Geology of the Inyo Mountains Volcanic Complex: Implications for Jurassic paleogeography of the Sierran magmatic arc in eastern California

George C. Dunne* Timothy P. Garvey† Department of Geological Sciences, California State University Northridge, Mark Oborne§ Northridge, California 91330-8266 Daniel Schneidereit# } A. Eugene Fritsche J. Douglas Walker Department of Geology, University of Kansas, Lawrence, Kansas 66045-2124

ABSTRACT from ca. 169 Ma to 150 Ma. The uppermost the eastern fringe of the carapace of the Sierran part of the complex remains undated but continental margin igneous arc, a dominant pa- An ~3.1-km-thick volcanic complex exposed probably accumulated prior to 140 Ma. leotectonic element in Mesozoic California (cf. in the southern Inyo Mountains, east-central The Inyo Mountains Volcanic Complex is Burchfiel et al., 1992). One of the better pre- California, records Jurassic subaerial deposi- part of a belt of volcanic complexes that are the served, previously little studied of these expo- tional environments along the east flank of the easternmost preserved Jurassic complexes of sures is in the southern Inyo Mountains (Fig. 1). Sierran arc. This complex, which we name the the Sierran arc. These complexes share suffi- In this paper we describe and interpret the origin Inyo Mountains Volcanic Complex, is subdi- cient similarities to suggest that they represent of these Inyo Mountains rocks, propose a formal vided into lower, middle, and upper strati- a distinctive arc-flank depositional province name—the Inyo Mountains Volcanic Com- graphic intervals. The 200–580-m-thick lower significantly different from that represented plex—for them, and contrast them with coeval interval comprises predominantly epiclastic by coeval volcanic complexes preserved in roof volcanic complexes exposed in the eastern strata deposited on alluvial fans and adjacent pendants farther west, closer to the magmatic Sierra Nevada to the west and with volcanic river flood plains that were inclined northeast. axis of the arc. Similarities among arc-flank complexes in the Mojave Desert to the south. Mafic lava flows and rare reworked tuff in this complexes include predominantly to exclu- Our report provides an important supplement to interval record the onset of Jurassic(?) volcan- sively subaerial settings, substantial (>30%) existing studies of Mesozoic volcanic complexes ism in this part of the arc. The 300–700-m- portions of epiclastic strata, and existence at in eastern California because (1) it fills a substan- thick middle interval is composed predomi- times of north- to northeast-inclined pale- tial gap between previously described reaches of nantly of intermediate to silicic lava flows and oslopes. We infer on the basis of the varying the arc to the northwest in the central Sierra (cf. tuffs representing a major episode of volcan- types and amounts of volcanic rocks that Fiske and Tobisch, 1978; Tobisch et al., 1986), to ism ending at ca. 169 Ma that is contempora- whereas most complexes in the arc-flank the south in the Mojave Desert (Schermer and neous with emplacement of numerous plutons province were rarely if ever proximal to major Busby, 1994; Wadsworth et al., 1995), and to the in the region. The >2260-m-thick upper inter- eruptive centers, complexes in two areas west (cf. Busby-Spera, 1984a, 1985; Kokelaar and val is composed of epiclastic strata with minor (White Mountains and eastern Mojave Desert) Busby, 1992); and (2) it describes a complex com- intercalations of volcanic rock. Most of this in- were at times located in or adjacent to such posed predominantly of epiclastic strata deposited terval accumulated on low-gradient flood centers. These differences lead us to speculate in nonmarine environments, a combination of plains that hosted evaporative lakes and that that the east flank of the Jurassic arc consisted characteristics uncommon in Mesozoic volcanic were episodically invaded by alluvial fan com- of eastward-projecting volcanic salients sepa- complexes in the eastern Sierra (cf. Saleeby et al., plexes. Three new U-Pb age determinations rated by arc recesses—typified by the Inyo 1990; Saleeby and Busby-Spera, 1992) and in the constrain the lower half of the upper interval Mountains area—in which epiclastic deposi- Mojave Desert (cf. Schermer and Busby, 1994). to have been deposited during the interval tion was dominant. Our results provide enhanced understanding of several issues related to the evolution of the arc in *E-mail: [email protected]. INTRODUCTION eastern California, including (1) the nature of non- †Present address: Groundwater Technology, marine depositional environments along the east Inc., 4820 McGrath St., Suite 100, Ventura, Cali- Sequences of interstratified volcanic and vol- flank of the arc during Middle to Late Jurassic fornia 93003. caniclastic sedimentary strata and cogenetic time; (2) transverse and longitudinal variations in §Present address: Sierra Geoscience, Inc., 23531 Stillwater Place, Newhall, California 91321. hypabyssal intrusive rocks of Mesozoic age are Jurassic paleogeography in eastern California; (3) # Present address: 34740 Jill Lane, Acton, Cali- exposed at several locations in east-central Cali- timing and regional extent of major eruptive epi- fornia 93510. fornia (Fig. 1). These sequences are remnants of sodes; and (4) modes of preservation of arc strata.

GSA Bulletin; November 1998; v. 110; no. 11; p. 1376–1397; 9 figures; 5 tables.

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INYO MOUNTAINS and hypabyssal rocks of Mesozoic age that are ex- of studies that focused on regional reconnaissance VOLCANIC COMPLEX posed on the west flank of the southern Inyo geology and economic geology, respectively. Co- Mountains. No previous formal or widely cited in- authors of this report presented brief, informal Introduction formal stratigraphic nomenclature exists for these summaries of earlier findings (Dunne et al., 1978; rocks, nor have they been the subject of extensive Oborne et al., 1983; Dunne, 1986; Schneidereit, We propose the name Inyo Mountains Volcanic previous study. Knopf (1918) and Merriam (1963) 1986; Garvey et al., 1993). Our most recent work Complex for the sequence of volcanic, epiclastic, provided brief descriptions of these rocks as parts on these strata has focused on determining isotopic ages for parts the complex (Dunne and Walker, 1993; Dunne et al., 1994), which we supplement here with three newly determined isotopic ages.

General Geology

Exposure and Stratigraphy. The Inyo Moun- tains Volcanic Complex is exposed in a northwest- trending belt ~35 km long and as wide as 4 km along the west flank of the southern Inyo Moun- tains (Fig. 2). The complex is broadly homoclinal, dipping from 40°–60°SW in most areas. Folds cre- ate east dips locally, especially near the faulted top of the complex. We divide the Inyo Mountains Volcanic Com- plex into lower, middle, and upper stratigraphic intervals that have a total maximum preserved thickness of ~3100 m. Two composite reference sections, one for the southern exposure of the complex and one for the northern exposure, are provided in Figure 3. A more detailed section measured through the volcanic-rich middle inter- val where it is least disturbed by faulting is pre- sented in Figure 4. We place the basal contact of the Inyo Moun- tains Volcanic Complex at the base of a resistant conglomerate that rests on fine-grained marine strata of the Union Wash Formation. This contact is unconformable in northern exposures (Oborne, 1983; Stone et al., 1991) but is of uncertain nature in southern exposures where it is faulted. The complex is limited upward by a major thrust fault and by Cenozoic valley fill (Fig. 2). Age. An older age limit for the complex is pro- vided by the underlying Union Wash Formation, which has yielded well-identified Early Triassic fossils from Inyo Mountains exposures, as well as questionable Middle Triassic fossils from up- permost strata in exposures south of the Inyo Mountains (Stone et al., 1991). The lower inter- val and lower half of the middle interval of the complex are undated. Dunne and Walker (1993) reported three U-Pb dates ranging from ca. 168 Ma to 169 Ma for samples of the upper half of the middle interval of the complex, and a U-Pb date of 148 Ma for the granite of French Spring, which intrudes the middle interval and lower part of the upper interval of the complex. We report here three additional U-Pb dates for the lower half of the upper interval of the com- plex (Fig. 5 and Tables 1, 2, samples SI-D55-2, Figure 1. Principal outcrops of Mesozoic volcanic complexes and Jurassic plutons along the SI-D57-2, SI-D3-94). Interpreted minimum ages east side of the Sierran continental margin arc in east-central California. for the three samples range from 163 to 148 Ma.

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These are consistent with the previously re- ported minimum ages noted here, and we inter- pret these new dates as constraining the time of deposition of the lower half of the upper interval to late Middle and Late Jurassic time using the scale of Harland et al. (1990). Strata of the upper half of the upper interval are older than intruding subvertical, northwest-trend- ing mafic dikes that have been correlated with the Independence dike swarm by Chen and Moore (1979) and Dunne (1986). Most dikes of this swarm are late Late Jurassic (ca. 148 Ma; Chen and Moore, 1979; Schermer and Busby, 1994) age, but some are Late Cretaceous (Coleman, 1994) age. These strata are also intruded by much- altered, fine-grained to aphanitic felsic intrusions. These intrusions are texturally and composition- ally similar to dikes and sills exposed near Cerro Gordo Road (Fig. 2); these in turn intrude mafic dikes correlated with the Independence dike swarm. One felsic dike yielded a minimum U-Pb date of ca. 140 Ma (Dunne and Walker, 1993). If felsic intrusions in the upper interval of the Inyo Mountains Volcanic Complex are correlative with this dated felsic dike, then the uppermost strata of the complex accumulated prior to ca. 140 Ma. Structure and Metamorphism. The Inyo Mountains Volcanic Complex contains structures formed by dominantly contractional deformation of Mesozoic age and by normal faults of late Cenozoic age. Contractional structures—reverse faults, conjugate strike-slip faults, rare folds with axial plane cleavage, and slight to moderate pen- etrative strain—evolved within the East Sierran thrust system (Dunne et al., 1983), whereas nor- mal faults are an expression of Basin and Range extension. Metamorphic mineral assemblages in rocks of the Inyo Mountains Volcanic Complex are repre- sentative of the albite-epidote hornfels facies (Dunne, 1986; Sorensen et al., 1998). Wide- spread veins as well as disturbed geochemical patterns in volcanic rocks (see following) indi- cate that hydrothermal fluids also affected the complex. As a result of these processes, rocks in the complex experienced variable loss of original textures and mineralogy. All glass in tuffs is re- crystallized, and recognizable glass shard out- lines have survived in only about 5% of samples examined in thin section. Virtually all ferromag- nesian minerals have been completely replaced by albite-epidote facies mineral assemblages. Many feldspars crystals have been similarly re- placed, although some retain relict igneous min- eralogy (Sorensen et al., 1998).

Rock Units

Tables 3 and 4 summarize principal field and Figure 2. Geologic sketch map of the Inyo Mountains Volcanic Complex. Numerous intra- thin section characteristics of the lithosomes that complex faults across which the section can be reconstructed are not shown.

