ABSTRACT The Eureka Valley Tuff consists of two Eureka Valley Tuff, major ash-flow sheets and a local over- lying sequence of ash-flow tuff erupted from vents within the Little Walker East-Central and caldera 11 mi west-northwest of Bridge- port, in east-central California. The lower of the two major ash-flow sheets, here named the Tollhouse Flat Member, is the Adjacent Nevada "biotite-augite-latite" of Ransome (1898). The overlying By-Day Member can readily be identified by the absence of phenocrystic biotite and by paleomagnetic Donald C. Noble and David B. Slemmons and other petrographic criteria. The Mackay School of Mines, University of Nevada, recognition of the distinctive By-Day Reno, Nevada 89507 Member above the Tollhouse Flat Member both in the Bridgeport area and west of Marjorie K. Korringa and William R. Dickinson the Sierra crest unequivocally demon- Department of Geology, Stanford University, strates the generally accepted correlation Stanford, California 94305 of the latitic ash-flow tuffs of the two Yehya Al-Rawi areas. K-Ar age determinations indicate that the three members of the Eureka Department of Geology, University of Baghdad, Valley Tuff were erupted within a very Adhamiya, Baghdad, Iraq short interval of time about 9.5 m.y. ago. and Edwin H. McKee INTRODUCTION U.S. Geological Survey, The Eureka Valley Tuff of the central , California, and adjacent Menlo Park, California 94025 Nevada was first studied by Ransome (1898) in the latter part of the 19th century. In 1948 Slemmons (1953) W. R. Dickinson and D. C. Noble, unpub. sheets. We therefore here raise the Eureka recognized, from the excellent description data). The caldera marks the source area Valley Member to formational rank, of the unit given by Ransome, that of the Eureka Valley Tuff. rename it the Eureka Valley Tuff, and Ransome's "biotite-augite-latite" was This paper describes the stratigraphy, subdivide the new formation into two composed of welded tuff. Smith (Ross field characteristics, and petrography of formal members (the Tollhouse Flat and Smith, 1961) independently recog- the Eureka Valley Tuff and proposes Member and the overlying By-Day Mem- nized the pyroclastic character of the revisions in the stratigraphic nomenclature ber) and an informal upper member. unit at about the same time. Slemmons of the unit. Subsequent papers will dis- Raising the Eureka Valley to forma- (1953) traced the biotite-augite-latite cuss the major element, minor element, tional status requires revision in the eastward from the Sierra foothills in the and isotopic geochemistry of the Eureka nomenclature of associated units. We vicinity of Sonora, California, across Valley Tuff; the geology of the Little therefore raise the Table Mountain Latite Sonora Pass on the Sierra crest. Johnson Walker caldera; and the geochemistry of Member and the Dardanelles Member of (1951), Halsey (1953), Gilbert and others genetically related potassium-rich the Stanislaus Formation to formational (1968), Chesterman (1968), and Al-Rawi intermediate lavas in the vicinity of the rank, renaming them the Table Mountain (1969) recognized the unit in the southern Little Walker caldera. Latite and Dardanelles Formation, and Sweetwater Mountains, the Bodie Hills, we raise the Stanislaus Formation to and other localities to the east of the group rank. The Stanislaus Group thus Sierra Nevada (Fig. 1). Radiometric age STRATIGRAPHY AND PHYSICAL includes, from bottom to top, the Table determinations by Dalrymple (1963; CHARACTERISTICS Mountain Latite, the Eureka Valley 1964; Dalrymple and others, 1967) and We have found that the Eureka Valley Tuff, and the Dardanelles Formation Al-Rawi (1969; Gilbert and others, 1968) Member of the Stanislaus Formation of (Table 1). supported these correlations. In his Slemmons (1966) consists of two major, The ridge between Bald Peak and Red review of the Cenozoic geology of the lit ho logically distinctive ash-flow sheets Mountain in the Dardanelles quadrangle central Sierra Nevada, Slemmons (1966) and a much smaller local overlying unit (scale, 1:62,500) was designated as the formally named the ash-flow sequence of ash-flow tuff. The recognition of type locality of the Eureka Valley that includes the biotite-augite-latite of these two major ash-flow sheets both Member by Slemmons (1966). Because Ransome the Eureka Valley Member of east and west of the Sierra crest un- of the relative inaccessibility of this type the Stanislaus Formation. equivocally proves the previously sug- locality and because rocks of the upper Reconnaissance mapping in the drain- gested correlation of the quartz latite member are not present there, we consider age area