POSTGLACIAL STRATIGRAPIC EVOLUTION OF THE DAMARISCOTTA RIVER ESTUARY, MAINE, THROUGH BASIN ISOLATION AND STEPWISE TRANSGRESSION

The Damariscotta River estuary in west-central coastal Maine is framed by Ordovician-Devonian Bucksport Fm. amphibolite schists, and crossed by several large pegmatite dikes. Late Pleistocene glaciers scouring SSE over the NNE-SSW striking isoclinally folded beds sculpted a narrow valley with basins separated by sills and narrows. Postglacial sea-level changes were dominated by isostatic rebound followed by Holocene transgression. During the postglacial fall, the Damariscotta valley was progressively uncovered, forming a series of isolation basin lakes and wetlands. After lowstand, sea-level rise progressively overtopped the intervening sills (Shipp, 1989). Processes changed from quiet lacustrine sedimentation to an interval of flushing by tidal currents during initial overtopping, to modern estuarine dynamic equilibrium. The Damariscotta River today is a tide-dominated estuary, with a spring tidal range of 3.3 m, and is slow flushing, with an average of only 1 m³/sec freshwater input from Damariscotta Lake. Wave energy is a relatively unimportant factor throughout the system. Identification of preserved lacustrine sediments has been an important goal of numerous seismic reflection profiling and vibracoring studies (Hannum, 1997). To date, several targets of a hypothesized lacustrine seismic facies have been identified, but not yet confirmed by sampling. During transgression, oysters were able to colonize and create large bioherms near the sills and within the basins, before failing as predation and other environmental factors caused their decline. Notably, the Johnny Orr rapids in Newcastle-Damariscotta were overtopped 2.2 cal ka, and a large bioherm there supplied Native Americans who built one of the largest shell middens on the east coast of North America – The Glidden Midden and The Whaleback. A similar natural bioherm was formed in Dodge Basin 6-4 cal ka, however detailed surveys have not discovered a midden (Davies, 1992; Leach, 2007). Evolution of each basin differs in part due to the variable time of isolation from marine conditions, including nearby modern Damariscotta Lake, Biscay Pond and Pemaquid Pond. These lakes are presently 15-25 m above sea level and will not be transgressed in the foreseeable future, but they do provide analogs for the hypothesized isolation basins.