Geochronologic and Geochemical Data from 

Mesozoic Rocks in the Black Mountain Area Northeast 

of Victorville, San Bernardino County, California

Open-File Report 2013–1146

U.S. Department of the Interior

U.S. Geological Survey
Cover: View east toward the Black Mountain limestone quarry northeast of Victorville, California, which is developed in 

limestone conglomerate of the Jurassic Fairview Valley Formation. Hills in distance are composed primarily of the Jurassic 

Sidewinder Volcanics. Photograph by Paul Stone, February 9, 2003.
Geochronologic and Geochemical Data from

Mesozoic Rocks in the Black Mountain Area Northeast 

of Victorville, San Bernardino County, California

By Paul Stone, Andrew P. Barth, Joseph L. Wooden, Nicole K. Fohey-Breting, Jorge A. Vazquez , and 

Susan S. Priest

Open-File Report 2013–1146

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2013 

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Suggested citation:

Stone, P., Barth, A.P., Wooden, J.L., Fohey-Breting, N.K., Vazquez, J.A., and Priest, S.S., 2013, 

Geochronologic and geochemical data from Mesozoic rocks in the Black Mountain area northeast of 

Victorville, San Bernardino County, California: U.S. Geological Survey Open-File Report 2013–1146, 31

p. 

Any use of trade, product, or firm names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government. 

Although this report is in the public domain, permission must be secured from the individual 

copyright owners to reproduce any copyrighted material contained within this report.
iii

Contents

Abstract ......................................................................................................................................................................... 1

Introduction.................................................................................................................................................................... 1

Geologic Framework...................................................................................................................................................... 2

Samples for Geochronologic and Geochemical Analysis .............................................................................................. 3

Geochronologic Data..................................................................................................................................................... 3

Methods ..................................................................................................................................................................... 3

Igneous Units............................................................................................................................................................. 4

Monzonite (Unit ^m) ............................................................................................................................................. 4

Sidewinder Volcanics, Rhyolite of Northern Sequence (Unit Jsnr) ........................................................................ 4

Sidewinder Volcanics, Dacitic to Rhyolitic Rocks, Coarse-grained (Crystal-rich) Facies (Unit Jsdc) .................... 4

Intermediate-composition Dike (Unit Jid) ............................................................................................................... 4

Quartz Monzonite Porphyry (Unit Jqmp) .............................................................................................................. 5

Sedimentary Units...................................................................................................................................................... 5

Fairview Valley Formation ...................................................................................................................................... 5

Quartz-rich Sandstone Unit (Jqs) .......................................................................................................................... 5

Provenance ............................................................................................................................................................ 5

Geochemical Data ......................................................................................................................................................... 6

Paleozoic to Jurassic History......................................................................................................................................... 6

Summary ....................................................................................................................................................................... 7

Acknowledgments ......................................................................................................................................................... 8

References Cited........................................................................................................................................................... 8 

Figures

1. Map showing location of Black Mountain area northeast of Victorville in San Bernardino County, 

California ......................................................................................................................................................................12

2. Geologic map of Black Mountain area......................................................................................................................13

3. Schematic diagram illustrating stratigraphic relations of Jurassic and older rocks in the Black Mountain 

area ..............................................................................................................................................................................15

4. Photographs showing characteristic outcrops of selected rock units, Black Mountain area......................................16

5. Photographs showing folds in unit 3 of Fairview Valley Formation ...........................................................................17

6. Photographs of stained slabs from selected igneous rock units, Black Mountain area.............................................18

7. Plots of U-Pb ages of individual zircon grains from selected igneous rocks in the Black Mountain area ..................19

8. Histograms and cumulative probability curves of U-Pb detrital zircon ages from sandstone samples in the 

Black Mountain area.....................................................................................................................................................20

9. Diagram showing chemical characteristics of igneous rocks analyzed in this study and by Fohey-Breting 

and others (2010) .........................................................................................................................................................21

10. Diagram showing chronology of Triassic and Jurassic magmatic and depositional events in the Black 

Mountain area...............................................................................................................................................................22
iv

Tables

1. Lithologic descriptions of pre-Cenozoic map units in Black Mountain area, San Bernardino County, 

California ......................................................................................................................................................................23

2. Samples used for geochronologic and geochemical analysis, Black Mountain area ................................................25

3. SHRIMP-RG U-Pb zircon data for igneous rocks, Black Mountain area...................................................................27

4. SHRIMP-RG U-Pb data for detrital zircons, Black Mountain area ............................................................................28

5. Chemical analyses of volcanic and intrusive rocks, Black Mountain area ................................................................31

Conversion Factors, Abbreviations, and Chemical Symbols

Conversion Factors

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

kilometer (km) 0.5400 mile, nautical (nmi) 

meter (m) 1.094 yard (yd) 

Abbreviations

CL Cathodoluminescence

Ma Mega-annum (a unit of time equivalent to one million years)

ppm parts per million

SHRIMP-RG Sensitive High Resolution Ion Microprobe–Reverse Geometry

TAS Total Alkali-Silica

XRF X-Ray Fluorescence

Chemical Symbols

Al Aluminum

Ba Barium

Ca Calcium

Fe Iron

K Potassium

Mg Magnesium

Mn Manganese

Na Sodium

O Oxygen

P Phosphorus

Pb Lead

Rb Rubidium

Si Silicon

Sr Strontium

Th Thorium

Ti Titanium

U Uranium

Y Yttrium

Zn Zinc

Zr Zirconium
Geochronologic and Geochemical Data from 

Mesozoic Rocks in the Black Mountain Area Northeast 

of Victorville, San Bernardino County, California

By Paul Stone, Andrew P. Barth, Joseph L. Wooden, Nicole K. Fohey-Breting, Jorge A. Vazquez, and 

Susan S. Priest

Abstract

We present geochronologic and geochemical data for Mesozoic rocks in the Black 

Mountain area northeast of Victorville, California, to supplement previous geologic mapping. 

These data, together with previously published results, limit the depositional age of the 

sedimentary Fairview Valley Formation to Early Jurassic, refine the ages and chemical 

compositions of selected units in the overlying Jurassic Sidewinder Volcanics and of related 

intrusive units, and limit the age of some post-Sidewinder faulting in the Black Mountain area to 

a brief interval in the Late Jurassic. The new information contributes to a more complete 

understanding of the Mesozoic magmatic and tectonic evolution of the western Mojave Desert 

and surrounding regions. 

Introduction 

Mountain ranges in the vicinity of Victorville, California (fig. 1) contain extensive 

outcrops of volcanic, sedimentary, and plutonic rocks that hold important clues for

reconstructing the Mesozoic paleogeographic and tectonic evolution of the western Mojave 

Desert. One significant outcrop area is centered around Black Mountain, 15 km northeast of 

Victorville, where two units of particular interest, the sedimentary Fairview Valley Formation 

and the overlying Sidewinder Volcanics, are well exposed. Several previous geologic mapping 

studies (Bowen, 1954; Dibblee, 1960, 1967; Stone, 1964; Miller, 1977, 1978, 1981) and topical 

investigations (Walker, 1985, 1987; Schermer, 1993; Schermer and Busby, 1994; Schermer and 

others, 2002; Fohey-Breting and others, 2010) have shown that rocks exposed at Black Mountain 

provide an unusually complete record of Triassic and Jurassic events in the Mojave Desert 

region. Structural complexities and insufficient geochronologic data in the area, however, have 

precluded a complete understanding of the geologic history. 

Stone (2006) remapped the geology of the Black Mountain area primarily to clarify the 

stratigraphic and structural relations of the Fairview Valley Formation and Sidewinder 

Volcanics. During and after that study, several map units were sampled for uranium-lead (U-Pb) 

zircon geochronology and geochemical analysis. In this report, we present the resulting 

geochronologic and geochemical data to supplement the preliminary geologic map of Stone 

(2006). 
2 

Geologic Framework

The Black Mountain area (fig. 2) consists of Mesozoic and minor Paleozoic rocks (table

1) surrounded by alluvial deposits of late Cenozoic age. The area is transected by the Helendale 

Fault (Bowen, 1954; Miller and Morton, 1980; Aksoy, 1993), one of several late Cenozoic, 

northwest-striking, right-lateral faults that make up the Eastern California Shear Zone (Dokka 

and Travis, 1990; Petersen and Wesnousky, 1994; Sauber and others, 1994). This study focuses 

on the Fairview Valley Formation, Sidewinder Volcanics, and associated rocks exposed 

northeast of the Helendale Fault. 

