Oligocene and Miocene Volcanic Rocks in the Central Pioche-Marysvale Igneous Belt, Western Utah and Eastern Nevada U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1433 Oligocene and Miocene Volcanic Rocks in the Central Pioche-Marysvale Igneous Belt, Western Utah and Eastern Nevada Stratigraphy of the Volcanic Oligocene Needles Range Group in Southwestern Utah and Eastern Nevada By MYRON G. BEST and S. KERRY GRANT Miocene Magmatism and Tectonism in and near the Southern Wah Wah Mountains, Southwestern Utah By MYRON G. BEST, HARALD H. MEHNERT, JEFFREY D. KEITH, and CHARLES W. NAESER U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1433A-B UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1987 DEPARTMENT OF THE INTERIOR DONALD PAUL HODEL, Secretary U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director Library of Congress Cataloging-in-Publication Data Best, Myron G. Oligocene and Miocene volcanic rocks in the central Pioche-Marysvale igneous belt, western Utah and eastern Nevada. (U.S. Geological Survey professional paper; 1433-A-B) Bibliography: p. Supt. of Docs, no.: I 19.16:1433 1. Geology, Stratigraphic Oligocene. 2. Geology, Stratigraphic Miocene. 3. Volcanic ash, tuff, etc. Nevada Pioche Region. 4. Volcanic ash, tuff, etc. Utah Marysvale Region. 5. Needles Range Group (Utah and Nev.) 6. Magmatism Utah Wah Wah Mountains. 7. Geology Utah Wah Wah Mountains. I. Grant, S. Kerry. II. Title. III. Series: Geological Survey professional paper ; 1433-A-B. QE693.B47 1987 551.7 35 '097924 86-600019 For sale by the Books and Open-File Reports Section U.S. Geological Survey Federal Center Box 25425 Denver, CO 80225 CONTENTS [Letters designate the chapters] Page (A) Stratigraphy of the volcanic Oligocene Needles Range Group in southwestern Utah, by Myron G. Best and S. Kerry Grant .................................. 1 (B) Miocene magmatism and tectonism in and near the southern Wah Wah Mountains, southwestern Utah, by Myron G. Best, Harald H. Mehnert, Jeffrey D. Keith, and Charles W. Naeser ................................................... 29 III Stratigraphy of the Volcanic Oligocene Needles Range Group in Southwestern Utah and Eastern Nevada By MYRON G. BEST and S. KERRY GRANT OLIGOGENE AND MIOCENE VOLCANIC ROCKS IN THE CENTRAL PIOCHE- MARYSVALE IGNEOUS BELT, SOUTHWESTERN UTAH AND EASTERN NEVADA U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1433-A Several ash-flow cooling units with a total volume of more than 6,600 km3 make up two compositional cycles, each of which began with rhyolite and terminated with dacite CONTENTS Page Page Abstract .......... 3 Ryan Spring Formation .............................. 13 Introduction ...... 4 Greens Canyon and Mackleprang Tuff Members ....... 13 Acknowledgments 4 Lava flow members ................................ 14 Lund Formation .................................... 14 General geology ... 4 Tuff member ..................................... 14 Stratigraphic nomenclature .................. 5 Andesite flow member ............................. 15 Change in Stratigraphic rank ............... 6 Distribution, volume, and sources of Needles Range Group 15 Abandonment of Wallaces Peak Tuff Member 6 Escalante Desert Formation ........................ 15 General comment on crystal-rich dacite tuffs ... 7 Cottonwood Wash Tuff ............................. 17 Wah Wah Springs Formation ....................... 17 Escalante Desert Formation ................. 7 Indian Peak caldera .............................. 18 Marsden Tuff Member .................... 8 Utah sector ................................... 19 Rhyolite flow member ..................... 8 Nevada sector ................................. 21 Andesite flow member .................... 8 Resurgence ................................... 21 Lamerdorf Tuff Member ................... 9 Total volume of Wah Wah Springs Formation ....... 22 Beers Spring Member ..................... 9 Ryan Spring Formation ............................ 22 Cottonwood Wash Tuff ..................... 9 Lund Formation .................................. 23 Wah Wah Springs Formation ................ 12 White Rock caldera .............................. 24 Outflow tuff member ..................... 12 Summary .......................................... 25 Intracaldera member ...................... 12 References cited .................................... 26 ILLUSTRATIONS Page FIGURE Al. Index map showing localities referred to in text ............................................. 5 A2. Chart showing Stratigraphic relations and nomenclature of the Oligocene Needles Range Group .... 6 A3. Geologic map and idealized cross section of the northeastern segment of the Indian Peak caldera ... 