The NSS Bulletin - ISSN 1090-6924
Volume 39 Number 2: 43-54 - April 1977

A publication of the National Speleological Society

Possible Exhumed Fossil Cavern in the Madison Group (Mississippian) of the Northern Rocky Mountains: A Discussion
Newell P. Campbell


A fossil karst on the upper surface of the Mission Canyon formation of the (MIssissippian) Madison group in Montana, northern Wyoming, and western South Dakota was infilled by late Mississippian or early Pennsylvanian sediments of the Amsden and Kibbey formations. The many caves in the Mission Canyon formation may represent exhumed fossil caverns that data back to Mississippian time. Evidence substantiating exhumed fossil caverns of Mississippian age is:

1. Closed, sediment-filled caves intersected by drill holes down to a depth of several hundred feet that have not yet been exhumed.
2. Remnants of a fossil sediment fill (filled "blindleads") in caves open to the surface.
3. Large caves with their passages oriented down dip in deepley dipping limestones but showing features common to phreatic caverns.

Evidence not applicable for proving the existence of exhumed fossil caverns in the study area is:

1. Fossil breccia in sinkholes and small caves which appears to encounter large cave passages only at random.
2. Microfossils (inconclusive, but recent fossil material was found only in the top few inches of the cave fill).
3. Age dating by radiometric or paleomagnetic means (not useful for dating caves that may date back to Mississippian).

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