UNIVERSITY OF NEVADA RENO General Geology of Triassic Rocks at Alaska Canyon in the Jackson Mountains, Humboldt County, Nevada A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in geology by Lynn Roy Fuller U\ November 1986 MINES 1 LIBRARY T^i e St'S 9k li % The thesis of Lynn Roy Fuller is approved: UNIVERSITY OF NEVADA RENO November 1986 i i ACKNOWLEDGMENTS I wish to thank Joseph Lintz, my advisor, for his constant encouragement and support. He introduced me to this investigation, accompanied me in the field and made countless valuable suggestions during the course of the investigation. During three summers of field work, many friends and family members offered their help and support. They gave up their vacation time to indure the alkali dust and heat of the Black Rock Desert in summer. To the following I wish to offer my sincere appreciation: Steve Hamilton, Dan Howe, James Wilkinson, Barclay Anderson and Mel, Diane, Monte and Steen Fuller. I also wish to thank Dan Howe, Tom Lugaski, and James Firby for many hours of stimulating, informal discussion on the problems and implications of these rocks. I am also appreciative of the financial support received for the project from the Sunmark Exploration Company of Denver, Colorado. i i i ABSTRACT Triassic mudrocks, ca1carenites, conglomerates and limestone of the Boulder Creek beds are exposed in a deformed stack of imbricate thrust nappes along the range front of Jackson Mountains in Humboldt County, Nevada. Deposition occurred in a terrane of basinal associations after the Sonoma Orogeny. A section of these rocks are exposed along the northeast wall of Alaska Canyon. There, a quartz-rich sequence of laminated and non- 1 aminated mudrocks with channelized debris flows and immature turbidites along with thick carbonates are interpreted as a middle to upper deep-sea fan regime. Ammonites (Arcestes, Pi scotropites) and pelecypods (Ha 1 ob i a ) collected from sediment gravity flows and mudrocks respectively correlate with fossils from the Pine Forest Range in Humboldt County, Nevada and the so-called Hosselkus Limestone in Shasta County, California. Structural data collected at and near Alaska Canyon agrees with previous research and indicates a north-northwest to south-southeast compressiona 1 orientation. TABLE OF CONTENTS Introduction............................................. Purpose and Scope of the Study.......................... 6 Geology of the Jackson Mountains........................8 McGi 11 Canyon unit................................... 10 Boulder Creek beds................................... 11 Happy Creek igneous complex..........................13 King Lear Formation.................................. 14 Alaska Canyon............................................ 15 Results of the Study.................................... 21 L i tho 1 ogy.............................................21 Environment of deposition............................25 Paleocurrent analysis................................ 34 Paleontology..........................................37 Structure.............................................44 Regional Setting.........................................48 Golconda allochthon.................................. 48 Sonomi a............................................... 49 Western Mesozoic marine province.................... 51 Rocks of the marine province........................ 52 Section at Alaska Canyon.............................54 Structural implications..............................56 Summary................................................... References............................................... 62 Appendix 1 - Lithologic Descriptions...................71 Appendix 2 - Systematic Paleontology...................85 INTRODUCTION The objective of this study was to determine the age, depositional nature and probable extent of limestone and related sedimentary rocks that occur along the western front of the Jackson Mountains in the vicinity of Alaska Canyon (figure 1). The Jackson Mountains are located in central Humboldt County, Nevada. This range forms the eastern boundary of the Black Rock Desert, an extensive alluvial and playa flat occupying parts of three counties in northwestern Nevada. Alaska Canyon (figure 2 and 3) is a short, steep drainage along the western side of the range (sections 12, 13; T.39N., R.30E.). It can be located on the U.S.G.S. 7.5 minute Hobo Canyon Quadrangle. Topography, climate and vegetation in this region is typical for the Great Basin. The relief is moderately high. The Jackson Mountains rise from an elevation of 1220 meters (4000 feet) on the Black Rock playa to 2716 meters (8910 feet) at King Lear Peak in about 4.8 linear kilometers (3 linear miles). The summers here are hot and the winters cold. Humboldt County is arid to semiarid with an annual precipitation of between about 12.7 and 25.4 centimeters (5 and 10 inches)(Wi1 den, 1 964). Desert shrubs 4 FIGURE 3: Mouth of Alaska Canyon (A) viewed from about one half mile south. Prominant thrust nappe (TN) at the top of the northeast wall terminates the section described in this paper. 