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compose the Inyo Mountains Volcanic Complex. The basal conglomerate (lithosome Ccb) of this Distinctive lithologic characteristics are illus- interval is composed almost entirely of nonvol- trated in Figure 6. Volcanic rocks are designated canogenic clasts derived from the Union Wash as either lava or pyroclastic deposits, and we use Formation and upper Paleozoic units, whereas the terminology of Fisher and Schmincke (1984, overlying strata of this interval are composed Tables 5-1 and 5-2) for the latter. Compositional mostly of epiclastic material containing rare layers names for volcanic rock units are based on relict and lenses of precomplex clasts. Above the paral- phenocrysts, overall rock color, and thin-section lel-bedded basal conglomerate, epiclastic rocks of textures. Trace-element chemistry of volcanic the lower interval in the southern exposure are rocks in the complex indicates that protolith com- composed of faintly bedded sandstone (Smc), positions were mildly alkalic to shoshonitic sparse 0.5–10-m-thick sheets of conglomerate (Sorensen et al., in press). Almost all sedimentary (Cmb, minor Cms), and rare 2–15-m-thick lenses rocks in the complex are interpreted to have been of pebble to cobble structureless conglomerate derived from erosion of volcanic or hypabyssal (Cms). Lensoidal deposits of Cms conglomerate rocks, thus we apply the term “epiclastic” to enclosed in faintly bedded sandstone (Smc) be- them. We typically use protolith sedimentary and come more abundant in the northern part of the volcanic rock names for rocks of the complex be- southern exposure and are the predominant litho- cause of the very mild metamorphism that these somes in the northern exposure (Figs. 3B and 7). rocks have experienced. In both areas, Cms conglomerate commonly con- tains boulder-size clasts that on average are more NATURE AND ORIGIN OF angular (subangular to locally angular) than clasts THE STRATIGRAPHIC INTERVALS in more southerly exposures. Volcanic rocks of the lower interval (Fig. 7) Overview consist of a horizon of felsic crystal tuff (Tdc?) and numerous, lensoidal bodies of mafic lava Figure 4. Supplemental reference column We describe the three stratigraphic intervals (Lba). The thoroughly calcitized tuff forms a hori- for the middle interval of the Inyo Mountains composing the Inyo Mountains Volcanic Complex zon as thick as 50 m that crops out discontinu- Volcanic Complex. See Figure 2 for location. in terms of their constituent lithosomes and inter- ously for ~10 km in the southern exposure (Fig. Irregular right edge of column is erosional pret their depositional environments. A schematic 7). It locally interfingers with enclosing epiclastic profile. cross section depicting the generalized distribution strata, indicating that it has been partly reworked. of lithosomes composing the complex is presented Lava is basaltic andesite (Lba, Table 3) that com- in Figure 7. In both Figure 7 and the descriptions monly occurs in broadly lensoidal, multi-flow canic edifices grew within the drainages. The that follow, we commonly refer to lithosomes by clusters interpreted to have flowed down and presence near the top of the lower interval of a their field codes as noted in Tables 3 and 4. eventually to have helped fill broad channels. few large cobbles derived from the basal con- All three intervals display moderate to substan- We interpret most of the lower interval to have glomerate indicates ongoing relative uplift and tial lateral changes in the relative abundances and been deposited in a lower alluvial fan setting, erosion of parts of the alluvial fan complex as the vertical arrangement of their constituent litho- which locally evolved into middle alluvial fan lower interval accumulated. Deposition of felsic somes (Fig. 7), a characteristic common to sub- settings in the northern part of the southern expo- tuff followed by numerous small tongues of aerial volcanic arcs (cf. Cas and Wright, 1987). sure and in the northern exposure. The parallel- mafic lava mark the onset of direct volcanic dep- The greatest lateral contrasts in lithostratigraphy bedded, clast-supported basal conglomerate osition in the Inyo Mountains Volcanic Complex. occur across the ~7-km-wide gap in exposure cre- (Ccb) containing lenses of parallel- to cross-bed- ated by the French Spring and New York Butte ded sandstone (Smc) was deposited in a low-gra- Middle Interval plutons and extensive talus-covered areas related dient braided-channel environment low on an al- to them (Fig. 7). This gap separates what we refer luvial fan. The sandstone-rich higher part of the The middle interval (Fig. 7), which ranges in to as northern and southern exposures of the com- interval in the southern exposure formed in broad thickness from 300 to 700 m, is composed of lava plex. Figure 3 presents representative strati- interdistributary parts of lower alluvial fans that (24%–53% of interval) and pyroclastic (20%– graphic columns for these two exposures. were traversed by local channels (Cmb). Rare de- 65%) and epiclastic deposits (15%–30%). The bris flows reached this lower fan environment. basal contact of this interval is mapped at the base Lower Interval The greater abundance of structureless conglom- of coarsely porphyritic andesite lava that extends erate (Cms) and the coarser and more angular na- the full length of the exposure belt, or at the base The lower interval, which ranges in thickness ture of its clasts in the northern part of the south- of a 1–2-m-thick lithic lapilli tuff that locally un- from 200 to 580 m, is composed of epiclastic rock ern exposure and most of the northern exposure derlies these lava flows. This basal contact is (~70% to 90% of interval), lava, and a single py- are interpreted to reflect transition to deposition sharp, with local erosional relief of a few to sev- roclastic deposit. Sandstone (Smc; ~80% of epi- predominantly by debris flow processes in lower eral meters. clastic strata), conglomerate (Cmb, Cms; ~15%), to middle fan environments in those areas. Across most of the southern exposure, the mid- and fine-grained sandstone and mudstone (Ssm, Provenance of the lower interval was predom- dle interval is composed of several 30–150-m- ~5%) compose the southern exposure, whereas inantly nonvolcanic at first, but changed abruptly thick sequences of andesite lava and dacite to rhy- conglomerate (Cms, Cmb; ~75% of epiclastic upward to predominantly volcanic as drainages olite tuff separated one from the next by thinner rocks) interspersed with sandstone (Smc; ~25%) supplying the complex with sediment extended sequences of epiclastic rocks (Fig. 4). The latter compose the northern exposure (Figs. 3 and 7). into existing volcanic terrain and/or as new vol- consist of planar-bedded and cross-bedded sand-

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Figure 5. Concordia diagrams for newly determined U-Pb dates for volcanic units in the Inyo Mountains Volcanic Complex, the volcanic complex of the Alabama Hills, and the Warm Spring Formation at Butte Valley. See Table 1 for data and Table 2 for discussion of interpreted ages. (A) Sam- ple of upper interval of Inyo Mountains Volcanic Complex, ~450 m above base. (B) Sample of upper interval of Inyo Mountains Volcanic Complex, ~1000 m above base. (C) Sample of upper interval of Inyo Mountains Volcanic Complex, ~1000 m above base. Collected from same cluster of ig- nimbrite lenses as sample plotted in B. (D) Two samples of lower interval of volcanic complex of Alabama Hills. Shaded error ellipses are those of sam- ple AH-D4-94, and nonshaded error ellipses are those of sample AH-D20-92. (E) Sample of lower part of Warm Spring Formation at Butte Valley.

stone (Smc), bedded, pebble, and cobble con- In the northern part of the southern exposure the removal of from 100–200 m of the middle glomerate (Ccb, Cmb), and rare massive, matrix- and in all of the northern exposure, the litho- interval by an unconformity (Fig. 7), upon supported pebble conglomerate (Cms). These epi- stratigraphy of the middle part of the middle in- which rest sequences of bedded and massive clastic rocks locally enclose 1–5-m-thick deposits terval differs substantially from that in more conglomerate (Ccb, Cms), minor sandstone of lapilli or crystal tuff (Trl, Tdc) and lava (Lba). southerly exposures. This difference stems from (Ssm), a few 5–20-m-thick layers and lenses of

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TABLE 1. ISOTOPIC DATA FOR VOLCANIC ROCKS ** ** ** # † § Fractions Weight U Pb 206 Pb* 207 Pb* 206 Pb % err 207 Pb % err 207 Pb % err 206 Pb 207 Pb 207 Pb corr. Pbc Pb /Pbc (mg) (ppm) (ppm) 204 Pb 206 Pb 238 U 235 U 206 Pb 238 U 235 U 206 Pb coef.** (pg) AH-D20-2 nm(1) m 0.216 541.9 15.4 2173 0.05673 0.02560 (0.50) 0.1764 (0.53) 0.04996 (0.17) 163.0 164.9 193.2 0.950 86.1 38.5 nm(0) c 0.132 446.4 12.8 213.7 0.1187 0.02585 (0.60) 0.1781 (1.71) 0.04998 (1.51) 164.5 166.4 194.3 0.488 492 3.4 nm(2) f 0.443 763.3 21.7 1492 0.05953 0.02614 (1.29) 0.1790 (1.40) 0.04968 (0.53) 166.3 167.2 179.9 0.926 378 25.4 nm(0) m 0.259 467.9 13.5 1999 0.05798 0.02580 (0.57) 0.1801 (0.59) 0.05063 (0.17) 164.2 168.2 224.0 0.959 98.0 35.6 AH-D4-94 nm(–1) c 0.234 323.0 9.4 5677 0.05208 0.02580 (0.49) 0.1760 (0.51) 0.04949 (0.14) 164.2 164.7 171.1 0.960 21.9 100 nm(0) f 0.019 528.3 14.6 1452 0.05991 0.02494 (0.75) 0.1711 (0.79) 0.04976 (0.21) 158.8 160.4 183.8 0.964 11.1 25.0 nm(0) m 0.099 506.1 14.1 2855 0.05468 0.02507 (0.54) 0.1712 (0.56) 0.04952 (0.14) 159.6 160.4 172.5 0.967 28.1 49.8 nm(0) c aa 0.140 267.5 7.4 2576 0.05540 0.02457 (0.58) 0.1683 (0.61) 0.04966 (0.17) 156.5 157.9 179.8 0.958 22.9 45.3 SI-D55-2 nm(–1) m aa 0.036 195.5 6.2 268.7 0.1048 0.02530 (2.47) 0.1741 (2.85) 0.04990 (1.32) 161.1 162.9 190.4 0.886 44.2 5.1 nm(–1) c 0.054 126.1 3.8 442.0 0.08310 0.02531 (1.77) 0.1736 (2.06) 0.04975 (0.99) 161.1 162.5 183.2 0.877 23.6 8.7 nm(0) f 0.079 153.3 4.5 349.6 0.09188 0.02466 (2.00) 0.1693 (2.19) 0.04980 (0.82) 157.0 158.8 185.8 0.927 55.0 6.5 nm(–1) m 0.176 150.9 4.5 389.7 0.08773 0.02587 (0.96) 0.1784 (1.13) 0.05001 (0.57) 164.7 166.7 195.5 0.864 115 6.9 nm(0) m 0.304 152.0 4.6 1507 0.06158 0.02624 (0.67) 0.1876 (0.71) 0.05185 (0.22) 167.0 174.6 278.7 0.951 49.5 28.1 nm(–1) m aa 0.021 248.4 7.3 71.73 0.2567 0.02541 (1.02) 0.1833 (3.28) 0.05232 (2.94) 161.8 170.9 299.6 0.471 158 1.0 nm(0) m aa 0.102 62.9 2.0 660.4 0.07462 0.02726 (1.01) 0.1976 (2.21) 0.05256 (1.89) 173.4 183.1 309.8 0.521 17.3 11.6 SI-D57-2 nm(2) f 0.140 182.8 4.6 368.6 0.09123 0.02270 (1.20) 0.1610 (1.57) 0.05142 (0.96) 144.7 151.6 259.8 0.790 103 6.2 nm(1) m 0.730 14.2 0.4 257.8 0.1006 0.02325 (2.74) 0.1576 (2.96) 0.04914 (1.01) 148.2 148.6 154.8 0.940 61.6 4.2 nm(2) m 0.113 161.6 4.0 442.4 0.08299 0.02285 (1.65) 0.1567 (1.96) 0.04974 (1.00) 145.7 147.8 182.6 0.861 60.3 7.5 SI-D3-94 nm(–1,0) b aa 0.066 379.8 8.9 767.2 0.06775 0.02210 (0.61) 0.1479 (0.82) 0.04854 (0.52) 140.9 140.1 125.6 0.769 47.0 19.7 nm(1) c aa 0.343 421.7 10.9 3167 0.05397 0.02341 (0.47) 0.1592 (0.50) 0.04932 (0.16) 149.2 150.0 162.9 0.949 68.3 2.8 nm(1) m 0.099 247.8 6.4 1392 0.05996 0.02343 (0.53) 0.1595 (0.59) 0.04937 (0.25) 149.3 150.2 165.2 0.908 26.6 19.3 nm(2) m 0.025 237.3 6.2 452.3 0.08144 0.02358 (1.14) 0.1588 (1.42) 0.04884 (0.80) 150.2 149.6 140.3 0.827 20.5 1.7 BV-D1-94 nm(0) aa 0.0870 282.8 11.52 4784.5 0.07657 0.03837 (0.49) 0.38955 (0.50) 0.07363 (0.12) 242.7 334.0 1031.2 0.971 12.6 79.5 nm(2) c 0.1120 246.5 9.1 657.0 0.09080 0.035114 (0.49) 0.33436 (0.56) 0.069061 (0.25) 222.5 292.9 900.6 0.893 96.3 10.6 nm(2) m 0.0850 296.8 9.72 1929 0.07115 0.03057 (0.50) 0.26833 (0.54) 0.06365 (0.18) 194.1 241.4 730.2 0.943 25.7 32.2 nm(1) m aa 0.0600 163.1 5.2 2457 0.06696 0.029340 (0.73) 0.24698 (0.74) 0.061053 (0.13) 186.4 224.1 641.1 0.985 7.5 41.1 nm(1) f aa 0.0280 270.6 8.31 460.6 0.08939 0.02817 (0.84) 0.22473 (1.04) 0.05785 (0.58) 179.1 205.8 524.3 0.830 30.7 7.6 Note: nm(#) = nonmagnetic on Frantz separator at angle of tilt (degrees); c = coarse size fraction (between 74 and 150 µm). m = medium size fraction (between 50 and 74 µm); f = fine size fraction (smaller than 50 µm); aa = air-abraded samples. Zircon dissolution followed the methods of Krogh (1973) and Parrish (1987). Elemental separation was done with a HBr anion column chemistry for lead and HCl column chemistry for uranium. Decay constants used were 238U = 0.15513 × 10–9 yr–1 and 235U = 0.98485 × 10–9 yr–1 (Steiger and Jäger, 1977). Isotopic analyses were determined on a VG Sector multicollector thermal ionization mass spectrometer. A mass fractionation correction of 0.10% ± 0.05%/amu, as determined by standard runs on NBS 981 (common lead) and NBS 982 (equal atom lead), was applied to the lead data. Samples were spiked with a mixed 205Pb/235U spike. Errors on 206Pb/204Pb were minimized by use of a Daly multiplier and are typically on the order of 1% or less. Errors for 206Pb/204Pb were reduced further on samples spiked with 205Pb by using a dynamic Daly calibration after the technique of Roddick et al. (1987, p. 115). Common lead corrections were made using values determined from Stacey and Kramers (1975) for the interpreted crystallization age. *Corrected for spike and mass fractionation. †Radiogenic component. § † Pb /Pbc is radio of radiogenic to total common Pb (blank and sample) in the analysis. # Pbc common Pb in analysis. **Numbers in parentheses are analytical errors in percent. Corr. coef. = correlation coefficient between the 206Pb/238U and 207Pb/235U errors. Errors and correlation coefficients were computed using the technique of Ludwig (1981).

