COUPLED MESOSCALE SEDIMENTARY RESPONSE DYNAMICS OF A BARRIER-BAY SYSTEM: NAUSET ISLAND AND PLEASANT BAY, OUTER CAPE COD

Nauset Island and Pleasant Bay on outer (SE) Cape Cod are a large (>20,000 acre) coastal parcel whose sedimentary environments and organismal habitats are intimately linked in a mesoscale sequence of response most clearly defined by cycles of inlet formation, migration, and repositioning. Inlet cycling at and near Chatham, MA has been described by Geise et al. and operates on an ~110 year period. The current cycle began in 1987 with the formation of New Inlet at Chatham over a four-day interval.

The coupled barrier/bay system was examined over the last 50-60 years of activity using historical records and photos; modern aerial photos and satellite images; and in-field sedimentology, geomorphology, and benthic ecology. The coupled system has undergone a three-stage evolution based on inlet behavior and bay response: a) inlet repositioning (essentially instantaneous during major storms), b) inlet structuring (lasting for years and producing highly dynamic conditions for both barrier and bay systems), and c) inlet migration (lasting for decades and characterized by waning tidal energy to back-barrier sites).

Data from Nauset Island shows that most existing barrier-island sedimentary environments and depositional terrains are linked with inlet migration and spit accretion. In the south Cape setting, both of these processes operate from north-to-south in response to net southerly littoral drift along the open-ocean shoreline. Pleasant Bay consists of three distinct sub-environments: an eastern portion dominated by tidal flows, a western portion characterized by littoral processes, and central ‘deeps’ which are mud sinks. Both the eastern and western sedimentary systems are energized during inlet repositioning and structuring with overall response waning during inlet migration. The present sequence of changes in the coupled system has also clearly been influenced by emplacement of coastal engineering structures after 1987. Numerous iterations of the coupled, barrier-bay, mesoscale response sequence have resulted in the morphology, sedimentology, and benthic habitats presently found throughout the southern OuterCape coastal zone.