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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

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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

Full-Resolution (JPEG)

Full-Resolution (SEG-Y)

1

line_0001.jpg

line_0001.sgy

2

line_0002.jpg

line_0002.sgy

3

line_0003.jpg

line_0003.sgy

4

line_0004.jpg

line_0004.sgy

5

line_0005.jpg

line_0005.sgy


CHIRP line

Full-Resolution (JPEG)

Full-Resolution (SEG-Y)

3

0003_2008_232_0951_3.5khz.jpg

0003_2008_232_0951_3.5khz.sgy

4

0004_2008_232_1020_3.5khz.jpg

0004_2008_232_1020_3.5khz.sgy

5

0005_2008_232_1029_3.5khz.jpg

0005_2008_232_1029_3.5khz.sgy

6

0006_2008_232_1053_3.5khz.jpg

0006_2008_232_1053_3.5khz.sgy

7

0007_2008_232_1110_3.5khz.jpg

0007_2008_232_1110_3.5khz.sgy

8

0008_2008_232_1146_3.5khz.jpg

0008_2008_232_1146_3.5khz.sgy

9

0009_2008_232_1228_3.5khz.jpg

0009_2008_232_1228_3.5khz.sgy

10

0010_2008_232_1247_3.5khz.jpg

0010_2008_232_1247_3.5khz.sgy

11

0011_2008_232_1303_3.5khz.jpg

0011_2008_232_1303_3.5khz.sgy

12

0012_2008_232_1330_3.5khz.jpg

0012_2008_232_1330_3.5khz.sgy

13

0013_2008_232_1339_3.5khz.jpg

0013_2008_232_1339_3.5khz.sgy

14

0014_2008_232_1439_3.5khz.jpg

0014_2008_232_1439_3.5khz.sgy

15

0015_2008_232_1449_3.5khz.jpg

0015_2008_232_1449_3.5khz.sgy

16

0016_2008_233_0923_3.5khz.jpg

0016_2008_233_0923_3.5khz.sgy

17

0017_2008_233_0940_3.5khz.jpg

0017_2008_233_0940_3.5khz.sgy

18

0018_2008_233_0950_3.5khz.jpg

0018_2008_233_0950_3.5khz.sgy

Downloading with Internet Explorer, Firefox, or Safari

To download a data file while using Internet Explorer, Firefox, or Safari, the user should select the desired file by placing the system pointer on the file name from the data list and pressing the right mouse button. This will display a pulldown menu from which the user should drag the mouse to highlight "Save Target As ..." and release the mouse button. Explorer will then prompt the user to specify the name and location of where the selected data file should be saved.

Click on figures for larger images.

Thumbnail image of Figure 2, index map of Edgartown Harbor, and link to larger figure.
Figure 2. Edgartown Harbor, the boundary of the study area (red line), and local physiographic features.
Thumbnail image of Figure 12, map of Boomer seismic-reflection profile tracklines, and link to larger figure.
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.
Thumbnail image of Figure 13, map of Chirp seismic-reflection tracklines, and link to larger figure.
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.
Thumbnail image of Figure 21, map showing interpretation of the sea floor within the survey area, and link to larger figure.
Figure 21. Interpretation of the sea floor surveyed in National Oceanic and Atmospheric Administration survey H11346.
Thumbnail image of Figure 25, seismic-reflection profile across the elongate bathymetric high that extends seaward from Middle Flats and interpretation, and link to larger figure.
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.
Thumbnail image of Figure 26, seismic-reflection profile across the buried channel that extends seaward beneath Edgartown Harbor and interpretation, and link to larger figure.
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.
Thumbnail image of Figure 27, seismic-reflection profile and interpretation from offshore of Edgartown Beach, and link to larger figure.
Figure 27. Segment of Chirp high-resolution seismic-reflection profile and interpretation from research vessel Rafael cruise 08034 line 3 offshore of Edgartown Beach.
Thumbnail image of Figure 28, seismic-reflection profile and interpretation across the buried channel that extends seaward beneath Edgartown Harbor, and link to larger figure.
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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