Click on figures for larger images

Figure 3. Segment of Uniboom seismic-reflection profile collected across a part of Block Island Sound.

Figure 17. Digital terrain model of the sea floor produced from the combined multibeam bathymetry collected during National Oceanic and Atmospheric Administration surveys H12009, H12010, H12011, H12015, H12033, H12137, and H12139 in Block Island Sound, offshore Rhode Island.

Figure 18. Locations of detailed planar views of the digital terrain model, Uniboom seismic-reflection profile shown in figure 3, bathymetric profiles of bedform morphology shown in figures 33 and 34, and directions of net sediment transport.

Figure 19. Detailed planar view of the combined National Oceanic and Atmospheric Administration surveys in Block Island Sound showing the bouldery sea floor off the eastern shore of Block Island and the location of station BIS83.

Figure 20. Detailed planar view of the combined National Oceanic and Atmospheric Administration surveys in Block Island Sound showing the bouldery sea floor off the western shore of Block Island and the location of station BIS41.

Figure 21. Detailed planar view of the combined National Oceanic and Atmospheric Administration surveys in Block Island Sound showing a submerged section of the Ronkonkoma-Nantucket end moraine, adjacent sand waves, and the locations of stations BIS34 and BIS35.

Figure 22. Detailed planar view of the combined National Oceanic and Atmospheric Administration surveys in Block Island Sound showing the bouldery sea floor southeast of Point Judith and the location of station BIS65.

Figure 23. Map showing the distribution of interpreted sea-floor features in Block Island Sound.

Figure 24. Detailed planar view of the combined National Oceanic and Atmospheric Administration surveys in Block Island Sound showing storm-wave induced scour on the flanks of the bathymetric high southeast of Point Judith and the locations of stations BIS62 and BIS63.

Figure 25. Detailed planar view of the combined National Oceanic and Atmospheric Administration surveys in Block Island Sound showing storm-wave induced scour on the flanks of the bathymetric high along the southeastern edge of the study area.

Figure 26. Schematic showing the depth to which waves can effectively move water and the wave-induced horizontal and vertical stresses that impact the seabed.

Figure 27. Detailed planar view of the combined National Oceanic and Atmospheric Administration surveys in Block Island Sound showing channels in the western part of the study area and locations of stations BIS22, BIS24, BIS26, BIS27, and BIS28.

Figure 28. Detailed planar view of the combined National Oceanic and Atmospheric Administration surveys in Block Island Sound showing channels in the eastern part of the study area and location of a shipwreck.

Figure 29. Detailed planar view of the combined National Oceanic and Atmospheric Administration surveys in Block Island Sound showing depressions formed by tidal scour in the passage north of Block Island and locations of stations BIS47, BIS48, BIS49, and BIS50.

Figure 30. Detailed planar view of the combined National Oceanic and Atmospheric Administration surveys in Block Island Sound showing a scour depression formed by tidal scour in the southwestern part of the study area and location of station BIS8.

Figure 31. Detailed planar view of the combined National Oceanic and Atmospheric Administration surveys in Block Island Sound showing large sand waves in the southwestern part of the study area and location of station BIS36.

Figure 32. Detailed planar view of the combined National Oceanic and Atmospheric Administration surveys in Block Island Sound showing barchanoid sand waves in the southwestern part of the study area and location of station BIS10.

Figure 33. Detailed planar view of the combined National Oceanic and Atmospheric Administration surveys in Block Island Sound showing a field of barchanoid megaripples in the western part of the study area.

Figure 34. Bathymetric profiles across sand waves and megaripples from the combined National Oceanic and Atmospheric Administration surveys in Block Island Sound from the eastern and western entrances to the passage north of Block Island.

Figure 35. Bathymetric profiles across sand waves from the combined National Oceanic and Atmospheric Administration surveys in Block Island Sound from the southwestern part of the study area.

Figure 36. Detailed planar view of the combined National Oceanic and Atmospheric Administration surveys in Block Island Sound showing the relatively flat, featureless appearance of the surrounding seabed composed of Holocene marine sediments, a shipwreck, and artifacts from acquisition and processing.

