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. Contacts between gneiss and granite are gradational largely because of very similar mineralogy. Folding, metamorphism, and intrusion have complicated the contact of the gneiss complex with overlying metasedimentary rocks and for this reason the contact was not mapped. Rocks belonging to the earlier Precambrian gneiss complex are included with both the Precambrian (?) metasedlmentary complex and with the Precambrian (?) metadioritic and metasedlmentary complex. Although the petrography of the rocks of the gneiss complex is somewhat variable, by far the most common rock type is quartzofeldspathic gneiss which is banded on a scale of 1/4 to 1 inch. The color is usually pink or white with black or gray bands. Quartz, microcline, and biotite are the major minerals present. Less abundant are, in N
Figure 2. Sample locality map (for explanation of lithologic symbols, see Plate 1). -7-
approximate order of decreasing abundance, muscovite,
orthoclase, and plagioclase. Accessory minerals include
apatite, chlorite, magnetite, titanite, and zircon. Garnet
and the alumino-silicates are absent. Orientation of
micaceous minerals gives the rocks a well defined foliation.
Features such as graphic granite, perthite, and very weakly
zoned plagioclase are often present. Potash feldspars
commonly contain blebs of quartz. Table 1 is a summary
of the mineralogical compositions of several representative samples. Sample numbers correspond to numbered localities
in Figure 2.
These older gneisses represent a terrain which has
been deformed and metamorphosed several times. In addition
to the deformation and metamorphism discussed by Kupfer,
evidence indicates a further deformation with perhaps two
i more episodes of metamorphism. In Kupfer*s area, the
Pahrump Group rocks which overlie the gneiss are essentially
unmetamorphosed while in the southern Silurian Hills the
overlying sedimentary rocks have been metamorphosed,
deformed, and intruded by a large granitic body. Essen¬
tially the same sequence of events has occurred in the
Halloran Hills. These metamorphic relationships will be discussed more fully in the sections concerning the
** Precambrian (?) metasedimentary complex and the Pre-
cambrian (?) metadlorltic and metasedlmentary complex. Table 1, Modal compositions for Precambrian gneissic rocks (in percent) O s -P -P •H H •H £2 ft rH rH P P rH •H PQ I! *P < •H •H •H •H O cd CO > PH c PH O O CO O O 0 0 0 O O 0 co O CO hO 0 O <1) O o 0 o CO PH CO CD O 0 VO 00 ■=t vo oo in H in oo rH p 0 ft a CO H OO H m vo P 0 0 U ft N •* n 00 rH 00 VO H in oo P 0 ft a N PH 0 n vo -=*
LATER PRECAMBRIAN PAHRUMP GROUP:' Later Precambrian
sedimentary rocks in the Kingston Range northeast of the
Silurian Hills were named the Pahrump Series by Hewett
(1940, 1956) and subdivided into the Crystal Spring For¬ mation, the Beck Spring Dolomite, and the Kingston Peak
Formationo Kupfer (i960) changed the name to the Pahrump Group in accordance with the rules of stratigraphic nomen¬
clature. In the Kingston Range, the Pahrump Group has an
aggregate thickness of approximately JjOOO feet and is
easily subdivided into the three formations. Shale and
dolomite with lesser amounts of conglomeratic quartzite,
sandstone, .and chert make up the Crystal Spring Formation.
Virtually all of the Beck Spring Dolomite is dolomite. The
Kingston Peak Formation•is composed predominantly of sand¬ stone and conglomerate. In the southern Death Valley region,
the Pahrump Group has an estimated thickness of nearly
10,000 feet (Noble, 194l). All three formations are recog¬
nizable in the Tecopa area, but their areal extent Is limited
(Mason, 1948). The Pahrump Group can be followed from the
Kingston Range westward, without significant structural break, for a distance of almost 75 miles.
In the northern Silurian Hills, all of the rocks of the lower plate of the Riggs thrust fault, with the exception
of the earlier Precambrian gneiss, were assigned to the
Pahrump Group by Kupfer. At least 11,000 feet of strati¬
graphic section were measured and subdivided into 35 members. -10-
Recently, however, Stewart (1970) identified and measured late Precambrian and lower Cambrian rocks of the Johnny
Formation, Stirling Quartzite, Wood Canyon Formation,
Zabriskie Quartzite, Latham Shale, and Chambless Limestone corresponding to Kupfer*s units 23 through 35* Rocks below unit 23 were not studied, but the upper part was assigned to the Kingston Peak Formation0 Quartzitic sandstone and carbonate rocks, both limestone and dolomite, with subor¬ dinate amounts of conglomerate and- shale make up the bulk of the northern Silurian Hills section. To the west, rocks of the Pahrump Group as well as the Johnny Formation, the
Stirling Quartzite, the Wood Canyon Formation, and the Zabriskie Quartzite have been identified by Stewart (1970) in the hills west of Silver Lake.
In the southern Silurian Hills, the rocks below the Riggs thrust fault are assigned to the Pahrump Group by
Kupfer (i960) who included the extreme northern part of the present study area in his more generalized map and assigned the rocks below the Riggs thrust to the Pahrump Group
"undifferentiated". It should be noted, however, that
Stewart did not study these rocks and there is a possi¬ bility that these rocks could be younger. These rocks have been metamorphosed but can be traced into unmeta¬ morphosed rocks to the north. Most of the rocks are quartzofeldspathic schists, but marble and quartzite are -11-
Important constituents. Quartz, K-feldspar, and blotlte are the most Important minerals In the schists. Some of the carbonate rocks have suffered little change In mineralogy while others have been transformed into calc-sllicate rocks containing such minerals as epldote-clinozoisite, tremolite- actinolite, diopside, and scapolite. Other minerals, important in some samples, include hornblende, plagioclase, muscovite, and chlorite. Accessory minerals include apatite, titanite, zircon, rutile, olivine, and magnetite.
