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Beneath the Western Canada plains in general, the stratigraphic record is fairly complete from Cambrian to Tertiary time, but important gaps, representing intervals of widespread uplift and erosion, occur over broad parts of the region. The Cenozoic tectonic features, the Sweetgrass arch and Williston basin, probably had early Paleozoic counterparts in the south. Middle Devonian-Silurian time was marked by development of an extensive evaporite basin on the central and eastern parts of the Alberta shelf, and coral reefs formed in late Middle Devonian on the north and east. The Peace River ridge, a positive element, located in northwestern Alberta, persisted as a lowland until inundated in late Upper Devonian time. Upper Devonian sediments were most widely distributed, with important coral reef development and, in the south, widespread evaporite deposition. Mississip- pian strata, though once widely distributed, were severely truncated during the long erosion interval which followed Post-Paleozoic emergence. Permo-Pennsylvanian, Triassic, and Jurassic beds have restricted distribution on the west and south, much of the shelf region having remained a quiescent lowland during these times. Following the Nevadan orogeny, Cretaceous and earliest Tertiary marine and fresh-water sediments covered the shelf; this inundation ended with the broad uplift which accompanied the Rocky Mountains, or Laramide, orogeny. Deep erosion then occurred followed by fresh-water sedimentation across the plains in late Eocene and Oligocene time. The record since then has been largely that of degradation, which has left only remnants of the formerly widespread Tertiary deposits on the plains.

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