In this area (figs. 2 and 3), Paleozoic marble and calc-silicate rocks (units |m, |mc; fig. 

4A) are intruded by late Early or early Middle Triassic monzonite (unit ^m) that Barth and 

Wooden (2006) dated as 244±2 Ma. These rocks are unconformably overlain by the Jurassic

Fairview Valley Formation, which consists of a basal conglomeratic unit (JfvE; fig. 4B); a thick 

middle part (units JfvF, JfvG) composed of mudstone, siltstone, sandstone, conglomerate, and 

minor limestone (fig. 4C–E); and an upper part (units JfvI, JfvL) characterized by massive

limestone conglomerate (fig. 4F). Geochronologic data (Stone and others, 2005; this report) 

indicate that the Fairview Valley Formation was deposited no earlier than about 194 Ma. The 

Fairview Valley Formation is largely homoclinal but is locally folded (figs. 2 and 5). A major 

syncline was mapped in the upper part of the Fairview Valley Formation by Bowen (1954), 

Dibblee (1960), and Stone (1964), but not by Miller (1977) or Stone (2006). 

The Fairview Valley Formation is overlain by a thin, unnamed sequence of sedimentary 

rocks (units Jvc, Jslc, Jch, Jqs) mainly composed of conglomerate and sandstone. The basal 

contact of this sequence is an angular unconformity that cuts westward and down section across 

unit 5, unit 4, and the upper part of unit 3 of the Fairview Valley Formation (figs. 2 and 3). 

Angular relations across the unconformity indicate that the Fairview Valley Formation was tilted 

about 35° eastward before deposition of the overlying strata. The western exposed part of the 

unconformity is bent to a northwesterly trend, in general alignment with bedding in unit 3 of the 

Fairview Valley Formation. The strata just below this part of the unconformity, however, are 

complexly folded into an inverted antiform, and the unconformity itself appears to be overprinted 

by a fault that truncates the northeastern limb of this fold (fig. 2). 

The quartz-rich sandstone (unit Jqs) that caps the thin sedimentary sequence is overlain 

by the oldest rocks of the Jurassic Sidewinder Volcanics (“northern sequence” of Stone, 2006) 

(figs. 2 and 3). The dominant unit (Jsnr) of this sequence, a rhyolitic ignimbrite (fig. 4G) that 

Schermer and Busby (1994) called the tuff of Black Mountain, was dated as late Early Jurassic 

(181±3 Ma) by Fohey-Breting and others (2010). The upper part of the northern sequence 

includes feldspathic sandstone and conglomerate (unit Jsns), quartzose sandstone (unit Jsnq), 

and finely laminated volcanic rocks (unit Jsnl) distinct from the rhyolitic ignimbrite (table 1).

Bodies of intrusive andesite (unit Jia) cut the northern sequence in several places (figs. 2 and 3). 

Faults separate the stratigraphic succession described above (Triassic monzonite, 

Fairview Valley Formation, and northern sequence of the Sidewinder Volcanics) from a complex 

assemblage of younger rocks that also are assigned to the Sidewinder Volcanics (figs. 2 and 3; 

table 1). Massive to weakly stratified, dacitic to rhyolitic rocks make up most of this assemblage. 

The relatively coarse grained (crystal-rich) rocks of units Jsdc and Jsdcq (figs. 4H and 6A–B) 

are ignimbrites that Schermer and Busby (1994) and Schermer and others (2002) assigned to 

their Middle Jurassic (~163–164 Ma) tuffs of Sidewinder Mountain and Turtle Mountain. An 

ignimbrite sample from unit Jsdc was dated at 164±2 Ma during this study. An associated fine-
3 

grained (crystal-poor) unit (Jsdf) could be either intrusive or extrusive. Also present are 

laminated rhyolite (unit Jslr) and conglomeratic rocks (unit Jscg). The younger Sidewinder 

rocks are intruded by undated quartz porphyry (unit Jqp, fig. 6C). All of these rocks, including 

the quartz porphyry, are intruded by Late Jurassic (153±3 Ma; this report) quartz monzonite 

porphyry (unit Jqmp, fig. 6D). 

Numerous northwest-striking dikes, some of which are dated as Late Jurassic, cut 

Triassic monzonite, the Fairview Valley Formation, and the Sidewinder Volcanics (figs. 2 and 

3). Felsic dikes (unit Jfd) predominate, but dikes of intermediate and mafic composition (units 

Jid, Jmd) are also present. Schermer and others (2002) dated a felsic dike in the area (fig. 2, loc. 

AV13) as 152±6 Ma, and a porphyritic dike of intermediate (monzonite to quartz monzonite) 

composition was dated as 154±2 Ma as part of this study. The dated felsic dike cuts a fault (fig. 

2) that separates Triassic monzonite (unit ^m) from the ~153 Ma quartz monzonite porphyry 

(unit Jqmp). This age relation limits movement on this fault to a brief time interval in the Late 

Jurassic. 

Bedrock outcrops southwest of the Helendale Fault are mostly granitic rocks of 

Cretaceous and (or) Jurassic age (unit KJg), which we did not examine in detail. In addition, 

however, Triassic monzonite (unit ^m), crystal-rich dacitic tuff of the Sidewinder Volcanics 

(unit Jsdc), and a thick felsic dike (unit Jfd) locally crop out adjacent to the fault. The presence 

of these rocks, particularly the Triassic monzonite (fig. 2), appears to limit right-lateral offset on 

the Helendale Fault to about 1 km or less (Miller, 1977).

Samples for Geochronologic and Geochemical Analysis

Table 2 lists and describes the twelve samples for which geochronologic and (or) 

geochemical data are presented in this report, and figure 2 shows the sample localities. Also 

shown on figure 2 are the localities of sample BM2 of Barth and Wooden (2006) and sample 

BM15 of Fohey-Breting and others (2010), which are discussed below.

Geochronologic Data

Methods

 The uranium-thorium-lead (U-Th-Pb) analyses reported here were carried out on the 

U.S. Geological Survey (USGS) SHRIMP-RG (Sensitive High Resolution Ion MicroProbe–

Reverse Geometry) ion microprobe at Stanford University. Zircons were separated from 2–5 kg 

samples of sandstone and igneous rocks using standard processing techniques. Individual zircons 

from mineral separates were mounted in epoxy, polished, and coated with gold prior to analysis. 

Homogeneous spots in the interiors of detrital zircons were analyzed based on 

cathodoluminescence (CL) and reflected light photomicrographs, without attempting to sort by 

grain size or shape. Relatively homogeneous spots within compositionally zoned zircons from 

igneous rocks were selected based on CL images. A small number of zircon grains in igneous 

rock samples 846 and 949 yielded statistically younger 238U/206Pb ages than the main 

populations, presumably due to loss of radiogenic Pb, and were excluded from the age 

calculations. 
4 

Igneous Units

Zircons from five igneous units in the Black Mountain area were analyzed. The sampled 

units are (1) monzonite (sample BM2, unit ^m), (2) rhyolitic tuff of the northern sequence of 

the Sidewinder Volcanics (sample BM15, unit Jsnr), (3) coarse-grained (crystal-rich) dacitic tuff

of the Sidewinder Volcanics (sample 840, unit Jsdc), (4) a dike of intermediate (monzonite to 

quartz monzonite) composition (sample 846, unit Jid), and (5) quartz monzonite porphyry 

(sample 949, unit Jqmp). Analytical data for samples 840, 846, and 949 are presented in table 3 

and depicted graphically in figure 7. Data for samples BM2 and BM15 are presented by Barth 

and Wooden (2006) and Fohey-Breting and others (2010), respectively, and are not included 

here. 

Monzonite (Unit ^m) 

Analysis of 12 zircon grains from sample BM2 yielded a weighted mean age of 244±2 

Ma for the Triassic monzonite (Barth and Wooden, 2006). Previous age determinations for this 

unit using thermal ionization mass spectrometry (TIMS) of bulk zircon fractions ranged from 

241±2 to 243±2 Ma (Miller and others, 1995; Barth and others, 1997). The preferred age of 

244±2 Ma straddles the currently accepted Early to Middle Triassic boundary of 245.0±1.5 Ma 

(U.S. Geological Survey Geologic Names Committee, 2010; Walker and Geissman, 2009). 

