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

Geological Interpretation of the Sea Floor Offshore of Edgartown, Massachusetts
Sediments Title             PageList of             FiguresConversion             FactorsAbstractIntroductionSettingMethodsBathymetrySidescan             Sonar Seismic             Profiles SedimentsBottom             Photography Data             CatalogSummaryAcknowledgmentsReferences             Cited Contacts
Click on figures for larger           images.
Figure 15. Locations of stations at which             bottom samples and photographs were taken during cruise 08012 of             research vessel Rafael to verify acoustic data. Figure 21. Interpretation of the sea floor             surveyed in National Oceanic and Atmospheric Administration survey             H11346.Figure 29. Locations of stations at which             sediment samples were collected during cruise 08034 of research             vessel Rafael to verify acoustic data.Figure 30. Sessile fauna and flora on the             bottom at station 18.Figure 31. The flat, sandy bottom at             station E9.Figure 32. The bottom at station E31.Figure 33. The bottom at station E26.Figure 34. The dense slippersnail             (Crepidula) shell bed on the bottom at station E16 off the             tip of the seaward extension of Middle Flats.Figure 35. The slippersnail             (Crepidula) shell bed on the bottom at station E22             northeast of the tip of the seaward extension of Middle Flats.Figure 36. The slippersnail             (Crepidula) shell bed on the bottom at station E10             northwest of the tip of the seaward extension of Middle Flats.Figure 37. The dense stands of seaweed that             grow on the flanks and crest of the seaward extension of Middle             Flats at station E17.Figure 38. The dense stands of seaweed that             grow on the flanks and crest of the seaward extension of Middle             Flats at station E13. 
Sediment Distribution

Gravel and gravelly sand are in the study area along exposed       shorelines, such as off Cape Poge and at the tip of the elongated       bathymetric high that extends northeastward from Middle Flats, where       scattered boulders were also observed in the bottom video (fig. 29). These are shallow, high-energy areas       characterized by sedimentary processes associated with erosion and       nondeposition. The Holocene section in these areas is thin or absent,       suggesting that finer grained sediments have been winnowed away leaving       coarser lag deposits. Boulders, where present, are typically covered by       sessile flora and fauna (fig. 30).

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. Medium and       fine-grained sand dominates the more protected areas (for example,       stations E5 and E7) and the deeper waters offshore (for example, stations       E9 and E31). The sea floor in the offshore sandy areas is typically flat       or only faintly rippled and burrowed (figs. 31, 32). Coarse and very coarse sand dominate bathymetric       highs (for example, station E17) and exposed offshore areas (for example,       stations E34 and E35). Sorting of the sandy sediments is generally poor,       but improves on bathymetric highs and off Cape Poge, where tidal- and       wave-driven currents rework the sediments.

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 (fig. 33).

Shell beds are common in the offshore part of the study area (fig. 21). Tidal and wave currents there are strong       enough to sweep the shells off bathymetric highs, such as the seaward       extension of Middle Flats, and to hydraulically concentrate the shells in       adjacent, more protected bathymetric lows, notably at stations E16 and E22       (figs. 34, 35, 36). These beds are typically thin and presumably       ephemeral. Although mussel, scallop, and whelk shells locally constitute a       significant portion of the shell debris, the densest beds are composed       predominantly of slippersnail (Crepidula) shells. These shell       beds are ecologically important because they form sea-floor habitats of       rough, relatively hard substrates that add considerably to the overall       benthic compositional complexity. Bottom photography shows that infauna       construct burrows in the shell deposits, juvenile finfish shelter between       the shells, and sessile fauna and flora attach themselves to the hard       substrate provided by the shells.

Dense beds of seaweed are on the crest and upper flanks of the seaward       extension of Middle Flats (figs. 37, 38). Conditions on this bathymetric high are probably       favorable for seaweed growth because the coarser sediments there provide a       more stable substrate, and because the shallow depths and lower turbidity,       owing to the offshore location, permit greater light penetration.       Elsewhere, bottom photography reveals that mats of dead drifting seaweed       collect in lower energy depressions and that pebbles with attached seaweed       are dragged along in the tidal currents. These seaweed beds are       ecologically important because they also add dramatically to the overall       benthic roughness.

Sediment Data 

The sediment grain-size dataset provided here contains information on       the collection, location, description, and texture of sediments at 37       stations occupied during the 2008 RV Rafael verification cruise       08012 (figs. 15, 29). All analyses were conducted in the sedimentation       laboratory at the USGS Woods Hole Coastal and Marine Science Center.       Records without textural data and statistics are based on visual       descriptions. The basic structure of the data is a flat-file format, a       matrix where records are rows representing individual samples and the       columns contain sample- and station-specific information. This matrix       consists of 42 fields which are defined in the Data Dictionary below.

The sediment data are provided in two formats: Microsoft Excel and       delimited ASCII text format. In the delimited ASCII text file, each field       or column of data is separated from the next by commas and can be       downloaded into many types of software.

raf08012_seddata.xls

raf08012_seddata.txt

Data Dictionary

An integral part of any database is the dictionary that explains the       structure and content. It contains a list of the fields and the       definitions of parameters measured. Data utilization is facilitated by       reference to this compilation because it defines abbreviations and lists       field names. 

