Stratigraphy and paleogeographic significance of metamorphic rocks in the Shadow Mountains, western Mojave Desert, California

Mark W. Martin* Isotope Geochemistry Laboratory and Department of Geology, University of Kansas, J. Douglas Walker } Lawrence, Kansas 66045

ABSTRACT work of the Late Proterozoic to early Mesozoic tectonic develop- ment of the southwestern Cordillera. In addition, the affinity of the Stratigraphic correlations presented here for the ductilely de- Shadow Mountain strata is critical to evaluating models for the formed and metamorphosed rocks exposed in the Shadow Moun- Mesozoic and Paleozoic paleogeography of the southwestern Cor- tains indicate that they formed on the North American continental dillera, which call for major tectonic boundaries to be present in the margin and are not exotic or significantly displaced from their site area (e.g., Walker, 1988; Stone and Stevens, 1988; Lahren et al., of origin. These strata represent a depositional history that spans 1990). Late Proterozoic and Paleozoic passive margin development, late The significance of the Shadow Mountains stratigraphy for un- Paleozoic transitional passive to active plate margin tectonics, and derstanding the regional geology can best be appreciated by review- late Paleozoic–early Mesozoic establishment of a convergent mar- ing our current knowledge of the Proterozoic and Paleozoic tectonic gin along the western edge of the North American craton. history of this region. The Late Proterozoic and Paleozoic western The stratigraphic sequence in the Shadow Mountains is rep- margin of North America, as defined by cratonal, miogeoclinal, and resented by a basal siliceous and calcareous section that is corre- eugeoclinal facies patterns, generally trends northeast (Fig. 1; lated with upper Proterozoic and Cambrian miogeoclinal strata. Burchfiel and Davis, 1975, 1981). Paleozoic miogeoclinal and cra- These strata are overlain unconformably(?) by a calcareous se- tonal facies project southwestward from southern Nevada and quence correlated with rocks of Pennsylvanian and Permian age of southeastern California into the Mojave Desert, where they inter- borderland affinity. The uppermost sequence comprises hornfels sect the northwest-trending Mesozoic plutonic arc at a high angle and calcareous rocks that rest unconformably across older strata (Hamilton and Myers, 1966; Burchfiel and Davis, 1975; Stewart and and are correlated with Permian and Triassic rocks that likely Poole, 1975). Rocks of eugeoclinal character occur from the El Paso record uplift and erosion of a magmatic arc. Mountains to Lane Mountain (Fig. 2; Poole, 1974; Carr et al., 1981; These stratigraphic correlations have several important Wust, 1981; Miller and Sutter, 1982). A notable observation from paleogeographic and tectonic implications for southwestern North these studies is that Paleozoic eugeoclinal rocks are juxtaposed America. First, passive margin, Late Proterozoic and early Paleo- against like-age transitional miogeoclinal-cratonal rocks with the zoic miogeoclinal facies extend as far west in the Mojave Desert as absence of intervening thick miogeoclinal sequences and middle the Shadow Mountains. Second, the presence of Pennsylvanian and Paleozoic clastic rocks related to the Antler orogeny (Walker, 1988; Permian borderland basin sediments suggests that the area was Martin and Walker, 1991, 1992). Both the extent and tectonic sig- affected by late Paleozoic tectonics, probably associated with the nificance of these eugeoclinal rocks in the Mojave Desert have been transition from a passive to an active plate-margin setting. Third, discussed and questioned by many workers. Davis et al. (1978) and inferred Late Permian–Triassic rocks record the onset of conver- Burchfiel and Davis (1981) attributed the presence of eugeoclinal gent-margin tectonism and magmatism in this region. rocks to strike-slip faulting during Permo-Triassic truncation of the continental margin. Alternatively, Poole (1974) considered the eu- INTRODUCTION geoclinal rocks to be essentially in place, but telescoped with mio- geoclinal rocks by late Paleozoic and Mesozoic thrusting. Various The Shadow Mountains comprise the largest exposure of meta- combinations of these interpretations have been utilized by later sedimentary rock in the central and western Mojave Desert. Previ- workers (e.g., Stone and Stevens, 1988; Walker, 1988; Snow, 1992). ous workers interpreted these rocks to range from Late Proterozoic The main obstacles to interpreting the nature of this juxtaposition to Mesozoic in age (Bowen, 1954; Troxel and Gunderson, 1970; have been (1) the difficulty in determining the age and paleogeo- Poole, 1974; Burchfiel and Davis, 1975, 1981; Miller and Cameron, graphic affinity of many exposures across the central Mojave Desert 1982; Brown, 1983). Despite this earlier work, no consensus exists due to degree of metamorphism and deformation associated with for either the age or correlation of these strata. A better under- development of the Mesozoic Cordilleran arc and (2) the lack of standing of these rocks is necessary to place them within the frame- exposures of contacts between known eugeoclinal and miogeoclinal- cratonal rocks within the same area. Since late Paleozoic–Early Triassic time, the central and west- *Present address: Servicio Nacional de Geologı´a y Minerı´a-Chile, ern Mojave has been along an active plate margin. As a result of Grupo de Geologı´a Regional, Avda. Santa Marı´a 0104, Casilla 1347, San- deformation and magmatism associated with this tectonic setting, tiago, Chile. pre-Mesozoic paleogeographic and tectonic trends present within

GSA Bulletin; March 1995; vol. 107; no. 3; p. 354–366; 10 figures; 1 table.

354 METAMORPHIC ROCKS, SHADOW MOUNTAINS, CALIFORNIA

Figure 1. Location map for the Mo- jave Desert and surrounding regions, with Paleozoic facies and tectonic trends. Note that the position of the inferred hinge line in the western Mojave Desert is approximate. The two heavy-stippled areas represent the Sierra Nevada, north of the Garlock fault, and the Peninsular Range, south of the San Andreas fault. SAF, San Andreas fault; GF, Garlock fault; MVF, Mojave Valley Fault (Martin et al., 1993).

