link to larger figure. A map of the location of bathymetric surveys completed in Long Island Sound.
Figure 1. Index map showing the locations of the National Oceanic and Atmospheric Administration H12012 study area and the other bathymetric and backscatter surveys completed in Long Island Sound by NOAA and by the U.S. Geological Survey and the Connecticut Department of Energy and Environmental Protection.
Thumbnail image of figure 11 and link to larger figure. An image of bathymetry in the study area.
Figure 11. Digital terrain model of the sea floor produced from the multibeam bathymetry collected during National Oceanic and Atmospheric Administration survey H12012, gridded to 2 m.
Thumbnail image of figure 12 and link to larger figure. An image of the interpretations of the sea floor features in the study area.
Figure 12. Interpretation of the digital terrain model from National Oceanic and Atmospheric Administration survey H12012.
Thumbnail image of figure 13 and link to larger figure. A map of figure and profile locations in the study area.
Figure 13. Locations of detailed planar views of the digital terrain model and profiles of bedform symmetry.
Thumbnail image of figure 14 and link to larger figure. Detailed bathymetry image of bedrock in the study area.
Figure 14. Detailed planar view from the eastern part of the digital terrain model produced from bathymetric data collected during National Oceanic and Atmospheric Administration survey H12012 showing extensively exposed bedrock and location of station 012-25.
Thumbnail image of figure 15 and link to larger figure. Detailed bathymetric image of bedrock outcrops.
Figure 15. Detailed planar view of the multibeam bathymetric data from National Oceanic and Atmospheric Administration survey H12012 showing relatively small bedrock outcrops on the basin floor south of Hatchett Point.
Thumbnail image of figure 16 and link to larger figure. Detailed bathymetric image of scoured areas in the study area.
Figure 16. Detailed planar view of the multibeam bathymetric data from National Oceanic and Atmospheric Administration survey H12012 showing broad scoured areas on the basin floor west of the extensive bedrock outcrops shown in figure 14 and the location of station 012-24.
Thumbnail image of figure 17 and link to larger figure. Detailed bathymetric image of sand waves in the study area.
Figure 17. Detailed planar view from the digital terrain model produced from bathymetric data collected during National Oceanic and Atmospheric Administration survey H12012 showing transverse sand waves on top of a shallow shelf platform and locations of stations 012-6 and 012-8.
Thumbnail image of figure 18 and link to larger figure. Sand wave profiles in the study area.
Figure 18. Cross-sectional views of transverse and barchanoid sand waves from the digital terrain model produced from bathymetric data collected during National Oceanic and Atmospheric Administration survey H12012.
Thumbnail image of figure 19 and link to larger figure. Detailed bathymetric image of sand waves in the study raea.
Figure 19. Detailed planar view of relatively straight-crested, non-bifurcating transverse sand waves from the digital terrain model produced from bathymetric data collected during National Oceanic and Atmospheric Administration survey H12012.
Thumbnail image of figure 20 and link to larger figure. A detailed bathymetric image of barchanoid sand waves in the study area.
Figure 20. Detailed planar view of barchanoid sand waves from the southwestern part of the digital terrain model produced from bathymetric data collected during National Oceanic and Atmospheric Administration survey H12012.
Thumbnail image of figure 21 and link to larger figure. A map showing how sand waves in the study area have shifted over time.
Figure 21. Barchanoid sand waves originally surveyed in September–October 2008 during National Oceanic and Atmospheric Administration survey H11997 and resurveyed in April–May 2009 during NOAA survey H12012.
Thumbnail image of figure 22 and link to larger figure. Detailed bathymetric image of dredge spoils in the study area.
Figure 22. Detailed planar view of dredge spoils of the Cornfield Shoals Disposal Site in the southwestern part of the digital terrain model produced from bathymetric data collected during National Oceanic and Atmospheric Administration survey H12012.

