The Bishop Tuff: New Insights from Eruptive Stratigraphy1

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ARTICLES The Bishop Tuff: New Insights from Eruptive Stratigraphy1

Colin J. N. Wilson and Wes Hildreth2 Institute of Geological & Nuclear Sciences, Private Bag 2000, Taupo 2730, New Zealand

ABSTRACT The 0.76 Ma Bishop Tuff, from Long Valley caldera in eastern California, consists of a widespread fall deposit and voluminous partly welded ignimbrite. The fall deposit (F), exposed over an easterly sector below and adjacent to the ignimbrite, is divided into nine units (F1–F9), with no significant time breaks, except possibly between F8 and F9. Maximum clast sizes are compared with other deposits where accumulation rates are known or inferred to estimate an accumulation time for F1–F8 as ca. 90 hrs. The ignimbrite (Ig) is divided into chronologically and/or geographically distinct packages of material. Earlier packages (Ig1) were emplaced mostly eastward, are wholly intraplinian (coeval with fall units F2–F8), lack phenocrystic pyroxenes, and contain few or no Glass Mountain-derived rhyolite lithic fragments. Later packages (Ig2) were erupted mostly to the north and east, are at least partly intraplinian (interbedded with fall unit F9 to the east), contain pyroxenes, and have lithic fractions rich in Glass Mountain-derived rhyolite or other lithologies exposed on the northern caldera rim. Recognition of the intraplinian nature of Ig1 east of the caldera and use of the fall deposit chronometry yields accumulation estimates of ca. 25 hrs for an earlier, less-welded subpackage and ca. 36 hrs for a later, mostly welded subpackage. Average accumulation rates range up to Ն1 mm/s of dense- welded massive ignimbrite, equivalent to Ն2.5 mm/s of non-welded material. Comparisons of internal stratification in Ig1 and northern Ig2 lobes suggest the thickest northern ignimbrite accumulated in Յ35 hrs. Identifiable vent positions migrated from an initial site previously proposed in the south-central part of the caldera (F1–8, Ig1) in complex fashion; one vent set (for eastern Ig2) migrated east and north toward Glass Mountain, while another set (for northern Ig2) opened from west to east across the northern caldera margin. Vent locations for Ig1 and Ig2 southwest of the caldera have not been identified. The new stratigraphic framework shows that much of the Bishop ignimbrite is intraplinian in nature, and that fall deposits and ignimbrite units previously inferred to be sequential are largely or wholly coeval. Fundamental reassessment is therefore required of all existing models for the eruption dynamics and the nature and causes of pre-eruptive zonations in trace elements, volatiles, and isotopes in the parental magma chamber.

Introduction The Bishop Tuff was erupted at ca. 0.76 Ma (Bo- Gardner et al. 1991), and documenting composi- gaard and Schirnick 1995), accompanying collapse tional zonation(s) in silicic magma chambers (e.g., of Long Valley caldera in eastern California (figure Hildreth 1979). Numerous other studies have built 1). Long Valley has been a focus of rhyolitic mag- on, modified or challenged the petrological studies matism for at least the past 2.1 m.y., but the Bishop of Hildreth (1979), and the nature and origins of the Tuff represents over 80% of the total erupted compositional variations are subjects of continuing magma during this time (Bailey et al. 1976; Hil- interest (e.g., Michael 1983; Cameron 1984; Halli- dreth 1979; Metz and Mahood 1985, 1989; Bailey day et al. 1984; Anderson et al. 1989; Wolff et al. 1989). The Bishop Tuff is important for its role in 1990; Anderson 1991; Dunbar and Hervig 1992; the development of concepts about ignimbrites Hervig and Dunbar 1992; Lu et al. 1992; Chris- (e.g., Gilbert 1938; Sheridan 1970; Sheridan and Ra- tensen and DePaolo 1993; Bogaard and Schirnick gan 1972), modeling large caldera-forming explo- 1995; Duffield et al. 1995). sive eruptions (e.g., Hildreth and Mahood 1986; In all published studies two crucial aspects of the Bishop Tuff eruption are assumed. 1. A widespread (plinian) fall deposit was fol- 1 Manuscript received July 1, 1996; accepted January 22, 1997. lowed by emplacement of an ignimbrite. 2 U.S. Geological Survey, Mailstop 910, 345 Middlefield 2. Based on the Fe-Ti oxide magmatic tempera- Road, Menlo Park, CA 94025. ture estimates of Hildreth (1979) and the lithic

