Insight into beach ridge formation using ground penetrating radar.

The internal architecture and formation of lacustrine beach ridges has long been a topic of debate, partly owing to the lack of continuous data through the ridges and their complex stratigraphy. Past interpretations were based on correlating isolated information from strategically placed vibracores across beach ridges. Today, ground penetrating radar (GPR) provides a method to view inside ridges and collect continuous data to define and correlate sedimentary units within beach ridges. Baedke and Thompson (1995) proposed a theoretical model explaining beach ridge development as a product of changing rates of sediment supply and water level change. This model was based on information from numerous vibracores and current shoreline processes. Thompson and Baedke (1995) redrew the Curray (1964) diagram, focusing on the positive rate of sediment supply side of the diagram and placing importance on water level changes crossing the aggradation line for the development of individual beach ridges. Some of these theoretical ideas of beach ridge formation and shoreline development have been verified using GPR. Several continuous GPR reflection surveys were collected across beach ridges in three embayments along the Lake Superior shoreline (Grand Traverse Bay, Tahquamenon Bay, and Au Train Bay). Digital GPR data was collected using a Noggin 250 SmartCart with a fixed 250 MHz antennae and a recording interval of 5 cm between traces. The depth of penetration was from 5 to 8 m. Information from vibracores were used to estimate the velocity of the radar signal and calibrate the GPR data before processing. Beach ridge topography was measured using a transit and used to correct GPR lines. Although each beach ridge has a unique GPR signature, they all contain a series of lakeward-dipping reflectors and a strong concave reflector that extends lakeward from the base of swales. The strong reflector is interpreted as an erosional surface (ravinement) created during lake-level rises while the other reflectors are interpreted as the offlapping part of the progradational development of beach ridges