and west of the central Mojave Desert are not preserved or are ered related to attenuation and ductile shearing associated with strongly obscured. To determine what these trends were, it is par- deformation, factors such as nondeposition and/or erosion cannot amount that we understand the age and paleotectonic affinity of the be precluded (see stratigraphic descriptions, Appendix, and follow- metasedimentary rocks exposed along the western part of this mar- ing discussion). gin such as those that crop out in the Shadow Mountains. The aim Development of the D1 foliation, D2 and D3 folds, and asso- of this paper, therefore, is to present stratigraphic descriptions and ciated metamorphism is older than 148 Ma, the age of the postkin- correlations from the Shadow Mountains in order to interpret the ematic, bimodal gabbro/granite complex in the Shadow Mountains Late Proterozoic to Mesozoic paleogeographic development of the (U-Pb zircon; Martin, 1992). Intrusion of the igneous complex im- southwestern Cordillera. parted a pervasive, static greenschist- to lowest amphibolite-facies retrograde metamorphism on the older amphibolite-facies dynamo- STRUCTURE AND METAMORPHISM thermal metamorphism. U-Pb geochronology on both metamorphic and detrital zircons that have experienced high-temperature Pb loss Sedimentary rocks in the Shadow Mountains are intensely de- suggests that dynamothermal metamorphism occurred at 165 Ϯ 11 formed and record metamorphism within the amphibolite facies. Ma (Martin, 1992). Based on the structural and metamorphic re- Because deformation and metamorphism limit the stratigraphic cor- lations and the U-Pb geochronology, D1 foliation, folding, and met- relations made in this paper, a brief discussion of these features is amorphism are interpreted to have formed during a period of pro- given here. Original sedimentary layering is transposed by foliation gressive contractile deformation; however, older deformation (formed during deformation event D1) that has been folded by at cannot be precluded. Based on a similar style of ductile contractile least two subsequent phases of coaxial north-south–trending, dom- deformation and permissive timing constraints, this deformation is inantly west-vergent ductile folds. The first folding event is repre- tentatively correlated with Middle to Late Jurassic contractile de- sented by a kilometer-scale overturned, nearly recumbent anticline formation identified in numerous ranges to the northeast, between (D2) in the northern Shadow Mountains (Silver Peak anticline, the Shadow Mountains and the Garlock fault (Walker et al., 1990; Fig. 3). At least one subsequent phase of folds (D3) overprints D2 Martin, 1992; Boettcher and Walker, 1993). folds. The foliation and folds are associated with amphibolite-facies metamorphism (Martin, 1992). Because of deformation, a signifi- STRATIGRAPHIC CORRELATION cant amount of shearing has occurred along lithologic contacts. This is particularly pronounced in the northern Shadow Mountains Metasedimentary rocks in the Shadow Mountains can be di- around the limbs of the map-scale Silver Peak and Shadow Valley vided into three assemblages, which can be further subdivided into anticlines (Fig. 3); lithologic units are discontinuous around these two or more map units (Fig. 4). The areal distribution of these units folds. Although the discontinuous nature of these units is consid- is shown in Figure 3. In the following section we present possible

Geological Society of America Bulletin, March 1995 355 MARTIN AND WALKER

Figure 2. Map of pre-Tertiary rocks in the central and western Mojave Desert. Mojave Valley fault (MVF) is marked by a dashed line; offset by numerous northwest-trending, late Cenozoic strike-slip faults is not shown. Map modified from Martin et al. (1993, Fig. 1). correlations for each of these stratigraphic assemblages. A detailed other studies (Kiser, 1981; Brown, 1983, 1991; Glazner et al., 1988; description of these units and their thicknesses is given in the Martin and Walker, 1991; Boettcher and Walker, 1993). We cor-

Appendix. relate quartz-biotite schists of unit –C1 with the Wood Canyon For- mation and the overlying quartzite of unit –C2 with the Zabriskie Assemblage 1 Quartzite (Fig. 3). Detrital zircon U-Pb geochronology and whole-

rock Nd and Sr isotopic studies on a meta-arkose bed from unit –C2 ε 87 86 Assemblage 1 is subdivided into five map units (Fig. 3): unit –C1 support a 1.7 Ga provenance age ( Nd ϭ –18, Sr/ Sr ϭ 0.7376), consists of quartz-biotite schist containing minor quartzite; unit –C2, in agreement with 1.35 and 1.8 Ga lower to middle Proterozoic pink to white quartzite and meta-arkose with minor pebble to cobble basement in the area (Martin and Walker, 1992) and the Zabriskie conglomerate and quartz-biotite schist; unit –C3, thin- to medium- Quartzite correlation. The calcareous rocks of unit –C3 are corre- layered siliceous calcite marble, calc-silicate rocks, and metasiltite lated with the Cambrian Carrara Formation. with minor dolomite marble and quartzite; unit –C4, massive dolo- Brown (1983) correlated rocks of unit –C4 with the Bonanza mite marble and, as mapped, structurally interleaved layers of sili- King Formation (Fig. 3), a correlation with which we concur. Al- ceous and buff colored calcite marble; and unit p–Cu, white to tan though not separated during mapping, siliceous and buff-colored dolomite marble intercalated with and overlain by compositionally marble layers transposed within and above the Bonanza King For- variable quartzites, meta-arkose, and quartz-biotite schist. mation may represent the Dunderberg Shale and Nopah Formation, Units of assemblage 1 are correlated with the upper Protero- and perhaps even Devonian and Mississippian strata. A thin middle zoic to Cambrian miogeoclinal sequence including the Wood Can- Paleozoic sequence lacking Ordovician and Silurian strata in the yon Formation, Zabriskie Quartzite, Carrara Formation, and Bo- Shadow Mountains would be consistent with the interpretation pre- nanza King Formation. This correlation is based on overall sented by Martin and Walker (1991, 1992, 1993), in which transi- stratigraphic succession and comparison with a nearly identical se- tional miogeoclinal-cratonal rocks continued west-southwestward quence exposed at Quartzite Mountain to the southeast (Fig. 2), through the central Mojave Desert. However, transposition and which has been identified as upper Proterozoic and Cambrian folding in the Shadow Mountains precludes unequivocal stratigraph- (Stewart and Poole, 1975; Miller, 1981), as well as on numerous ic determinations for these rocks.

356 Geological Society of America Bulletin, March 1995 METAMORPHIC ROCKS, SHADOW MOUNTAINS, CALIFORNIA

Figure 3. Simplified geologic map of the Shadow Mountains showing the distribution of stratigraphic units discussed in the text. Map modified from Martin (1992, Plate 1).

