AN INTEGRATED ANALYSIS OF BACK-BARRIER SANDS, PHILLIPS BEACH, SWAMPSCOTT, MA, USA

Beaches experience cycles of erosion and accretion in response to meteorologic conditions. In New England, hurricanes and Nor’easters are the primary episodic events that shape the coasts. The most severe storms can be accompanied by overwash, which transports sediment to the back-barrier environment. The aim of this study is to find evidence of washover deposits or paleochannels within the back barrier sands of Phillips Beach, Swampscott, MA in order to reconstruct its morphological history. Phillips Beach is a south-east facing type-2 welded barrier beach with small, continuous dunes and three back-barrier lagoons. A preliminary, shore-parallel, 288 m ground penetrating radar (GPR) line was run at 400 MHz with a 50 ns window between the dunes and lagoons in the back-barrier environment. The radargram shows a symmetrical 20 m breach in the barrier with its thalweg approximately 5 m below the current berm crest. Based on the radar profile, two vibracores (PB11-VC#) were taken from the back- barrier. Magnetic susceptibility (MS) was measured at a 1 cm resolution. MS for PB11-VC1 averages 615.9 SI, whereas PB11-VC2 displays a mean of 446.0 SI. Point counting data (n=42) for both cores shows similar modal mineralogy for all samples. Sieving was performed using U.S. Standard sieves at a 0.5 φ scale and at 2 cm resolution down-core. PB11-VC1 shows a mean grain size of -0.10 φ and standard deviation values up to 1.31 φ, whereas PB11-VC2 has a mean grain size of 0.44 φ and displays standard deviation values up to 0.76 φ. The coarsest zone within PB11-VC1 is composed of very coarse sands and occurs between 3 and 10 cm down-core. Samples within this zone show a mean grain size of -0.58 φ and an average SD of 1.19 φ. Based on these data and the close proximity of the two cores, PB11-VC1 is interpreted as being located within the confines of an overwash fan or paleochannel while PB11-VC2 falls outside of its boundaries.