INTERPRETING HISTORIC LAKE LEVELS FROM THE INTERNAL ARCHITECTURE OF A TOMBOLO: A GROUND PENETRATING RADAR INVESTIGATION OF THE GRAND ISLAND TOMBOLO, MI, USA

Grand Island, Michigan, USA is the largest island on the southern shore of Lake Superior and consists of several geomorphic features including bedrock highs connected by a narrow sandy tombolo (coastal deposit connecting two pieces of land). The island is part of the Hiawatha National Forest and development has been limited to hiking and biking trails, though there are several small inholdings. The protected status of Grand Island provides an excellent environment for the preservation of the tombolo’s internal architecture otherwise disturbed by human influences. A ground penetrating radar (GPR) survey taken on the Grand Island tombolo provides insight into its internal architecture. Two transects, parallel and perpendicular to the shoreline, using a pulseEKKO 100 GPR system with 100 MHz antennae were collected on the Grand Island tombolo. Step size was 0.5m with an antennae separation of 1.0m. A common midpoint survey collected in the field, determined a velocity of 0.1 m/ns. Topographic measurements were collected using a Topcon RL-H3CL laser level to adjust profiles to reflect changes in relief. Processing and plotting of the data utilized trace-to-trace and down trace averaging with the pulseEKKO software. Reflections are interpreted as bedding surfaces and divided into several reflection packages interpreted as different depositional environments created by fluctuating lake levels in Lake Superior which allows for approximate dating of the tombolo’s evolutionary phases to be determined. Bottom reflections, below 12m depth, relate to beach faces deposited during the Houghton Low. Middle reflections, from 5m to greater than 20m depth relate to waterlain aggradation associated with the Nippising High. Upper reflections, to a depth of 5m, relate to the development of the modern northward prograding strandplain.