PRELIMINARY INTERPRETATIONS OF A TOMBOLO’S DEPOSITIONAL HISTORY: A GROUND PENETRATING RADAR INVESTIGATION, GRAND ISLAND, MI

Grand Island, part of the Hiawatha National Forest, is the largest island on the southern shore of Lake Superior. Grand Island consists of two north-south trending sandstone bedrock highs connected by a 2 km long, 0.75 km wide sand dominated tombolo. Limited development and a freshwater, sandy coastal depositional landform make the Grand Island tombolo a good location to conduct a ground penetrating radar (GPR) survey. GPR is a noninvasive method to image subsurface stratigraphy using electromagnetic waves. The purpose of the GPR survey was to collect and analyze over 2000 m of shore parallel radar profiles, gaining insights about the Grand Island tombolo’s internal architecture to better understand its depositional history. The GPR system used for this study was a pulseEKKO 100 with 50 and 100 MHz antennae and a 1000 V transmitter. Step size varied from 0.5 m to 0.25 m, and antennae separation varied from 2 m to 1 m respectively. Common midpoint surveys collected in the field were analyzed to determine near surface velocities in order to estimate depth. Relief measurements were collected using a Topcon RL-H3CL laser level to adjust the profile to reflect changes in topography. Processing and plotting utilized EKKO_Project software and applied horizontal and vertical averaging, dewow filtering and automatic gain control to the data. Depth of penetration varied from 5 m to 20 m. Profiles are separated into various radar facies based on relation of reflections to one another and described using radar stratigraphic terminology. Previous GPR data on coastal depositional landforms aids our interpretation of the radar facies. Our results suggest progradation from both the eastern and western bedrock lobes, a trough structure near the center of the tombolo, and aggradation completely filling the trough. Erosional contacts are also mapped within the tombolo. Additionally, a beach ridge sequence can be observed on the tombolo’s surface. Our preliminary interpretations document the evolution of the Grand Island tombolo based on analysis of the internal stratigraphy imaged with a GPR survey.