Table 2 is a list of modal analyses of several representative samples.
Although relic sedimentary structures are sometimes preserved, metamorphic fabrics predominate. Large-scale compositional variations are due to original bedding, but it is not clear whether the microscopic banding observed is the result of metamorphic segregation or of original sedi¬ mentation, A high degree of mineral orientation gives these rocks their schistose character. Micas are most strongly oriented, but other minerals such as quartz, amphiboles, and pyroxenes often show preferred orientation. Lineations are less common, but are locally conspicuous. In one case, biotite occurs in distinct rod-like masses which all trend in the same direction. Streaks of other mafic minerals also are present. Most rocks are fine-grained, but some, especially the carbonate layers, are medium- to coarse¬ grained. The few conglomerate layers found contained 12-
Table 2o Modal compositions for Precambrian Pahrump Group rocks (in percent).
Sample Quartz K-feldspar Plagioclase Muscovite Biotite
3a 99 - tr tr 3b 80 tr
3c 60 - 5 3d 89 3 tr 7
3f ' 18 38 2 - 25a 30 3 2 25c 65 5 tr - 25 25d 25 25 10 25e 46 28 4 7 13 25 j 95 tr 27a 1 38 11 - 40
27b - -
27c 30 39 5 24
132a - -
137 1 - 138 30 33 28 -13-
Table ! 20 Continuedo
Sample Chlorite Hornblende Epidote Carbonate Accessories
3a - - - - ap, mt, zr
3b - - - 10 ap, mt, zr, Pe-ox-10^
3c - - 10 5 ap, mt, ti, zr, trm-20^
3d - - - - ap, mt, ru, ti, zr
3f tr 38 - - ap, mt, ti, z
25a - - 60 5 ap, mt 25c - - - - ap, mt
25d 2 15 20 1 ap, mt, ti 25e tr tr - - ap, mt, zr
25J - - tr 1 ap, mt, ti, zr, Fe- ox
27a 8 - - - ap, mt
27b tr - - 5 mt, ti, di- 30$i scp-65$
27c tr - - - ap, mt, zr
132a - - - ’ 99 ol, Fe- ox
137 1 - - 95 stp-l$ 138 - - . -' - ap, mt, zr ap = apatite, di = diopside, Fe -ox = Iron oxide, mt = magnetite ol = olivine, ru = rutile, scp = scapollte, stp = = stilpnomelane ti = tltanlte, trm = tremolite, zr = zircon
© -14-
quartz pebbles in a quartzofeldspathic matrix. Three
carbonate layers, each 3 to 5 feet thick, occur just beneath
the Riggs .thrust at the northwestern end of the area
(Plate l). The carbonate beds are isoclinally folded and
the fold limbs are parallel to the general strike of the
bedding, thus the attitude of the bedding is fairly constant„
Very small bodies of basic and acidic rocks locally intrude
the Pahrump Group in this area®
These rocks are regionally metamorphosed to high
greenschist-low amphibolite grade, but some assemblages
indicate a high temperature-low pressure type of meta¬
morphism to hornblende hornfels grade0 The following assemblages are present:
A.. Quartzofeldspathic
lo quartz-calcite 2® quartz-muscovite-epidote-calcite-tremolite
(titanite)
3« quartz-microcline-muscovite (titanite) 4® quartz-orthoclase-plagioclase-hornblende
(titanite, rutile)
5® quartz-orthoclase-biotite 6® quartz-microcline-orthoclase-muscovite-biotite-
chlorite-hornblende-epidote (calcite, titanite)
7® quartz-orthoclase-microcline-muscovite-biotite (chlorite, hornblende) -15-
8, orthoclase-plagioclase-muscovite-biotite-
chlorite (quartz)
9. quartz-orthoclase-microcline-muscovite-biotite 10. quartz-orthoclase-microcline-plagioclase-biotite B o Calcareous
1. calcite-diopside-scapolite (tltanlte) 2. calcite-epidote-quartz (orthoclase, muscovite)
3. calcite-olivine (biotite?)
4. calcite-muscovite-chlorite-stilpnomelane? Most assemblages, containing such minerals as epidote,
tremolite-actinolite, biotite, chlorite, muscovite, and
calcite, indicate a high greenschist facies (Turner, 1968).
However, the presence of diopside and hornblende are indicative of amphibolite grade metamorphism, and the
occurrence of scapolite in one sample and a small amount
of olivine in another suggest the possibility of local
contact metamorphism. The absence of wollastonite in
calc-silicate rocks and almandine in quartzofeldspathic
rocks places an upper limit on the degree of metamorphism.
PRECAMBRIAN (?) METASEDIMENTARY COMPLEX: Rocks
assigned to this unit occur in the talc mine area (Plate l).
Here they are complexly folded, intruded by both basic and acidic plutonic rocks, metamorphosed, and perhaps meta-
somatized. Wright (1955) studied the talc mines and presented the composite stratigraphic section which is -16- reproduced In Table 3. The present author’s observations
are consistent with Wright's findings,, Table 4 Is a
tabulation of modal analyses of several representative
thin sections from the talc mine area. Metaigneous rocks, usually dioritic in composition,
were also found locally intruding the metasedimentary complex as dikes or sills. Sample 6d (Table 4) is a microscopic analysis of one of these dikes. The amount
of metadiorite in the talc mine area is quite small, but becomes much more common in the metadioritic and meta¬
sedimentary complex discussed below.
