VESSEL-BASED, ACOUSTIC INVESTIGATION OF GLACIAL LAKEFLOOR DEPOSITS OFF THE COAST OF OUTER CAPE COD, MASSACHUSETTS

High resolution data were collected using phase-measuring sidescan sonar and a seismic sub-bottom profiler approximately 500 m off the Atlantic Ocean shoreline in the North Truro quadrangle. These data were interpreted to be outcrops of glacial lakefloor sediments deposited by retreating lobes of the Laurentide ice sheet. Outer Cape Cod was constructed during the retreat of the Cape Cod Bay lobe, located in present day Cape Cod Bay and the South Channel lobe which bounded Cape Cod to the east. As the Cape Cod Bay lobe retreated north to a position in the center of Cape Cod Bay and the South Channel lobe retreated northward and eastward, a proglacial lake, Glacial Lake Cape Cod Bay, formed between the two lobes with moraine deposits to the south. A series of outwash plains including the Truro Plain were deposited into Glacial Lake Cape Cod Bay via the South Channel Lobe. The readvance of the Cape Cod Bay Lobe pushed glaciolacustrine sediments to form a moraine in Cape Cod Bay. Further retreat of the Cape Cod Bay Lobe resulted in the northward expansion of Glacial Lake Cape Cod Bay.

Swath bathymetry and backscatter imagery were collected using a Teledyne Benthos C3D and an Edgetech 6205. Both instruments collect coincident bathymetry and acoustic backscatter imagery from the same pulse. The C3D has an operating frequency of 200 kHz (pulse rate 30 Hz). The 6205 has an operating frequency of 550 kHz (pulse rate 150 Hz) and collects two frequencies of backscatter imagery (550/1600 kHz). Offshore seismic reflection profile data were collected offshore using an EdgeTech 216S full spectrum sub-bottom profiler operated at a frequency sweep of 2-10 kHz.

The irregular and anomalous pattern seen in the backscatter imagery was not initially identified as exposed, deformed lakefloor. The outcrops appear in shore normal troughs as seen in the swath bathymetry. However, after the seismic survey was conducted the hypothesis of exposed glacial lakefloor was supported. A review of the literature further corroborated these findings. This suite of instruments can be rapidly deployed on relatively small boats, with minimal crew and deliver useful data for various types of nearshore investigations. These findings can elucidate reconstructions of deglaciation, differential sediment transport pathways and rates as well as coastal and nearshore evolution.