The study area covers a narrow (varying from 1.1 to 2.8 km wide), elongate (about 22.4 km long), northeasterly trending area of the sea floor in eastern Long Island Sound (figs. 1 and 11). In general, depths were shallower along its northern edge where the study area extends over thicker marine deltaic deposits (Horne and Patton, 1991; Lewis and DiGiacomo-Cohen, 2000). The sound narrows toward the eastern part of the study area, constricting the tidal flow and causing stronger bottom currents (Signell and others, 2000; White and White, 2012). These currents have scoured the sea floor and increased the depth through erosion. The greatest water depths surveyed within the study area exceed 74 m and occur at its eastern end near the entrance to The Race, the passage between Orient Point and Fishers Island. The shallowest areas, which are less than 15 m deep, occur along the northern edge of the study area on top of the marine delta. The high resolution of the digital terrain model (DTM) allows us to identify specific morphological features that indicate distinct sedimentary processes (fig. 12). Features identified in this dataset are reviewed in the following sub-sections.

Bedrock Outcrops and Boulder Lag Deposits

Paleozoic bedrock is exposed both as a fairly large area and in small isolated outcrops across the eastern part of the study area that together cover about 4 percent of the sea floor (figs. 13, 14, and 15). Features identified on the bedrock surface can be correlated with those of onshore formations. The northwest-southeast bedrock trends conspicuously visible in the multibeam data (fig. 14) are similar to glacially smoothed bedrock ridges on land that trend between N 15° and N 20° W immediately west of the Thames River. These elongate ridges, which are similar in size, orientation, and morphology to their inferred offshore counterparts, indicate the direction of ice movement interpreted from grooves and striations (Goldsmith, 1962). The second set of ridges that trend northeast-southwest (N 65° E and N 80° E) are similar to features on the east side of the Thames River, which parallel the strike of foliation and the orientation of some jointing of the Avalonian anticlinorium (Poppe and others, 2006c). These ridges and their offshore counterparts were probably formed by glacial plucking, and they resemble strike ridges formed in dipping layered rock. These lineations show that glacial erosion of the bedrock continue offshore under Long Island Sound.

Small (2 to 10 m across), individual, rounded bathymetric highs that give the sea floor a rough appearance are interpreted to be boulders. Boulders occur on and around the bedrock outcrops and at scattered locations along the northeastern and southern edges of the study area (fig. 15). They occur individually, protruding through the surrounding surface sediments and as more spatially extensive accumulations. The boulders and accumulations of boulders are considered lag deposits that remain from winnowed Pleistocene glacial drift.

Scour Features

Scour provides evidence of ongoing erosion by strong, oscillating tidal currents. For example, swirling currents develop around bedrock outcrops and boulders that protrude through surficial sediments. Turbulence causes increased flow velocity sufficient to scour the sea floor around and downstream of obstructions and produces sharp-pointed depressions with coarse-sediment floors called obstacle marks, also known as comet marks (Werner and Newton, 1975). Good examples of scour features are present on the western side of the bedrock outcrops south of Hatchett Point (fig. 15) and around boulders south-southwest of Black Point. Asymmetry of the scour around the obstacle marks reflects the stronger flood tide (White and White, 2012) and indicates that net transport is predominantly toward the west and farther into the Long Island Sound estuary (Werner and Newton, 1975; Reineck and Singh, 1980).

In addition to the large scour depression present in the easternmost part of the study area, strong tidal currents have also scoured a broad area of the sea floor south-southeast of Black Point, producing a depression that in places exceeds 10 m below the surrounding sea floor (figs. 11 and 16). Sediments excavated from this depression have been mounded along its western edge, forming a long narrow ridge similar to a levee.

Gravelly Pavements

Almost 89 percent of the study area appears to be relatively flat and featureless in the DTM. These flat areas, which are shown in dark yellow in the interpretive figure (fig. 12), have strong tidal currents that prevent the deposition of modern marine sediments and erode the finer fraction from the older sediments, leaving exposed lag deposits of pea- to pebble-sized gravel and gravelly sediment to armor the sea floor. Although scattered small encrusting faunal forms are present on the rock surfaces of gravel pavements, larger sessile forms are absent, suggesting that the gravel is periodically mobilized.

Sand Waves and Megaripples

Transverse sand waves occur in the northwestern part of the study area on top of deposits associated with the marine delta (fig. 17). Crestlines of these bedforms are variably straight to sinuous, trend north-northwest, and, in places, bifurcate. Average crest to trough relief is 2 to 2.5 m, but the largest transverse sand waves approach 4 m of relief. These bedforms are asymmetrical with stoss slopes invariably oriented toward the east-northeast, indicating transport toward the west-southwest (fig. 18A; Allen, 1968, Ludwick, 1972; Belderson and others, 1982). Transverse megaripples, with orientations and symmetries similar to those of the transverse sand waves, occur on the stoss slopes of the sand waves, border the transverse sand-wave fields, and occur in small isolated patches across the western part of the study area (figs. 12 and 17).

