ELECTRICAL RESISTIVITY SURVEY AND INTERPRETATION OF GLACIAL DEPOSITS AT JONES POINT, LOWER HUDSON RIVER VALLEY, NY

Little investigation has focused on deposits near the Hudson River to reconstruct the paleoenvironment of Woodfordian glacier retreat in the Hudson Highlands area. Recent attention to the Jones Point location, previously described as a terrace made up of stratified boulders, gravel, sand and clay, involved an attempt to date these deposits using the optically-stimulated luminescence method (OSL), and sedimentological analysis to explain the context of geomorphic/sedimentary environments. The site, south of a steep promontory at a bend in the Hudson River opposite Peekskill, lends itself to geophysical investigation of sediments exposed in ridges adjacent to the Precambrian gneiss and granite bedrock. Field observations revealed very distinctive clast-supported horizons at several locations in the exposed bluffs. A geophysical survey over the top of the deposit was designed to identify the subsurface structure, particularly the geometry of a boulder horizon, across the studied location.

The survey was conducted using an AGI Supersting 28-electrode DC resistivity unit, with 2D inverse modeling of the subsurface, and included two perpendicular profiles along one ridge and across a saddle to the other. The goal of the investigation was to use the resulting imagery of apparent resistivity in combination with the bluff exposures to visualize the 3D subsurface spatial configuration of the boulder layer. Understanding the geometry of these distinctive glacial deposits can help in interpretation of the environment of deposition and possible links to other glacial sediments farther south along the Hudson.

Results revealed distinct zones of high apparent resistivity consistent with clast-supported boulder horizons in bluffs, however they don't appear to be spatially contiguous. Analysis of subsurface data and local geomorphology at the site support preliminary identification of glacial deposits as kame, kame-kettle, kame-moraine or kame-deltaic units. Using available sedimentary data from previous analysis of sediments at Jones Point, we can conclude that its sediment facies are consistent with ice-contact formation: initially subaqueous fine material, followed by boulder horizon of high-energy delta or supraglacial collapse origin, topped with glaciofluvial sandy sediments.