andesite (Lba) and siliceous tuff (Trc, Tdc), and luvial fan setting. The interleaving of these litho- ever, increased relief apparently existed during a small lensiform deposit of cross-bedded sand- somes suggests that depositional environments deposition of the middle part of the interval in stone (Sfc). The northern exposure of the mid- fluctuated between flood plain and lower alluvial northerly exposures, reflected there by the cre- dle interval also differs from its southern coun- fan or were entirely the latter. In the vicinity of ation of a pronounced unconformity and the local terpart in that a rhyolite lava flow (Lr) replaces the Burgess Mine, Csm conglomerate is locally deposition on it of coarse debris flow deposits. We pyroclastic deposits above the basal andesite more abundant and coarser, and its deposits are observed no evidence of volcanic vents within the lava sequence. more lensoidal. These characteristics reflect dep- present outcrop area of the complex; however, at In exposures south of the Burgess Mine (Fig. 7), osition by debris flows, perhaps in middle allu- least one vent probably existed within a few to the lithostratigraphic characteristics and consid- vial fan settings. several kilometers of the northern exposure based erable lateral extent—commonly 1–3 km—of In sum, the middle interval accumulated during on the presence there of a rhyolite lava flow; such the bedded sandstone (Smc) and conglomerate a time of abundant volcanic activity interspersed flows rarely travel greater distances than this (cf. (Ccb, Cmb) lithosomes separating volcanic se- with episodes of erosion and sedimentary deposi- Cas and Wright, 1987). quences suggest that they were deposited in ei- tion by fluvial, less-common debris flows, and ther flood-plain or lower alluvial fan settings rare eolian processes. The great lateral extent and Upper Interval (Table 4). Sparse deposits of massive, matrix- relatively uniform thickness of many lithosomes supported pebble conglomerate (Cms) probably suggests that topographic relief across the region The upper interval, which is at least 2260 m were deposited by rare debris flows in a lower al- commonly was modest during this time. How- thick, has distinctly different lithostratigraphies in

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TABLE 2. LOCATION, GEOLOGIC SETTING AND U-Pb AGE INTERPRETATIONS Sample Location Geologic setting U-Pb age interpretation number(s) Lat/Long Inyo Mountains: SI-D55-2 36°35′52″ Andesite lava flow, All zircon fractions of this sample are discordant and show evidence of both Pb loss 117°53′51″ upper interval ~450 m and variable inheritance (Fig. 5A; Table 1). A discordia drawn through the uppermost above base points in the array gives a poorly defined lower intercept of 163 Ma. Our preferred minimum age for this sample is 163 ± 3 Ma; this is consistent with the poorly defined lower intercept and with the uncertainty attached to the most concordant points. This rock must be younger than 170 Ma on the basis of previously determined ages for underlying units (Dunne and Walker, 1993).

SI-D57-2 36°38′19″ Dacite tuff, upper All zircon fractions show evidence for inheritance and possibly of minor Pb loss (Fig. 5B) 117°58′16″ interval. ~1,100 m One fraction is concordant at 148 Ma. We consider this latter a minimum age for the below fault-truncated sample. Our preferred minimum age for this sample is 148 ± 2 Ma. top

SI-D3-94 36°38′50″ Dacite tuff, upper Zircons from this sample show behavior similar to that of SI-D57-2, except that one 117°59′22″ interval, approximate fraction shows clear evidence of Pb loss. One fraction is concordant at 150 Ma and stratigraphic equiva- two others are nearly so (Fig. 5C, Table 1). On the basis of the concordant point and the lent of SI-D57-2 206Pb/238U ages of two other related samples, our preferred minimum age for this sample is 150 ± 2 Ma.

Alabama Hills: 36°38′57″ Rhyolite tuff, All zircon fractions are discordant, and the data show clear indications of variable Pb AH-D20-2, 118°05′48″ approximate middle loss (Fig. 5, Table 1). In addition, the 207Pb-206Pb ages scatter widely, indicating that zircon AH-D4-94 of lower interval; two fractions have inherited variable amounts of zircon either as xenocrysts or as cores. We assign a minimum sample sites age of 167 ± 2 Ma to this interval on the basis of the 206Pb/238U age of the nm(2) fine fraction of separated sample AH-D20-2; this fraction plots at the upper end of the data and thus presumably stratigraphically by has undergone the least amount of Pb loss. That our preferred age is a minimum ~50 m, so are treated age is reflected by the fact that this rock interval underlies a unit that has yielded a minimum U-Pb together here crystallization age of 170 ± 4 Ma (Dunne and Walker, 1993).

Butte Valley 35°57′39″ Dacite(?) tuff, ~30 m Zircons from this sample show evidence of inheritance but little evidence of Pb loss. BV-D1-94 117°02′45″ above top of basal We regressed a line through five fractions to get a lower intercept age of 154 ± 7 Ma conglomerate (model 2 of Ludwig, 1980). We believe this to be a reasonable approximation of the crystallization age of the sample, but some caution is in order, given the large MSDW.

northern and southern exposures (Figs. 3 and 7). nor limestone (Shc) that can be traced laterally less abundant Cmb) that form ill-defined bodies The southern exposure consists of fine-grained to for 12 km (Fig. 7) is a distinctive feature of the of subrounded to subangular clasts. Sandstone, coarse-grained sandstone with interbeds of mud- upper part of this sandstone-dominated sequence. siltstone, and bedded conglomerate (Smc, Ssm, stone (Smc, Ssm; ~70% of interval), matrix-sup- It is gradationally bounded above and below Ccb) form laterally discontinuous intervals to ported and minor clast-supported conglomerate by—and in places interbedded with—sandstone 250 m thick within the conglomerate. (Cms, Cmb, Ccb; ~20%), a horizon of calcareous and desiccation-cracked siltstone and mudstone A few 5–30-m-thick lenses of welded dacite shale, siltstone, and minor limestone (Shc, ~5%), (Ssm), and it locally contains lenses of interbed- tuff (Tdc) are present near the mouth of Long and volcanic rock (Lba, Trc, Tdc; ~5%). The ded cross-bedded sandstone and conglomerate John Canyon (Fig. 7). This dacite commonly dis- northern exposure is composed of matrix-sup- (Smc and Ccb). plays eutaxitic layering, and in one area complex ported conglomerate (Cms, Cmb; ~60% of inter- The sandstone-dominated lithology of the folds in the eutaxitic layering suggest rheomor- val); fine-grained to coarse-grained sandstone and southern exposure gives way in two areas to con- phic remobilization during welding (cf. Cas and clast-supported conglomerate (Smc, Ssm, Ccb; glomerate-rich sequences (Fig. 7) characterized Wright, 1987, p. 255). Another body of tuff is ~30%), and pyroclastic rock (Tdc, ~10%). In both by upward-coarsening and upward-thickening, composed of angular blocks of tuff set in a tuff exposures, the base of this interval is mapped at laterally interleaving sheets and lenses of poorly matrix, and it may have been emplaced as a the base of an interbedded sandstone and mud- sorted, structureless, matrix- and clast-supported block-and-ash flow. If correctly interpreted, these stone sequence (Ssm) containing abundant desic- conglomerate (Cmb, Cms) with less-abundant two unusual textures may indicate that these tuffs cation cracks. interbeds of clast-supported conglomerate (Ccb) were deposited a few to several kilometers from Sandstone composing most of the southern ex- and sandstone (Smc). Individual conglomerate their source. posure is well bedded, locally pebbly, and con- beds in this sequence are commonly amalga- Poorly preserved assemblages of bivalve and tains abundant intercalated, desiccation-cracked mated and extend laterally <200 m. gastropod fossils are present within fine-grained siltstone and mudstone beds (Ssm) and minor In the northern exposure, partial sections of the sandstone and siltstone sequences (Ssm) at two bedded conglomerate (Ccb). Individual marker upper interval are preserved on either side of the locations in the northern exposure of the upper beds in the lower part of this interval can be traced Duarte fault (Fig. 7). A wedge of interbedded interval (Fig. 7). John Hanley (U.S. Geological laterally to 3 km. The lower few hundred meters sandstone and desiccation-cracked mudstone and Survey, 1984, personal commun.) provisionally of the interval contain scattered 3–50-m-thick minor tuff that compose the basal strata of the in- assigned the bivalves to the Unionidae and lenses of andesite and dacite lava (Lba) and rhyo- terval is preserved east of the fault. West of the Sphaeriidae and the gastropods to the Neritidae, lite tuff (Trc) that extend laterally <1 km (Fig. 7). fault, the upper interval is composed predomi- an assemblage consistent with a freshwater en- A 50–100-m-thick sequence composed of nantly of massive to faintly bedded, pebble, cob- vironment of middle or late Mesozoic age. thin-bedded, calcareous shale, siltstone, and mi- ble, and rare boulder conglomerates (Cms and In summary, low-gradient flood-plain environ-

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TABLE 3. OUTCROP AND PETROGRAPHIC CHARACTERISTICS AND INTERPRETED ORIGINS OF VOLCANIC LITHOSOMES, INYO MOUNTAINS VOLCANIC COMPLEX Lithosome Outcrop characteristics Petrography Interpretation Lba— Occurs as individual sheets 3 to 8 m thick, more Grayish red-purple, grayish-purple, medium dark gray,and very dusky purple Most lava sheets basalt and commonly as sequences of sheets up to 25 m rock containing medium to very light gray plagioclase, and rare, small K- were emplaced as andesite thick. Both are broadly lensoidal, and typically feldspar phenocrysts.* lava flows, based lava extend laterally 0.2 to 0.5 km, rarely to 1 km. Phenocrysts on presence of Exception to above is basal lava sequence of Plagioclase: Commonly composes 2% to 8% of rock, rarely as much as 15%. rubbly, vesicular middle interval, which is 150 to 290 m thick, Most grains altered to non-zoned albite; rare “survivors” are An20 to An45 tops and/or bases, extends laterally 35 km. Individual sheets of and show faint oscillatory and progressive zoning.Typically subhedral together with sequences commonly separated by irregular stubby laths to 3 mm long. Glomerocrysts common. interleaved and veneers of detritus eroded from underlying Ferromagnesian: Completely replaced by anhedral clots of magnetite(?), overlying detritus flow. Many sheets have rubbly, vesicular chlorite, epidote, and calcite that compose 2% to 7% of rock derived from bases and tops; vesicles otherwise sparse, K-feldspar: Present in a few samples, where it composes 1% to 2% of rock. underlying lava. A with average diameter 1 mm. Interiors of Forms subhedral, subequant grains ~1 mm long. few sheets of this sheets commonly massive, with very rare Matrix: unit lack these intervals of flow breccia. Pale plagioclase Pilotaxitic to felted plagioclase microlites, abundant opaque minerals. features and may phenocrysts sparse in basaltic lava, have been moderately to very abundant in andesite lava. emplaced as sills. Basal flow sequence of middle interval contains plagioclase laths to 9 mm long that locally form abundant, equant glomerocrysts resembling popped corn. Unit forms moderately to very resistant ridges.

