INTERPRETATION OF A PALEO-INLET’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 2km long, 0.75km 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 2000m of shore parallel radar proles, 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 100 Mhz antennae and a 1000 V transmitter. Step size was 0.25m, and antennae separation was 1m. Common midpoint surveys collected in the field were analyzed to determine near surface velocities in order to estimate depth. Topographic measurements were collected using a Topcon RL-H3CL laser level to adjust the profile to reflect changes in relief and GPS coordinates were collected along each profile. Processing and plotting utilized pulseEKKO software and applied horizontal and vertical averaging, dewow filtering and automatic gain control to the data. Depth of penetration varied from 25 to 28m. Profiles are separated into various radar facies based on relation of reflections to one another and described using radar stratigraphic terminology. A full 2 km profile of the tombolo contained radar facies interpreted as progradational sand spits from both the eastern and western bedrock lobes separated by an inlet connecting Trout and Murray bays near the center of the tombolo. A closer examination of the profile containing the paleo-inlet reveals complex radar facies including a sequence of trough and fill structures up to 10m thick and 90m wide resting unconformably above hummocky sequences. Our interpretation of these radar facies suggests NW summer breezes caused sediment carried by waves refracted around the western bedrock lobe to choke the paleo-inlet, conversely, shore parallel NE storm winds eroded that sediment. The modern beach ridge sequence was deposited above the sand spits and paleo-inlet forming the tombolo that is observed today.