The NSS Bulletin - ISSN 1090-6924
Volume 45 Number 3: 55-75 - July 1983

A publication of the National Speleological Society

Clastic Sediments in Mystery Cave, Southeastern Minnesota
Jodi Milske, E. Calvin Alexander, Jr., and Richard S. Lively


Mystery Cave is a complex, joint-controlled maze cave developed in the (Ordovician) Dubuque and Gelena formations in the fluviokarstic terrain of southeastern Minnesota. The cave system functions as a subterranean meander cut-off for a portion of the entrenched South Branch of the Root River. The cave contains a clastic sedimentary record that can be correlated with radioisotope ages obtained from calcite speleothems. These deposits record the following history: 1) initial development of the cave system in the shallow phreatic zone proximal to a water table level which pre-dates surface valley entrenchment; 2) deposition of a thick bed of finely laminated silt by backflooding of the cave by the South Branch of the Root River; 3) lowering of base level and draining of the cave, beginning at about 160 ka, resulting in headward erosion of the silt fill by vadose streams captured from the entrenching surface valley; and 4) deposition of stream gravels at about 145 ka and 13 ka at levels 15 to 25 m above the present stream level in the cave. The gravels are derived primarily from surface deposits of pre Late-Wisconsian 'Old Gray' drift, placing an upper limit of 145 ka on the age of this drift in southeastern Minnesota. Episodes of fluvial deposition in the cave correlate approximately with the onset of major cycles of speleothem growth. The stream gravels are interpreted as representing late-stage alluvial terrace deposits corresponding to the retreatal phases of the Wisconsinan and Illinoian Glaciations. This interpretation suggests a potential method for dating surface valley terraces in karst basins by correlation with cave terraces.

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