The Seafloor Revealed

Physiography

Introduction

The inner continental shelf of the western Gulf of Maine is a submerged extension of the northern Appalachian Mountains, and its bathymetry is as complex as the topography on the adjacent upland. The orogenic belt consists of a series of suspect terranes of varying bedrock lithology and structure that have undergone erosion since at least the Mesozoic Era, and possibly longer (more than 100 million years; Kelley and Kelley, 1993). Rocks exposed on land and on the seafloor were formed kilometers beneath the earth's surface during continental interactions hundreds of millions of years ago. The overall geomorphology of the region is controlled by the spatial distribution of lithology, faults, folds, and other structural features imparted to the rocks long ago.

Glacial erosion and deposition modified the bedrock skeleton and added to the regional geomorphic complexity. Almost all of the "recent" sedimentary material along the coast and offshore is derived from contemporary erosion of glacial deposits. Although bedrock defines the overall shape of the coastal region, glaciation provided the materials for contemporary processes, like waves and currents, to shape into the dynamic habitats of the inner shelf.

The coast of Maine and New Hampshire may be subdivided into several geomorphic compartments largely defined by bedrock (Kelley and others, 1995c; Kelley, 1993). From New Hampshire to Cape Elizabeth, the Arcuate Embayments are a series of rocky headlands separating sandy bays of varying sizes. Extensive salt marshes and sand beaches are the principal geomorphic elements of this stretch of shoreline. From Cape Elizabeth to Port Clyde, elongate bedrock peninsulas of the Indented Shoreline are separated by relatively deep, but narrow estuaries. Deposits of muddy glacial sediment blocked many of the bedrock valleys that were probably carved by ancient rivers, and most of the modern coastal embayments possess little freshwater discharge. The largest stream in the area, the Kennebec River, enters the sea through a rocky gorge that probably never supported a river prior to the last Ice Age (Kelley and Kelley, 1989; Hannum, 1997). The Island-Bay Coast extends from Port Clyde to Cross Island and is shaped by numerous, granitic islands sheltering broad embayments. Like the Indented Shoreline compartment, mud and mixed mud and gravel flats are the most common intertidal settings. From Cross Island to Canada, bedrock faults along both the Atlantic and Oak Bay shorelines define the Cliffed Shoreline and create a straight coast with abundant bedrock habitats in the intertidal zone. The 6 m tidal range in this area has led to considerable scouring of the seafloor by tidal currents as well as to the formation of extensive tidal flats.

As discussed below, there are similarities between the morphology of the shoreline and the adjacent inner shelf. Seaward of the sandy beaches of the Arcuate Embayments are sandy Nearshore Ramps. Similarly, seaward of the muddy tidal flats of the Indented Shoreline and Island-Bay coastal compartments, there are muddy Nearshore Basins on the shelf. The high cliffs of the Cliffed Shoreline plunge rapidly into deep water near shore where a vast Rocky Zone and Hard-Bottom Plain exists. The fault line along the Cliffed Coast, the Fundy fault, trends to the southwest and forms a major bathymetric discontinuity along the inner shelf (Johnson, 1925; Shepard, 1930).

Nearshore Ramps

Nearshore Ramps are regions that slope gently seaward with widely spaced, shore-parallel bathymetric contours (Figure 11, Figure 22; Kelley and others, 1989a, 1997). They comprise only about 5% of the study area and are most common in New Hampshire and southwestern Maine, where they abut large beaches (Table 4). Most Nearshore Ramps begin at the shoreline and continue to the 30 or 40 m isobath; less than 25% continue into water deeper than 40 m (Figure 22, Table 5). These deep water exceptions include the large Nearshore Ramps off the Kennebec and Merrimack River mouths, which reach to depths of 55 m, and those in Narrraguagus and Machias Bays, which occur in deeper water.

Nearshore Ramps are predominantly covered with sand (66% of their area, Table 4) with subordinate gravel (27%) and rare outcrops of bedrock (4%; Figure 18). Mud becomes increasingly important in depths greater than 70 m (Figure 23). Bottom samples are largely well sorted sand or mixtures of fine-coarse sand and gravel (Figure 24, Figure 25).

In areas <30 m deep, Nearshore Ramps represent the shoreface of the adjacent beaches. In deeper water seaward of the Kennebec, Merrimack, and Narraguagus Rivers, the Nearshore Ramps are complex, reworked deltaic deposits of late Pleistocene-early Holocene age (Figure 11; Oldale and others, 1983; Barnhardt, 1994). There is no deltaic feature in Saco Bay, but sand covering the Nearshore Ramp in that bay is derived from the Saco River (Figure 26; Kelley and others, 1989b; 1995a). There is no fluvial input to the Nearshore Ramps in and south of Wells Embayment, but reworking of glacial deposits has provided a sand and gravel veneer for that Nearshore Ramp (Figure 8), as well as for a ramp south of Mt. Desert Island (Barnhardt and Kelley, 1991). Reworked shell deposits cover a Nearshore Ramp off southeast Mt. Desert Island (Barnhardt and Kelley, 1995). The Nearshore Ramp south of Machias Bay is poorly understood, but appears to be a steep bedrock slope covered with a thick deposit of glacial-marine sediment (Shipp, 1989).