Lr— Occurs as one, unconformity-capped tabular Light to medium gray, predominantly aphyric rock. Bands 2 to 4 mm thick are Provisionally rhyolite lava body >200 m thick, >1.7 km long. Basal defined by diffuse, darker laminae. interpreted as a breccia zone locally present. Ubiquitous fine Phenocrysts lava flow, based on flow banding gives unit thin-bedded Present in only a few specimens, where they compose <1% of rock. Consist abundant erosional appearance; is mostly parallel, but locally of subequal amounts of albite and quartz in equant to slightly elongate detritus from unit forms complex flow folds. Unit supports steep grains 0.1 to 0.5 mm long. present in ridge. Matrix immediately Extremely fine grained granular quartz, feldspar, and sparse opaque overlying minerals, the latter concentrated in diffuse layers that help define flow sedimentary strata. banding.

Trc— Forms predominantly massive sheets <175 m Light gray to medium light gray rock, containing abundant quartz phenocrysts Emplaced as pumice- rhyolite thick extending laterally from 1.5 to 17 km. and less abundant pink K-feldspar, light gray plagioclase phenocrysts. poor ignimbrites. crystal tuff Bases relatively planar and sharp; tops Phenocrysts Matrix textures eroded, veneered with detritus derived from Quartz: Composes 7% to 22% of rock. Occurs mostly as rounded, suggest that basal underlying tuff sheet. Flattened pumice lapilli magmatically embayed crystals up to 4 mm wide and as crystal fragments. parts of some locally form up to 8% of rock near base of K-feldspar: Composes 5% to 18% of rock. Forms subequant, subhedral ignimbrite sheets some sheets, but are absent in most outcrops. crystals up to 2 mm long and less common crystal fragments. are welded. Lithic lapilli <1 cm long compose 1% to 5% of Plagioclase: Composes 0.5% to 5% of rock. Forms subhedral laths to 1 mm most outcrops; are medium to medium dark long. An mostly <10, rarely as high as 20. gray, aphanitic to microporhyritic rock of likely Biotite: Composes trace to 1% of rock. Forms thoroughly altered booklets 0.5 volcanic and hypabyssal origin. Unit forms to 1 mm long. steep ridges. Matrix Mostly recrystallized to massive, fine-grained texture. Rare shard outlines locally preserved; some appear flattened. Faint, millimeter-scale fluidal banding molded about phenocrysts in about one-third of specimens is reminiscent of eutaxitic layering; especially common near base of sheets.

ments, represented by laterally extensive intervals accumulation of the lower part of the upper inter- U-Pb age determinations for parts of the Alabama of interbedded Smc, desiccation-cracked Ssm, and val, and small, aerially restricted tongues of pyro- Hills and Butte Valley complexes, as well as an sparse Ccb lithosomes, prevailed across much of clastic material, perhaps deposited within a few to expanded description of the Alabama Hills com- the region as the upper interval began to accumu- several kilometers of their vents, formed during re- plex supplemental to that presented in Dunne and late, and continued to exist throughout deposition newed magmatic activity as the middle part of the Walker (1993). Lithosomes in these complexes of most of the southern exposure. Relief across the interval was being deposited. are broadly similar to those in the Inyo Mountains region was especially subdued as calcareous silty Volcanic Complex, and we apply the same litho- shale and limestone (Shc) was deposited in what REGIONAL logic names and inferred depositional environ- we interpret to have been slightly evaporative STRATIGRAPHIC CORRELATIONS ments to them, although in a more speculative lakes into which rivers episodically built small sense that reflects our less detailed study of them. fan-delta complexes (lenses of Ccb and Smc). Al- Correlative Volcanic Complexes luvial fan complexes (Cms, Cmb lithosomes) pro- Volcanic Complex of the Alabama Hills graded onto this low-relief environment twice in Jurassic volcanic complexes that have at least the southern area, establishing lower and locally some lithostratigraphic similarity with the Inyo We initially considered the volcanic complex middle fan environments. Lower and middle allu- Mountains Volcanic Complex are exposed in the of the Alabama Hills to be an outlying exposure vial fan settings predominated in the northern ex- White Mountains, Alabama Hills, Argus and of the Inyo Mountains Volcanic Complex. Our posure. Local, low-volume eruptions of pyroclas- Slate Ranges, and at Butte Valley in the southern work has revealed, however, that the two com- tic material and lava occurred episodically during Panamint Range (Figs. 1 and 8). We present new plexes display important differences in litho-

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TABLE 3. (Continued). Lithosome Outcrop characteristics Petrography Interpretation Tdc— Most occur as individual lenses and sheets 8 Colors range from medium light gray to grayish purple or very dusky red Emplaced as pumice- dacite, to 50 m thick, traceable laterally 0.5 to 3 km. purple. Very light gray to very light orange euhedral plagioclase poor ignimbrites. rhyodacite Less common are thicker, probably composite phenocrysts ubiquitous; pinkish-gray to moderate orange-pink K-feldspar Widespread crystal tuff sheets up to ~175 m thick extending laterally and dark gray biotite phenocrysts widespread but sparse. eutaxitic texture at least 10 km. Darker colored units Phenocrysts suggest basal parts commonly eutaxitic throughout, locally contain Plagioclase: Composes 2% to 10% of rock. Forms 1–3-mm-long, euhedral, of most sheets fiammi. Upper parts of thicker, lighter colored equant to stubby laths plus locally abundant broken fragments. Most are welded and some units commonly massive; eutaxitic texture and twinned, unzoned albite (secondary?), but locally slightly zoned to An30 sheets welded concentrations of flattened pumice lapilli K-feldspar: trace to 6% of rock. Forms equant, perthitic, euhedral crystals 1–2 throughout. increase near base. Complexly folded mm, plus locally abundant angular fragments. Gradational vertical eutaxitic layering in rare outcrops suggests Biotite: trace to 1% of rock. Forms 1 mm elongate booklets, thoroughly changes in rheomorphic remobilization. Flattened pumice altered to white mica + opaque minerals. phenocryst lapilli up to 6 cm long compose to 15% of rock Quartz: Locally present in trace amounts, as equant crystals to 0.5 mm. abundance, and near base of some tuffs, but are rare Matrix color suggest elsewhere. Lithic lapilli compose 1% to 4% of Mostly granular and recrystallized, with rare flattened shard outlines. Eutaxitic thicker flows most exposures. Typically are medium to dark texture obvious in some samples; faint fluidal layering molded around compositionally gray, aphanitic, subangular to subrounded, phenocrysts reminiscent of eutaxitic texture in many samples. zoned. 0.5 to 1.5 cm long, composed of volcanic and epiclastic rock types.

Tri—rhyolite, Forms 0.5–10-m-thick sheets, plus 1 Diverse, pale splotchy (mostly alteration-induced?) colors include pinkish Emplaced as pumice- rhyodacite composite(?) 150-m-thick sheet. Lateral gray, yellowish-gray, moderate orange-pink, grayish-orange pink; less poor, locally lithic- lithic lapilli extent 0.5 to >10 km. Mostly sharp, planar common darker colors mostly moderate red to very dark red. Sparse, tiny, rich ignimbrite. tuff bases, eroded tops. Massive, with uniformly pinkish-gray to light greenish-gray feldspar phenocrysts and less-common Matrix textures distributed lithic lapilli of diverse composition quartz phenocrysts are scattered uniformly throughout matrix. suggest base of suspended in homogeneous aphanitic matrix Phenocrysts some ignimbrite that is dusted with sparse, tiny phenocrysts. K-feldspar: Forms 0.5% to 4% of rock, as stubby, subhedral laths, sheets are welded. Lithic lapilli compose 8% to 18% of rock. average size 1 mm, plus abundant broken crystal fragments. Typically subrounded to subangular, Plagioclase: Forms trace to 8% of rock, as slightly elongate, subhedral laths maximum diameter <2 cm. Diverse class with An commonly <10, locally 25 to >30; slight progressive zoning; broken types include volcanic, epiclastic, and crystal fragments common. hypabyssal. Pumice lapilli compose 0% to 5% Quartz: 0% to 2% of rock, rarely as intact, equant, magmatically corroded of rock, occurring in rare bands of slightly grains ~0.5 mm, mostly as angular crystal fragments. flattened, weathered-out cavities to 3 cm. Biotite(?): Forms 0% to >1% of rock, as plates and booklets to 1 mm. Now entirely Units form resistant ledges and one prominent altered to white mica and opaque grains. ridge. Matrix Mostly recrystallized aphanitic granular with rare non-flattened shard shapes. Faint fluidal layering molded around phenocrysts observed near base of thicker sheets.

H— Occurs as concordant and discordant tabular Diverse compositions, textures. Most abundant are pale olive to grayish olive Most are certainly hypabyssal bodies and irregular masses. Locally intruded and dusky green rocks, lighter colored ones bearing pale colored intrusive because rock by northwest-trending mafic dikes that may plagioclase phenocrysts to 3 mm, and darker colored ones bearing they are discordant, represent Late Jurassic (148 Ma) part of hornblende phenocrysts to 2 mm. Less common are very light gray to are uniformly regional Independence dike swarm. yellowish-gray rocks, mostly aphanitic or with sparse, tiny feldspar and less massive, and shed common quartz phenocrysts. Least common are dikes of diverse mixed red no erosional debris and purple rocks, bearing feldspar phenocrysts into adjacent Not systematically examined in thin sections. sedimentary rocks. Some concordant tabular bodies could be lava flows, but most likely are sills. Bodies with surviving hornblende phenocrysts probably post date the Inyo Mountain volcanic complex. Note: Color names are those of the Geological Society of America rock color chart (Goddard, 1980).

stratigraphy. Moreover, as noted by Dunne and viding the complex into informal lower and up- abundant hypabyssal intrusions, mostly in the Walker (1993), the original location of the Al- per stratigraphic intervals. form of subvertical, northwest-trending dikes of abama Hills exposure relative to the Inyo Moun- Lower Interval. The lower interval is com- diverse textures and compositions. Some dikes tains Volcanic Complex is unknown, owing to posed of a lower, massive-appearing rhyolite are demonstrably part of the Independence dike the possible existence between them of cryptic crystal tuff and an upper part of approximately swarm (Chen and Moore, 1979). Tuff forms the thrust and/or strike-slip faults of Mesozoic age. equal thickness that consists of epiclastic strata lower ~400 m (estimated thickness with hyp- For these reasons, we treat this sequence as a sep- interleaved with sparse, thin intervals of rhyolite abyssal intrusions removed) of the lower interval. arate volcanic complex to which we give the in- crystal tuff. Sparse bedding attitudes suggest that The apparent great thickness and massive nature formal name volcanic complex of the Alabama the lower interval has an minimum gross thick- of this tuff sequence suggest that it was emplaced Hills. We follow Dunne and Walker (1993) in di- ness of ~2000 m, ~60% of which is composed of as one or more ash flows.

