Chapter 1

Maps Showing the Shape of the Marine Transgressive Surface and the Thickness of Postglacial Sediments in Long Island Sound

By
Mary L. DiGiacomo-Cohen1 and Ralph S. Lewis2
 
Table of Contents
Introduction
Methods
Marine Transgressive Surface
Thickness of Postglacial  Sediments
References
Figure Captions
Digital Data and Metadata
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INTRODUCTION

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 press).
 

METHODS

Over 3,500-line km of high-resolution seismic-reflection profiles were collected during multiple cruises in LIS (Fig. 1).  Data from the original Figure 1 - Tracklinesanalog 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 data page.  Further detailed information can be found on the ArcView Project Filepage.
 

THE MARINE TRANSGRESSIVE SURFACE

Figure 2 - Transgressive surfaceThe 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 LIS.
 

THE THICKNESS OF POSTGLACIAL SEDIMENTS

Figure 3 - Thickness of postglacial sediments and Figure 4 - Isopach of postglacial sedimentsThe 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, in press).

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.
 
 

REFERENCES

Fenster, M.S., 1995, The Origin and Evolution of the Sand Sheet Facies: Eastern Long Island Sound. Ph.D. Thesis, Boston University, Boston, Massachusetts, 328 p.

Knebel, H.J. and Poppe, L.J., Sea-floor environments within Long Island Sound: A regional overview: Journal of Coastal Research, Thematic Section, in press.

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 CAPTIONS

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.


1 Long Island Sound Resource Center, University of Connecticut, Avery Point, Groton, CT
2 Connecticut Geological and Natural History Survey, Department of Environmental Protection, Hartford, CT [an error occurred while processing this directive]