Assignment of unit p–Cu is problematic (Fig. 3). This unit is tercalated with and overlain by compositionally variable quartzites, lithologically dissimilar to the overlying assemblages 2 and 3, and to meta-arkose, and quartz-biotite schist is most similar to upper Prot- other middle Paleozoic to Mesozoic sequences in the Mojave erozoic to Cambrian rocks of the Johnnie Formation, Stirling Desert. The gross succession of white to tan dolomite marble in- Quartzite, and the lower to middle Wood Canyon Formation

Geological Society of America Bulletin, March 1995 357 MARTIN AND WALKER

Figure 4. Diagrammatic stratigraphic column of the Shadow Mountains and explanation. Sections A and B represent transects across the eastern and western halves of the Silver Peak anticline, respectively, in the northern Shadow Mountains (Fig. 3). Section C represents a transect taken in the southwestern Shadow Mountains (Fig. 3).

(Stewart, 1970). This correlation is favored here and is consistent Miller and Cameron (1982) noted similarities of the stratigra- with this unit occupying the lowest apparent stratigraphic position phy in the southern Shadow Mountains (assemblage 2) and upper in the northern Shadow Mountains, although exact correlation is Paleozoic rocks in the Lane Mountain–Goldstone area, El Paso precluded by extreme deformation. Mountains, and the Inyo Mountains (Fig. 1). On the basis of our new interpretations (Appendix), such as the likely existence of rocks of Assemblage 2 mass-flow origin in assemblage 2 and the limited fossil control (see above), we concur with the assessment made by Miller and Cameron The rocks of assemblage 2 are generally well exposed and have (1982). Basinal calcareous turbidites in the Inyo Mountains and been divided into four map units: Unit PP1 consists of thin- to me- Darwin Hills (Fig. 5) are of Pennsylvanian and Permian age dium-layered calcite marble that contains numerous sedimentary (Stevens and Stone, 1988). These strata are said to have ‘‘border- structures indicative of mass-flow deposits; unit PP2, orange to land’’ affinity because they were deposited in isolated, rapidly sub- brown schistose feldspathic quartzite; unit PP3, white to tan massive siding and shoaling basins interpreted to have been related to left- dolomite to calcite marble; and unit PP4, a heterogeneous mix of lateral strike-slip and thrust faults associated with truncation of the various types of calcareous marbles, locally containing sedimentary southwest margin of North America during Pennsylvanian and Per- structures indicative of mass-flow deposits. mian time (Stone and Stevens, 1984, 1988; Stevens and Stone, 1988; Two poorly preserved brachiopods suggestive of a Pennsylva- Walker, 1988). nian age have been the only known fossils collected from the Besides lithologic similarities to rocks of borderland affinity, Shadow Mountains. These fossils were collected by Bowen (1954) the regional stratigraphic occurrence of these rocks is also important from the southeastern part of the area (W1⁄2 sec. 25, T. 7 N., R. 6 W.; for our correlation of assemblage 2 rocks. Rocks of borderland af-

Dibblee, 1967) and appear to have been taken from unit PP1–3 of finity rest conformably on Lower Pennsylvanian and Mississippian this assemblage (Fig. 3). Thirty-four samples were collected during miogeoclinal rocks in the southern Inyo Mountains and Darwin this study from various marble beds throughout assemblages 2 and Hills (Fig. 5; Stone and Stevens, 1988). In the Soda Mountains, 3 in an effort to recover conodonts. All samples were barren (B. R. Lower Permian borderland sedimentary rocks rest conformably on Wardlaw, 1992, personal commun.), probably due to the high de- transitional miogeoclinal-cratonal Pennsylvanian and Mississippian gree of metamorphic recrystallization. rocks (Figs. 2 and 5; Walker and Wardlaw, 1989). Lower Permian

358 Geological Society of America Bulletin, March 1995 elgclSceyo mrc ultn ac 1995 March Bulletin, America of Society Geological

Figure 5. Correlation diagram incorporating composite sections from the El Paso Mountains (Carr et al., 1984), Quartzite Mountain area (Miller, 1981), Soda Mountains (Walker and Wardlaw, 1989), southern Inyo Mountains and Darwin Plateau area (Stone and Stevens, 1984). Figure modified from Walker (1988). 359 MARTIN AND WALKER

TABLE 1. ANALYTIC DATA FOR EL PASO MOUNTAIN ANDESITES

Fraction Sample U 206Pb* Measured ratios†,§ Radiogenic ratios§ Ages§ (Ma) wt. (mg) (ppm) (ppm) 206Pb/204Pb 207Pb/206Pb 208Pb/206Pb 206Pb*/238U 207Pb*/235U 207Pb*/206Pb* 206Pb*/238U 207Pb*/235U 207Pb*/206Pb*

And-A. Location: 35Њ 21Ј 20Љ N, 117Њ 56Ј 55Љ W nm(Ϫ1) 4.415 409.7 15.16 4419 (31) 0.05522 (3) 0.1532 (2) 0.04308 (20) 0.3083 (14) 0.05190 (4) 271.9 272.9 281.2 (2) nm(Ϫ1) aa 2.532 402.8 15.02 6713 (130) 0.05417 (7) 0.1537 (2) 0.04341 (21) 0.3112 (16) 0.05198 (8) 274.0 275.1 284.6 (4) nm(0) 3.704 455.6 16.75 4317 (33) 0.05529 (3) 0.1596 (2) 0.04279 (20) 0.3062 (14) 0.05190 (4) 270.1 271.2 280.9 (2) nm(0) (aa) 4.748 408.8 15.25 7953 (95) 0.05366 (3) 0.1548 (2) 0.04343 (20) 0.3103 (14) 0.05182 (4) 274.0 274.4 277.3 (2)

And-D. Location: 35Њ 26Ј 10Љ N, 117Њ 45Ј 15Љ W nm(0)Ͻ240 0.731 473.6 17.36 3780 (23) 0.05631 (3) 0.1627 (2) 0.04267 (21) 0.3084 (15) 0.05243 (4) 269.3 272.9 304.0 (2) nm(0)Ͼ240 1.158 409.5 15.41 4436 (38) 0.05648 (3) 0.1547 (2) 0.04380 (16) 0.3212 (16) 0.05318 (5) 276.4 282.8 336.4 (2) nm(1)Ͻ240 1.278 443.2 16.37 3445 (33) 0.05698 (3) 0.1669 (2) 0.04301 (20) 0.3127 (15) 0.05272 (5) 271.5 276.2 316.9 (2) nm(2)Ͻ240 aa 0.897 463.6 17.87 869.4 (6.1) 0.07094 (8) 0.2069 (2) 0.04487 (23) 0.3348 (20) 0.05412 (15) 282.9 293.2 376.1 (6)