Sidewinder Volcanics, Rhyolite of Northern Sequence (Unit Jsnr) 

Analysis of 10 zircon grains from sample BM15 in the rhyolite unit (tuff of Black 

Mountain of Schermer and others, 2002) yielded individual ages of ~167–184 Ma and a coherent 

group of 7 grains with a weighted mean age of 181±3 Ma (late Early Jurassic), the interpreted 

magmatic age of the sample (Fohey-Breting and others, 2010). This age is comparable to a TIMS 

age of 179.4±3.4 Ma reported by Schermer and others (2002) for rocks they assigned to the tuff 

of Black Mountain 6 km east of the present study area.

Sidewinder Volcanics, Dacitic to Rhyolitic Rocks, Coarse-grained (Crystal-rich) Facies (Unit Jsdc) 

Analysis of eight zircon grains from sample 840 yielded a weighted mean age of 164±2 

Ma (late Middle Jurassic). Schermer and others (2002) assigned the sampled rocks to their 164-

Ma tuff of Sidewinder Mountain. Our data are consistent with this correlation but also would be 

consistent with correlation to the lithologically similar 163-Ma tuff of Turtle Mountain of 

Schermer and others (2002). We infer the sampled rocks to be juxtaposed against the Fairview 

Valley Formation by a fault that is intruded by a felsic dike (unit Jfd; fig. 2), an interpretation 

previously suggested by Miller (1977). 

Intermediate-composition Dike (Unit Jid) 

Analysis of 10 zircon grains from sample 846 yielded a coherent group of 9 grains with a 

weighted mean age of 154±2 Ma (Late Jurassic). The age calculation excluded the youngest 

analyzed grain, which is presumed to have undergone Pb loss. The sampled dike is cut by a felsic 

dike (unit Jfd) a short distance south of the sample locality (fig. 2), consistent with the ~152 Ma 

age reported for another felsic dike in the study area by Schermer and others (2002). 
5 

Quartz Monzonite Porphyry (Unit Jqmp) 

Analysis of 14 zircon grains from sample 949 yielded a coherent group of 11 grains with 

a weighted mean age of 153±3 Ma (Late Jurassic). The age calculation excluded the three 

youngest analyzed grains, which are presumed to have undergone Pb loss. Because the ~153 Ma 

age of this sample is within analytical error of the ~154 Ma age of sample 846, it is possible that 

the quartz monzonite porphyry (unit Jqmp) and the intermediate-composition dikes (unit Jid), 

which are lithologically similar, were emplaced during the same intrusive event. The zircon 

analysis also shows that the quartz monzonite porphyry may be only slightly older than the ~152 

Ma felsic dike that cuts it (fig. 2), although the dike could be as young as 146 Ma based on the 

reported analytical error of ±6 Ma (Schermer and others, 2002). 

Sedimentary Units

We analyzed detrital zircons from one arkosic sandstone sample (178) in unit 2 of the 

Fairview Valley Formation (JfvF), two arkosic sandstone samples (129 and 159) in unit 3 (JfvG), 

and one sample (215) in the overlying quartz-rich sandstone unit (Jqs). We measured the ages of 

24–50 randomly selected grains from each sample, sufficient to broadly characterize the age 

distribution, but not sufficient to ensure that all minor age populations were identified. 

Analytical data are presented in table 4 and depicted graphically in figure 8. 

Fairview Valley Formation

We analyzed 50 detrital zircon grains from sample 129, 24 from sample 159, and 30 from 

sample 178. All three samples are dominated by Paleoproterozoic grains with ages between 

about 1640 and 1750 Ma (75 percent in sample 129; 56 percent in sample 159; 60 percent in 

sample 178). The remaining grains are mostly Mesoproterozoic to latest Paleoproterozoic in age 

(~1022–1632 Ma). In addition, however, sample 178 contains one Permian grain, one Triassic 

grain, and most significantly, 7 grains (23 percent) of Early Jurassic age (~191–198 Ma). These 7 

grains constitute a statistically significant population (coherent group) that yields a weighted 

mean age of 194±2 Ma. We consider this to be a reasonable estimate of the maximum 

depositional age of unit 2 of the Fairview Valley Formation, assuming that all 7 grains represent

a single igneous source rock. 

Quartz-rich Sandstone Unit (Jqs) 

We analyzed 25 detrital zircon grains from sample 215. This analysis yielded a 

heterogeneous distribution of ages, including Archean (2 grains), Paleoproterozoic (5 grains), 

Mesoproterozoic (9 grains), Neoproterozoic (5 grains), Paleozoic (2 grains), and Jurassic (2 

grains). These ages do not precisely limit the maximum depositional age of the sandstone unit, 

but the two Jurassic ages of 172±2 and 185±4 Ma are within the range of ages reported by 

Fohey-Breting and others (2010) for individual zircons from the overlying rhyolite of the 

Sidewinder Volcanics (sample BM15). Thus, it is likely that the sandstone unit is not 

significantly older than the rhyolite.

Provenance

The predominance of Paleoproterozoic zircons in the samples from the Fairview Valley 

Formation is consistent with derivation of detritus from local crystalline basement rocks 

(Wooden and Miller, 1990; Barth and others, 2009). By contrast, the more heterogeneous 
6 

distribution of ages in the overlying quartzose sandstone unit is similar to that reported by 

Dickinson and Gehrels (2003) from Jurassic eolian sandstones of the Colorado Plateau, which 

they interpreted to be derived from various distant sources including the Appalachian orogen. 

The Early Jurassic (~191–198 Ma) zircons in Fairview Valley Formation sample 178 are 

of unknown provenance. Because plutons of this age have not been identified in the Mojave 

Desert region, these zircons may have been derived from local volcanic sources that are either 

not preserved or not yet dated. Detrital zircons of similar age also have been reported in the 

Jurassic(?) and Cretaceous McCoy Mountains Formation about 250 km southeast of Black 

Mountain (Barth and others, 2004; Spencer and others, 2011).

Geochemical Data

Whole-rock X-ray fluorescence (XRF) analyses of seven samples of igneous rocks from 

the Black Mountain area were performed in December 2006 at Michigan State University using a 

Bruker S4 Pioneer spectrometer. These analyses provide a general idea of the composition of 

selected map units for comparison with rocks of the Sidewinder Volcanics previously analyzed 

by Fohey-Breting and others (2010). Data from our analyses are presented in table 5, and 

selected data are plotted graphically (fig. 9) on a total alkali–silica (TAS) diagram (Best and 

Christiansen, 2001; LeMaitre and others, 2002) and the cation plot of Jensen (1976). 

Four samples are from Middle Jurassic dacitic to rhyolitic rocks of the Sidewinder 

Volcanics. Two of these samples (840, 1072) are from the coarse-grained facies (unit Jsdc) and 

one sample (1435) is from the coarse-grained facies with abundant quartz (unit Jsdcq). These 

three samples plot on or just above the trachydacite-dacite boundary on the TAS diagram and on 

or near the tholeiitic rhyolite to calc-alkaline dacite boundary on the Jensen plot (fig. 9). By 

contrast, sample 1104 from the fine-grained facies (unit Jsdf) is of rhyolitic composition. The 

other three samples are from fine-grained to porphyritic intrusive units that are closely associated 

with Sidewinder volcanic rocks. On the TAS diagram these samples range in composition from 

andesite (sample 1250, unit Jia) to borderline dacite/rhyolite/trachydacite (sample 949, unit 

Jqmp) to rhyolite (sample 935, unit Jqp), or their plutonic equivalents (fig. 9). 

Also shown on both the TAS and Jensen plots (fig. 9) are samples of the tuffs of Black 

Mountain and Turtle Mountain that were analyzed by Fohey-Breting and others (2010). These 

analyses confirm the dominantly rhyolitic composition of the tuff of Black Mountain (equivalent 

to our unit Jsnr) in contrast to the dacitic composition of the younger tuff of Turtle Mountain. 

Our samples 840, 1072, and 1435 compare closely with samples analyzed by Fohey-Breting and 

others (2010) from the tuff of Turtle Mountain, but are slightly more alkaline. 

Paleozoic to Jurassic History

During the Paleozoic the Black Mountain area was part of the western North American 

continental shelf, where carbonate strata accumulated in shallow marine water (Miller, 1977, 

1981). In late Early or early Middle Triassic time (~244 Ma), the Paleozoic rocks were intruded 

by a monzonite pluton that was part of the nascent Mesozoic magmatic arc of western North 

America (Barth and others, 1997; Barth and Wooden, 2006). The Triassic arc is thought to have 

developed after truncation of the Paleozoic miogeocline along a northwest-striking, sinistral fault 

zone at the edge of the continent (Stevens and others, 2005). Deformation and metamorphism of 

the Paleozoic strata preceded magmatism (Miller, 1977, 1981). 