407 408 C. J. N. WILSON AND W. HILDRETH

Figure 1. The eastern-central Sierra Nevada–Long Valley area, showing localities named in the text. The patterned area represents generalized, surface or near-surface outcrops of the Bishop ignimbrite (modiﬁed from Hildreth 1979; Bailey 1989), while all localities of Bishop plinian fall deposit in this area lie east of the line marked. The Long Valley caldera margin in this and subsequent maps is from Bailey (1989), while the ‘‘initial vent site’’ is the area within which the vent for early phases of the eruption were sited (from Hildreth and Mahood 1986). Marginal ticks in this and all other maps represent 10 km squares in the UTM metric grid. All 6-digit grid references in the paper are given to 100 m in the UTM grid; the ﬁrst 3 digits are east, and second 3 north, coordinates; e.g., Crestview is at 253801, i.e., 25.3 km east, 80.1 km north. Abbreviated locality names are: BC ϭ Birchim Canyon; BHS ϭ Benton Hot Springs; BSH ϭ Blind Spring Hill; BSV ϭ Blind Spring Valley; CB ϭ Chalk Bluffs; DC ϭ Dexter Canyon; FS ϭ Fish Slough; LRV ϭ Little Round Valley; NC ϭ North Canyon; PV ϭ Pleasant Valley; RCG ϭ Rock Creek Gorge; RRC ϭ Red Rock Canyon; WC ϭ Wet Canyon. studies of Hildreth and Mahood (1986), the ignim- southeast (Tableland lobe), partly overlapping in brite consists of three parts: (a) Pyroxene-free units time with (c) pyroxene-bearing units of higher- to of lower magmatic temperature (723–737°C) em- highest-magmatic temperature (749–790°C) emplaced mostly to the south and southeast of the cal- placed mostly to the north (Mono, Adobe, and up- dera (Gorges, Chidago, and lower San Joaquin lobes per San Joaquin lobes). However, these two crucial of Hildreth 1979). These were followed by (b) pyrox- assumptions have never been critically tested by ene-bearing units of higher magmatic temperature detailed stratigraphic study. Here we present a de- (733–763°C) emplaced mostly to the south and tailed re-examination of the stratigraphy of the Journal of Geology T H E B I S H O P T U F F 409

Table 1. Summary of Diagnostic and Characteristic Features Used to Deﬁne a Bishop Fall Deposit Stratigraphy

Fall Grid Tmax, unit Diagnostic and characteristic features Localitya reference (mm)

