Woods Hole Science Center
Sea-Floor Character and Sedimentary Processes in the Vicinity of Woods Hole, Massachusetts
The multibeam-echosounder (fig. 16) and sidescan-sonar (fig. 17) data collected from Woods Hole displayed as grids and images of the sea floor, which provide detailed information on benthic character, combined with the subbottom seismic-profile and verification data are used to interpret the surficial geology (fig. 18).
Surveyed depths within the study area range from less than 3 m to almost 27 m (fig. 16). The shallowest areas are found along the shorelines and on isolated bathymetric highs such as Nonamesset Shoal. Many small, individual, rounded bathymetric highs are also scattered across these shallow areas, around Great Ledge, and on the sea floor in Vineyard Sound in the southeastern part of the study area. These features that give the sea floor in these areas a rough appearance are interpreted to be boulders (figs. 19, 20). Although the boulders average 1 to 5 m in width, the largest, known as Coffin Rock, exceeds 12 m across. These rocky areas are lag deposits remaining from the winnowed sediments of the Buzzards Bay moraine.
At least five large, isolated depressions (greater than 100 m in nominal diameter) are present in Great Harbor and within the Inner Harbor. One of these depressions (fig. 21), a complex northwest-trending elongate feature that lies west of Juniper Point, is the deepest spot in the study area, approaching 27 m in depth. Bathymetric changes throughout most of the study area are typically gradual. However, notably along the edge of Parker Flat, depth can change dramatically (fig. 22). In this area, relief is more than 5 m and the slope is nearly vertical in places.
Many smaller depressions, which we interpret to be pockmarks (see the Character and Processes section), are present at the northern end of Little Harbor (figs. 23, 24). These depressions average about 0.5 m deep and 7 m wide, but range up to 14 m in diameter. The morphology of the features is quite varied. Some are rounded, others are irregular; some are flat floored, others have a central peak; some have steep sides, whereas the sides of others slope more gently.
South of Woods Hole in Vineyard Sound, the sea floor slopes gently to the southeast. Most of the benthic complexity in this part of the study area consists of the alternating narrow elongate bathymetric highs and lows revealing the crests and troughs of adjacent sand waves (fig. 25). Crests of the sand waves in the Woods Hole channel between Nonamesset Shoal and Great Ledge trend roughly east-west. Elsewhere, crest orientation in the larger sand-wave fields varies from 315° to 45°, but averages north-south. Amplitudes of the larger of these bedforms exceed 1.5 m, and wavelengths can exceed 25 m.
Distinctive tonal patterns revealed on the sidescan-sonar mosaic (fig. 17) 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), and (4) alternating bands of high and low backscatter in a “tiger-stripe” pattern. Boundaries between patterns are commonly gradational; backscatter is not uniform throughout these areas. Water-column phenomena, such as turbulence around obstructions on the bottom, are also present on the image.
The complex patches of high and low backscatter with individual high-backscatter targets delineate the rocky, bouldery areas of the Buzzards Bay moraine (fig. 26). Rocky areas are concentrated along shorelines, such as around Nobska and Juniper Points and Grassy Island, and around Great Ledge. Areas characterized by relatively high backscatter are concentrated adjacent to the bouldery deposits and on the crests of bathymetric highs. The higher backscatter tends to be produced by coarser grained sediments, typically gravel and gravelly sand, and reflects higher energy environments. Patches characterized by relatively low backscatter (fig. 27) are found at the northern end of Little Harbor and in the Inner Harbor formed by the convex side of Penzance Point. These areas coincide with lower energy environments that prevail in areas protected from the strong tidal- and storm-driven currents. The lower backscatter tends to be produced by finer grained Holocene marine sediments, typically silty sand and clayey silt.
Areas characterized by alternating bands of high and low backscatter include much of the southern part of the study area in Vineyard Sound and in the channel between Nonamesset Shoal and Great Ledge. This “tiger-stripe” pattern (fig. 28), which is produced by sand waves and megaripples, results from a combination of topographic changes affecting the angle of incidence of the sidescan sonar and the differences in sediment texture commonly present between crests and troughs of sand waves (Reineck and Singh, 1980).
Click on figures for larger images.