of the Little Walker River and ash-flow tuffs of these areas (compare it desirable to designate a reference vicinity, prompted by consideration of Mackin, 1960; Christiansen and others, section. This section is located on the the regional distribution of the Eureka 1968). east bank of the at Valley Tuff and other geologic relations, It is now common practice (for Tollhouse Flat (lat 38°25'35" N., led to the discovery of the Little Walker example, see Christiansen and others, long 119°26'45" W.;Fig. 1). The Toll- caldera, located 11 mi west-northwest of 1968) to assign formational status to a house Flat and By-Day Members also are Bridgeport, California (Fig. 1; Noble and sequence of genetically related ash-flow well developed at the following selected others, 1969; Priest and others, 1974; sheets and member status to the individual localities: lat 38°25'05" N., long

GEOLOGY 139 119°25'55" W.; lat 38°23'20" N., The member is composed typically of magnetite, and apatite. Hornblende laths long 119°21'45" W.; and lat 38°20'45" N., densely to moderately welded, light- to of apparent intratelluric origin have been long 119° 14'45" W. Figure 1 shows the dark-gray tuff containing abundant observed in several thin sections. No grains general present distribution of the Eureka phenocrysts and lithic fragments. Eutaxi- of hypersthene, olivine, or quartz have Valley Tuff; details of these exposures tic structure is strikingly well developed, been found in thin sections of collapsed are shown on the maps of Ransome with some collapsed pumice fragments pumice, and thus, rare grains of these (1898), Koenig (1963), Strand and exceeding a foot in maximum diameter, minerals observed in some thin sections Koenig (1965), Strand (1967), Klein- particularly near the Little Walker caldera. of tuff are interpreted as accidental hampland others (1974), D. B. Slemmons Some of the pumice fragments show material incorporated during eruption (unpub.), W. R. Dickinson (unpub.), D. C. relatively little compaction, even in and (or) transport. Prisms of apatite Noble (unpub.), and in various of the densely welded portions of the unit, and typically are included within other other references cited here and in the some appear to have been nearly un- phenocryst minerals, as are small grains above-listed publications. vesiculated lumps of molten magma of magnetite, biotite, and clinopyroxene. (compare Gibson and Tazieff, 1967; Phenocryst minerals commonly occur in Tollhouse Flat Member Gibson, 1970; Noble and others, 1968; incipient glomeroporphyritic clusters in The Tollhouse Flat Member, here Korringa, 1972). In some intervals, the collapsed pumice fragments. Phenocrysts named for exposures at the reference inner portions of such large equant blocks are much less abundant in the pumice section located on the east side of Toll- are primarily devitrified and partly fragments (about 15 volume percent) than house Flat, is the most voluminous and expanded by the release of volatiles on in the remainder of the nonlithic portion widespread ash-flow sheet of the Eureka crystallization. This produces large, of the welded tuff, indicating that appre- Valley Tuff; areal extent in an east-west irregular lithophysal cavities, whereas the ciable amounts of fine-grained glassy direction is more than 90 mi (Fig. 1). The margins of the blocks have remained material were removed during eruption member is identical with the biotite- glassy. and transport (compare Lipman, 1967; augite-latite of Ransome (1898; Table 1). Walker, 1972; Korringa, 1972). Lithic Phenocrysts consist of oscillatory- The Tollhouse Flat Member is 210 ft fragments, composed dominately of zoned plagioclase (about An )1 with thick at the reference section and locally 45 lavas of intermediate composition, and smaller amounts of biotite, clinopyroxene, reaches a thickness of more than 300 ft. less common fragments of pre-Cenozoic The unit varies markedly in thickness granitic and metamorphic rocks, make because of the unevenness of the surface up about 15 to 20 percent of most rocks. on which it was deposited, but in general, 'The extinction angle measure to (010) it thins away from the Little Walker in the a-normal position is 30° for plagioclase In many outcrop sections, as many as caldera. from the Tollhouse Flat Member and 34° for five or more partings between successive plagioclase from the By-Day Member. ash flows or sequences of ash flows can

119°

38°

Figure 1. Map showing present distribution of Eureka Valley Tuff. Dashed line marks location of Little Walker caldera (LWC). Arrows show direction of remanent magnetization of units at type (west) and reference sections of Eureka Valley.

