TRANSITION FROM ICE SHELF TO TIDEWATER MARGIN IN THE GULF OF MAINE 20 – 14 KA

The Gulf of Maine (GoM) during LGM was filled by Laurentide Ice Sheet to Georges and Browns Banks. Basins and margins contain a record of deglaciation accessible to high-resolution seismic reflection profiling and piston coring. Till is overlain by proximal glaciomarine (PGM) with a draped geometry, distinct stratification, and abundant internal hyperbolic reflections from iceberg dump. Distal glaciomarine (DGM) has a draping geometry, with less distinct reflectors and fewer iceberg dumps. Ice shelves developed ca. 20 ka as warming and sea-level rise led to progressively back-stepping grounding lines. At the grounding lines wedge-shaped till tongues define ice shelf prograding and receding grounding lines exquisitely interbedded with PGM. There is little deformation at the interface, strongly suggesting a grounding line as opposed to ice push or outwash fan. Ice streams draining the Bay of Fundy brought reddish sediments as far west as Wilkinson Basin, and through the eastern basins from Jordan to Crowell into Georges Basin, producing a transitional (TGM) unit, 15.7-16.9 ka, near the time of Heinrich Event 1 (16.5 ka). Coarse dumps and well-stratified reflectors suggest rapid advection and meltout south of Truxton Swell. Central and southern GoM is completely plowed by iceberg furrows to a bathymetric depth of 200 m, with some deeper in Georges Basin. The deglacial sequence in central Jordan Basin, north of and younger than the TGM, records ca. 14.5 ka transition from red-brown gravelly, sandy mud of Bay of Fundy provenance to gray mud from crystalline basement in New England. No till tongues are found in northern Jordan Basin, suggesting a transition to calving tidewater front also around 14.5 ka. This tidewater margin was characteristic of the ice sheet front as it retreated across the isostatically depressed coasts of Northeastern New England and New Brunswick, until becoming a terrestrial margin at the marine limit ca. 14-13 cal. ka. The tidewater margin demonstrates short (ca. 1 km) coarse fans interfingering with glaciomarine mud, actively deformed till and stratified sediment, and fossil evidence of shallow environments 14.5 to 12.5 cal ka. This is one of the world's best studied examples of termination of a marine-based ice margin, with extensive high-resolution geophysical records and correlative on-land exposures.