CHARACTER AND DISTRIBUTION OF EXPOSED GLACIOLACUSTRINE SEDIMENTS OFF OUTER CAPE COD, MASSACHUSETTS

High-resolution seismic-reflection, multibeam echosounder bathymetry and backscatter, bottom photography, and sediment sampling surveys were conducted to examine the surficial geology and map seafloor distributions of surficial sediment and sedimentary environments east of Cape Cod, Massachusetts. One result of this effort was the identification of narrow, irregular, somewhat discontinuous outcrops of fine-grained glaciolacustrine sediments at three locations. The regional extent of these low-permeability beds is important because they effect the outer Cape’s hydrogeologic framework and because the exposures locally provide unique habitats within an area of seafloor dominated by sand and gravel. These outcrops have been observed to benefit the benthic fauna by providing shelter under overhangs and a substrate amenable to burrow construction.

The glaciolacustrine sediments are gray, cohesive, and composed primarily of silty clay with some very fine sand. Ongoing degradation of the outcrops, which can exceed 1 m in relief, is mainly by wasting along varves, as evidenced by abundant slabs and blocky rip-up clasts present on the surrounding seafloor, and to a lesser extent by bioerosion. The glaciolacustrine strata record distal bottomset deposition within deltaic systems that prograded southwestward into glacial lakes in Cape Cod Bay and Nantucket Sound from the South Channel lobe about 18 ka. The two southernmost exposures, which record deposition in the Eastham outwash plain and perhaps another older delta, are found about 1.5 km east of southeastern Wellfleet and about 5.7 km off Chatham, respectively. The northernmost outcrop, which occurs about 1.9 km off North Truro, probably represents distal deposition along the northern edge of the Truro outwash plain. Glaciolacustrine clasts observed elsewhere in the study area are admixed into tills and Holocene sediments and are probably allochthonous.