MYSTERY CAVE, MINNESOTA: A WINDOW INTO THE PALEOHYDROLOGY OF THE UPPER MISSISSIPPI VALLEY

Mystery Cave in Fillmore County Minnesota is a 19+ km, joint controlled maze cave. For at least the last 11 ka, the cave has functioned as an epigene subsurface meander cut off of the South Branch of the Root River (SBRR). The groundwater flow is downward, roughly west to east toward the base level established by the Mississippi River. The SBRR under moderate and low flow conditions sinks entirely beginning near the Mystery I Entrance, flows through the lower levels of Mystery Cave, in the process bypassing several km of surface entrenched bedrock meanders and resurges along a series of springs. Those resurgences are the head of an important trout fishery in the SBRR.

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.