Survey Number 3--SIDESCAN SONAR IMAGE, SURFICIAL GEOLOGIC INTERPRETATION, AND BATHYMETRY OF THE LONG ISLAND SOUND SEA FLOOR IN NIANTIC BAY AND VICINITY, CONNECTICUT
Lawrence J. Poppe1, Ralph S. Lewis2, Jane F. Denny1,
Kenneth F. Parolski1,
Mary L. DiGiacomo-Cohen2, and Douglas S. Tolderlund3
1Coastal and Marine Geology Program, U.S. Geological Survey, Woods
Hole Field Center, Woods Hole, MA 02543
2Long Island Sound Resource Center, Connecticut Geological and Natural History Survey, Department of Environmental Protection, 1084 Shennecossett Road, Avery Point, Groton, CT 06340
3U.S. Coast Guard Academy, 15 Mohegan Avenue, New London, CT 06320
Ongoing research by the U.S. Geological Survey (USGS) in Long Island Sound, a major East Coast estuary surrounded by the most densely populated region of the United States, is building upon cooperative research with the State of Connecticut that was initiated in 1982. During the initial phase of this program, geologic framework studies in Long Island Sound were completed and published (Lewis and Needell, 1987; Needell and others, 1987; Lewis and Stone, 1991). The emphasis of the present program in the Sound is directed toward studies of the sediment distribution and the processes that control it, nearshore environmental concerns, and the relation of benthic community structures to the sea-floor geology.
Because of the enormous surrounding population, large inputs of anthropogenic wastes (e.g. sewage and fertilizer) and toxic chemicals have produced stresses on the environment of the Sound, causing degradation and potential loss of benthic habitats (Long Island Sound Study, 1994). To address this problem, we are constructing sidescan sonar mosaics (complete-coverage acoustic images) of the sea floor within specific areas of special interest (fig. 1). The mosaic presented herein, which covers the sea floor in and adjacent to Niantic Bay, enables us to define the geological variability of the sea floor, which is one of the primary controls of benthic habitat diversity. It also provides a detailed framework for future research, monitoring, and management activities, and it improves our understanding of the processes that control the distribution of bottom sediments, benthic habitats, and associated infaunal community structures. Because precise information on environmental setting is important to the selection of sampling sites and to the accurate interpretation of point measurements, the sidescan sonar mosaics also act as base maps for subsequent sedimentological, biological, and geochemical observations.
Long Island Sound is about 182 km long and up to 32 km wide. It is bordered on the north by the rocky shoreline of Connecticut and on the south by the eroding sandy bluffs of Long Island, N.Y. The study area (figs. 1 and 2), which covers about 28.7 km2, is located in the northeastern part of the Sound and just offshore from both the Millstone nuclear power station and the marine recreational facilities of the Niantic River.
The bedrock beneath the study area is believed to be composed primarily of gray to dark-gray gneissic and schistose metamorphic rocks of pre-Silurian age (Goldsmith, 1967; Rodgers, 1985). The Tatnic Hill Formation comprises the bedrock on most of Black Point and the bedrock beneath Twotree Island and Bartlett Reef. The Rope Ferry Gneiss, and, to a much lesser extent, dikes of the Westerly Granite, are exposed on Millstone Point and White Point, along the northeast shore of Niantic Bay, and on Waterford Island.
The bedrock across much of southeastern Connecticut is unconformably overlain by two tills, one of pre-Wisconsinan age and one of late Wisconsinan age (Stone and others, 1992). The tills surrounding Niantic Bay typically have smooth surfaces, are loose to compact, are sandy and stony with many large subrounded boulders, and contain thin lenses of stratified material (Goldsmith, 1964). Glacial stream deposits blanket the tills and bedrock in the Niantic River valley and along Jordan Cove. Deltaic and varved lake deposits of glacial Lake Connecticut overlie the bedrock and glacial drift under much of Niantic Bay (Lewis and Stone, 1991). A marine mud facies, which occurs in quiet-water areas throughout the Long Island Sound basin, overlies these earlier deposits and records deposition during the postglacial Holocene eustatic rise. Although the northward retreat of the late Wisconsinan ice sheet is marked in southeastern Connecticut by a northward succession of minor recessional moraines (i.e. the Mystic, Old Saybrook, and Hammonasset-Ledyard moraines; see fig. 1) no end moraines have been identified along the shore of Niantic Bay (Goldsmith, 1980).
In eastern Long Island Sound, strong tidal currents have extensively eroded and reworked both the glacial and postglacial deposits and continue to influence the surficial sediment distribution. The irregular bottom topography and the presence of extensive lag deposits in the eastern Sound reflect scour, transport, and reworking of the sediments (Lewis and Stone, 1991).
