Trilobites and brachiopods of Late Cambrian and Early Ordovician ages were obtained from seven wells that penetrated the Deadwood Formation in the Williston basin in Montana and North Dakota. The sections contain a basal transgressive sandstone overlain by interbedded limestone pebble conglomerate, dark shale, and light colored sandstone. On the east in North Dakota the section becomes markedly sandy. In these eastern sections it is difficult to place the Deadwood-Winnipeg boundary exactly, but in Montana there is a sharp lithic contact which represents a significant subaerial erosion break in the section.

In Montana Dicellomus of Dresbachian age is present in the basal sandstone. The Elvinia, Conaspis, Ptychaspis-Prosaukia, and Saukia zones of Franconian and Trempealeauan ages occur in the overlying 287 feet. There is no evidence of a depositional break between the Upper Cambrian and the Lower Ordovician, and beds at this level carry the transitional Apoplanias faunule. An arbitrary boundary is placed at the first appearance of Symphysurina and (or) the associated Euloma. Faunas of the Lower Ordovician trilobite Zones A, B, C, D, E, and G (of Ross [1951] and Hintze [1953]) have been found. Intercalated graptolite shales permit correlation with Berry's Lower Ordovician graptolite Zones 1, 2, and 3. On the basis of the faunas the Deadwood can be correlated with formations throughout the anadian Rockies, Montana, Wyoming, Utah, Nevada, Colorado, New Mexico, Texas, and Missouri.

Lithofacies exhibit many lateral changes within the basin. No persistent units can be recognized for the entire basin, but local facies members useful for mapping in smaller areas are distinguished.

Reconstruction of paleogeographic conditions for the Late Cambrian shows the region as a wide, shallowly inundated coastal shelf with a west-northwesterly slope. A positive transcontinental arch between Laurentia and Siouxia forms a low sandy eastern shore or a maze of islands and shallow straits connecting with the upper Mississippi Valley region. The recurring transgressions of the middle Dresbachian, the early Franconian, and the middle Trempealeauan were not accompanied by upwarping of the land areas and may have been caused by eustatic sea-level changes. Shoaling in the Late Cambrian moved the shoreline gradually westward to western North Dakota. The Saukia zone trilobite genera died out, but dendroid graptolites persisted on the oxygenated shoals. Slow deposition of carbonate oo es, muds, and glauconites continued in the basin sites. The appearance of Euloma, Symphysurina, and Hystricurus marks the beginning of a new marine transgression which altered the shore topography and eventually carried the Zones A and B assemblages into the upper Mississippi Valley. Depositional thickening of the basal Lower Ordovician strata from the Richey to the Pine and Cabin Creek fields near the Montana-North Dakota border suggests that the axis of the earliest basin depocenter was in the direction of the Pine and Cabin Creek fields. During Zones D-E time, widespread carbonate shoals supported a gastropod-brachiopod fauna allied to assemblages of the central interior states. By Zones F-G time, the eastern sections were markedly sandy. Zone G faunas are the youngest Early Ordovicia assemblages known in the basin. The continued shoaling and eventual cessation of deposition probably occurred throughout the Williston basin during Zone H time, or definitely before the end of the Early Ordovician.

Although no hydrocarbons are known in the Deadwood, it is suggested that the Deadwood shales may be source rocks. Stratigraphic entrapment of these postulated hydrocarbons may have occurred in Winnipeg strata or in sandy Deadwood facies during Early to Middle Ordovician time along the then newly formed eastern rim of the basin.