The Quaternary Geology of the Lake Johanna Region West-Central Minnesota

Abstract

The lake Johanna Region, encompassing an area of 625 square miles in west-central Minnesota, includes major portions of Swift, Pope, Kandiyohi and Stearns counties. The area has experienced a complex glacial history; all of the surficial deposits are ascribed to depositional events of the Wisconsin stage of the Pleistocene Epoch. If deposits of older glaciations exist in the area, they have eluded recognition. Bedrock is not exposed because of the thick mantle of glacial sediments. The landscape of the Lake Johanna Region is composed of three major elements: a rolling-ground moraine of low relief to the south, that is separated from an extensive outwash plain to the north by a long, linear, hummocky, stagnation moraine complex that runs diagonally in a northwest-southeasterly trend through the area. Numerous hypotheses have been presented during the past century in an attempt to interpret the depositional history of the drift in the Lake Johanna Region. All previous workers have based their interpretations on the spatial distribution of the various drift units throughout the entire state of Minnesota. . This study provides detailed stratigraphic and lithologic descriptions of all drift units in the Lake Johanna Region. Two different tills and outwash ranging from thick bedded, boulder gravels to laminated silts, constitute most of the surface and subsurface drift of the area. Numerous exposures show a variety of drift types in superposition including: outwash over till, till over outwash, till over till, and silt over all other sediment types. These stratigraphic relationships, coupled with topography, provide the key to an interpretation of the geologic history. The two tills, both calcareous; are distinguishable on the basis of rock fragment content and texture. Generally, the lower till is a buff, calcareous, sandy, shale-poor till (less than 5 percent contained shale fragments) having a textural compositon of 58 percent sand, 30 percent silt and 12 percent clay (average of 10 samples), whereas the upper till is a buff, calcareous, silty, shale-rich unit (more than 5 percent contained shale fragments) with a textural composition of 51 percent sand, 30 percent silt and 19 percent clay (average of 36 samples). Regionally, on the basis of composition and texture, the lower till of the Lake Johanna Region is correlated with the till exposed in the Wadena drumlin field to the north. The upper till is correlated with the drift that is the surface deposit over a large area in the Minnesota River Valley to the south. The lower till was deposited by the Wadena lobe, which made its way from its source in the Winnipeg lowland down through north-central Minnesota to its final terminus south of the Lake Johanna Region. The upper till was deposited by the Des Moines lobe, which originated further west and followed the Red and Minnesota River Valley lowlands to its terminus in central Iowa. This divergence in paths accounts for the difference in lithologies of the two types of till. Ice stagnation played an important role in molding the major geomorphic features of the Lake Johanna Region. Controlling factors included: the amount of debris carried by the ice, the position of the debris in the ice, the volume of meltwater produced, the rate of melting and differential erosion and deposition. The major stagnant ice features include an extensive kame complex, perched lake plains, ice walled outwash plains, the Blue Mounds Ridge System (a perched drainageway), and an extensive esker system. Flanking the linear ice stagnation complex are ground moraines, outwash plains, and proglacial lake-beds. The origin of all these features is intimately related to the activity of two ice lobes and their final disintegration. The Wisconsin history of the Lake Johanna Region consists of five phases. During the first phase, Wadena lobe ice completely inundated the Lake Johanna Region, depositing a buff to yellow, sandy, calcareous, shale-poor till. The material from the lithologically distinctive upper Cretaceous Pierre Formation was incorporated into this ice, because it flowed along a course east of the Pierre outcrop belt. In phase two, the Wadena lobe wasted back across the region to a point northeast of the study area, leaving massive blocks of dead ice in pronounced topographic lows which are probably the remains of a pre-Wisconsin drainage system. Meltwater from these large areas of ice swept great volumes of outwash material to the north and south of the ice masses until mantling by this debris slowed the ablation process. The main sheet of ice to the north continued to melt, back through Minnesota, leaving a complex mixture of outwash material and ablation till. In phase three, the Lake Johanna Region, including the buried dead ice, was covered by an advance of the Des Moines lobe. Because the Des Moines lobe crossed the eastern edge of the Cretaceous Pierre Formation, it deposited, in the study area, a ground moraine comprised of buff to yellow, calcareous, clayey, shale-rich till. During phase four, both the thin Des Moines lobe and the buried Wadena lobe ice gradually wasted. Water from this melting ice ponded to produce numerous small proglacial lakes along the northeastern margin of the stagnant ice. Large volumes of water poured through an ice-walled gorge south of Starbuck (the Blue Mounds Sluiceway) along a drainage line produced by a crevasse system or a sag in the continental ice sheet along this linear topographic low. This water eventually drained into Glacial Lake Benson which occupied a low-lying area in the southwestern portion of the Lake Johanna Region. In the last episode of the Quaternary history, the Des Moines lobe had completely wasted, except for a large block of ice that occupied the Lake Minnewaska basin. Meltwater from this ice initially flowed north, but as lower outlet levels were found, the waters flowed south and west into the Lake Emily system. The Wadena dead ice complex was also in its final stages of melting, which caused it to lower a complicated melange of stagnant ice features onto an older stable till surface. Extensive river systems draining this complex again swept copious volumes of debris onto the Bonanza Valley outwash plains. Lake Benson continued to expand until it finally breached an ice-cored morainic dam near Redwood Falls, and then it was quickly drained. As all ice melted, the drainage channels were deprived of their discharge; they have since been gradually filling with paludal deposits.