Exhumation of the Triassic monzonite was followed by Early Jurassic deposition of the 

Fairview Valley Formation. Detrital zircons from the Fairview Valley Formation indicate that 
7 

deposition began no earlier than about 194 Ma. The abundance of conglomerate and the lack of 

in situ marine fossils suggest a nonmarine or marginal marine depositional setting (Miller, 1978; 

Schermer and others, 2002). Arkosic sandstone in the lower and middle parts of the formation 

was derived from local Proterozoic crystalline rocks and subordinate Early Jurassic volcanic 

rocks. Massive conglomerate in the upper 500 m of the formation probably represents alluvialfan deposits close to an uplifted source area of Paleozoic limestone. 

After deposition, the Fairview Valley Formation was tilted eastward, eroded, and 

unconformably overlapped by a thin sedimentary sequence that includes conglomerate and 

quartz-rich sandstone. The presence of conglomerate suggests a nonmarine depositional 

environment. The provenance of the quartz-rich sandstone was distinct from that of arkosic 

sandstone in the underlying Fairview Valley Formation, but comparable to that of 

contemporaneous eolian sand sheets represented by the Navajo Sandstone on the Colorado 

Plateau (Schermer and others, 2002). 

Deposition of the quartz-rich sandstone was closely followed by the extrusion of rhyolitic

ignimbrite that constitutes the oldest part of the Jurassic Sidewinder Volcanics (tuff of Black 

Mountain) at about 181 Ma. Voluminous late Middle Jurassic (~163–164 Ma) dacitic ignimbrites 

(equivalent to the tuffs of Sidewinder and/or Turtle Mountain) were extruded later, although the 

stratigraphic relations between these ignimbrites and the tuff of Black Mountain are obscured by 

faulting. Late Jurassic magmatic activity in the area included the intrusion of northwest-striking 

dikes of felsic to mafic composition and a pluton of quartz monzonite porphyry between about 

152 and 154 Ma. 

Regionally, the Sidewinder Volcanics are interpreted as a nested caldera complex that 

includes four successive, widespread ignimbrites ranging in age from ~181 to ~151 Ma 

(Schermer and Busby, 1994; Schermer and others, 2002; Fohey-Breting and others, 2010). The

Sidewinder Volcanics and the associated Jurassic intrusive rocks formed as part of a northwesttrending Andean arc that extended the length of the western North American continental margin 

(Tosdal and others, 1989; Saleeby and Busby-Spera, 1992; Barth and others, 2008).

 Middle(?) and Late Jurassic deformation of rocks in the Black Mountain area included 

northward tilting, faulting, and folding. Many details of the structural chronology remain unclear, 

and conflicting interpretations have been proposed (Miller, 1977; Schermer, 1993; Schermer and 

others, 2002). Most of the deformation, however, likely predated intrusion of the Late Jurassic

dikes, although Schermer (1993) and Schermer and others (2002) interpreted the folding to be 

younger. Some faulting postdated intrusion of ~153 Ma quartz monzonite porphyry and 

predated intrusion of the ~152 Ma felsic dike dated by Schermer and others (2002), and other 

faulting could be even younger. 

Summary 

U-Pb geochronologic data presented here and elsewhere (Barth and Wooden, 2006; 

Fohey-Breting and others, 2010) provide an updated chronology of Mesozoic magmatic and 

depositional events in the Black Mountain area (fig. 10): (1) intrusion of a late Early to early 

Middle Triassic (~244 Ma) monzonite pluton; (2) Early Jurassic (no earlier than ~194 Ma) 

deposition of the Fairview Valley Formation, a thick sequence of mudstone, siltstone, sandstone, 

and conglomerate; (3) late Early Jurassic deposition of a thin sedimentary sequence, primarily 

conglomerate and sandstone, followed closely by extrusion of a thick rhyolitic ignimbrite (tuff of 

Black Mountain) at about 181 Ma; (4) late Middle Jurassic (~164 Ma) extrusion of crystal-rich, 

dacitic ignimbrite (tuff of Sidewinder Mountain or tuff of Turtle Mountain); and (5) Late Jurassic 

intrusion of intermediate-composition dikes (~154 Ma) and quartz monzonite porphyry (~153 
8 

Ma). Intrusion of younger (152±6 Ma) felsic dikes (Schermer and others, 2002) followed

faulting that postdated intrusion of the ~153 Ma quartz monzonite porphyry. The geochronologic 

data presented in this report, together with new geochemical data for selected rock units, 

supplement the geologic map of Stone (2006) and help clarify the tectonic evolution of the 

western Mojave Desert region. 

Acknowledgments

 We extend our appreciation to the staff of CEMEX for granting access to Black 

Mountain Quarry and the adjacent company-owned lands. The logistical assistance of Gerry 

Frankovich, quarry manager in May 2006, was especially helpful. Cal Stevens (San Jose State 

University) visited the map area and contributed to some of the stratigraphic and structural 

interpretations. Discussions with Brett Cox and David Miller (U.S. Geological Survey) led to 

significant improvements in the final manuscript. Geochronologic work was in part supported 

by the National Science Foundation through grants EAR-0408730 and EAR-0711119. 

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11

Figures 1–10 and Tables 1–5 follow.
12

Figure 1. Map showing location of Black Mountain area northeast of Victorville in San Bernardino 

County, California.
13

Figure 2. Geologic map of Black Mountain area (see figure 1 for location of area). Reduced and modified from Stone (2006).
14

Figure 2 (explanation).
15

Figure 3. Schematic diagram illustrating stratigraphic relations of Jurassic and older rocks in the Black 

Mountain area. Unit symbols and colors as in figure 2. Approximate numerical ages, based on U-Pb 

geochronology, are shown in parentheses.
16

Figure 4. Photographs showing characteristic outcrops of selected rock units, Black Mountain area. A, 

Paleozoic marble and calc-silicate rocks (unit |mc); B, basal conglomerate composed of plutonic clasts, 

Fairview Valley Formation, unit 1 (JfvE); C, banded mudstone and siltstone, Fairview Valley Formation, 

unit 2 (JfvF); D, calcareous siltstone, Fairview Valley Formation, unit 3 (JfvG); E, conglomerate containing 

white marble clasts, Fairview Valley Formation, unit 3 (JfvG); F, limestone conglomerate, Fairview Valley 

Formation, unit 5 (JfvL); G, crystal-poor rhyolitic tuff with flattened clasts, Sidewinder Volcanics (unit 

Jsnr); H, crystal-rich dacitic tuff with flattened clasts, Sidewinder Volcanics (unit Jsdcq).
17

Figure 5. Photographs showing folds in unit 3 (JfvG) of Fairview Valley Formation. A, isolated outcropscale synformal fold in middle part of unit, view to the southeast; and B, hinge of map-scale syncline in 

uppermost part of unit, directly below contact with unit 4, view to the northwest.
18

Figure 6. Photographs of stained slabs from selected igneous rock units, Black Mountain area. Potassium 

feldspar was stained yellow using sodium cobaltinitrite; plagioclase was stained red using amaranth. 

Quartz is unstained. A, crystal-rich dacitic tuff, Sidewinder Volcanics (unit Jsdc). Angular plagioclase 

phenocrysts (stained red) and other grains in a felsic groundmass. B, crystal-rich dacitic tuff with abundant 

quartz, Sidewinder Volcanics (unit Jsdcq). Large, embayed quartz phenocrysts (dark gray) and angular 

plagioclase phenocrysts (stained red) in a felsic groundmass. Angular texture of plagioclase crystals in A 

and B is ascribed to fragmentation during explosive venting of magma. C, quartz porphyry (unit Jqp). 

Quartz phenocrysts (gray) in a felsic groundmass. D, quartz monzonite porphyry (unit Jqmp). 