A strong similarity in rock type and mineralogy exists
between the rocks of the talc mine area and those of the Pahrump Group beneath the Riggs thrust fault. It is
probable that the rocks of the talc mine area belong to
either the Pahrump Group, the late Precambrian-lower
Cambrian formations, or both. These rocks could also represent a previously unknown series of Precambrian rocks.
Metamorphic relationships in the talc mine area indicate
that, like the rocks beneath the Riggs thrust fault, these
rocks were subjected to both regional and contact meta¬
morphism. Metasomatism may also be an important process
here. o Table 3. Composite stratigraphic section in the western part of the Silver Lake Talc Mine area (Wright, 1955)» '—/ ft P ft ft P P ft A-4 525 0 U ft o c 0 O 0 0 o G 0 co 0 K g o • rH 0 ft CP H O Nd*H -P to 'd rH •H T3CO f* X! <*H Hu -P *H+i •d ato rl'd'd c X>
Table 4. Modal analyses for Preeambrian (?) metasedimentary complex (in percent).
Sample Quartz K-feldspar Plagioclase Muscovite Biotite
6b
6e 16 - mm
6d - 2 46 - 2
6e 76 18 - 6 -
6g •46 28 3 6f 50 13 10
8b 53 23 2 1 19 8c 62 17 - tr 20 16c 15 - - l6e 20 - 10 — l6g 1 mm mm 3 -
17a tr - - 5 -
17b 35 - - 35 -
17d 5 - - 10 -
34a - - - tr -
34c 43 14 / tr - tr 50c 42 16 10 tr 32
63a 5 M 5 -20-
Table 4. Continued.
Sample Hornblende Carbonate Chlorite Diopside Accessories
6b - - - - trm-6($, tlc-4<#
6c - 11 tr 71 gr-2$, ti, ep
6d 48 - - - ti-2$, ap, il
6e - - - - Fe-ox, mt, ru, zr
6g 23 tr - - ap, mt, zr
6f - 2 - 17 trm-8^, ap
8b - - 2 - ap, zr
8 c - tr tr , mm Fe-ox, ep
16c - 85 - - Fe-ox, mt l6e - 70 tr mm Fe-ox, mt l6g - 95 - - Fe-ox, mt
17a mm 95 tr mm Fe-ox, mt
17b - - 25 - Fe-ox, ap, mt
17d - 80 5 - Fe-ox, mt
34a - 70 - - ol-30$, hum, idd, mt, se]
34c 25 tr - 17 ap, ep, mt, ti
50c - - - - ap, mt, zr
63a - 75 15 - - ap =* apatite $ ep = epidote, Pe -ox = iron oxide, gr = garnet, hum » humite* idd = = iddingslte , 11 = ilmenite, mt : = magnetite, ol =s olivine, ru = rutile, serp = serpentine, ti = titanlte, tic = talc, trm = tremolite, zr = zircon -21-
The following assemblages are present in the talc mine area:
A. Quartzofeldspathic
lo quartz-orthoclase-microcline-muscovite (rutile) 2. quartz-orthoclase-plagioclase-hornblende
3o quartz-orthoclase-plagioclase-diopside-calcite- tremolite
4, quartz-orthoclase-microcline-biotite (plagioclase,
muscovite, chlorite)
5o quartz-orthoclase-biotite (plagioclase, epidote)
60 quartz-muscovite-chlorite 7o quartz-microcline-hornblende-diopside (titanite, epidote)
8. quartz-orthoclase-plagioclase-biotite
B0 Calcareous
lo calcite-diopside-orthoclase-garnet (titanite, epidote)
20 tremolite-talc 3. calcite-quartz 4. calcite-muscovite
5. calcite-quartz-muscovite-chlorite 6. calcite-olivine (humite)
7* calcite-quartz-chlorite-muscovite The mineralogical assemblages above are Indicative of low amphibolite grade of regional metamorphism with an over¬ print of hornblende hornfels grade of contact metamorphism -22-
(Turner, 1968). As in the Pahrump Group, wollastonite or
almandine are not present0 Chlorite and epidote are present, but in much smaller abundances than in the Pahrump
Group. This is due to the higher intensity of metamorphism
in the talc mine area than in the Pahrump Group rocks
beneath the Riggs thrust. Metasomatism has been instru¬
mental in the formation of the talc-tremolite rocks (Wright,
1955) and may have been responsible for some of the other assemblages.
PRECAMBRIAN (?) METADIORITIC AND METASEDIMENTARY
COMPLEX: Metadioritic and metasedimentary rocks underlie
an east-west trending terrain bounded on the north by the Riggs Formation and on the south by a large granitic pluton (Plate l). This pluton separates these rocks from the metasedimentary rocks of the talc mine area. Meta¬
dioritic rocks of the complex intrude both the Riggs
Formation and the metasedimentary part of the Precambrian (?) complex. The original relations between the metasedimentary
rocks and the Riggs Formation are obscure, however they are
now intimately infolded.
