       |
| |
[Ron Blakey has a series of pairs of maps on his website showing the both the tectonic features and distribution of sedimentary rocks (on the left side) and the paleogeography (on the right side) for the period of 600 million years ago to the present in the Western United States. These maps will be used extensively in the following discussions. The maps need a set of symbols to be interpreted.]
After the Rodinia supercontinent split apart, western North America (Laurentia) had a new margin. For the next 350 million years (i.e. late Precambrian to Middle Devonian) this margin was passive, similar to the present Atlantic margin of the United States. It slowly subsided and a large "wedge" of sediments was deposited along the margin.
In North Dakota the Williston Basin started subsiding very slowly, a process which is still continuing. The Williston Basin is a large circular basin located on top of the roots of the Trans-Hudsonian Suture Zone.
| |
Plate Tectonics of Montana (Page 6 of 14)
