The Seafloor Revealed

Geological History

The geological history of Maine, between the time of bedrock formation and the start of the Ice Age, is not well known. For hundreds of millions of years prior to the Ice Ages, the Appalachian Mountains were eroded, mainly by fluvial processes. The landscape of Maine, both on and offshore, reflects that period of erosion. Topographic and bathymetric high points are generally erosion-resistant types of bedrock (Hanson and Caldwell, 1989). Weak areas within the bedrock, such as contacts between rock bodies, faults and folds, were etched out by erosive processes and are today marked by low areas like bays and coves. Linear bays like Oak Bay, Eggemoggin Reach, and Cutler Harbor clearly mark fault traces (Osberg and others, 1985) where fractures in bedrock were preferentially removed by erosive processes. The Shelf Valleys are also associated with weaknesses in bedrock (Kelley, 1993). The remarkable gorge at the mouth of Penobscot Bay (Figure 30), for example, appears to be developed on the trace of a bedrock fault (Osberg and others, 1985).

Multiple Pleistocene glaciations probably smoothed the landscape and seafloor without altering the overall bedrock influence on major landforms (Kelley, 1987). Thus, bays and peninsulas that existed before glaciation remained as bays and peninsulas after glaciation. The most important influence of glaciation was to introduce sediment to the inner shelf, and to alter the level of the sea and the focal point of coastal erosive and redistributive processes.

Melting glaciers reached the Maine coast around 14,000 years ago. Moraines, marking the position of the retreating ice, are well developed in many places. The largest field of moraines is located in the Wells Embayment in 20 to 50 m water depth (Figure 8, Figure 9; Miller, 1997). Here, the landforms are 5 to 10 m high, 10 to 50 m wide, and stretch for kilometers with occasional breaks. Their erosion has produced sand and gravel for the beaches of the embayment. Prominent moraines also exist near Prouts Neck (Kelley and others, 1989b), parallel to rock peninsulas in Casco Bay (Kelley and others, 1987b), in western Penobscot Bay (Kelley and Belknap, 1991; Knebel, 1986), at the entrance to Somes Sound (Barnhardt and Kelley, 1995; Figure 36), and within Gouldsboro and Machias Bays (Shipp, 1989; Belknap and others, 1987b). In all locations the moraines are mapped as a gravel-dominated bottom on the surficial maps (Barnhardt and others, 1996a-g) and are located in Rocky Zones (Figure 41) or Hard-Bottom Plains on the physiographic map (Kelley and others, 1997). Despite changes in sea level and early reworking by waves, the boulder-littered surfaces of the moraines have served as a protective armor against further erosion (Figure 36). Where the till deposits were thin, as in the Hard-Bottom Plains near the Canadian border, small bedrock outcrops often appear through the till.

Vast quantities of fine-grained, glacial-marine sediment were deposited along with the till. This well-bedded sediment blanketed the seafloor and still supports extensive areas of smooth bathymetry in the Nearshore and Outer Basins. During the fall in the level of the sea after about 11,000 yr B.P., the glacial-marine sediment was exposed to subaerial weathering processes and removed from high areas to lower locations. Rivers carved out channels in the glacial-marine sediment which were often later filled with gas-rich Holocene mud. Many of the Shelf Valleys were completely excavated during this time of emergence (Figure 42), and truncated seismic reflectors from the glacial-marine material are imaged along the margins of modern estuaries (Figure 7).

Some rivers, like the Saco, Kennebec, Narraguagus, and Penobscot, delivered large quantities of sandy and muddy sediment to the sea during the time of lower sea level (Kelley and others, 1992; Barnhardt, 1994; Barnhardt and others, 1997). The mud, plus fine-grained sediment reworked from glacial deposits, collected in water depths greater than 60 m. Where enough sediment existed, relief associated with rock outcrops was smoothed or eliminated by burial in the Outer Basins (Figure 43). The sediment supply to these areas has decreased as contemporary sea level rises and the source of sediment becomes distant.

Rising sea level has eroded bluffs of glacial sediment along the shoreline from 60 m depth to the present coast (Figure 2, Figure 4, Figure 5). This bluff-erosion process forms the transgressive unconformity, or erosion surface, on the top of the glacial-marine sediment. Where the unconformity is buried by mud, it forms a prominent seismic reflector (Figure 10), especially beneath muddy Nearshore Basins and in some Shelf Valleys (Figure 44).

Immediately seaward of the Merrimack, Saco, Kennebec, and Narraguagus Rivers, and a few other places where abundant quantities of sand existed during the rise of sea level, waves formed beaches. The beaches have migrated landward with rising sea level and left behind the gently seaward-sloping surface of the Nearshore Ramps (Figure 45). In most areas, sand on the Nearshore Ramps is only a thin surface veneer separated from glacial-marine sediment by the transgressive unconformity (Figure 26). The muddy glacial-marine sediment is commonly exposed just seaward of the shoreface (Barber, 1995; Kelley and others, 1995a).

Along the shoreline near Canada, and in a few other smaller areas, either no glacial-marine sediment was deposited or it was eroded and nothing else buried the bedrock and till-mantled seafloor. Here, the Hard-Bottom Plain is covered by a gravel and rock surface in a deep-water setting (Figure 46). It is possible that the strong tidal currents associated with the large tidal range near the Bay of Fundy have eroded older, muddy sediment and prevented new material from accumulating. The physiographic environments of the western Gulf of Maine have been shaped by a complex history of glaciation, sea level fluctuation, and modern processes - all of which have acted over an irregular foundation of ancient rocks. Rocky Zones, Shelf Valleys, Outer Basins, Nearshore Basins, Nearshore Ramps, and Hard-Bottom Plains have formed over thousands of years into distinct types of seafloor. Revealed in the work of marine geologists, these marine environments are the underwater landscapes or "seascapes" offshore of the Maine coast.

Contents Introduction Methods Physiography Surficial Geology Geological History Summary Glossary References

Last updated on October 6, 2005