F1 poorly bedded, coarsest in middle to upper parts, capped by distinctive 19 537578 300 thin very-fine-ash, finer-grained than F2 F2 moderately bedded; three coarser bands merging into one distally; top 19 537578 350 and base defined by characteristic beds in F1 and F3; maximum clast sizes similar to F3 and F4 F3 two normally graded beds of subequal thickness 19 537578 120 F4 three beds: lower is non-graded, coarser than top of F3; middle is mas- 25 594564 370 sive to normally graded, coarsest part of F4; upper is finer-grained, merging with normally graded top to middle bed Ͼ30–35 km from vent. All beds have maximum clast sizes similar to those of F2 and F3 F5 base coarser than F4; basal thin, normally graded bed overlain by thick, 19 537578 1700 massive to very poorly bedded unit, sometimes inverse graded in basal 20–30% F6 basal contact marked by finer bed; moderately bedded alternations of 57 691402 1870 massive fall material and very low-angle, cross-bedded units; top marked in eastern localities by distinct finer-grained low-angle cross- bedded band; no systematic overall grading; maximum clast sizes similar to F5 F7 base defined by marked increase in abundance and size of lapilli- to 436 659814 3900 block-sized pumices; marked increase in overall grainsize over F6 at sites east and northeast of vent; moderately bedded alternations of massive fall material and very low angle, cross-bedded penecontempo- raneously wind-redeposited units; no systematic overall grading; coarsest unit in Bishop fall deposit F8 base defined by marked decrease in abundance and size of lapilli- to 94 789465 610 block-sized pumices; plane-parallel bedded but internally very low angle cross-stratified; no systematic grading except for pronounced nor- mal grading in top 50–100 mm with admixed fine to very fine ash; level of incoming of Glass Mountain rhyolite lithics is 60–80% through F8 thickness F9 base defined by ashy top to F8; Glass Mountain rhyolite fragments in 436 659814 930 lithic fraction; whole unit is plane-parallel bedded but internally very low angle cross-stratified; no systematic grading; full thickness not seen as always truncated by erosion surface or overlain by ignimbrite a Locality where each unit is thickest; grid reference and thickness are given.

Bishop Tuff to arrive at a more accurate picture for fragments in the deposits (cf. Hildreth and Mahood the eruption. We use ‘‘Bishop Tuff’’ as the formal 1986) were studied to give rapid quantitative char- term for all deposits from the eruption (Bateman acterization of ignimbrite whatever the welding, 1965), together with ‘‘Bishop fall deposit’’ and crystallization, or alteration state. Lithic abun- ‘‘Bishop ignimbrite’’ informally to denote the two dances were measured by counting the lithic frag- major products of the eruption. Our description ments Ͼ5 mm long per square meter of exposure, concentrates on the ignimbrite, the unit which has regardless of the welding state. Lithic proportions been previously the most studied and sampled. were calculated by counting the main lithologies present as Ͼ5 mm-long clasts at an exposure to a total of 100–300 clasts. Key lithic lithologies that Methods demarcate eruptive packages are obsidian and devi- We used three approaches to identify and correlate trified-rhyolite which, from their crystal-poor na- marker planes in the tuff. (a) Detailed documenta- ture, are inferred to be derived from the Glass tion of the fall deposits enabled a consistent stratig- Mountain edifice and its associated volcaniclastic raphy to be recognized and correlated over a 90° fans (Metz and Bailey 1993). Errors in field counting sector E of the caldera. We found many localities of lithics are likely to be larger than those in the where fall deposits and ignimbrite were interbed- laboratory techniques of Hildreth and Mahood ded, giving the relative timing of events. (1986). However, variability within the ignimbrite (b) The amounts, types, and proportions of lithic (lithic abundances between Ͻ5 and Ͼ500 clasts/ 410 C. J. N. WILSON AND W. HILDRETH m2; proportions of rhyolite lithics from 0 to Ͼ90 rial interbedded in the ignimbrite, to calibrate ar- count %) has proved in practice to be much greater rival of flow units within the fall deposit accumula- than differences between results from different ob- tion, to correlate eruptive and emplacement events servers, or between sites in emplacement units of where the fall deposits are preserved, and to esti- uniform composition but different welding state. mate the duration of this part of the eruption. Lithic proportions were similarly determined in The Bishop fall deposit is of plinian dispersal, the fall deposits, but were also measured in weight well to moderately sorted, contains fine ash in mi- percent on unconsolidated samples. nor quantities at only three levels (two are useful (c) The field appearance, crystal content, miner- markers, capping F1 and F8; figures 2 and 3), and alogy, and chemistry (notably Ba, Sr, Rb, Zr con- shows an overall inverse grading. The apparent tents) of pumices were used to define the incoming similarity at different sites, however, can be mis- of crystal-rich (Ͼ20–25 wt %) pyroxene-bearing leading as the uppermost, coarsest fall material clasts that represent the hottest, least-evolved may represent different levels in the overall deposit magma erupted (Hildreth 1979). These pumices are (cf. Gardner et al. 1991, who assumed the top of relatively dense, have a characteristic crimped fi- the fall deposit was at the same stratigraphic level). brous matrix texture, and have many of the quartz Some intervals in fall units F6 to F9 show a discon- and feldspar phenocrysts strung out as schlieren. tinuous low-angle cross-bedding defined by a divi- sion of the material into finer crystal-rich versus coarser pumice-rich beds on a 1–5 cm scale. Al- Fall Deposit Stratigraphy though sometimes described as surge deposits (e.g., In the study area, the Bishop fall deposit is exposed Wohletz and Sheridan 1979; Cas and Wright 1987; around, beneath, and locally within the ignimbrite Izett et al. 1988; Gardner et al. 1991), these inter- in a ca. 90° sector from SE to NE of the caldera, but vals show features more consistent with a primary is absent N of the caldera (figure 1). We subdivide fall origin, such as a regionally correlatable dm- to it on the basis of consistent grainsize and grading m-scale plane-parallel bedding, a scarcity of mega- characteristics into nine fall units (F1 to F9; table ripples, a lack of consistent paleocurrent direc- 1) recognizable over the area examined (figure 2). tions, the angularity of many pumices, and a lack This stratigraphy enables us to correlate fall mate- of clast imbrication. We attribute the cross-bedding

