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U.S. Geological Survey Open-File Report 2009-1001

Geological Interpretation of the Sea Floor Offshore of Edgartown, Massachusetts
Setting Title             PageList of             FiguresConversion             FactorsAbstractIntroductionSettingMethodsBathymetrySidescan             Sonar Seismic             Profiles SedimentsBottom             Photography Data             CatalogSummaryAcknowledgmentsReferences             Cited Contacts
Click on figures for larger           images.
Figure 3. Regional paleogeographic map             showing the extent of the Laurentide ice sheet over Cape Cod and the             islands about 20–22 thousand years ago (ka).Figure 4. Regional paleogeographic map of             Cape Cod and the islands just before 18 thousand years ago         (ka).
The configuration and geology of Nantucket Sound, Cape Cod, and the       adjacent islands are products of glacial processes during the late       Pleistocene and a storm- and tide-dominated marine regime during the       Holocene sea-level rise. The late Wisconsinan Laurentide ice sheet reached       its maximum extent and southernmost position about 20 to 22 thousand years       ago, as marked by terminal moraines on Block Island, Martha's Vineyard,       and Nantucket and the southern limit of abundant gravel on the intervening       sea floor (Schlee and Pratt, 1970; Oldale and O'Hara, 1984; Uchupi and       others, 1996; fig. 3). This lobate ice front was formed by the       Buzzards Bay lobe that deposited the moraine along the western part of       Martha's Vineyard, the Cape Cod Bay lobe that deposited the moraines       across eastern Martha's Vineyard and Nantucket, and the South Channel lobe       that extended east toward Georges Bank (Oldale and Barlow, 1986; Oldale,       2001).

Shortly before 18 thousand years ago, the ice sheet had retreated, and       the Buzzards Bay lobe occupied a position along the Elizabeth Islands. The       Cape Cod Bay lobe was along the northern edge of Cape Cod, and the South       Channel lobe was adjacent to the Cape Cod Bay lobe (Larson, 1982; Uchupi       and others, 1996; fig.       4). About this time, meltwaters from the Cape Cod bay lobe were dammed       by the terminal moraine, forming a large proglacial lake in Nantucket       Sound, and the main part of Cape Cod, the Barnstable outwash plain, was       built by a delta prograding into this proglacial lake. Subsequent collapse       of the dam allowed the proglacial lake to drain, exposing the lake-floor       sediments to subaerial erosion. During this time and prior to sea-level       rise, the glacial drift beneath the study area was eroded by streams that       flowed northeastward toward the dendritic fluvial system that drains the       area beneath present day Nantucket Sound (O'Hara and Oldale, 1987; Oldale,       2001; Uchupi and Mulligan, 2006). Initially during Holocene submergence,       the postglacial valleys were partly filled by estuarine, marsh, and other       transitional deposits (Gutierrez and others, 2003). Eventually, the       transgressing seas covered, reworked, and smoothed the sea floor and       marine sediments accumulated over the wave-cut surface. Shallow       stratigraphy interpreted from high-resolution seismic profiles, collected       as part of this and an earlier study (O'Hara and Oldale, 1987), reflects       this Quaternary history.

Strong tidal currents in Nantucket Sound are driven by the different       filling and flushing rates and sizes of the adjacent water bodies. For       example, the ebb tide off East Chop flows west and exceeds 2.1 kts (1.08       m/s); the flood tide adjacent to Cape Poge flows north and exceeds 2.2 kts       (1.13 m/s) (Eldridge-White and Foster-White, 2008). Although the nearshore       parts of the survey area are partially protected, they are shallow and       above wave base. 

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