Geology of Montana

The structure was created due to the collision of tectonic plates about 59-75 million years ago that drove a several mile thick wedge of Precambrian rock 50 mi (80 km) eastwards, causing it to overlie softer Cretaceous age rock that is 1300 to 1400 million years younger.

Regional low-grade metamorphism altered sandstone, carbonates and silty shale to quartzite, argillite and dolomite.

Most Paleozoic rocks originated in a marine environment, particularly dolomite and limestone together with shale, siltstone, sandstone, and evaporites such as gypsum, anhydrite and salt.

Tuff and andesite lava flows resulting from volcanic eruptions is found interbedded in sedimentary rocks.

The Fort Union Formation in the east, made up of the Tongue River, Lebo and Tullock members is a remnant of Paleocene deposition.

Intermontane valleys preserve thick sequences of Oligocene and Miocene rocks that are rare in other parts of the Eastern Plains.

These sediments vary considerably, from claystone, mudstone and siltstone to bentonite, diatomaceous earth, coal and tuff.

In places bentonite has been mined from altered volcanic ash beds in Cenozoic intermontane sedimentary basins.

The region also has significant production of oil and natural gas from the Cat Creek anticline, the Kevin-Sunburst dome and the Sweet Grass arch.

The eastern region produces petroleum, principally in the western Williston Basin and the Cedar Creek Anticline.