U.S. Geological Survey Open-File Report 2005-1048
Geological Interpretation of Bathymetric and Backscatter Imagery of the Sea Floor Off Eastern Cape Cod, Massachusetts
This brief description of sea-floor geology off outer Cape Cod covers a narrow elongate area starting near Provincetown and continuing around the Cape’s northern tip and southward along its eastern shore to just northeast of Monomoy Island. The discussion focuses on the distributions of surficial sediment and sedimentary environments, but includes comments on the bathymetry, bottom character, Quaternary framework geology, and benthic habitats.
Depths increase rapidly offshore from 6 m to 14 m about 1 km off Provincetown. Sand, which dominates near shore, progressively gives way to a narrow band of silty sand and finally clayey silt on the floor of Cape Cod Bay (Figure 11). The muddy sea floor in this part of the Bay is characterized by long-term deposition and displays uniform backscatter intensity, implying a relative homogeneity of the bottom sediment. Northward from Long Point toward Race Point the environmental energy is greater and mud gives way to silty sand and sedimentary environments characterized by sorting and reworking (Knebel and others, 1996; Figure 12). Along this stretch sand dominates the nearshore and in areas adjacent to points, but also dominates the offshore part of the study area by Race Point where environments of coarse bedload transport dominate.
A sandwave field extends eastward for over 5 km from near Race Point (Figure 13). The larger waves in this field exceed 7 m in height and have wavelengths averaging over 100 m. Wave crests strike generally perpendicular to the shoreline, but deflect westward about 1 km offshore. Sandwave asymmetry and the presence of megaripples on the sandwaves indicate net transport is westward into Cape Cod Bay and active. Two other smaller sandwave fields are present further offshore and to the east (centered around 42o6.4’N, 70o11’W and 42o6.6’N, 70o6’W). These fields, which have a similar orientation and asymmetry to the field near Race Point, have crests that are predominantly sand and troughs characterized by gravelly sediments and harder acoustic returns.
A field of linear scour depressions is centered about 1.7 km offshore at 42o5.6’N, 70o9.5’W (Figure 14). These depressions, the largest of which exceeds 230 m in length and 35 m in width, are oriented roughly perpendicular to the shoreline and are floored by coarser sediments as evidenced by higher backscatter. Although located somewhat far offshore, these features are closely associated with and apparently at least partly owe their existence to Peaked Hill Shoal, a shallow long-shore sand body. Described elsewhere in a variety of settings including to the south off Nauset (Needell and others, 1982; Schwab and others, 1995), these linear scour depressions may provide pathways for net offshore, cross-shelf sediment transport (Ferrini, 2004) or may be large wave-generated ripples reflecting large-scale sorting and along-shore transport (Murray and Thieler, 2004).
Narrow, somewhat discontinuous exposures of gray semi-consolidated silty clay, which are centered near 42o5’N, 70o6.5’W, are interpreted to be outcrops of glaciolacustrine sediments (Figure 14, Figure 15). We believe these sediments were distally deposited as bottomset beds as part of the delta that prograded into Cape Cod Bay glacial lake and the produced the Truro outwash plain slightly less than 18 ka, but before the Eastham plain delta (Figure 3). Multibeam data show this deposit rises about 1 m above the surrounding sea floor and, as such, it probably constitutes an erosional outlier. Bottom photography shows the flat areas on top of the beds are partially covered by a thin veneer of rippled sand and that outcrops have a rough honeycombed appearance (Figure 16). Although this rough appearance may be due to differences in grain size or induration, it is probably the result of bioerosion.
Much of the sea floor off the towns of Truro and Wellfleet is characterized by low (<10 cm) broad (about 0.5 m) ripples. Gravel and brownish organic debris are concentrated in the troughs; sand dollars congregate on the sandier crests (Figure 17). Smaller current ripples are occasionally superimposed on the low broad ripples. Generally, areas of the sea floor characterized by lower backscatter and sandier sediments are more common in the western, nearshore part of this stretch of the study area; higher backscatter and more gravelly sediments are more common in the offshore part (Figure 18). Sandwaves are also variably present along the nearshore part of this stretch and form a relatively large sandwave field centered at 42o1.2’N , 70o0.5’W. Stoss slopes of the sandwaves have higher backscatter intensity and sandwave asymmetry indicates net northward sediment transport. Gravelly areas of the sea floor are littered with cobbles and boulders, interspaced with patches of finer gravel and rippled sand (Figure 19). A large elongate patch of bouldery gravel (centered near 42oN, 69o58’W) probably records more ice-proximal glaciofluvial topset deposition within the delta that produced the Wellfleet outwash plain. These topset beds have been winnowed leaving a gravelly lag deposit. Sessile organisms attached to many of the rocks indicate that transport is not active and suggests that sedimentary environments within the coarser gravelly areas are characterized by erosion and nondeposition.