Note: nm(#) ϭ nonmagnetic on Frantz separator at angle of tilt # degrees; Ͼ240 ϭ size in standard mesh; aa ϭ air abraded fraction. 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. Air abrasion followed the methods of Krogh (1982). Decay constants used were 238U ϭ 0.15513 ϫ 10Ϫ9 yrϪ1 and 235U ϭ 0.98485 ϫ 10Ϫ9 yrϪ1 (Steiger and Ja¨ger, 1977). Isotopic analyses were determined on a VG Sector multicollector thermal ionization mass spectrometer. A mass fractionation correction of 0.10% Ϯ 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. Errors on 206Pb/204Pb were minimized by use of a Daly multiplier. Common lead corrections were made using values determined from Stacey and Kramers (1975) for the interpreted crystallization age. *Radiogenic component. †Ratios corrected for spike and mass fractionation only. §Numbers in parentheses are analytical errors. For measured and radiogenic ratios this is the 2-␴ value for the ratio, e.g., 0.1750 (15) means 0.1750 Ϯ 0.0015. For the 207Pb/206Pb age the value is in m.y. Errors were computed using data reduction program PBDAT of Ludwig (1989).

turbidites in these areas are unconformably overlain by Upper Per- tains with miogeoclinal and/or transitional miogeoclinal-cratonal mian to Lower Triassic strata (Fig. 5). In the El Paso Mountains strata is consistent with regional stratigraphic relationships. (Figs. 2 and 5), Ordovician and Devonian eugeoclinal rocks (Antler belt) and Mississippian Antler foreland rocks are deformed and Assemblage 3 overlain by Pennsylvanian and Permian rocks (Poole, 1974; Carr et al., 1981, 1984). Pennsylvanian strata in this area comprise shal- Assemblage 3 comprises two units: PT1 and PT2 (Fig. 4). Unit low-water carbonates that grade upward into Pennsylvanian and PT1 consists of massive hornfels containing lesser amounts of Lower Permian turbidites (Carr et al., 1984). Lower Permian rocks quartzite, schistose feldspathic quartzite, calc-silicate rocks, calcite contain Guadalupian to Wolfcampian(?) mass-flow and olisto- marble, and talc-chlorite schist. Unit PT1 is gradational into unit strome deposits. These rocks are overlain by volcanogenic debris PT2 (Fig. 4), which is composed of calcite marble, calc-silicate rocks, and andesitic volcanic rocks of late Early to Late Permian age (Carr and lesser amounts of siliceous hornfels, quartz-biotite schist, and et al., 1984; U-Pb data presented below). Similar successions as quartzite. Unit PT1 protolith was made up predominantly of imma- described for the El Paso Mountains are probably also exposed at ture fine- to coarse-grained clastic rocks with minor amounts of Pilot Knob, Goldstone, Paradise Range, and Lane Mountain in the marine carbonate rocks. Although equivocal and sparse, rocks of north-central Mojave Desert (Fig. 2; McCulloh, 1952, 1960; Smith volcanic origin may also be present. Unit PT2 is dominantly marine and Ketner, 1970; Poole, 1974; Carr et al., 1981, 1984, 1992; Burch- in origin. Assemblage 3 is interpreted to rest unconformably on fiel et al., 1980; Wust, 1981; Miller and Sutter, 1982). assemblage 2 and possibly also on assemblage 1 (Appendix).

Neither Pennsylvanian nor Permian rocks have been recog- Our interpretation is that units PT1 and PT2 of assemblage 3 nized in any of the transitional miogeoclinal-cratonal metasedimen- (Fig. 3) correlate with Upper Permian rocks in the El Paso Moun- tary sequences between the Soda Mountains and the Shadow Moun- tains and Lower Triassic rocks in the central Mojave Desert. To tains (Martin and Walker, 1991). However, in the Quartzite appreciate this correlation we must discuss the stratigraphic rela- Mountain area, Lower Triassic(?) conglomerate containing Per- tions in the El Paso Mountains. In addition, because relations in the mian fossiliferous limestone clasts rests unconformably on a 241 Ma Shadow Mountains only bracket these rocks to be between Penn- monzonite and deformed Mississippian and older Paleozoic and sylvanian-Permian and Middle Jurassic in age, we present new age Proterozoic rocks (Fig. 5; Miller, 1981; Walker, 1987). These rela- data for El Paso Mountains strata that helps establish the age of tions indicate that Permian rocks were once present in the area in assemblage 3 rocks. order to have been redeposited into the Triassic conglomerate. To In the central and eastern El Paso Mountains, the succession the south in the San Bernardino Mountains, Pennsylvanian and Per- from Pennsylvanian and Lower Permian borderland rocks gives way mian Bird Spring Formation (nonborderland affinity) rests uncon- upward to Upper Permian volcanic rocks (Carr et al., 1984); this formably on the Mississippian Monte Cristo Limestone (Brown, transition is considered the earliest indication of the Sierran arc 1991). (Burchfiel and Davis, 1981; Miller and Sutter, 1982; Carr et al., 1984; Although the mechanism by which eugeoclinal and transitional Walker, 1988; Burchfiel et al., 1993). In the Goler Gulch area this miogeoclinal-cratonal rocks were juxtaposed is debated (Stevens transition is marked by an abrupt change from borderland carbon- and Stone, 1988; Walker, 1988; Snow, 1992), it is clear that Penn- ate rocks to a thick succession of andesite flows. In the Iron Canyon sylvanian and Permian borderland sedimentation was associated area, west of Goler Gulch, the contact between borderland sedi- with both allochthonous and autochthonous strata (Stevens and ments and andesite flows is gradational across a succession of vol- Stone, 1988; Walker, 1988). Therefore, aside from their overall lith- canic and volcanogenic rocks with intercalated carbonates (Fig. 5; ologic similarity to borderland rocks, the association of inferred Carr et al., 1984). Pennsylvanian and Permian borderland strata in the Shadow Moun- A similar succession exposed in the western El Paso Mountains,

