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Structural Geology of the Southern Silurian Hills, San Bernardino County, California

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Structural Geology of the Southern Silurian Hills, San Bernardino County, California RICE UNIVERSITY STRUCTURAL GEOLOGY OP THE SOUTHERN SILURIAN HILLS SAN BERNARDINO COUNTY* CALIFORNIA by Earl William Abbott 3 1272 00095 0491 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF ARTS Thesis Director's signature: Houston, Texas May, 1971 TABLE OF CONTENTS Page INTRODUCTION 1 Purpose i Location 1 Previous Investigations 2 Economic Geology „ 2 Acknowledgements 3 LITHOLOGIC UNITS 4 o Introduction 4 Earlier Precambrian Gneiss Complex 4 Later Precambrian Pahrump Group 9 Precambrian (?) Metasedimentary Complex 15 Precambrian (?) Metadioritic and Metasedimentary Complex 22 Paleozoic Riggs Formation 27 Mesozoic (?) Basic Intrusives 28 Mesozoic Acidic Intrusives 29 STRUCTURAL GEOLOGY 34 Introduction 34 Folding 34 Riggs Thrust Fault 37 Other Faults 43 CONCLUSIONS 45 REFERENCES CITED 46 ILLUSTRATIONS Following Page 1 Regional geographic location map 1 2 Sample locality map 7 3 Photograph of minor doming by forceful intrusion of granitic rock 35 4 Stereographic projection of poles to bedding or foliation In the talc mine area 36 5 Stereographic projection of lineations in the talc mine area 36 6 Stereographic plot of poles to bedding of the Riggs Formation 37 7 Stereographic plot of poles to bedding of the Pahrump Group ... 37 8 Photograph of the Riggs thrust fault near the western mouth of Through Canyon.. 37 LIST OF PLATES .a 1 Geologic map of the southern Silurian Hills in pocket LIST OP TABLES Table Page 1 Modal compositions for Precambrian gneissic rocks (in percent) 8 2 Modal compositions for Precambrian Pahrump Group rocks (in percent) . 12 3 Composite stratigraphic section in the western part of the Silver Lake Talc Mine area 17 4 Modal analyses for Precambrian (?) meta¬ sedimentary complex (in percent) 19 5 Modal analyses of metadioritic rocks from Precambrian (?) metadioritic and metasedimentary complex (in percent) . 24 6 Modal analyses for metasedimentary rocks from the Precambrian (?) metadioritic and metasedimentary complex (in percent) 25 7 Modal analyses of Mesozoic basic intrusives (in percent) 31 8 Modal analyses of Mesozoic acidic intrusives (in percent) 33 ABSTRACT Structural Geology of the Southern Silurian Hills, San Bernardino County, California Earl William Abbott Geology Department Rice University Houston, Texas In the southern part of the Silurian Hills, seven lithologic units are delineated. They are an earlier Precambrian gneiss complex, the later Precambrian Pahrump Group, a Precambrian (?) metasedimentary complex, a Precambrian (?) metadioritic and metasedimentary complex, the Riggs Formation, basic plutonic rocks, and acidic Plutonic rocks. Metamorphism reaches high greenschist to low amphibolite grade with an overprint of contact metamorphism caused by the intrusion of the acidic plutonic rocks. The Riggs thrust fault separates the Pahrump Group, which is in the lower plate, from all of the other units. The upper plate rocks show a more complex folding history than the lower plate rocks. Two quite different types of movement have occurred along the Riggs thrust. The first occurred in late Mesozoic time, was a compressional event, moved in a north or northeast direction, formed a sharp contact with plastic deformation rather than brecciation, and can be seen in the southern Silurian Hills. The second occurred in the Tertiary, was a gravitational event, moved south, formed a chaos structure in the lower plate, and can be seen in the northern Silurian Hills. INTRODUCTION PURPOSE: A study of the southern part of the Silurian Hills was undertaken to produce a large scale map of the area and to establish the relationship between the Riggs Formation, the Pahrump Group, and the Halloran Hills metamorphic complex0 This information will be useful in analysis of the Riggs Thrust (Kupfer, i960) and possible relationships between the rocks of this area and the major thrust plates exposed near Mountain Pass and mapped by Burchfiel and Davis (1971). Thrusts mapped by Burchfiel and Davis were formed during the Sevier and earlier (as yet unnamed) orogenic pulses and an understanding of their geometry will aid in the evaluation of any theories con¬ cerning the development and history of the Cordilleran Orogeny. LOCATION: The Silurian Hills are located in south¬ eastern California approximately 15 miles north-northeast of Baker (Fig. l). Topographic coverage is provided by the Baker, Silurian Hills, Kingston Peak, and Halloran Spring 15-minute quadrangle sheets. The area can be reached by driving 8 miles north from Baker on California highway 127 then northeast on a good dirt road. About 8 miles from the highway is the Silver Lake Talc Mine located near the southwest corner of the study area. The southern part of the Silurian Hills consists of a group I. Regional r.ooj'raphic location map. -2 of low hills ranging in elevation from 2300 to 3700 feet and bounded on three sides by alluvium. Only a very small amount of vegetation and almost no soil cover are charac¬ teristic of this hot, dry climate. PREVIOUS