John J. Anderson

Age of structural differentiation between the Colorado Plateaus and Basin and Range provinces in southwestern Utah

Distribution of Tertiary regional ash-flow tuffs indicates that the Colorado Plateaus and Basin and Range provinces in southwestern Utah became separate structural terrains some time after 29 m.y. ago; the structural differentiation probably was underway by 26 m.y. ago and had produced significant topographic contrasts between the two provinces by 24 m.y. ago. Initial differentiation may be older than the main phase of basin-range faulting in southwestern Utah, which appears to have begun about 21 to 20 m.y. ago.

Geology of the Southwestern High Plateaus of Utah: Bear Valley Formation, an Oligocene-Miocene Volcanic Arenite

The area which today comprises the southern High Plateaus of Utah was the locale during the early and middle Tertiary of extensive volcanic activity which formed a volcanic pile many thousands of feet thick. During the time it took for this vast build-up of lava flows, volcanic mudflow-breccias, and ignimbrites, there occurred, in the late Oligocene and early Miocene, a cessation of regional volcanic activity which was marked by the accumulation of volcanic arenite and associated clastic sediments in and around what is today the northern Markagunt Plateau. The rock stratigraphic unit thus formed is defined here as the Bear Valley Formation. The Bear Valley Formation was deposited in an extensive structural and physiographic basin within which faulting and volcanism were contemporaneous with sediment accumulation. Little volcanism accompanied the early stages of sand deposition; the later stages, however, were marked by local eruptive activity which admixed a considerable quantity of glass shards in the sand and produced local ignimbrite and tuff strata. Deposition of the arenite was accomplished mostly by the wind, and took place under arid climatic conditions. This is indicated by the mineralogy and texture of the sand and by the large-scale cross-bedding throughout most of the section. After deposition and subsequent burial by renewed volcanism, the sand was cemented by clinoptilolite. Today the Bear Valley Formation crops out over an area of more than 1000 sq mi and has a maximum exposed thickness in excess of 1000 ft. Typically, it may be described as moderately to well sorted, fine- to medium-grained, zeolite-cemented submature to mature volcanic arenite. No fossils have been recovered from this rock unit.