Plagioclase phenocrysts (stained red) and clots of mafic minerals (black) in a felsic groundmass. 
19

Figure 7. Plots of U-Pb ages of individual zircon grains from selected igneous rocks in the Black Mountain 

area (sample 840, Sidewinder Volcanics, crystal-rich dacitic tuff, unit Jsdc; sample 846, intermediatecomposition dike, unit Jid; sample 949, quartz monzonite porphyry, unit Jqmp). A, Tera-Wasserburg 

concordia plots. B, bar diagrams of 206Pb*/238U ages with 2-sigma errors; boxes show weighted mean 

crystallization ages. Note that the youngest grain in sample 846 and the youngest three grains in sample 

949 (table 3) were excluded from the calculations of weighted mean age; we infer these grains to have 

undergone minor Pb loss. 
20

Figure 8. Histograms and cumulative probability curves of U-Pb detrital zircon ages from sandstone 

samples in the Black Mountain area. A–C, Fairview Valley Formation. A, sample 129, and B, sample 159, 

both from unit 3 (JfvG); C, sample 178, from unit 2 (JfvF); D, sample 215, quartz-rich sandstone unit 

(Jqs) above Fairview Valley Formation. 
21

Figure 9. Diagram showing chemical characteristics of igneous rocks analyzed in this study and by FoheyBreting and others (2010). A, total alkali-silica (TAS) plot (Best and Christiansen, 2001; LeMaitre and 

others, 2002). B, Jensen cation plot (Jensen, 1976). Analytical data normalized to 100 percent (volatilefree) with all Fe as FeO. 
22

Figure 10. Diagram showing chronology of Triassic and Jurassic magmatic and depositional events in the 

Black Mountain area. Note that ages of events are approximate and are depicted without error bars for 

simplicity. Time scale from U.S. Geological Survey Geologic Names Committee (2010).
Symbol Unit Lithology Age

KJg Granite Medium- to coarse-grained, light-colored granitic rocks Cretaceous and/or Jurassic

KJps Plutonic rocks near Sidewinder Mine

Medium- to coarse-grained granitic rocks, fine- to mediumgrained quartz porphyry, and porphyritic dikes of 

intermediate composition

Cretaceous and/or Jurassic

KJmp Monzonite porphyry

Granitoid rocks containing 50 percent potassium feldspar 

and plagioclase phenocrysts ≤10 mm across in a finegrained groundmass Cretaceous or Jurassic

Dikes Northwest-striking dikes of felsic, intermediate, and mafic 

composition

Jfd Felsic dikes Very fine grained felsite dikes

Jid Intermediate-composition dikes Porphyritic monzonite to quartz monzonite dikes

Jmd Mafic dikes Diorite to quartz diorite dikes

Jqmp Quartz monzonite porphyry

Granitoid rocks containing 30–50 percent plagioclase, 

potassium feldspar, and quartz phenocrysts in a finegrained groundmass. Feldspar phenocrysts ≤10 mm 

across, quartz phenocrysts ≤3 mm across

Late Jurassic

Jqp Quartz porphyry

Very fine grained to microcrystalline felsic rocks containing 

10–40 percent anhedral to subhedral phenocrysts of quartz 

≤3 mm across

Late Jurassic(?)

Jia Intrusive andesite

Massive, dark, very fine grained rocks composed primarily 

of plagioclase, mafic minerals, and minor quartz; scattered 

plagioclase phenocrysts ≤4 mm across

Late or Middle Jurassic(?)

Sidewinder Volcanics Volcanic and minor sedimentary rocks originally called 

“Sidewinder Volcanic Series” by Bowen (1954) Jurassic

Dacitic to rhyolitic rocks Thick, largely unstratified masses of dacitic to rhyolitic 

rocks

Jsd Undivided Mixed rocks similar to units Jsdf and Jsdc

Jsdf Fine-grained (crystal-poor) facies

Massive, dark-colored rocks characterized by a 

microcrystalline groundmass and 10–40 percent 

phenocrysts, primarily plagioclase, generally ≤3 mm 

across

Jsdc Coarse-grained (crystal-rich) facies

Massive rocks characterized by a microcrystalline 

groundmass and 40–75 percent phenocrysts ≤5 mm 

across. Phenocrysts primarily plagioclase; potassium 

feldspar, quartz, and mafic phenocrysts rare

Jsdcq Quartz-rich coarse-grained facies Similar to unit Jsdc, but with 10–15 percent embayed 

quartz phenocrysts ≤5 mm across

Jslr Laminated rhyolite Thinly laminated, felsic rock of probable rhyolitic 

composition Middle Jurassic(?)

Jscg Conglomeratic rocks Dark-brown lithic sandstone containing angular siliceous 

rock fragments Middle Jurassic(?)

Northern sequence Rocks exposed northwest of Black Mountain Quarry

Jsnr Rhyolite

White to very light gray, microcrystalline rhyolite that 

contains scattered plagioclase phenocrysts ≤3 mm 

across

Jsnl Laminated volcanic rocks

Very fine grained, thinly laminated, dark-gray rocks 

containing scattered plagioclase phenocrysts ≤2 mm 

long

Jsnq Quartzose sandstone

Medium- to coarse-grained, massive to laminated, 

quartzose sandstone that contains minor lithic grains 

and feldspar

Jsns Sandstone and conglomerate

Medium- to coarse-grained grained feldspathic 

sandstone and conglomerate containing volcanic(?) 

clasts mostly ≤2 cm across

Table 1. Lithologic descriptions of pre-Cenozoic map units in Black Mountain area, San Bernardino County, California (fig. 2). 

Modified from Stone (2006).

Late Jurassic

Middle Jurassic

Early Jurassic

23
Symbol Unit Lithology Age

Unnamed sedimentary rocks Rocks between unit Jsnr of Sidewinder Volcanics and 

Fairview Valley Formation

Jqs Quartz-rich sandstone

Fine- to medium-grained, quartz-rich sandstone that 

contains as much as 50 percent volcanogenic (felsic) 

sand and matrix

Jch Calc-hornfels

Massive, light green, very fine grained calcareous 

hornfels, probably metamorphosed calcareous 

mudstone

Jslc Sandy limestone conglomerate

Conglomerate composed of poorly sorted, light-gray 

pebbles and cobbles of limestone (marble) in a darkbrown, sandy matrix

Jvc Volcanic conglomerate

Dark-brown to greenish-gray, volcaniclastic(?) rocks 

composed of siliceous (volcanic rock and/or chert?) 

clasts generally ≤4 cm across in a dense, fine-grained 

matrix

Fairview Valley Formation Thick sequence of weakly metamorphosed sedimentary 

rocks named by Bowen (1954)

JfvL Unit 5

Massive to very thick bedded limestone-clast 

conglomerate; clasts range in size from small pebbles to 

large cobbles and boulders; calcareous matrix contains 

≤5 percent quartz sand

JfvI Unit 4

Gray limestone, brown-weathering calcareous siltstone 

to fine-grained sandstone, silty limestone, pebbly 

sandstone, and lenses of limestone conglomerate 

lithologically similar to that of unit 5

JfvG Unit 3

Calcareous siltstone to fine-grained sandstone and silty 

to fine-grained sandy limestone; subordinate limestone, 

medium- to coarse-grained arkosic sandstone, and 

pebbly sandstone. Middle part includes numerous 

conglomerate beds, typically 1–10 m thick, composed 

primarily of limestone or marble clasts 1–30 cm across in 

a sandy, calcareous matrix; clasts also include plutonic 

and volcanic rocks, quartzite, and sandstone

JfvF Unit 2

Non-calcareous mudstone (argillite) and siltstone that 

form distinctive banded outcrops, and medium- to 

coarse-grained arkosic-lithic sandstone and pebbly 

sandstone

JfvE Unit 1

Conglomerate and sandstone. Conglomerate largely 

composed of poorly sorted, angular to subangular 

pebbles, cobbles, rare boulders in a coarse-grained 

sandstone matrix; clasts are plutonic and fine-grained 

siliceous rocks

^m Monzonite

Medium- to coarse-grained monzonite composed of 35–50 

percent potassium feldspar, 10–30 percent plagioclase, 

and 30–40 percent mafic minerals; quartz rare to absent

Middle or Early Triassic

}|m Siliceous marble Small pod of siliceous marble and calc-silicate rocks within 

intrusive andesite (Jia)

Mesozoic or Paleozoic

}|q Quartzite Small outcrop of reddish-brown, fine-grained quartzite that 

underlies unit JfvF of Fairview Valley Formation Mesozoic or Paleozoic

|mc Marble and calc-silicate rocks Light-gray marble and brown calc-silicate rocks, locally 

interbedded to produce a banded appearance Paleozoic

|m Marble Light-colored marble, generally massive but locally thinly 

laminated Paleozoic

Early Jurassic

Early Jurassic

Table 1 (continued).

