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By 1.8 billion years the ancestral North American continent had grown quite large indeed and was continuing to grow. It was part of a super-continent called Rodinia which would be completely assembled by 1.1 billion years. About 1.5 billion years ago a rift system developed within this large continent in Montana and Idaho. (See map in Fig. 1 of Harlan's article for the U.S. distribution of the Middle Proterozoic Belt Supergroup; and the Atlas map for the distribution in adjacent Canada.) The rift was initally similar to the East African rift, being confined to within the continent, but quickly established a connection with the ocean so that marine sediments were deposited in the rift, called the Belt basin. The Red Sea rift is perhaps a modern analog. The mountain chain along the Montana-Dakota border was a major source for sediments which were carried westward to the Belt Basin. An incredible thickness of 17 kilometers of sediments, known collectively as the Belt Supergroup, accumulated in the Belt Basin with about two thirds of this thickness being deposited during the period of 1.48 to 1.44 b.y. [According to Jim Sears' 1994 article, the Belt Basin trended east-west with the Lewis & Clark Line forming its northern boundary and its eastern extension forming the Helena embayment. These two features are well-known to Montana geologists. Large displacements during the formation of the Rocky Mountains created the present pattern.] Similar east-west trending rifts formed the Vulcan Low (also known as the Calgary Trough) in Alberta and along the Wyoming/Utah border (now the Uinta Mountains).
The Mid-Continent Rift System formed within the Rodinia Supercontinent 1.1 billion years ago and extended from the Great Lakes down through Iowa and Kansas. It is well exposed in northern Minnesota.
The rifting process in the Belt Basin finally succeeded in splitting the Rodinia Supercontinent forming the Panthallassic Ocean. Continental crust which was formerly west of Montana and Idaho is now part of eastern Anarctica. Ian Dalziel's map shows Rodinia just as it began splitting apart at 725 million years ago. Chris Scotese's map shows the Rodinia Supercontinent after splitting apart at about 650 million years. (On both maps ancestral North America is rotated 90º clockwise and is labeled "Laurentia.") The rifting was accompanied by widespread basaltic volcanism extending from the Canadian Arctic to Wyoming. Steve Harlan and co-workers determined radiometric ages of 780 to 770 million years for diabase (basaltic) intrusions (dikes and sills) in the Beartooth and Tobacco Root Mountains and in Wolf Creek Canyon, northeast of Helena.
The long period of rifting and uplift during the Proterozoic (from 1,500 to 550 million years ago) caused regional uplift of Montana, Wyoming, and Alberta. Up to a third or half of the crust was eroded. For example, granulites which were at 20 kilometers depth at 2.8 billion years ago in Tobacco Root Mountains in southwestern Montana and at about 12 kilometers at 1.8 billion years, were exposed on the surface by Cambrian time (500 million years ago). The large mountain chain along the Montana-Dakota border that had formed along the Trans-Hudson Suture Zone was eroded to a peneplain and covered by flat-lying sediments in the Cambrian.
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Plate Tectonics of Montana (Page 5 of 14)