DATA COLLECTION AND PROCESSING
Sidescan sonar imagery, bathymetry, 3.5-kHz high-resolution seismic-reflection profiles, and navigational data were collected along tracks spaced 150 m apart aboard the RV Asterias during June 7-9, 1995 (figs. 2, 3, and 6). The bathymetric data were collected by means of a 200-kHz echo sounder, and the data were logged digitally. The subbottom data were collected in analog form using an Ocean Research Equipment 3.5-kHz profiler transmitting at a 0.25-s repetition rate. The sidescan sonar data were collected using a 100-kHz Klein sidescan sonar system set to sweep 100-m to either side of the ship's track. These data were logged digitally to 8-mm tape on a QMIPS data acquisition system (Danforth and others, 1991). Ship position was determined with a differential Global Positioning System (GPS) and was logged on a computer at 10-s intervals. The bathymetric data were corrected for the approximately 0.82-m tidal range in the study area. These data were corrected by adjusting the measured depth values to the predicted mean sea-level for New London, Conn., approximately 8.5 km east of the study area.
The sidescan sonar data were processed according to procedures summarized by Danforth and others (1991) and Paskevich (1992a,b). Briefly, the sonar data were multiplexed and filtered to convert them into a format useable by the processing routines and to remove speckle noise. The data were corrected for slant-range distortions, and striped noise was removed. Shading was corrected to compensate for signal attenuation, and a filtering procedure was used to correct for dropped lines in the sonar data. Statistics from the sonar data were accumulated to determine the dynamic range of the data, and a contrast enhancement was applied. Corrected ship navigation, which accounted for changes in ship speed, was merged with the sonar data, and the data were geographically oriented and displayed on a Mercator grid.
After this preliminary processing, the data were used to make the composite digital mosaics. Alternating strips of sonar data were placed in their proper geographic location at the appropriate scale and projection (Paskevich, 1992c). Adjacent sonar images were matched for tone, and unwanted data where two images overlapped were digitally trimmed off and progressively combined into the composite image. Although registration between the shiptrack navigation and the strips of sidescan sonar data are generally good throughout most of the study area, the northeasternmost strip was over-compressed during the digital stitching and has been offset slightly toward the west (fig. 2).
The final image was a digital mosaic with a 1-m pixel size (a pixel represents 1 m on the sea floor). This image was output to film and was photographically enlarged to a scale of 1:10,000.
Surficial sediments were sampled and bottom photography was attempted at 69 locations during March 1996 aboard the RV John Dempsey by means of a Van Veen grab sampler equipped with video and still camera systems (fig. 5). These photographic systems were also used to appraise intra-station bottom variability and to observe boulder fields and bedrock outcrops where sediment samples could not be collected. Surficial sediments (sediments from the sediment-water interface down to 2 cm below the interface) were collected from the grab sampler; these subsamples were frozen and stored for later analysis.
In the laboratory, the sediment samples were corrected for salt content, disaggregated, and wet sieved through a number 230, 62 micro (4 phi) sieve to separate the coarse and fine fractions. The fine fraction was analyzed by Coulter Counter (Shideler, 1976); the coarse fraction was analyzed by sieving (gravel) and by rapid sediment analyzer (sand; Schlee, 1966). Because biogenic carbonates commonly form in situ, they usually are not considered to be representative of the depositional environment from a textural standpoint. Therefore, bivalve shells and other biogenic carbonate debris were manually removed from the gravel fraction. Size classifications are based on the method proposed by Wentworth (1929). The verbal equivalents were calculated using the inclusive graphics statistical method (Folk, 1974) and are based on the nomenclature proposed by Shepard (1954). A detailed discussion of the laboratory methods employed is given in Poppe and others (1985); the raw grain-size data, associated statistics, and detailed descriptions of the bottom photography are reported in Poppe and others (1996).
The map of surficial sediment distribution shown in figure 5 is based on data from the sediment sampling and bottom photography stations, on tonal changes in backscatter on the sidescan sonar image, and on the bathymetry.
Near-Surface Echo Character
The 3.5-kHz seismic-reflection measurements provide information on the subsurface geology and its relation to surficial processes and sediment distribution. Four different acoustic facies were identified on these subbottom profiles (fig. 3): prolonged, opaque, transparent, and layered facies. More than one of these facies may occur together; the layered and transparent facies commonly grade into each other. Outcropping subbottom units of the prolonged facies and surficial sedimentary features, such as sand waves, were also recorded on the 3.5-kHz profiles.
