A DEEP-TIME APPRECIATION OF KARST FROM MINNESOTA TO THE MID-ATLANTIC: HONORING THE IDEAS OF CALVIN ALEXANDER

The evolution of karst systems through deep geologic time, and consideration of the associated geologic context, is often given less attention than is deserved. Dr. E. Calvin Alexander, Jr. has been a champion of placing conceptual models of karst development into a proper deep-time perspective by examining the geologic evidence of fluid migration through carbonates that may not, at first glance, appear to be related to modern surficial karst processes. Some of the geologic evidence for deep-time influence on karst formation in southeastern Minnesota that has been highlighted by Dr. Alexander include stratigraphic controls on high transmissivity horizons such as at the contact between the Shakopee Formation and underlying Oneota Dolomite of the Prairie du Chien Group, and the occurrence of Pb-, Zn- and Fe-sulfide ores within solutional voids at that horizon which indicate very old ages (~270 Ma) for the karst features that contain these deposits.

Evidence for a deep-time influence on the evolution of karst extends from the mid-west into the Appalachian Mountains, and exhibits some similarities. For example, Pb-Zn sulfide ores are hosted within voids in carbonates at various locations within the Appalachians, extending from Tennessee through Virginia and into Pennsylvania. Deeply weathered bauxite and lignite-bearing sediments within karstic depressions are found from Georgia to Pennsylvania, many of which were mined. Mn- and Fe-oxide ore deposits are common throughout the Appalachian karst areas, and several show evidence for a deep-seated rather than supergene source. Lastly, rare but important fossil discoveries indicate that the age of portions of the Appalachian surficial karst landscape likely extend back to the Cretaceous, a situation similar to the karst of southeastern Minnesota. Future work utilizing absolute age-dating techniques that can extend beyond the Pleistocene on mineral deposits in karst features is needed to better constrain karst-forming processes across deep-time.