STRATIGRAPHY AND GEOLOGIC HISTORY OF PHILLIPS BEACH (MA) BACK-BARRIER LAGOONS

Overwash deposits associated with Nor’easters and hurricanes are known to occur within New England back-barrier environments. Occasionally a breach in the dune will lead to the formation of a tidal channel connecting a back-barrier lagoon with the ocean. The purpose of this study was to analyze the stratigraphy and interpret the Holocene geologic history of three back-barrier lagoons (north, central, and south) associated with the southeast-facing Phillips Beach welded barrier system, Swampscott, MA. Specifically, we test the hypothesis that past overwash deposits and/or evidence of paleochannels are present in this location.

Two Livingstone cores were recovered in April 2010 from the north lagoon, one proximal and one distal from the barrier. Three additional cores were recovered in February 2011 from the central and southern lagoons. The central lagoon had one proximal core and the southern lagoon had one proximal and one distal core. The samples were analyzed down-core for lithology, wet and dry bulk densities, loss on ignition, and magnetic susceptibility (MS), as well as for grain size, and mineralogy of sand beds.

The results obtained show the two southern lagoon cores having the highest average organic matter content (OM), which likely relates to a stable depositional environment with high productivity, organic accumulation, and a relatively consistent sedimentation rate. The proximal and distal cores taken from the north lagoon have some of the lowest average organic matter and the highest wet and dry bulk densities, which can be attributed to sand beds present within the core. The central lagoon has gyttja followed by interbeds of coarse to very fine-grained sand. A near- basal radiocarbon date of 2573 ± 144 cal yrs BP on a terrestrial macrofossil from the north lagoon proximal core constrains the history to the past three millennia.

The deposition rates of sand layers are interpreted to be higher than those representative of organic deposition. It is likely that the thicker sand layers seen within the distal cores relate to deposits associated with paleochannels at Phillips Beach due to the energy requirements of such transport. The thinner layers of sand with smaller grain sizes are likely from overwash events because they require less energy to move than coarse-grained sediment.