The acoustically prolonged facies, which typically represents acoustic basement, has a highly reflective upper surface and is interpreted to be bedrock (figs. 3A, 3C, and 4). This facies is exposed or only thinly covered across much of the southern part of the study area and in Twotree Island Channel, where strong tidal currents presumably have removed the overlying sediments. In the southwestern part of the study area the acoustically prolonged facies delineates the steep gradients of a massive bedrock outcrop south of Black Point (fig. 3A); toward the eastern part of the study area the bedrock surface slopes up toward Twotree Island and Bartlett Reef (figs. 3B and 4). The subbottom profiles show that the acoustically prolonged facies has a somewhat irregular surface, which generally dips toward the south and commonly controls the distribution of surficial sediments (figs. 3B and 3C). Channels and basins incised into this surface are commonly filled with more recent sediments.
The acoustically opaque facies is interpreted to result from gas-charged sediment (figs. 3B, 4). The distribution of this facies is primarily limited to areas where the depth to acoustic basement is relatively great and where the depositional environment is one of lower energy, such as the area between Black Point and Twotree Island and the area within Niantic Bay. The top of this facies occurs between 3-9 m beneath the sediment-water interface. Gas-charged sediment may be associated with both the transparent and layered acoustic facies, but, where detected, gas obscures most of the underlying units and acoustic basement in these areas.
The transparent and layered facies, which are too gradational and discontinuous to be mapped throughout the study area, overlie and are younger than the prolonged acoustic facies and probably represent Holocene deposition. The transparent facies dominates beneath the sand-wave fields (fig. 3D) and in other higher energy environments. The layered facies is more prevalent in areas of lower energy, such as in sediment ponds protected by bedrock highs (figs. 3B and 3C) and in the interior of Niantic Bay.
Sand waves and megaripples cover much of the sea floor east of Bartlett Reef and along the southern part of the study area (fig. 3D). The average crest-to-trough height of these features is less than 0.5 m, but occasionally exceeds 1 m; asymmetry of the sand waves suggests that they migrate. Based on the orientations of this asymmetry, the residual (net) transport of the surficial sediments is toward the west along the southern boundary of the study area and toward the northwest in Twotree Island Channel (fig. 4).
The corrected bathymetry was contoured at 2-m intervals (fig. 2) to facilitate an understanding of the geology and general benthic character. Maximum water depths exceed 40 m and occur at the southern edge of the study area. Water depths shoal shoreward within Niantic Bay and around Black Point, Twotree Island, and Bartlett Reef. The steepest bathymetric gradients occur south of Black Point, along the southern boundary of the study area, and in Twotree Island Channel. Elsewhere, especially at depths shallower than 14 m, the gradients are much gentler.
Major isolated bathymetric highs occur in the southwestern and eastern parts of the study area. The former delineates a massive bedrock outcrop off Black Point (fig. 3A); a depression around this outcrop suggests scour by the strong tidal currents. Other isolated depressions having relief greater than 2-m, that are also related to scour by strong tidal currents, occur within and at the western end of Twotree Island Channel. The highs in the eastern part of the study area are associated with the rising bedrock surface around Twotree Island and Bartlett Reef (fig. 2) and with a sand wave-covered shoal (fig. 3D). Although there are a few other outlying areas of elevated relief greater than 2 m, the sea floor is generally smooth.
Sidescan Sonar Mosaic
The 100-kHz sidescan sonar mosaic (fig. 6) recreates an acoustic image of the sea floor that, when used in combination with the 3.5-kHz seismic and bathymetric data (figs. 2 and 3), provides an overall perspective of the benthic character (fig. 7). Distinctive acoustic patterns revealed by this mosaic include the following: complex patches of high and low backscatter that coincide with areas where the prolonged facies crops out (bedrock, and boulders and gravel); elongate patches of alternating high and low backscatter (sand waves); areas of low backscatter (dark tone on image; probably finer grained sediment); and isolated spots of high backscatter (light tone on image; interpretation is problematic). Ship wakes are common on the image, notably in Niantic Bay, and faint trawl marks are present east of Black Point. Turbulance associated with the strong tidal currents appears as streaks of high backscatter around Millstone Point.
Complex patches of high and low backscatter that coincide with outcrops of subbottom reflectors are present in the southern and northeastern parts of the study area and along the margins of Niantic Bay. The outcrops in the southwestern part of the study area are primarily bedrock, probably of the Tatnic Hill Formation (Rodgers, 1985). Bedrock also crops out off Millstone Point and in the channel between Bartlett Reef and White Point. Elsewhere in the study area, this acoustic pattern is associated with the presence of boulders and course gravel. Boulders and gravel cover the underlying bedrock along the eastern shore of Black Point, across much of the southern boundary of the study area, and around Twotree Island and Bartlett Reef. Scattered boulders are also present adjacent to some of the bedrock outcrops.