360 Geological Society of America Bulletin, March 1995 METAMORPHIC ROCKS, SHADOW MOUNTAINS, CALIFORNIA

sidered equivalent to the andesites described above (Carr et al., 1984). To determine the age of the post–Lower Permian stratigraphic successions in the El Paso Mountains, two volcanic samples were collected for U-Pb zircon geochronology: Andesite-A and Ande- site-D (Table 1, Fig. 6). Andesite-D was collected from the base of the volcanic section in Goler Gulch and yields an early Late Permian age of 262 Ϯ 2 Ma (Model 1 of Ludwig, 1990). This age is consistent with the age of underlying Lower Permian turbidites (Carr et al., 1984) and establishes a minimum age for the volcanic sequence exposed in the central and eastern El Paso Mountains. Andesite-A was collected from the Bond Buyer Sequence exposed in the west- ern El Paso Mountains and yields an Early Permian age of 281 Ϯ 8 Ma (Fig. 6; Model 1 of Ludwig, 1990, and forced through 0 Ma to average the Pb/Pb ages). This latter age implies that the Bond Buyer Sequence repre- sents yet another transitional section from the borderland to the arc succession. This age, combined with ages from nearby plutons, makes it unlikely that the Bond Buyer correlates with the Fairview Valley Formation or any other Lower Triassic rocks in the Mojave Desert region (e.g., Walker, 1988); rather, it is entirely an upper Paleozoic succession (e.g., as interpreted by Carr et al., 1984). We consider the transition from Pennsylvanian and Lower Per- mian borderland rocks to Upper Permian arc-derived volcanic and sedimentary rocks with intercalated marine carbonates in the cen- tral and eastern El Paso Mountains to be grossly similar to the transition from assemblage 2 calcareous mass-flow deposits to as-

semblage 3 siliceous hornfels and intercalated marbles (unit PT1)in the Shadow Mountains (Figs. 4 and 5). Although the presence of volcanic rocks in assemblage 3 is equivocal, it is permissible for the siliceous hornfels of assemblage 3 to be arc-derived clastic rocks. The angular discordance between assemblages 2 and 3 described here is not observed across the El Paso Mountain transition. How- ever, considering the differences between the two sections described in the central and eastern El Paso Mountains (discussed above), we do not believe that exact lithologic or stratigraphic similarities should be expected between ranges. The following paleogeographic setting is proposed for arc-de-

rived units of assemblage 3. Meta-clastic rocks of unit PT1 are cor- related with Upper Permian volcanogenic and volcanic rocks in the central El Paso Mountains (Fig. 5) and probably also in the Gold- Figure 6. U-Pb concordia diagram for samples Andesite-A and stone and Lane Mountain area. This unit represents Upper Permian Andesite-D. Sample locations and analytic data are presented in clastic rocks derived from the proto-Sierran magmatic arc and de- Table 1. posited in a marine or near-marine environment. Local and/or re- gional uplift associated with arc development and associated defor- mation accounts for the angular unconformity between assemblage 3 and older strata. the Bond Buyer Sequence, contains, in ascending order, andesitic Although a reasonable maximum age for unit PT2 has not been flows, clastic, and calc-silicate rocks that are intruded locally by established, it must be older than 165 Ma (age of metamorphism in

Early to Middle Triassic granodiorites (227–239 Ma, Cox and Mor- the Shadow Mountains, Martin, 1992). Because unit PT2 appears ton, 1986, written commun.; 246 Ma, Miller et al., 1993, unpub. data; lithologically gradational with unit PT1 (Appendix), we infer that it Walker, 1988). Unfortunately, the correlation of the Bond Buyer was deposited relatively close in time to unit PT1. Based on this Sequence with the more intact upper Paleozoic succession in the inference, marbles and calc-silicate rocks of unit PT2 suggest either central El Paso Mountains is uncertain (Carr et al., 1984; Walker, the diversion of arc sediments (represented by unit PT1)orthe 1988). The clastic and calc-silicate rocks in the Bond Buyer Se- erosion of the arc to near sea level by this time. In this context, unit quence have been correlated with rocks of similar composition in PT2 is tentatively correlated with the Lower Triassic overlap se- the Goldstone–Lane Mountain area, which have been correlated quence, the Fairview Valley Formation, exposed in the Quartzite with the Lower Triassic Fairview Valley Formation, exposed in the Mountain area, and correlative rocks in the Lane Mountain–Gold- Quartzite Mountain area (Fig. 5; Burchfiel et al., 1980; Walker et al., stone area and Soda Mountains (Fig. 5; Burchfiel et al., 1980; Miller, 1984; Walker, 1988). Alternatively, the Bond Buyer has been con- 1981; Walker et al., 1984; Walker, 1988).

Geological Society of America Bulletin, March 1995 361 MARTIN AND WALKER

Figure 7. Late Proterozoic to early Mesozoic development of the western Mojave Desert region. State boundaries and the San Andreas and Garlock faults are shown in present-day coordinates for reference only. Antler belt rocks displaced into the El Paso Mountains–Lane Mountain area in the Permian contain lower to middle Paleozoic eugeoclinal facies as well as Mississippian Antler foreland and overlap rocks (Carr et al., 1981). Continuation of trends and structures to the west of the Mojave Desert and San Andreas fault are uncertain.

DISCUSSION rocks and Antler foreland deposits with miogeoclinal-cratonal rocks in the Mojave Desert (Burchfiel and Davis, 1975; Poole and Chris- Figure 7 represents a series of time slices that show schemat- tiansen, 1980), and (3) the tectonic setting during the deposition of ically the paleogeographic and tectonic setting in the region of the Pennsylvanian-Permian borderland rocks defined by Stone and central and western Mojave Desert from Late Proterozoic to early Stevens (1984) and Stevens and Stone (1988). Mesozoic time. The paleogeography presented in Figure 7 incor- Late Proterozoic to Mississippian paleogeography in the Mo- porates many aspects of earlier models that call on late Paleozoic– jave Desert region was characterized by a northwest-facing passive Early Triassic truncation of the southwestern Cordillera to explain margin (Fig. 7A). On the basis of regional stratigraphic data dis- (1) the angular discordance between the Late Proterozoic–Paleo- cussed by Martin and Walker (1991, 1992, 1993), we extend the zoic facies trends and the trend of later Mesozoic features (Burchfiel miogeoclinal-cratonal hinge line (defined as those sections where and Davis, 1975; Walker, 1988), (2) the juxtaposition of eugeoclinal lower to middle Paleozoic sequences are relatively thin or missing,

