Fabric alteration and isotopic signature of carbonates in a recharge area of the Madison Aquifer, Montana, USA

The Mississippian Mission Canyon Formation (Madison Group) is a shallow-wat er carbonate unit that has intermittently acted as an aquifer over geologic time. Outcrops in a modern recharge area provide physical evidence of cond uit flow in the form of paleocaves, bedding plane conduits, and solution ch annels. Principal alterations of the rock matrix are microspar neomorphism, leaching, and dedolomitization of carbonate mudstones. Petrographic and is otopic data indicate that the modern aquifer is equilibrating with ground w ater and over printing characteristics that may have been inherited from pa leoaquifers. (C) 2000 Elsevier Science B.V. All rights reserved.