A PRELIMINARY GROUND PENETRATING RADAR SURVEY EXAMINING THE GEOLOGIC HISTORY OF PRESQUE ISLE, PENNSYLVANIA

To date, no study has systematically examined the subsurface stratigraphy of Presque Isle, a migrating sand spit in Lake Erie, Pennsylvania. Digital elevation models of the peninsula clearly distinguish prehistoric beach and eolian dune ridges, which were used to define study sites. A MALA^® ground penetrating radar (GPR) unit with a 250 Mhz antenna, vibracoring, and sediment analyses were undertaken to compare 3 prehistoric beach/dune systems with the modern analogue, to better understand the geologic evolution of Presque Isle.

Dunes are composed of structureless to crossbedded fine, well sorted quartzose sand that is frequently penetrated by roots. Eolian processes blow fine sand off the beach, which then accumulates in vegetation downwind (generally southeast) to form dune ridges directly adjacent to the active beach. These same processes occurred episodically in the past, resulting in several prominent dune ridges separated by low beach and wetland environments.

On GPR profiles, beach facies show strong, dipping reflectors that are likely due to the presence of alternating, laminated quartzose and heavy mineral sands, and gravel. Beach facies reflectors downlap basinward. Dune sediments are characterized by more complex, often weaker reflectors that 1) downlap onto basinward beach facies, 2) onlap older sediments on the stoss side of bedforms, and 3) downlap in the down-wind direction. Modern beach facies have steeper beds than older equivalents, which may be due to changing depositional patterns caused by the placement of erosion control structures. Longshore bars are evident in the subsurface, as is glacial till, marked by the occurrence of large rocks at depth.