362 Geological Society of America Bulletin, March 1995 METAMORPHIC ROCKS, SHADOW MOUNTAINS, CALIFORNIA

Figure 10. Photo of thinly layered calcite and siliceous calcite Figure 8. Angular to slightly rounded carbonate clast conglom- marbles from unit PP1, assemblage 2 (similar rocks found in unit erate from unit PP1, assemblage 2 (similar rocks found in unit PP4, PP , assemblage 2). 35 mm lens cap for scale. assemblage 2). Sharpie marker for scale. 4 but upper Precambrian to Cambrian sections are relatively well de- during this time period Antler belt rocks moved southward along a veloped) through the Mojave Desert at least as far west as the south-southeast–striking, sinistral strike-slip fault or fault system, Shadow Mountains. The Mississippian Antler orogenic belt paral- truncating older northeast-striking passive margin facies patterns. lels Paleozoic facies trends through central Nevada into central Cal- Transpressional and/or transtensional structures associated with ifornia. This interpretation implies that these facies and tectonic this tectonic setting most likely resulted in local uplift and basin trends continued to the southwest. formation (e.g., Stone and Stevens, 1988; Stevens and Stone, 1988). Pennsylvanian and Early Permian time was a period of transi- Pennsylvanian and Permian borderland sediments (diagonal ruling tion from passive to active plate-margin setting along the south- in Figs. 7B and 7C) were deposited in basins associated with both western Cordillera (Figs. 7B and 7C). Following Walker (1988), allochthonous Antler belt rocks (El Paso Mountains and areas in the north-central Mojave Desert) and autochthonous miogeoclinal and transitional miogeoclinal-cratonal rocks (Inyo Mountains, Darwin Hills, Soda Mountains, and Shadow Mountains). In the central and western Mojave Desert, local uplift and erosion associated with this tectonism may have caused erosion of preexisting Paleozoic strata, which may in part explain missing section (not ignoring structural modification) between assemblages 1 and 2 in the Shadow Moun- tains. Observations and interpretations presented here for the Shadow Mountains imply that the central and western portions of the Mojave Desert were affected by late Paleozoic basin formation and deformation. Considering older facies trends, this suggests en- croachment of these modifications into more interior parts of the Cordillera and implies that the major structure(s) that truncates the margin must lie outboard of the Shadow Mountains. By Late Permian time (Fig. 7D), allochthonous Antler belt rocks docked with Paleozoic transitional miogeoclinal cratonal rocks in the Mojave Desert, in part explaining the absence of a fully developed miogeoclinal facies in the central and western Mojave Desert. Proximal to a marine environment, magmatic arc activity and associated contractile deformation began in the Mojave Desert area by Late Permian time with the onset of convergent margin activity (diagonal ruling, Fig. 7D). Rocks belonging to assemblage 3 and the apparent angular discordance between assemblages 2 and 3 in the Shadow Mountains are interpreted to record the uplift and erosion of this arc, probably in a marine or near-marine environment.

Figure 9. Recrystallized grainstone from unit PP1, assemblage Snow (1992) proposed an alternative model that calls on the 2. Note layering and fining(?) upwards texture. Mechanical pencil Permian(?) Last Chance Thrust System to explain late Paleozoic for scale. structural and stratigraphic relations in the region west of Death

Geological Society of America Bulletin, March 1995 363 MARTIN AND WALKER

Valley and in the Mojave Desert. Although our observations pre- that locally contains thinly layered, medium-grained quartzite. Unit –C1 is – sented here do not contradict this model, the truncation model re- conformably overlain by unit C2 (Fig. 3), which consists of 30–40 m of lies on fewer inferred structural projections (southward bend of the coarse- to fine-grained, pink to white quartzite and meta-arkose. This unit contains discontinuous layers of granite-pebble to cobble conglomerate, and Antler belt and southern continuation of the Last Chance Thrust minor amounts of quartz-biotite schist. Possible relict Skolithos tubes were

System into the Mojave Desert) and changes in facies trends (mio- observed in one quartzite bed. Unit –C3 (Fig. 3) rests conformably on unit –C2 geocline becoming narrower southward into the Mojave Desert). In and consists of thin- to medium-layered, siliceous calcite marble, calc-silicate addition, east- or southeast vergent structures that are present in the rocks, and metasiltite with minor dolomite marble. The upper portion of this calcareous sequence contains thin, laterally extensive, fine- to medium- Mojave Desert are demonstrably Middle to Late Jurassic (Walker grained green and pink quartzite and quartz-biotite schist horizons. The et al., 1990; Boettcher and Walker, 1993; Glazner et al., 1994) and thickness of unit –C3 is somewhat variable due to ductile attenuation and not late Paleozoic (Snow, 1992) in age. Finally, U-Pb geochronology shearing related to folding, but the maximum thickness is ϳ120 m. – and isotopic data from Permian to Mesozoic plutonic rocks suggest Unit C4 (Fig. 3) consists of massive dolomite marble ϳ120–150 m thick. that eugeoclinal rocks in the Mojave Desert area cannot have been Structurally interleaved within the dolomite marble are laterally discontin- uous layers of siliceous and buff-colored calcite marble that are included thrust eastward over Proterozoic continental crust (as implied by within unit –C4 because of their transposition with the dolomite marble. The Snow, 1992) until between Middle Triassic and Middle Jurassic time contact between unit –C3 and unit –C4 has been strongly modified due to the (Miller et al., 1993, unpub. data). competence contrast between these two units and attenuation related to ductile folding. As a result, unit –C4 is only present along the large rootless SUMMARY parasitic fold (Shadow Valley anticline) in the upright limb of the Silver Peak anticline (Fig. 3; Martin, 1992). In addition to this sequence of four units, a sequence of rocks contain- The correlations presented here for the Shadow Mountain stra- ing, in ascending structural order, white to tan dolomite marble with thin tigraphy help elucidate the Late Proterozoic to early Mesozoic pa- interlayered quartzite and meta-arkose beds, quartz-biotite-staurolite schist leogeography of the central and western Mojave Desert. Assem- and dark-green to blue, medium- to coarse-grained (locally pebbly) mica- blage 1 rocks are correlated with Late Proterozoic–Cambrian ceous (locally calcareous) and amphibole-bearing quartzite, and pink to white quartzite and quartz-pebble quartzite is exposed in the eastern Silver miogeoclinal rocks indicative of passive margin development. As- Peak area (unit p–Cu, Fig. 3). This sequence of rocks forms the core of the semblage 2 is correlated with Pennsylvanian to Early Permian bor- overturned anticline in an area that is complicated by folding and structural derland sedimentary rocks. This correlation implies that deforma- attenuation of these rocks. This group of rocks has a combined thickness of – tion associated with the deposition of these rocks had encroached on ϳ300–400 m and is designated unit pCu. interior parts of the Cordillera. Siliceous hornfels, marbles, and Assemblage 2 calc-silicate rocks of assemblage 3 are correlated with Late Permian and Triassic sequences elsewhere in the region that represent the Description. Rocks assigned to assemblage 2 consist of calcite marble, development of a volcano-magmatic arc in or near a marine envi- conglomeratic marble, siliceous calcite marble, calc-silicate, dolomite mar- ronment. This sequence represents the onset of convergent margin ble, quartzite, and meta-laminite. Assemblage 2 (units PP1–4, Fig. 3) is widely exposed in the southern Shadow Mountains. In the northern Shadow Moun- tectonics in this part of the Cordillera. These correlations indicate tains, a thin sequence assigned to assemblage 2 lies between Cambrian Car- that autochthonous continental North America extends westward rara Formation and assemblage 3 rocks and in high-angle fault contact from the eastern Mojave Desert to the Shadow Mountains. against Bonanza King Dolomite (Fig. 3). Rocks in the southern Shadow Mountains are at lower metamorphic grade and less deformed than those in ACKNOWLEDGMENTS the northern Shadow Mountains (although the base of assemblage 2 is not exposed). For this reason, the following description is based mainly on the southern exposures where assemblage 2 can be divided into four units (Fig. 4

