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The Sauk Sequence in Indiana and adjacent states is composed of two supergroups in mutual facies relationship, the Potsdam below and the Knox above. The Potsdam Supergroup contains, in ascending order, predominantly siliciclastic rocks that include the Mount Simon Sandstone,
Eau Claire Formation, Galesville Sandstone, Ironton Sandstone, and Franconia Formation. The Davis Formation replaces the latter three where they cannot be differentiated. The age of the Potsdam ranges from pre-Dresbachian through Franconian. The Knox Supergroup consists in ascending order of predominantly carbonate rocks of the Potosi Dolomite, Oneota Dolomite, Shakopee Dolomite, and Everton Dolomite. The age of the Knox rocks ranges from Franconian through Whiterockian.
Cambrian sedimentation in pre-Dresbachian time is poorly known and may be restricted to fault-block basins. Terrigenous sand and mud were deposited in peritidal and shallow subtidal, predominantly marine, environments in the early Dresbachian. By the late Dresbachian, carbonate sand and mud, including oolite shoals, accumulated in the south, whereas marine terrigenous sand and mud were deposited in the north. In the Franconian, a bi-lobed delta complex, not yet well understood, dominated by terrigenous sand and mud, formed to the north, and the earlier shallow marine environment of carbonate deposition in the south expanded northward, so that by the end of the Franconian, pervasive carbonate deposition was extremely widespread.
The Oneota and Shakopee rocks were deposited in widespread shallow marine environments, and the record of marine sedimentation of Everton rocks is restricted to the southwestern part of the study area. Extensive post-Knox erosion has greatly reduced the original distribution of the Sauk Sequence, and rocks of the Chazyan are unconformably juxtaposed with rocks as old as Franconian. The distribution of rocks at the top of the Sauk and the consideration of the internal stratigraphy of the Sauk should influence future decisions for petroleum exploration, underground gas storage, and deep-injection liquid-waste disposal.
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