Abstract

The second-order Madison sequence is a late Devonian to middle Mississippian, carbonate and evaporite-dominated unit that is bounded by unconformities in landward areas (Black Hills) and conformable stratal surfaces in the Williston Basin. A relative sea level fall in the late Meramecian (middle Visean) initiated subaerial exposure that caused erosion in landward areas, and lower Kibbey silt and carbonate-evaporite deposition in the Williston Basin. The change to a more humid climate during transgression in the early Chesterian (middle to late Visean) led to local sinkhole formation and deposition of quartz sand in karst cavities in the Black Hills region, concurrent with upper Kibbey sand deposition in the Williston Basin. A late Mississippian interregional sea level fall initiated development of a first-order unconformity (the sub-Absaroka unconformity) that continued during transgression in the early Pennsylvanian. This caused peneplain erosion throughout the southern Williston Basin area, while a high-rainfall climate led to regional dissolution of Madison anhydrite beds that reached 100 m (328 ft) or more below the erosion surface. Most Madison paleokarst development occurred during this exposure event. An area of massive dolomitization occurs within the anhydrite dissolution area.