Click on figures for larger images

Figure 14. Map showing the station locations used to verify the acoustic data with bottom sampling and photography in Block Island Sound during U.S. Geological Survey cruise 2011-006-FA.

Figure 19. Detailed planar view of the combined National Oceanic and Atmospheric Administration surveys in Block Island Sound showing the bouldery sea floor off the eastern shore of Block Island and the location of station BIS83.

Figure 21. Detailed planar of view the combined National Oceanic and Atmospheric Administration surveys in Block Island Sound showing a submerged section of the Ronkonkoma-Nantucket end moraine, adjacent sand waves, and the locations of stations BIS34 and BIS35.

Figure 22. Detailed planar view of the combined National Oceanic and Atmospheric Administration surveys in Block Island Sound showing the bouldery sea floor southeast of Point Judith and the location of station BIS41.

Figure 37. Map showing the station locations within Block Island Sound collected during U.S. Geological Survey cruise 2011-006-FA that were used to verify the acoustic data, color coded for sediment texture.

Figure 38. Distribution of sedimentary environments based on the digital terrain model from combined National Oceanic and Atmospheric Administration (NOAA) surveys H12009, H12010, H12011, H12015, H12033, H12137, and H12139; unpublished NOAA survey H12299 located adjacent to the southwestern corner of the (DTM); and the sampling and photography data from U.S. Geological Survey cruise 2011-006-FA that were used to verify the acoustic data.

Figure 39. Bottom photographs from stations BIS25 and BIS10 showing the sessile fauna that commonly cover boulders in Block Island Sound.

Figure 40. Bottom photographs from station BIS86 showing the sessile flora that commonly cover boulders in high-energy shallow-water environments within Block Island Sound.

Figure 41. Bottom photographs from stations BIS73 and BIS48 showing the gravel sea floor that commonly surrounds bouldery areas and armors the sea floor in higher energy sedimentary environments.

Figure 42. Bottom photographs showing a comparison of the rippled gravelly sand in a scour depression at station BIS85 with the adjacent undulating, muddy sand outside the scour depression at the same station.

Figure 43. Bottom photographs from stations BIS6 and BIS8 showing views of current-rippled sea floor composed of coarse to medium sand.

Figure 44. Bottom photographs from stations BIS3 and BIS5 showing views of faintly rippled to flat sea floor composed of fine-grained sand.

Figure 45. Bottom photographs from stations BIS22 and BIS28 showing views of shell-debris accumulations.

Figure 46. Bottom photographs from stations BIS31 and BIS15 showing a view of the flat to undulating, bioturbated sea floor composed of muddy sand typical of those areas of Block Island Sound characterized by processes associated with deposition.

Sediments and Sedimentary Environments

Boulders and gravel dominate the surficial sediment in high-energy sedimentary environments, such as along shorelines, on bathymetric highs, and on the floors of scour depressions (figs. 37, 38; for example, stations BIS83, BIS65, and BIS35). These are areas where the Holocene section is thin or absent, indicating that finer grained sediments have been eroded, exposing the coarser lag deposits of Pleistocene drift (figs. 19, 21, 22; Needell and Lewis, 1984). Boulders and cobbles, where present, are typically covered by sessile fauna, including hydrozoans, anemones, encrusting sponges, and hydroids (fig. 39). In shallow water (less than about 15 m), the boulders and cobbles are covered with green and red seaweed and, in places, kelp (fig. 40). The attached fauna and flora are ecologically important because they add to the overall benthic roughness and complexity and indicate that the coarser gravel is immobile even during severe storms. Pea- and pebble-sized gravels and gravelly sediments surround and are present within the bouldery deposits, armoring the underlying sea floor (fig. 41). Episodic mobilization of the finer grained gravel during storm events is suggested by the presence of only sparse sessile biota and, in deeper water, thin veneer of fine-grained sediment. Together, the gravels and gravelly sediments delineate areas of the sea floor characterized by sedimentary environments associated with processes dominated by long-term erosion or nondeposition (fig. 38; Knebel and others, 1982; Knebel and Poppe, 2000).

While similar to linear scour depressions reported elsewhere (Cacchione and others, 1984; Garnaud and others, 2005) and on the shoreface along the south shore of Rhode Island (Oakley and others, 2009), floors of the storm-induced scour depressions in shallower waters offshore in Rhode Island Sound (Poppe and others, 2011; McMullen and others, 2011) and Block Island Sound are most commonly not rippled. We contend that the coarseness of exposed winnowed Pleistocene deposits is responsible for the absence of bedforms on the floors of these features. Ripples, however, become much more common on the floors of the storm-induced scour depressions in Block Island Sound as depth increases and the composition of the underlying deposits changes. In any case, the sediments on the floors of the scour depressions are almost always coarser that those on the surrounding sea floor (fig. 42).

The storm-induced scour discussed above and in the bathymetry section of this report results in the juxtaposition of sea-floor areas in Block Island Sound with distinct boulder, gravel, sand, and muddy sand textures (fig. 38). This textural heterogeneity in turn creates a complex patchwork of contrasting sedimentary environments and, presumably, habitats. Our observations of local variations in community structure suggest that this small-scale textural heterogeneity adds dramatically to the regional sound-wide benthic biological diversity.

Sand is the prevalent textural class across much of Block Island Sound (fig. 37). In shallower areas or near constrictions in the tidal flow, the sand is coarse- to medium-grained, clean (silt plus clay less than 1 percent), moderately well to moderately sorted, and ubiquitously rippled (fig. 43; for example stations BIS34, BIS37, and BIS82). Fields of sand waves and megaripples are variably present; moon snails, hermit crabs, cancer crabs, and sand dollars are varibly present; shell hash and gravel are concentrated in the ripple troughs; and sedimentary environments characterized by processes associated with coarse bedload transport prevail (fig. 38). In the deeper areas and areas away from constrictions in the tidal flow, the sand is typically fine- to very fine-grained, moderately to poorly sorted, muddier (silt plus clay 5-15 percent), and faintly rippled to undulating in appearance (fig. 44; for example, stations BIS70 and BIS80). Patchy distributions of small burrows, worm tubes, amphipod communities, skate, and flounder are common; thin, ephemeral accumulations of shell hash are variably present (fig. 45); and sedimentary environments characterized by processes associated with sorting and reworking prevail (fig. 38).

Silty sand is the prevalent textural class only in the deeper, predominantly lower energy western parts of the study area (fig. 37; for example, stations BIS31 and BIS43). This area, which constitutes about 9 percent of the study area, is more protected from Atlantic storm events because it is bordered by Long Island to the west, the shallow top of the terminal moraine to the south, and Block Island to the southeast. The sand in this area is very fine grained, mud averages over 40 percent of the samples, and the sediments are poorly to very poorly sorted. The sea floor in these muddy areas has an undulating appearance and is heavily bioturbated, large and small burrows and worm tubes are abundant (fig. 46), and sea-floor sedimentary environments are characterized by the processes associated with deposition.

Sediment Data

The sediment-grain-size dataset provided in the Data Catalog section of this report contains information on the collection, location, description, and texture of sediments at 86 stations occupied during the 2011 RV Rafael verification cruise 2011-006-FA (figs. 14, 37). All analyses were conducted in the Sedimentation Laboratory at the USGS Woods Hole Coastal and Marine Science Center in Woods Hole, Mass. 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 collected 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))

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; Schlee, 1973; Poppe and others, 2004):

where 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

and where 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 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 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 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 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 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 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 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 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