Off both the Saco and Kennebec Rivers, and Wells Embayment, many vibracores, along with extensive seismic reflection and side-scan sonar data suggest that the profiles of the Nearshore Ramps are maintained by waves. The seafloor in these areas is a relatively thin, and actively reworked wedge of sandy shoreface material overlying glacigenic sediment (Figure 26). Discrete patches of sand and gravel form complex patterns on these ramps and apparently remain stable over many seasons despite powerful storm activity (Barber, 1995).

Nearshore Basins

Nearshore Basins are shallow marine depressions protected from the open sea by the mainland, peninsulas, islands, or shoals (Kelley and others, 1989a, 1997). They are generally bordered by tidal flats on the landward side, and 65% of the Nearshore Basins extend seaward to the 30 m isobath where they merge with Shelf Valleys (Table 5; Figure 7, Figure 21, Figure 22). Nearshore Basins occupy 17% (Table 4) of the seafloor and are concentrated off central Maine where shelter from waves is afforded by the peninsulas and chains of islands in the Indented Shoreline and Island-Bay coastal compartments (Kelley and others, 1995c). The Nearshore Basins occur over local linear depressions in bedrock that are mapped as faults in parts of Casco, Sheepscot, Penobscot, Muscongus, Oak, and Cobscook Bays (Osberg and others, 1985). Many of the Nearshore Basins terminate against thick deposits of glacial sediment on land which cover deep, bedrock buried valleys (Upson and Spencer, 1964; Tolman and others, 1986; Kelley and others, 1987b).

Nearly 80% of the Nearshore Basins are floored by mud (Table 4), with about 20% rock and gravel and almost no sand. Rock exposures are common along the margins of Nearshore Basins (Figure 27), and outcrops commonly punctuate the smooth seafloor (Figure 21). Nearshore Basins contain sediment coarser than mud where bedrock constrictions accelerate tidal currents, especially in shallow nearshore passages. Sediment samples emphasize the dominant nature of mud, however; all samples contained some mud (Figure 24), even the outer reaches of bays with deep channels (Figure 10, Figure 28). In some Nearshore Basins, unstable muddy material has slumped into channels; in others natural gas eruptions have disturbed the seabed with pockmarks (Figure 12, Figure 29; Kelley and others, 1989c). Nearshore Basins on the western side of Penobscot Bay and northern Blue Hill Bay have highly irregular bathymetry because of gas-escape pockmarks (Kelley and others, 1994; Barnhardt and Kelley, 1995).

Rocky Zones

Occupying half of the study area (Table 4), Rocky Zones are the most abundant physiographic region along all areas of the inner shelf, except off the Cliffed Shoreline near the Canadian border (Dickson and others, 1994; Kelley and others, 1989a, 1997). Rocky Zones are areas with exposed or shallow bedrock and gravel, associated with rapid and extreme changes in bathymetric relief (Figure 19). Cliffs ranging from 3 to 10 m appear as commonly as on land. The Rocky Zones are generally less than 60 m deep and locally form shoals. Rocky Zones surround many of the large islands in the Island-Bay coastal compartment and trend parallel to peninsulas in the Indented Shoreline compartment (Figure 27).

Although bedrock dominates Rocky Zones, sediment ponds infill many fractures in the rock (Figure 19), and gravel aprons often form halos around more isolated bedrock outcrops and islands (Figure 9, Figure 27). Sediment in these "ponds" and "halos" is usually coarse-grained, and commonly enriched (with as much as 100%) with shells from nearby encrusting organisms (Kelley and Belknap, 1991; Barnhardt and Kelley, 1995). Large boulders, up to several meters in diameter, commonly occur on areas of exposed bedrock, and moraines are associated with bedrock knobs in some locations. Despite the generally coarse nature of Rocky Zones, sediment samples from these regions contain the greatest range of particle sizes of any place on the inner shelf, and at least some muddy sediment occurs in most samples (Figure 24).

During times of lower sea level, most of the areas that are mapped as Rocky Zones were islands or part of the mainland. They may have been once covered with till and glacial-marine sediment, but have lost some or all of their sedimentary cover due to wave and current action at times of lower sea level. Ripples associated with these regions suggest continuing erosive activity. Some moraines were not completely removed by these processes, but became armored with boulders and gravel until they could not be eroded any further (Figure 8, Figure 9).

