U.S. Geological Survey Open-File Report 2011–1004
Sea-Floor Geology and Character of Eastern Rhode Island Sound West of Gay Head, Massachusetts
Surveyed depths within the study area range from less than 24 m to almost 43 m (fig. 20). The deepest part of the study area occurs in its southwestern corner, from which depths progressively shallow both shoreward to the north and northeastward to where the shallowest depths occur. One exception is a large isolated bathymetric high located in the south-central part of the study area that rises 6-13 m above the surrounding sea floor. Regardless of location, most large-scale gradients are relatively gentle.
Sea-floor features visible in the digital terrain model (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 in patches on and around larger bathymetric highs. These features, which give the sea floor in these areas a rough appearance, are interpreted to be boulders (figs. 21, 22, 23, 24). Although the boulders average 1 to 3 m in width, several exceed 7 m and the largest exceeds 15 m across. Seismic profiles show that these rocky areas are lag deposits remaining from winnowed Pleistocene deposits (fig. 4).
The bouldery patches are commonly confined to scour depressions, but the shallowest parts of the bouldery accumulations can extend above the depressions' rims (figs. 22, 23, 24). These depressions average about 0.5 m deep, have relatively steep (4-6°) well-defined sides, and occur in a variety of shapes, sizes, and configurations. Some are elongate and narrow; others are broad and rounded. In a 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 (figs. 22, 25). In other places, turbulence from the strong bottom currents has scoured the sea floor around individual boulders and small outcrops (fig. 26). Typically, similar amounts of scour are present on all sides of the individual boulders, and asymmetrical scour features, such as comet or obstacle marks, are uncommon.
The coincidence of the bouldery deposits with the floors of the scour depressions indicates that surface sediments have been removed to reveal the underlying Pleistocene deposits. The lack of asymmetrical scour features and the relatively weak tidal currents (White and White, 2009) suggests that storm-generated currents are more important than bi-directional tidal currents in the formation of these features, and the presence and morphological variety of the depressions in the study area suggests 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 the pumping action of storm-driven waves and down-welling currents can remove overlying fine-grained bottom sediments (Cacchione and others, 1984; Garnaud and others, 2005). The concentration of these scour features on gentle seaward-facing slopes and around bouldery deposits, which would enhance turbulence, suggests that similar processes may be at work in eastern Rhode Island Sound.
Elsewhere, most of the study area's benthic bathymetric complexity consists of the alternating narrow, elongate bathymetric highs and lows revealing the crests and troughs of adjacent megaripples (figs. 22, 24). These bedforms, which together cover less than 1 percent of the study area, occur in three fields, two in the northeastern corner and one east of the large isolated bathymetric high in the south-central part of the study area. Most of the megaripples in the northeastern fields are asymmetrical, and indicate northwestern to northeastern net sediment transport (fig. 27, profiles A and B, Reineck and Singh, 1980). Megaripples in the south-central part of the study area (fig. 27, profile C) are predominantly symmetrical, suggesting no net transport. All of the megaripples have a transverse morphology and none of their crest to trough amplitudes exceed 0.5 m.
Approximately 86 percent of the sea floor within the study area has a relatively smooth featureless appearance at the 2-m grid size of the DTM (figs. 23, 26). Seismic profiles show that these smooth areas delineate Holocene marine deposits (fig. 4). Where present, visible features are generally artifacts produced by oceanographic conditions prevalent during multibeam acquisition and made more conspicuous by the sun-illumination and 5x vertical exaggeration of the imagery.