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The Norphlet Formation is an important oil and gas reservoir in Mississippi, Alabama, and Florida, and is also one of the key units for interpreting the early history of the Mississippi salt basin. In Clarke County, Mississippi, the Norphlet comprises a nonfossiliferous terrigenous clastic sequence which overlies Louann evaporites (Middle to Late Jurassic) and underlies Smackover carbonate rocks (Late Jurassic). The thickness of the formation ranges from 50 to 500 ft. It consists of three lithofacies: a basal black shale, a middle red siltstone, and an upper quartz sandstone. The black shale facies was deposited in topographic depressions in the surface of the regressive Louann evaporite. The overlying red siltstone facies, which was deposited in arid intertidal-to-suprat dal environments, represents the shoreline equivalent of the Louann evaporite. The quartz sandstone facies at the top of the Norphlet Formation was deposited along a low-lying desert coast, and includes sands inferred to be both eolian and intertidal.
The petrology and stratigraphic relations of the Louann-Norphlet-Smackover sequence can be explained best in terms of a deep-water basin periodically isolated from the world ocean. Thick deposits of Louann evaporites at the base of the sequence suggest that the area was at least partly cut off from the open sea during its early history (Schmalz's deep-water evaporite basin model). The overlying, mainly regressive Norphlet siltstones and sandstones are thought to indicate a time of nearly complete isolation and lowering of water level in the basin. The transgressive Smackover carbonate rocks at the top of the sequence represent a period of more open connection with the ocean and flooding of the basin. Full opening and establishment of holo-oceanic conditions did not occur until deposit on of the fossiliferous and oolitic carbonate sediments in the upper part of the Smackover Formation.
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