The metadiorite is medium-grained, dark in color,
always foliated and sometimes lineated. Plagioclase
(An^o-An^o), hornblende, and lesser biotite characterize the metadiorite. Quartz and potash feldspar, while present, make up only a few percent of the rocks. Titanite is a
conspicuous accessory mineral usually making up 1 or 2 -23- percent of the rocks. Table 5 is a tabulation of modal analyses of some representative thin sections of the dioritic rocks. The age of the metadiorite is not Precambrian, but the difficulty of mapping separately these rocks and the metasediments they intrude has led the author to lump them together. Because the Paleozoic (?) Riggs Formation is intruded by these rocks, they must be younger than the Riggs, but they predate the major deformation and granitic intrusion in this area. Biotite schist, metaquartzite, and marble, in descending order of abundance, are the major metasedimentary rock types in this complex. The biotite schist is often abun¬ dantly injected with intermediate through acidic igneous material in a "lit par lit" fashion. Metaquartzite and marble are similar to that of the talc mine sequence, but the presence of injected biotite schist is a major difference Table 6 is a tabulation of modal analyses of several thin sections of the metasedimentary rocks. Metamorphic mineral assemblages in the metasedimentary rocks of this complex, like those of the two previous meta¬ sedimentary terrains, indicate both regional and contact metamorphism. The following mineral assemblages are present in the metasedimentary part of this complex: A. Quartzofeldspathlc 1. quartz-orthoclase-muscovite-epidote (rutile) -24- H 0 •» * H •* * P •* •H p •* •rH ^ - P 0 0 *4 H P< $ co •=J* D) 0 H 1 0 1 1 H i •> 1 0 •\ •»0 c d 0 0 ^ o ^ 0 o ^ o 0 c 0 H rH rH H H -ef H rH H OJ rH p cd O n 1 •» 1 1 i 1 1 1 1 1 •* 0 1 1 1 »t •H O p1*H H Pi •H rH Pi P-H P*H rH P P H P •H H P P P o <: 0 P •H 0 P •H 0 0 P 0 P •H 0 g •H 0 P *H 0 0 •n II p •H •H SH 0 -P O p •H •H n d U OJ -=t . P 1 U O rH P H in 0 cd O P P H P P -p H •rl 0 P > S O o o 0 c^* N—^ 0 d i c G II cd 0 •H rH OJ 00 tv O O -=t -ef in CVJ eh
2. quartz-orthoclase-epidote-tremolite (titanite)
3. quartz-microcline-muscovite-chlorite (biotite) 4 o quartz-microcline-plagioclase-muscovite-biotite
5. quartz-microcline-plagioclase-biotite-chlorite (muscovite)
60 quartz-orthoclase-plagioclase-biotite (muscovite) B. Calcareous
1» calcite-quartz 2. calcite-quartz-epidote-garnet (titanite)
3. calcite-muscovite-olivine (humite) 4. calcite-quartz-chlorite
5. calcite-muscovite These assemblages are indicative of high greenschist regional metamorphism with contact metamorphism reaching hornblende hornfels grade locally (Turner, 1968). The absence of hornblende and the presence of abundant epidote in these rocks suggest a lower grade of regional metamorphism here than in the talc mine area where hornblende is common and epidote is present only in small amounts.
Although the metadioritic rocks have a tectonite fabric, their mineralogy is that of the original diorite.
Either these rocks were not metamorphosed or their original mineralogy was stable under the pressure-temperature condi¬ tions present during metamorphism. Because of their fabric, the author favors the latter explanation. -27-
PALEOZOIC RIGGS FORMATION: Kupfer (i960) provisionally suggested the name Riggs Formation for a series of carbonate rocks which composes the upper plate of the Riggs thrust fault. The unit is about 2500 feet thick and is composed of approximately equal proportions of limestone and dolomite. No fossils were found and the Riggs Formation is everywhere highly recrystallized so an Indisputable age determination is presently impossible. Four possible correlatives with the Riggs Formation were considered by Kupfer, the Beck
Spring Dolomite, the Noonday Dolomite, upper Paleozoic carbonate rocks, and post-Paleozoic rocks. Kupfer*s analysis led him to believe the Riggs Formation was most likely late Paleozoic in age. The present author agrees but no new evidence was found in the southern part of the
Silurian Hills to support this interpretation.
Wilson (1966) thought the Riggs Formation to be correlative with the Goodsprings Dolomite and to be the source of dolomite blocks for the dolomite megabreccia in the Shadow Mountains. The Riggs Formation, however, consists of approximately 50 percent limestone whereas the Goodsprings Dolomite is largely dolomite. On the other hand, Hewett
(1956) found rocks which he designated "undifferentiated Paleozoic (?)" to be similar to rocks of the Riggs Formation and states that they most closely resemble portions of the
Sultan, Monte Crlsto, and Bird Spring Formations all of late Paleozoic age. -28-
The thickness of the Riggs Formation in the southern
Silurian Hills is not known because the base of the Riggs is marked by a thrust fault, while the top of the section
is intruded by both metadioritic and granitic rocks0 The contact between the Riggs Formation and the metasedimentary complex is folded and the foliation of the metasediments is parallel to the axial plane of the folds0 Axial plane foliation does not extend into the limestone of the Riggs
Formation, but this may be due to lack of platey minerals in the limestone. The original nature of this contact is obscured by deformation and metamorphism and will be dis¬ cussed belowo
Several thin sections of the Riggs Formation were examined and no mineralogical changes other than recrys¬
tallization were observed. Either temperature and pressure were not high enough to form wollastonite, garnet, diopside, or any other metamorphic minerals or else insufficient
silica was available. Some of the previously mentioned carbonate rocks showed development of calc-silicate minerals while others did not even though quartz was present in the rock.
MESOZOIC (?) BASIC INTRUSIVES: Small bodies of basic
intrusive rocks occur scattered throughout the area of the upper plate. All of the bodies were too small to map except three in the talc mine area. They are porphyritic and -30- unfoliated and their composition lies near the diorite- gabbro boundary. Hornblende, plagloclase (Ang^ to An^), and auglte are the major minerals. Biotite, orthoclase, orthopyroxene, and quartz are minor constituents. Accessory minerals include titanite, apatite, and magnetite. Chlorite, epidote, muscovite, and calcite are alteration minerals.