Figure 2. Scaled stratigraphic columns of units in the Bishop fall deposit. Here and in figure 7, clasts are drawn to scale, with stippling and dashes for coarse ash (2 mm–63 µm) and fine ash (Ͻ63 µm), respectively. Open and filled shapes represent pumice and lithic clasts, respectively. Ignimbrite flow units (where present) are scaled to their correct thickness but are left unornamented for clarity (see text for explanation of ignimbrite labeling); ps ϭ paleosol. a. Index map. b. Proximal to medial sites. c. Medial to distal sites to the southeast. d. Medial to distal sites to the northeast. Grid references for sites are: 661684 (loc. 12), 719781 (15), 787486 (16), 790475 (17), 537578 (19), 661413 (24), 594564 (25), 541607 (26), 636638 (54), 691402 (57), 789465 (94), 701753 (95), 659880 (371), and 628558 (437). Distances from vent are from the nearest edge of the initial vent area proposed by Hildreth and Mahood (1986). Figure 2. (Continued) 412 C. J. N. WILSON AND W. HILDRETH

Figure 3. Bishop fall deposits, showing units introduced here, and the overall inverse grading of the deposit. a. Basal parts of the fall deposit at loc. 57 (figure 2c) showing overall inverse grading up to F5. b. Middle to upper parts at loc. 94 (figure 2c); I ϭ ignimbrite (Ig2Ea, see text). to localized penecontemporaneous reworking of roxene-free, rhyolite-lithic-free ignimbrite (Gorges falling pyroclasts by strong swirling winds (analo- and Chidago lobes of Hildreth 1979) is coeval with gous to firestorm winds) associated with the em- the fall deposits (units F2–F7) exposed in Chalfant placement of coeval pyroclastic flows. Valley (figure 4). In addition, the incoming of rhyo- The fall deposit correlations (figure 2b–d) show lite lithics coincides with the first appearance of that the level of onset of continuous pyroclastic pyroxene-bearing pumices in the fall deposits and flow deposition is strongly diachronous (figures 2 coeval ignimbrite on the eastern side. From this we and 4). This diachronous onset does not simply re- infer that the onset of rhyolite lithics reflects a sult from erosion by the flows, as we have found change in vent position that caused the eruption fall units (which show little sign of disturbance) in- to tap different (pyroxene-bearing) portions of the tercalated with ignimbrite in both proximal and magma body. One implication of this is that the distal areas of the ignimbrite (e.g., figure 5). In gen- pyroxene-free and pyroxene-bearing magmas might eral, fall material beneath the ignimbrite was not necessarily have been in simple vertical succes- eroded and disturbed only where the paleotopogra- sion in the magma chamber but could have been in phy was steep. The fall stratigraphic framework part laterally juxtaposed. also demonstrates unequivocally that large-scale flow deposition did not simply follow after plinian Introduction to Ignimbrite Stratigraphy fall activity, as is widely assumed. Fall deposits are intercalated with ignimbrite up to the highest Hildreth (1979) referred to five major ignimbrite stratigraphic levels preserved east of the caldera. In bodies as lobes named Gorges, Chidago, Adobe, particular, recognition of the level at which rhyo- Mono, and San Joaquin from their broad geographic lite lithic fragments first appear in the eruption se- position (table 2). One additional lobe (Tableland) quence shows that essentially all of the early, py- overlies and extends beyond the Gorges and Chi- Journal of Geology T H E B I S H O P T U F F 413