24
Sample Map Unit Symbol Latitude (N) Longitude (W) Lithologic Description

129 Fairview Valley 

Formation, unit 3

Jfv3 34°38'18" 117°08'34" Coarse-grained sandstone to granule conglomerate 

composed of quartz, feldspar, and lithic grains to 4 mm 

in diameter. Plagioclase ~40% of rock, potassium 

feldspar (including microcline) ~25%. Plagioclase 

largely altered to sericite. Lithic grains include 

polycrystalline quartz, chert, siltstone, and micritic 

limestone. Grains are moderately sorted, mostly 

subangular. 

159 Fairview Valley 

Formation, unit 3

Jfv3 34°37'43" 117°07'03" Coarse-grained sandstone composed of quartz, 

feldspar, and polycrystalline quartz. Plagioclase and 

potassium feldspar each ~30% of rock. Sedimentary 

texture largely obliterated; most grain boundaries 

sutured.

178 Fairview Valley 

Formation, unit 2

Jfv2 34°37'24.2" 117°07'27.0" Coarse-grained sandstone composed of quartz, 

feldspar, and lithic grains to 2 mm in diameter. 

Plagioclase ~70% of rock; almost no potassium feldspar 

is present. Lithic grains include volcanic rock fragments 

and polycrystalline quartz. Clastic texture of quartz and 

feldspar grains is well preserved; surrounding matrix or 

pseudomatrix could contain altered volcanic grains. 

215 Quartz-rich 

sandstone unit

Jqs 34°38'04" 117°06'51" Fine- to medium-grained quartzose sandstone. Quartz 

grains 0.1–0.3 mm in diameter are separated by finegrained volcanogenic matrix or pseudomatrix that 

comprises ~50% of rock. Microscopic inspection shows 

that this interstitial material is composed mostly of 

volcanic sand grains about the same size as the quartz, 

but true volcanogenic matrix may be present as well.

840 Sidewinder 

Volcanics, dacitic to 

rhyolitic rocks, 

coarse-grained 

(crystal-rich) facies

Jsdc 34°38'08.8" 117°08'50.4" Crystal-rich porphyritic rock composed of ~60% 

phenocrysts in a cryptocrystalline felsic groundmass. 

Most phenocrysts are anhedral to subordinate euhedral 

plagioclase crystals that range from <1 to 4 mm long. 

Plagioclase generally well preserved, but some is 

altered. Small (~0.2–0.5 mm) quartz phenocrysts are 

sparse; one partially resorbed quartz crystal 1.5 mm in 

diameter was observed. No potassium feldspar 

phenocrysts identified. Altered biotite phenocrysts 

common (~10% of rock). 

846 Intermediatecomposition dikeJid 34°38'14.4" 117°08'31.0" Porphyritic rock composed of altered plagioclase 

phenocrysts as much as 5 mm long (~25% of rock); 

diffuse mafic clots of uncertain composition as much as 

7 mm in diameter (~25%); and fine-grained felsic 

groundmass (~50%). 

935 Quartz porphyry Jqp 34°37'24.2" 117°06' 34.0" Porphyritic rock composed of quartz phenocrysts as 

much as 2 mm in diameter (~25% of rock) in a felsic 

groundmass. Quartz crystals are light gray, anhedral, 

not embayed. No other phenocrysts are present.

Table 2. Samples used for geochronologic and geochemical analysis, Black Mountain area, San Bernardino County, 

California.

25
Sample Map Unit Symbol Latitude (N) Longitude (W) Lithologic Description

949 Quartz monzonite 

porphyry

Jqmp 34°37'00.4" 117°07'10.4" Porphyritic rock composed of plagioclase phenocrysts 

<1 to 9 mm in diameter (~40% of rock); irregular masses 

of epidote(?) (~15%), rare quartz phenocrysts, and a finegrained felsic groundmass (~45%). Most plagioclase is 

mildly sericitized, and some plagioclase crystals have 

zones of potassium feldspar replacement.

1072 Sidewinder 

Volcanics, dacitic to 

rhyolitic rocks, 

coarse-grained 

(crystal-rich) facies

Jsdc 34°36'45.0" 117°06'49.2" Crystal-rich porphyritic rock containing ~50% plagioclase 

phenocrysts as much as 4 mm long, conspicuous 

embayed quartz phenocrysts as much 4 mm in 

diameter, and ~5% mafic phenocrysts in a felsic 

groundmass. Sample also includes a volcanic(?) lithic 

clast 10 mm long. 

1104 Sidewinder 

Volcanics, dacitic to 

rhyolitic rocks, finegrained (crystal-poor) 

facies

Jsdf 34°36'41.0" 117°06'24.4" Fine-grained felsic rock containing ~25% phenocrysts as 

much as 2 mm in maximum dimension, mostly 

plagioclase and subordinate quartz. 

1250 Intrusive andesite Jia 34°38'29.5" 117°07'15.0" Fine-grained rock composed of interlocking plagioclase 

(~65%), hornblende or augite (~30%), and quartz (~5%) 

crystals ~0.1–0.5 mm in diameter. 

1435 Sidewinder 

Volcanics, dacitic to 

rhyolitic rocks, quartzrich coarse-grained 

facies

Jsdcq 34°37'58.7" 117°06'07.8" Crystal-rich porphyritic rock containing abundant 

plagioclase phenocrysts as much as 2 mm long, 

subordinate potassium feldspar phenocrysts as much as 

4 mm long, and abundant quartz phenocrysts as much 

as 5 mm in diameter in a fine-grained groundmass.

Table 2 (continued).

26
Zircon

grain

U 

(ppm)

Th 

(ppm)

238U/

206Pb

error 

(%)

207Pb/

206Pb

error 

(%)

206Pb*/238U 

age (Ma)

error 

(Ma)

840-1 576 482 38.44 0.7 0.0498 2.2 165.5 1.1

840-2 619 537 38.43 0.7 0.0513 2.3 165.2 1.1

840-3 517 407 39.34 0.7 0.0500 2.4 161.7 1.2

840-4 529 366 39.03 0.7 0.0515 2.7 162.6 1.1

840-5 528 400 38.97 0.8 0.0505 2.6 163.1 1.3

840-6 519 626 38.60 0.7 0.0499 2.4 164.8 1.2

840-7 239 263 39.31 1.1 0.0488 3.7 162.0 1.8

840-8 167 154 38.33 1.3 0.0473 5.3 166.5 2.2

846-1 60 45 41.14 2.1 0.0509 7.2 154.5 3.3

846-2 60 70 43.38 2.2 0.0454 7.9 147.6 3.2

846-3 81 70 41.33 1.9 0.0458 6.8 154.8 2.9

846-4 75 89 42.18 1.9 0.0529 6.3 150.3 2.9

846-5 91 112 40.85 1.7 0.0567 5.6 154.4 2.7

846-6 147 198 40.83 1.4 0.0515 4.8 155.5 2.2

846-7 66 54 41.35 2.0 0.0504 6.9 153.8 3.1

846-8 93 121 41.14 1.7 0.0505 7.3 154.6 2.6

846-9 77 97 41.47 1.8 0.0525 6.2 153.0 2.9

846-10 95 89 40.97 1.7 0.0518 5.7 154.9 2.6

949-1 92 95 41.49 2.3 0.0483 10.7 153.7 3.7

949-2 120 154 42.13 1.9 0.0471 6.5 151.6 2.8

949-3 56 54 42.04 2.7 0.0588 8.3 149.7 4.1

949-4 61 54 43.81 2.7 0.0503 12.1 145.3 4.0

949-5 117 123 42.50 1.9 0.0528 6.4 149.2 2.9

949-6 94 95 40.29 2.1 0.0498 7.1 157.9 3.3

949-7 102 115 40.00 2.0 0.0533 6.7 158.4 3.3

949-8 109 184 44.32 2.4 0.0520 6.9 143.3 3.4

949-9 91 85 42.28 2.1 0.0536 6.7 149.8 3.1

949-10 92 71 40.58 2.1 0.0494 7.3 156.9 3.4

949-11 80 100 41.39 2.2 0.0528 7.4 153.2 3.5

949-12 110 125 44.53 2.0 0.0514 6.5 142.7 2.8

949-13 83 80 40.64 2.1 0.0553 6.9 155.5 3.3

949-14 84 82 44.45 2.2 0.0527 7.7 142.7 3.2

Table 3. SHRIMP-RG U-Pb zircon data for igneous rocks, Black Mountain area, San 

Bernardino County, California.

