AN OVERVIEW OF THE MESOZOIC AND CENOZOIC GEOLOGICAL HISTORY OF THE WESTERN UNITED STATES

How does the geology of the Pacific Northwest fit into the current models of the active margin of the Western Cordillera from Jurassic to the present? How do the Mesozoic orogenies and post-Laramide volcanics relate to the western Cordilleran margin? Where is the rest of Wrangellia? How did the Columbia River Basalt Group (CRBG) develop?

The Sevier orogeny (140-85 Ma) was the overthrusting of sedimentary formations. The Laramide orogeny (84-48 Ma) was the compression from west of the Rocky Mountain Foreland and Colorado Plateau and breakup of the previously fractured basement blocks with a development of mountain ranges and intermontane basins east of the overthrust belt. There was progressive post-Laramide alkaline vulcanism under the broken basement blocks in the Western Cordillera. The combined Rocky Mountain Foreland and Colorado Plateau is bounded by the Lewis-Clark FZ extending from the Black Hills to Washington State on the north and by the Texas-Walker FZ from southern Texas through central Arizona on the south. Late Cretaceous granites and volcanics are present along these fracture zones.

The Wrangellia Terrane on Vancouver Island has a positive gravity anomaly that may extend under the Coast Range. The Olympic Mountains has a strong negative anomaly interrupting the positive anomaly but not the Paleogene volcanics and clastics.

The northern boundary of the Great Basin extends from the Newberry Crater through the Oregon Lava Plain and the Snake River Plain of Idaho to the Yellowstone Caldera. Streck and others age-dated rhyolites interbedded with basalts in the Oregon Lava Plain and found that the oldest were around 14 Ma. From this line west there is a westward decrease in age to the 0.5 Ma Newberry Crater. An eastward decrease in age is present in the Snake River Plain to the Yellowstone caldera.

The Blue Mountains have had post-CRBG westward extension as the Great Basin expanded over the past 14 Ma. Neogene extension is present on the east side of the Idaho Batholith and connects with the east-west Lewis-Clark FZ and then turns north joining normal faults in Montana, Idaho and the Canadian Rocky Mountains.

There is a thick Paleogene sedimentary section in the Columbia Basin overlain by the CRBG with numerous reverse-faulted anticlinal ridges. The Olympic Mountains were folded into an antiform. Both areas were compressed from the south.