Geologic Setting:

Within the Black Mountains block of the Death Valley region of eastern California is a terrane composed primarily of Cenozoic intrusive and extrusive rocks, which covers an area of about 1,300 sq km. The volcanic rocks of this terrane are predominantly rhyolitic but also include dacitic, andesitic, and basaltic units. They appear to be genetically related to the Cenozoic deformational features of the block (Noble and Wright, 1954; Wright and Troxel, 1971a, 1971b). The extrusive rocks that have been mapped thus far are mostly lava flows and air-fall tuffs, but ash-flow tuffs form much of the lower part of the volcanic sequence in the southern part of the field (L. A. Wright, personal commun.). Included within the rhyolitic rocks is an accumulation of lava flows and tuffs, as much as 900 m thick, to which the name “Shoshone Volcanics” is here applied and which is the subject of this summary report.

High-altitude imagery of southern and central Death Valley. Image NASA ERTS 1125-17554 band 7, 25 November 1972; copy produced by Drew P. Smith, California Division of Mines and Geology.

Although the field is incompletely explored, it appears to consist dominantly of acid intrusive and extrusive rocks; rocks of intermediate to basic composition are also present but subordinate,

Clastic sediments derived from the igneous rocks, together with evaporites, were deposited in local basins (Drewes, 1963). Some of these rocks contain commercial borate deposits. Two major sediment accumulations have been named Copper Canyon Formation (Curry, 1941; Drewes, 1963, p. 32) and Furnace Creek Formation (Noble, 1941, p. 956).

DESCRIPTION

The Shoshone Volcanics are exposed almost continuously in a belt about 26 km long, which extends northward (Fig. 1). They also are exposed in more westerly parts of the Greenwater Range and in the Black Mountains.

Figure 1. Distribution of the Shoshone Volcanics in southeast Greenwater Range (Eagle Mountain quadrangle) and Dublin Hills (Shoshone quadrangle).

In the area of Figure 1, the Shoshone Volcanics overlie Cambrian sedimentary rocks, Cenozoic quartz monzonite plutons, and older acid volcanic rocks with an erosional unconformity. The contact between the two accumulations of volcanic rock shows marked angular discordance. Southwest of the area shown in Figure 1, the Shoshone Volcanics also overlie ash-flow tuffs and andesite lava flows. The eroded surface of the Shoshone Volcanics is in turn overlain by air-fall tuffs and lava flows of the Greenwater Volcanics. This contact commonly also shows angular discordance. As the dip direction of all three volcanic accumulations is generally eastward, the angular unconformities suggest continued eastward tilting of fault blocks during emplacement of the volcanic rocks. Dark colored lava flows of basaltic and andesitic composition overlie the Shoshone Volcanics in some places.

The Shoshone Volcanics are prominently layered, the layers producing a striped landscape colored in various shades of pink, yellow, and gray. A regular sequence of layers is obvious, even from a distance. A yellow layer is generally overlain successively by a gray layer, a pink layer, locally a second gray layer, and another yellow layer (Table 1). At many localities, three or four of these sequences are exposed in layer-cake fashion on a single slope, individual sequences being 45 to 120 m thick. The sequences on steeper slopes produce cliffs (gray and pink layers), alternating with benches (yellow layers).

TABLE 1. TERMINOLOGY AND INTERPRETATION OF LAYERING IN SHOSHONE VOLCANICS

As the volcanic rocks dip eastward, the best views of the layer sequences are from the west side of the mountains. The sequences in the Dublin Hills are among the best exposed (Chesterman, 1973) and are also easily accessible. These dominate the view eastward as one travels toward Salsberry Pass from Greenwater Valley.

CORRELATION AND AGE

The Shoshone Volcanics are equivalent to a part of Drewes’s “older volcanics” in the Funeral Peak quadrangle to the west of the area shown in Figure 1 (Drewes, 1963). This unit is lithologically similar to the Shoshone Volcanics even to the zones within individual lava flows, and it also overlies eroded quartz monzonite plutons and underlies Greenwater Volcanics.

