ANALYSIS AND INTERPRETATION OF PIPELINE EXCAVATIONS IN THE SOUTHERN MARGIN OF THE GLACIAL LAKE AGASSIZ BASIN, WESTERN MINNESOTA

The geologic history of the southern margin of the Glacial Lake Agassiz basin in western Minnesota, is not very well understood, partly due to the lack of exposures. Fortuitous excavation by Alliance Pipeline for a new natural gas pipeline produced a series of exposures of the uppermost sediments in the area. The primary goal of the project was to investigate the sediments and stratigraphy exposed. Pollen analysis was performed in an effort to reconstruct paleo-environments and to determine if laminated clay units were varves.

Timing was key to the project. A small window of opportunity existed between the time the pits were open, and when they were the site of active construction. Nine exposures 2-5m in depth spaced approximately every half-mile along a northwest transect were analyzed. The exposures revealed a northwest progression of clayey till to laminated clay sediments with one exposure containing cross-bedded sands. This stratigraphy is consistent with a glacial lake and post-glacial drainage depositional environment. The laminated clays were not varves. Pollen analysis of the clays showed no significant amount of pollen grains present, suggesting that the depositional environment was either not conducive to pollen preservation, or else there were no pollen producing plants in the vicinity at the time of deposition.

Varve exposures are rare in the southern margin of Glacial Lake Agassiz basin, thus not finding varves in this study was not a surprise. The project did however, take advantage of time sensitive geological exposures in areas typically not revealed. Additional studies of the units below, and to the northwest and southeast are needed.

Research for this study was funded by a grant from the National Science Foundation - Research Experiences for Undergraduates (REU) Program (NSF-EAR 9820249).