Elongate patches of alternating high and low backscatter (sand waves) occur the southern and southeastern part of the study area (figs. 6 and 7). Smaller patches of this acoustic pattern also occur in the southwestern part of the study area and in Twotree Island Channel. The sand waves that produce this pattern range in wavelength up to 40 m and are the result of the strong tidal currents (46 to 98 cm/s) present in this part of the Sound (Fenster and others, 1990; Quarrier, 1993; Eldridge Tide and Pilot Book, 1994). The alternation in backscatter that creates this pattern is due to a combination of topographic changes (because the angle of incidence of the sonar varies) and to the differences in sediment texture between crests and troughs. The asymmetry of the bedforms in the 3.5-kHz seismic profiles and the lobate curvature of the crests in the sidescan sonar image suggest the net transport direction is to the southeast over the southernmost part of the sand wave fields south of Twotree Island and east of Bartlett Reef and to the west over the remainder of these fields (fig. 4). This distribution suggests that the flood tide is most intense on the north side of the sand wave fields, and that the ebb tide is strongest on the south side (figs. 6 and 7). The presence of gravel-floored linear scour depressions and linear ridges of finer grained sediment that extend both eastward and westward from isolated boulders also demonstrates the strong nature of the tidal currents.
Areas of relatively uniform low backscatter, caused by finer grained sediment, occur primarily within Niantic Bay and west of Twotree Island (fig. 7). These areas are more protected sites that have generally lower energy.
Isolated spots of high backscatter are concentrated within Niantic Bay and in a discontinuous belt southwest of Twotree Island. Both areas are associated with parts of the sea floor that are characterized by relatively low backscatter. The spots of high backscatter within Niantic Bay range up to 40 m in diameter and are obviously elevations in the sea-floor bathymetry. These features probably represent disposal sites for dredge spoils from the Niantic inner harbor. The spots of high backscatter that occur in the discontinuous belt southwest of Twotree Island are smaller (5 to 20 m in diameter) and appear to have little relief. Interpretation of these smaller backscatter features is difficult because they may be related to several processes. For example, they occur adjacent to the area underlain by the acoustically opaque facies (figs. 3B and 4) interpreted to be caused by gas-charged sediment, and, thus, they may be pockmarks. Conversely, they occur along a change in slope at the mouth of Niantic Bay, and, therefore, they may be shallow scour features related to current action. Inasmuch as no water column disturbances on the 3.5-kHz or sidescan sonar records coincide with the spots of high backscatter in this belt, it is unlikely that the spots are related to schools of fish or to the resuspension of bottom sediments.
Surficial Sediment Distribution
The distribution of surficial sediment and the locations of the sediment sampling and bottom photography stations are shown in figure 5. Gravelly sands dominate the surficial sediment distribution across the southern part of the study area and in the channel between Twotree Island and White Point, areas that are exposed to strong tidal currents. These gravelly sediments, which are usually very poorly sorted and coarsely skewed, are typically characterized by abundant mussel-shell detritus, sponges, and hydrozoans. Patches of boulders and gravel are common in the gravelly sands and around the bathymetric highs associated with Black Point, Twotree Island, and Bartlett Reef. The boulders are typically covered by mussel beds, and starfish, crabs (cancer), anemones, and hydrozoans are common. Barnacles grow on the boulders in the shallower areas, and, especially east of Bartlett Reef, kelp grows among the boulders. Bedrock outcrops were observed in the bottom video off both Black and White Points.
The gravelly sands in the southern part of the study area grade northward into sands and finer-grained sediments. The sands are moderately to poorly sorted, finely skewed to nearly symmetrical, and leptokurtic. The ubiquitous presence of bedforms (ripples and sand-wave fields) indicates that tidal currents are strong enough to move the sands. Shell debris, gravel, and hydrozoans occur in the troughs of the bedforms within the higher energy areas; amphipod communities occur in the bedform troughs in areas of lower energy.
Silty sands occur at the mouth of Niantic Bay and between Twotree Island and Bartlett Reef. These silty sands are primarily moderately sorted, finely skewed, and leptokurtic. They also are characterized by faint ripples and scattered, patchy concentrations of amphipod and worm tubes. The sediments grade to sandy silts toward the more protected area of Niantic bay. The sandy silts, which are poorly to moderately well sorted and leptokurtic, are characterized by undulating, bioturbated bottoms, extensive worm and amphipod communities, and shrimp burrows. Many of the faint current-modified bedforms observed on the bottom video within the areas of sandy silts appeared to originate from biological activity. For example, feeding crabs and burrowing mollusks mound the sediments. These mounds are then modified by the tidal currents.