A tentative age for the Shoshone Volcanics may be inferred from isotope ages of rock units elsewhere in the volcanic field. Fleck (1970, p. 2810) reported seven K-Ar ages for “older volcanics” from the Funeral Peak quadrangle: three from Dante’s View and four from Hidden Springs, The dates from Dante’s View (6.32 ± 0.13, 6.34 ± 0.13, 6.49 ± 0.13) are clearly younger than those from Hidden Spring (8.02 ± 0.16, 7.60 ± 0.30, 7.77 ± 0.15, 8,02 ± 0.16). These yield an early Pliocene age for Hidden Spring and a middle Pliocene age for Dante’s View, based on the time scale of Evernden and others (1964). Although the “older volcanics” of Drewes include more than the equivalent of the Shoshone Volcanics, the early to middle Pliocene dates fit the permissible interval allowed the Shoshone Volcanics by radiometric dating of the quartz monzonite plutons and the Greenwater Volcanics. The former have yielded middle to late Miocene K-Ar dates (Stern and others, 1966; Armstrong, 1966). The Greenwater Volcanics, on the other hand, have yielded late Pliocene K-Ar dates (Fleck, 1970). The early to middle Pliocene age for the Shoshone Volcanics thus indicated may be refined when more detailed correlations of volcanic units are made, and when the age of the volcanic rocks underlying the Shoshone Volcanics is better known.

PETROGRAPHY

The Shoshone Volcanics consist primarily of four lithotypes: (1) gray perlitic vitrophyre, (2) red-brown to pinkish-gray felsophyre, (3) yellow felsophyre, and (4) yellow devitrified tuff (Table 1).

Phenocrysts of the Vitrophyre and Felsophyre. Phenocrysts comprise 3 to 20 percent of the total rock volume (Table 2); the remainder is glass or cryptocrystalline groundmass. Some of the layer sequences are crystal rich, others are crystal poor (Haefner, 1972). The Dublin Hills sequences are crystal poor. Crystal-rich sequences overlie crystal-poor sequences in the vicinity of Brown Peak.

TABLE 2. APPROXIMATE MODAL COMPOSITIONS, BY CONVENTIONAL POINT COUNTING

Plagioclase, the only feldspar, forms the largest (4 mm) and most abundant phenocrysts. Most crystals are andesine, but the composition ranges from oligoclase to labradorite.

Hornblende forms euhedral prisms to 1.5 mm long. It occurs as ordinary hornblende (pleochroic in green and brown), and as the varieties variously known as oxyhornblende, lamprobolite, or basaltic hornblende (pleochroic in red, green, and brown). Crystals of the latter variety are commonly rimmed with opaque minerals. Oxyhornblende occurs only in the pink felsophyre; ordinary hornblende occurs in the vitrophyre and yellow felsophyre.

Biotite occurs as euhedra to 1.5 mm in maximum dimension. It, too, occurs as two varieties: ordinary biotite (pleochroic in green and brown), and a type which, by analogy with hornblende, may be called oxybiotite (pleochroic in red, green, and brown). Oxybiotite is commonly rimmed with opaques, and it occurs exclusively with oxyhornblende in the pink felsophyre.

Opaque minerals, largely magnetite, occur as small anhedral to euhedral grains from dust size to 0.3 mm, and as larger masses partly to completely replacing oxyhornblende and oxybiotite. Orthopyroxene and clinopyroxene form euhedral phenocrysts that may be as large as the hornblende and biotite but are much less common.

A xenocrystic character is inferred for some grains that are rounded or embayed. Quartz grains as much as 1 mm in diameter occur in a few thin sections and are consistently rounded or embayed. Species that occur as phenocrysts also commonly show such outlines.