Transverse sand waves that are relatively straight crested, display no bifurcation, and have ends marked by distinct scour zones or moats occur within a small field along the northern edge of the study area (fig. 19). Although these sand waves have moats similar to bedforms described in Great South Channel west of Georges Bank (Valentine and others, 2002) and in Long Island Sound south of the present study area (Poppe and others, 2011a), they are asymmetrical in profile (fig. 18B) and occasionally have megaripples on their eastward-facing stoss slopes.

Although less common than transverse sand waves, barchanoid morphologies occur along the base of the eroded delta (fig. 17), along the southern edge of the transverse sand-wave fields, and in isolated fields on the gravelly pavement in the southwestern part of the study area (fig. 20). Individual barchanoid sand waves average less than 100 m wide, but the largest exceeds 195 m. These waves commonly align into elongate fields having smaller isolated waves concentrated in the eastern ends. Westward within barchanoid wave fields, wavelengths increase, the fields widen, and the barchanoid waves become larger and coalesce into complex forms and highly curved ridges (fig. 20). The observed widening of the barchanoid sand-wave fields is probably caused by the propagation of younger waves off the horns of pre-existent waves situated up-drift in the transport regime. Relief of the barchanoid waves is typically 1 to 2 m. However, scour depressions, probably formed by roll vortexes, commonly occur in the troughs within the convex areas of the larger waves (fig. 18C). If trough-to-crest amplitudes are measured from these depressions the waves appear larger, and the largest barchanoid wave exceeds 13 m. The presence of depressions is evidence for intense scour that occurs on the down-current sides of these sand waves. Because of this scour, the slip faces of both small individual and large complex barchanoid sand waves are commonly at or near the angle of repose. In places, transverse waves grade laterally into barchanoid morphologies. Regardless of size, location, and complexity, all of the barchanoid sand waves are concave westward, indicating net westward sediment transport because the horns migrate more rapidly than the wave's main body (McKee, 1966).

The distribution of sand-wave morphologies within the study area is controlled at least to some extent by the availability of sand. Transverse morphologies dominate on top of the marine delta where the sediment supply is most abundant. Seismic-reflection profiles confirm that the eroding marine deltaic deposit is the probable source of the sediment necessary for these bedforms (Lewis and Needell, 1987; Horne and Patton, 1991; Lewis and DiGiacomo-Cohen, 2000; Poppe and others, 2002; Poppe and others, 2011a). Conversely, barchanoid wave morphologies typically form in environments where the substrate is firm and the sediment supply is limited (Reineck and Singh, 1980), such as those in isolated fields on the gravelly pavement. Seismic profiles confirm that barchanoid sand-wave fields in the study area coincide with areas where the Holocene section is thin.

The presence of megaripples on the sand waves (fig. 18A) and the steepness of the slip faces suggest that sediment transport is active and that the sand waves are propagating under the present hydraulic regime (Dalrymple and others, 1978; Reineck and Singh, 1980; Allen, 1982). Also, where sand waves on the present DTM overlap those from a previous survey (Poppe and others, 2011a), the rate of transport can be measured (fig. 21). Comparisons between datasets show that individual bedforms change shape and size, but that almost all migrate west-southwestward. Average crest-line shift of barchanoid waves in the southwestern part of the study area exceeds 14 m per year.

Dredge Spoils

The Cornfield Shoals Disposal Site (Wiley, 1996) in the northwestern part of the study area is about 6.1 km south of Cornfield Point, Conn. (fig. 22). Although this is a dispersive disposal site (that is, material dumped here is expected to be transported out of the area by bottom currents), diffuse spoil mounds and circular impact structures can still be identified in the DTM. Vertical relief of the largest spoil mound exceeds 1.1 m above the surrounding sea floor.

Multibeam Artifacts

Due to a faulty multibeam receiver, a systematic artifact appears throughout the DTM as dual along-track striping (fig. 22; National Oceanic and Atmospheric Administration, 2009a). Although less than 20 cm high, these acquisition-related artifacts are made more conspicuous by the sun-illumination and fivefold vertical exaggeration of the imagery.

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