ICE-MARGINAL DELTAIC DEPOSITS OF THE CROMWELL DELTA IN CENTRAL CONNECTICUT MAPPED USING GROUND PENETRATING RADAR

Glacial ice from the last major advance covered the state of Connecticut and reached its glacial maximum on Long Island about 21,000 years ago. As glacial ice melted and retreated northward, meltwater streams deposited a variety of sediments, like gravel, sand, silt, and clay, in low-lying areas, especially in the Hartford Basin. Many paraglacial lakes formed in central Connecticut as meltwater accumulated into valleys that were dammed by ice or glacial sediments. The Cromwell delta was deposited into glacial Lake Middletown. Previous studies have identified seven types of morphosequences, which are sedimentary facies that were each deposited contemporaneously. The Cromwell delta has been labeled an ice-marginal deltaic morphosequence, with proximal deposits (e.g., topsets and foresets) laid atop stagnant glacial ice and distal deposits (e.g., topsets, foresets, and bottomsets) built up as free fronts in the open waters of glacial Lake Middletown. Proximal deposits experienced collapse as the stagnant ice block melted. Thus, each ice-marginal deltaic morphosequence within the Cromwell delta includes two sections divided by an east-west-trending topographic high outlining a previous ice-margin position: north of this boundary are collapsed deltaic deposits dipping northward with evidence of post-deposition deformation; south are noncollapsed deltaic deposits dipping southward.

Ground penetrating radar (GPR) was used at River Highlands State Park (RHSP) to map the collapsed and noncollapsed sections of the Cromwell delta. The purpose of the study is threefold: (1) to demonstrate the capabilities of GPR at identifying deltaic structures to locate noncollapsed and collapsed regions and, therefore, previous ice-margin positions in the area; (2) to provide insight into collapsed delta formation by examining the areal extent and geometry of collapsed structures; (3) to show how GPR can be used in an exploratory manner, with limited access to direct subsurface data, by combining GPR with shallow auger samples and results from previous studies in areas near RHSP, the Cromwell delta, and other contemporaneous deltas. Results show that GPR can be used to map deltaic morphosequences and ice marginal positions to provide a more detailed understanding of the Cromwell delta history at RHSP.