Paper No. 4
Presentation Time: 11:15 AM
STORM-DOMINATED BARRIER EMPLACEMENT OVER IRREGULAR GLACIAL TOPOGRAPHY, BUZZARDS BAY, MASSACHUSETTS
The shoreline of Buzzards Bay is subject to high storm surge elevations and offers an opportunity to study the role of storms in the evolution of the flanking beaches. Along the eastern shore of the bay, a microtidal Old Silver Beach barrier evolved within an embayment bordered by irregular ice-contact topography (10-30 m-high ridges with 2-8 m-deep depressions). The recent history of the less developed, northern barrier is reconstructed based on historical data, 2.5 km of ground-penetrating radar (GPR) surveys, sediment cores, and diatom analyses. In geophysical profiles, glacial sediments produce a diagnostic chaotic reflection pattern that contrasts with overlying coastal deposits. Along the northern part of the barrier, GPR records reveal a channel-fill sequence (width: 90 m; max. depth: 6 m) within an antecedent depression. Widespread gravel and debris in the upper 0.5-1.0 m likely represent washovers from Hurricane Carol (1954) that raised the height of this barrier segment. A shallow dune depression adjacent to this washover area contains a 2 m-thick sand sequence with two peat horizons. Diatom content indicates that the lower peat unit (1.7-1.9 m depth) represents a saltmarsh environment, with the assemblage strongly dominated by brackish (Diploneis sp.) and marine (Paralia sulcata) taxa. This unit can be traced landward to the modern marsh, where similar diatoms occur. In contrast, the upper organic horizon (0.4-0.7 m) is devoid of diatoms, suggesting subaerial deposition in a dune swale. An AMS age of 1470-1640 cal AD on a rhizome from the lower peat provides a maximum age for the overlying sand. This sand may represent overwash deposition by intense hurricanes of 1635 and/or 1638. A shore-normal GPR profile across the southern headland-anchored section of the barrier shows a shallow glacial depression under the crest of the modern barrier. In contrast to the paleo-channel to the north, this topographic low does not connect to a backbarrier marsh in a landward direction. A 2-3-m-thick package of landward-dipping reflections within the depression represents overwash deposits that are in turn capped by 1-2 m of aeolian sands. Our study demonstrates the important role of antecedent topography and storm deposition in both landward migration and aggradation of coastal barriers within large paraglacial embayments.