A Baraboo vexation: Applying a new landform development model to a classic area in Wisconsin

Steven Gerald Kenaga, Purdue University

Abstract

For more than a century, men of science have investigated the singular geological features of the Baraboo, Wisconsin, area, which has illustrated fundamental principles of geology to generations of students. The Baraboo rock-record there spans more than 1.5 billion years of geologic time, and because 87% of that time is represented by erosional unconformities, geomorphology plays an important role in what students are taught. However, frameworks for accepted geomorphic histories were established by the early twentieth century, and these histories have become dogma that few have dared, or bothered, to question. Remapping geology around Devil's Lake fails to support the traditional Precambrian age of gorge-cutting. Instead, gorges of Devil's Lake, Upper Narrows, Lower Narrows, and Narrows Creek, are thought to have been cut by spill-over of ponded glacial meltwater during development of Glacial Lake Wisconsin. Thin-sections of samples from an outcrop in the north end of Devil's Lake gorge, previously mapped as Cambrian, appear to be neither Baraboo Quartzite nor Cambrian sandstone and, therefore, is thought to be a younger Proterozoic quartzite. The flat, summit plain of the Baraboo Range, and bedrock benches cut into the range flanks, traditionally are thought to result from either early Paleozoic wave-planation, or by Tertiary peneplanation. Paleozoic bedrock underlying some of these surfaces makes impossible wave-planation of the same age, and peneplanation is not considered a workable explanation. These bedrock surfaces are reinterpreted as having developed by localized pedimentation under a humid, tropical/subtropical savanna environment, thought to exist in parts of Wisconsin throughout most of the Tertiary. This interpretation closely follows a model outlined by Budel (1957). Associated with pedimentation is the Windrow Formation. The Windrow is believed to be the product of repeated episodes of intense weathering and erosion prior to final cementation by iron oxides; the degree of weathering and type of cement also are preferentially developed under a tropical/subtropical climate. Accelerated erosion, triggered by Pleistocene climatic change, has accentuated differences in bedrock lithology, creating the appearance of retreating escarpments, and established the Wisconsin River by glacial diversion, leaving the landscape as seen today.

Degree

Ph.D.

Advisors

Melhorn, Purdue University.

Subject Area

Geology

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