Maps Showing the Shape of the Marine Transgressive
Surface and the Thickness of Postglacial Sediments in Long Island Sound
Mary L. DiGiacomo-Cohen1
and Ralph S. Lewis2
A 20 year cooperative between the U.S. Geological Survey and the Connecticut
Geological and Natural History Survey has produced an overall characterization
of the Quaternary stratigraphy and history of the Sound (Lewis and Stone,
1991; Stone and others, 1998; Lewis and DiGiacomo-Cohen, in press).
Our purpose in presenting maps of the marine transgressive surface and
the thickness of postglacial sediments in Long Island Sound (LIS) is two
fold. The eastern and east-central LIS portions of these maps were published
by Lewis and Needell (1987); and Needell and others (1987), but that series
was never completed as ideas evolved as the investigations continued westward.
So in this product an effort was made to unify the previous mapping and
present the Sound in its entirety. Secondly by releasing the data in a
useful GIS format we hope to both encourage its use and offer digital evidence
in support the sediment budget described Lewis and DiGiacomo-Cohen (in
Over 3,500-line km of high-resolution seismic-reflection
profiles were collected during multiple cruises in LIS (Fig.
1). Data from the original analog
records were hand plotted and contoured at 1:80,000 scale in sections.
These contours were digitized using Arc/Info software and modified where
necessary along section boundaries to create maps of the marine transgressive
surface and the thickness of postglacial sediments (Figs.
2 and 3).
In order to generate a total volume from the thickness contours Arc/Info
TIN software was used to create a triangulated irregular network data model
representing a continuous surface. This method provided a volume estimate
that accounted for the sediment believed to exist where we had gaps in
the data matrix due to the presence of gas in the sediment. In areas
where the transgressive surface was not detected, obscured by gas, or eroded,
such as in the eastern sound (Fig. 2),
the distinctive seismic character of the marine section was sufficient
to map its thickness. New contours were then created from the TIN
(with minor edits where necessary); these are shown in Lewis and DiGiacomo-Cohen
(in press) and are also shown shaded in color in Figure
4. The new contours eliminated the problem that different contour
intervals were used the eastern verses the western portions of LIS in the
original mapping efforts.
To open a georeferenced display of the marine transgressive surface
and thickness of postglacial sediment themes in ESRI's ArcView program
make sure the application is loaded on your computer. Users should
go to the lisound directory located on the top level of this
CD-ROM and double click on the lisound.apr project file.
The individual ArcView shapefiles may also be opened directly with any
Arc application (e.g. ArcInfo, ArcExplorer) and can also be found on the
page. Further detailed information can be found on the ArcView
THE MARINE TRANSGRESSIVE SURFACE
depth of the marine transgressive surface (Fig.
2) ranges from 72 m in the east-central Sound to 6 m along the
shore. This is a time-transgressive erosional surface erosion that formed as
the sea transgressed westward across the sub-aerially exposed bed of Glacial
Lake Connecticut (Lewis and Stone, 1991; Stone and others., 1998). Along
the north and south shores of the underlying bedrock (north shore), coastal-plain
and moraine (south shore). In the vicinity of the Housatonic River the
expression of a large delta built into Glacial Lake Connecticut can be
seen. In several places along the LIS shoreline valleys associated
with streams and rivers also influence the shape of the unconformity (e.g.
Quinnipiac, Housatonic rivers). Beaches and/or spits associated with structural
highs appear to have formed on the marine transgressive surface in numerous
places (Mills, 1995). South and slightly west of the Connecticut River
the unconformity is truncated by modern erosion and is not present in eastern
THE THICKNESS OF POSTGLACIAL SEDIMENTS
postglacial sediments (Figs.
3, 4) of LIS rest on the marine transgressive (Fig.
2) . These marine sediments
range in thickness from 0 to 45 m and represent a total volume of
22.7 billion m3 (Lewis and DiGiacomo-Cohen, in press). The thickest
accumulation of postglacial sediment in LIS is found just west of the mouth
of the Connecticut River. This thick deposit represents the remnant of
a marine delta that was built into LIS as glacial lake Hitchcock drained;
Stone and others (1998) estimated that the original volume of this delta
was 11.5 billion m3.
West of the Lake Hitchcock delta remnant, the marine deposits rarely
exceed 16 m in thickness. Along the north and south shores of the Sound,
the distribution of these sediments is influenced by the shape of the underlying
bedrock, moraine, and perhaps coastal plain (Lewis and DiGiacomo-Cohen,
in press). Large glacial-lake deltas and underlying river valleys also
influence the sediment distributions in the nearshore (Lewis and DiGiacomo-Cohen,
The abrupt thinning of the marine section east of the mouth of the Connecticut
River is inferred to be the result of extensive tidal scour in the eastern
Sound (Fenster, 1995; Knebel and Poppe, in press). The marine delta of
the Thames River lies just south of the River mouth.
Fenster, M.S., 1995, The Origin and Evolution of the Sand Sheet Facies:
Eastern Long Island Sound. Ph.D. Thesis, Boston University, Boston, Massachusetts,
Knebel, H.J. and Poppe, L.J., Sea-floor environments within Long Island
Sound: A regional overview: Journal of Coastal Research, Thematic Section,
Lewis, R.S. and DiGiacomo-Cohen, M.L., A Review of the Geologic Framework
of the Long Island Sound Basin, With Some Observations Relating To Postglacial
Sedimentation: Journal of Coastal Research, Thematic Section, in press.
Lewis, R.S. and Needell, S.W., 1987, Maps showing stratigraphic framework
and Quaternary geologic history of eastern Long Island Sound: U.S. Geological
Survey Miscellaneous Field Studies Map MF-1939-A, scale 1:125,000, 3 sheets.
Lewis, R.S. and Stone, J.R., 1991, Late Quaternary stratigraphy and
depositional history of the Long Island Sound Basin: Connecticut and New
York. Journal of Coastal Research, Special Issue No. 11, p. 1-23.
Mills, S., 1995, The Classification of Marine Transgressive Holocene
Sand Bodies Found in Seismic Profiles of Long Island Sound: Williamstown,
Mass., unpublished honors thesis, Williams College, 105 p.
Needell, S.W., Lewis, R.S., and Colman, S.M., 1987, Maps showing the
Quaternary geology of east-central Long Island Sound. U.S. Geological Survey
Miscellaneous Field Studies Map MF-1939-B, scale 1:125,000, 3 sheets.
Figure 1. Index map showing the location
of high-resolution seismic-reflection profiles, color-coded by cruise/year.
Figure 2. Map showing the depth
below present sea level to the marine transgressive surface. Contour interval
2 m; contours dashed where uncertain or inferred.
Figure 3. Map showing the thickness
of postglacial sediments. Original contour interval varies; contours dashed
where uncertain or inferred.
Figure 4. Isopach map showing
the thickness of postglacial sediments. Isopachs derived from TIN model;
5 m interval.
Island Sound Resource Center, University
of Connecticut, Avery Point, Groton, CT
2 Connecticut Geological
and Natural History Survey, Department
of Environmental Protection, Hartford, CT
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