MYSTERY CAVE, MINNESOTA: A WINDOW INTO THE PALEOHYDROLOGY OF THE UPPER MISSISSIPPI VALLEY
Numerous speleogens in the upper, dry levels of the cave record a much older hypogene, phreatic speleogenesis phase of Mystery Cave. Groundwater flow in this early stage was upward and appears to have been east to west. The age of this phase of the cave’s development is only constrained to older than the oldest dated speleothem in Mystery Cave, >~300 ka and younger than the Ordovician when the carbonates containing the cave were deposited. But the initial phase of the cave’s development may well date to the early Cenozoic or Mesozoic times.
This interpretation, if correct, provides pathway to reconstruct the regional groundwater flow before the establishment of the current Mississippi River drainage. The east to west drainage pattern is consistent with the pattern of thin, discontinuous Cretaceous deposits between the Ordovician carbonates and the unconformably overlying Pleistocene glacial sediments in Fillmore County. Hypogene flow provides a simple explanation for the problematic iron ore deposits, developed in paleosinkholes that were mined in Fillmore County in the 1940s through the 1960s. One of those ore bodies was directly over Mystery Cave.