i \ University of Nevada Reno (jreeology of Paleozoic Basinal Rocks in the Northern Fox Range; Washoe County, Nevada A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology. by Mark F. Thiesse V i August 1988 11 MINES LIBRARY The thesis of Mark F. Thiesse is approved: 2H03 Thesis Advisor Department Chairman Dean, Graduate School University of Nevada Reno August 1988 iii ACKNOWLEDGEMENTS I would like to thank Dr. R .A. Schweickert for his help, encouragement and patience throughout this project. Dr. Bruce Wardlaw provided the conodont identification which suggested the Carboniferous (?) age of the metamorphic rocks. Other thanks go to Andy Markos and Bob Strobel for help with the thin sections and interesting discussions. And finally, thanks to Carol for typing and for funding this project. iv ABSTRACT The Fox Range in Northwestern Nevada is a typical Basin and Range fault block mountain range which has been tilted about 30° to the east, accelerating the erosion of Tertiary volcanic rocks along the steep western flanks, and exposing Paleozoic metamorphic rocks. The Paleozoic rocks consist of a very fine grained, siliceous mudstone which was regionally metamorphosed from mid-greenschist to mid-amphibolite facies producing argillite, schist, limestone, quartzite, and gneiss. These Paleozoic rocks exhibit three generations of structures. Di created the schistose and gneissic layering common throughout the range, along with abundant tight to isoclinal folds. D2 structures are dominated by two large, map-scale antiforms and occasional smaller, open to tight, east to northeast trending folds and assorted lineations. D3 structures are limited to a few north-south trending, open to gentle folds which affect all pre-Tertiary lithologies. These rocks were previously correlated with Upper Triassic to Lower Jurassic mudstones common in northern Nevada. Lithologic, metamorphic, structural and fossil data suggest this correlation is not valid. Instead, these rocks are Carboniferous, and most probably are correlative with clastic rocks of the upper Paleozoic Golconda allochthon. V TABLE OF CONTENTS Page Introduction ........................................ 1 Geographic Setting ................................ 5 Geologic Setting ................................... 9 Rock U n i t s ...........................................12 Structure ............................................ 39 Geologic History ................................... 74 Recommendations for Further W o r k ...................................... 85 References 87 1 INTRODUCTION The Fox Range of north central Washoe County contains some of the westernmost exposures of metamorphosed sedimentary rocks in Nevada. Several thousand feet (1000 m) of metasedimentary rocks are exposed in the range and are composed primarily of argillite, phyllite, schist, marble, quartzite and scarce gneiss. These metasedimentary rocks resemble pelitic rocks that are common throughout central and western Nevada, and are separated by an extensive tract of Tertiary volcanic rocks from the Klamath Mountains and northern Sierra Nevada. The pelitic rocks in central Nevada have been studied exten­ sively by numerous authors who have worked out the depositional and tectonic history in some detail. The rocks in far western Nevada, however, have been less extensively studied and those in the Fox Range have only been studied in reconnaissance by a few workers. Because of the lack of research and the potential significance of these pelitic rocks for understanding the regional tectonic framework, a study of the metamorphic rocks in the Fox Range was undertaken to provide information about the stratigraphy, structural history, and metamorphism. Scope of Investigation This project primarily involved detailed mapping and structural analysis of the metasedimentary rocks. Particular 2 attention was focused on style, orientation, and overprinting of folds and other structures. Thin sections were prepared of the metasedimentary rocks and the surrounding granitic and volcanic rocks to aid in lithologic indentification and to identify possible sources and depositional environments of the original sedimentary rocks. In addition, attempts were made to extract conodonts to date the metasedimentary rocks. Method of Investigation Approximately 30 square miles (78 km2 ) of the northern Fox Range were mapped at a scale of 1:24,000 during the Fall of 1983, and the orientation and style of cleavage, other foliations, lineations, and folds were noted in detail. Since the best exposures of the metasedimentary rocks are in deep narrow canyons along the range front, all of the canyons were walked and a majority of the ridges were also examined to map this area accurately. The areas with the best exposure and the greatest structural detail were re-examined in an attempt to measure sections and to determine specific generations of observed structures. The attitudes of folds were plotted on equal area stereonets to aid in the recognition of different