Surveyed depths within the study area range from less than 1 m to over 58 m (fig. 17). The shallowest parts of the study area occur along the shoreline of Block Island, on bathymetric highs along the southern Rhode Island coast, and on a discontinuous ridge extending both southwest from Block Island toward Montauk Point and, to a less pronounced extent, northeast toward Martha's Vineyard. The deepest part of the study area occurs in an isolated depression along its western edge. Other depressions occur at the eastern and western entrances to the passage between Block Island and the mainland, and just north of the elongate bathymetric high southwest of the Island.

Sea-floor features visible in the DTM can be geologically interpreted and ongoing sedimentary processes identified because they are morphologically distinct. For example, many small, individual, rounded bathymetric highs are also concentrated along the shoreline around Block Island (figs. 18, 19, 20), on the submerged ridge extending west-southwest and east-northeast off southern Block Island (fig. 21), and around bathymetric highs, such as the one extending south-southeast from Point Judith (fig. 22). These features, which give the sea floor in these areas a rough appearance, are interpreted to be boulders (fig. 23). Although the boulders average 1 to 3 m in width, many exceed 10 m across. These rocky areas are lag deposits remaining from winnowed Pleistocene deposits. The ridge extending off southern Block Island is a submerged part of the Ronkonkoma-Nantucket terminal moraine; the bathymetric high extending southeast of Point Judith is a submerged part of the Point Judith moraine, a segment of the Buzzards Bay moraine line; and the boulders along the shoreline of Block Island are a product of the contact between the Buzzards Bay and Rhode Island-Connecticut lobes of the retreating ice sheet.

Some of the bouldery patches are confined to scour depressions (figs. 24, 25). These depressions, which are most common in the eastern Sound, are erosional features that average 0.5 to 0.8 m deep, have relatively steep (4 to 6°) well-defined sides, and occur in a variety of shapes, sizes, and configurations. Some are elongate and narrow, others are broad and rounded, but most cap bathymetric highs or extend down the gentle (less than 1°) slopes. Although most of these scour depressions occur in depths shallower than 35 m, some on southward-facing slopes extend down to depths as great as 42 m. In several places, individual scour depressions have expanded to combine with adjacent depressions, forming larger eroded areas that commonly contain outliers of the original sea-floor sediments. Where boulders adjacent to these depressions protrude through the surface sediment, similar amounts of scour are present on all sides of the rocks, and asymmetrical scour features, such as comet or obstacle marks, are uncommon in these areas.

The coincidence of the bouldery deposits with the floors of the scour depressions indicates that surface sediments have been removed to reveal underlying Pleistocene-age deposits of winnowed till and outwash. The lack of asymmetrical scour features and the relatively weak tidal currents in these areas (White and White, 2009) suggest that storm-wave induced currents are much more important than bi-directional tidal currents in the formation of these features, and the presence and morphology of the depressions in the study area suggest that they are in equilibrium with the present hydrodynamic regime. Earlier work from shallow marine environments has shown that these depressions form and expand under high-energy shelf conditions when bottom stress from storm-driven waves and down-welling currents remove overlying fine-grained bottom sediments (Cacchione and others, 1984; Garnaud and others, 2005; Oakley and others, 2009; Poppe and others, 2011). Wave motion can effectively move water, and therefore move sediment and erode the seabed, down to the wave base, a water depth equal to one half of the wavelength (fig. 26). As large storm waves repeatedly pass over a seabed that is shallower than the wave base, cyclic vertical and horizontal shear stresses and pore-water pressure, compounded by gases from biologic activity, cause progressive deformation, liquefaction, and transport of seabed sediments. We contend that the concentration of these scour features on gentle slopes and around bouldery deposits, which would enhance turbulence, suggests that similar processes are at work offshore in eastern Block Island Sound.

