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

Geological Interpretation of the Sea Floor Offshore of Edgartown, Massachusetts
Seismic Profiles Title             PageList of             FiguresConversion             FactorsAbstractIntroductionSettingMethodsBathymetrySidescan             Sonar Seismic             Profiles SedimentsBottom             Photography Data             CatalogSummaryAcknowledgmentsReferences             Cited Contacts
The seismic profiles acquired during the 2008 RV Rafael cruise       08034 (figs. 12, 13) provide insights into the stratigraphic framework       and Quaternary geologic history of the sea floor off Edgartown,       Massachusetts. Four sedimentary units are inferred from the subbottom       profiles: coastal-plain deposits, glacial drift, estuarine and       transitional deposits, and marine deposits. The deepest seismic reflectors       observed beneath the study area are interpreted to be coastal-plain       deposits of Late Cretaceous to Tertiary age. These reflectors are most       conspicuous beneath the elongate bathymetric high that extends seaward       from Middle Flats (fig. 25). On the basis of earlier interpretations of       seismic data (O'Hara and Oldale, 1987), cuttings recovered from an onshore       borehole (Hall and others, 1980), and outcrops on western Martha's       Vineyard (Skehan, 2001), these deposits are inferred to consist       predominantly of unconsolidated to semi-consolidated sand and silty clay.       Although not resolved by our shallow high-resolution seismics, these       deposits overlie a bedrock surface reported to be composed of       metamorphosed rocks of the Avalon or Meguma terranes (Oldale, 1969;       Hallett and others, 2004). Unconformably overlying the coastal-plain       strata are glacial-drift deposits of Pleistocene age (figs. 25, 26, 27, 28). Although the seismic profiles collected as part of       this study did not permit regional differentiation of these deposits,       stratigraphic position and seismic character suggest local deposits of       subglacial till, outwash, and glaciolacustrine sediments. The till is       characterized by hard reflectors that mimic the underlying unconformity.       The outwash deposits are characterized by flat-lying and gently dipping       reflectors put down under fluvial and deltaic environments. The       glaciolacustrine deposits are characterized by finely laminated rhythmic       reflectors, which we interpret to be varved, muddy sediments put down in       proglacial lakes. 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 (figs. 21, 26, 27). The seismic signal in the channel beneath Edgartown       Harbor is locally attenuated by the presence of gas (figs. 26, 28). This gas is probably biogenic methane and suggests       that the estuarine and transitional deposits there contain organic-rich       sediments. Although small and patchy nearshore, areas covered by the gassy       sediments expand offshore, obscuring much of the underlying stratigraphy       within this channel. 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 (fig.       2). 

The seismic profiles collected during RV Rafael cruise 08034       (figs. 12, 13) can be accessed through the tabular data listing       below.  To view the seismic-reflection profiles through the data       listing, click on the line number in the first column to display a       down-sampled Graphics Interchange Format (GIF) image of that seismic line.       The image will be displayed in a new browser window. All preview images       have been reduced to 72 dpi and a maximum width of 650 pixels for optimum       viewing with a web browser.  Because the original sizes of the       preview images varied, please note that the images are not the same scale.       To access a full-resolution image of the seismic profile in either Joint       Photographic Experts Group (JPEG) or SEG-Y format, the user is advised to       download the desired image, exit their browser, and open the file in an       appropriate application (such as Corel Draw or Adobe Illustrator for a       JPEG file). Instructions for downloading the full-resolution images follow       the data listing. 

Boomer line 

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2

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4

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5

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3

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5

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9

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0009_2008_232_1228_3.5khz.sgy

10

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11

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12

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15

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16

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0016_2008_233_0923_3.5khz.sgy

17

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18

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0018_2008_233_0950_3.5khz.sgy

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Click on figures for larger           images.
Figure 2. Edgartown Harbor, the boundary of             the study area (red line), and local physiographic features.Figure 12. The locations of the tracklines             (blue lines) along which the 5 Boomer high-resolution             seismic-reflection profiles were collected during cruise 08034 of             research vessel Rafael.Figure 13. The locations of the tracklines             (green lines) along which the 16 Chirp high-resolution             seismic-reflection profiles were collected during the August 2008             cruise 08034 of the research vessel Rafael.Figure 21. Interpretation of the sea floor             surveyed in National Oceanic and Atmospheric Administration survey             H11346. Figure 25. Segment of Boomer             high-resolution seismic-reflection profile and interpretation from             research vessel Rafael cruise 08034 line 4 across the             elongate bathymetric high that extends seaward from Middle         Flats.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 27. Segment of Chirp high-resolution             seismic-reflection profile and interpretation from research vessel             Rafael cruise 08034 line 3 offshore of Edgartown         Beach.Figure 28. Segment of Chirp high-resolution             seismic-reflection profile and interpretation from research vessel             Rafael cruise 08034 line 18 across the buried channel that             extends seaward beneath Edgartown Harbor.   
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