Geological Society of America Bulletin, November 1998 1385

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/110/11/1376/3382822/i0016-7606-110-11-1376.pdf by guest on 27 September 2021 ain. urrents, and episodic ris-flow lobes, levees, and environments is flow. l exposure of silts and muds with nal-lacustine sedimentation near a lake ans. ments: Channel bedload and channel uent drying and cracking. posits in flood-plain and lower alluvial : Wind traction deposition. ment: Eolian sand dune field. ses: Aqueous traction transport and nvironments: Braided river deposits on a scouring and plucking of bottom by strong episodically disturbed by storms. flood-plain and braided distributary channel Environment: Large, shallow evaporative lake or gravitational settling of suspended load. and Process: Aqueous traction transport. upper load in aqueous environment, plus chemical s inter- precipitation of calcite; rare traction transport and nd lower Processes: Gravitational settling of suspended anar, Process: Aqueous traction transport. ts gradational channel fill on middle and lower parts of and convolute laminations).laminations. Rare horizons of flat-pebble intraclasts and dessication-cracked beds. vals traceable laterally to 7 km. to distances >200 m; one 10-m-thick reworking by gentle c Environments: (1) Overbank and sheet-flood soft-sediment deformation structures (fluid-escape structures, flame bedsets are commonly laterally continuous subseq medium gray. planar cross-bedding and cross-lamination. Abundant dessication cracks. Local adjacent to beds of Smc or Ccb. Individual Subaeria to 1 m thick. Inversely graded foreset beds. deposit ~0.3 km long. grained. Locally pebbly. grained. Locally pebbly. grayish red, greenish gray in upper part of lower interval; light white to brown in upper interval. beds are lenticular and grade laterally into interdistributary deposits on an alluvial fan. upward grading at bases of beds, fining near middle or top. Matrix: mudstone to coarse, angular sandstone. Colors: dusky purple to brown. reddish-brown. for a distance of >2500 m. with a flat, fluctuating shoreline. diameter. Upward-fining sequences commondiameter. Matrix: Predominantly coarse sandstone. Colors*: Limestone conglomerate in lower interval is predominately pale yellowish-brown: volcanic conglomerates are dusky purple and reddish-brown. Smc. fan settings. Colors: sandstones are light bluish gray to reddish purple; mudstones of the upper interval is exposed along strike (2) Margi Sandstone grains: subangular to subrounded. sequence of lithosome Ssm in the lower part deposits on a shallow-gradient fluvial pl Colors: shale is light greenish-gray to pale yellowish-green; limestone currents. Colors: grayish red-purple to grayish-brown. Grains: subangular to subrounded, quartz, feldspar, and lithic fragments.Grains: subangular to subrounded, quartz, feldspar, Colors: pale yellowish brown and greenish gray in lower part of interval distances of 10 m to 30 m. finer - or coarser-grained beds over TABLE 4. LITHOSTRATIGRAPHIC CHARACTERISTICS AND INTERPRETED ORIGINS OF SEDIMENTARY LITHOSOMES, INYO MOUNTAINS VOLCANIC COMPLEX LITHOSOMES, INYO MOUNTAINS CHARACTERISTICS AND INTERPRETED ORIGINS OF SEDIMENTARY 4. LITHOSTRATIGRAPHIC TABLE - Structureless and/or bedded, matrix-supported limestone- (lower part of lower Sharp, erosional basal contacts most Process: Debr Cms and - Sandstone Laminated to thin-bedded, fine- medium-grained sandstone and minor Generally sharp, planar contacts. Locally Proces - Sandstone Laminated to thick-bedded sandstone. Local trough and planar cross bedding: Sharp, planar to slightly irregular basal - Conglomerate: Horizontally and cross-bedded limestone-clast volcanic-clast conglomerate. Basal contacts generally sharp, pl - Calcareous Laminated calcareous silty shale (90%) locally grading laterally and vertically to Predominantly gradational upper a - Cross-bedded Fine- to medium-grained, rounded well-rounded, well- very well-sorted All contacts sharp and probably Process *Color names are those of the Geological Society America rock color chart (Goddard, 1980). silty shale andlimestone. laminated or massive-appearing limestone; both lithofacies in beds 0.5 to 2 cm thick. Beds and laminations mostly parallel, with local millimeter-scale trough cross- contacts with Ssm, and rare sharp contacts with Ccb and Cmb. Form Sfc and mudstone. siltstone with intercalated mudstone. Local ripple lamination and low-angle scoured tops where overlain irregular, sandstone. arkosic and lithic sandstone forming grouped, large-scale, trough cross-beds unconformable. Forms one lensiform Environ Ssm Conglomeratematrix-supportedand bedded (Cmb)or structureless(Cms). Beds: 1 m to 20 (?) thick, locally amalgamated; laterally discontinuous (areal interval) and volcanic-clast conglomerate, rare breccia. extent of beds and/or outcrops generally less than 150 m). Clasts: typically subangular to subrounded, locally angular; 1.5 m diameter, locally to 20 m. Lateral contac medium to coarsegrained. sharp, planar to convex upward. Random clast orientation. Local coarsening- typically 4 cm to 6 diameter. and cross lamination. Generally horizontal beds 0.5 m to 2 thick. Common to sandstone. Upper contacts gradational common. Downcutting into underlying strata top contacts. Most bedsets laterally interstratification of beds with varying grain sizes; generally medium to coarse alluvial f Environments: Deb individuaL continuous >200 m; however, E Smc Shc Cmb clast-supported andbedded. Beds: 3 cm to 1.5 m thick; laterally continuous 50 m. typically 2 cm to 4 Clasts: subangular to rounded; 0.75 m diameter, grade upward and laterally into lithosome locally irregular (erosional). Beds generally bar de Environ Lithosome Stratigraphic and lithologic features Contacts and stratigraphic relations Inferred depositional processes and Ccb

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A B

C D

Figure 6. Outcrop photographs of lithosomes of the Inyo Mountains Volcanic Complex. Lengths of hammer head and pencil in photos are 16 cm and 13 cm, respectively. (A) Interbedded layers of andesitic basaltic lava and cinders (Lba lithosome) in upper part of lower interval. Lack of pillows and altered glass rinds in lava and nonsorted nature of cinder horizons are consistent with subaerial deposition. (B) Mudcracked silt- stone bed (Ssm lithosome) in lower part of the upper interval. Such beds are numerous and laterally extensive throughout much of southern ex- posure, and attest to repeated wetting and drying cycles on a subaerial plain of low relief. (C) Cms lithosome in northern exposure of lower in- terval, displaying poor sorting, lack of bedding, and lack of preferred clast orientation, characteristic features of deposits we interpret as debris flows. Angular to subangular clast textures shown here are rare; most Cms deposits feature subangular to subrounded clasts. (D) Cross-bedded epiclastic sandstone (Sfc lithosome) of inferred eolian origin in northern exposure of middle interval.

Epiclastic strata overlying the lower tuff se- taining a greater abundance of moderately well ern exposure include rare bedded tuff and vesicu- quence are predominantly poorly bedded to mas- bedded, variably pebbly volcanogenic sandstone lar mafic to intermediate lava flows in sequences sive-appearing pebbly fine sandstone and siltstone (Smc), and mostly clast-supported pebble to cob- less than 30 m thick. (Ssm), and pebble to cobble, matrix-supported, ble conglomerate (Ccb). Clasts of medium gray Upper Interval. The upper interval crops out in nonbedded conglomerate (Cms). An isolated ex- limestone, probably derived from Early Triassic or two areas of the northern Alabama Hills (see Fig. 2 posure of correlative(?) strata in the southern Al- Paleozoic units that underlie the complex, are of Dunne and Walker, 1993). In the southern out- abama Hills (Fig. 1) is somewhat different, con- present in a few beds. Volcanic strata in this south- crop, this upper interval clearly rests uncon-

