ABSTRACT

During early Tertiary time, varied volcanic rocks were deposited upon the eroded Laramide uplifts of the Yellowstone region. During the middle Tertiary, these uplifts and the volcanic rocks, were segmented obliquely by block faults, and the present mountain ranges were inaugurated. The fault blocks plunge or dip under the Snake River basin and the Yellowstone Plateau; the Plateau represents the high, lava-filled end of the Basin.

During the Pliocene, an ancestral Yellowstone Plateau, at about the same elevation as the present one, was formed by welded tuffs of alkaline rhyolite, accompanied by flows of the same composition. A caldera 30 miles in diameter probably formed by the collapse of this rhyolite sheet into the underlying magma chamber.

During the late Quaternary, vast flows of viscous rhyolite filled this caldera and overflowed to the west across a buried connection between the Madison and Teton Ranges onto the Snake River plain. Some of these flows have been dated relative to glaciations. These flows brought the level of the Yellowstone Plateau back up to its previous position. Now, the Pliocene rhyolites form an outer ring, nearly 10 miles wide, bounding the Quaternary flows on all sides save the southwest. The magma chamber beneath the caldera was in the center of the pre-Pliocene structural and topographic basin. Rhyolites of Pliocene and Quaternary ages are alkaline rocks, compositionally similar to silicic differentiates of basaltic magmas. Field and petrologic relations suggest that the Yellowstone Plateau may be the silicic crust of an extrusive basaltic lopolith.

The Snake River basin, adjacent to the Yellowstone Plateau, is formed of basalt flows with subordinate flows and tuffs of rhyolite. Island Park basin is a caldera, 18 miles in diameter, rimmed by rhyolite and filled with basalt.

The Snake River basin (and its east end, the Yellowstone Plateau) cuts across most Mesozoic and Cenozoic structures. Around its upper part are encountered many of the highest mountains of the northern Rocky Mountains. So many structures, Laramide as well as Late Cenozoic, are systematically related to the upper end of the Basin that the factors which produced it must operate well beyond its borders.