Precambrian igneous and metamorphic rocks underlie the Powder River basin. Their distribution is uncertain because of lack of subsurface control.

For the purpose of this paper the geologic section is divided into eight rock units. Some of these rock units cross time boundaries between periods. Rock units are used to relate the sediments deposited within each major sea transgression in the area. The rock units are separated by major unconformities. Sedimentation in the Powder River basin was controlled by regional tectonic features prior to Late Cretaceous time and by local tectonic features in post-Cretaceous time.

Rock Unit I includes strata of Cambrian and Early Ordovician age. The strata of Rock Unit I were eroded from the extreme southeastern part of the Powder River basin during post-Early Ordovician time. The strata contain facies changes which in other formations produce effective stratigraphic traps.

Rock Unit II contains strata of Ordovician age which thin southward because of depositional onlap and Devonian-Silurian erosion. The strata of Rock Unit II contain facies changes and porosity pinch-outs in beds of similar age to the productive Red River Formation of the adjacent Williston basin.

Rock Unit III is composed predominantly of carbonate rocks of Devonian and Mississippian age. These strata thin south-eastward as a result of depositional onlap and truncation. Widespread karst topography characterizes the upper surface of the unit. The possibilities of oil entrapment in the upper portion appear to be limited whereas the lower portion appears to have some potential.

Rock Unit IV includes the Pennsylvanian and Early Permian rocks. The terminology and age of the formations change on each flank of the basin. Lower Permian tectonics and erosion produced breached folds which were subsequently buried under Unit V. The types of oil traps differ; most of the production occurs in Late Permian beds.

Rock Unit V includes the Permian and Triassic redbeds. The boundary between the Paleozoic and Mesozoic is within Rock Unit V. Large-scale erosion occurred at the close of Rock Unit V deposition. The truncated upper sandstones offer possibilities for future oil exploration.

Rock Unit VI includes all Jurassic rocks and the transition strata at the base of the Cretaceous. Shoreline sandstones of the Jurassic and Cretaceous are potential oil reservoirs.

Rock Unit VII contains the strata deposited in the Cretaceous sea. The unit is divided on the basis of five regressive cycles. A western source area contributed most of the sediments in Rock Unit VII. Shoreline sandstones of the Cretaceous contain oil and gas. Many miles of known Cretaceous shorelines are yet unexplored.

Rock Unit VIII includes all beds of Tertiary age. The present-day configuration of the Powder River basin and the major structures and fault systems were formed at this time. Tertiary sedimentation was controlled by the present basin outline. Sandstones associated with lacustrine deposits are potential oil and gas reservoirs which are unexplored to the present time.