Abstract

The Mesozoic Era of the Western Interior of the United States was dominated by sedimentation in shallow marine and continental settings behind a magmatic arc developed along the western margin of the continent. Part of the history of this magmatism is preserved as altered volcanic ash enclosed in sedimentary rocks. For the Mesozoic of the Western Interior, we have compiled from the literature the ages and locations of discrete volcanic ash beds (mostly bentonites) and ash–rich rocks (e.g., analcime–rich or bentonitic mudstones). These data are plotted on a series of eight paleogeologic maps and compared with the locations of contemporaneous Mesozoic sedimentary and plutonic rocks. From these maps we infer:

(1) Most ash was erupted from volcanoes in the arc during plinian eruptions of silicic magma associated with caldera–collapse and emplacement of ignimbrite. The ash was deposited as pyroclastic fall, and then reworked physically and chemically by sedimentary processes.

(2) Ash–producing volcanism was intermittent with pulses separated by as few as several million years to as many as forty million years.

(3) Major periods of volcanism occurred during the Late Triassic (about 225 Ma), during the Middle Jurassic (about 160 to 140 Ma), and during two periods in the Late Cretaceous, peaking in the Cenomanian (about 95 Ma) and again near the Campanian–Maastrichtian boundary (about 75 Ma). Volcanic quiescence prevailed during the Early and Middle Triassic, from the latest Triassic to the Middle Jurassic, and perhaps during the early Cretaceous.

(4) Volcanic episodes correlate closely with the temporal pattern of pluton emplacement in the western United States.

(5) The location and extent of the arc plutonic belt and the distribution of ash in each time interval constrain the locations of the volcanoes from which the ash erupted. For example, the bentonitic mudstones of the Late Triassic Chinle Formation contain ash probably erupted from volcanoes in what is now southeastern California, southern Arizona, and northern Mexico. Middle and Late Jurassic ashes probably erupted from near the continental margin from vents in what is now the Sierra Nevada Batholith, whereas Cretaceous ashes erupted from the Idaho and Boulder batholiths as well as the Sierra. As a result of the distribution of these magmatic systems, no volcanic ash is preserved in sedimentary rocks in Montana that are older than Aptian (mid- Cretaceous).

(6) Two general types of vent regions may have erupted compositionally distinct ashes. Continental margin regions are typified by the northern or western Sierra igneous suites and the Idaho and Boulder batholiths comprise a continental interior suite.

(7) In the Western Interior, Mesozoic sedimentary rocks are not widespread during times of magmatic quiescence. This relationship can be explained if periods of slow sea-floor spreading increased the volume of the ocean basins, caused elevation of the continental margins, and decreased rates of intrusion and volcanism above the slab being subducted beneath the western margin of North America.