THE MITCHELL AQUIFER KARST HYDROSTRATIGRAPHY CORRELATED WITH EROSIONAL HISTORY OF THE SPRINGVILLE ESCARPMENT IN INDIANA

The hydrostratigraphic name Mitchell Aquifer for Indiana was introduced by Conner (2015) and encompasses three stratigraphic cave tiers defined by three spring stratigraphic levels in the Kirby Watershed based on 55 karst springs in an 80 M limestone sequence. This paper expands the documented aquifer range to include an area of approximately 15,999 Km² (6,172 Mi²) in south central Indiana, extending 240 Km (150 Mi) southward from Putnam County to Harrison County bordering on the Ohio River. The range follows the trace of the Springville Escarpment through the Crawford Upland and Mitchell Plateau. The three cave tiers are stratigraphically confined and are correlated with three transitional periods of base level decline. The bedrock dip is 7 M/Km (37 Ft/Mi) to the southwest and the Springville Escarpment’s erosional retreat is westward during the periods of base level decline correlated with the speleogenic enlargement of each of the three cave tiers. The speleogenic enlargement process was described and Palmer (2002). This study adds to the Gardner model (1935) by providing the surface erosional context related to the static water zones. This karst geology was important in the design and construction of a 23 Mi segment of Interstate 69 traversing karst terrain in south central Indiana (Kutschke, Conner, and Krothe, 2014).

Upper tier cave speleogenic enlargement at Kirby Watershed, and with variations throughout the expanded range of the Mitchell Aquifer, progresses with surface water from headland tributary fissures underflow in the Crawford Upland. Middle tier cave enlargement progresses with recharge from well defined sinking stream basins recognizable as localized outliers of the Mitchell Plateau. Lower tier cave enlargement progresses with recharge from sinkholes in the karst plains of the Mitchell Plateau. Additionally, karst spring cutoffs link between cave tiers. The cutoffs begin upstream from a cave spring and divert flow downward and below the tributary floor toward another cave in the next lower tier.

The implied periods of enlargement for caves in each of the cave tiers are related to the erosional retreat of the Springville Escarpment and suggests that enlarging caves in each of the tiers are younger from east to west and upper tier caves enlarge to the west while/after enlarged upper tier caves to the east are eroded.