REFINING THE MT. CARMEL FAULT ZONE WITH NEW BEDROCK MAPPING OF THE BEDFORD QUADRANGLE, INDIANA

The Mt. Carmel Fault zone bounds the eastern margin of the Illinois Basin for 80 km north-northwest across south-central Indiana. Its northern extent is obscured by glacial sediments, while the southern extent is challenging to define using traditional field mapping methods due to carbonate-on-carbonate offsets and denudated karst geomorphology.

Ongoing bedrock geological mapping efforts in the 1:100k Bedford quadrangle are recognizing and extending these mapped fault structures to the south. North of Campbellsburg, a pair of 60-m-long drill holes completed in November 2021 show 30 m of offset south of the previously mapped fault zone. Approximately 8.5 km east and 15 km south, a second pair of drill holes show about 15 m of offset even outside the mapped fault zone. LiDAR data suggests fault traces as sinkhole lineations on the Mitchell Plateau, extending the Mt. Carmel Fault zone southward another 5 km and outlining almost 24 km of new fault to the south and east. Investigation of mapped caves in Cave River Valley and Heltonville revealed undescribed normal faults down-stepping eastward in a graben to the most recognized fault of the Mt. Carmel Fault zone—possibly related to flower structures in a transtensional basin.

Future work to constrain fault geometry and mechanics may aid studies of the Illinois Basin, cave formation, Precambrian Basement structure, and earthquake hazard assessments. For example, the newly proposed fault to the south aligns with abutting Precambrian basement provinces identified in previous aeromagnetic surveys. The inference of the newly mapped fault extending to Precambrian basement is consistent with earlier interpretations of the Mt. Carmel Fault zone. Similarly, the east-west offset between the sections of the broad fault zone may help explain the eastward shift of the physiographic provinces in the Indiana Uplands, including major sections of the Mitchell Plateau.