RICE UNIVERSITY STRUCTURE AND STRATIGRAPHY OF THE POTOSI MOUNTAIN AREA, SOUTHERN SPRING MOUNTAINS, NEVADA by CHRISTOPHER SCOTT CAMERON A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF ARTS THESIS DIRECTOR'S SIGNATURE: HOUSTON, TEXAS DECEMBER, 1977 ABSTRACT Structure and stratigraphy of the Potosi Mountain area, southern Spring Mountains, Nevada Christopher Scott Cameron The Potosi Mountain area is located 25 miles southwest of Las Vegas, Nevada, along the eastern margin of the Cordilleran foreland thrust belt. Three structural blocks are exposed, including, from lowest to highest, the autoch¬ thon, the Contact thrust plate and the Keystone thrust plate. Both thrust plates contain a similar sequence of Middle Cambrian through Permian shelf carbonates. The presence of the Upper Ordovician (?) Mountain Springs Formation in both indicates a similar marginal miogeoclinal paleogeographic affinity. Triassic to Latest Jurassic (?) strata of cratonic facies are exposed in the autochthonous block. The Contact thrust, exposed east of Potosi Mountain, carries Paleozoic carbonate rocks east-northeastward over autochthonous Jurassic Aztec Sandstone, or locally, Latest Jurassic (?) synorogenic conglomerates. The Contact thrust plate is truncated north of Potosi Mountain by the northwest trending Cottonwood fault. The Keystone thrust plate over¬ rides the Contact thrust plate south of this fault, and the autochthon to the north. The Cottonwood fault downdrops the Keystone thrust only 200 feet. The following sequence of structural events was deduced: 1) inferred high angle faulting of the autochthon; 2) east vergent folding within the future Contact thrust plate; 3) emplacement of the Contact and intraplate Potosi thrusts; 4) high angle faulting of the autochthon and Contact thrust plates (development of Cottonwood fault and Ninetynine fault zone); 5) emplacement of the Keystone thrust; 6) minor high angle faulting; and 7) minor gravity sliding. Event 1 prob¬ ably predates Latest Jurassic (?) synorogenic conglomerates. Event 3 postdates these deposits and probably correlates with a folding and thrusting event dated at 135±5 M.Y.B.P. in the Clark Mountains (Burchfiel and Davis, 1971). Event 5 pre¬ dates a 95±5 M.Y.B.P. post-tectonic pluton in the Clark Moun¬ tains (Burchfiel and Davis, 1971). The Keystone thrust is localized near the base of the Banded Mountain Member of the Middle Cambrian Bonanza King Formation, and similar stratigraphic control is inferred for the Contact thrust in subsurface. Cross sections indicate thrust faulting has produced a minimum 10 miles of shortening of miogeoclinal rocks in the Potosi Mountain area. The occurrence of synorogenic chert and quartzite pebble conglom¬ erate restricted to a zone below the Contact thrust and un- conformably overlying Aztec Sandstone is interpreted as evidence that the Contact thrust plate moved over an erosional surface. i TABLE OF CONTENTS Page INTRODUCTION 1 Purpose of Study 1 Geographic Setting and Access 2 Previous Investigations 4 Present Study 5 Acknowledgments 6 STRATIGRAPHY 8 Introduction 8 Allochthonous Rocks 8 Bonanza King Formation 8 Nopah Formation 11 Mountain Springs Formation 12 Sultan Limestone 15 Monte Cristo Limestone 19 Bird Spring Formation 24 Autochthonous Rocks 27 Moenkopi Formation 27 Shinarump Conglomerate and Chinle Formation . 27 Aztec Sandstone 28 Channel conglomerates 29 Regional Stratigraphic Setting 31 STRUCTURE 34 Introduction 34 Autochthon 34 Contact Thrust Plate 36 General statement 36 Contact thrust 37 Major folds 40 Potosi thrust 41 Small folds north of Ninetynine fault zone ... 43 ii Page STRUCTURE (Continued) Small folds south of Ninetynine fault zone ... 44 High angle faults 45 Gravity slides 46 Ninetynine Fault Zone 48 Keystone Thrust Plate 53 General statement 53 Keystone thrust 54 Folds 56 High angle faults 57 Landslides 57 TIMING OF STRUCTURAL EVENTS 58 REGIONAL TECTONIC CORRELATIONS 62 THRUST FAULT GEOMETRY AT DEPTH 64 Interpretative Cross Sections 64 "Hidden Thrust" Interpretation 68 THRUST SURFACES AS EROSION SURFACES 71 THE MECHANICAL PROBLEM OF EROSIONAL THRUSTS 73 MODEL FOR THE EMPLACEMENT OF THE CONTACT AND KEYSTONE THRUST PLATES AT POTOSI MOUNTAIN 77 CONCLUSIONS 79 REFERENCES 82 LIST OF ILLUSTRATIONS FOLLOWING PAGE FIGURE 1. Index map to the eastern Cordilleran foreland thrust belt . 