Geological Survey Professional Paper 554—D Cenozoic Volcanic Rocks of the Devils Postpile Quadrangle, Eastern Sierra Nevada California

A rhyolitic ash-flow tuff, in part welded, is exposed in the vicinity of Reds Meadow and in other erosional remnants on the valley sides of the Middle Fork of the San Joaquin River. This tuff near Reds Meadow was briefly noted by Erwin (1934, p. 50), but its rather large original area and volume have not previously been appreciated because of the scattered and remote nature of other outcrops. The rhyolitic tuff is very similar to the Bishop Tuff, first described by Gilbert (1938), the nearest exposure of which is along Deadman Creek in the Mount Morrison quadrangle 10 miles northeast of Reds Meadow. The tuff of Reds Meadow is a crystal-vitric tuff composed of 15 to 30 percent crystals, 15 to 25 percent pumice fragments, and 45 to 70 percent fine ash and shards. The crystals consist of quartz and sanidine in nearly equal amounts, minor oligoclase, and very minor biotite; similar crystals also occur sparsely in the pumice fragments. The sanidine has a potassium-sodium ratio of about 2 (estimated from a potassium analysis made in connection with a potassium-argon age determination) and is generally in euhedral crystals with an average size of 1 millimeter. The quartz occurs both as euhedral crystals and as rounded embayed grains, also about 1 millimeter in average size. Except for minor variation in relative percentages of the constituents, the tuff appears to be quite uniform in composition throughout the stratigraphic section examined in detail near Reds Meadow. An estimate of 72 percent silica was obtained from a refractive-index determination on a fused sample of pumice from the base of the tuff.

ZONATION IN THE TUFF

Exposures of ash-flow tuff in a small stream gully on the slope east of the Reds Meadow Ranger Station, together with an exposure of the approximate base of the tuff near Sotcher Lake half a mile to the north, permit the division of the tuff into nearly horizontal zones similar to those described by Smith (1960) as characterizing welded ash flows. In the 600+ foot exposed section of tuff, two distinct zones of dense welding, indicating two flow units, can be distinguished.

These localities are the only ones where two zones were recognized, and even there, the zones are not sharply defined. Hence, they are not separately delineated on the geologic map.

LOWER FLOW UNIT

Lower zone of no welding (about 100 ft thick).—A zone of no welding consists of unconsolidated ash containing millimeter-size crystals of quartz and sanidine, pumice fragments, numerous pebbles of granitic and metamorphic rock, and andesite of probable Pliocene age. The pumice fragments and exotic pebbles are commonly grouped in layers which define a rude bedding that dips 5° to 10° W. toward the center of the Middle Fork valley. Crossbedding is also evident at one exposure. Bedding appears to be limited to this zone and is not found higher in the section. Tuff in the lower half of this zone is unconsolidated, whereas that in the upper half becomes progressively more indurated upward. The color also grades upward from a very light gray to a reddish orange.

Lower zone of partial welding (about 50 ft thick).—Although outcrops are not continuous, the transition between the zone of no welding and the zone of partial welding appears gradational over a few tens of feet. The rock in the zone of partial welding is highly indurated and pale red. The pumice fragments, collapsed and flattened in a nearly horizontal plane, are black and obsidianlike; but on a fresh surface many have a brownish tinge and exhibit a fibrous structure indicating incomplete welding. In thin section the pumice fragments and shards can be seen to be locally molded around quartz and sanidine crystals and other fragments but are otherwise not strongly deformed (fig. 7A).

FIGURE 7.—Photomicrographs of tuff of Red Meadow, lower flow unit, A (upper), Lower zone of partial welding showing shards and flattened pumice fragment. B (middle), Zone of intense welding showing flattened shards bent around crystals and flattened pumice fragment, C (lower), Upper zone of partial welding showing undeformed shard structures.

Zone of intense welding (about 100 ft thick).—The transition between the lower zone of partial welding and the zone of intense welding is not exposed; but across a span of about 25 feet of stratigraphic section, the change in appearance of the tuff is striking. The dull-red matrix of the partially welded zone gives way to a vitreous brownish- or purplish-black matrix; the pumice fragments are jet black and obsidianlike and commonly fracture concoidally. Complete welding, as defined by Smith (1960, p. 155), was not achieved, however, for the pumice fragments and glass shards are not completely homogenized. The pumice fragments and shards are more strongly flattened than in the zone of partial welding and are tightly molded around the crystal fragments (fig. 7B.) Incipient devitrification has not progressed far enough for the formation of spherulitic or axiolitic structures, except adjacent to quartz crystals. Columnar jointing is locally present.