140 MARCH 1974 TABLE 1. CHART SHOWING EVOLUTION OF NOMENCLATURE OF ROCKS OF THE STANISLAUS GROUP member, which is essentially identical

Ransome (1898) Slemmons (1966) to the "pumice tuff-breccia member" of Halsey (1953), are nonwelded or very Dardanelle flow Dardanelles Member Dardanelles Formation poorly welded, although densely welded ash flows are interstratified with less Upper Member compact strata in most sections of the unit. The upper member is best developed Eureka Valley Member By-Day Member north and east of the Little Walker caldera (for example, east of Tollhouse biotite-augite-latite Tollhouse Flat Member Flat) and locally is intercalated with

Table Mountain faciès; flow Table Mountain Latite Menfoer Table Mountain Latite postcollapse lavas, tuffs, volcanic sand- stones, and so forth, within the caldera. The tuffs, which everywhere are separated be recognized. At many localities, the ber by the absence of phenocrystic from the underlying By-Day Member by upper 5 to 20 ft of the member consists biotite. Although a flake or two of a complete cooling break, vary in content of one or more distinctive ash flows that foreign biotite, probably derived from of phenocrysts and lithic fragments. contain appreciably fewer and smaller volcanic or granitic lithic fragments, Most, and perhaps all, of the ash flows phenocrysts, smaller pumice fragments, occasionally may be seen in rocks of the of the upper member contain abundant and a much smaller percentage of lithic By-Day, biotite has not been observed in biotite but appear to lack the large, fragments than the tuffs which compose collapsed pumice fragments of the unit. slightly vesiculated pumice blocks found most of the unit. No other evidence of (Halsey [1953] was the first to recognize, in the Tollhouse Flat Member. Another vertical compositional zonation has yet in the vicinity of Tollhouse Flat, a biotite- distinctive feature is the presence of been recognized. free ash-flow unit in the Eureka Valley.) spongy plagioclase phenocrysts compris- Measurement of the direction of In thin section, the presence of a ing from about one-fourth to more than natural thermoremanent magnetization second generation of lath-shaped plagio- three-fourths of the total plagioclase by portable flux-gate magnetometer clase microphenocrysts is diagnostic of in the rocks. Finally, an ash flow at or (Al-Rawi, 1969) has shown that the rocks of the By-Day. The larger first near Tollhouse Flat, apparently belonging Tollhouse Flat Member has reverse generation of oscillatory-zoned plagioclase to the upper member, has normal magnetic polarity. phenocrysts is somewhat more calcic magnetic polarity (Al-Rawi, 1969), (about An60) than those of the Tollhouse suggesting that the direction of remanent By-Day Member Flat Member. First-generation plagio- magnetization may provide a means of The By-Day Member is here named for clase and clinopyroxene phenocrysts are distinguishing rocks of the unit from exposures on the north bank of By-Day slightly smaller and less abundant than those of the reversely magnetized Toll- Creek (lat 38°16'20" N., long those of the Tollhouse Flat Member, and house Flat Member. Although no 119° 18'45" W.; Fig. 1). At the reference pumice and lithic fragments also are complete cooling breaks have yet been section of the Eureka Valley Tuff at smaller on the average. As in the Toll- recognized within the upper member, the Tollhouse Flat, the By-Day Member is house Flat Member, there is a greater lighologic variability of the unit suggests 115 ft thick and is separated by about concentration of crystals in the fine- the possibility that it may consist of 10 ft of