The southwestern High Plateaus of Utah and an adjacent portion of the Great Basin were the site of extensive and prolonged volcanism during much of the Cenozoic Era. The volcanic pile that is centered near Marysvale, Utah, consists mostly of intermediate lava flows and volcanic breccia and dominates the vast accumulation of eruptive deposits that blankets this region; this study includes the southern half of the Marysvale pile. Regional emplacement of ash-flow tuff sheets, probably erupted from sources in the Great Basin, along with local intrusive activity and deposition of continental sediments, was contemporaneous with Marysvale volcanism. The Cenozoic stratigraphy of the area is complex, but the regional ash-flow tuff sheets serve as time-stratigraphic marker units throughout most of the area. The Cenozoic rocks of the southwestern High Plateaus are subdivided into three stratigraphic sequences: lower Tertiary, middle Tertiary, and upper Tertiary and Quaternary. The lower Tertiary (Eocene and Oligocene) sequence is regional in extent and consists of continental sedimentary strata of the Claron Formation and ash-flow tuff of the Needles Range Formation. These formations covered almost the entire area of the southwestern High Plateaus and nearby Great Basin, indicating that there was at that time neither marked structural differentiation nor significant buildup of the Marysvale pile. The middle Tertiary (uppermost Oligocene-lower Miocene) sequence includes most Cenozoic deposits in the area. It is made up of two intertonguing rock assemblages that were deposited contemporaneously but derived from different sources. The western assemblage, best exposed in the southwestern Black Mountains but representative of much of the southeastern Great Basin, consists almost entirely of regional ash-flow tuff derived from Great Basin sources. In ascending order, it includes the Isom Formation and the Quichapa Group, the latter comprising the Leach Canyon Formation, Condor Canyon Formation, and Harmony Hills Tuff. East of these rocks lies the contemporaneous Marysvale pile of volcanic and sedimentary strata, most of which were derived and distributed locally. This eastern assemblage is exposed in the northern Black Mountains, southern Tushar Mountains, northern Markagunt Plateau, and southern Sevier Plateau. The lower rocks consist of lava flows, volcanic breccia, and ash-flow tuff of the Mount Dutton Formation and its lateral correlatives, which are, in ascending order, volcanic breccia and ash-flow tuff(?) of the Buckskin Breccia, sandstone of the Bear Valley Formation, and ash-flow tuff of the Osiris Tuff. All are overlain by rhyodacitic lava flows of the Dry Hollow Formation, basalt of the older basalt flows, intermediate lava flows and volcanic breccia of the Horse Valley Formation, rhyolite flows of the Mount Belknap Rhyolite, and ash-flow tuff of the Joe Lott Tuff. The Mount Dutton Formation, which makes up most of the Marysvale pile, is divided into a vent facies, composed largely of andesitic and basaltic(?) lava flows and flow breccia near the eruptive center of the pile, and an alluvial facies, composed largely of volcanic mudflow breccia derived from the vent facies. Most of the vent facies is restricted to the southern Tushar Mountains, whereas the alluvial facies occurs radially outward in the northern Black Mountains, northern Markagunt Plateau, and southern Sevier Plateau. The upper Tertiary (upper Miocene-Pliocene) and Quaternary sequence is made up of the Sevier River Formation, which consists mostly of detritus shed from upfaulted structures of the High Plateaus, and younger basalt flows, which represent the latest volcanism in the area. New radiometric age determinations demonstrate that most of the Tertiary volcanism occurred during a 5-m.y. time span, in latest Oligocene and early Miocene time (about 25 to 20 m.y. B.P.). The younger basalt flows and, by inference, the closely associated Sevier River Formation may be older (latest Miocene) in part than was believed heretofore. Deposition of the Sevier River Formation and extrusion of the younger basalt flows probably were synchronous with the faulting that led to the structural formation of the High Plateaus; therefore, it appears that this faulting began at least as early as latest Miocene time and has continued intermittently.