Late Quaternary morphogenesis of a marine-limit delta plain in southwest Maine

Abstract

Detailed study of the Sanford-Kennebunk sand plain using sediment analysis of 125 samples from 20 locations and 37 km of ground-penetrating radar (GPR) indicates that much of the feature can trace its origins to regressive-phase deltaic deposition during the late Pleistocene.The Sanford-Kennebunk sand plain is an ϳ125 km 2 elongate, west-east-trending landform overlying a large, deep bedrock trough. With the exception of the eastern coastal margin, the entire sand plain is bordered by bedrock uplands.The plain is composed of coarsening-upward sand and minor gravel. Sediment sorting ranges from poor to moderate in the western plain, to moderate to good in the eastern sector. Grain size generally decreases from west to east.Extensive ground-penetrating radar coverage shows that many areas display packages of clinoforms, the thicknesses of which range from shallow, Ͻ1 m thick, channel fill-type deposits to the more common 5-14-m-thick, rhythmic, inclined (4Њ-15Њ average dip) beds. Dip directions generally radiate outward from the west-central plain, and farther eastward, from the south-central plain. These GPR records in conjunction with the sediment analyses strongly suggest a deltaic origin for the orderly dipping beds of the sand plain.A model for development and evolution of the sand plain encompasses deglaciation, associated marine incursion, and subsequent regression. As ice retreated from the area and isostatic recovery led to marine regression, sediments from the glacier's margin were transported in energetic meltwater channels to be deposited in a bedrock-enclosed, marine bathymetric low, forming the Sanford-Kennebunk sand plain.