Shelf Valleys

Shelf Valleys occupy only 9% of the study area, but are conspicuous, bedrock-framed, seaward sloping troughs that generally are offshore extensions of Nearshore Basins (Kelley and others, 1989a, 1997). Because of their sheltered setting, they contain mostly mud, but rock, gravel, and sand are common in less than 30 m depth (Table 5, Figure 23). In areas of late Pleistocene/early Holocene sand deposition, such as off the Kennebec, Narraguagus, and Saco River mouths, Shelf Valleys terminate against Nearshore Ramps. Here, the deeply eroded bedrock valleys are buried by sediment (Barber, 1995; Barnhardt and others, 1997; Dickson and others, 1994). In more exposed locations where tidal currents are strong, Shelf Valleys have no sediment and may be very deep, as off Penobscot Bay (Figure 30). In many locations Shelf Valleys are apparently not accumulating modern sediment, but are floored by glacial-marine mud. In addition, these troughs cut across the regional trend of the bedrock and may serve as conduits for material escaping nearshore regions. In Penobscot Bay, for example, sediment erupted by gas escape in the upper Nearshore Basin in Belfast Bay may travel down the channel of the basin, along the deep Shelf Valley, and out into the Gulf of Maine. Contaminated nearshore sediment may find its way into the Gulf of Maine through these pathways, but offshore nutrient-rich water may also enter estuaries via the same route. The origin of the Shelf Valleys, and the deep bedrock troughs beneath Nearshore Basins, is unknown. They were once considered to be ancient products of fluvial erosion (Johnson, 1925), but were later viewed as glacially scoured features (Shepard, 1931). More recently, the deep bedrock valleys nearshore were described as composite features resulting from initial fluvial erosion and later glacial deepening (Uchupi, 1968). The branching nature of some of the Shelf Valleys appears fluvial (Figure 31), but there are more valleys than modern-day rivers and embayments, and many more valleys than present levels of precipitation require (Kelley and Kelley, 1993). Although possibly some of the Shelf Valleys were deepened by glaciers, they are not consistently lined up with the direction of ice advance (generally northwest to southeast, Thompson and Borns, 1985), and some of them are perpendicular to that direction. It is possible that subglacial meltwater was involved in the formation of these deep troughs as inferred elsewhere (Boyd and others, 1988).

Outer Basins

The Outer Basins occur in water generally deeper than 40 m and are poorly studied, in part, because of their remoteness (Kelley and others, 1989a, 1997). They generally border Rocky Zones, and although they comprise only 16% of the study area, they extend without interruption into the deeper water of the Gulf of Maine. In most locations Outer Basins have a more subdued bathymetry than Rocky Zones, with 77% of their area covered by mud with occasional outcrops of rock (Figure 32). Seaward of Saco, Muscongus, and Narraguagus Bays, where the Outer Basin is a broadened extension of several Shelf Valleys, Outer Basins contain more than 50 m of glacial sediment with natural gas. Off Penobscot Bay, Mt. Desert Island, and Machias Bay, Outer Basins have an irregular bathymetry, and may, with more investigation, be reclassified as Rocky Zones.

Because they exist in relatively deep water, Outer Basins experience currents and wave activity too weak to erode muddy sediment. For this reason, modern mud accumulates in these deep settings and is only disturbed by fishing practices like dragging. At times of lower sea level, the Outer Basins below 60 m depth were not exposed subaerially and accumulated material eroded from Rocky Zones. Thus, only in deep Outer Basins has sediment continuously accumulated to provide a complete geologic record of oceanographic events since the Ice Age.

Hard-Bottom Plains

Hard-Bottom Plains are extensive areas of low bathymetric relief that are covered by gravel (83%), sand (14%), and rock (3%) (Table 4, Table 5, Figure 23, Figure 24). The Hard-Bottom Plains occupy a large area off eastern Maine (Dickson and others, 1994) and a smaller region in southern Maine at depths ranging from 60-90 m and 10-30 m, respectively (Figure 22). Bottom samples contain mostly sand and gravel, but minor amounts of mud also occur.

The Hard-Bottom Plain in southern Maine is associated with a large series of moraines and till deposits (Figure 8, Figure 9). During a time of lower sea level, waves removed gravel from the moraines and spread it around, creating the Hard-Bottom Plain. The large Hard-Bottom Plain in northeast Maine is also covered by a deposit of till, but it is thin (Figure 33) and largely occurs deeper than 60 m, the depth of the lowstand of the sea. The bedrock in this area is also extremely subdued in relief. It is possible that the rocks here are Mesozoic sandstones that are flat-lying (Ballard and Uchupi, 1975; Tagg and Uchupi, 1966) or easily eroded like those in the Bay of Fundy (Hutchinson and others, 1988). Glaciers may have smoothed them into the low-relief rock presently exposed. Strong tidal currents may be responsible for reworking the till and a lack of recent sediment accumulation over that material. Little observational data exist from this area.

Contents Introduction Methods Physiography Surficial Geology Geological History Summary Glossary References

Last updated on October 6, 2005