Poikiolitic textures predominate with hornblende surrounding plagloclase. Modal analyses of a few samples are presented in Table 7.
Structural relations such as lack of foliation and metamorphic mineral assemblages suggest that the basic intrusives postdate the major metamorphism and associated deformation; field relations show that the acidic Intrusives are later. Thus, the basic intrusives are here regarded as
Mesozoic in age.
MESOZOIC ACIDIC INTRUSIVES: Granitic intrusive rocks are common throughout the area of the upper plate of the Riggs thrust. The largest area of granitic rock separates the talc mine area from that of the Precambrian (?) meta- dioritic and metasedimentary complex (Plate l). Small bodies of granitic rock, too small to map, are also found in the lower plate of the Riggs thrust and along the thrust fault itself. Dikes of granitic rock are also very wide¬ spread in the area of the upper plate. Table 7. Modal analyses of Mesozoic basic intrusives (in percent) < •H < P w •H 'd •H P *p H •H P (H •H H rH w H o? CO P
These rocks are medium- to coarse-grained and pink to light gray in color„ No obvious foliation is present0
Quartz, microcline, orthoclase, plagioclase (An2Q-An^Q) are the most important minerals. Potash feldspar-plagioclase ratios range from approximately 1:1 to 2:1 in the larger granitic bodies and greater than 2:1 in the dikes0 Biotite and muscovite are present in lesser amounts and the acces¬ sory minerals are apatite, magnetite, and zircon0 Chlorite and sericite are alteration minerals. Potash feldspars are often perthitic. Graphic granite occurs in most samples and is sometimes very common, for example in sample 2d graphic granite makes up over 50$ of the rock. Myrmekite occurs less frequentlye Table 8 is a tabulation of modal analyses of several representative samples. Samples 2d,
9a, 10c, and 8lb are dikes.
The granitic rocks intrude all of the other rock units in this area. A Mesozoic age is assigned to them because of their intrusive relationships and because the youngest of these, namely those intruded along the Riggs thrust fault, have been isotopically dated and give a Cretaceous age (see below). Table 8. Modal analyses of Mesozoic acidic intrusives (in percent). •H < p •H s P P .g •H m •H •H rH PH rH O CO P rH 0 XJ U > O CO o 0 ifl O •H H 0 3 CO O O p •H O CD CQ 0 O CO cd bO O W cd 0 O O c O U O O 0 N u 00 P X3 OJ •% 00 H 00 OO OJ OO cd p a O rH OJ 1 H o n N U x; *H •H p O 0 Pi a bO cd OJ in H OJ OJ X) in i 00 X3 rH P VO 00 OJ H H cd in Pi O OO OJ U OJ STRUCTURAL GEOLOGY INTRODUCTION: The structure of the Silurian Hills is dominated by the Riggs thrust fault (Kupfer, I960), the lower plate of which is exposed only in the northern part of the southern Silurian Hills. A folded and intruded metamorphic terrain makes up the rest of the southern part of the Silurian Hills and this terrain is proposed to be part of the upper plate of the Riggs thrust0 A complex history of folding characterizes the talc mine area (upper plate) while the folding in the Riggs Formation and in the lower plate of the Riggs thrust is simpler. High angle faulting, while present, is not well known due to lack of stratigraphic control. A north or northeastward movement direction occurring for the last time at approximately 90 million years ago is inferred for the thrusting along the Riggs thrust, followed by Tertiary landsliding forming chaos structure in the northern Silurian Hills. A corre¬ lation between the upper plate of the Riggs thrust fault and the Winters Pass plate (Burchfiel and Davis, 1971) Is suggested. An earlier fault, possibly a thrust, is postu¬ lated for bringing together the rocks of the talc mine area and the Riggs Formation. FOLDING: Folding relationships in the southern Silurian Hills are difficult to interpret in part because of its complex nature and in part because of lack of stratigraphic -35- control. Some generalizations can be made, however. Four areas can be delineated on the basis of folding pattern and history. They are, from south to north, the talc mine area, the metasediments adjacent to the Riggs Formation and assigned to the Precambrian (?) metadioritic and meta- sedimentary complex, the terrain of the Riggs Formation, and the lower plate rocks north of the Riggs thrust. At least two and probably three types of folds are present in the talc mine area. . The first type is iso¬ clinal and can only be recognized if seen at the hinge area. The folds are small, the largest being only a few feet across. The second type of fold is more open and larger is size, measuring a few tens of feet in amplitude. They can be recognized by tracing individual beds and are usually asymetric. Folding on a larger scale is strongly indicated by the outcrop pattern of the carbonate beds and by the attitudes of the bedding (Plate l). Deformation by forceful intrusion of granitic rocks is also present. Figure 3 shows granitic rocks, just above the wash, which appear to have pushed the overlying carbonate bed into a small dome-shaped fold. All of these factors combine to give a very complex folding picture. Figure 4 is a sterographic projection of poles to bedding or foliation in the talc mine area. It can be seen that there is no single fold axis. Although the data is sparse, a plot of lineatlons in the Figure 3 Photograph of minor doming by forceful intrusion of granitic rock. -36- talc mine area (Pig. 5) indicates refolding of a previous lineation about a northwest axis. Data in the area encompassed by the Precambrian (?) metadioritic and metasedimentary complex is too limited for fold analysis. However, an examination of the outcrop pattern of the carbonate units (Plate l) indicates that one type of folding that was present in the talc mine area is missing here. The outcrop pattern of the carbonate beds is straight compared to the sinuous outcrop pattern in the talc mine area. Also, attitudes