Groundmass of the Vitrophyre and Felsophyre. The glass groundmass of the vitrophyre commonly is gray to black. Brown glass occurs as a local variation that is mixed in swirls and patches with gray glass. The glass contains abundant pyroxene crystallites of several habits. Perlitic structure is nearly ubiquitous. In thin section, concentric fractures outline spheres with centers spaced at intervals of 0.5 to 1.0 mm and associated with a boxwork of planar fractures. Spherulites, 0.1 to 30 cm, occur in the glass but are confined to the several meters of vitrophyre along the margins of the red felsophyre (Fig. 2). Some spherulites are hollow and contain quartz crystals, agate, or opal.

Figure 2. Features of typical eruptive unit in Shoshone Volcanics.

The groundmass of the yellow felsophyre is cryptocrystalline. The presence of the alteration mineral jarosite is indicated by x-ray diffraction analysis. Magnetite commonly is absent. Relict perlitic fractures are locally present, indicating that the yellow felsophyre groundmass was originally glass, and that it acquired its present crystallinity through devitrification.

The groundmass of the red felsophyre is cryptocrystalline. The red to pink color, which is particularly conspicuous around magnetite microphenocrysts and around the opacitized rims of oxyhornblende and oxybiotite, is produced by abundant patches of hematite. In contrast to the vitrophyre and yellow felsophyre, the red felsophyre is vesicular; small vesicles to 2 mm are nearly ubiquitous, but large vesicles to 1 cm also occur, most of them adjacent to the vitrophyre (Fig. 3).

Figure 3. Distribution of megascopic vesicles in red felsophyre.

Devitrified Yellow Tuff. The same phenocryst species as found in the vitrophyre and felsophyre are found in the tuff matrix. The matrix has been devitrified and is now cryptocrystalline, with only rare traces of the original glass shards. Magnetite appears to be absent in the matrix, whereas jarosite is present. Essential pumice lapilli (devitrified) and accidental lapilli and blocks of volcanic rocks are the most common clasts; clasts of sedimentary and metamorphic rocks are rare. Vitrophyre clasts are absent nearly everywhere, but yellow and red felsophyre clasts are common.

Two localities have been found where the tuff is not devitrified. The most accessible of these is the lowest tuff exposed on the west side of the Dublin Hills where vitrophyre lapilli and glass shards are abundant, and jarosite does not occur in the matrix.

Paleomagnetic Properties of the Rocks. Ninety-six determinations of paleomagnetic direction were made on vitrophyre and red felsophyre of the layer sequences only because yellow felsophyre and tuff commonly are not magnetically susceptible. In each case, strike and plunge of the remanant magnetization was determined. As the rocks are relatively young, the orientation of the remanant magnetization should approximately parallel the magnetic direction of the Earth’s present field, after correcting for postcooling deformation.

All the layer sequences examined exhibit normal polarity, except for specimens from exposures of one layer sequence, which are reversed (NW1/4 sec. 3, T. 22 N., R. 5 E.).

All the vitrophyre samples, including those from the reversed sequence, have directions of remanant magnetization which parallel that predicted from the present field (Fig. 4). However, among specimens from the red felsophyre zone, fewer than half have remanant directions that parallel the predicted direction; the remainder have remanant directions with widely varying strikes and plunges. It appears that only the vitrophyre can be relied upon to be a faithful recorder of the Earth’s magnetic field at the time of emplacement.

Figure 4. Determination of paleomagnetic direction.

INTERPRETATION OF THE LAYER SEQUENCES

Types of Deposits. Within each yellow layer is a deposit, about 9 m thick, of well-stratified tuff. The remainder of the layer is yellow felsophyre (Table 1). The tuffs, which consist of interbedded lapilli-rich and lapilli-poor strata, are interpreted as air-fall tuffs. The tuff deposits divide the yellow color bands into additional mappable units, or zones (Table 1).

The sequence of zones between tuff deposits is interpreted as a single rhyolite lava flow (Fig. 2; Haefner, 1969). Glass shards, fiamme, and other characteristics of ash-flow deposits are absent in these zones. In addition, laminae in the red felsophyre zone are locally warped into folds, indicating that the magma flowed as a coherent mass. That this sequence of zones constitutes only one lava flow is evidenced by exposures of flow margins. Four flow margins are exposed in the Dublin Hills; at such localities, the red felsophyre constitutes a core, around which are wrapped the upper vitrophyre and yellow felsophyre zones to become the lower vitrophyre and yellow felsophyre zones (Fig. 5).

