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U.S. Geological Survey Open-File Report 2012-1157

Shallow Geology, Seafloor Texture, and Physiographic Zones of the Inner Continental Shelf from Nahant to Northern Cape Cod Bay, Massachusetts


Summary

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The interpretations presented in this report represent a unique geologic dataset. Each of the interpretive maps contributes new insight to the evolution and sedimentary environments of the Massachusetts inner continental shelf at a resolution that was previously not possible due to a lack of high-resolution geophysical data. Interpretations of high-resolution geophysical data and sediment samples suggest that the shallow geologic framework and surficial geology from Nahant to northern Cape Cod Bay is a complex and variable distribution of sediments and geomorphic features that can be primarily attributed to the advances, occupations, and retreats of Wisconsinan glaciation and reworking during Holocene sea-level change. Glacial, marine, and terrestrial processes have acted on this region during the last 20,000 to 24,000 years, creating a complex geologic history and a heterogeneous seafloor character. Thick deposits of glacial and nonglacial sediment bury bedrock over most of Cape Cod Bay and Boston Harbor, but on the unprotected western margin of Massachusetts Bay, rocky zones are prevalent and are attributed to eroded bedrock or winnowed glacial deposits, which are virtually indistinguishable in shallow seismic records. Glacial sediments within the entire region are locally overlain by nearshore marine (beach and bar deposits), fluvial, estuarine, and marine muds. The wide variety of sedimentary environments within the study area provide habitat for many marine organisms. The high-resolution geologic interpretations provided in this report are valuable input for identifying marine habitats on the Massachusetts inner continental shelf.


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