Support for our research in the Mojave Desert has come from and units PP1–4 of Fig. 3). National Science Foundation grants EAR-8816628 and EAR-8916802 Unit PP1 (Fig. 3) is composed of thin- to medium-layered, dark blue to awarded to J. D. Walker, and from a Sigma Xi research grant and gray, resistant calcite marble. Within this unit are 0.5- to 2.0-m-thick layers Shell Oil Company Graduate Fellowship awarded (1990–1991) to of angular carbonate-pebble to cobble-conglomerate overlain by metagrain- stones and finer-grained, laminated marbles. These fining-upward variations M. W. Martin. Acknowledgment is made to the donors of the Pe- in grain size generally occur within the same bed. Clasts within conglomerate troleum Research Fund, administered by the American Chemical beds are Ͻ0.3 m in diameter and are composed of white and gray calcite Society, for partial support of this research. We thank J. M. Bartley, marble (Fig. 8). Metagrainstone beds commonly preserve apparent fining D. S. Coleman, R. P. Fillmore, J. M. Fletcher, A. F. Glazner, M. M. upward trends (Fig. 9). Although recrystallized, these grainstone horizons locally contain possible relict fusulinids and/or crinoid ossicles. Locally, fin- Lahren, J. S. Miller, E. R. Schermer, and R. A. Schweickert for their er-grained, thinly laminated siliceous marbles rest on grainstone beds insights on Mojave geology. In addition, we thank E. L. Miller, F. G. (Fig. 10). Unit PP1 also contains 2- to 10-m-thick layers of alternating thinly Poole, R. A. Schweickert, and P. Stone for critical and helpful re- layered to platy, siliceous calcite marble and calcite marble as well as local, views of earlier versions of this manuscript. Editorial comments pure white quartzite or calcareous quartzite layers (Ͻ0.5 m thick). Several from Mischa L. Enos are greatly appreciated. white calcite marble beds contain rounded, medium-grained quartz grains. Another distinctive, but rare rock type is brown to dark green, quartz-mica- amphibole-andalusite hornfels and schist similar to some rocks of assem- APPENDIX: STRATIGRAPHIC DESCRIPTION blage 3 (described below). The mineralogy suggests the protolith was an

Al-rich pelite. Unit PP1 has a minimum thickness of 500 m; the base is not Note: All thicknesses reported for the units described in the Shadow exposed.

Mountains are tectonic. Unit PP2 (Fig. 3) rests on unit PP1 and consists of orange to brown, thin- to medium-layered, medium-grained, schistose feldspathic quartzite; this

Assemblage 1 unit is 5–30 m thick (Fig. 4). The contact between units PP1 and PP2 is gradational over a few meters from siliceous calcite marble to metacalcar-

Description. Assemblage 1 is exposed in a nearly recumbent anticline in eous siltstone to meta-arkose. The upper 1–2 m of unit PP2 consists of the northern Shadow Mountains (Fig. 3) and is divided into five map units. interlayered metasiltite and calcareous marble. Unit PP2 is overlain by 20–30 Unit –C1 (Fig. 3) occurs in the core of the anticline at Silver Peak. It is 20–30 m of featureless, white to tan, massive dolomite to calcite marble of unit PP3 m thick and composed of coarse- to medium-grained, quartz-biotite schist (Figs. 3 and 4).

