U.S. Geological Survey Open-File Report 2011–1004
Sea-Floor Geology and Character of Eastern Rhode Island Sound West of Gay Head, Massachusetts
The study area lies in Rhode Island Sound, located offshore of Rhode Island and southeastern Massachusetts. Rhode Island Sound, which covers an area of over 2,500 km², is bordered on the west by Block Island, R.I., and on the east by Martha's Vineyard, Mass. (fig. 2). The sedimentary section beneath Rhode Island Sound overlies a south-southeasterly dipping basement of mostly pre-Mesozoic gneiss and schist. Overlying the metamorphic bedrock are late Cretaceous - Tertiary coastal plain sediments in the southern part of the sound and Pleistocene glacial drift and Holocene fluvial, estuarine, and transitional sediments throughout Rhode Island Sound (figs. 3 and 4; McMaster and others, 1968; O'Hara and Oldale, 1980; Needell and others, 1983a). The coastal plain sediments, which contain deeply eroded channels, form a cuesta across southern Rhode Island Sound (McMaster and others, 1968; McMaster and Ashraf, 1973). These coastal plain sediments were also penetrated by a well drilled on Martha's Vineyard (Hall and others, 1980).
Two glacial advances across Rhode Island Sound are recorded by the Ronkonkoma-Nantucket terminal end moraine and the Harbor Hill-Roanoke Point-Charlestown-Buzzards Bay recessional end moraine, and their submerged sections are evidenced by bands of gravelly sediment and submarine ridges (fig. 2; McMaster, 1960; McMaster and others, 1968; Schlee and Pratt, 1970). The Ronkonkoma-Nantucket moraine, which marks the maximum extent of the Laurentide Ice Sheet at around 20-24 ka (Uchupi and others, 1996), dislocated and thrust faulted the coastal plain deposits. The Gay Head cliffs on western Martha's Vineyard provide an exposed example of the disturbed coastal plain deposits (fig. 5). The Harbor Hill-Roanoke Point-Charlestown-Buzzards Bay moraine, which partially underlies the northwestern part of the study area, records a readvance at about 18 ka during the overall retreat of the Laurentide Ice Sheet (Larson, 1982; Oldale and O'Hara, 1984; Oldale and Barlow, 1986; Uchupi and others, 1996; Oldale, 2001). Where exposed on the sea floor, the moraines beneath Rhode Island Sound are capped by a lag deposit of gravel and boulders resulting from winnowing of the till by marine processes (fig. 4; O'Hara and Oldale, 1980).
During glacial recession, proglacial lakes formed behind the end moraines (Needell and others, 1983b), and outwash sediments deposited in the lakes added to the total thickness of glacial drift that in places now exceeds 40 meters (m) (O'Hara and Oldale, 1980). During and after lake drainage, while sea level was near lowstand, rivers cut drainage channels into the glacial drift, often in the locations of previously eroded valleys (McMaster and Ashraf, 1973; Needell and others, 1983b). A late Pleistocene fluvial unconformity, which underlies much of the study area, marks this erosional episode (fig. 3). As sea level rose, estuarine and transitional sediments were deposited, infilling many of the depressions left by the late Pleistocene fluvial erosion, and, as the transgression migrated across the shelf, the unconformity that now separates the Holocene marine sediments from the underlying older deposits was created (figs. 3 and 4; O'Hara and Oladale, 1980; Needell and others, 1983b).