Central Montana has had a complex structural and sedimentary history, especially in the area of the present-day Central Montana uplift.

Precambrian and Cambrian subsidence was accompanied by the deposition of more than 13,000 feet of clastics in an east-west-trending trough roughly coincident with the present-day uplift.

Pre-Devonian uplift and erosion took place after stable depositional conditions during Ordovician time. Ordovician strata have been eroded from the western half of the study area. Silurian beds are absent from the entire study area.

The Central Montana uplift area remained high during Early, Middle, and part of Late Devonian time. Upper Devonian rocks lap onto the uplift, and uppermost Devonian sediments finally covered the area. Pre-Mississippian uplift and erosion removed latest Devonian carbonates and shales completely from a large area of the uplift.

The Mississippian System is composed of the carbonate-evaporite Madison Group and clastic Big Snowy Group. Stable conditions prevailed through most of Madison deposition, but central Montana began to subside in late Madison time. Distribution of the Big Snowy Group was restricted by continued subsidence which downwarped central Montana into a synclinorium.

Early Pennsylvanian streams draining eastern Montana cut valleys in the Central Montana trough. These valleys were filled, as the streams attained old age, primarily with sandstones and shales derived from the underlying Big Snowy Group. These stream-channel deposits, comprising the lower member of the Tyler Formation, contain the major reservoirs of central Montana. Middle and Upper Pennsylvanian sediments covered central Montana, but all of the Upper Pennsylvanian was eroded before Jurassic time. Pre-Jurassic folding accentuated Mississippian structure.

During Jurassic time uplift occurred in the Belt Mountain area at the west. Jurassic strata lap onto this high and thicken regionally eastward and in the Central Montana trough area.

Lower Cretaceous deposition was controlled by uplift at the south. These sediments thicken from south to north.

The Laramide orogeny upwarped the old trough to form the Central Montana uplift and generally folded the old synclines into anticlines as, for example, at the Sumatra trend. Differential epeirogenic movements, which probably resulted from isostatic adjustment of basement fault blocks, caused the down-up-down-up movements of central Montana.

The wedging and truncation of Ordovician, Devonian, Mississippian, and Pennsylvanian reservoir beds created numerous prospects for oil and gas in and around central Montana. Zones of stratigraphic changes in the Pennsylvanian, Jurassic, and basal Cretaceous are also prospective. The area should be rewarding to the explorationist.