INTRODUCING THE FERTILE DUNE FIELD, MINNESOTA, USA: CONTEXT, GEOMORPHOLOGY AND CHRONOLOGIC CONSTRAINTS

The Fertile Dune Field is located immediately southwest of the town of Fertile in northwestern Minnesota. It overlies the Sand Hill Delta, where the Sand Hill River fed into the eastern shoreline of Glacial Lake Agassiz (GLA). The delta was abandoned during a major drawdown event of GLA, which exposed the surface of the delta allowing eolian mobilization of the sand. Although it has been studied ecologically, the dune field has not been widely studied in a geological context. The objectives of this project were to identify what types of dunes are present, evaluate dominant paleo-wind direction(s), develop a chronology for the dune field and determine if there were multiple eolian activity phases represented by the dunes. This presentation deals with the geologic context, geomorphology, and chronologic constraints for the dune field. The project utilized GIS for spatial analysis, field work including sample collection for lab analysis and SEM grain imaging. The delta is nearly circular in expression and covers an area of ~10.5 km2. The delta lies west of and lower in elevation than the Herman shoreline of GLA, suggesting that the delta formed when GLA occupied the Herman shoreline. The delta is overprinted along the western side by a beach ridge previously inferred to be a Norcross or Tintah shoreline of GLA. These geomorphological relationships constrain the formation of the Sand Hill delta between 14.2 ka and 13.4 ka. Formation of the Fertile dune field began shortly after delta abandonment. The dune field exists along the southern, eastern, and northern periphery of the Sand Hill delta, with the largest accumulations of dune forms atop the northern part of the delta. Both barchan and complex parabolic dune forms are present in the dune field in small (< 50 m wide), medium, and large sizes (> 250 m wide). Many dune crests have been noticeably modified by what are interpreted to be later stage blowouts and small scale parabolic dunes. Paleosols identified within the dunes give evidence for at least two activation periods.