INVESTIGATIONS: Early geologic investigations of the southwestern United States included the Silurian Hills, but all resulting maps were extremely generalized (Spurr, 1903; Waring, 1915; Thompson, 1929; Tucker and Sampson, 1931; and Jenkins, 1938)„ Miller (1946) specif¬ ically mentioned the rocks in the vicinity of the talc mines in his report on the crystalline rocks of southern California,, The Silurian Hills are briefly discussed by McCulloh (1954) and by Hewett (1954 a,b) in his summary of the geology of the Mojave region„ A detailed geologic map was made by Kupfer (1951* 1953* 1954, i960) who dis¬ cussed the geology of the main body of the Silurian Hills north of the present study area. The geology of the talc deposits and their host rocks was studied by Wright (1954). More recently Warnke (1965* 1969) mapped the Halloran Hills to the south. ECONOMIC GEOLOGY: Although the area was prospected for silver in the early 1900*s, talc is the only material resource of lasting Importance. The Silver Lake Talc Mines in the southwestern part of the area have been -3- operating almost continuously since 1915* but production has never been large. In the period from 1915 to 1952 production was just over 210,000 tons (Wright, 195^) <> The mines are currently operating, but on a very irregular basis. ACKNOWLEDGEMENTS: Financial support for this project has been provided by NSF grant GA 1079 awarded to B. C. Burchfiel, by Rice University, and by an NDEA fellowship. The author wishes to acknowledge the assis¬ tance of Dr. Burchfiel who directed the project and provided much help in both the field and the laboratory. Drs. H. C. Clark and D. R. Baker critically read the manuscript and offered much valuable advice. Assistance in the field was ably provided by D. Johnson and D. Pearson. The author is also indebted to the staff and families of the Silver Lake Switching Station of the Department of Water and Power for their hospitality while he was in the field. Above all, the author wishes to thank his wife, Diane, for her help and encouragement. -4- LITHOLOGIC UNITS INTRODUCTION: Seven lithologic units were recognized in the study area. They include an earlier Precambrian gneiss complex, later Precambrian Pahrump Group, a Pre¬ cambrian (?) metasedimentary complex, a Precambrian (?) metadioritic and metasedimentary complex, metamorphosed Paleozoic (?) rocks of the Riggs Formation, Mesozoic intrusive rocks of basic composition, and Mesozoic intrusive rocks of acidic composition. The earlier Precambrian gneisses were recognized below the Precambrian (?) meta¬ sedimentary complex and below the Precambrian (?) meta¬ dioritic and metasedimentary complex, but were not mapped separately. EARLIER PRECAMBRIAN GNEISS COMPLEX: The eastern part of the Mojave Desert region is underlain by a highly metamorphosed complex of gneisses, schists, and various types of associated Intrusive rocks» High grade gneisses and schists are recognized as the oldest rocks in all of the previously mapped areas surrounding the southern part of the Silurian Hills. Basement rocks in nearby areas have been isotopically dated by K-Ar and Sr-Rb methods (Wasserburg and others, 1959; Lanphere, 1964; and Wasserburg and Lanphere, 1964) and give ages of 1400 to 1700 million years old. -5- In the main body of the Silurian Hills, Kupfer (i960) recognized a series of metamorphosed sedimentary rocks intruded by granitic rocks lying unconformably below the sedimentary rocks of the Pahrump Group„ The most common rocks are subschistose gneisses although true schists are reported to be present„ According to Kupfer, the rocks were subjected to more than one metamorphism0 Farther north in the Tecopa area Mason (19^8) assigned a granite gneiss rich in biotite and containing porphyroblasts of pink feldspar to the Archean "age"„ The gneiss is over- lain by sedimentary rocks of the Pahrump Group, but the depositional contact is not exposed because of faulting. To the east, the Shadow Mountains are largely Precambrian granitic gneiss in fault contact with late Tertiary sedi¬ ments (Hewett, 1956 and Wilson, 1966). Where rocks of the Pahrump Group are missing the gneiss is nonconformably overlain by Noonday Dolomite, but at Shadow Mountain a depositional contact is present between the Pahrump Group and the older gneisses (Burchfiel, personal communication). In a report on the Halloran Hills, Warnke (1969) did not recognize the earlier Precambrian gneiss as such, however the Silver Lake Peak Formation in the Halloran Hills may be the metamorphosed equivalent of the gneiss in the southern Silurian Hills. Warnke described the Silver Lake Peak Formation as predominantly quartzofeldspathic gneiss and -6- metadiorite0 It is overlain by the Cree Camp Formation, a series of several types of schists which may be equivalent to the Precambrian (?) metasedlmentary complex of this reporto In the Soda Mountains, to the west, a series of gneisses and schists are overlain by a series of meta- sedimentary rocks. All are assigned to the Precambrian by Grose (1959) <> In the area covered in this report, the earlier Pre¬ cambrian gneiss complex is present although its exact areal extent and its exact contacts are difficult to determine.
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