The Seafloor Revealed

Surficial Geology

Introduction

The surficial materials of the inner continental shelf of the western Gulf of Maine are the most complex of any place along the Atlantic Margin of the United States. Igneous, metamorphic, and sedimentary rocks spanning almost a billion years of earth history form the regional basement. Glacial deposits containing all clast sizes from boulders to mud partly mantle the rocks. These materials, in turn, have been reworked by coastal processes during extreme excursions of sea level over the past few thousand years to locally create better texturally sorted deposits of modern sediment. Biological processes, including shell formation and gas eruption, have added to and disturbed the sediments, respectively. As discussed above, the selection of map units to describe this complexity involves a compromise between providing detailed information where it exists and generalizing where data are scarce or absent. For this reason we have chosen the 16-unit classification for areas where side-scan sonar coverage exists (Figure 17), and 4 "end-member" units where it is absent. Off the entrance to the Bay of Fundy, no data exist, and so only bathymetry is depicted. Grain size analyses were performed on many of the bottom samples (Figure 24), and although no new maps based on these were made, older publications did produce such maps (Kelley and Belknap, 1991).

Rocky Areas

Rocky seabeds occupy 41% of the inner continental shelf bottom (Figure 34) and are the most abundant surface material in all parts of the study area except extreme eastern Maine, and at all depths less than 50 m (Figure 34, Table 4, Table 5; Barnhardt and others, 1996a-g). Rocky areas are most abundant in water less than 40-50 m deep and become less common with increasing depth. Where little data exist and the seafloor is very irregular, a rocky bottom was inferred. Thus, large areas of rocky bottom are mapped off extreme southern Maine, Penobscot Bay, and off Petit Manan Point (Barnhardt and others, 1996a-g), despite the lack of many direct observations in those places. Large areas of rock are also inferred to occur surrounding the many granitic islands in Blue Hill and Frenchman Bays, and elongate bodies of rock apparently follow the linear trend of the peninsulas north of Cape Elizabeth.

No effort was expended to identify the nature of the bedrock, but side-scan sonar images clearly depict parallel fractures and elongate outcrop patterns common in layered rocks, as well as more rounded bodies of rock often associated with plutonic (granitic) rocks. The surfaces of rock outcrops are usually covered with algae (seaweed) and encrusting organisms in shallow water, and with only encrusting organisms at greater depths (Figure 35). Fractures in rock and regions surrounding rock outcrops are commonly covered with shells of dead organisms formerly attached to the rock surface, as well as with angular fragments of rock. It is for this reason that large, "pure" rock outcrops (R) were less frequently mapped with side-scan sonar than "rock greater than gravel" (Rg) or "gravel greater than rock" (Gr, Figure 27). Together, these are among the most common units mapped with side-scan sonar on the surficial maps. Fractures filled with gravel are often called "sediment ponds" and are generally thin deposits (Kelley and others, 1989a).

The map unit, "rock greater than mud" (Rm), is most common in Outer Basins where outcrops poke through the contemporary mud that is presently mantling the seafloor. Rm also occurs as small areas seaward of tidal flats in the Nearshore Basins. Rock greater than sand (Rs) exists only in a few locations on Nearshore Ramps.

Gravelly Areas

Gravel (G) is a common constituent of inner shelf sediment, but extensive areas of gravelly sediment cover only 12% of the seabed (Figure 34, Table 4, Table 5; Barnhardt and others, 1997). Gravel dominates the seafloor off the Kennebec River mouth (Figure 18) where palimpsest deltaic sediments are exposed, and near reworked glacial moraines off Wells (Figure 9) and Saco Bays, and off Machias Bay in eastern Maine. In many instances the gravel has a rippled surface (Figure 18, Figure 25), and may contain minor amounts of coarse sand. In areas where scouring of the seabed has occurred, a gravel-lag deposit armors the seafloor (Figure 9, Figure 20, Figure 36, Figure 37) at least temporarily. Gravel also occurs in small linear bands where moraines exist on the seabed (Figure 9, Figure 36). Gravel is most abundant in the 20 to 40 m depth range, except in eastern Maine where the Hard-Bottom Plain is covered with gravel to depths of at least 100 m (Table 5).