364 Geological Society of America Bulletin, March 1995 METAMORPHIC ROCKS, SHADOW MOUNTAINS, CALIFORNIA

Unit PP4 (Fig. 3), which overlies unit PP3 and has a minimum thickness sandstone, and graywacke. It is possible, though inconclusive from thin- of ϳ750 m, is lithologically heterogeneous. The contact between units PP3 section studies, that the talc-chlorite schists exposed in the northeastern part and PP4 is relatively abrupt and is marked by the first appearance of siliceous of the area have a volcanic protolith. material, typically represented by thinly layered, alternating siliceous marble Unit PT1 is gradational into unit PT2, which is composed of calcite and pure marble layers. In one exposure near the contact, cross bedding in marble, calc-silicate rocks, and lesser amounts of siliceous hornfels, quartz- the marble layers is truncated by thin, quartz-rich marble layers. Cross- biotite schist, and quartzite. The transition from unit 1 to unit 2 occurs over bedding in these rocks and fining-upward sequences discussed for unit PP1 several alternating 5- to 50-m-thick intervals of siliceous hornfels (unit PT1) both indicate that the section is upright. The lower half of unit PP4 consists and calcite marble and calc-silicate rocks (unit PT2). This transition is only predominantly of fine-grained, thinly layered, calcareous metasiliciclastic observed across the nose of the Silver Peak anticline in the northern Shadow rocks. Locally, metalaminite consisting of alternating thinly layered (1– 10 Mountains (Fig. 3 ). In addition, mapped lithologic units within the transition mm) marble and siliceous marble is present. Calcite-pebble conglomeratic from unit PT1 to unit PT2 pinch out along strike, although it could not be beds, although rare, also occur in this unit along with beds of orange to demonstrated if this was primarily a function of original deposition or sub- brown feldspathic quartzite, identical to the schistose, feldspathic quartzite sequent deformation. Although exposure is poor because of alluvial cover of unit PP2. Encased within the metasiliciclastic rocks are numerous brown and pervasive intrusion of Late Jurassic igneous rocks, unit PT2 becomes to dark-blue calcite marble layers (Ͻ1 m thick) as well as distinctive, alter- dominated by calc-silicate rocks up section (Fig. 4). Unit PT2 is of marine nating thin- to medium-layered, white calcite marble and siliceous marble origin. beds similar to those observed in unit PP1. Unit PP4 is dominated by thin- The basal contact of assemblage 3 with assemblage 2 in the southern to medium-layered calcite marble and calc-silicate upward. Shadow Mountains appears to have been originally discordant. This is sug- In the northern Shadow Mountains, along the limbs of the Silver Peak gested by two field relationships observed in the central Shadow Mountains anticline, a thin (5–20 m), discontinuous sequence lithologically similar to (sections 28 and 34, Fig. 3): (1) the high angle between the consistently units PP1 and PP2 of assemblage 2 (unit PPu, Fig. 3) rests on rocks correlated subvertical layering in the underlying unit PP4 of assemblage 2 (Fig. 3) and with Cambrian Carrara Formation (unit –C3, Figs. 3 and 4). Strata above and the lower-angle outcrop trace of the contact between units PP4 and PT1 below this contact are concordant on the upright limb of the anticline; (Fig. 3), and (2) the apparent local downcutting channels(?) of basal assem- around the hinge, strata inferred to belong to lower assemblage 2 are dis- blage 3 into underlying strata (section 28). This contact was interpreted by continuous and are missing on the overturned limb of the fold where as- Dibblee (1967, p. 30, sections 34, 26, and 23) as a thrust fault. However, on semblage 3 rests against Carrara Formation (unit –C3, Fig. 3). Elsewhere in the basis of observation 2 above, we consider this contact depositional. In the northern Shadow Mountains, calcite and siliceous calcite marbles of addition, in the northern Shadow Mountains, unit PT1 (Fig. 3) rests on rocks assemblage 2 lie beneath assemblage 3 and in high-angle fault contact with of assemblages 1 and 2. As discussed previously, there is ample evidence for

Bonanza King Formation (unit –C4, Fig. 3). In part, the apparent absence and structural attenuation and ductile shearing along lithologic contacts in the discontinuous nature of assemblage 2 units around the folds in the northern northern Shadow Mountains. As a result, an erosional or nondepositional Shadow Mountains is due to attenuation and ductile shearing related to unconformity along this contact (in the northern Shadow Mountains) cannot deformation, although nondeposition and/or erosion of these units prior to be uniquely demonstrated. Based on the available data, assemblage 3 is deformation cannot be precluded. interpreted to rest unconformably on assemblage 2 and possibly also assem- Interpretation. The presence of fining-upward carbonate beds (con- blage 1 rocks. glomerate to laminated marble) in unit PP1 suggests a mass-flow origin. The Because the top of assemblage 3 is not observed, the minimum exposed periodic influx of siliceous material associated with unit PP1 may suggest a structural thickness is 2000 m in the northwest Shadow Mountains (Fig. 3). complex depositional setting. The presence of sparse siliceous hornfels sim- ilar in mineralogy to that in the stratigraphically higher assemblage 3 may suggest a magmatic-volcanic arc source for these rocks (discussed below). REFERENCES CITED The absence of sedimentary structures in the schistose feldspathic Boettcher, S. S., and Walker, J. D., 1993, Geologic evolution of Iron Mountain, central Mojave Desert, quartzite of unit PP2 prevents exact determination of its depositional envi- California: Tectonics, v. 12, p. 372–386. ronment. However, the abundance of metamorphic muscovite and andalus- Bowen, O. E., Jr., 1954, Geology and mineral deposits of the Barstow quadrangle, San Bernardino ite suggests that Al-rich clays are present in the protolith and that a win- County, California: California Division of Mines Bulletin, v. 165, 208 p. Brown, H. J., 1983, Possible Cambrian miogeoclinal strata in the Shadow Mountains, western Mojave nowed beach sand or eolian sand origin can be discounted. The vertical Desert, California: Geological Society of America Abstracts with Programs, v. 15, p. 413. gradation from marble to quartzite and then back into marble suggests a Brown, H. J., 1991, Stratigraphy and paleogeographic setting of Paleozoic rocks in the San Bernardino gradual transition to a siliciclastic regime and back to carbonate deposition Mountains, California, in Cooper, J. D., and Stevens, C. H., eds., Paleozoic paleogeography of the western United States—II: Pacific Coast Section, Society of Sedimentary Geologists, Field within a marine environment. Trip Guidebook, v. 67, p. 193–207. We interpret the massive marble of unit PP as indicative of a carbon- Burchfiel, B. C., and Davis, G. A., 1975, Nature and controls of Cordilleran orogenesis, western United 3 States: Extensions of an earlier synthesis: American Journal of Science, v. 275-A, p. 363–396. ate-platform depositional environment. In unit PP4, the cross-bedded nature Burchfiel, B. C., and Davis, G. A., 1981, Mojave Desert and environs, in Ernst, W. G., ed., The of the marbles and the sharp truncation of these structures by siliceous geotectonic development of California: Englewood Cliffs, New Jersey, Prentice-Hall, marbles indicates a relatively high-energy environment. These structures p. 217–252. Burchfiel, B. C., Cameron, C. S., Guth, P. L., Spencer, J. E., Carr, M. D., Miller, E. L., and McCulloh, may be turbiditic in origin. The dominant siliciclastic component and the T. H., 1980, A Triassic overlap sequence in northern Mojave/Death Valley region, California: Geological Society of America Abstracts with Programs, v. 12, p. 395. variety of rock types associated with unit PP4, from conglomerate to meta- Burchfiel, B. C., Cowan, D. S., and Davis, G. A., 1993, Tectonic overview of the Cordilleran orogen laminite to clean marble, suggests either a tectonically active region with in the western United States, in Burchfiel, B. C., Lipman, P. W., and Zoback, M. L., eds., The rapidly changing sources and/or variable and rapidly changing water depths. Cordilleran orogen: Conterminous U.S.: Boulder, Colorado, Geological Society of America, Geology of North America, v. G-3, p. 407–479. Carr, M. D., Poole, F. G., Harris, A. G., and Christiansen, R. L., 1981, Western facies Paleozoic rocks Assemblage 3 in the Mojave Desert, California: U.S. Geological Survey Open-File Report 81–503, p. 15–17. Carr, M. D., Christiansen, R. L., and Poole, F. 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