INTEGRATION OF ONSHORE-OFFSHORE GLACIAL GEOLOGY IN MASSACHUSETTS BAY USING HIGH-RESOLUTION SEISMIC REFLECTION PROFILES: NEW INSIGHTS INTO GLACIAL LAKES, SYNGLACIAL MARINE INCURSION, AND SOUTH CHANNEL ICE LOBE IN THE GULF OF MAINE

Glacial geology mapped on land for the new Quaternary Geologic Map of Massachusetts is extended offshore by re-interpretation of more than 11,500 line-km of high-resolution seismic reflection profiles collected during many previous studies. Map units include: glacially modified coastal plain-cored hills, drumlins, end moraine deposits, ice-marginal lacustrine/marine fans, glacial lake deltas, glacial lake clay, fluvial-estuarine channel-fill deposits, glaciomarine clay, and glaciomarine deltas and spits.

Mapped deposits offshore record the existence of Glacial Lake Stellwagen (GLS) at altitudes lowering from about -30 m to -50 m. Lake containment requires persistence of an active South Channel ice lobe (SCL) in the Gulf of Maine impinged against Georges Bank to the south, allowing lake waters to escape through Great South channel. GLS deposits include successive ice-marginal lacustrine fans, thick glacial lake clay, and an extensive delta that prograded northward from meteoric streams draining Cape Cod. GLS may have spilled out at Race Point channel, but more likely it extended through that channel and occupied the area east of Cape Cod and south to a spillway at Great South channel. Lake drainage and an exposed lake bottom is indicated by an extensive channel system cutting the lake bed down to an altitude of -75 m.

Inundation by the synglacial sea could not occur until the SCL retreated enough to let the sea into the Gulf of Maine via the deep Northeast channel. Our mapping indicates this happened when the mainland ice margin stood at Boston, extending northeast to Cape Ann and across the south end of Jeffreys Ledge. This timeframe is estimated to be approximately 17.2 cal ka by correlation with ice margin positions in the Connecticut Valley. Eustatic sea level (based on the Barbados record) was -108 m at that time, requiring 116 m of glacio-isostatic depression to produce the +8 m sea level recorded by glaciomarine deltas at Boston.

NE-SW trending linear grooves seen on multibeam bathymetry north and east of Stellwagen Bank occur on glaciated K/T coastal plain strata outcropping at the ocean bottom; we interpret these as glacial grooves carved by the SW moving SCL ice. More irregular grooves cross-cut the linear ones and may be scours made by icebergs as the SCL became an ice shelf accompanying inundation by the sea.