Upper zone of partial welding (about 200 ft thick).—The lower 50 feet of the upper zone of partial welding is similar to the lower zone of partial welding, and hand specimens are identical. Higher, however, a gradation begins which continues to the top of the zone. The color of the matrix changes upward from a pale red to a very light gray. The pumice fragments become lighter in color and less strongly welded and flattened until their fibrous structure is readily apparent in hand specimen. In the upper part the glass shards are largely undeformed (fig. 7C.) Incipient devitrification is noticeable but not extensive. There is no completely nonwelded tuff preserved above this zone.

UPPER FLOW UNIT

Division of the upper flow unit into distinct zones is more difficult than with the lower flow unit; in fact, the placement of the contact between the two flow units is somewhat arbitrary. A continuous and fairly homogeneous zone of partially welded tuff extends from the densely welded zone of the lower flow unit to the densely welded zone of the upper flow unit. No obvious textural or mineralogic change is apparent in this interval. The base of the upper flow unit is about 50 feet below the zone of dense welding, where the number of exotic rock fragments increases and the pumice fragments become somewhat darker, and more strongly welded.

Lower zone of partial welding (about 50 ft thick).—In the lower part of this zone, pumice fragments are largely collapsed, but these fragments and the shards are otherwise little deformed. The degree of welding and compaction increases upward, but the rock does not assume the reddish hues typical of comparable zones in the lower flow unit. Some pumice fragments are partly devitrified.

Zone of intense welding (about 130 ft thick).—The change in the rock at the base of the zone of intense welding is gradational. The two most important changes used to define the boundary are darkening of the grayish matrix and welding of the pumice fragments into dense obsidianlike pellets that fracture concoidally. The most striking microscopic change is the nearly pervasive but incomplete devitrification—the only extensive devitrification within the entire section of tuff. This is evidenced by the formation of spherulitic and axiolitic structures, which here and there selectively envelop the pumice fragments, and by the obliteration of shard structures (fig. 8). The top of this zone and higher zones, which presumably were present following eruption and cooling of the tuff, have been removed by erosion; and the tuff is unconformably overlain by the andesite of the Devils Postpile.

FIGURE 8.—Photomicrographs of tuff of Reds Meadow, upper flow unit. A (upper), Zone of intense welding showing flattened pumice fragments and opaque materials in groundmass, with obliteration of shard structure. B (lower), Same, cross nicols. Note devitrification.

THICKNESS AND LATERAL EXTENT

A complete section of the ash-flow tuff is nowhere preserved, and therefore only an estimate of its original thickness can be obtained. Approximately 600 feet of tuff is preserved in the thickest section on the valley wall east of Reds Meadow Ranger Station. If the upper flow unit had originally been a virtual duplicate of the lower, the total original thickness of the tuff at this locality would have been about 900 feet. Reconstruction of the postulated nearly flat flow surface across the Middle Fork of the San Joaquin River on the basis of outcrops of tuff on the west side of the valley suggests a minimum thickness of 1,000 feet before erosion (fig. 9). Further extrapolation of this surface upvalley and downvalley suggests a minimum areal extent of 22 square miles and a volume of 4 cubic miles for the tuff east of Lion Point. In arriving at the area and volume, allowance was made for posttuff deepening of the canyon; the figures are believed to represent reasonable minimums. Because at least 500 feet of tuff are preserved nearly 2,000 feet above the present stream level near Lion Point, the ash flow must have originally extended down the Middle Fork Canyon well beyond the limit of present outcrop, a condition adding considerably to the estimates of areal extent and volume noted above.

