Geophysical evidence of a late Pleistocene glaciation and paleo-ice stream on the Atlantic Continental Shelf offshore Massachusetts, USA

Interpretations of seismic reflection data collected offshore Massachusetts, USA, reveal the first conclusive geophysical evidence of a pre-Wisconsinan glaciation that extended beyond the limits of the Last Glacial Maximum (LGM) in the region. The data image numerous glacial geomorphic features that define the extent of a paleo-ice stream, including: (1) a regionally distributed erosion surface that forms a 50 km wide trough, with steeply eroded sidewalls (4°–18°) and nearly 100 m in relief at the margins; (2) a network of sub-ice sheet meltwater channels; and (3) a transparent, glacigenic seismic unit. The orientation of the paleo-ice stream trough indicates that the ice stream flowed to the south-southwest, toward the shelf break. This suggests that the ice stream formed further to the north, where it appears that Georges Bank (southeast of the Gulf of Maine, USA) redirected ice flow. Limited well data constrain the glacial erosion event (up to 300 m below sea-level) to occur within the Pleistocene. The glacial event represents a time of larger ice volume on the northern Atlantic continental shelf, as compared to the LGM; thus, we suggest that the event corresponds to marine oxygen isotope stage 12 (late Pleistocene) when the first major Pleistocene shelf-crossing glaciation began offshore southeastern Canada. These geophysical constraints on a late Pleistocene glaciation offshore Massachusetts have important implications for: (1) models of the Laurentide Ice Sheet, as the geomorphic evidence of pre-LGM ice streams are difficult to characterize yet account for most of the ice sheetʹs mass flux; and (2) the pore water salinity pattern offshore New England, as sedimentary basins near an ice sheet margin often contain large volumes of glacially emplaced freshwater.