Sedimentation in the Duluth-Superior Harbor, Lake Superior

Abstract

The sedimentary environment of the Duluth-Superior Harbor was investigated by examination of surface sediment samples, borehole stratigraphy, seismic reflection profiles, and aerial photographs. Harbor sediments are predominantly silt. Sand is found in a narrow band along the shores of the embayment, on the lower reaches of the St. Louis river bed, and at the base of constrictions on the floor of the dredged channels in Superior and St. Louis Bays. Lag is found upstream in the St. Louis River and in the ship entryways. Clay lies in isolated depressions on the harbor floor and in tributary embayments lining St. Louis Bay. Total organic carbon concentrations correlate well with grain size variation. Percentages range from 0.06 to 5 percent dry weight, with the highest concentrations occurring in the fine-grained sediments of the inner bays. Mineralogy of the harbor sediments is relatively uniform. The medium sand fraction is dominated by lithic fragments. Opaque minerals, amphiboles, and pyroxenes constitute the majority of heavy minerals in the fine fraction. In the clay-sized fraction, relative percentages of smectite, illite, kaolinite, and chlorite vary systematically with bulk-sediment textures. Kaolinite and chlorite are concentrated in the finer sediments, whereas illite is more abundant in coarser sediments. The distribution of surface sediment texture reflects exposure to currents produced by seiches, river currents, ship traffic and wind generated waves. Engineering borehole data and 3.5 kHz seismic reflection profiles were used to reconstruct stratigraphy and Holocene history of the area. The variation of sediment types records changing water levels and environments in western Lake Superior. Boreholes 10 to 61 m deep contain 7 lithologic units: till, glacial outwash sand, glaciolacustrine clay, postglacial peat, silt, clay, and nearshore sand. Seismic profiles contain 3 main reflectors which roughly correspond to the lithologic units observed in borehole logs. The two sets of spits outlining the harbor are Holocene features, possibly formed by emergence of offshore bars, submergence of coastline ridges, or spit progradation by longshore currents or "self generation". Post-formational modifications of the spits include breaching of inlets and inland sediment transfer by wind and water. Numerous comparisons can be drawn between the St. Louis River estuary and coastal estuaries. The St. Louis River estuary is small in comparison to marine embayments, having boundaries determined by the geology of the region. Marine estuaries predate the St. Louis River embayment by a few thousand years, inundated in response to glacial melting and isostatic rebound, respectively. Circulation in a marine estuary, driven by tidal currents and wind, is based on density stratification caused by the mixing of saline and fresh water. In the freshwater St. Louis River estuary, there is no density stratification, and periodic currents are driven more by seiches than tides. Shoreline changes over the past 120 years were described from historic maps and aerial photographs. Development of the shoreline and alteration of the environment through dredging have been accompanied by a rising water level on the southwestern shore of Lake Superior.