PALEODELTAS AND PRESERVATION POTENTIAL–HOLOCENE EVOLUTION OF THE MAINE SHELF

The inner shelf of the northern Gulf of Maine is underlain by Pleistocene glacial and deglacial sediments. Lowstand paleodeltas created 12-10 ka occur at the mouth of the Kennebec River, the Merrimack in Massachusetts and perhaps the St. John in New Brunswick. Reworking of the deglacial and lowstand deposits, and small new inputs of fluvial sand, created a complex mosaic of Holocene transgressive deposits, a palimpsest of modern and relict units. Controls include bedrock framework, local sediment sources, rapid local sea-level changes, and variable waves and tidal currents. We have investigated shelf and littoral evolution using seismic reflection profiling, side-scan sonar, and vibracoring. On the Maine coast, sea level fell during deglaciation 14-11 ka from 70 m above present to 60 m below present, and then rose 10 ka to present. In SW Maine a thin shoreface wedge of barrier and backbarrier facies, overlies the basal unconformity over Presumpscot Fm. Moraines are preserved below 30 m depth because of rapid sea-level changes, but are planed off at shallower depths. In WC coastal Maine the lowstand Kennebec Paleodelta is the product of abundant coarse sediment supply. Transgression reworked the surface into barrier and estuarine facies. In EC coastal Maine, Penobscot Bay lacks abundant sand, and is filled with estuarine mud with abundant pockmarks. Eroding bluffs of till and glaciomarine mud, as well as submerged moraines are common. Evolution of Penobscot Bay after lowstand included tidal channel migration, infill, avulsion, and overstepping. Two ravinement surfaces are found: tidal ravinement and open bay wave ravinement surfaces. The mud-buried mid-estuary Penobscot Paleodelta occurs at –30 m, creating ca. 9-8 ka during a hesitation in sea-level rise during transgression. The stratigraphic evolution of this complex paraglacial coast requires identification of the primary erosional surfaces: basal unconformity formed by fluvial and littoral processes, tidal ravinement unconformity formed in inlets and estuaries, and wave ravinement unconformity, formed at the shoreface. Facies may have higher or lower preservation potential between these surfaces depending on paleotopography, sediment type, exposure to waves and currents, and rate of sea-level change.