The Belt Supergroup, is an assemblage of Mesoproterozoic

Mesoproterozoic

The Mesoproterozoic Era is a geologic era that occurred between 1600 Ma and 1000 Ma . The Mesoproterozoic was the first period of Earth's history with a respectable geological record. Continents existed in the Paleoproterozoic, but we know little about them...

 sedimentary rocks which outcrop chiefly in western Montana

Montana

Montana is a state in the Western United States. The western third of Montana contains numerous mountain ranges. Smaller, "island ranges" are found in the central third of the state, for a total of 77 named ranges of the Rocky Mountains. This geographical fact is reflected in the state's name,...

, but also exposed in Idaho

Idaho

Idaho is a state in the Rocky Mountain area of the United States. The state's largest city and capital is Boise. Residents are called "Idahoans". Idaho was admitted to the Union on July 3, 1890, as the 43rd state....

, Washington, Wyoming

Wyoming

Wyoming is a state in the mountain region of the Western United States. The western two thirds of the state is covered mostly with the mountain ranges and rangelands in the foothills of the Eastern Rocky Mountains, while the eastern third of the state is high elevation prairie known as the High...

, and British Columbia

British Columbia

British Columbia is the westernmost of Canada's provinces and is known for its natural beauty, as reflected in its Latin motto, Splendor sine occasu . Its name was chosen by Queen Victoria in 1858...

. It is most famous as the formation that makes up Glacier National Park in northwest Montana. It has been geologically linked to the Purcell Supergroup in British Columbia and rocks of the Udzha Basin in Siberia

Siberia

Siberia is an extensive region constituting almost all of Northern Asia. Comprising the central and eastern portion of the Russian Federation, it was part of the Soviet Union from its beginning, as its predecessor states, the Tsardom of Russia and the Russian Empire, conquered it during the 16th...

. It was named after Belt, Montana

Belt, Montana

Belt is a town in Cascade County, Montana, United States. The population was 635 at the 2000 census. It is part of the Great Falls Metropolitan Area.The town's main street is called Castner after its founder, John Castner...

.

Characteristics

The Belt Supergroup was deposited from about 1470 to 1400 million years ago and is as much as 18 km thick. It is mostly made of fine-grained quartzites, argillite

Argillite

An argillite is a fine-grained sedimentary rock composed predominantly of indurated clay particles. Argillaceous rocks are basically lithified muds and oozes. They contain variable amounts of silt-sized particles. The argillites grade into shale when the fissile layering typical of shale is...

, carbonates, and mafic

Mafic

Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...

 sills

Sill (geology)

In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet...

. In the primary location, western Montana and northern Idaho, the Belt is divided into four groups (youngest to oldest):

• Missoula Group - More fluvial deposition of sands and muds, similar to the Rivalli group but from the south
• Piegan Group (Middle Belt Carbonate) - Carbonate muds alternating with lamina of clastic muds
• Ravalli Group - Subaerial-deposited sands and muds, mostly fluvial, also from the southwest
• Lower Belt - Heterogeneous coarse to fine grained, clastic to carbonate rocks, mostly subaqueous, deep water deposition, with sediments derived from the southwest, with common mafic sills

Formation

It is generally believed that the Belt Supergroup was formed in a fault-bounded (rift

Rift

In geology, a rift or chasm is a place where the Earth's crust and lithosphere are being pulled apart and is an example of extensional tectonics....

) basin within continental crust

Continental crust

The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to more felsic, or granitic, bulk composition, which lies in...

, and that the rocks are "lacustrine

Lake

A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...

", or at least, not completely open marine. The basin was a part of a 1.45 Ga supercontinent that predated Rodinia

Rodinia

In geology, Rodinia is the name of a supercontinent, a continent which contained most or all of Earth's landmass. According to plate tectonic reconstructions, Rodinia existed between 1.1 billion and 750 million years ago, in the Neoproterozoic era...

. It shows evidence of these basin-bounding faults on all sides, except the west, which rifted away in the breakup of Rodinia. Siberia, Australia, and Antarctica have all been theorized to contain the extension of the Belt Supergroup.

Sedimentology

The Belt Supergroup is known for the strange sedimentary structures it displays. The sedimentation in the Belt basin was strange because 1) there is an abundance of fine-grained sand, but not much sediment coarser, 2) lack of sequence boundaries that are common in Phanerozoic

Phanerozoic

The Phanerozoic Eon is the current eon in the geologic timescale, and the one during which abundant animal life has existed. It covers roughly 542 million years and goes back to the time when diverse hard-shelled animals first appeared...

 sediments, and 3) cyclic and rhythmic deposition over long periods of time. The Belt Supergroup is also noted for "Molar Tooth" structures in carbonates (a bacterial degassing structure) and abundant stromatolite

Stromatolite

Stromatolites or stromatoliths are layered accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms of microorganisms, especially cyanobacteria ....

s of various types.