in the western part of the area have the same general northwest-southeast strike and a high dip angle. Any large-scale folding present here must have an axis with this general orientation. In the Riggs Formation, the folding, at least on a large scale, is much gentler. Most attitudes strike east-west and dip south. Kupfer (i960) has suggested a broad warping of the Riggs Formation and the underlying Riggs thrust fault. This warping is present here and Figure 6, a plot of poles to bedding in the Riggs Forma¬ tion, shows a configuration which, although only a few points are plotted, could be explained by this warping. Beneath the Riggs thrust fault in the southern Silurian Hills, the only folds recognized were small isoclinal folds similar to those of the talc mine area. Like those of the talc mine area, the folds are very tight and can only be seen near the axis when individual beds can be traced. Figure 4. Stereographic projection of poles to bedding or foliation in the talc mine area (60 points; contours on 1, 3* and 5 percent). Figure .5. Stercographic projection of lineatlons in the talc mine area (21 points; circles are microfold axes, triangles are fold axes, and squares are sllckensldes). -37- In the northern Silurian Hills, folds with northwest plunging axes are present in the lower plate rocks (Kupfer, 1960)0 Figure 7 is a sterographic plot of poles to bedding of the lower plate rocks using Kupfer*s attitudes. Even considering the error Inherent in transferring attitudes from a map, a well-defined girdle with a southeast axis is apparent. It is evident, then, that the folding history of the rocks of the talc mine area and the sediments of the meta- dioritic and metasedimentary complex is different from that of the Pahrump Group beneath the Riggs thrust. The rocks of the upper plate are more intensely deformed and meta¬ morphosed. Figure 8 is a photograph illustrating the difference in character of the two terrains. To the left are the less deformed rocks of the lower plate, to the right the rocks of the upper plate. The trace of the Riggs thrust is to the left of the highest ridge. The photograph was taken from near the western mouth of Through Canyon, RIGGS THRUST FAULT: The Riggs thrust fault is the major structural feature of the Silurian Hills. Precambrian, late Precambrian, and lower Cambrian rocks compose the lower plate which is exposed most widely in northern part of the Silurian Hills. Here, earlier Precambrian gneisses are overlain by the Pahrump Group, Johnny Formation, Stirling Quartzite, Latham Shale, and Chambless Limestone (Stewart, Figure 6 , Stereographic plot of poles to bedding of the Riggs Formation (21 points; contours on 1, 2 and 3 percent). Figure 7 of the (196 points; Figure 8. Photograph of the Riggs Thrust Fault near the western mouth of Through Canyon. -38- 1970)o These rocks, exclusive of the earlier Precambrian gneisses, are deformed into folds with northwest plunging axes (see above and Kupfer, i960). In the northern part of the Silurian Hills these rocks are unmetamorphosed, but near Citadel Ridge, to the south, the metamorphism increases to high greenschist-low amphibolite grade. In the northern part of the Silurian Hills, the upper plate of the Riggs' thrust is composed only of Riggs For¬ mation and granitic intrusive rocks. In the southern Silurian Hills, the upper plate also includes a Precambrian (?) metadioritic and metasedimentary complex, the Pre¬ cambrian (?) metasedimentary complex of the talc mine area, and basic intrusive rocks. Granitic intrusive rocks occur in the upper plate, in the lower plate, and along the thrust itself, but by far the largest amount of granitic rock is in the upper plate and is cut by the Riggs thrust. The Riggs Formation generally separates the granitic rock from the thrust fault, but at two localities the thrust contact with the granitic rock is exposed at the surface. One locality is near the eastern end of Through Canyon near sample locality 25 (see Figure 2). The other locality is at the extreme northeastern end of the present study area (Plate l). The rocks of the talc mine area are included with the upper plate because: l) of their similarity with the metadioritic and metasedimentary complex; 2) of the complexity of folding in this area compared to the simple -39- folding in the lower plate rocks to the north; and 3) they are intruded by a large granitic body which is itself cut by the Riggs thrust0 In the southern Silurian Hills, the Riggs thrust fault separates two metamorphosed units, the Riggs Formation and the Pahrump Group. They appear to have been metamorphosed together forming a "welded” contact. No brecciation of the lower plate is present. Instead, a thin, highly sheared zone containing boudinage, isoclinal folds, mullions, and deformed granitic rock separate the two plates. The upper plate is often fluted and lineations in both plates are parallel. In contrast, the Riggs thrust in the northern Silurian Hills is characterized by extreme brecciation of the lower plate forming the Riggs chaos (Kupfer, i960). The term chaos was first used to describe an extensive sheet of brecciated rock in the southern Death Valley region (Noble, 194l). Noble called this sheet the Amargosa chaos and described it as "a fault breccia on a cyclopean scale". In both the Silurian Hills and Death Valley, the fault blocks range in size from a few feet in diameter to more than half a mile in length and are separated by little or no matrix. No parallelism of lineations or foliations is found between blocks. Although the blocks have a chaotic arrangement, an overall order is present. It is Important to note that in the Amargosa chaos the blocks rest on a -40- fault surface and consist entirely of allochthonous material„ Hypotheses for the origin of chaos structure, especially applied to the Amargosa chaos, have Included thrust faulting (Noble, 