A small exposure of light bluish gray mud observed at station CC69 off Wellfleet (41o54.6’N, 69o56.6’W) is interpreted to be an outcrop of glaciolacustrine sediment that was distally deposited in Cape Cod Bay glacial lake as part of the delta that built Eastham outwash plain (Figure 3). The mud is cohesive, semi-consolidated, and varies from silty clay to clayey silt with small amounts of very fine sand. While limited within the present study area, larger exposures of these sediments, which were verified as part of this study (station CC182 at 41o54.1’N, 69o56.8’W and station CC183 at 41o53.9’N, 69o56.85’W), have been reported by Foster and Poppe (2003) just shoreward to the west. High-resolution seismic-reflection data collected by Foster and Poppe (2003) mapped this unit on the Atlantic side of South Wellfleet and North Eastham and on the Cape Cod Bay side of North Eastham and revealed a seismic facies characterized by closely spaced (rhythmic) internal reflectors within glacial drift (Figure 20). East-west seismic lines on the Atlantic side show that this unit dips landward and that it is truncated to the east by the Holocene transgression (Figure 21); north-south lines show its lateral extent (Figure 22). Poor subbottom penetration and discontinuous-to-chaotic reflectors in these records represent sandy and gravelly drift deposited under more ice-proximal, higher energy conditions, glaciofluvial conditions, probably as topset beds in the delta that built the Eastham outwash plain. Good penetration and resolution of internal reflectors suggests muddy sediments deposited under more distal, lower energy glaciolacustrine conditions, probably as bottomset beds in this delta.
Evidence from onshore boreholes shows these glaciolacustrine deposits are variably present within all of the outwash plains on the outer Cape (Figure 23). Bottom photography shows that these outcrops exceed 1 m in relief and that degradation, as evidenced by talus piles beneath ledges and the presence of abundant rip-up clasts on the surrounding sea floor, appears to primarily be along weak planes or varves and presumably occurs during high-energy storm conditions (Figure 24). Bioerosion, while probably not as important a mechanism of degradation, is ongoing.
The glaciodeltaic deposits of the Eastham outwash plain, and those elsewhere along the outer Cape, are extremely important for both hydrological and ecological reasons. Hydrologically, the impermeable glaciolacustrine units can affect the position of the salt water/freshwater interface in well fields on Cape Cod, limiting recharge and causing saltwater intrusion, downconing of surface waters in wells drilled to above the fine-grained units, upconing of salt water in wells drilled to below these units, and poor flow rate in wells drilled into the glaciolacustrine sediments (Figure 25). Ecologically, exposures of these strata provide unique relatively stable benthic habitats in an area of the shelf dominated by mobile sand and fine-grained gravel. Bottom photography shows that lobsters use the muddy cohesive outcrops to construct burrows and that finfish use the ledges and overhangs for shelter (Figure 26).
Other facies of the delta which formed the Eastham outwash plain are exposed farther to the south about 1.7 km off the villages of North Eastham and Eastham (Figure 27). A large area of bouldery gravel interspaced with small patches of gravelly sediment and sand (centered around 41o51.5’N, 69o54.9’W) records ice-proximal glaciofluvial topset deposition. Most of the finer sands and gravels that were originally present have been winnowed from the surface under sedimentary environments characterized by erosion and nondeposition, leaving the bouldery lag deposit (Figure 28). These boulders provide a substrate for the sessile fauna and, as with the glaciolacustrine outcrops described above, shelter for finfish and lobsters.
A band of tabular blocks composed of moderately well sorted very fine-grained sand, which rise over 1 m above the surrounding sea floor and can exceed 400-m across, are concentrated along the southern edge of the bouldery area (Figure 27). These slabs become smaller and more widely spaced both westward and northward. Based on their intermediate sediment grain size and low backscatter intensity, we interpret these features to be the remnants of foreset beds from the delta that produced the Eastham outwash plain. As the sandy ice-distal topset beds were eroded away by the Holocene current regime, the foreset beds over which they had prograded were exposed. The more cohesive, muddier sands of these foreset beds were more resistant and a part of this facies is now preserved as erosional outliers. Although the sea floor on these blocks is characterized by very fine sand and an undulating track-marked appearance (Figure 29), preservation of these blocks suggests that denser, more cohesive sediments (i.e. muddy sand and sandy mud) probably compose the interiors. Abundant skate observed on the tabular blocks suggest that the blocks constitute an important habitat for these fish.
Much of the sea floor off the town of Orleans is relatively flat and characterized by ubiquitous low (<10 cm) broad (about 0.5 m) ripples and sedimentary environments dominated by coarse bedload transport. Gravel and brownish organic debris are concentrated in the troughs; sand dollars congregate on the sandier crests (Figure 30). Smaller current ripples are superimposed on the low broad ripples in places.
Sandwaves become more common on the sea floor to the south off Chatham. These bedforms, which can locally exceed 4 m in height, trend N 30-40o E and typically have higher backscatter and coarser sediments on their southeast-facing slopes (Figure 31). Although roughly symmetrical, sandy areas on and near the sandwaves are current rippled, suggesting transport is active. In general, however, sediments coarsen southward, gravel and gravelly sediment paves much of the bottom, and sandy patches, where present, are thin, as evidenced by patches and individual grains of gravel poking through the sand (Figure 32).
Narrow, somewhat discontinuous outcrops off Chatham, which are centered near 41o41.4’N, 69o52.1’W, are also interpreted to be exposures of glaciolacustrine sediments (Figure 33). Although these outcrops have not been verified with bottom photography or sampling, we interpret them to be composed of fine-grained glaciolacustrine sediments based primarily on their morphology. Similar to the other distal glaciolacustrine sediments off outer Cape Cod (e.g. Figure 15), the outcrops off Chatham have low backscatter intensity, abnormally steep sides, and abrupt edges. Surficial sediments on the surrounding sea floor are dominated by coarse noncohesive sands and gravels that have gradational boundaries and will not support such steep slopes.
Location suggests that the glaciolacustrine outcrops off Chatham are older than those associated with the Truro and Eastham outwash plains and may record glaciodeltaic sedimentation prograding from the South Channel lobe into Glacial Lake Nantucket Sound just prior to 18 ka (Figure 2B). If this is the case, then these sediments were probably deposited as part of the eastern edge of the Barnstable outwash plain.