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Devonian of the Northern Rocky Mountains and Plains
The Devonian System, represented predominantly by shallow-water marine carbonate, is widespread in Montana, Wyoming, eastern Idaho, North Dakota, South Dakota, and northwestern Nebraska. It comprises cratonic rocks in the east and miogeosynclinal rocks in the west.
The cratonic rocks thicken generally northward from their southern limit in Wyoming across a broad shelf that occupies most of Wyoming and Montana. In northern Montana, they are as much as 1,250 feet thick. Cratonic rocks also thicken eastward from areas of Early Mississippian erosional thinning in central and eastern Montana to as much as 2,000 feet in the intracratonic Williston basin centered in northwestern North Dakota.
The miogeosynclinal rocks, which moved eastward on low-angle thrust faults, abut against cratonic rocks along a north-trending disturbed belt in western Wyoming, western Montana and eastern Idaho. The miogeosynclinal rocks thicken abruptly westward from 1,000 feet near this belt to about 3,000 feet near the east edge of the Idaho batholith. Farther west they have been buried beneath younger rocks, altered by the batholith, or eroded.
Five subdivisions of the Devonian System are treated separately:
1. Upper Lower Devonian (Coblenzian) marginal and nearshore marine carbonate rocks and related continental and estuarine discontinuous sinkhole and channel-fill deposits.
2. Upper Middle Devonian (Givetian) carbonate rocks that contain a 525-foot-thick evaporitic sequence in the Williston basin.
3. Lower Upper Devonian (Frasnian, toI) cyclically deposited carbonate rocks that include thick beds of dolarenite and dolomitized calcarenite on the west.
4. Upper Upper Devonian (Famennian, toII-IV) evaporitic rocks overlain by fossilferous open-marine shale and limestone.
5. Undivided uppermost Devonian (Famennian, to V-VI) and lowermost Mississippian (Tournaisian, cuI-lower cuIIα) carbonaceous and clastic rocks deposited in six shallow basins interspersed among areas uplifted during the penecontemporaneous Antler orogeny.
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