sorted clastic rock of possible grained portion of the tuff than in the several distinct small volume ash-flow base-surge origin from the underlying pumice fragments (about 12 percent), sheets of limited areal extent. Tollhouse Flat Member. At the type indicating winnowing of glass shards and locality of the Eureka Valley Tuff, the dust-sized particles. By-Day is separated by a latite lava flow AGE OF THE EUREKA VALLEY from the underlying Tollhouse Flat The By-Day Member is composed of TUFF at least three, and probably many more, Member. At many localities, however, K-Ar age determinations on 13 rocks individual ash flows. Maximum observed particularly near the Little Walker caldera, belonging to the Eureka Valley Tuff are thickness is about 200 ft. The unit tends the contact between the two members is available (Table 2). Single age determina- to thin away from the Little Walker inconspicuous. Air-fall tuff commonly is tions also are available on rocks of the caldera. Both original areal distribution absent, and the complete cooling break Table Mountain Latite and Dardanelles and volume were less than half that of between the two units may be indicated Formation (Table 2). We have obtained the Tollhouse Flat Member. For example, only by several inches of porous, partly new K-Ar ages of 9.9 ± 0.4 and 10.0 the By-Day Member has not yet been welded glassy ash-flow tuff at the base of ± 0.3 m.y. on two ash-flow units of the recognized northeast of Sonora or in the the By-Day Member. This thin and upper member (Table 3), using standard Bodie Hills. relatively nonresistant glassy zone may isotope-dilution techniques (Dalrymple be hidden by soil, vegetation, and At the type locality of the Eureka and Lanphere, 1969). Valley Tuff and in the vicinity of Toll- detached blocks of welded tuff; at many The average of the radiometric age house Flat, the By-Day Member is places, a careful search must be made to determinations on the Eureka Valley normally magnetized (Al-Rawi, 1969). locate the cooling break. Tuff, excepting the two new ages on the The direction of remanent magnetization Exposures of the By-Day Member are upper member and the date of 10.7 m.y. thus may provide yet another method of similar in color and general appearance reported by Evernden and others (1961), distinguishing the Tollhouse Flat and to those of the Tollhouse Flat Member, is 9.2 m.y. With the four ages (from By-Day Members. 4 although weathered surfaces tend to be 8.8 to 9.2 m.y.) reported by Dalrymple somewhat smoother and the average Upper Member (1963) also excluded, the average is size of pumice fragments somewhat The upper member of the Eureka 9.43 m.y. Additional information on the smaller. Rocks of the By-Day Member Valley Tuff is composed of ash-flow age of the Eureka Valley is provided by may be distinguished quickly and un- tuffs similar in general appearance to numerous ages of from 9.1 to 9.5 m.y. ambiguously in the field from those of the those of the Tollhouse Flat and By-Day obtained by Al-Rawi (1969; Gilbert and Tollhouse Flat Member and upper mem- Members. Many of the tuffs of the upper others, 1968) and by Silberman and

