STRUCTURE AND DEVONIAN STRATIGRAPHY OF THE TIMPAHUTE RANGE, NEVADA Volume I by Alan K. Chamberlain © Copyright, 1999 A thesis submitted to the Faculty and Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Geology). Golden, Colorado Date____________ Signed:__________________________ Alan K. Chamberlain Approved:__________________________ Dr. John E. Warme Professor and Thesis Advisor Golden, Colorado Date____________ ____________________________________ Dr. Roger Slatt, Professor and Head, Department of Geology and Geological Engineering ii ABSTRACT Sequences of Devonian rocks are advantageously exposed along a unique 40- mile-long east-west traverse in the greater Timpahute Range, southeastern Nevada. Study of these rocks casts light upon Devonian paleogeography, the Devonian Antler orogeny, an Upper Devonian cosmolite impact basin, this part of the Cretaceous Sevier fold-and- thrust belt, and the effects of Cenozoic extension. The greater Timpahute Range lies within the Timpahute Range 30' X 60' quadrangle and includes the region from Tempiute Mountain on the west to the Pahroc Range on the east. Concealed major north-south trending normal faults caused by Cenozoic extension have been proposed to disrupt the Paleozoic rocks of the region. However, a structural interpretation using a new geologic map of the quadrangle requires no major north-south striking normal faults. Furthermore, the greater Timpahute Range is interpreted as a salient of stacked thrust sheets within the Sevier fold-and-thrust belt. The range is bounded on the north and south by thrust tear faults that may be related to basement fractures caused by the cosmolite impact. Evidence for the Late Devonian cosmolite impact includes shocked quartz, iridium anomalies, ejecta spherules, and disturbed shallowing-upward sequences exhibiting intrasequence folding, brecciation, carbonate liquefaction, and graded bedding. Impact breccia thins radially from 510 feet at Tempiute Mountain to zero within 80 miles north and south of Tempiute Mountain, but within only 60 miles northeast and southeast of the impact site. Bed length measurements from a geometrically balanced cross section of the greater Timpahute Range show at least 64 miles of cumulative crustal shortening. When restored, the diameter of the concentric impact basin, centered near Tempiute Mountain, is 160 miles. A restored cross section suggests that the impact site is likely about 20 miles west of Tempiute Mountain. iii Thrust sheets within the salient contain rocks of three contrasting facies above the cosmolite impact breccia marker: facies 1 is a peculiar, anoxic, deepwater, thin-bedded limestone characterized by soft-sediment slump folds and interbedded turbidites that may represent a crater fill that now is exposed in a proposed fenster on the west end of the range; facies 2 is a thick shallow-water quartz sandstone (as much as 1070 net feet thick) deposited on the west side of the impact basin that now is in an interpreted klippe in the middle of the range; facies 3 is a shallow-water platform carbonate deposited on the east side of the impact basin. It contains a large stromatoporoid reef. Characterized by shallowing-upward cycles, a reference measured section for the carbonate platform facies 3 was correlated to surface and subsurface sections of the region using sequence characteristics and gamma-ray log patterns. Only 19 of 21 sequences are exposed in the reference section that lies in the footwall and east of the thrust sheet containing sandy facies 2. When viewed in order, isopach maps of the 21 sequences show a large (200 X 400 miles, unrestored) depression, the Sunnyside basin, the axis of which migrated from central Nevada to western Utah during the Devonian. A composite isopach map of the 21 sequences shows that the intrashelf Sunnyside basin is thickest and therefore centered over Sunnyside, 60 miles north of the Timpahute Range. Quartz sandstone isolith maps show that Devonian quartz sandstones were deposited on the edges of the Sunnyside basin. It is proposed that sandstones on the west side were derived from the Devonian Antler orogenic forebulge. Sandstones on the east side were probably derived from the craton and more specifically from an east-west highland developed on the Uinta aulacogen in north-central Utah. Isopach patterns suggest that the highland or arch, herein called the Monitor-Uinta arch, extended into central Nevada. Dolomite, a common constituent of much of the Great Basin Devonian, commonly occurs as penecontemporaneous dolomite caps on shallowing-upward cycles in facies 3. Sections of Upper Devonian strata near the edges of the Sunnyside basin are iv more dolomitic than those within the basin. Dolomites, basinward of the sandy facies 2, are rich in the thin, stick-like stromatoporoid, Amphipora. Basinward of the Amphipora- rich dolomites are limestones rich in corals and bulbous and tabular stromatoporoids. A major unconformity, characterized by deep (100's feet) karst cavities, separates pervasively dolomitized Middle and Lower Devonian carbonates from Upper Devonian limestones. Of the three Upper Devonian facies in the greater Timpahute Range, only the facies 3 reference section contains significant dolomite as caps on shallowing-upward carbonate cycles, suggesting supratidal depositional environments in the shallower parts of the Sunnyside basin. v TABLE OF CONTENTS ABSTRACT ........................................................... iii LIST OF FIGURES.................................................... xvii LIST OF TABLES...................................................... xx LIST OF PLATES...................................................... xxi ACKNOWLEDGMENTS ............................................... xxii DEDICATION......................................................... xvi CHAPTER 1 ............................................................1 INTRODUCTION ....................................................... .1 Author’s Background Concerning Timpahute Range Research ..............4 Purpose and Scope.................................................5 Format ......................................................... .8 Location of the greater Timpahute Range..............................10 Geologic Setting of the greater Timpahute Range ........................13 Tectonic Setting ............................................14 Stratigraphic Setting .........................................18 vi CHAPTER 2 ..........................................................26 RELATIONSHIP OF NEW RESEARCH TO PREVIOUS STUDIES ..............26 Mapping........................................................27 Stratigraphy .....................................................27 Pre-Devonian Stratigraphy ....................................28 Devonian Lithostratigraphy ...................................31 Sevy Dolomite .......................................32 “Oxyoke Formation” ..................................33 Simonson Dolomite ...................................34 Guilmette Formation ..................................38 West Range Limestone ................................39 Pilot Formation ......................................40 Regional Comparison..................................41 Younger Paleozoic Rocks of the Study Area ......................43 Sevier Synorogenic Sediments.................................44 Age of Thrusting .....................................45 Distribution and Stratigraphy............................48 Structure and Tectonics............................................49 Pre-Antler Orogeny.........................................49 Monitor-Uinta Arch.........................................52 Devonian-Mississippian Antler Orogeny.........................54 Mesozoic Sevier Orogeny ....................................54 Sevier Fold-and-Thrust Belt.............................56 Sevier Fold-and-Thrust Belt in Nevada....................57 Sevier Fold-and-Thrust Belt Analogue ....................58 Thrust Detachment....................................59 vii Tectonic Model of Thin-Skinned Deformation..............60 Cenozoic Volcanism and Extension ............................64 Summary .......................................................65 CHAPTER 3 ...........................................................65 METHODS USED ......................................................65 Data Identification................................................66 Mappable Sequences........................................67 Measured Sections..........................................74 Stratigraphic Terminology....................................75 Data Collection ..................................................76 Global Positioning Systems (GPS) .............................77 Aerial Photographs and Field Work Maps........................78 Geographic Information System (GIS) ..........................79 Gravity Data...............................................79 Computer-aided Structural Cross Sections .......................81 Surface Gamma-Ray Logs ....................................84 Gamma-Ray Logs vs. Fischer Plots.......................85 Gamma-Ray Field Data Acquisition ......................87 Data Analysis....................................................87 Regional Correlations .......................................88 Data Manipulation..........................................90 viii CHAPTER 4 ...........................................................90