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formably on the lower interval with an apparent blende and biotite K/Ar dates of ca. 149 Ma for a leogeographic settings of volcanic complexes in angular discordance of between 5° and 10°; the granitoid pluton that intrudes the upper part of the these regions in Middle Jurassic time (Fig. 9). two intervals are separated by a discontinuous, Warm Spring Formation south of Warm Spring Figure 9 reveals that the Inyo Mountain Vol- <12-m-thick veneer of muscovite schist, quartzite, Canyon, thereby suggesting a Late Jurassic canic Complex was part of a linear belt of com- and sparse volcanic-pebble conglomerate. younger age limit for the formation. plexes extending from the White Mountains to The upper interval consists of >450-m-thick To better constrain the age of the Warm Spring the eastern Mojave Desert, whereas complexes in sequence of welded(?) lithic lapilli crystal rhyolite Formation, we determined a U-Pb date for a the eastern Sierra Nevada, Alabama Hills, and tuff. Flattened pumice lapilli compose as much as dacite tuff collected ~30 m above the basal con- the central Mojave Desert formed a second, par- 10% of the lowest several meters of this tuff but glomerate of this formation (sample BV-D1-94, allel belt to the west. We apply the terms arc- are sparse higher in the unit. Subangular to sub- Tables 1 and 2; Fig. 5E). The interpreted crystal- flank paleogeographic province to the eastern rounded, aphanitic lithic lapilli compose from 1% lization age of this tuff is 154 ± 7 Ma. Considered belt and arc-core paleogeographic province to the to 10% of the tuff; these locally define diffuse, together with the 148 Ma concordant K/Ar cool- western belt. Complexes in the southern reaches lithic-rich bands that may mark contacts between ing ages for the intruding granitoid, this new U-Pb of these two belts differ enough from those in different eruptive sequences or zones of conver- date suggests the possibility that the bulk of the more northerly reaches that we place them into gence of different lobes of an ash flow. The great Warm Spring Formation was deposited during a separate central and eastern Mojave paleogeo- thickness and near-massive nature of this unit limited interval of Late Jurassic time. graphic provinces. The central Mojave province suggest that it was emplaced as thick ash flow(s). is represented by the Sidewinder volcanic series Age. Dunne and Walker (1993) reported an in- IMPLICATIONS FOR (Karish et al., 1987; Schermer and Busby, 1994), terpreted minimum U-Pb date of ca. 170 ± 4 Ma MESOZOIC EVOLUTION whereas the eastern Mojave province is repre- for tuff composing the upper part of the upper OF EAST-CENTRAL CALIFORNIA sented by volcanic complexes in the Cowhole stratigraphic interval, which is consistent with a Mountains (Wadsworth et al., 1995) and Soda U-Pb date of 148 Ma reported by Chen and Paleogeography Mountains (Grose, 1959; Walker and Wardlaw, Moore (1979) for a northwest-trending felsic dike 1989). Table 5 summarizes key characteristics of that intrudes both the lower and upper intervals. Few studies have addressed the Jurassic paleo- these four provinces and Figure 8 provides We report here a new U-Pb date for the rhyolite geography of east-central California. Hanson schematic stratigraphic columns for the best-doc- tuff composing the lower part of the lower inter- (1986) and Dunne and Walker (1993) commented umented complexes in each area. val of the volcanic complex of the Alabama Hills. on similarities between the volcanic complexes Marine volcanic complexes of Mesozoic age Two samples (AH-D20-2 and AH-D4–94) were exposed in the White Mountains and the Inyo located west of the area depicted in Figure 9, collected ~50 m apart in the same rock unit and Mountains and briefly contrasted these com- such as those at Mineral King (Busby-Spera, are treated here as a composite sample (Fig. 5, Ta- plexes with partly coeval volcanic complexes pre- 1984b) and Lake Isabella (Saleeby and Busby- bles 1 and 2). The preferred minimum age for this served in a collinear belt of pendants in the east- Spera, 1993), may represent a more westerly sample, 167 ± 2 Ma, overlaps the previously re- ern Sierra Nevada. Saleeby et al. (1990) included part of the arc-core province. However, they are ported 170 ± 4 Ma preferred minimum age of the in this latter group volcanic complexes in the Rit- composed of lithosome assemblages distinctly tuff forming the upper interval. If correctly inter- ter Range, Goddard, and Oak Creek pendants different from those composing arc-core com- preted, these ages, together with the significant (Fig. 1), and inferred that these complexes are plexes in the eastern Sierra. These differences composite thickness of lower and upper intervals, remnants of a common geologic province based may reflect substantial latitudinal displacement suggest the possibility that the exposed part of the on several shared lithostratigraphic characteris- of these more westerly complexes by intra-arc complex accumulated over a short span of late tics. We include the Mesozoic volcanic complex strike-slip faults and/or their deposition on very Middle Jurassic time during a prominent episode in the Mt. Morrison pendant (Rinehart and Ross, different basement that is located west of a cryp- of explosive volcanism. 1964, Fig. 14) in this group. tic lithospheric boundary within the arc (Saleeby Busby-Spera (1988) and Busby-Spera et al. and Busby-Spera, 1993, Fig. 1). We restrict our Butte Valley (1990b) briefly addressed the Jurassic paleo- discussion of arc-core complexes to those lo- geography of east-central California as part of a cated within the area of Figure 9. The volcanic complex exposed at Butte Valley, regional paleogeographic and tectonic model for Arc-Flank Province. We assign the volcanic named the Warm Spring Formation by Johnson the Sierran arc. For Late Triassic to Middle Juras- complexes in the Inyo and White Mountains, Ar- (1957), is composed of a 10–100-m-thick basal in- sic time, Busby-Spera et al. (1990b, Fig. 1) in- gus Range, and Slate Range to the arc-flank terval of laterally discontinuous beds of limestone- ferred that the Inyo Mountains and adjacent parts province because they share several important pebble conglomerate, epiclastic sandstone, and of east-central California were part of a north- lithostratigraphic characteristics that suggest a felsic tuff and lava, overlain by an ~1300-m-thick west-trending “peninsula” forming the east flank common paleogeographic setting. Most impor- upper interval of andesite lava flows locally sepa- of an arc core that was experiencing predomi- tantly, all arc-flank complexes contain from 30% rated by thin intervals of epiclastic strata (Fig. 8). nantly marine to coastal plain conditions and to 70% epiclastic sandstone, conglomerate, and Johnson (1957) and Cole (1986) inferred the rapid subsidence. siltstone, all or almost all of which accumulated Warm Spring Formation to be of Triassic(?) age This study permits the construction of a more in nonmarine environments in which pronounced based on their interpretation that the formation is detailed view of the Jurassic paleogeography of topographic relief developed episodically. Devel- conformable with the underlying Butte Valley east-central California, and in turn brings into opment of such relief is indicated by the abun- Formation of Early Triassic age. In contrast, sharper focus contrasts between the paleogeogra- dance—especially in the Inyo Mountains com- Wrucke (1966) and Walker (1985) interpreted the phy of that region and that of the eastern high plex—of conglomerate, much of which is basal contact to be unconformable. Chet Wrucke Sierra Nevada and of the Mojave Desert. To fa- interpreted to have been deposited by debris (U.S. Geological Survey, 1996, personal com- cilitate these comparisons, we have synthesized a flows and debris floods on alluvial fans (Nilsen, mun.) reported approximately concordant horn- first-order reconstruction of the locations and pa- 1982; Ritter et al., 1995). Nonvolcanogenic sedi-

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Geological Society of America Bulletin, November 1998 1389

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/110/11/1376/3382822/i0016-7606-110-11-1376.pdf by guest on 27 September 2021 Walker (1993) and this study; Argus Range: Moore (1976), and our reconnaissance map- (1976), and our reconnaissance (1993) and this study; Argus Range: Moore Walker unpublished mapping and geochronol- ping; Slate Range: Dunne et al. (1994), and Walker, (1966), Johnson (1957), and this study; Ritter Range: Cole (1986), Wrucke ogy; Butte Valley: et al. (1998); central Mojave Desert: Schermer and Busby (1994); eastern Sorensen (1989); Busby-Spera et al. (1990b); Saleeby and and Wardlaw (1959); Walker Desert: Grose et al. (1995). Busby (1992); Wadsworth Figure 8. Schematic stratigraphic columns for Mesozoic volcanic complexes in east-cen- Figure tral California and the Mojave Desert. Inyo Mountains column is a composite of character- Eastern Mojave Desert column is an in- both northern and southern exposures. istics from composite of sections exposed in the Cowhole Mountains and Soda Mountains. terpretive as follows. White Mountains: Hanson (1986), et al. for the columns are Data sources (1987), McKee and Conrad (1996); Alabama Hills southern Inyo Mountains: Dunne

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mentary material typically constitutes no more tuff; (2) composed of quartz grains having textural Parnell, 1995). Peterson (1994) speculated that than a few percent of most sections, but is locally features—such as worm-like embayments typical these areas constituted parts of a long-lived, north- more abundant in the upper third of the White of partly resorbed phenocrysts—indicating that east-inclined “Mogollon slope” that extended into Mountains section, which contains grains of they are of volcanic origin; or (3) inferred to be east-central California. We think that an alternative well-rounded quartz sand and pebbles of chert, younger than the youngest known Jurassic quartz- view is equally likely, namely that such slopes de- quartzite, and limestone, some of which bear Pa- rich eolianites of the southwestern Cordillera (cf. veloped locally and episodically through Meso- leozoic fossils (Fates, 1985; Hanson, 1986). Non- Riggs et al., 1993), as in the case of well-rounded zoic time as coalescing aprons flanking clusters of volcanogenic pebbles were presumably derived quartz grains of possible extrabasinal origin that northwest-trending volcanic centers, at least one of from Early Triassic or late Paleozoic strata ex- are mixed in with volcanogenic sand in the upper which was located in the core of the Jurassic arc posed either in uplifted blocks in the arc-flank or (post-147 Ma) part of the White Mountains com- southwest of the arc-flank province. arc-core provinces, or, less likely (see following), plex (Fates, 1985). However, the possibility that Both uplift of a source area and subsidence of from areas east of the arc-flank province. quartz sand in the younger (Late Jurassic?) part of the basin floor seem likely to have been involved Rock units in arc-flank volcanic complexes are the White Mountains complex was recycled from in maintaining—or episodically reestablishing— interpreted to have accumulated principally on older eolianites cannot be ruled out. the nonmarine depositional setting of the arc- river flood plains and alluvial fans and less com- Volcanic strata in arc-flank complexes consist flank province over a period of >20 m.y. Uplifted monly in slightly evaporative lakes and local eo- of subequal amounts of lava and pyroclastic de- source areas in the arc-core province could have lian dune fields. The latter two settings are consist- posits. Most lava is andesite; basalt is relatively been created by any combination of the follow- ent with the dry climate likely to have prevailed in abundant only in the White Mountains complex, ing processes: (1) construction of large, high- southwestern North America during Jurassic time possibly reflecting the location of that area near standing volcanic edifices, perhaps supple- (Parrish, 1992). Marine environments have been the outer, thinned edge of the continental crust (cf. mented episodically by significant magmatic proposed for limited parts of two complexes. In Greene et al., 1997). Pyroclastic rocks are pre- inflation such as that inferred for the Toba caldera the White Mountains, Hanson (1986) inferred that dominantly dacite and rhyolite in composition; of Sumatra (~600 m of uplift over 2500 km2; an isolated exposure of felsic volcanic rocks in- most are massive in appearance and contain frag- Aldiss and Ghazali, 1984); (2) isostatic response terbedded with marble layers as thick as 50 m are mented quartz phenocrysts, features consistent to heating and thickening of the arc-core region of marine origin and represent the oldest surviving with emplacement as ignimbrites. The modest by widespread Middle Jurassic magmatism; and part of that volcanic complex. He inferred that the thicknesses of these ignimbrite sequences—typi- (3) isostatic response to crustal thickening caused remainder of the complex was of nonmarine ori- cally less than 150 m—and restriction of included by contractional deformation, which began along gin. In the central Argus Range, Moore (1976) in- lithic fragments to lapilli size suggest that they the western margin of the arc at and to the north terpreted one ~300-m-thick, thrust-fault–bounded were emplaced as runout sheets not immediately of the latitude of the study area beginning ca. 175 lens of calcareous feldspathic sandstone, siltstone, proximal to major pyroclastic eruptive centers. Ma (Saleeby and Busby-Spera, 1992; Bjerrum and sparse shale and limestone to have been de- However, volcanic vents were probably located and Dorsey, 1995). posited in a shallow-marine environment, princi- within a few kilometers of some present expo- Subsidence of the basement of the arc-flank pally on the basis of the abundance of calcareous sures of arc-flank complexes, based on the pres- province almost certainly facilitated accumula- material. He inferred that the remainder of the vol- ence of locally abundant hypabyssal intrusions, tion of volcanic and epiclastic strata. Subsi- canic complex was nonmarine. Our examination some of which are lithologically similar to nearby dence within arcs may have multiple origins of the calcareous strata leads us to conclude that volcanic units, and on the presence of a rhyolite (cf. Ingersoll and Busby, 1995), but isostatic re- while one cannot preclude a marine origin for this lava flow in the Inyo Mountains complex. sponse to surface sediment loading is likely to interval, it is equally likely that it is composed of We infer that Middle and Late Jurassic pale- have been a principal contributor, given that the interstratified lacustrine and fluvial strata that be- oslopes of the arc-flank province were commonly province was subjected to ongoing influx of came thoroughly calcified during hydrothermal al- inclined to the north and northeast, based on sediment from higher source areas to the south teration, a phenomenon observed in both the Inyo (1) limited paleocurrent and paleodrainage infor- and west. Mountains (Oborne, 1983) and White Mountains mation from the Inyo and White Mountains com- Data from the Inyo Mountains Volcanic (Fates, 1985) complexes. We favor this latter in- plexes; (2) paucity in arc-flank complexes of non- Complex indicate that sediment accumulation terpretation and consider the Argus Range com- volcanogenic clastic sediment that might have rates in at least that part of the arc-flank basin plex to be nonmarine. been derived from pre-arc rocks potentially ex- were moderate to low during late Middle Juras- Although known or inferred ages of most arc- posed east of the arc; and (3) analogies with in- sic and Late Jurassic time. Comparison of flank complexes span the Middle Jurassic interval ferred correlatives of the arc-flank province in stratigraphic thickness—corrected for an aver- when eolian quartz sand spread southwest from southern Nevada, southeastern California, and age tectonic flattening of ~20% (M. DeFrisco, the craton into parts of the arc now located north western Arizona. We infer that epiclastic material 1995, personal commun.)—and isotopic dates (Wyld and Wright, 1993; Fisher, 1990), south and ignimbrites moved down these slopes away defining two different reaches of the lower half (Busby-Spera et al., 1990a; Riggs et al., 1993) and from topographically high volcanic edifices that of the upper interval in the complex (Fig. 7) west (Busby-Spera, 1984a, 1984b) of the arc-flank developed atop Jurassic intrusions (Lipman, 1984) yield minimum accumulation rates of 63 and province, sparse quartz sand in arc-flank volcanic now widely exposed to the southwest (Fig. 1). 67 m /m.y. The complex experienced an aver- complexes is not obviously related to these eolian- North- to northeast-inclined paleoslopes in the age accumulation rate of 105 m/m.y. if the ites. Quartz sand and sandstone in the arc-flank arc-flank province are consistent with predomi- complex accumulated between ca. 186 Ma, the province are (1) of indeterminate origin, as in the nantly northeast-directed Jurassic paleodrainage age of oldest plutons in the province (Dunne case of granoblastic quartzites marking the uncon- directions interpreted for the east flank of the arc in and Walker, unpub. data), and 148 Ma. These formity between upper and lower intervals of the southern Nevada (Marzolf, 1991), the eastern Mo- accumulation rates are near or below the low Alabama Hills complex, which may have origi- jave Desert (Grose, 1959), and northwestern end of the range of rates reported for basins in nated by weathering of underlying quartz-phyric Arizona (Riggs and Blakey, 1993; Blakey and continental-margin arcs by Smith and Landis

