High-resolution subsurface (GPR) imaging and sedimentology of coastal ponds, Maine, USA: Implications for holocene back-barrier evolution

Ground-penetrating radar (GPR) transects and sediment cores have been used to examine the basement morphology, stratigraphy, and environmental history of maritime ponds along the peninsular coast of Maine. Silver Lake, Lily Pond, and North Pond are shallow (< 3 m) water bodies bordered by steep bedrock ridges in the north, east, and west, and sandy barriers to the south. The bedrock basins of the ponds are formed in metasedimentary rocks surrounded by resistant pegmatitic intrusions. A dense network of GPR traverses obtained over the ice-covered Silver Lake reveals a series of prominent wavy-parallel and basin-fill reflector geometries terminating against the bedrock or grading into the barrier sediments and interpreted as organic lake-bottom facies. The transparent units represent sand-rich horizons, mostly eolian in origin. Convex-up structures found both on the surface and within the basin-fill sequence are interpreted as preserved parts of coastal dunes. The present study indicates that freshwater conditions prevailed since at least 4.6 ka, with an initial sedimentation rate of 1.7 mm/yr. The position of this unit below the contemporary sea level suggests presence of a welded barrier by that time. Radar profiles taken along the shores of Lily Pond, a small water body behind the Sand Dune Barrier, indicate a significantly larger areal extent of the pond in the past. A succession of organic deposits overlying a Pleistocene glaciomarine unit indicates progressive inundation of the paleo-lagoon by rising sea level. Saltwater peat seaward of Lily Pond was buried by washover sands, about 1.2 ka, and a narrow pond existed here prior to dredging and artificial infilling of its eastern part in the 1950s-60s. The organic and eolian units are absent in the North Pond, where sedimentary fill consists of glaciomarine clay overlain by marine sands. A proposed three-stage model of pond evolution along an embayed coastline consists of: (1) organic accumulation in an upland depression during lower sea level; (2) predominantly washover or tidal deposition in a lagoon (Stage 2a) or blocked coastal pond (Stage 2b) during initial transgression, and (3) mainly eolian and organic deposition behind a prograded or aggraded barrier. Future accelerated rise in relative sea level and inadequate sediment supply will cause many back-barrier ponds to reenter Stage 2 of the proposed model.