SEDIMENTOLOGICAL SIGNATURES OF RIVERINE-DOMINATED PHASES IN ESTUARINE AND BARRIER EVOLUTION, MID-COASTAL MAINE

An extensive sedimentological and geophysical database was used to examine the interaction of fluvial and marine processes and the resulting stratigraphic records in mid-coastal Maine. Estuarine valley fills, coastal accumulation forms, and shelf deposits of this formerly glaciated, highly indented coastal compartment exhibit distinct sedimentological signatures of former and present-day contributions by the Kennebec and Androscoggin Rivers. The diagnostic sedimentological attributes of fluvial sands (lithofacies F) include: 1) subangular texture; 2) moderate sorting; 3) medium to very coarse mean grain size; 4) variable gravel component, and 5) relatively low quartz content (<50% by volume of coarse-sand fraction). This lithology was observed in the deposits of the submerged Late Pleistocene paleodelta (seismic-stratigraphic facies D) and characterizes sediments flooring the modern estuarine channel. Onshore, similar riverine-derived units are confined to older, landward portions of the largest barriers at 1.5 -6.0 m depth below mean sea level (MSL). Underlying facies F are fine-grained, micaceous estuary/bay sands presently found in adjacent tidal embayments. In a seaward direction and extending onto the inner shelf, facies F grade into shell-rich Late Holocene estuary/nearshore deposits (facies E). Radiocarbon dates place the deposition of facies F westward of the Kennebec River mouth at 4.5-3.5 ka yBP.

Several fluvio-deltaic depocenters that were abandoned on the inner shelf (10-30 m below MSL) west and east of the main channel in Late Pleistocene became the only sources of sediment to coastal lithosomes during the Holocene transgression. As a result, even the younger beach/nearshore facies of barriers supplied by these palimpsest shelf deposits (partially reworked facies D) are lithologically similar to facies F. A noticeable downcore change in the mineralogy of this facies was documented at several locations and may be related to an abrupt shift in the relative contribution of the two fluvial sources. Lithostratigraphic units analogous to facies F comprise lower portions or nuclei of many barrier lithosomes throughout the world, particularly those characterized by episodic or continuous riverine contribution to the coastal ocean.