10 A4-A10. Maps showing distribution and thickness of units in the Needles Range Group: A4. Marsden Tuff Member of Escalante Desert Formation and location of Pine Valley caldera 16 A5. Lamerdorf Tuff Member of Escalante Desert Formation ............................. 17 A6. Cottonwood Wash Tuff ......................................................... 18 A7. Wah Wah Springs Formation and location of Indian Peak caldera 19 A8. Greens Canyon Tuff Member of Ryan Spring Formation .................................... 22 A9. Mackleprang Tuff Member of Ryan Spring Formation ...................................... 23 A10. Lund Formation ...................................................................... 24 All. Map showing nested sources of tuff cooling units in the Needles Range Group .......................... 26 TABLES Page TABLE Al. Modal composition of samples of the Three Creeks Tuff Member of the Bullion Canyon Volcanics .... 7 A2. Isotopic ages of dacite tuffs in the Needles Range Group ...................................... 12 A3. Presently exposed areal extents and estimated volumes of ash-flow tuffs of the Needles Range Group 15 OLIGOCENE AND MIOCENE VOLCANIC ROCKS IN THE CENTRAL PIOC HE-MARYS VALE IGNEOUS BELT, WESTERN UTAH AND EASTERN NEVADA STRATIGRAPHY OF THE VOLCANIC OLIGOCENE NEEDLES RANGE GROUP IN SOUTHWESTERN UTAH By MYRON G. BEST and S. KERRY GRANT ABSTRACT Dominantly explosive mid-Oligocene magmatic activity 33-28 m.y. about half of which are plagioclase. In the Cottonwood Wash Tuff, ago along the southern Utah-Nevada border produced several ash-flow large phenocrysts of biotite and lesser quartz are characteristic. The cooling units with a total volume of at least 6,600 km3 and formed source of the Cottonwood Wash Tuff appears from indirect evidence the large Indian Peak caldera complex. These units constitute two to be concealed beneath a broad, mostly alluviated area immediately eruptive-compositional cycles, each of which began with crystal-poor, west of the Utah-Nevada State line. lithic, low-silica rhyolite tuff and culminated with a larger volume of The first eruptive-compositional cycle of the Needles Range Group crystal-rich, mostly nonlithic dacite tuff. Pyroxene andesite flows were culminated 29.5 m.y. ago with the eruption of at least 1,500 km3 of erupted from many widely scattered vents contemporaneously with crystal-rich dacite tuff of the outflow tuff member of the Wah Wah first-cycle rhyolite flows and tuffs, and locally with second-cycle tuffs. Springs Formation, which spread over an area of at least 22,000 km2 Five major pyroclastic units deposited during the two eruptive cycles in Utah and Nevada. The tuff is distinctively rich in hornblende and are here accorded formation rank (some were previously assigned poor in quartz, and it is the only reversely magnetized tuff in the member rank) within the revised Needles Range Group (formerly the Needles Range Group. The Indian Peak caldera produced by this erup­ Needles Range Formation). From the base upward, these formations tion had an estimated area of at least 1,200 km2 and is partly filled are the Escalante Desert Formation (newly added to group), the Cot- by 2,400 km3 or so of the intracaldera member of the Wah Wah tonwood Wash Tuff (formerly a member), and the Wah Wah Springs Springs Formation, which comprises landslide and fault debris that Formation (formerly a member) in the older cycle, and the Ryan Spring sloughed off the topographic wall as well as lithic dacite tuff. Formation (new unit) and the Lund Formation (formerly a member) Two cooling units of crystal-poor, lithic-rich rhyolite ash-flow tuff in the younger cycle. The Wallaces Peak Tuff (formerly a member in the Greens Canyon and Mackleprang Tuff Members of the Ryan the Needles Range Formation) is herein abandoned because of Spring Formation, all newly named in this report initiated the se­ synonymy with the Three Creeks Tuff Member of the Bullion Canyon cond cycle. The more than 200 km3 of ash-flow tuff is almost entire­ Volcanics, whose source lies far to the east in south-central Utah. ly confined within the Indian Peak caldera, where its source was The Escalante Desert Formation consists of at least two low-silica located. The pattern of deposition implies that the core of the older rhyolite ash-flow tuff cooling units, rhyolite and pyroxene andesite caldera had been resurgently uplifted and block faulted before the lava flows, and local lenses of epiclastic deposits. The crystal-poor, Ryan Spring Formation was deposited. Andesitic and rhyolitic lava lithic rhyolite tuff units are given formal status as the older Marsden flows of the Ryan Spring Formation occur locally. Tuff Member (new unit) and the younger Lamerdorf Tuff Member. The 27.9-my.-old crystal-rich dacite tuff of the Lund Formation com­ The source of the Marsden, whose total volume is about 300 km3, was pletely