TRANSGRESSIVE VALLEY FILL SEQUENCES ON THE MASSACHUSETTS INNER SHELF: IMPLICATIONS FOR LATE QUATERNARY COASTAL EVOLUTION

The Massachusetts inner continental shelf north of Cape Cod has been shaped by a complicated history of relative sea level (RSL). After deglaciation of the region, isostatic rebound caused RSL to fall to a lowstand depth of approximately -50 m at 12,000 ¹⁴C yr B.P. RSL then rose to its present elevation, driving the shoreline across wide areas of the inner shelf. Details of the transgression remain elusive but new geophysical data (swath bathymetry, sidescan sonar and subbottom profiling), bottom samples and cores are revealing the geomorphic and stratigraphic record generated by these RSL changes. Seaward of the Merrimack River, a large sandy paleodelta formed that is graded to the RSL lowstand. Buried channels up to 750-m wide and 15-m deep are incised into the paleodelta forming a dendritic pattern that extends 6 km offshore to the seaward edge of the paleodelta. Ongoing transgression has truncated the channel-fill deposits and eroded a prominent unconformity that is overlain by thin, discontinuous deposits of sandy Holocene sediment. In Cape Cod Bay, a braided system of large (2-km wide, 25-m deep) buried channels are incised into Pleistocene sediment and converge towards the center of the bay in water depths of 40-50 m. Cores from these transgressive valley fills contain stiff, muddy sediment with abundant oyster shells. The estuarine deposits probably accumulated behind early to middle Holocene barriers that were pinned to drumlins or other glacial deposits, much like Plum Island and Duxbury Spit are today. Reconstructing coastal evolution in the region is critical to understanding potential shoreline responses to variations in future RSL rise and sediment supply that might accompany climate change.