As described above, gravel greater than rock (Gr) is a common feature adjacent to bedrock outcrops and in fractures in the rock. Here the gravel may have a high shell content (calcium carbonate) because shells are the only modern sediment being introduced to the area (Barnhardt and Kelley, 1995). Gravel greater than rock and gravel greater than sand (Gs) are major features of the seafloor from the Canadian border to Englishman Bay (Barnhardt and others, 1996g). Here, low-relief bedrock is mantled by till, which fills in rock depressions, but lacks much relief itself (Figure 38). Gravel greater than mud (Gm) is very rare along the inner shelf because the two sizes of clasts are not deposited under the same hydrodynamic conditions.

Sandy Areas

Sandy areas (S) are relatively rare along the inner shelf of the western Gulf of Maine, and occupy only 8% of the seafloor (Figure 34, Table 4; Barnhardt and others, 1996a-g). The sandiest region is in southern Maine and New Hampshire where sand is concentrated in Nearshore Ramps at water depths of less than 40 m. In eastern Maine, sand is more common in water deeper than 40 m, so that the overall abundance of sand remains nearly constant with changing depth (Figure 34). Sand is acoustically uniform and strongly contrasts with bordering areas of gravel and rock (Figure 18). Although many samples from shallow water contain "clean," well-sorted sand, areas mapped as "sand" or sand with other materials frequently contain sediment in which the sand is mixed with mud or gravel.

Sand greater than gravel (Sg) is an important seafloor component off the Kennebec River mouth and possibly off Machias Bay, although observational data in this area are scarce (Barnhardt and others, 1996g). Outer portions of Narraguagus and Pleasant Bays also have a mixture of sandy gravel and gravelly sand (Dickson and others, 1994). On the Kennebec River paleodelta many small rippled gravel patches are intermingled with sand in a complex, but persistent mosaic (Figure 39) that has not changed appreciably in several years (Barnhardt, 1994; Dickson and others 1995 a,b). Large storms presumably move the sand as bars across the rippled, gravel substrate, but this has not yet been demonstrated (Dickson and others, 1993, 1995 a,b; Barber, 1995).

Sand greater than rock (Sr) is a minor component of the seafloor that exists adjacent to small bedrock outcrops scattered across the study area. It is possible that more such areas exist, especially in the sandy southern part of the study area, but few observations have been made there.

Sand greater than mud (Sm) is a very difficult unit to map because mixtures of mud and sand look alike on acoustic imagery. The only large areas of sand greater than mud are located in Saco Bay, where many closely spaced bottom samples confirmed the presence of both mud and sand. Similar occurrences of Sm may occur at the seaward margin of other Nearshore Ramps.

Muddy Areas

After rocky areas, muddy regions are the most abundant areas on the inner shelf, covering 39% of the study area (Figure 34, Table 4; Barnhardt and others, 1996a-g). Mud covers most of the Nearshore and Outer Basins, and so it is a common map unit in both shallow and deep water, with a minimum abundance at 30 to 40 m depth range (Figure 34). It is the dominant seabed material in all nearshore areas except for southern Maine and near Canada. It is also the major deep-water surficial material in all locations except off southern Maine.

Mud accumulates where there is an available supply of fine-grained sediment and quiet conditions, that favor the slow settling of small particles, or their entrapment by sessile organisms (Figure 40). In the nearshore regions, mud probably comes from eroding glacial bluffs and seasonally from rivers. Deep-water mud must be derived from erosion of deposits in shallow water plus the particulate organic remains of formerly living things.

Muddy seafloors are featureless on acoustic records unless they have been disturbed or contain anomalous, "hard" (dark) objects (Figure 12, Figure 21, Figure 27, Figure 29, Figure 32). Drag marks left by fishing boats are very common in all sedimentary environments along the inner shelf, but are most noticeable over muddy seabeds. Gas-escape pockmarks are more localized disturbances, but, where they occur in abundance, they profoundly alter the seabed (Figure 12, Figure 29). In Penobscot, Blue Hill, and Passamaquoddy Bays thousands of hemispherical depressions, up to hundreds of meters in diameter and tens of meters deep, mark the muddy bottom (Fader, 1991; Kelley and others, 1994; Barnhardt and Kelley, 1995).

Mud greater than rock (Mr) occurs in some deep-water locations, but mud greater than gravel (Mg) is as rare as gravel greater than mud (Gm) because of the hydrodynamic differences between the two sizes of materials. Mud greater than sand (Ms) occurs seaward of the sandy area of Saco Bay, and is mapped, not because it is acoustically recognizable, but only because of the large number of bottom samples that encountered this mixture.

Contents Introduction Methods Physiography Surficial Geology Geological History Summary Glossary References

Last updated on October 6, 2005