The Permo-Triassic Spearfish Formation of the southeastern Black Hills, South Dakota, consists of evaporite, clastic, and carbonate sediments which formed as the result of the complex depositional history.

The lithologies that occur as the result of primary deposition are (in decreasing order of abundance) gypsum, siltstone, shale, sandstone, conglomerate, limestone, dolomite, and a highly

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organic-rich marlstone (oil shale). The gypsum and limestone were precipitated in a low energy, hyper-saline, subaqueous environment, while the shale and organic-rich marlstone were deposited in a relatively fresh, low energy, subaqueous environment. The siltstone and dolomite were deposited in intertidal to supratidal conditions. Sandstone and conglomerate were deposited in a high flow regime fluvial environment. Although salt casts are common, no halite was observed in the area. Breccias occur as the result of post-depositional processes.

During the late Middle Permian, the depocenter of the Minnekahta sea shifted westward causing the beginning of Spearfish deposition in the area. Subsequent local fluctuations of the shoreline altered environments from shallow marine to terrestrial. Throughout accumulation of the formation, deposition continued in this fashion, similar to conditions currently observed in the Persian Gulf region.

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