Sample 840. Sidewinder Volcanics, dacitic to rhyolitic rocks, coarse-grained facies (unit 

Jsdc). Weighted mean age=164±2 Ma.

Sample 846. Intermediate-composition dike (unit Jid). Weighted mean age=154±2 Ma.

[Note: grain 2 excluded from calculation of weighted mean age.]

Sample 949. Quartz monzonite porphyry (unit Jqmp). Weighted mean age=153±3 Ma.

[Note: grains 8, 11, and 14 excluded from calculation of weighted mean age]

27
Zircon

grain

U

(ppm)

Th

(ppm)

238U/

206Pb

error

(%)

207Pb/

206Pb

error

(%)

206Pb*/

238U

age (Ma)

error

(Ma)

207Pb*/

206Pb*

age (Ma)

error 

(Ma)

 Preferred 

Age

(Ma)

error

(Ma)

129-1 128 86 3.4204 1.5 0.1027 1.1 1652 21 1660 20 1660 20

129-2 805 315 6.9531 1.2 0.1054 0.6 857 10 1557 33 1557 33

129-3 125 54 4.1729 1.5 0.1032 1.2 1376 19 1583 44 1583 44

129-4 2081 217 3.9755 1.2 0.0908 0.5 1444 16 1412 11 1412 11

129-5 147 67 3.3936 1.4 0.1018 1.0 1663 21 1644 20 1644 20

129-6 130 58 3.6980 1.5 0.1065 1.1 1543 20 1740 20 1740 20

129-7 269 182 4.5560 1.4 0.1100 0.8 1266 16 1650 36 1650 36

129-8 253 220 3.3807 1.3 0.1049 0.8 1671 20 1714 14 1714 14

129-9 724 322 3.5899 1.2 0.1045 0.5 1581 17 1673 11 1673 11

129-10 185 116 3.3877 1.4 0.1029 0.9 1665 20 1660 19 1660 19

129-11 225 150 3.5714 1.4 0.1042 1.2 1589 19 1678 23 1678 23

129-13 273 136 3.3647 1.3 0.1044 0.7 1676 20 1691 14 1691 14

129-14 703 503 7.0984 1.2 0.1178 0.6 833 10 1661 57 1661 57

129-15 193 135 3.4953 1.4 0.1017 0.9 1622 20 1655 16 1655 16

129-16 218 157 3.6560 1.4 0.1146 0.8 1537 19 1675 46 1675 46

129-17 941 218 5.7679 1.3 0.1088 0.5 1017 12 1588 33 1588 33

129-18 235 132 3.4333 1.3 0.1024 0.8 1647 20 1663 15 1663 15

129-19 208 126 3.3729 1.4 0.1032 0.8 1673 20 1679 16 1679 16

129-20 441 193 3.7951 1.3 0.1070 0.6 1506 17 1729 13 1729 13

129-21 185 93 3.4417 1.4 0.1033 0.9 1641 20 1657 21 1657 21

129-22 172 97 3.7514 1.5 0.1044 1.0 1521 20 1679 18 1679 18

129-23 223 287 4.2413 1.4 0.1197 0.8 1344 17 1745 52 1745 52

129-24 960 490 8.4949 1.2 0.1081 1.2 705 8 1510 61 1510 61

129-25 214 113 3.4066 1.4 0.1039 0.9 1658 20 1687 17 1687 17

129-26 825 398 5.2171 0.3 0.1061 0.5 1125 4 1667 14 1667 14

129-27 358 177 3.3160 0.5 0.1067 0.6 1698 7 1739 11 1739 11

129-28 359 163 3.5816 0.5 0.1245 0.5 1554 7 1737 35 1737 35

129-29 1640 649 15.8246 0.4 0.1068 1.7 383 2 1273 71 1273 71

129-30 1733 829 9.9173 0.3 0.1250 0.4 590 2 1362 69 1362 69

129-31 586 338 4.2422 0.4 0.1049 0.5 1361 5 1680 13 1680 13

129-32 477 202 3.5122 0.4 0.1042 0.5 1612 6 1668 12 1668 12

129-34 305 228 3.8330 0.5 0.1168 0.6 1472 7 1703 35 1703 35

129-35 236 131 3.3175 0.6 0.1034 0.7 1697 9 1675 13 1675 13

129-36 129 65 3.3892 0.8 0.1038 0.9 1665 12 1680 19 1680 19

129-37 95 97 3.4768 0.9 0.1537 3.5 1528 17 1632 173 1632 173

129-38 380 154 3.4978 0.5 0.1027 0.6 1620 7 1667 11 1667 11

129-39 282 125 3.2818 0.5 0.1066 0.6 1714 8 1737 12 1737 12

129-40 879 521 3.8303 0.3 0.1072 0.4 1493 4 1725 9 1725 9

129-41 196 173 4.4435 0.7 0.1235 0.8 1277 10 1682 78 1682 78

129-42 164 96 3.4802 0.7 0.1035 0.8 1625 10 1656 20 1656 20

129-44 115 65 3.4196 0.9 0.1046 1.0 1653 12 1705 19 1705 19

129-45 124 59 3.4714 0.8 0.1066 1.0 1630 12 1723 18 1723 18

129-46 560 187 7.2402 0.5 0.0939 1.4 822 4 1245 48 1245 48

129-47 107 79 3.2951 0.9 0.1023 1.0 1707 13 1655 20 1655 20

129-48 316 163 3.3831 0.5 0.1044 0.7 1668 8 1691 14 1691 14

Table 4. SHRIMP-RG U-Pb data for detrital zircons, Black Mountain area, San Bernardino County, California.

Sample 129

Fairview Valley Formation, unit 3 (Jfv3)

28
Zircon

grain

U

(ppm)

Th

(ppm)

238U/

206Pb

error

(%)

207Pb/

206Pb

error

(%)

206Pb*/

238U

age (Ma)

error

(Ma)

207Pb*/

206Pb*

age (Ma)

error 

(Ma)

 Preferred 

Age

(Ma)

error

(Ma)