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Figure 9. Paleogeographic sketch of eastern margin of the Sier- ran arc in east-central California and Mojave Desert regions dur- ing Middle to Late Jurassic time. Location of arc margin is after Barton et al. (1988) and Riggs and Blakey (1993). Jurassic marine settings and drainage direction are derived from Busby-Spera (1988) and Riggs and Blakey (1993). Jurassic marine waters in arc core may have retreated westward by the time the interior Jurassic sea transgressed to depicted location. Volcanic complexes have been restored to inferred Late Jurassic locations utilizing the reconstruc- tions of the Mojave Desert and eastern California of Marzolf (1994) and Walker et al. (1990). We have also restored ~65 km of right sep- aration across Owens Valley, the estimated net separation (Stevens and Greene, 1995) caused by Mesozoic (Saleeby and Busby, 1993, Fig. 1a; Kistler, 1993, Fig. 1b) and Cenozoic (Beanland and Clark, 1994) faulting. The Mesozoic fault zone is cryptic, but available con- straints permit it to be located between the two belts of volcanic complexes. Restoration of its slip leaves the parallel arrangement of these two belts intact, but produces a more compact latitudinal jux- taposition of complexes. The projection of this zone south of the Garlock fault is unconstrained, so we have not attempted to restore its slip in the Mojave Desert. Circled letter code for locations of vol- canic complexes is same as Figure 1, with addition of sw (Side- winder Mountains), sm (Soda Mountains), and c (Cowhole Moun- tains) in the Mojave Desert.

(1995), leading us to infer that basin floor sub- provinces during Middle and Late Jurassic (ca. subaqueous—in part marine—conditions in the sidence in the arc-flank province was not accel- 170 to 147 Ma) time, and also reveal similarities former, to predominantly low-relief subaerial erated by extensional tectonism, as may have in lithostratigraphic characteristics that lead us to conditions in the latter (Fig. 9). Busby-Spera been the case in the arc-core (Busby-Spera, assign the volcanic complex of the Alabama (1988) ascribed this change to the presence of 1988; Busby-Spera et al., 1990b) and central Hills to the arc-core province and the volcanic thicker cratonic lithosphere under the central Mo- Mojave (Schermer and Busby, 1994) provinces. complex of Butte Valley to the eastern Mojave jave province in contrast to thinner miogeoclinal Paleogeographic Comparisons Among province. lithosphere under areas to the north (cf. Schermer Provinces. Table 5 and Figure 8 summarize data Longitudinal changes in stratigraphy and in- and Busby, 1994, Fig. 1), an interpretation we that provide the principal bases for assessing lon- ferred paleogeography along the arc-core and think reasonable. gitudinal and transverse changes in paleogeogra- central Mojave provinces are relatively modest, Decidedly greater longitudinal variations in phy among the arc-flank, arc-core and Mojave reflecting primarily the change from low-relief stratigraphy are apparent in the arc-flank and east-

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ern Mojave province (Fig. 8), and are expressed the lower part of the Sidewinder volcanic series in flank province. These volcanic rocks have yielded principally in differing proportions of sedimen- the central Mojave province at ca. 170 Ma, extra- minimum U-Pb ages of ca. 148 Ma and ca. 150 tary and volcanic deposits. We infer that these dif- caldera volcanic rocks were deposited and then Ma (Fig. 7). As in the case of the earlier volcanic fering proportions reflect two aspects of volcan- eroded, but they noted that no significant deposits outburst, numerous plutons were emplaced in the ism that are common along the back-arc side of of resulting epiclastic material are known in the region about the same time. One of these, the 148 modern arcs (Marsh, 1979; Tatsumi and Eggins, central Mojave region. We infer that at least some Ma French Spring pluton (Dunne and Walker, 1995): (1) volcanic centers are more widely and of that epiclastic material was transported north- 1993), is clearly epizonal, as reflected by its mi- more irregularly spaced than in the magmatic core east into the arc-flank and eastern Mojave regions. arolitic texture and by the fact that, like the older of arcs; and (2) their eruptive volumes are less Topographic relief and erosion increased in the Barcroft pluton in the White Mountains (Hanson, when compared to volcanoes in the magmatic central Mojave at ca. 166 Ma in response to wide- 1986), it intrudes its own, slightly older volcanic core of these arcs, with major explosive pyroclas- spread plutonic intrusion accompanied by exten- and epiclastic cover (Fig. 7). tic eruptions being less common. Varying propor- sional deformation (Schermer and Busby, 1994), Elsewhere in eastern California, Late Jurassic tions of volcanic and epiclastic deposits that are and some of the epiclastic sediment thus generated volcanic rocks and plutons yielding U-Pb ages apparent among volcanic complexes in the arc- may also have been transported northeast into the ranging from ca. 147 to ca. 152 Ma have been re- flank and eastern Mojave provinces seem to re- eastern Mojave and arc-flank provinces. ported from the White Mountains (Hanson flect a more widely spaced, less-regular distribu- et al., 1987), southern Panamint Range (McKenna tion of volcanoes along the backarc side of the Magmatism et al., 1993; Wrucke et al., 1995), Coso Range Sierran arc. Eastward-projecting volcanic salients (R. Whitmarsh, 1996, personal commun.), south- may have formed where local volcanic centers The Inyo Mountains Volcanic Complex and an ern Slate Range (Dunne et al., 1994), Spangler developed, as in the White Mountains, Butte Val- intruding pluton record two principal phases of Hills (Chen and Moore, 1982), southeast Sierra ley, and Soda Mountains areas, whereas epiclas- magmatism, a volumetrically larger one peaking (Dunne et al., 1991), and the central Mojave tic-dominated recesses may have formed where at or somewhat earlier than ca. 170 Ma and a vol- Desert (Martin, 1991; Schermer and Busby, 1994). volcanic centers were rare and/or of small vol- umetrically smaller one peaking at or somewhat These rocks range in composition from basalt- ume, as in the southern Inyo Mountains (Fig. 9). earlier than ca. 148 Ma (Fig. 7). Both phases are gabbro to rhyolite-granite, and represent a wide- Table 5 and Figure 8 also reveal significant of unknown duration. In the Argus Range, vol- spread, compositionally expanded, volumetrically transverse changes in arc volcanism and sedi- canism began prior to intrusion of a 173 Ma plu- restricted magmatic episode in eastern California mentation. In the arc-core and central Mojave ton (Fig. 8), and initial volcanism in the Inyo that mirrors a coeval magmatic episode in the provinces, Middle Jurassic volcanism often took Mountains complex may be this old, or perhaps western foothills of the Sierra (e.g., Wolf and the form of large-volume pyroclastic eruptions as old as the oldest plutons in the arc-flank Saleeby, 1995). and was commonly accompanied by caldera col- province, which are ca. 186 Ma. lapse. In contrast, large-volume pyroclastic erup- This older volcanic phase is coeval with em- Preservation of Volcanic Complexes tions were apparently rare in the arc-flank and placement of numerous Jurassic plutons along eastern Mojave provinces, where mafic to inter- the east flank of the arc in east-central Califor- Volcanic complexes of the arc-flank province mediate lava flows form a larger part of these nia (Fig. 1), some of which demonstrably in- of east-central California have been preserved in complexes than is the case for complexes farther truded their own, slightly older volcanic cover, a manner distinct from those common for com- west. These contrasts are consistent with patterns as in the case of the Barcroft Granodiorite in the plexes elsewhere in the Sierran arc. Preservation of volcanism observed in many modern arcs White Mountains (Hanson et al., 1987). U/Pb of Mesozoic volcanic complexes elsewhere has (Marsh, 1979; Tatsumi and Eggins, 1995). Trans- dates for these plutons range from ca. 160 to been attributed to the following mechanisms. verse contrasts in epiclastic deposits are equally 186 Ma (cf. Stern et al., 1981; Chen and Moore, (1) Where plutonic intrusions were numerous evident. In the arc-core and central Mojave 1982; Bateman, 1992; Dunne and Walker, un- and rose into the volcanic cover, parts of vol- provinces, topographic relief was commonly published data) with the greatest volume of canic complexes were caught between rising modest outside of the calderas themselves, and magma emplaced from ca. 166 to 174 Ma. plutons and translated downward via “return epiclastic strata did not accumulate in any signif- Many of these plutons—like the volcanic rocks flow” required by mass-balance considerations icant quantity. In contrast, arc-flank and eastern of the complex—are reported to have alkalic (Tobisch et al., 1986). (2) Complexes accumu- Mojave provinces contain variable—locally sig- compositions (Dunne et al., 1978; Sylvester lated and were preserved locally in caldera com- nificant—amounts of epiclastic material that ac- et al., 1978; Dunne, 1979). These temporal, spa- plexes (Schermer and Busby, 1994). (3) Com- cumulated at relatively slow rates, and topo- tial, and compositional similarities suggest that plexes formed in broad, mildly subsident basins graphic relief was at times great enough to the volcanic rocks of the complex and these plu- (Schermer and Busby, 1994). (4) Complexes ac- promote development of alluvial fans. tons are cogenetic and together reflect an im- cumulated and were preserved in normal- Evidence of north- to northeast-inclined pale- portant phase of Middle Jurassic magmatic ac- fault–bounded, subsiding basins of regional oslopes in the arc-flank and eastern Mojave tivity in the arc-flank province that has also (Busby-Spera, 1988; Fisher, 1990; Fackler- provinces suggest that much of the epiclastic ma- been recognized throughout the Sierran and Adams et al., 1997) or local (Wadsworth et al., terial deposited in these provinces originated in the Klamath arcs (Barton et al., 1988; Wright and 1995) extent. arc-core and central Mojave provinces. Increased Fahan, 1988; Saleeby and Busby-Spera, 1992). In contrast, preservation of arc-flank volcanic rates of intrusion and contractional deformation This magmatic phase is reflected by abundant, complexes commonly was facilitated by thrust during Middle Jurassic time are well documented wind-borne volcanic ash deposited in the backarc faulting. All of the arc-flank complexes lie—or, in the arc-core province (Saleeby and Busby, (Blakey and Parnell, 1995). in the case of the Butte Valley complex, are in- 1992), and both processes would have facilitated Volcanic strata in the middle part of the upper ferred to lie (Wrucke et al., 1995; Davis and uplift and erosion in that area. Schermer and interval reflect a second widespread, although Burchfiel, 1997)—in the footwalls of thrust Busby (1994) concluded that during deposition of less-voluminous, phase of magmatism in the arc- faults of the East Sierran thrust system. As noted

