Reconstructing the bay-side geomorphic evolution of a freshwater baymouth bar in response to lake level change using three dimensional (3D) ground-penetrating radar (GPR) data

Abstract

Situated at the southwestern tip of Lake Superior, Minnesota and Wisconsin Points' form a 16-kilometer baymouth bar between Duluth, Minnesota and Superior, Wisconsin, providing the breakwater for the largest and farthest inland freshwater seaport in North America. Comprised of sandy sediment, this baymouth bar's formation is attributed largely to littoral drift from the Wisconsin South Shore and minor sediments from the outflows of the St. Louis and Nemadji River's. Due to continuing differential isostatic rebound of the basin, local lake level at Duluth is presently rising at approximately 25 centimeters/century. The objective of this thesis was to understand the geomorphic evolution of the bay-side of the baymouth bar in response to lake level change in relation to the system overall. Using an approach akin to energy industry seismic studies, the geomorphic expression of the bay-side baymouth bar’s response to lake level change was investigated with 39.62 kilometers of Three-Dimensional (3D) Ground-Penetrating Radar (GPR) data. 8 vibracores’, 11.24 meters’ in total, were advanced within the 3D GPR data to supplement subsurface understanding both spatially and temporally. Radiocarbon material taken from the vibracores were analyzed, dated, and used to establish a chronology for the evolution of the study area (~520.50 Cal yrs. BP to 181.25 Cal yrs. BP). Whole core logger data (including p-wave amplitude and velocity, gamma ray density, acoustic impedance, and fractional porosity) along with Loss on Ignition (LOI) samples taken from each core were used in conjunction with the GPR data to supplement the subsurface stratigraphic and facies identification. In total, 5 facies units were interpreted on the bay side of the baymouth bar, exhibiting a thick clay layer, shoreface ridge and shallow offshore sediments, overwash deposition, fluvial-flood like deposition, and man-made dredge deposits. Historical information and photographs dating back to the 1930’s provide evidence supporting interpretations of dredge material versus natural material. Although this baymouth bar is a young, non-marine system, reconstructing its geomorphic evolution in response to lake level change has become a useful analogue for similar, larger systems involved with base level change. In addition, stratigraphic findings of how such a system’s architecture is configured yield insightful clues towards vintage conventional exploration reservoirs. Finally, these data support a stronger understanding of how such a system geomorphically evolved in the context of the Lake Superior region post glaciation and aid in reshaping knowledge of how other geomorphic features and processes have developed throughout the region, perhaps providing tangible framework for future engineering and environmental management undertakings.