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The Belt Supergroup, is an assemblage of Mesoproterozoic sedimentary rocks, primarily mudstones, which outcrop chiefly in western Montana, but also exposed in Idaho, Washington, Wyoming, and British Columbia. 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, rocks of the Udzha Basin in Siberia, and the Rocky Cape Group in Tasmania. It was named after the Big Belt Mountains, Little Belt Mountains and town of Belt, all in western-central Montana. ==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, carbonates, and mafic sills. 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 Ravalli 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 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Belt Supergroup」の詳細全文を読む スポンサード リンク
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