DETAILED STRATIGRAPHY AND EVOLUTION OF A PARAGLACIAL BARRIER, CASTLE NECK, NORTHERN MASSACHUSETTS

Thirty percent of the world’s coasts are glaciated and yet little is known about the stratigraphic framework of sandy barriers in these settings. Previous investigations have provided simplistic models based solely on core data. Recently Ground-Penetrating Radar (GPR) has been utilized to define a high-resolution subsurface stratigraphy based on large-scale erosional/depositional surfaces and facies boundaries. A comprehensive study of Castle Neck Barrier was performed using a total of 27 kilometers of ground-penetrating radar, 120 meter seismic line and 49 sediment cores. The resulting stratigraphy reveals a complex evolution delineated by ten barrier and associated facies: drumlin, reworked drumlin, dune, marsh, forest, spit, bar, channel fill, storm lag deposits, and beach/nearshore.

A chronostratigraphic evolution for this barrier begins with an underling topography consisting of drumlins deposited over bedrock. Seismic reflection data show bedrock at an average depth of 13 meters in the north and dipping slightly to the south with a maximum core depth of 20 meters. The basal sedimentary unit is composed of barrier and estuarine sediments anchored to drumlin and bedrock highs. The landwardmost portion of the barrier consists of shoreface accretionary facies. Initially these progradational units contain numerous heavy mineral deposits denoting periodic storm erosion. Continued progradation to the south appears rapid an uninterrupted with the absence of storm lag deposits. As this northern portion of the barrier built seaward much of the architecture was influenced by Parker River Inlet processes. Migrations of the main ebb-channel have produced extensive cut and fill structures. Overlying these facies is a gently seaward dipping coarse-grained layer that was deposited during a large storm event. The seawardmost portion of the barrier is dominated by bar welding facies, which result from the landward migration of swash bars during inlet sediment bypassing. The southern part of the island consists mostly of large-scale southerly dipping beds created by spit accretion into Essex River Bay. All of the barrier units are capped by aeolian facies varying in thickness between 1-6 meters. Buried within these dune facies are two paleo-forest floors and a paleo-marsh/lagoon, that formed late within the evolutionary history.