The detailed 3.5-kHz subbottom, bathymetric, and sidescan-sonar data presented herein show that the surficial geology of the sea floor in and adjacent to Niantic Bay is extremely variable over short distances. This variability is the product of bedrock lithology, late Quaternary glaciation, and the cumulative effects of sediment transport, reworking, and deposition since the Holocene marine incursion. Although the surficial geology within the study area is clearly too complex to be mapped by measurements made solely at discrete sample locations, the integrated acoustic surveys presented provide a detailed base map for future studies of sedimentary environments, biological communities, and geochemical processes.
The product was prepared in cooperation with the State of Connecticut, Department of Environmental Protection, Geological and Natural History Survey and was originally published as US Geological Survey Geologic Investigations Series Map I-2625. We thank Dave Olmsted, who captained the Woods Hole Oceanographic Institution research vessel Asterias; Peter Simpson and Miles Peterle, who operated the Connecticut Department of Environmental Protection research vessel John Dempsey; the University of Connecticut for providing support; and Kathy Scanlon-Catanach and Harley Knebel (USGS) who critically reviewed this map. Mean sea-level tidal-height measurements used to correct the bathymetric data were computed by the Tidal Analysis Branch of the National Oceanographic and Atmospheric Administration, Silver Spring, Maryland. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
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Figure 1.---Map showing the location of the study area (diagonal stripe-filled polygon). Also shown, as open polygons, are the locations of other surveys completed to date that are part of or preparatory to this series: [Norwalk---Twichell and others (1997); Milford---Twichell and others (1998); Roanoke Point---Poppe and others (1998); Falkner Island---Poppe and others (1997c); Hammonasset---Poppe and others (1997a); New London---Poppe and others (1992), Moffett and others (1994), Zajac and others, 1995; Fishers Island Sound---Poppe and others (1994, 1997b). Major onshore moraines are indicated.
Figure 2.---Bathymetric map of the study area. Depths are in meters, have been corrected for tides, and are adjusted to mean sea level. Fine dashed lines represent tracks along which geophysical data were collected.
Figure 3.---Subbottom profiles (3.5-kHz) showing different echo-character types. (A) Profile from south of Black Point examplifies of the acoustically prolonged facies, here interpreted to be a massive bedrock outcrop. The upper, highly reflective surface of the prolonged facies typically represents acoustic basement. (B) Profile shows the acoustically opaque facies interpreted to be the result of gas-charged sediment, and also shows the layered facies that tends to dominate in the lower-energy, more protected environments. (C) Profile shows the control exerted on surficial sediment distribution by the shallow, irregular bedrock surface in Twotree Island Channel. Ponded sediments fill depressions in the bedrock surface. (D) Profile shows sand waves and the transparent facies that dominates east of Bartlett Reef. Locations of the profiles are shown in figures 2 and 4.
Figure 4.---Near-surface echo character map showing the distribution of the prolonged facies and the acoustically opaque facies. The prolonged facies is expressed as subbottom depth to acoustic basement; contour interval is 5 m. This facies crops out on the sea floor within the 0-m contour. It is interpreted to be bedrock that is exposed or only thinly covered by sediment. The acoustically opaque facies is interpreted to be the result of gas in the sediments. Figure also shows the net direction of transport (arrows) interpreted from sand waves, and the location of profiles shown in figure 3. Profile C is plotted in its actual position, rather than at the over-compressed position shown in the sidescan sonar image (fig. 6; see discussion in the section on data collection and processing).
Figure 5.---Map showing the distribution of surficial sediments and the locations of the sediment sampling and bottom photography stations. Interpretation of the sediment distribution is based on data from these stations, on tonal changes in backscatter on the sidescan sonar image (fig. 6), and on the correlation of the textural and backscatter data with the bathymetry (fig. 2).
Figure 6.---Sidescan sonar mosaic of the sea floor in the study area. Light tones on the image represent areas of high backscatter (typically coarser grained sediments); dark tones represent areas of low backscatter (typically finer grained sediments).
Figure 7.---Interpretation of the sidescan sonar mosaic. Map shows the distribution of bedrock, boulders and gravel, areas characterized by low backscatter, sand waves, and spots of high backscatter. The different patterns are discussed in the text.
The sidescan-sonar mosaic is a digital image with a 1-m pixel size ( a pixel represents one meter on the sea floor). The 55Mbyte digital image is available for viewing as a graphic in "tif" format with its associated Arcview geo-reference world file.
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