Click on figures for larger images

Figure 19. Map showing the locations of sediment samples taken during U.S. Geological Survey cruise 2012-002-FA on the research vessel Rafael.

Figure 20. Illustration showing the maximum depth that is affected by orbital wave motion, called the wave base, as it is related to wavelength.

Sediment Distribution

Sea-floor sediments in the study area range from sand to gravelly sediment to gravel, which includes boulders (fig. 19). Sand is present in a narrow band along the coastal areas and in the deeper, offshore area along the southern edge of the study area. The majority of the sea floor in the central part of the study area is made up of gravel or gravelly sediment; bathymetry, sidescan-sonar, and bottom photography clearly indicate the presence of abundant boulders.

Several bottom sample stations were planned to compare surface sediment textures in scour depressions with those on the adjacent sea floor or an erosional outlier. Stations NBIS8 and NBIS9 are about 120 m apart from one another in the southern part of the study area; NBIS8 is on the modern marine sediment surrounding the scour depression where NBIS9 is located. While both samples are classified as sand, the sample from NBIS8 is much finer grained with a mean grain size of 2.56 phi, whereas the sample from NBIS9 has a mean grain size of 0.65 phi. NBIS11, another station on an erosional outlier of modern marine sediment, has a mean grain size of 2.68 phi. Unfortunately, we could not obtain a sample from NBIS10, in a nearby scour depression, but photographs from the station show coarser grained sediment similar to that found at NBIS9. Sediments in scour depressions tend to be coarser grained than those of the Holocene sediments on the surrounding sea floor and erosional outliers. Scour depressions are present throughout Block Island and Rhode Island Sounds (McMullen and others, 2007, 2011, 2012; Poppe and others, 2011, 2012). Based on studies of wave climate showing that large waves regularly impact the sea floor of Rhode Island Sound (Thompson, 1977; Grilli and others, 2010; Ullman and Codiga, 2010) and an earlier geologic study showing evidence of wave impacts (Knebel and others, 1982), we believe these features are formed from and maintained by vertical and horizontal stresses and changing pressure fields associated with the repeated passing of large, long-period storm waves. For example, orbital motion alone can resuspend and erode the sea floor down to the wave base, a depth equal to half the wavelength (fig. 20). Thus, the orbital motion from storm waves with 60-m wavelengths affects the sea floor to about 30-m depths where the deep scour depressions are within the study area. Once resuspended by storm waves, the sediment can then be transported away by the relatively weak tidal currents (Haight, 1942; White and White, 2012). The coarser areas of the sea floor also increase the water turbulence immediately above them, which probably inhibits finer grained sediment from being deposited (Murray and Thieler, 2004).

Sediment Data

The sediment grain-size dataset provided in this report contains information on the collection, description, and texture of sediments at the 26 stations in the study area. Sediments were sampled at only 11 of the stations due to either a coarse-grained sea floor or rough sea state that prevented a sample from being obtained. Stations where samples could not be obtained were classified based on video and still photography obtained at the station. These stations have no-data values (-9999) in the textural and statistical sections of the sediment dataset. The basic structure of the dataset is laid out in a matrix where rows represent individual samples and columns (fields) contain sample- and station-specific information. This matrix consists of 42 fields that are defined in the Data Dictionary section.

In the Geographic Information System Data Catalog section, a .zip file contains the sediment data in three formats (a shapefile, a Microsoft Excel spreadsheet, and a comma-delimited ASCII text file) along with metadata.

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 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 (north latitudes are positive values)

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

DEPTH_M - Depth of water overlying sediment at the time of sampling, measured by a hull-mounted fathometer, corrected for draft, but 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 mm; -1 phi and larger)

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

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

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

SEDCLASS - Sediment description based on a rigorous definition (Shepard, 1954; Schlee, 1973; Poppe and others, 2004)

For sediments with gravel equal to or greater than 10 percent:

GRAVEL – gravel equal to or greater than 50 percent

GRAVELLY SEDIMENT – gravel equal to or greater than 10 percent, but less than 50 percent

For sediments with gravel less than 10 percent:

SAND – sand equal to or greater than 75 percent

SILTY SAND - sand less than 75 percent and equal to or greater than 50 percent, silt is greater than clay, and clay less than 20 percent

CLAYEY SAND - sand less than 75 percent and equal to or greater than 50 percent, clay is greater than silt, and silt less than 20 percent

SILT - silt equal to or greater than 75 percent

SANDY SILT – silt less than 75 percent and equal to or greater than 50 percent, sand is greater than clay, and clay less than 20 percent

CLAYEY SILT - silt less than 75 percent and equal to or greater than 50 percent, clay is greater than sand, and sand less than 20 percent

CLAY – clay equal to or greater than 75 percent

SANDY CLAY - clay less than 75 percent and equal to or greater than 50 percent, sand is greater than silt, and silt less than 20 percent

SILTY CLAY - clay less than 75 percent and equal to or greater than 50 percent, silt is greater than sand, and sand less than 20 percent

SAND SILT CLAY – all of these components greater than 20 percent

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

KURTOSIS - Degree of curvature near the mode of the grain-size distribution

PHI_11 - Weight percent of the sample in the 11-phi fraction (nominal diameter of particles greater than or equal to 0.0005 mm but less than 0.001 mm); 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 mm but less than 0.002 mm); 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 mm but less than 0.004 mm); 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 mm but less than 0.008 mm); 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 mm but less than 0.016 mm); 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 mm but less than 0.031 mm); 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 mm but less than 0.0625 mm); coarse silt

PHI_4 - Weight percent of the sample in the 4-phi fraction (nominal diameter of particles greater than or equal to 0.0625 mm but less than 0.125 mm); 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 mm but less than 0.25 mm); 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 mm but less than 0.5 mm); 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 mm but less than 1 mm); coarse sand

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

PHIM1 - Weight percent of the sample in the -1-phi fraction (nominal diameter of particles greater than or equal to 2 mm but less than 4 mm); 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 mm but less than 8 mm); fine pebbles

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

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

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