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Sand and gravel are found in several geological settings on the northern New England inner shelf: shorefaces, lowstand shorelines, stratified moraines, and paleodeltas. (Table 1)(Kelley and others, 2003). Seaward of major beaches, shoreface-sand deposits represent a significant repository of material. Off Saco Bay, for example, 56 million m3 of sand line the shoreline (Kelley and others, 2003, 2005). This sand is geologically linked to the adjacent beaches, however, and is not an appropriate source for replenishment.
A considerable body of geologic literature exists for Saco Bay, in part because it contains the largest beach in Maine (Kelley and others, 1986a, 1989a). Beginning in the late 1980's, cooperative work with the MMS led to several publications that define outer bay sand bodies with seismic reflection and bottom sample observations (Kelley and others, 1986b, 1989b). Vibracores added ground truth to the remotely sensed data and led to revisions of sand abundance (Kelley and others, 1990, 1992, 1995). Insight gained from this work on inner shelf sand bodies led to some more general discussions on offshore sand in Maine (Kelley and others, 1989b, 1998, 2003) and specifically sand offshore Saco Bay (Kelley and others, 2005).
Several problems became apparent during early investigations. Although lowstand deltas are known from other large rivers in the region (Barnhardt and others, 1997; Belknap and others, 2005), none has yet been found off the Saco River in outer Saco Bay (Kelley and others, 2003). In addition, sand deposits observed in seismic records and vibracores from the outer bay are all relatively thin (<2 m), and many shorelines are erosional features (Shipp and others, 1991; Kelley and others, 2003). During the Years 1 and 2 Cooperative projects described herein, our focus was directed at locating a lowstand delta and thicker lowstand shoreline deposits in an area of the outer bay that had received little prior work.
Paleozoic bedrock crops on the headlands framing Saco Bay and is recognized as acoustic basement in seismic reflection profiles (Kelley and others, 1998). Rocky islands exist in the central part of the bay and an extensive shallow area surrounds the islands and is supported by near-surface bedrock. The bedrock consists mostly of metamorphic rocks that trend in a northeast direction (Osberg and others, 1985). A Carboniferous granite crops out just south of Saco Bay.
Bedrock locally exhibits several meters of relief over tens of horizontal meters. Numerous fractures exist in bedrock on land and are distinctive features on side-scan sonar records (Kelley and others, 1998).
Till and glacial-marine muddy sediment locally bury bedrock. Glacial sediment was deposited by melting ice approximately 14,000 radiocarbon years ago (Borns and others, 2004) when local, relative sea level was approximately 75 m above present level (Barnhardt and others, 1997). Till represents a strong acoustic reflector that often resembles bedrock (Barnhardt and others, 1997). Glacial-marine sediment possesses strong and coherent acoustic reflectors that extend for hundreds of meters or more. The reflectors are draped over the underlying topographic elements. In water depths less than about 65 m, the glacial sediment is often eroded where it was apparently exposed to wave action at the sea-level lowstand between approximately 11,000 radiocarbon years ago and present. It was during the time of the lowstand of the sea that large rivers deposited significant bodies of sand in present water depths up to 70 m (Kelley and others, 2003). As sea level rose to the present time, sand has continued to be added to the inner shelf even as older deposits are reworked. The present seafloor sediment is, thus, modern close to shore, and palimpsest or relict in depths between about 30 m and 70 m.
Last updated on November 1, 2006
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