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U.S. Geological Survey Open-File Report 2010–1150

Sea-Floor Geology and Topography Offshore in Eastern Long Island Sound


Sediment Distribution

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Click on figures for larger images
Thumbnail image of figure 24 and link to larger figure. A map of sedimentary environments in the study area.
Figure 24. Distribution of sedimentary environments based on the digital terrain model from National Oceanic and Atmospheric Administration survey H11997 and verification data from U.S. Geological Survey cruise 2010-015-FA.
Thumbnail image of figure 25 and link to larger figure. A map of station locations in the study area.
Figure 25. Station locations from U.S. Geological Survey cruise 2010-015-FA used to verify the acoustic data from National Oceanic and Atmospheric Administration survey H11997, color-coded for sediment texture. Hotter colors are coarser grained sediment; cooler colors are finer grained sediments.
Thumbnail image of figure 26 and link to larger figure. Photographs of hydroids in the study area.
Figure 26. Hydroids and sponges covering boulders at stations 997-1 and 997-24, respectively.
Thumbnail image of figure 27 and link to larger figure. Photographs of gravel in the study area.
Figure 27. Gravel armors the sea floor in the high-energy environments at stations 997-13 and 997-26 and throughout much of the study area.
Thumbnail image of figure 28 and link to larger figure. Photographs of current-rippled sand in the study area.
Figure 28. Bottom photographs from stations 997-8 and 997-10 of current-rippled sand that is prevalent in areas characterized by sedimentary environments of coarse-bedload transport.

The funnel-shaped geometry of eastern Long Island Sound constricts tidal flow, producing bottom currents of increased strength (Signell and others, 2000), coarse-grained sediments (Poppe and others, 2000), and sedimentary environments characterized by processes associated with erosion or non-deposition (fig. 24; Knebel and Poppe, 2000). Gravel, which is all sediment greater than 2 mm in nominal diameter including boulders, is the dominant surficial sediment on exposed bathymetric highs and submerged sections of the Harbor Hill-Roanoke Point-Fishers Island moraine and in scour depressions and constricted areas (fig. 25). Boulders, which are present as lag deposits on bedrock outcrops and the winnowed surfaces of moraines, are typically covered by sessile fauna (fig. 26), suggesting that they are immobile even during severe storms. The sessile fauna is ecologically important because it adds to the overall benthic roughness and commonly includes sponges, hydroids, bushy hydrozoans, and anemones. Finer grained gravels (that is, pea and pebble gravels) and gravelly sediment dominate on relatively flat areas of sea floor throughout the study area (fig. 27). These sediments armor the sea floor and limit erosion of the underlying finer grained sediment. Hydrozoans grow on the larger gravel, and scour features are often present around these obstructions. The Holocene section in the eastern part of the study area is thin or absent (Lewis and Needell, 1987; Poppe and others, 2002), suggesting that finer grained sediments have been winnowed away leaving coarser lag deposits.

Tidal-current speed decreases westward as the sound widens, and sedimentary environments characterized by processes associated with coarse bedload transport and sandy sediments become more prevalent (fig. 24; Knebel and Poppe, 2000; Poppe and others, 2000; Signell and others, 2000). Sand is much more common in the western part of the study area, where much of the sea floor is covered with sand waves and megaripples, and the stoss slopes of these bedforms are covered in current ripples (figs. 25, 28). The sand is generally moderately to poorly sorted regardless of location; shell hash, organic debris, and coarser sediments are commonly concentrated in the ripple troughs. The Holocene section in the western part of the study area is thicker (Lewis and Needell, 1987; Poppe and others, 2002), suggesting that erosion is less prevalent there.

Sediment Data

The sediment grain-size dataset provided here contains information on the collection, location, description, and texture of sediments at 28 stations occupied during the USGS 2010 OSV Bold verification cruise 2010-015-FA (fig. 25). 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 three formats: ESRI shapefile, 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. These files are available through the Data Catalog section of this report.

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 or equal to 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 millimeter; 0 through 4 phi)

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

ZCLAY - Clay content in percent dry weight of the sample (particles with nominal diameters less than 0.004 millimeter; 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

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 millimeter but less than 0.001 millimeter); 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 millimeter but less than 0.002 millimeter); 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 millimeter but less than 0.004 millimeter); 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 millimeter but less than 0.008 millimeter); 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 millimeter but less than 0.016 millimeter); 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 millimeter but less than 0.031 millimeter); 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 millimeter but less than 0.0625 millimeter); coarse silt

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

PHI_0 - Weight percent of the sample in the 0 phi fraction (nominal diameters of particles greater than or equal to 1 millimeter 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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