LATE HOLOCENE BREACHES OF A MESOTIDAL WELDED BARRIER: PHILLIPS BEACH, SWAMPSCOTT, MA

Phillips Beach, Swampscott, MA is part of a mesotidal welded barrier complex with three small back-barrier lagoons. The purpose of this study is to constrain the geomorphic expression and timing of hypothesized breaches associated with previously identified GPR anomalies in the barrier and anomalous coarse-grained deposits in the back barrier lagoon. This study utilized a two-part strategy: shallow earth geophysics constrained the stratigraphy of the barrier, and geochronology of back-barrier sediments constrained the timing of identified events. A grid pattern was established for GPR lines run parallel and perpendicular to the shore at 200MHz and 400MHz. An anomaly is found in multiple lines with the geomorphic expression of a channel thalweg, approximately 2.5m below the sediment surface. Electrical resistivity results display a feature with resistivity values around 120 Ωm—presumably fresh water draining through the paleochannel, in the same location as the GPR anomaly. A macrofossil sampled in situ within lagoonal muds exposed on the beach face during a storm in 2011 dated to 2151 ± 38 cal BP. Since the barrier must have been seaward of this location at the time of deposition, any breach observed in the modern barrier must be younger than ~2100 cal BP. Landward of the geophysical anomaly, a coarse-grained sand unit was observed within a sediment core from the back-barrier lagoon. The sediment core is composed of fine – coarse-grained sands and mud. Radiocarbon dates obtained above and below the sand unit are 714 ± 25 cal BP (34-37cm) and 711 ± 24 cal BP (145cm), respectively. These dates indicate that a high-energy event took place approximately 710 cal BP (ca. 1240 CE). Below the sand unit, the ages are 2552 ± 26 cal BP (165cm) and 2476 ± 46 cal BP (211.5cm). These dates suggests that down-cutting into the preexisting lagoonal sediment occurred resulting in an erosional unconformity. The New England coast experiences many storms and it is likely that storm activity is responsible for creating the breach and depositing the coarse-grained sand unit. Coinciding GPR data and electrical resistivity data along with the radiocarbon dates show that the barrier has been breached in the past and that coarse-grained sand units in the back-barrier are likely associated with the breaching events.