UNLOCKING THE LOWSTAND: GEOMORPHOLOGY OF MAINE’S INNER CONTINENTAL SHELF AND THE HOLOCENE TRANSGRESSION REVEALED BY MULTIBEAM

Relative sea level in the Gulf of Maine has varied over 120 m vertically since the Laurentide Ice Sheet retreated. Marine transgression over the last 12 ka reworked glacial sedimentary deposits infilling depressions and exposing Paleozoic bedrock. Multibeam data, collected by the Maine Coastal Mapping Initiative (MCMI) since 2014 and earlier by NOAA, provides new geomorphic evidence confirming a paleo-shoreline to at least a 60-m depth. Off Kennebunkport, we discovered an esker with a N-S sinuous 48-m deep crest truncated in a 12-m bluff with a base-of-slope from -60 to -67 m along a 220-m shoreline. This feature is in an embayment facing SW into a 95-m deep basin with distinct sediment depocenters, possibly submerged deltas. A smooth sediment apron seaward of the bluff’s base suggests wave reworking to an 80-m depth along a distance of at least 1.4 km. Beginning around 12 ka, rapid sea-level rise at about 7.5 m/century would have submerged the bluff’s toe and crest below wave base to a 15-m depth in 500 years, contributing to its preservation.

During the lowstand, the Penobscot River discharged to the sea through one or more deep bedrock valleys between Rockland and Vinalhaven. Bedrock constrictions apparently caused sediment deposition along the west shore, seaward of present-day Rockland to Port Clyde. Located here is a large sedimentary body with a level surface at -35 m which could have formed during rapid sea level rise. A bedrock shelf valley 80 to 90 m deep exits the ancestral Penobscot Bay, curving to the southeast of Vinalhaven Island. This pronounced shelf valley is sediment-starved and draped with moraines unburied by Holocene fluvial or marine sediment. This paleo-channel through coastal highlands would have provided a narrow marine connection to a broader, possibly euryhaline, paleo-Penobscot Bay. Adjacent shoals have 20 preserved De Geer moraines across 3.2 km. If the moraines were annual, regional ice-margin retreat to the NW would have been on the order of 160 m/year.