PALEOECOLOGY OF THE GLACIOMARINE PRESUMPSCOT FORMATION (LATEST PLEISTOCENE) OF SOUTH-CENTRAL MAINE

The Presumpscot Formation was deposited during the deglaciation of the northern Gulf of Maine between 15 and 11 ka. Exposures on land originally depressed well below sea level were raised by postglacial isostatic rebound to present elevations of as much as 140 m above present. Fossil mollusks were collected from various exposures of Presumpscot Formation in southern and central Maine, with particular attention paid to Whitney Corners and North Waldoboro, pits near the town of Waldoboro. Both localities are on the flanks of the Waldoboro moraine, and expose tongues of glaciomarine mud that interfinger with till and subaqueous fan sand and gravel. A radiocarbon date from the mollusks in the Whitney Corner pit, at 46 m a.s.l. is 11,720 +/- 125 BP. More than 500 specimens were identified to 13 species. Taxa are composed of 12 species of mollusks, 8 bivalves and 4 gastropods. In addition, we identified one species of barnacle (Balanus sp.). The paleocology of the collection indicates a muddy embayment in cold water conditions equivalent to those found off Labrador today. The lower units contain a mixed fauna of deep and shallow marine types, while the uppermost sandy units are inhabited almost exclusively by Mytilus edulis, the inter-to-subtidal blue mussel. This suggests and emergent succession. All specimens were examined for shell distribution, shell size, and evidence of predation, in the form of boreholes from gastropod predators. Bivalves are the most abundant fossils seen (n=230), with Hiatella arctica being the most abundant (n=144). Natica clausa is the most abundant gastropod (n=8 out of 16). Shell lengths range in size from 12.1 mm (Natica clausa) to 83.4 mm (Neptunia despecta). Symmetry of bivalves indicates only slight preference towards right-handed shells, suggesting little post-death redistribution of shells. Evidence of predation indicates that Macoma balthica is the preferred prey species (25%). There appears to be no preference as to prey size; many different species have radula drill holes in them.