The Sundance Formation is a western North American sequence of Upper Jurassic age marine shales, sandy shales, and sandstones. The formation underlies the western North American Morrison Formation, the most fertile source of dinosaur fossils in the Americas, and is separated by a disconformity from the underlying Upper Triassic Chugwater Formation red beds. The Sundance Formation is known for fossils of an extinct species of marine cephalapod, Belemnites densus.