EROSIONAL GEOINDICATORS IN ATLANTIC BEACH SEQUENCES: IMPLICATIONS FOR COASTAL ADAPTATION FROM MAINE TO THE BAHAMAS

Geological signatures of landfalling storms dating from Late Holocene (3-2 ka BP) to Hurricane Sandy (2012) are variously expressed along sand-rich Atlantic coastlines. Depositional features (tempestites), such as washovers, ensure long-term retrogradation and aggradation of narrow barriers and allow short-term monitoring using ground and aerial surveys. Along prograded systems (functionally precluding overwash) or retrograding barriers stabilized by engineering structures, erosional geoindicators (EGIs) offer relevant benchmarks for event hindcasting and adaptation strategies. Often, subsurface imaging using georadar (GPR) is the only means of EGI detection. We present examples of EGIs along a spectrum of natural to heavily developed beaches from Maine, Massachusetts, New Jersey, Maryland, the Carolinas, and the Bahamas. Regardless of beach lithology, a morphological EGI is exemplified by a dune or berm scarp with erosional surface gradients ranging from ψ~20 to ψ>90°(over-steepened scarps). Paleo-inlet and overwash scour channels add a dimension of alongshore complexity. In dip-sections of unconsolidated deposits and lithified (e.g., tropical carbonate) lithosomes, post-storm sedimentary packages are characterized by downlap or onlap onto an EGI. These units offer natural references for the style and extent of recent beach and dune recovery following intense erosional episodes. Where lithological anomalies (size or density lags) are absent, spatially extensive truncations can be traced using near-surface imaging. GPR surveys helped identify buried pre-1900s storm scarps and paleo-channels beneath existing engineering structures and residential developments. Such features may guide site-specific zoning laws and construction setbacks, although time-sensitive ordinances require a reliable EGI chronology. In quartz-rich systems, optical luminescence dating is increasingly applied to constraining the timing of pre-historic storms and for post-1950s event correlation. Even without adequate age control, accurate mapping of landward and alongshore extent of storm impact is crucial for establishing local and regional coastal management practices, laws, and zoning strategies (e.g., flood levels, surge velocity zones, property boundaries, and inlet hazard areas).