SPATIAL AND BATHYMETRIC CONTROLS ON DROWNED SHORELINES IN THE WESTERN GULF OF MAINE

Bedrock, saltmarsh/tidal flats, bluffs and beaches of many morphologies are eroding along the rocky, glaciated, drowning coast of Maine. There was even greater variety of landforms at times of lower sea level, but most are not preserved. Preservation depends on the rate of sea-level change, sediment supply, accommodation space, antecedent geology and degree of exposure to waves. In the western Gulf of Maine, sea level reached a –60 m lowstand by 12.5 ka, rose rapidly to -22 m by 11.5.ka and then rose slowly to -17 m at 7.5.ka. It then rose again rapidly before rising progressively slower until recent. Preservation of Pleistocene landforms was best at times of rapid sea-level rise (SLR) and poorest during the 11.5-7.5 ka “slowstand”. Thus, erosional coastal landforms (notches cut into Pleistocene deposits), are prevalent near the lowstand, with numerous shorelines cut into older material, but sediment deposits from this time are few. Exceptions are the large paleodeltas of the Kennebec, Merrimack and Penobscot Rivers. These deltas represent times of large sediment input with, in part, falling sea level. Landward of the lowstand position, the inner shelf contains a condensed section with patchy Holocene deposits marking rapid SLR and accompanying transgression. The slowstand depth range, -17 m to -22 m, is well represented by excellent examples of constructional landforms (spits, tombolos) derived from slow erosion of glacigenic materials along with accumulations of sediments from coastal environments (tidal flat, salt marsh, freshwater wetland). In many locations, the retreat path of eroding bluffs is marked by preserved intertidal sediments. In all locations, preservation depends on the sheltering effect of bedrock; lacking protection from open-ocean waves, most sediment is removed to bedrock or a coarse lag deposit.