The Quaternary Stratigraphy and Glacial History of the Duluth-Superior Area

Abstract

Three major lithostratigraphic units of Pleistocene age are exposed in the Duluth - Superior area, all of which are the result of Late-Wisconsin glacial activity. The lowermost unit, a reddish-brown, sandy-textured diamicton (sand/silt/clay ratio: 52/38/10) containing abundant clasts of Precambrian red sandstone, is interpreted to be subglacial till of the Lower Cromwell Formation, which was deposited by the Superior lobe during the St. Croix phase of glaciation approximately 20,000 years B.P. Its main exposure within the study area is at the base of stream valleys and roadside cuts in the southern portion of the Esko Quadrangle. Overlying the Lower Cromwell Formation is a very compact reddish-brown, silt-rich diamicton (sand/silt/clay ratio: 30/43/27) interpreted to be subglacial till of the Upper Cromwell Formation, which was deposited by the Superior lobe during the Automba phase of glaciation, approximately 14,000-18,000 years B. P. This unit is the dominant stream valley and roadside exposure in the southern portion of the Eska Quadrangle. The Upper Cromwell Formation also contains a supraglacial facies composed of flow tills and outwash sediments deposited during the retreat of Automba phase ice. These flow tills have been previously interpreted as subglacial tills associated with a later "Split Rock" advance. This supraglacial facies is the dominant surficial deposit in the northern portion of the Esko Quadrangle. The youngest lithostratigraphic unit grades vertically from a basal laminated silt and clay into massive red clay in its upper portions, and grades laterally into sands and gravels. Topographically, it is confined to elevations below 1115 feet, and is found throughout the southern portion of the Esko Quadrangle and most of the Berea Quadrangle. Based on its areal extent, facies relationships, grain-size trends, stratigraphic relationships, geomorphic expression, sedimentary structures, and engineering properties, the entire unit is interpreted to be glaciolacustrine sediment and is assigned to the Wrenshall Formation. It was deposited in Glacial Lake Duluth approximately 10,000 years B. P. The massive clay facies of this unit is not the equivalent of the Douglas Till Member of the Miller Creek Formation deposited during a late-glacial ice expansion referred to as the "Marquette" phase approximately 9900 years B. P. That advance affected portions of northern Wisconsin and Upper Michigan, but did not reach as far southwest into the Superior Basin as the Duluth - Superior area.