generations of folding. Three base maps were used; only one, the Smith Canyon 7-1/2 minute quadrangle, was readily available. The other two maps, the Pyramid Lake 1 NE Nevada and the Peephole 3 SW Nevada sheets are older 7-1/2 minute T-maps which were used as preliminary maps to construct 15 minute series maps. These maps had to be specially ordered from the U.S.G.S. in Menlo Park, California. Air photos covering the area were obtained from the Eros Data Center in Sioux Falls, South Dakota and were used to trace large scale features and to assist in mapping lithologic contacts. Previous Investigations The first mention of this area was by Hague and Emmons in 1877 during the 40th Parallel Survey under the heading of the Lake Range. They provided a general description of the rock types and the physiography of the area north of Pyramid Lake, and suggested the metasedimentary rocks were Jurassic in age because of their similarity with other rocks to the east that overlie Jurassic limestones. The first mention of the Fox Range was by Hill (1915), who described the mining districts in the area. Overton (1947) also used the name Fox Range while describing the location of the Cottonwood mining district. Both of these authors briefly described the rock types and mineralogy present in the mining districts. Bonham (1969) discussed the Fox Range in general, together with structure, lithology, and mining history, and mapped the area at a scale of 1:250,000 in his report on Washoe and Storey Counties. He informally named the metasedimentary rocks the Nightingale sequence because they 4 resembled rocks in the Nightingale Range about 50 miles (81 km) southeast of the Fox Range. Bonham suggested that these rocks are probably correlative with the upper part of the Winnemucca Sequence of Silberling and Roberts (1962), and are uppermost Triassic and Lower Jurassic in age. Dixon (1977) studied the structural, stratigraphic, and economic relationships between the metasedimentary and volcanic rocks located in Wild Horse Canyon and the immediate vicinity, about 4 miles (6.5 km) south of the present study area. He agreed with Bonham’s (1969) correlation with the Winnemucca Sequence of Silberling and Roberts (1962). 5 GEOGRAPHIC SETTING Location and Accessibility The Fox Range is located in central Washoe county (Fig. 1) with its southern end about 7 miles (11 km) north of Pyramid Lake. The range is about 30 miles (48 km) in length and is up to ten miles (16 km) wide at its widest point. The range is oriented approximately North-South, being slightly convex to the west, and is bounded to the west by the Smoke Creek Desert and to the east by the smaller San Emidio Desert (Fig. 2), which is a southern arm of the Black Rock Desert to the northeast of Figure 2. The mapped area encompasses approximately 30 square miles (78 km2 )in the northern part of the range. The west side of the range is reached by traveling to Gerlach along State Highway 34, and then taking State Highway 81 about one mile northwest to the Gerlach landfill road. The landfill road intersects the Western Pacific Railroad tracks about 4.5 miles (7.25 km) to the south. The road that parallels the tracks skirts the northern tip of the range, curves around to the southwest and follows the western flank of the range. Access to the eastern portion of the mapped area is much easier. State Highway 34 is taken as far north as the Empire Hay Ranch road, a dirt road about 7 miles (11.3 km) south of Empire and 13 miles (21 km) south of Gerlach. This road intersects a powerline road after four miles (6.5 km). Following the powerline road west across the San Emidio 6 7 8 Desert, the eastern flank of the Fox Range is about 6 miles (10 km) farther. This entire range is criss-crossed by old .jeep trails and roads, so it is wise to use the previously mentioned topographic maps as a guide. Few of the dirt roads are maintained, so caution is advised when traveling the area. Physiography The Fox Range, typical of mountain blocks in the Basin and Range province, is a moderately rugged fault-block mountain surrounded by relatively low playas. The range is dissected by intermittent streams which carried water throughout 1983 because of the record amounts of precipitation during the winter of 1982-83. These small streams have cut deep narrow canyons that provide good exposures of the metamorphic rocks. Vertical cliffs over 200 feet (61 m) high were observed in many of the canyons, making it difficult to study these exposures thoroughly. The highest part of the range, the Pah Rum peak area, just south of the central part of the range, reaches a maximum height of 7600 feet (2320 m). The base of the range has an elevation of approximately 4000 feet (1220 m). The highest elevation in the study area is about 6800 feet (2043 m) and is located near the southern boundary. 9 GEOLOGIC SETTING Tertiary to Recent volcanic and minor sedimentary rocks, and Quaternary lacustrine and alluvial deposits are the predominant rock types in northwestern Nevada.