GEOLOGY 141 TABLE 2. K-AR AGE DETERMINATIONS ON THE EUREKA VALLEY The new age determinations on rocks Koenig, J. B., 1963, Geologic map of California: TUFF AND OTHER ROCKS OF THE STANISLAUS GROUP of the upper member appear to be Walker Lake sheet: California Div. Mines and Geology, scale 1:250,000. Sample Nuiriber Age (m.y.) Reference slightly too old. Nevertheless, they Korringa, M. K., 1972, Vent area of the strongly suggest that no appreciable Soldier Meadow Tuff, an ash-flow sheet Dardanelles Foimztion intervals of geologic time elapsed between in northwestern Nevada [Ph.D. dissert.): eruption of the successive ash-flow sheets Stanford, Calif., Stanford Univ., 100 p. KA-969 9.3+0.4 1 Lipman, P. W., 1967, Mineral and chemical of the Eureka Valley, thus providing Eureka Valley Tuff variations within an ash-flow sheet from Aso additional support for the close genetic Caldera, southwestern Japan: Contr. Upper Menijer relations of the units. Mineralogy and Petrology, v. 16, p. 300-327. SC-3T 10.04O.3 2 Mackin, J. H., 1960, Structural significance of Tertiary volcanic rocks in southwestern TF-U 9.9+0.4 2 REFERENCES CITED Utah: Am. Jour. Sci., v. 258, p. 81-131. Tollhouse Flat Member Noble, D. C., Bath, G. D., Christiansen, R. L., Al-Rawi, Y. T., 1969, Cenozoic history of the and Orkild, P. P., 1968, Zonal relations KA-961 8.8+0.2 3 northern part of Mono Basin, California and paleomagnetism of the Spearhead and Nevada [Ph.D. dissert.|: Berkeley, and Rocket Wash Members of the Thirsty KA-1124 9.2+0.2 3 Univ. California, Berkeley, 163 p. Canyon Tuff, southern Nevada: U.S. Geol. W23 9.52+0.38 4 Berggren, W. A., 1972, A Cenozoic time- Survey Prof. Paper 600-C, p. C61-C65. scale—Some implications for regional Noble, D. C., Dickinson, W. R„ and Clark, W24 9.19+0.28 4 geology and paleobiogeography: Lethaia, M. M., 1969, Collapse caldera in the Little KA-1907 9.8+0.2 5 v. 5, no. 2, p. 195-215. Walker area, Mono County, California: Chesterman, C. W., 1968, Volcanic geology Geol. Soc. America, Abs. for 1968, Spec. KA-1908 9.6+0.3 5 of the Bodie Hills, Mono County, Paper 121, p. 536-537. Uncertain * California, in Coats, R. R., Hay, R. C., and Priest, G. R., Bowman, H. R.. Hebert, A. J., Anderson, C. A., eds., Studies in vol- Silberman, M. L., Street, Kenneth, Jr., and KA-132 10. 7+? 6 canology: Geol. Soc. America Mem. 116, Noble, D. C., 1974, Eruptive history and KA-9 76 8.9+0.2 3 p. 45-68. geochemistry of the Little Walker volcanic Christiansen, R. L., Noble, D. C , Orkild, P. P., center, east-central California: A progress 9.0+0.2 KA-977 3 and Sargent, K. A., 1968, Cogenetic report: Geol. Soc. America, Abs. with W25 9.35+0.37 4 sequences and source areas in silicic vol- Programs, v. 6 (in press). 9.01-W.27 4 canic terranes: Geol. Soc. America, Abs. Ransome, F. L., 189 8, Some lava flows of the for 1966, Spec. Paper 101, p. 39. western slope of the Sierra Nevada, KA-1909 9.5+0.3 5 Dalrymple, G. B., 1963, Potassium-argon dates California: U.S. Geol. Survey Bull. 89, Table Mountain Latite on some Cenozoic volcanic rocks of the 74 p. Sierra Nevada, California: Geol. Soc. KA-1110 9.0+O.2 1 Ross, C. S., and Smith, R. L., 1961, Ash-flow America Bull., v. 74, p. 379-390. tuffs: Their origin, geologic relations, and 1964, Cenozoic chronology of the identification: U.S. Geol. Survey Prof. References: 1, Dalrymple (1964); 2, Table 3, this Sierra Nevada, California: California Univ. paper; 3, Dalrymple (1963); 4, Dalrymple and others Paper 366, 81 p. (1967); 5, Gilbert and others (1968), Al-Rawi (1969); Pubs. Geol. Sci., v. 47, 41 p. Silberman, M. L., and Chesterman, C. W., 6, Everndefl and others (1961). Dalrymple, G. B., and Lanphere, M. A., 1969, 1972, K-Ar age of volcanism and minerali- * These specimens probably all are from the Tollhouse Potassium-argon dating: Principles, tech- Flat Member. zation, Bodie mining district and Bodie niques, and applications to geochronology: Hills volcanic field, Mono County, San Francisco, W. H. Freeman and Co., California: Isochron/West, no. 3, p. 13-22. 