Unkpapa Sandstone (Jurassic), Black Hills, South Dakota: An Eolian Facies of the Morrison Formation

Depositional history of Jurassic rocks in the area of the Powder River basin, northeastern Wyoming and southeastern Montana

“…Upper Jurassic continental rocks in this area were originally referred to as the "Atlantosaurus beds" or the Beulah Clay (Jenney, 1899). At the turn of the century, Darton (1901a), recognizing the similarities between these rocks and rocks described by Eldridge (1896) Szigeti and Fox (1981), I consider the Unkpapa a member of the Morrison.…”

“…In Wyoming and western South Dakota, the Morrison generally is 125-370 ft thick (Imlay, 1980). Szigeti and Fox (1981) reported that the formation has an average thickness of 100 ft in the Black Hills (less where the Unkpapa is present), and Mapel and Gott (1959) reported thicknesses of 70-120 ft; Tank (1955), however, reported a maximum thickness of 223 ft. On the southeastern side of the Black Hills, the Morrison can be totally represented by the Unkpapa. In the southern Powder River Basin, Love (1958) reported that the Morrison is 150-220 ft thick, but Faulkner (1950) reported that it is 264 ft thick southwest of Casper, Wyoming, in the northern Laramie Range.…”

“…Watenvorn bone and teeth fragments (mostly reptilian), polished pebbles interpreted by some workers to be gastroliths, and fragments of petrified wood are common (Tank, 1955;Sohn, 1958;Moberly, 1960;Szigeti and Fox, 1981).…”

“…Darton (1899) named these deposits the Unkpapa Sandstone and, because he considered them older than the Morrison, gave the unit formational status. Reeside (in Yen, 1952), Sacrison (1958), and Szigeti and Fox (1981) demonstrated, however, that the Unkpapa Sandstone is a contemporaneous facies of the main body of the Morrison, and Szigeti and Fox (1981) recommended lowering its status to that of a member of the Morrison. Based on the established relation between these two units, I consider the Unkpapa a member of the Morrison.…”

“…Where the Unkpapa is overlain by the main body of the Morrison, the contact is either conformable and gradational or disconformable and sharp with as much as several inches of erosional relief. Where carbonaceous rocks of the Lakota lie on the Unkpapa, the contact is conformable and gradational; where a fluvial channel in the Lakota has cut down into the Unkpapa, the contact is disconformable and sharp (Szigeti and Fox, 1981).…”