FIGURE 9.—West-east section in vicinity of Rainbow Falls showing reconstructed surface of the tuff of Reds Meadow. (click on image for an enlargement in a new window)

AGE, SOURCE, AND CORRELATIONS

Similarities between the lithology and welding characteristics of the tuff of Reds Meadow and the Bishop Tuff lead us to speculate on possible correlations between the two tuffs. A potassium-argon age of approximately 0.7 million years has been reasonably well established for the Bishop Tuff (Dalrymple, Cox, and Doell, 1965). Three age determinations—0.66±0.02 million years,¹ 1.4 million years, and 1.1 million years—have been made on sanidine from the tuff of Reds Meadow. The two older ages were determined on sanidine separated from samples of the lower zone of partial welding in the lower flow unit. This material was contaminated with exotic fragments of granitic rock (approximately 80 million years old) including fragments of feldspar in the same size range as the sanidine. Every precaution was taken to get a pure sanidine separate, but we believe that some exotic material was included which biased the age determinations toward an older age. The third potassium-argon determination, 0.66 million years, was made on sanidine concentrated from uncontaminated pumice fragments collected from the unconsolidated basal portion of the tuff. This type of material is the most satisfactory for potassium-argon dating of ash-flow deposits (Dalrymple, Cox, and Doell, 1965), and 0.66 million years is accepted as the most reliable age determination for the tuff of Reds Meadow.

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¹The analytical data for this determination are as follows (G. B. Dalrymple, written commun., 1966): K₂O, 10.72 percent (average of two analyses); radiogenic An₄₀, 1.043X10^-11 mole per g (average of two analyses); radiogenic Ar₄₀, 56 and 62 percent for the two argon extractions.

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The remanent paleomagnetic polarity (normal) and orientation of the tuff of Reds Meadow is identical with that of the Bishop Tuff (A. V. Cox, oral commun., 1963). The Brunhes normal-polarity epoch, which extends from approximately 0.7 million years ago to the present, was preceded by the Matuyama reversed-polarity epoch from approximately 2.5 to 0.7 million years ago (Dalrymple, Doell, and Cox, 1965). This suggests that the tuff of Reds Meadow is not appreciably older than 0.7 million years and could well be the same age as the Bishop Tuff.

Cobbles of gray ash-flow tuff and pumice pebbles are common in alluvial deposits at Friant, where the San Joaquin River emerges from the foothills of the Sierra Nevada and flows onto the alluvial plain of the San Joaquin Valley. The tuff pebbles are similar to material from the tuff of Reds Meadow, although some petrographic and chemical differences do exist; no other source of similar tuff is known in the San Joaquin drainage basin. A potassium-argon age determination on sanidine phenocrysts from pumice pebbles associated with the tuff pebbles gave and age of 0.60±0.02 million years (Janda, 1965), which is compatible with the age determined for the tuff of Reds Meadow.

No source for the tuff of Reds Meadow was found within the present drainage basin of the Middle Fork of the San Joaquin River. Possible vents could be hidden beneath the younger rocks in the Mammoth Mountain area or farther northeast. Past areal contiguity with the Bishop Tuff is a possibility, but one for which direct evidence is lacking.

Glacial deposits lie beneath the Bishop Tuff (Putnam, 1960, 1962; Wahrhaftig, 1965; Sharp, 1965); hence if the tuff of Reds Meadow is equivalent to the Bishop Tuff, it also should overlie glacial debris. Because the base of the tuff of Reds Meadow is nowhere exposed, we do not know whether or not the surface on which it rests was glaciated.

Indirect evidence suggests pretuff glaciation, however. Numerous pebbles and cobbles of older andesite and granitic and metamorphic rocks from the headwaters of the Middle Fork (Huber and Rinehart, 1965a) occur in the unconsolidated ash deposit at the base of the tuff near Sotcher Lake. The pebbles are rounded and must have been present as gravels in the Middle Fork valley at the time of eruption of the tuff. R. J. Janda (oral commun., 1965), in his study of the stream regimen of the upper San Joaquin basin, has concluded that these pebbles are too heterogeneous and too large to have been transported to this site by modern stream processes; hence the pebbles may represent incorporated glacial materials.

Prior to the eruption of the andesite of the Devils Postpile, a minimum volume of 4 cubic miles of the tuff had probably been removed from the Middle Fork valley during an interval of a few hundred thousand years at most. This feat would seem to require some process, presumably glaciation, in addition to normal stream erosion. Owens River, about 15 miles east of the quadrangle, receives drainage from a 25-mile segment of the eastern part of the Sierra Nevada, yet in a much longer time has only been able to cut a narrow, steep-walled gorge into the Bishop Tuff. Thus we favor glacial erosion as the agent of removal of the tuff of Reds Meadow prior to the eruption of the andesite of the Devils Postpile; the eruption probably occurred about half a million years ago.