Quaternary history and ice dynamics of the late Wisconsin Rainy and Superior lobes, central Minnesota

The contrasts in glacial landforms and sedimentology of the glacial deposits associated with terminal and recessional moraines of the Rainy and Superior lobes of the Laurentide Ice Sheet in Minnesota have been used in conjunction with computer modeling to develop a spatially and temporally dependent glacial-process facies model. The western sector of the St. Croix morainic system and associated deposits in central Minnesota are divided into two regions, based on contrasts in glacial landforms. The northern section is characterized by a large frontal outwash apron, broad ice-stagnation topography, glacial thrust systems, and an absence of streamlined topography and features produced by subglacial drainage. Recessional ice margins are similar to the terminal moraine, with large scale glacial-thrust systems being even more common. The southern section of the moraine is composed of closely spaced recessional ridges with little ice stagnation topography. Recessional ice margins are marked by ice-contact slopes and small outwash fans with eskers commonly forming the fan handles. Drumlins are abundant and a large system of tunnel valleys and eskers indicates that englacial and subglacial drainage was well developed. Segments of tunnel valleys frequently terminate at the recessional margins and eskers often head in distinct moulin kames.;The field relationships were used to constrain boundary conditions for numerical modeling of the thermal regime and geometry. The results suggest a slow rate of recession of the ice from the northern section, the area glaciated by the Rainy lobe. A frozen toe was maintained during ice retreat, allowing freezing-on and thrusting of large blocks of subglacial sediment. Drumlin formation and the development of subglacial drainage features were not favored. A climatic regime characterized by cold (ca. ;Farther to the south, in the area glaciated by the Superior lobe, the glaciological conditions were substantially different. A narrow frozen toe existed at the maximum limit of the ice. However, at recessional ice margins the frozen zone was absent. Surface meltwater was able to penetrate to the glacier bed, forming tunnel valleys and eskers. Drumlin formation was occurring behind the retreating ice margin.