A thick succession of Mississippian mud rocks in the eastern Great Basin has historically been identified as the Chainman Shale and interpreted as a foreland basin deposit associated with the Antler orogeny. Our work shows that this section is two distinct mud-rock units separated by a Late Mississippian unconformity. The Gap Wash Formation is identified as the mud rock below the unconformity, and Chainman Shale is restricted to mud rock above the unconformity. Poor exposure, sparse age control, and limited lithologic contrast have hampered identification and correlation of the mud-rock units. New research identifies subtle differences in rock type, and mineralogical and source-rock analyses show that the two formations have contrasting characteristics. These are associated with petrophysical properties that can be correlated across the region. The data show that the Gap Wash Formation has significantly more organic carbon and better pyrolysis results than the Chainman Shale. The Gap Wash also has mineralogy that could make it more suitable for treatment as a shale reservoir. The new understanding of Mississippian mud rocks in the Great Basin can focus future hydrocarbon exploration. The mud-rock units record regional Mississippian paleogeographic evolution. The Gap Wash Formation was deposited into accommodation space created by the loading of the continental margin during the Antler orogeny. The Chainman Shale was deposited in a successor basin that formed in response to resumed continental margin contraction in the late Meramecian to early Chesterian and is not clearly associated with the Antler orogeny.