Styles of Coastal Progradation Revealed in Subsurface Records of Paraglacial Barriers: Duxbury, Massachusetts, USA.

Evidence of barrier progradation is found throughout the world, although the modes of seaward growth and their stratigraphic signatures can be quite variable. Based on high-resolution ground-penetrating radar (GPR) images of several New England barrier systems, a mesoscale model of coastal progradation is proposed which consists of four styles: 1) uniform; 2) non-uniform; 3) punctuated, and 4) complex. The first two modes are typically represented by sigmoidal-oblique geometry of reflectors similar in angle to beachface slope, and are differentiated based on observable uniformity of reflector spacing. Punctuated progradation is manifested by: 1) sharp tangential-oblique reflector geometry representing the paleo-beach surface and 2) steeply dipping straight reflector configuration interpreted as a buried upper berm/dune scarp. Significant accumulations of heavy minerals (>30% by volume) resulting from severe storm erosion or riverine floods are responsible for strong return of the electromagnetic radar signal. Complex progradation-aggradation mode reflects an abundant sediment supply and can be recognized by complex sigmoidal-oblique radar facies with occasional toplap. The geometry of individual bounding surfaces may be used to document shifts in sediment supply. The present study indicates that seaward accretion of a jetty fillet may proceed with a time-averaged rate of over 3 m/yr, ultimately producing a progradational sequence comparable in scale and reflector characteristics to that of many natural coastal systems. Along paraglacial coastlines, changes in sediment supply (fluvial, offshore, or local glacial deposits) may introduce various amounts of texturally and compositionally heterogeneous sediments, which can alter the rates of seaward accretion and preserve a record of individual progradation episodes. Local sea-level history and storm and flood frequency exert a major control on the magnitude of accretionary phases and the extent of barrier erosion and retreat. A survey of prograded sequences reported in the literature indicates widespread application of the proposed classification. [ABSTRACT FROM AUTHOR]