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

Geological Interpretation of the Sea Floor Offshore of Edgartown, Massachusetts
Summary Title             PageList of             FiguresConversion             FactorsAbstractIntroductionSettingMethodsBathymetrySidescan             Sonar Seismic             Profiles SedimentsBottom             Photography Data             CatalogSummaryAcknowledgmentsReferences             Cited Contacts
Click on figures for larger           images.
Figure 19. Digital terrain model of the sea             floor produced from the single-beam and multibeam bathymetry             collected during National Oceanic and Atmospheric Administration             survey H11346 and gridded to 25 m.Figure 20. Sidescan-sonar imagery produced             from data collected during National Oceanic and Atmospheric             Administration survey H11346.Figure 26. Segment of Boomer             high-resolution seismic-reflection profile and interpretation from             RV Rafael cruise 08034 line 4 across the buried channel             that extends seaward beneath Edgartown Harbor.Figure 29. Locations of stations at which             sediment samples were collected during cruise 08034 of research             vessel Rafael to verify acoustic data.
The data and interpretations in this report are based on NOAA survey       H11346 completed during 2004 and two USGS verification cruises completed       during 2008. These combined data provide a fundamental framework for       research and management activities along this part of the Massachusetts       coastline, show the composition and terrain of the seabed, and provide       information on sediment transport and benthic habitat. 

Surveyed depths within the study area range from less than 4 m to       almost 20 m (fig. 19). The shallowest areas are along the shorelines       and on the elongate bathymetric high that extends northeastward from       Middle Flats toward the central part of the study area. This elongate       bathymetric high is flanked by depressions. To the southeast, an elongate       depression connects Edgartown Harbor with the deepest part of the study       area north-northwest of Cape Poge. Most gradients are relatively gentle       except in constricted areas, such as between the elongate bathymetric high       in the center of the study area and Cape Poge. There, tidal currents       maintain steeper slopes. 

Distinctive tonal patterns revealed on the sidescan-sonar mosaic can be       related to seabed features (fig. 20). Complex patches of high and low backscatter       with individual high-backscatter targets delineate the rocky, bouldery       areas. Bouldery areas on the mosaic are limited to the tip of the seaward       extension of Middle Flats. Areas characterized by alternating bands of       high and low backscatter include much of the western part of the study       area between Oak Bluffs and Middle Flats and small patches off Cape Poge       and the seaward extension of Middle Flats. This tiger-stripe pattern is       produced by bedforms like sand waves and megaripples. Areas characterized       by relatively high backscatter are concentrated along shorelines, on the       crests and flanks of bathymetric highs, and in depressions east and west       of the seaward extension of Middle Flats. Most of the areas of high       backscatter are produced by coarse-grained sediments, typically coarse       sand and gravelly sand; however, the areas of high backscatter east and       west of the seaward extension of Middle Flats are caused by shell beds       that produce strong acoustic backscatter. Patches characterized by       relatively low backscatter are in the inner part of Edgartown Harbor and       offshore in the deeper parts of the study area. These areas coincide with       areas protected from strong storm- and tide-driven currents. The low       backscatter is produced by fine-grained Holocene marine sediment. These       sediments are clayey silt in the inner harbor and fine sand in deeper       water. 

The seismic profiles provide insights into the stratigraphic framework       and Quaternary geologic history of the sea floor off Edgartown,       Massachusetts (fig. 26). The deepest seismic reflectors observed       beneath the study area are interpreted to be coastal-plain deposits of       Late Cretaceous to Tertiary age. Unconformably overlying the coastal-plain       strata are Pleistocene glacial deposits of subglacial till, outwash, and       glaciolacustrine sediments. Overlying the undifferentiated glacial drift       is a fluvially cut unconformity created by streams flowing generally       toward the northeast. Channels created by these streams, which are now       partly to mostly filled by estuarine and transitional deposits, are best       developed beneath Edgartown Harbor and offshore of Edgartown Beach, where       they coalesce into a dendritic pattern. A marine unconformity cut during       the last transgression separates the estuarine and transitional deposits       from the overlying modern marine deposits. These marine sediments are       thickest within the fluvially cut channels and other more protected areas       and thin or absent in more exposed areas, such as on the elongate       bathymetric high that extends seaward from Middle Flats. 

The surficial-sediment distribution is a product of the Quaternary       history and modern environmental conditions (fig. 29). Gravel and gravelly sand are in the       high-energy areas along exposed shorelines, such as off Cape Poge, and at       the tip of the elongated bathymetric high that extends northeastward from       Middle Flats. Sand is the dominant textural class of the sediment along       the more protected shorelines, such as off Edgartown Beach, along the       flanks of the elongated bathymetric high that extends northeastward from       middle flats, and in the offshore parts of the study area in Nantucket       Sound. Finer grained sediments are limited to deeper, more protected       areas. Patches of silty sand are both east and west of the elongated       bathymetric high that extends northeastward from Middle Flats. The finest       grained sediment is clayey silt, which dominates in Edgartown Harbor where       low-energy conditions characterized by sedimentary processes associated       with deposition prevail. 

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