129-49 109 57 3.4168 0.9 0.1029 1.0 1653 13 1656 24 1656 24

129-50 508 174 3.3822 0.4 0.1024 0.5 1668 6 1653 10 1653 10

129-51 719 251 4.8532 0.4 0.1024 0.5 1205 4 1627 12 1627 12

129-52 293 144 3.7713 0.5 0.1028 0.7 1514 7 1655 15 1655 15

129-53 153 93 3.3366 0.7 0.1027 0.9 1693 11 1698 20 1698 20

159-1 367 147 3.3536 1.3 0.1059 0.6 1681 19 1715 13 1715 13

159-2 197 58 5.2320 1.4 0.0774 1.2 1128 15 1131 24 1131 24

159-3 203 149 3.4564 1.5 0.1024 0.9 1637 21 1654 18 1654 18

159-4 415 212 3.2549 1.3 0.1060 0.6 1727 19 1728 12 1728 12

159-6 151 52 3.4880 1.4 0.1064 1.0 1620 20 1694 25 1694 25

159-7 219 116 3.2224 1.4 0.1053 0.8 1741 21 1712 15 1712 15

159-8 169 87 3.4806 1.4 0.1069 0.9 1626 20 1731 18 1731 18

159-9 206 86 3.3693 1.4 0.1067 0.8 1675 20 1738 15 1738 15

159-10 1418 104 10.4211 1.2 0.0950 0.5 586 7 1380 28 1380 28

159-11 206 65 5.4479 1.4 0.0752 1.2 1086 14 1060 27 1060 27

159-12 337 152 3.4465 1.3 0.1069 0.7 1641 19 1735 13 1735 13

159-13 649 245 4.3551 1.2 0.1157 0.5 1311 15 1660 41 1660 41

159-14 901 106 8.3874 1.2 0.1209 1.9 707 8 1610 77 1610 77

159-15 1489 101 9.4066 1.2 0.1030 1.7 642 8 1459 58 1459 58

159-16 235 91 5.5068 1.4 0.0755 1.4 1073 14 1022 35 1022 35

159-17 205 95 3.2347 1.4 0.1065 0.8 1735 21 1730 16 1730 16

159-18 407 210 3.1522 1.3 0.1161 0.6 1754 20 1721 33 1721 33

159-19 505 98 4.3292 1.3 0.1059 0.8 1334 15 1667 21 1667 21

159-20 1024 24 9.5087 1.2 0.1112 0.7 629 8 1469 63 1469 63

159-21 234 96 5.4046 1.4 0.0777 1.1 1095 14 1151 23 1151 23

159-22 1527 109 9.3428 1.2 0.1038 0.5 643 8 1380 60 1380 60

159-23 241 92 3.5796 1.3 0.1063 0.8 1586 19 1717 17 1717 17

159-24 350 148 4.0217 1.3 0.1061 0.7 1429 17 1708 16 1708 16

159-25 1481 331 11.7745 1.2 0.0964 1.0 520 6 1370 39 1370 39

178-1 446 281 4.4323 1.3 0.1088 0.6 1305 15 1707 20 1707 20

178-2 445 185 3.2110 1.3 0.1072 0.6 1747 20 1746 10 1746 10

178-3 185 97 3.3178 1.4 0.1034 0.9 1697 21 1675 17 1675 17

178-4 204 144 3.4416 1.4 0.1047 0.9 1638 20 1654 23 1654 23

178-5 790 145 4.0754 1.2 0.1016 0.5 1414 16 1647 9 1647 9

178-6 364 167 4.5795 1.3 0.1064 0.7 1268 15 1677 21 1677 21

178-7 102 110 3.7460 1.5 0.1053 1.3 1523 21 1701 26 1701 26

178-8 207 136 3.3954 1.4 0.1040 0.8 1664 21 1693 15 1693 15

178-9 412 184 3.6484 1.3 0.1050 0.6 1556 18 1656 19 1656 19

178-10 173 81 32.9864 1.8 0.0504 4.0 193 3 345 133 193 3

178-11 240 132 3.3075 1.3 0.1056 0.8 1700 20 1697 18 1697 18

178-12 130 50 33.3779 2.0 0.0509 4.5 193 4 615 238 193 4

Sample 159

Fairview Valley Formation, unit 3 (Jfv3)

Sample 178

Fairview Valley Formation, unit 2 (Jfv2)

Table 4 (continued).

29
Zircon

grain

U

(ppm)

Th

(ppm)

238U/

206Pb

error

(%)

207Pb/

206Pb

error

(%)

206Pb*/

238U

age (Ma)

error

(Ma)

207Pb*/

206Pb*

age (Ma)

error 

(Ma)

 Preferred 

Age

(Ma)

error

(Ma)

178-13 999 232 5.8921 1.2 0.1158 0.6 988 11 1580 53 1580 53

178-14 290 139 7.1210 1.4 0.0949 1.1 846 11 1514 22 1514 22

178-15 203 105 3.4306 1.4 0.1019 0.9 1649 20 1656 16 1656 16

178-16 23 28 3.3033 2.4 0.1058 2.5 1705 36 1728 46 1728 46

178-17 111 46 22.1057 1.9 0.0649 3.6 285 5 771 75 285 5

178-18 306 103 31.9334 1.7 0.0516 2.9 198 3 186 90 198 3

178-19 207 94 3.3308 1.4 0.1034 0.8 1692 20 1686 15 1686 15

178-20 112 89 3.6741 1.5 0.1039 1.2 1552 21 1694 23 1694 23

178-21 215 112 33.1962 1.7 0.0534 3.5 191 3 202 141 191 3

178-22 156 81 3.3616 1.4 0.1028 1.0 1678 21 1665 19 1665 19

178-23 138 89 3.3536 1.4 0.1030 1.0 1682 21 1679 19 1679 19

178-24 306 159 3.3274 1.3 0.1025 0.7 1693 20 1665 14 1665 14

178-25 225 121 3.4393 1.3 0.1041 0.8 1644 20 1684 19 1684 19

178-26 236 147 32.4899 1.8 0.0484 3.4 194 4 -305 249 194 4

178-27 135 57 32.9197 1.9 0.0530 4.4 192 4 193 134 192 4

178-28 1242 149 25.6583 1.3 0.1239 0.7 232 3 1103 182 232 3

178-29 258 96 6.3145 1.4 0.0980 1.1 947 12 1576 20 1576 20

178-30 189 35 31.5753 1.7 0.0586 3.5 196 4 -540 610 196 4

215-1 309 114 3.5699 1.3 0.1036 0.7 1588 18 1652 18 1652 18

215-2 110 94 5.8669 1.6 0.0732 1.8 1013 15 974 40 974 40

215-3 211 147 9.5051 1.5 0.0611 2.0 646 9 681 51 681 51

215-4 53 20 5.5167 2.0 0.0723 2.7 1071 20 930 111 930 111

215-5 59 52 6.0473 1.9 0.0728 2.5 984 17 958 62 958 62

215-6 144 212 1.9258 1.4 0.1868 0.6 2695 31 2710 11 2710 11

215-7 220 28 15.1713 1.5 0.0552 2.3 411 6 420 51 411 6

215-8 273 69 3.5789 1.3 0.0974 0.8 1588 19 1570 15 1570 15

215-9 118 59 4.6053 1.7 0.0832 1.5 1265 19 1249 29 1249 29

215-10 98 36 2.5651 1.5 0.1311 1.0 2121 28 2108 17 2108 17

215-11 423 162 4.5858 1.3 0.0826 0.8 1271 15 1252 15 1252 15

215-12 482 242 3.6865 1.3 0.1062 0.6 1547 18 1729 11 1729 11

215-13 382 179 5.1309 1.3 0.0780 0.9 1147 14 1134 18 1134 18

215-14 24 31 3.9768 2.4 0.0954 2.8 1451 32 1594 69 1594 69

215-15 120 35 6.5784 1.6 0.0739 1.9 911 13 1016 42 1016 42

215-16 303 174 2.5837 1.3 0.1965 0.5 2100 23 2769 10 2769 10

215-17 79 28 4.5773 1.7 0.0811 1.8 1278 19 1286 51 1286 51

215-18 794 210 16.4477 1.3 0.0542 1.3 380 5 375 32 380 5

215-19 93 44 2.6277 1.6 0.1242 1.9 2075 28 1995 37 1995 37

215-20 709 447 36.9919 1.4 0.0507 2.0 172 2 180 59 172 2

215-21 245 83 3.1488 1.3 0.1088 0.7 1776 21 1766 14 1766 14

215-22 323 202 5.7250 1.3 0.0736 1.0 1037 13 1001 23 1001 23

215-23 52 21 5.2053 1.9 0.0742 2.5 1131 20 1011 57 1011 57

215-24 222 207 5.9656 1.4 0.0726 1.5 997 13 959 34 959 34

215-25 99 182 33.9262 2.3 0.0523 5.6 185 4 -242 274 185 4

Sample 215

Quartz-rich sandstone unit (Jqs) above Fairview Valley Formation

Table 4 (continued).

30
Sample 840 935 949 1072 1104 1250 1435

Map Unit Jsdc Jqp Jqmp Jsdc Jsdf Jia Jsdcq

SiO2 64.27 77.39 67.70 64.88 70.32 57.03 65.83

TiO2 0.64 0.13 0.48 0.62 0.47 0.77 0.52

Al2O3 16.64 11.62 15.72 16.67 15.09 16.37 15.44

Fe2O3* 4.90 0.83 3.19 4.84 2.18 6.38 4.26

MnO 0.12 0.02 0.06 0.06 0.06 0.12 0.10

MgO 1.23 0.00 0.80 0.80 0.44 3.71 0.87

CaO 3.14 0.22 2.42 2.94 1.90 6.71 3.02

Na2O 3.56 2.74 4.00 4.02 3.47 3.21 3.17

K2O 3.73 5.00 3.71 3.87 4.36 2.13 4.75

P2O5 0.20 0.02 0.22 0.21 0.09 0.20 0.15

Total 98.43 97.97 98.30 98.91 98.38 96.63 98.11

Zn 56 12 44 122 45 81 50

Rb 121 170 100 129 99 60 165

Sr 449 23 366 442 329 422 288

Y 50 72 43 49 50 35 65

Zr 278 121 292 259 350 217 293

Ba 1174 95 1098 1132 1657 771 1056

Major Element Oxides (weight percent)

Selected Trace Elements (parts per million)

Table 5. Chemical analyses of volcanic and intrusive rocks, Black Mountain area, 

San Bernardino County, California. [X-ray fluorescence analyses. All iron 

assigned to Fe2O3.]

31