Figure 5. Cross-section sketch of lava flow margin in exposure of Shoshone Volcanics near Miller Spring. Flow margin overlain by another eruptive unit of Shoshone Volcanics. Not drawn to scale; length of section approximately 250 ft.

Alteration. The yellow felsophyre is interpreted as altered vitrophyre (Haefner, [969, 1973). Evidence for this inference is the presence of relict perlitic fractures and the observation that yellow felsophyre merges with unaltered vitrophyre inward from both the upper and lower flow surfaces. Thus, a lava flow appears to consist essentially of a red felsophyre core encased in a vitrophyre sheath; the outer portions of the sheath have been altered, as have adjacent tuff deposits. Alteration is expressed as devitrification, the growth of jarosite, and the disappearance of magnetite.

Altered acid volcanic rocks are usually distributed about centers of fumarolic or hydrothermal activity. The Shoshone lava flows are unusual in that the altered rock occurs as a sheet like body at the glassy top and at the glassy base of each lava flow. However, such alteration may not be unique to Death Valley; there are at least two localities in the Soviet Union from which apparently similar altered lava flows of acid composition have been described (Nasedkin, 1963).

A hypothesis for the origin of this alteration in the Shoshone Volcanics, which involves the interaction of magmatic with meteoric volatiles during the cooling of the flows, has been presented elsewhere (Haefner, 1969, 1973).

EMPLACEMENT OF A TYPICAL ERUPTIVE UNIT

Crust of the Active Lava Flow. At the top of each lava flow that is overlain by a tuff deposit, veinlets of tuff extend downward into the flow, outlining individual blocks of yellow felsophyre (altered vitrophyre); locally, the blocks are pumiceous. These blocks apparently constituted a rubble crust formed by the chilling and breaking of lava on the upper surface of the active flow. The crust, about 4.5 m thick, composes only 3 to 9 percent of the total flow thickness; furthermore, crustal thickness does not depend on the thickness of the lava flow (Fig. 6).

Figure 6. Thickness of upper crust of rubble in six lava flows of Shoshone Volcanics.

Evidence of a rubble crust at the base of the lava flow would be generally difficult to detect, because tuff veinlets do not outline the rubble blocks. Nevertheless, ghostly remnants of angular blocks in the yellow felsophyre (altered vitrophyre) are observable at some localities and apparently originated as rubble. The lower crust is relatively thin, and massive, unaltered vitrophyre composes most of the part of the flow that underlies the red felsophyre zone (Fig. 2). If the lower crust was about as thick as the upper crust, then 80 to 95 percent of the lava flow remained liquid while the flow was active.

This large proportion of liquid magma contrasts with that inferred for many other acid lava flows, which are block lavas. These contained a much higher proportion of rubble during their active stage than do the flows of the Shoshone Volcanics. The overlying flows of the Greenwater Volcanics, exposed near Brown Peak, are block lavas. One measured section shows a flow to have been 70 percent rubble and only 30 percent liquid during its active stage.

Formation of Massive Vitrophyre. Adjacent to the rubble crusts are thick layers of massive gray vitrophyre and massive yellow felsophyre (altered vitrophyre). Together they form a sheath that encases the red felsophyre core of the flow. Patches of gray vitrophyre within the massive altered vitrophyre resemble rubble crust (Fig. 2), but they are not, because flow laminae or layers of spherulites can be traced across the patches into the adjacent vitrophyre, thus demonstrating that the patches are not rotated blocks.

