DEFINING LAKE HITCHCOCK

We used a chirp subbottom profiler and side-scan sonar system to define the margins and sedimentological history of lower Glacial Lake Hitchcock in the Connecticut River near Hartford Connecticut. Glacial Lake Hitchcock extended 350 kilometers from Rocky Hill, CT to Burke, Vermont between approximately 15,000 and 12,000 years ago as the Laurentide Ice Sheet retreated.

The subbottom profiles show up to 50 meters of parallel reflectors interpreted to be varved sediment deposited in Glacial Lake Hitchcock. The parallel reflectors (varves) are darker (higher impedance contrast) and more widely spaced in the lower section of the sequence and become lighter and more closely spaced near the top. We interpret this to indicate a fining upward sequence, with more and coarse-grained sediment deposited when the glacier was closer to Hartford. As the retreat progressed finer-grained and lower quantities of sediment were deposited near Hartford. The sharp distinction between the two types of varves, further suggests a relatively abrupt change in the sediment source and probably the location of the Ice Sheet.

The varves may also be used to define the lake margins. Lake margin locations are identified topographically, by the thinning of the varves and an increase in the amount of soft-sediment deformation (slumping). Deep reflectors are exposed close to or at the surface in some locations, suggesting up to 15 meters of sediment removal. In addition, an up to eight-meter deep, roughly north-south trending channel is incised into the varves. As, there is no evidence that the lake drained northward or that it filled in gradually with sediment, the erosion and channel formation may have occurred during a catastrophic drainage event, possibly during the failure of the sediment dam, which formed the southern margin of the lake. The varved sediments are overlain by sand and gravel, which form extensive bedform fields over much of the Connecticut River bottom. These sediments too, especially the meter high dunes, may be remnants of the rapid lake drainage.