258 p. Slemmons, D. B., 1953, Geology of the Dalrymple, G. B., Cox, Allan, Doell, R. R., Sonora Pass region [Ph.D dissert.]: Chesterman (1972) on lavas of the Mount and Gromme, C. S., 1967, Pliocene Berkeley, Univ. California, Berkeley, 222 p. geomagnetic polarity epochs: Earth and Biedeman Formation, the Silver Hill 1966, Cenozoic volcanism of the central Planetary Sci. Letters, v. 2, p. 1 63-1 73. Sierra Nevada, California, in Bailey, E. H., Volcanic Series, and related units that Evernden, J. F., Curtis, G. H., Obradovich, J., ed., Geology of northern California: overlie the Eureka Valley Tuff in the and Kistler, R., 1961, On the evaluation of California Div. Mines and Geology Bull. 190, glauconite and illite for dating sedimentary Bodie Hills. These data suggest that the p. 199-208. rocks by the potassium-argon method: radiometric age of the Eureka Valley Strand, R. G., 1967, Geologic map of California: Geochim. et Cosmochim. Acta, v. 23, Tuff is approximately 9.5 m.y., slightly Mariposa sheet: California Div. Mines and p. 78-99. Geology, scale 1:250,000. greater than previously inferred. Accord- Gibson, I. L., 1970, A pantelleritic welded Strand, R. G., and Koenig, J. B., 1965, Geologic ing to the Cenozoic time scale of Berggren ash-flow tuff from the Ethiopian Rift map of California: Sacramento sheet: Valley: Contr. Mineralogy and Petrology, (1972), the Eureka Valley Tuff is of late California Div. Mines and Geology, v. 28, p. 89-111. Miocene age. scale 1:250,000. Gibson, I. L., and Tazieff, H„ 1967, Additional Walker, G.P.L., 1972, Crystal concentration theory of origin of fiamme in ignimbrites: in ignimbrites: Contr. Mineralogy and Nature, v. 215, p. 1473-1474. Petrology, v. 36, p. 1 35-1 46. TABLE 3. NEW K-AR AGE DETERMINATIONS ON ROCKS FROM Gilbert, C. M., Christensen, M. N., Al-Rawi, THE UPPER MEMBER OF THE EUREKA VALLEY TUFF Yehya, and Lajoie, K. R., 1968, Structural and volcanic history of Mono Basin, ACKNOWLEDGMENTS California-Nevada, in Coats, R. R., Hay, Sample k2o Radiogenic Ar^ Radiogenic Ar Age Reviewed by H. F. Bonham, Jr. Number (VC. %> (moles/gm) (percent) (m.y.) R. C., and Anderson, C. A., eds., Studies in volcanology: Geol. Soc. America Mem. Noble and Korringa supported by National Science Foundation Grants GA-1546 and 10 116, p. 275-329. 1 7.83 1.17xl0~ 41.9 10.0+0.3 Halsey, J. G., 1953, Geology of parts of the GA-35765 and by National Aeronautics and -10 2 7.76 1•14x10 27.3 9.9+0.4 Bridgeport, California and Wellington, Space Administration Giant NGR-22-007-103; Nevada quadrangles [Ph.D. dissert.] Slemmons' work supported in part by NASA 1. Biotite from densely welded crystallized tuff Berkeley, Univ. California, Berkeley, SC-3T from 600 ft N.80°w. of spot elevation 9972 Grant NGR-29-001-015 and by NSF Grant (Fales Hot Spring 15' quadrangle) (lat 38°23'20"N. 506 p. GP-5034; Al-Rawi supported by Ministry of long 119°21'«5"U.). Johnson, R. F., 1951, Geology of the Masonic Education, Baghdad, by NSF Grant GP-4567, 2. Biotite from vitrophyre TF-U from the base of the mining district, Mono County, California upper member at Tollhouse Flat (lat 38°25'35"ti., and by the Department of Geology and long 119°26'45"w.). [M.A. thesis]: Berkeley, Univ. California, Berkeley, 5 5 p. Geophysics, University of California, Berkeley. Potassium analyses by L. B. Schlocker; argon deter- Kleinhampl, F. J., Davis, W. E., Silberman, minations by E. H. McKee M. L., Chesterman, C. W., and Chapman, MANUSCRIPT RECEIVED DECEMBER 17, 40 10 K decay constants: \ = 0.585xl0~ /yr R. H., 1974, Aeromagnetic and geologic 1973 -10 map of the Bodie Hills and surrounding - 4.72x10 /yr region, Nevada and California: U.S. Geol. MANUSCRIPT ACCEPTED JANUARY 3, Abundance ratio: K40/K = 1.19xl0~4 atom/atom Survey Geophys. Inv. Map (in press). 1974

142 PRINTED IN U.S.i MARCH 1974