The massive vitrophyre, fresh and altered, represents lava which must have chilled to a glassy rock only after movement of the lava ceased. Otherwise, the chilled glassy rock would have been broken and become part of the rubble crust. This chilling took place at least in part after the flow margin stopped advancing, because the massive vitrophyre sheath extends to the flow margin (Fig. 5), The chilling to form vitrophyre could have begun in less mobile parts of the flow during the active stage, particularly in portions of the flow near the vent (Fig. 7).

Figure 7. Interpretation of emplacement of an eruptive unit. Massive vitrophyre may have formed in stable crust areas of active lava flow but also formed after lava flow ceased advancing. Not drawn to scale.

Formation of the Felsophyre Core. An emplaced and cooling lava flow consisted of a molten core surrounded by a vitrophyre sheath (Fig. 7). The layers of massive vitrophyre must have continued to thicken as cooling proceeded, until conditions favored creation of features that characterize the lava flow core, namely, vesicularity, hematite stain, oxyhornblende and oxybiotite, and primary cryptocrystallinity, as contrasted with crystallinity caused by devitrification.

Except for local pumiceous rock in the rubble crust, only the felsophyre core is vesicular. This indicates that the lava, instead of degassing completely when it reached the Earth’s surface, retained an appreciable quantity of gas in solution until after the flow was emplaced and had cooled for some time.

Lava flows of basic composition are generally devoid of evidence of delayed vesiculation, probably because they have a lower melt viscosity. The proportion of acid lava flows that display evidence of delayed vesiculation, is unknown, because detailed descriptions of the interiors of acid lava flows are rare. The delayed vesiculation probably is responsible for the other characteristic features of the red felsophyre, based on the following evidence: On heating in air to about 750°C, ordinary hornblende changes to oxyhornblende (Belovsky, 1891; Kozu and others, 1927; Barnes, 1930). This process is one of oxidation and is accompanied by resorption around the crystal margins, the products generally including iron oxides and pyroxene (Deer and others, 1962).

The conversion of ordinary hornblende to oxyhornblende and of biotite to oxybiotite in the flow interior probably occurred with vesiculation, the exsolved gases providing the necessary oxidizing environment. Because hematite, the colorant of the lava flow core, is concentrated around oxyhornblende and oxybiotite crystals, it probably formed as a resorption product.

Because oxidation is an exothermic process, additional heat probably was generated in the molten lava flow core upon vesiculation. The oxidation of hornblende, experimentally, suggests that the temperature of the molten core was at least 750°C when the oxidation began. The additional heat provided by the oxidation apparently was sufficient to maintain the vesiculating molten core in a liquid state long enough for ions to migrate through the miscous melt and organize themselves as minute crystals. Thus the cryptocrystallinity of the red felsophyre groundmass also appears to have been favored by the delayed vesiculation.

Vesicle Collapse in the Felsophyre Core. The walls of vesicles are more coarsely crystalline than the rest of the ground mass. Streaks of similar material, observed in thin section, demonstrate the partial to complete collapse of vesicles (Fig. 8). Collapsing may have occurred when exsolving gases became depleted in the melt, or when the falling temperature caused a reduction in gas pressure to the point where it could no longer support the vesicle walls. The relatively large vesicles, which occur at the top and base of the red felsophyre zone, may have become “frozen” in highly viscous lava close to the chilled vitrophyre.

Figure 8. A. Photomicrograph of collapsed vesicles in red felsophyre (crossed nichols). Coarsely crystalline walls of vesicles show as light streaks. Arrows point to tridymite, which filed in partially collapsed vesicles, Light streak above tridymite patch on right is completely collapsed vesicle, without tridymite filling. Length of photo, 3 mm. B. Large vesicles in red felsophyre near bottom of zone. Vesicles are lined with druses of silica minerals.

