The characterization and possible origins of two loess sheets in the upper Great Lakes region, USA.

摘要

In this thesis, I investigate two loess deposits - one in north-central Wisconsin and the other in northeastern Wisconsin and the western Upper Peninsula of Michigan. Loess thickness is typically less than 1 m across each of these two regions. The goal of this research is to determine the possible origins of these loess deposits by examining their spatial characteristics. Five sampling transects were utilized, each extending away from a possible loess source area, onto the loess sheet proper. A total of 113 loess samples were collected from these transects. Data indicate that the loess in north-central Wisconsin had two likely source areas: 1) the Late Wisconsin terminal moraine and 2) Cambrian sandstone outcrops in Clark County, WI. The Late Wisconsin terminal moraine was the primary silt source; fine sands were largely derived from the sandstone landscape. The northeastern Wisconsin and western Upper Peninsula loess deposit is surrounded by four potential loess sources: 1) the Watersmeet Moraine, 2) the Republic Moraine, 3) the Sagola Moraine, and 4) the Vilas County Outwash Plain. Loess data indicate that all of these landforms may have been contributing various types of eolian sediment to the loess sheet. This study provides evidence that landscapes that have previously not been recognized as loess sources may, in fact, have provided substantial amounts of eolian sediment to stable, upland landscapes in immediate post-glacial times.