5 such as the common sagebrush (Artemisia tridentata) and rabbit brush (Chrysothamnus nauseosus) along with the juniper (Juniperus utahensis) characterize the vegetation. Wilden (1958, 1963, 1964) did most of the preliminary work in the Jackson Mountains. His geologic map (1964) of Humboldt County includes the Jackson Mountains at a scale of 1.250,000. Russell (1981) studied the pre-Tertiary pa 1eogeography and tectonic history of the central Jackson Mountains. Russell also mapped the central portion of the range on a scale of 1:24000. The middle Paleozoic to upper Mesozoic rocks of the Jackson Mountains represent a variety of depositional environments including: basinal, slope, subaqueous and subaerial magmatic, alluvial fan, and f1uviati1e-1acustrine (Russell, 1981). These rocks may provide important clues that could increase our understanding of the Paleozoic and Mesozoic tectonic development of this portion of the Cordillera. 6 PURPOSE AND SCOPE OF THE STUDY This study began in the summer of 1981. Sunmark Exploration Company of Denver, Colorado, graciously offered financial assistance toward a thesis that would undertake a study of the Triassic-Jurassic(?) carbonates around Alaska Canyon in the Jackson Mountains and compare them with those in the Pine Forest Range across the Black Rock Desert. During that first summer measurements and descriptions were made of the sections in the two ranges. With the obligation to Sunmark fulfilled, the research was directed to the carbonates and related rocks in the vicinity of Alaska Canyon in the Jackson Mountains. Part of a second summer was spent mapping the area near Alaska Canyon, and a portion of a third summer was used to look at sedimentary features, collect paleocurrent data and measure structural orientations also in and around Alaska Canyon. Both the structural complexity and the rugged topography in and around Alaska Canyon provided an interesting challenge. Mesozoic folding and thrusting produced a deformed stack of thrust nappes. The nature of the rocks also added to the problem. The highly fractured mudrocks, thick-bedded sandstones and relatively unfossi1iferous , massive limestones when structurally 7 imbricated, deformed and exposed discontinuous1y on shear cliffs and steep talus slopes are difficult to map and correlate. Because of the structural complexity no realistic cross section could be generated to accompany the geologic map. At the beginning of the second summer of study, a probable continuous section was located on the northeast wall of Alaska Canyon. The description of the lithological and depositional nature of the rocks of this section is a major objective of this paper. While it appears that the section is continuous, it is by no means complete. The section does not contain all the rock units present at this locality, but it is believed representative of depositional conditions for similar and depositionally related carbonate and clastic rocks found between Alaska Canyon and Bliss Canyon to the north. 8 Geology of the Jackson Mountains The rocks of the Jackson Mountains have been divided into two tectonostratigraphic units by Russell (1984). These are the McGill Canyon unit and the Jackson Mountains unit. A tectonostratigraphic unit (Russell, 1984 after Jones and others, 1981) is a distinctive stratigraphic sequence or assemblage which is fault bounded and markedly differs from nearby partly or entirely coeval units. The older tectonostratigraphic unit in the Jackson Mountains is the McGill Canyon unit. It contains Paleozoic volcanogenic and siliceous turbidites, sediment gravity flows, hemipelagic and pelagic rocks, and some carbonate units (Russell, 1984). The Jackson Mountain tectonostratigraphic unit consists of Mesozoic rocks of Upper Triassic to Lower Cretaceous age. This unit is subdivided into three subunits. They are: (1) the Upper Triassic Boulder Creek beds composed of carbonates, pelitic to conglomeratic sedimentary rocks along with minor volcanic rock; (2 ) the Upper Triassic and Jurassic Happy Creek igneous complex containing basaltic andesite, andesite, diorite, quartz diorite with minor sedimentary rocks; and (3) the Lower Cretaceous King Lear Formation containing alluvial fan, 9 fluvial and lacustrine rocks. Both the McGill Canyon and Jackson Mountains units share a similar deformationa1 history (Russell, 1984). Two episodes of folding and cleavage