The peculiar paleomagnetic properties of the red felsophyre may be related to vesicle collapse. About 40 percent of determinations on red felsophyre yield magnetic directions that parallel the predicted directions of the rocks. The red felsophyre, even though it has been oxidized by exsolved gases, evidently can be a faithful recorder of the Earth’s magnetic field, just as the vitrophyre can. Thus, the remaining determinations that do not yield the predicted magnetic direction seem best explained by mechanical rotation of mineral grains perhaps related to vesicle collapse. No other rotation is apparent as the paleomagnetism of the massive vitrophyre, particularly in the flow margins, demonstrates that the Curie point in the outer part of the lava flow was crossed only after the lava flow had stopped advancing. If this interpretation is correct, then the collapse of vesicles in the red felsophyre took place below 580°C, the measured Curie point of the rock.

Mechanism of Lava Flow Advance. Since freshly fallen tuff is notoriously susceptible to erosion but is conformable with the overlying flows, the lava flows must have moved over the tuff immediately after the tuff was deposited. Therefore, tuff is taken to represent deposits of explosions that cleared the vent just prior to a flow effusion.

The tuff deposits, which were unconsolidated lapilli and ash at the times the lava flows moved over them, are virtually undisturbed. Typically, the tuff fragments that are mixed into the overlying lava flow rock occur within 10 cm of the contact. In addition, the surfaces of the tuff layers apparently are unscoured by the overlying flows. Individual layers at the tops of tuff deposits can be traced for nearly 1 km. Therefore, a typical lava flow advanced without disturbing the underlying loose tuff. The nature of the flow margins and the existence of rubble crust at the base of the typical flow indicate that it advanced with a simple rolling tractor-tread motion, the crust that formed at the upper surface being dragged beneath the flow as it advanced (Fig. 7).

Although the areal extent of individual lava flows is obscured by abundant faults, individual flows have been traced continuously for as much as 5.5 km. A lava flow in the Dublin Hills that contains distinctive fragments of basaltic rock may be correlative with a similar lava flow exposed as far as 19.3 km north of the Dublin Hills.

Considered as a group, the lava flows of the Shoshone Volcanics appear to have filled the valleys. A map of the area near Brown Peak in the Eagle Mountain quadrangle, for example, shows flows accumulated which filled an ancient valley that possessed a relief of about 300 m (Haefner, 1972, Pl. 2).

The tractor-tread motion, the large proportion of liquid lava in the active flows, and the apparent extensiveness of some lava flows suggest a high fluidity for the Shoshone lava flows. This might be an effect of high initial temperature (Gibbon, 1964), but the presence of phenocrysts of the lava suggests a limited temperature range comparable to the range of the relatively viscous lava assumed for the porphyritic flows of block rhyolite. Thus, apparently high fluidity of the Shoshone rhyolite lava flows is more likely an effect of a high content of dissolved volatiles than of a high initial temperature, the volatiles being largely retained in solution during the advance of the flows.

REFERENCES CITED

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Chesterman, Charles w., 1973, Geology of the northeast quarter of Shoshone quadrangle, Inyo County, California: California Div. Mines and Geology, Map Sheet 18.

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Haefner, Richard, 1969, Emplacement and cooling history of a rhyolite lava flow and related tuff at Deudman Pass, near Death Valley, California [M.S. thesis]: University Park, Pennsylvania State Univ., 82 p.

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Wright, L. A., and Troxel, B. W., 1971a, Thin-skinned megaslump model of Basin Range structure us applicable to the southwestern Great Basin: Geol. Soc. America, Abs. with Programs (Ann. Mtg.), v. 3, no, 7, p. 758.

______ 1971b, Evidence for tectonic control of volcanism, Death Valley: Geol. Soc. America, Abs. with Programs (Cordilleran Sec.), v. 3, no. 2, p. 221.

High-altitude vertical aerial photograph near southeastern end of Greenwater Range. Pale-colored racks in left of center of photograph are marine Tertiary volcanic rocks described by Haefner [see preceding article]. The rocks also crop out on northwest [left] side of Dublin Hills [right of center]. Amargosa River flows south across photograph. Pale-colored area on right is underlain by flat-lying lake sediments deposited during an interval when the flow of the Amargosa River was blocked. Part of photo U.S. Air Force 374V 197, 6 September 1968; courtesy of the U.S. Geological Survey.