1 2. Index map to the Potosi Mountain area: major thrust faults of the region 1 3. Index map to the Potosi Mountain area: major thrust plates of the region 2 4. Stratigraphic sections from French¬ man Mountain to the northwest Spring Mountains 31 5. Lower hemisphere projection of slickenside lineations from anas¬ tomosing slip surfaces below the Contact thrust 39 6. Model for the emplacement of the Contact and Keystone thrust plates at Potosi Mountain 77 PLATE 1. Geologic map of the Potosi Moun¬ tain area, Southern Spring Moun¬ tains, Nevada back pocket 2. Geologic cross section AA'A" . back pocket 3. Geologic cross section BB' . back pocket 1 INTRODUCTION Purpose of Study The Potosi Mountain area is located along the eastern margin of the Cordilleran foreland thrust belt (see figure 1) . The thrust belt of southern Nevada and southeastern California, a portion of which is shown in figure 2, repre¬ sents the continuation of the Sevier orogenic belt of Armstrong (1968) southward across the Las Vegas shear zone. Several of the major east-directed thrust faults of the southern Nevada-southeastern California are identified in figure 2. One of the principal throughgoing structural elements in this area is the Keystone thrust. The Keystone thrust is exposed in the east and southern Spring Mountains and can be traced southwards to the Clark and New York Mountains of California. It is offset and continues northward across the Las Vegas shear zone as the Muddy Mountain thrust. It was long believed that the Keystone thrust repre¬ sented the easternmost and lowest major structural element in the region. However, Davis' (1973) study of the Red Spring thrust indicated that this was not the case. He concluded, returning to the original idea of Longwell (1926), that the emplacement of the Red Spring thrust predated the emplacement of the Keystone thrust, which overrides it. Further, Davis (1973) correlated the Red Spring thrust with the Contact thrust which crops out east of Potosi Mountain and suggested that they were remanents of a once continuous thrust. He Figure 1: Index map to the eastern Cordilleran foreland thrust belt. Figure 2: Index map to the Potosi Mountain area: major thrust faults of the region. Solid lines delimit bed¬ rock exposures; random tick marks indicate granitic rocks. 2 further believed that after emplacement, the Contact-Red Spring thrust plate was cut by high-angle faults. During this episode of high-angle faulting, the central portion of this thrust plate was uplifted on a horst and removed by erosion prior to the emplacement of the higher Keystone thrust plate (see figure 3) . The major objective of this study was to determine geo¬ metric and timing constraints on the emplacement of the Contact and Keystone thrusts at Potosi Mountain by detailed geologic mapping of the area. It was hoped that this study might provide important data concerning the tectonic evo¬ lution of the Cordilleran thrust belt in this region. A second objective was to examine the stratigraphy of the allochthonous and autochthonous rocks at Potosi Mountain. Geographic Setting and Access Potosi Mountain marks the northern boundary of the southern Spring Mountains. Mountain Springs Pass, immediately north of the map area, divides the east Spring Mountains and southern Spring Mountains. Potosi Mountain is located 25 miles southwest of Las Vegas, Nevada, and immediately south of the village of Mountain Springs Pass. Topographic relief within the map area is fairly typical of the Basin-Range physiographic province, varying from about 4400 feet on the eastern and western slopes to 8512 feet at the summit of Potosi Mountain. Potosi Mountain is actually a broad massif which occupies the central third of the map Figure 3: Index map to the Potosi Mountain area: major thrust plates of the region. Solid lines delimit bed¬ rock exposures; random tick marks indicate granitic rocks. 3 area. The terrain is generally rugged, particularly the steep to vertical approaches to the central massif. Vegetation is fairly abundant and diverse as the area lies within the high desert floral zone. Joshua trees and numerous species of cactus are present at lower elevations. Juniper trees are encountered at approximately 6000 feet and a forest of cedar and pinon pine covers Potosi Mountain above approximately 7500 feet. Notable wildlife includes wild burros near Potosi Spring, wild horses in the vicinity of Cottonwood Pass, moun¬ tain sheep at the higher elevations, an occasional cougar and numerous species of snakes and lizards. The summer climate is relatively pleasant. Daytime summer temperatures seldom exceed 100 degrees F., and usually range between 80 to 100 degrees. Evenings are cool. Thunder¬ storms are infrequent. The northern and western sections of the Potosi Mountain area are best approached from Las Vegas or Pahrump, Nevada, via State Highway 16, which crosses Mountain Springs Pass. The "Camp Potosi" road leading southwest from Highway 16 at a point approximately 1 mile east of Mountain Springs Pass provides good access as far south as Potosi Spring, and further south for 4-wheel drive vehicles. The eastern sec¬ tion of the area is best approached by the Cottonwood Pass road, which connects Highway 16 in Cottonwood Valley with the town of Goodsprings, Nevada. From this road, a gravel road and a washed out track lead to the Aztec Tank quarries and 4 the Ninetynine Mine respectively (the latter begins at the first westward turn-off south of Cottonwood Pass). A 4-wheel drive road leads from the Aztec Tank area to a microwave station 3/4 mile south of the summit of Potosi Mountain (elevation approximately 8250 feet). Access is restricted by a locked gate and permission to pass and a key were obtained from Centel of Nevada.