The Spearfish Formation is a geologic formation, originally described from the Black Hills region of South Dakota, United States, but also recognised in North Dakota, Wyoming, Montana and Nebraska.

[3] Other rock types include claystone, conglomerate, dolomite, and oil shale.

[4] It is typically regarded as Permian–Triassic in age,[4] although its original description[5] included Jurassic rocks.

[6] The Spearfish Formation is interpreted as representing shallow marine to coastal terrestrial deposition, like the modern Persian Gulf.

[7] Depositional environments may have included restricted marine bodies and ephemeral lakes (gypsum), hypersaline waters (limestone), hypersaline microbial mats (oil shale), and sabkhas (dolomite).