Geological Society of America Bulletin, November 1998 1393

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/110/11/1376/3382822/i0016-7606-110-11-1376.pdf by guest on 27 September 2021 # Schermer and al extension mplex; small amounts site to rhyolite lava flow rock ral higher levels; one ~1-km- thin lenses of fluvial epiclastic : dacite to rhyolite ash flow, minor : dacite to rhyolite ash flow, : basalt, basaltic andesite, minor igraphic position present in Rodman on Early Triassic? to early Middle on Early Triassic? Central Mojave province ower sequence), and in broader, minor tuff breccia compose ~90% of minor tuff he remainder derived quartz sand present in basal e, siltstone, minor conglomerate k eolian quartz sandstone (Aztec?) of (upper sequence) sequence); hypabyssal intrusions environment Low-relief, shallow-marine environment at arc southwest initiation, succeeded by low-relief subaerial 70% to Lower sequence § Busby et al., 1990; Saleeby and Busby, 1992; Wadsworth et al., 1995. 1992; Wadsworth Busby et al., 1990; Saleeby and Busby, , 1990; Saleeby and Busby, 1992; Schweickert and Lahren, 1993; , 1990; Saleeby and Busby, een ~30% to ~50% thicker than listed values prior flattening. et al. (1998) and correcting section et al., thickness for 50% ductile flattening (Tobische 1977) and ~1.2 km of fault repetition(Sorensen et al., 1998), maximum average (MIddle Jurassic) partof the Sidewinder volcanic in Ritter Rangeaccumulation rate was ~120 m/m.y. complex nested calderas depositional setting of these rocks in a series Busby (1994), accumulation rate in the lower a rate consistent with the series was >225 m/m.y., Mountains Complexes Mt. Morrison, Goddard Early Triassic strata in SodaEarly Triassic Mountains, disconformably on late Paleozoic strata in Cowhole Mountains miogeoclinal Paleozoic strata in central complexes exposures TABLE 5. SUMMARY OF CHARACTERISTICS OF JURASSIC VOLCANIC COMPLEXES OF THE ARC-FLANK, ARC-CORE, AND MOJAVE PROVINCES OF CHARACTERISTICS JURASSIC VOLCANIC COMPLEXES THE ARC-FLANK, ARC-CORE, AND MOJAVE 5. SUMMARY TABLE rare limestone is of inferredbase of White Mountains complex (Aztec Sandstone) are present in to ~550 m thick of craton- common in Ritter Range (where two beds, limy tuff few localities, but indicators of depositional setting uncertain strat strata and at seve flood-plain and alluvial fan settings;lacustrian origin, except for that at of complexes; layers and lenseswhich may be marine derived, eolian quartz sandstone local channeling and crossbedding reported in a generally lacking; thin, laterally extensive limestone of craton- Cowhole Mountains and Soda thic have yielded marine fossils), also present locally in Mountains conglomerate deposited in conglomerate compose 5% to 20% sandstone, siltstone, less common conglomerate; compose <10% of entire co and rhyolite lava flow rockcomposes 25% -> 40%, dacite to composesrhyolite ash-flow tuff typically occur inremainder; tuffs intervals thinner than 150 minferred to represent runout sheets abundant lava flow rock dominant;distant from their sources; basalt rhyolite lava flow breccia, and less- in intervals a dacite and rhyolite tuff relatively abundant only in White few meters to 70 m thick estimatedMountains commonly forms intervals the remainder; tuff rocks several hundred meters thick interpreted to have 80%, basalt andesite, and rhyolite lave flow rock formed in or near calderas to compose ~20% of volcaniccomplexes interpreted to be compose t volcanic rocks; ande air-fall tuff, subaerial, and to have accumulatedin settings of variable relief thatsloped north to northeast at times inclined to northeast at times Upper sequence alternating subaqueous and subaerial environments of uncertain kind, mostly low relief except near calderas rhyolite lava flow rock, feeder dikes, and Data for arc-core complexes derived from Rinehart and Ross, 1964; Fiske and Tobisch, 1978; Tobisch et al., 1986; Saleeby al. 1978; Tobisch Data for arc-core complexes derived from Rinehart and Ross, 1964; Fiske Tobisch, 1994. Data for central Mojave province from Karish et al., 1987; Schermer and Busby, *Data for eastern Mojave province derived from Grose, 1959; Dunne, 1977; Marzolf, 1983; Walker, 1985; Walker and Wardlaw, 1989; and Wardlaw, 1985; Walker *Data for eastern Mojave province derived from Grose, 1959; Dunne, 1977; Marzolf, 1983; Walker, § # **Present “structural” thickness, not corrected for faulting or penetrative flattening; arc-core sections interpreted to have b Volcanic rocks:Volcanic Andesite and less abundant basalt Andesite, dacite, and less-common breccia compose tuff Dacite to rhyolite tuff, Mode of In footwalls of Late Jurassic? In fault grabens in Cowhole As screens of vertically stretched and flattened In calderas (l CharacteristicDepositionalsubstrate(s): marine strata Arc-flank province Early Triassic preservation: 1985) or Conformably (Walker, Documented age range: thrusts >173 Ma to <147 Range of East Mojave province* Allochthonous eugeoclinal Paleozoic strata inthicknesses**:Estimatedaccumulationrates: 1.4 km -> ~3.8 Conformably? ~173 Ma to ~148? for details) (see text 63 to 105 m/m.y. disconformably (Grose, 1959) onSedimentaryrocks: Arc-core province northern complexes, in situ transitional to Mountains; unknown for other Unknown ~2.2 km (estimate from Soda Form ~30% to 70% of sections; ~222 Ma to ~143 “return-flow” material between large intrusions Jurassic? marine strata predominantly epiclastic sandstone, Lenses of fluvial epiclastic ~3.5-> 5.5 km Mountains) sandstone, siltstone, minor shallow basins related to region compose <10% of sections;poorly Typically Utilizing new isotopic dates reported by Sorensen described, but seem to consist mostly of epiclastic ~170 Ma to ~148? Utilizing age and thickness data of sandston Scattered, mostly ~4.5 km (lower sequence), ~0.8 (upper Greene and Schweickert, 1995; Sorenson et al., 1998. depositionalsetting White Mountain complex; remainder of interpreted for all complexes; in White Mountain complex and all other Soda Mountains, paleoslopes were adjacent low-relief coastal plain that sloped at times; other complexes inferred to have formed in Inferred Possibly marine for oldest part of Low-relief subaerial environments Ritter Range section formed in shallow marine and

1394 Geological Society of America Bulletin, November 1998

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matism: The volcanic record in the eolian Page Sandstone for the Inyo Mountains (Dunne and Walker, pal Mesozoic deformational events evident else- and related Carmel Formation, Colorado Plateau, in 1993), emplacement of thrust plates would both where in the region temporally straddled deposi- Miller, D., and Busby, C., eds., Jurassic magmatism and depress the complexes and provide a resistant tion of these complexes, with some occurring be- tectonics of the North American Cordillera: Geological Society of America Special Paper 299, p. 393–411. cover for them, thereby aiding in their preserva- fore (Dunne, 1986; Schweickert and Lahren, Boettcher, S., and Walker, J. D., 1993, Geologic evolution of tion. Dunne and Walker (1993) hypothesized that 1987; Greene et al., 1995) and others after (Dunne Iron Mountain, central Mojave Desert, California: Tec- these thrust plates were emplaced in Late Juras- and Walker, 1993); (2) significant deformation tonics, v. 12, p. 373–386. Burchfiel, B. C., Cowan, D. S., Davis, G. A., 1992, Tectonic sic time, shortly after the youngest strata in the occurred during accumulation of the complexes overview of the Cordilleran orogen in the western United sections were deposited. Ages for the youngest but was irregularly distributed, happening not to States, in Burchfiel, B. C., Lipman, P. W., and Zoback, M. L., eds., The Cordilleran Orogen: Conterminous U.S.: dated strata in these sections (Fig. 8) are permis- affect the limited areas where these complexes are Boulder, Colorado, Geological Society of America, Ge- sive of this interpreted timing, as are the ages of preserved; or (3) structures related to such defor- ology of North America, v. G-3, p. 407–479. crosscutting intrusive rocks in the Slate Range mational events are present within and/or beneath Busby-Spera, C. J., 1984a, The lower Mesozoic continental margin and marine intra-arc sedimentation at Mineral part of the thrust system (Dunne et al., 1994). some complexes, but are relatively subtle and King, California, in Crouch, J. K., and Bachman, S. B., Preservation of Mesozoic volcanic complexes have not been recognized during field studies eds., Tectonics and sedimentation along the California by thrust faults may have occurred elsewhere in completed to date. margin: Pacific Section, Society of Economic Paleontol- ogists and Mineralogists, v. 38, p. 135–156. eastern California as well. East-vergent thrust Busby-Spera, C. J., 1984b, Large-volume rhyolite ash flow faults that developed broadly synchronously with ACKNOWLEDGMENTS eruptions and submarine caldera collapse in the lower Mesozoic Sierra Nevada, California: Journal of Geophys- accumulation of the Triassic and Jurassic vol- ical Research, v. 89, p. 8417–8427. canic complex exposed in the Saddlebag Lake Support for isotopic dating was provided Busby-Spera, C. J., 1985, Depositional features of rhyolitic and pendant (Schweickert and Lahren, 1993) locally by National Science Foundation grants EAR- andesitic volcaniclastic rocks of the Mineral King sub- marine caldera complex, Sierra Nevada, California: Jour- cut through and duplicated parts of the volcanic 9204703 to Dunne and EAR-9205096 to nal of Volcanology and Geothermal Research, v. 27, complex and may have helped preserve it. At Walker, and by a grant to Walker from the Pe- p. 43–76. Iron Mountain in the central Mojave Desert, the troleum Research Fund, administered by the Busby-Spera, C. J., 1988, Speculative tectonic model for the early Mesozoic arc of the southwest Cordilleran United Middle Jurassic(?) Hodge volcanics are inter- American Chemical Society. David Anderson, States: Geology, v. 16, p. 1121–1125. preted to lie in the footwall of a Late Jurassic Cathy Busby, Dick Fiske, Brooks Hanson, Sam Busby-Spera, C. J., Mattinson, J. M., and Schermer, E. R., 1990a, Stratigraphic and tectonic evolution of the Juras- thrust fault that was later steepened to near-verti- Longiarue, Liz Schermer, Jason Saleeby, and sic arc: New field and U-Pb zircon geochronological data cal orientation (Boettcher and Walker, 1993). Sorena Sorensen generously shared with us data from the Mojave Desert: Geological Society of America and insights regarding the development of vol- Abstracts with Programs, v. 22, no. 3, p. 11. Busby-Spera, C. J., Mattinson, J. M., Riggs, N. R., and Schermer, CONCLUSIONS canic complexes. Cathy Busby, Rachel Gulliver, E. R., 1990b, The Triassic-Jurassic magmatic arc in the Nancy Riggs, and Dick Tosdal provided helpful Mojave-Sonoran Deserts and the Sierra-Klamath region; Although our study has illuminated several as- reviews of various versions of the manuscript. Similarities and differences in paleogeographic evolution, in Harwood, D. S., and Miller, M. M., eds., Paleozoic and pects of Middle to Late Jurassic geology of east- Dunne acknowledges the stimulation provided early Mesozoic paleogeographic relations; Sierra Nevada, ern California, there remain important questions by California State University Northridge Klamath Mountains, and related terranes: Geological Soci- ety of America Special Paper 255, p. 325–337. deserving attention in future studies of this region, (CSUN) students who participated in field map- Cas, R. A. F., and Wright, J. V., 1987, Volcanic successions: the two most important of which are as follows. ping classes and senior thesis projects in the Boston, Allen & Unwin, 528 p. 1. Are there systematic differences in the age study area. Financial support for geochemical Chen, J., and Moore, J., 1979, The Late Jurassic Indepen- dence dike swarm in eastern California: Geology, v. 7, of the oldest preserved strata in different loca- analyses and field work was provided to Dunne p. 129–133. tions, and if so, can a likely cause of such patterns by a CSUN Faculty Research Grant and by the Chen, J., and Moore, J., 1982, Uranium-lead isotopic ages from be deciphered? Oldest arc-related volcanic strata CSUN Department of Geological Sciences. the Sierra Nevada batholith, California: Journal of Geo- physical Research, v. 87, p. 4761–4784. in the central and southern Sierra are ca. 222 Ma Dunne thanks Kirk Peek of Lone Pine, Califor- Cole, R. 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