SIDESCAN SONAR IMAGE AND GEOLOGIC INTERPRETATION OF THE SEAFLOOR NEAR
FALKNER ISLAND, CONNECTICUT: A COASTAL NATIONAL WILDLIFE REFUGE
Falkner Island is part of the Connecticut National Wildlife Refuge, an array of five federal tracts set aside to protect unique ecosystems, to manage biodiversity, and to benefit environmentally sensitive fauna and flora. We have conducted continuous-coverage sidescan sonar, bathymetric, and CHIRP subbottom surveys over a 12.5 km2 area around Falkner Island to assess the variability of the seafloor and to improve our understanding of the processes that control the complex distribution of bottom sediments and benthic habitats.
The seafloor geology around Falkner Island is extremely variable over short distances and controlled by a combination of the subsurface stratigraphy and the modern hydraulic regime. Areas of high backscatter on the sidescan image, which reflect bouldery outcrops of glacial till and are environments of erosion or nondeposition, occur adjacent to Falkner and Goose Islands and over shoals extending northward and southward from the islands. These coarse deposits overlie an irregular bedrock surface which generally dips southward. The strong oscillatory nature of the tidal currents around the islands is demonstrated by the linear accumulations of finer-grained sediments that extend both eastward and westward from isolated boulders. Areas of lower backscatter, which surround the outcrops of till, represent relatively fine-grained Holocene sands and silty sands. The finest-grained sediments occur just west and southeast of Goose Island and in other protected sites characterized by lower-energy depositional environments. Sand waves occur in small patches both north and south of the islands, but they never exceed 0.5 m in height or 20 m in wavelength.
This map shows the location of the study area (striped polygon) around
Falkner Island, Connecticut. Map also shows the locations (open polygons) of
the Norwalk Islands, Milford, Hammonasset, Roanoke Point, Niantic Bay, New
London, and Fishers Island Sound sidescan sonar surveys which are part of
this map series, the other components of the Stewart B. McKinney National
Wildlife Refuge, and the major onshore moraines.
Click on the figures for larger images.
A bathymetric map of the study area. Depths are contoured in 2-m
intervals,
have been corrected for tides, and are adjusted to mean sea level. Small
dashed lines are the tracklines along which the geophysical data were
collected. Figure also shows the locations of seismic profiles shown in the
next figure.
CHIRP subbottom profiles show the stratigraphy and
different echo character types.
(A) Profile from south of Falkner Island showing an example
of the prolonged acoustic facies interpreted to be glacial drift and the
opaque acoustic facies interpreted to result from gas-charged sediments.
Sidescan sonar mosaic of the Falkner Island study area.
Light tones on
the image represent areas of high backscatter (usually coarser-grained
sediments); dark tones represent areas of low backscatter (usually finer-
grained sediments).
Click on the figures for larger images.
The
accumulations on the westward side of the boulders are usually longer in this
area and around Falkner Island than on the accumulations on the eastern side.
This asymmetrical distribution is evidence for a predominantly westward
direction of net bottom sediment transport. The near absence of these
features north of the islands suggests that the currents there are not as
strong.
This is <http://woodshole.er.usgs.gov/epubs/posters/falkner/index.html>
(B)
Profile across the shoal north of Falkner Island showing the infilling of
topographic lows in the irregular surface of the prolonged acoustic facies by
glaciolacustrine sediments.
(C) Profile across the shoal north of Goose
Island. Boulders are clearly present on the surface of the prolonged acoustic
facies.
(D) Profile across the north-central part of the study area showing
the infilling of topographic lows in the irregular surface of the prolonged
acoustic facies by glaciolacustrine sediments. Examples of the transparent
and layered facies are also labeled on these profiles. Locations of the
profiles are shown on the bathymetric and echo-character maps.
A near-surface echo character map shows the distribution
of the prolonged
seismic facies (depth to acoustic basement, m) and the acoustically-opaque
seismic facies interpreted to be the result of gas-charged sediments. The
map also shows the areas within which the seismic facies of the Holocene
sediments are predominantly layered or transparent. The prolonged facies
outcrops on the sea floor within the 0-m contour. This Figure also shows the
location of subbottom profiles shown above.
Interpretation of the Falkner Island sidescan sonar mosaic
shows the distribution of bedrock, gravel and boulders, areas characterized
by low backscatter, and sand waves. Heavy dashed line marks the extent of
the sidescan sonar coverage; the coastline is stippled.
Surficial sediments were sampled and bottom photography was attempted at
50 locations during March 1996 aboard the RV John Dempsey by means of a Van
Veen grab sampler equipped with video and still camera systems. The
photographic systems were also used to appraise intra-station bottom
variability and to observe boulder fields and outcrops where sediment samples
could not be collected.
Map showing the distribution of surficial sediments and the
locations of the 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,
and on the correlation of the textural and backscatter data with the
bathymetry.
The strong, oscillatory nature of the tidal currents in the study
area is demonstrated by the linear accumulations of
sediment extending both eastward and westward from isolated
boulders on the sidescan images. These accumulations, which form in
the lee of the boulders, are best developed southwest of Stony Island.
Falkner Island has decreased by 2.8 acres or 50 percent since 1818, when
accurate observations on island size were first conducted. Rapid erosion
continues because the surrounding bluffs are steep, only partly covered by
vegetation, composed of unlithified glacial drift, and fronted by narrow
beaches.
Maintained by webmaster-woodshole@usgs.gov
Modified Monday, 01-Mar-1999 09:41:38 EST