THE CONTRIBUTION OF REWORKING EXTENSIVE OFFSHORE GLACIOFLUVIAL DEPOSITS IN THE GEOMORPHOLOGICAL DEVELOPMENT OF THE WESTERN BUZZARD'S BAY COAST

Deglaciation of southern New England produced drainage through many of the valleys that characterize the northwestern Buzzards Bay (MA) coastal physiography. During this time vast quantities of sand and gravel were deposited offshore by meltwater streams at a lower stand of sea level. As sea level rose, storm waves and tidal currents reworked sand onshore within drowned valleys enclosed by till-covered bedrock peninsulas. This landward movement of sediment was especially integral in shaping: 1. the Horseneck Beach barrier, 2. the spit systems fronting Allen's Pond and its abutting backbarrier marsh, and 3. the dune system east toward Barney's Joy Point (next eastward promontory). It is estimated that this onshore movement of an estimated >20 million m³ of sand, here labeled the "Great Sand Event" occurred relatively rapidly during a period initiated 2,000-4,000 years ago. It is noteworthy that much of the sand comprising Horseneck Beach is highly mature (90% quartz and very well sorted) indicating the source was dominantly quartz. Analysis of photographs and ground penetrating radar (GPR) profiles of the study area reveal paleo-tidal inlets, prograding beach ridges, and spit systems, which have been used to identify the paleo-entrance to the Westport River Estuary and broad sand platform on which the surface geomorphology evolved. Differences observed in elevation and width between drowned backbarrier beach ridges and sand ridges that comprise the main Horseneck Barrier system delineate not only differences in age, but also abundance in sand. The most landward beach ridges are low and widely spaced indicating sparse sediment characterizing the early period of the Great Sand Event. This contrasts with the continuous thick beach ridge units marking the progradation and formation of Horseneck Beach. Following the end of the Great Sand Event, extensive backbarrier infilling and decreasing tidal prism in the Westport River Estuary allowed spit accretion and western extension of the Horseneck Barrier. This study has unraveled a complex geomorphologic history of an isolated sand-rich region of Buzzards Bay, accounting for a massive onshore sand event.