Although much less common, storm-wave induced scour also occurs in western Block Island Sound, where scour depressions are present on the upper flanks of the bathymetric highs along the study area's northern edge. A few of these depressions, however, connect with single-channel and dendritic channel systems that extend downslope (figs. 27, 28). Individual channels average 1 to 1.5 m deep, can continue upslope and over minor bathymetric highs along their courses, and can exceed 5 km in length. The mechanisms governing formation and maintenance of these features are uncertain, but interpretations of seismic-reflection profiles show that there is no correlation between these channel systems and the underlying channel systems cut during either pre- or post-glacial subaerial exposure (Needell and others 1983b; Needell and others, 1984). Because of this lack of correlation, we would argue that these channels are related to modern processes, such as the return flow of bottom waters pushed up against the shoreline during storms.

Where constricted, tidal flow is enhanced and turbulence from the strong currents scours the sea floor. This scour can create large depressions, such as those at the eastern and western entrances to the passage between Block Island and the Rhode Island mainland (fig. 29), as well as smaller depressions around individual boulders and other obstructions, such as shipwrecks (fig. 30). Where scour around obstructions is asymmetrical, comet or obstacle marks are created. Also, tidal flow may be responsible for or contribute to the formation of a few cross-slope features similar to those discussed above. For example, southward-flowing ebb-tidal currents along the west side of Block Island are deflected offshore by boulder ridges (fig. 20), and channels are cut as the enhanced currents encounter finer grained sediments.

Alternating narrow, elongate bathymetric highs and lows reveal the crests and troughs of adjacent sand waves and megaripples (figs. 21, 23). These bedforms, which together cover approximately 10 percent of the study area, occur primarily in the southwestern part and near the entrances to the passage between Block Island and the Rhode Island mainland. Most of the sand waves and megaripples exhibit transverse morphologies (that is, sand waves trend at right angles to the direction of the currents) and occur where sediment supply is abundant, such as near shoals (figs. 21, 31). The largest of these tranverse bedforms exceed 4 m in crest-to-trough relief and 450 m in wavelength. Bedforms characterized by barchanoid morphologies are less common but occur in two fields where sediment supply is limited, as evidenced by nearby gravel pavements, scour, and boulders protruding through the surface sediment (figs. 32, 33).

Asymmetry of obstacle marks and sand waves can be used to interpret directions of net sediment transport (fig. 18). For example, the dominant (longer) tail of an obstacle mark extends downstream in the transport direction (Werner and Newton, 1975) and stoss slopes of sand waves face upstream (Allen, 1968; Ludwick, 1972; Reineck and Singh, 1980). Although bedform asymmetry on a shoal at the eastern entrance to the passage between Block Island and the Rhode Island mainland indicates that transport is predominantly out of the passage (fig. 34), bedform asymmetry on the flanks of the shoal at the western entrance indicates counterclockwise sediment transport is important to shoal morphology and maintenance. Both shoals were probably formed as tidal deltas (Dalrymple and others, 1978), but the eastern shoal is larger. Bedform asymmetry in the southwestern part of the study area indicates that transport is predominantly toward the northwest and into western Block island Sound (fig. 35), but sandwaves on top of the largest shoal in this area are symmetrical. Horns of barchanoid megaripples in the field located about 10 km southeast of Watch Hill Point extend eastward, indicating net transport in that direction (fig. 33; McKee, 1966). Elsewhere, bedform asymmetry within the study area is uncommon.

Much of the sea floor within the study area has a relatively smooth featureless appearance at the 4-m grid size of the DTM (figs. 17, 23), and seismic profiles show that these smooth areas delineate Holocene marine deposits (fig. 3; Needell and Lewis, 1984). Where present, features within the areas characterized by Holocene deposits are generally (1) artifacts produced by the equipment used and oceanographic conditions prevalent during multibeam acquisition or (2) anthropogenic in origin. The acquisition-related artifacts are made more conspicuous by the sun-illumination and 5x vertical exaggeration of the imagery. Anthropogenic features include slight arcuate depressions interpreted to be trawl marks and shipwrecks. Shipwrecks located away from constricted areas are not typically associated with scour features, suggesting that the tidal currents at these locations are weak (fig. 36).