194l), landsliding (Sears, 1953 and Noble and Wright, 1954), internal adjustments between large strike-slip faults (Rrewes, 1963)> and anastomosing normal faults flattening into one master normal fault (Wright and Troxel, 1969). The Riggs chaos contains all of the features of the Amargosa chaos except one, In the Riggs chaos it is the lower plate which is brecciated and involved in the chaos formationo It is this feature more than any other which led Kupfer to a thrust fault origin. Kupfer determined a south¬ ward movement direction by replacing the individual chaos blocks into their original position and placed the age of movement in the Tertiary by stratigraphic means. The age and direction of movement can be determined by other means, however, and the two types of thrust faults have different ages and different movement directions. In the southern Silurian Hills, the Riggs thrust fault postdates the major volume of granitic intrusion, but is contemporaneous with intrusion of small pods of granitic rock which are intruded into and are deformed by the thrusting. These rocks yield K-Ar ages of 84 to 95 million years (Sutter, 1968). This is a minimum age representing uplift or cooling of the granitic rock. This is older than the Tertiary age obtained by Kupfer. The discrepancy between -41- the two ages Is interpreted to mean that the major thrusting occurred at approximately 90 million years ago and at some time in the Tertiary another movement, probably gravitational and unrelated to the thrusting, occurred forming the chaos. There are several reasons to believe that the main thrusting movement along the Riggs thrust was from south or southwest to north or northeast: l) Fluting on the lower surface of the upper plate indicates a final movement direction of NE-SW. 2) The upper plate of the Riggs thrust is everywhere metamorphosed while metamorphism of the lower plate occurs only to the south. 3) Rocks interpreted as being older than the Riggs Formation occur in the southern part of the upper plate. 4) Regional movement directions determined by Burchfiel and Davis (personal communication) for thrust faults with which the Riggs thrust may be correlated are northeastward. Almost 4 miles of overlap between the upper and lower plates of the Riggs thrust are present in the Silurian Hills. This established an absolute minimum distance of movement. No upper limit on the distance of movement is known. Tentatively, the Riggs thrust may outline a window in the Winters Pass plate, a thrust plate interpreted by Burchfiel and Davis (personal communication) to be a very widespread feature in this area. Hewett (1956) first described the Winters thrust fault and Burchfiel and Davis -42- (l97l) are able to trace the upper plate of the thrust, now the Winters Pass thrust, over an area which may be as much as 2000 square miles. Precambrian rocks of the Halloran Hills complex are currently interpreted by Burchfiel and Davis to be part of the Winters Pass plate. These rocks are apparently continuous with rocks of the talc mine area. Metamorphism increases to the southwest in the Winters Pass plate and areas toward the rear of the plate, such as the Silurian Hills, would be expected to be metamorphosed. i Precambrian and late Precambrian rocks are included in both the upper and lower plates of the Winters Pass thrust in the Clark Mountain thrust complex. The lower plate of the Riggs thrust in the Silurian Hills is postu¬ lated to be in the same structural position as the lower plate of the Winters Pass thrust, but whether it is con¬ tinuous with any of the plates in the Clark Mountains farther east is not known. A possibility exists that the Riggs thrust belongs to a higher structural unit than the Winters Pass thrust. In this case, the lower plate of the Riggs thrust would correspond to the upper plate of the Winters Pass thrust and the upper plate of the Riggs thrust would correspond to a hitherto unrecognized structural unit. More work is needed, especially in the area of the Halloran Hills, if this question is to be resolved. -43- OTHER FAULTS: The mechanism involved in the super¬ position of the Riggs Formation with the Precambrian (?) metasedimentary complex of the talc mine area and the Precambrian (?) metadioritic and metasedimentary complex remains a problem. It is certainly a fault, but it has been obliterated by intrusion of first diorite, then granitic rocks. A structural discrepancy between the Riggs Formation, which has been only gently folded and domed, and the complexly folded talc mine area, is obvious. This suggests that the rocks of the talc mine area have gone through at least one deformational event before they came together with the Riggs Formation. Also, the Riggs Formation appears to underlie another complexly folded terrain, the meta¬ dioritic and metasedimentary complex. A contact between the Riggs Formation and metasediments of the Precambrian (?) metadioritic and metasedimentary complex can be seen near the western mouth of Through Canyon. This contact is a folded surface above which the metasedimentary rocks exhibit an axial plane foliation. Whatever is the nature of this contact, the two terrains were metamorphosed and deformed together during a deformational phase preceding or in part contemporaneous with thrusting along the Riggs thrust. The age of the postulated fault is not known, but if the Riggs Formation is middle or upper Paleozoic it must be late Paleozoic or early to middle Mesozoic. It is -44- cert a inly older than the intrusion of the diorite which intrudes both the Riggs Formation and the metasediments and is cut by the Riggs thrust. -45- 1 CONCLUSIONS This study of the southern Silurian Hills leads the author to the following conclusions: l) There are four structurally distinct terrains, the Pahrump Group beneath the Riggs thrust fault, the Precambrian (?) metasedimentary complex of the talc mine area, the Precambrian (?) meta- dioritic and metasedimentary complex, and the Riggs For¬ mation. 