Stratigraphy of the Lower Mississippian Lodgepole Formation of the Madison Group in the centralWilliston Basin, North Dakota, U.S.A.

The Lower Mississippian Lodgepole Formation in the Williston Basin of North Dakota is a carbonate succession that reaches a maximum present-day thickness of*900 ft (275 m) near the center of the basin. Twenty cores and more than 1,450 petrophysical well logs from theWilliston Basin in North Dakota and South Dakota were used to evaluate the large-scale stratigraphic architecture of the Lodgepole Formation in the south-central part of the Williston Basin. Six sequence stratigraphic cycles and numerous sub-cycles are recognized within the Lodgepole Formation. Maximum flooding surfaces mark the boundaries between these cycles, and these surfaces are identified in petrophysical logs by having higher gamma ray API values that correspond with organic shale, fossiliferous marl, and wackestone. The intervening strata between the flooding surfaces have lower gamma ray API values that correspond with less argillaceous carbonate facies including fossiliferous wackestones and packstones. The Lodgepole cycles collectively comprise a carbonate ramp system and have laterally variable thicknesses due to their lenticular-shaped dimensions and compensational stacking patterns. The Lodgepole Ramp was constructed in three phases, an early northward progradation phase (cycles 1 and 2), a middle aggradation phase (cycle 3), and a late west-northwestward progradation phase (cycles 4–6). An unconformity surface at the top of the Lodgepole impacts the total thickness of the Lodgepole Formation in such a way that some paleoerosional patterns of the subaerially exposed Lodgepole Ramp can be observed in parts of the basin.