LABNO - Unique sample identifier assigned in the laboratory 

STATIONID - Sample name or number assigned in the field 

PROJECT - Project under which samples were taken or data generated 

CRUISEID - Name or number of cruise on which sample was collected or       station occupied 

PRINCIPAL - Name of principal investigator 

LATITUDE - Latitude in decimal degrees 

LONGITUDE - Longitude in decimal degrees (west longitudes are negative       values) 

DEPTH_M - Depth of water measured by a hull-mounted fathometer       overlying sediment at the time of sampling, not corrected for tides, in       meters 

T_DEPTH - Top depth of the sample below the sediment-water interface,       in centimeters 

B_DEPTH - Bottom depth of the sample below the sediment-water       interface, in centimeters

DEVICE - Device used to collect the sample 

MONTH - Number of calendar month during which the sample was collected       

DAY - Calendar day on which the sample was collected 

YEAR - Calendar year during which the sample was collected 

WEIGHT - Dry weight of sample, in grams 

ZGRAVEL - Gravel content in percent dry weight of the sample (particles       with nominal diameters greater than 2 millimeters; -1 phi and larger) 

ZSAND - Sand content in percent dry weight of the sample (particles       with nominal diameters less than 2 millimeters but greater than or equal       to 0.0625 millimeters; 0 through 4 phi) 

ZSILT - Silt content in percent dry weight of the sample (particles       with nominal diameters less than 0.0625 millimeters but greater than or       equal to 0.004 millimeters; 5 through 8 phi, inclusive) 

ZCLAY - Clay content in percent dry weight of the sample (particles       with nominal diameters less than 0.004 millimeters; 9 phi and smaller) 

SEDCLASS - Sediment description based on a rigorous definition       (Shepard, 1954) 

MEDIAN - Middle point in the grain-size distribution, in phi units 

MEAN - Average value in the grain-size distribution, in phi units 

STDDEV - Standard deviation (root mean square of the deviations) of the       grain-size distribution, in phi units (that is, sorting) 

SKEWNESS - Deviation from symmetrical form of the grain-size       distribution, in phi units 

KURTOSIS - Degree of curvature near the mode of the grain-size       distribution, in phi units 

PHI _11 - Weight percent of the sample in the 11 phi fraction (nominal       diameter of particles greater than or equal to 0.0005 millimeters but less       than 0.001 millimeters); fine clay 

PHI_10 - Weight percent of the sample in the 10 phi fraction (nominal       diameter of particles greater than or equal to 0.001 millimeters but less       than 0.002 millimeters); medium clay 

PHI_9 - Weight percent of the sample in the 9 phi fraction (nominal       diameter of particles greater than or equal to 0.002 millimeters but less       than 0.004 millimeters); coarse clay 

PHI_8 - Weight percent of the sample in the 8 phi fraction (nominal       diameter of particles greater than or equal to 0.004 millimeters but less       than 0.008 millimeters); very fine silt 

PHI_7 - Weight percent of the sample in the 7 phi fraction (nominal       diameter of particles greater than or equal to 0.008 millimeters but less       than 0.016 millimeters); fine silt 

PHI_6 - Weight percent of the sample in the 6 phi fraction (nominal       diameter of particles greater than or equal to 0.016 millimeters but less       than 0.031 millimeters); medium silt 

PHI_5 - Weight percent of the sample in the 5 phi fraction (nominal       diameter of particles greater than or equal to 0.031 millimeters but less       than 0.0625 millimeters); coarse silt 

PHI_4 - Weight percent of the sample in the 4 phi fraction (nominal       diameters of particles greater than or equal to .0625 millimeters but less       than 0.125 millimeters); very fine sand 

PHI_3 - Weight percent of the sample in the 3 phi fraction (nominal       diameter of particles greater than or equal to 0.125 millimeters but less       than 0.25 millimeters); fine sand 

PHI_2 - Weight percent of the sample in the 2 phi fraction (nominal       diameter of particles greater than or equal to 0.25 millimeters but less       than 0.5 millimeters); medium sand 

PHI_1 - Weight percent of the sample in the 1 phi fraction (nominal       diameter of particles greater than or equal to 0.5 millimeters but less       than 1 millimeters); coarse sand 

PHI_0 - Weight percent of the sample in the 0 phi fraction (nominal       diameters of particles greater than or equal to 1 millimeters but less       than 2 millimeters); very coarse sand 

PHIM1 - Weight percent of the sample in the -1 phi fraction (nominal       diameter of particles greater than or equal to 2 millimeters but less than       4 millimeters); very fine pebbles (granules) 

PHIM2 - Weight percent of the sample in the -2 phi fraction (nominal       diameter of particles greater than or equal to 4 millimeters but less than       8 millimeters); fine pebbles 

PHIM3 - Weight percent of the sample in the -3 phi fraction (nominal       diameter of particles greater than or equal to 8 millimeters but less than       16 millimeters); medium pebbles 

PHIM4 - Weight percent of the sample in the -4 phi fraction (nominal       diameter of particles greater than or equal to 16 millimeters but less       than 32 millimeters); coarse pebbles 

PHIM5 - Weight percent of the sample in the -5 phi fraction (nominal       diameter of particles greater than or equal to 32 millimeters); very       coarse pebbles to boulders 

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