2) Three of these terrains, namely the Precambrian (?) metasedimentary complex of the talc mine area, the Pre¬ cambrian (?) metadioritic and metasedimentary complex, and the Riggs Formation, were brought together at some time in the past, probably the late Paleozoic or early Mesozoic, by faulting. 3) The three previously mentioned terrains now compose the upper plate of the Riggs thrust fault and have been displaced northward or northeastward by the thrust over the Pahrump Group in late Mesozoic time. 4) The Riggs thrust fault is probably a window in the Winters Pass plate and thus, if this is true, the Winters Pass thrust must "root" to the southwest of the Silurian Hills. 5) The development of chaos and the southwest movement along the Riggs thrust in the northern Silurian Hills is a Tertiary event. -46- REFERENCES CITED Burchfiel, B. C. and Davis, G. R., 1971, Clark Mountain thrust complex in the Cordillera of southeastern California, p. 1-28, in Elders, W. A., Editor, Geological excursions in southern California: Univ. Cali'f., Riverside, Campus Museum Contrib., no, 1. Drewes, H., 1963, Geology of the Funeral Peak Quadrangle, California, on the east flank of Death Valley: U. S. Geol. Survey Prof, Paper 413, 78 p. Grose, L, T., 1959, Structure and petrology of the northeast part of the Soda Mountains, San Bernardino County, California: Geol. Soc. America Bull., v. 70, p. 1509-1548. Hewett, D. F., 1940, New formation names used in the Kingston Range, Ivanpah quadrangle, California: Wash. Acad. Sci. Jour o, v. 30, p. 239-240. , 1954a, A fault map of the Mojave Desert region, California, chap. 4, p. 15-18, ir^ Jahns, R. H., Editor, Geology of southern California: Calif. Div. Mines Bull. 170. , 1954b, General geology of the Mojave Desert region, California, chap. 2, p. 5-20, in Jahns, R. H., Editor, Geology of southern California: Calif. Div. Mines Bull. 170. , 1956, Geology and mineral resources of the Ivanpah quadrangle, California and Nevada: U. S. Geol. Survey Prof. Paper 275, 172 p. Jenkins, 0. P., 1938, Geologic map of California: Calif. Div. Mines, scale 1:500,000. Kupfer, D. H., 1951, Thrusting and chaos structure in the Silurian Hills, San Bernardino County, California (Abstract): Geol. Soc. America Bull., v. 62, p. 1456. , 1953, Origin of chaos structure (Abstract): Geol. Soc. America Bull., v. 64, p. 1509. , 1954, Geology of the Silurian Hills, San Bernardino County, map sheet 19, in Jahns, R. H., Editor, Geology of southern California: Calif. Div. Mines Bull. 170. -47- Kupfer, D, H., I960, Thrust faulting and chaos structure, Silurian Hills, San Bernardino County, California: Geol. Soc. America Bull., v. 71* P. 181-214. Lanphere, M. A., 1964, Geochronologic studies in the eastern Mojave Desert, California: Jour. Geology, v. 72, p. 381-389. Mason, J. F., 1948, Geology of the Tecopa area, southeastern California: Geol. Soc. America Bull., v. 59, p. 333-352. McCulloh, To H., 1954, Problems of the metamorphic and igneous rocks of the Mojave Desert, chap. 7, p. 13-24, in Jahns, R. H., Editor, Geology of southern California: Calif. Div. Mines Bull. 170. Miller, W. J., 1946, Crystalline rocks of southern California: Geol. Soc. America Bull., v. 57* p. 457-542. Noble, L. F., 1941, Structural features of the Virgin Spring area, Death Valley. California: Geol. Soc. America Bull., v. 52, p. 941-1000. * and Wright, L. A., 1954, Geology of the central and southern Death Valley region, California, chap. 2, p. 143-160, in Jahns, R. H., Editor, Geology of southern California: Calif. Div. Mines Bull. 170. Sears, D. H., 1953* Origin of Amargosa chaos, Virgin Spring area, Death Valley, California: Jour. Geology, v. 6l, p. 182-186. Spurr, J. E., 1903, Descriptive geology of Nevada south of the fortieth parallel and adjacent portions of California U. S. Geol. Survey Bull. 208, 229 p. Stewart, J. H., 1970* Upper Precambrian and lower Cambrian strata in the southern Great Basin California and Nevada: U. S. Geol. Survey Prof. Paper 620, 206 p. Sutter, J. F., 1968, Geochronology of major thrusts, southern Great Basin, California: M. Sc. Thesis, Rice University, 32 p. Thompson, D. G., 1929, Mojave Desert region California, a geographic, geologic, and hydrologic reconnaissance: U. S. Geol. Survey Water-Supply Paper 578, 759 p. Tucker, W. B. and Sampson, R. J., 1931* San Bernardino County, p. 262-401, iin Mining in California: Calif. Div. Mines Rept., v. 27. -48- Turner, P. Jo, 1968, Metamorphic Petrology: McGraw-Hill Co., New York, 403 p. Waring, G. A., 1915, Springs of California: U. S. Geol. Survey Water-Supply Paper 338, 410 p. Warnke, D. A., 1965, A geologic study of the Halloran Hills, central Mojave Desert, California: Ph. D. Dissertation, University of Southern California, 226 p. , 1969* A geologic investigation of the Halloran Hills, central Mojave Desert, California: Geol. Rundschau, v. 58* no. 3* p. 998-1047. Wasserburg, G. J. and Lanphere, M. A., 1965, Age determi¬ nations in the Precambrian of Arizona and Nevada: Geol. Soc. America Bull., v. 76, no. 7, p. 735-758. , Wetherill, G. W9, and Wright, L. A., 1959* Ages in the Precambrian terrain of Death Valley, California: Jour. Geology, v. 67, p. 702-708. Wilson, R. C., 1966, The structural geology of Shadow Mountains area, San Bernardino County, California: M. A. Thesis, Rice University, 45 p. Wright, L. A., 1954, Geology of the Alexander Hills area, Inyo and San Bernardino counties, California, map sheet 17, in Jahns, R. H., Editor, Geology of southern California: Calif. Div. Mines Bull. 170. , 1955, Geology of the Silver Lake talc deposits San Bernardino County, California: Calif. Div. Mines Spec. Report 38, 30 p. , and Troxel, B. W., 1969, Chaos structure and Basin and Range normal faults: Evidence for a genetic relationship (Abstract): Geol. Soc. America Meeting 1969, p. 242.