NEW INSIGHTS INTO THE DEGLACIAL EVOLUTION OF THE NEW MEADOWS RIVER, NORTHEASTERN CASCO BAY, GULF OF MAINE

The New Meadows River in northeastern Casco Bay, Gulf of Maine is a deeply indented embayment and deeply incised paleovalley with no modern fluvial connection. The river is also framed by two deltaic deposits at the head and mouth, the Brunswick sand plain (Weddle and Retelle, 1995) and western lobe of the Kennebec River paleodelta (Belknap et al., 1989; Barnhardt, 1994; Barnhardt et al., 1997). Previous studies have suggested that the Androscoggin River flowed into the Gulf of Maine through the New Meadows (Kelley and Hay, 1986; Belknap et al., 1989; Crider, 1998; Buynevich et al., 1999). This investigation primarily utilized seismic stratigraphy constrained by archival vibracore data to reconstruct the paleogeography, deglacial evolution, and determine if there was a connection between the Androscoggin River, New Meadows River, and the deltas. The interpretations of seismic stratigraphy suggest that the New Meadows River was never a large fluvial system. The channel is filled largely with glaciomarine Presumpscot Formation (Bloom, 1960, 1963), truncated by a basal unconformity. Salt marsh deposits are present mid-estuary coincident with the -20 m shoreline depth of the “slowstand” period, between 11,500-7,500 cal. years BP (Kelley et al., 2010). These salt marsh deposits fringe a 45 m deep basin that contains no evidence of fluvial sediments. The sequence throughout the river is predominantly capped with estuarine mud. When the subsequent rapid sea-level rise occurred after the “slowstand” period, the salt marsh deposits were drowned. Sandy deposits in the upper part of the New Meadows River are correlated to tidal channel incision and migration in the Thomas Bay area, with the Brunswick sand plain as the probable sediment source. This sand and gravel is trapped in the upper part of the system by a greater flood-to-ebb ratio in tidal currents, shallowness, and lack of fluvial input.