By: S. R. Mattox
Volcanic rocks of the eastern flank of the Marysvale volcanic field are exposed on the Awapa Plateau, one of the easternmost High Plateaus in the Utah Colorado Plateau - Basin and Range transition zone. Initial mapping by Williams and Hackman showed the plateau to be similar in lithology and stratigraphy to those to the west. Mudflow breccia of the alluvial facies of the volcanic rocks of Langdon Mountain are exposed at the base of the stratigraphic section. The mudflow breccia records the eastward migration of volcanic debris from andesitic stratovolcanoes on the northern Sevier Plateau in late Oligocene to early Miocene time. Lava flows of William and Hackman's basaltic andesite unit overlie and are interlayered with the mudflow breccia. Isotopic ages determined by the potassium-argon method support synchronous volcanic mudflow deposition and lava flow emplacement. Chemical analyses of the basaltic andesite unit show the lava flows to be trachyandesite in composition. Lava flows of similar age and composition were erupted across the High Plateaus. Trachyte lava flows of William and Hackman's latite unit overlie the mudflow breccia and the basaltic andesite and are early Miocene in age. The Osiris Tuff, an ash-flow of regional extent, overlies all of these units and is interlayered with the mudflow breccia on the western margin of the plateau. By middle Miocene time volcanic sediments had accumulated to produce the Sevier River Formation. Effusive volcanic activity resumed by the middle Miocene and peaked in Miocene - Pliocene time. The resulting shield volcanoes, lava flows, cinder cones, and volcanic necks range from basalt to basaltic trachyandesite in composition and have geochemical characteristics distinct from older mafic lavas.
The primary objectives of this study are: (1) to characterize the petrologic and geochemical variation of the middle and late Cenozoic volcanic rocks of the Awapa Plateau, (2) to constrain the age of the stratigraphic units, (3) to compare the volcanic rocks of the Awapa Plateau to the formally or informally designated stratigraphic units of the High Plateaus to the west, and (4) to outline the middle to late Tertiary geologic history of the Awapa Plateau.
Pages: 46 p.
Plates: 1 pl.
Location: Garfield County, Piute County, and Wayne County
Media Type: Paper Publication