Figure 4. Scaled summary stratigraphic columns to show relationships east of the caldera between the ignimbrite units that lack pyroxene phenocrysts and rhyolite lithics (Ig1Eb here; Gorges and Chidago lobes in Hildreth 1979), and those with them (Ig2E here; Tableland lobe of Hildreth 1979). The basal contact, where exposed, is denoted by wavy vertical lines; fall deposits are unornamented; ignimbrite is shaded; ‘‘R’’ ϭ level of onset of Glass Mountain rhyolite lithics; e ϭ erosion horizon. The dashed lines marked in some fall deposit sections just above the ‘‘R’’ line represent the fine-ash-bearing marker level separating F8 and F9 (figure 2). Grid references for sites are: 787486 (loc. 16), 790475 (17), 792466 (21), 661413 (24), 772567 (80), 701753 (95), 631454 (368), 659880 (371), 657816 (435), 659814 (436), 609554 (461), 608540 (481) and 697621 (640). dago lobes and was recognized as a separate cooling and a solitary exposure north of the caldera at Clark unit having pyroxene-bearing pumices. We here in- Canyon (figure 6). We group all the demonstrably troduce a new temporally and geographically con- early, pyroxene-free ignimbrite together as a com- strained labeling system for the major ignimbrite mon unit, Ig1, and denote the different geographic units or ‘‘packages’’ (table 2). We use the term and chronological packages by letters (table 2). The ‘‘package’’ for ignimbrite with similar components spatial relationships between the major subpack- and lithological characteristics (but independent of ages of Ig1 east of the caldera are illustrated in fig- welding zonation) emplaced as a series of pulses or ures 4 and 6–8. flow units. We label these packages with an Ig (for Package Ig1E. This package corresponds to the ignimbrite), a number (1 or 2) for earlier (pyroxene- Gorges and Chidago lobes of Hildreth (1979), to- free) or later (pyroxene-bearing), respectively, and gether with distal correlatives around Blind Spring then capital letters for the sector direction where Hill and the Benton Range. It is subdivided on the the deposit is best developed. Subpackages, where basis of correlations with underlying and interbed- recognized, are labeled a, b, c, in stratigraphic order. ded fall units into two subpackages, Ig1Ea and Ig1Eb (figures 5–7). Subpackage Ig1Ea, recognized in upper Owens Ignimbrite Ig1 Gorge and around Lake Crowley (figures 5 and 6), Ignimbrite east and southeast of the caldera—dom- is of limited extent, reaches a maximum thickness inated by pumices that lack pyroxene, are generally of ca. 80 m and is characterized by the following crystal-poor and yield Fe-Ti oxide temperatures in features: (a) High lithic abundances (as many as the narrow range 725–737°C—was grouped into 200–300 clasts/m2). Lithologies are dominated by the Gorges and Chidago lobes by Hildreth (1979). metapelite with subordinate other metamorphic Similar ignimbrite forms a lower cooling unit along rocks and granitoids indicating a vent site in the the Middle Fork of the San Joaquin River (figure 1), south-central part of the caldera, as proposed for the Figure 5. Field relationships in Ig1Ea ignimbrite and associated fall deposits. a. The most proximal fall material yet found (part of F6), separating Ig1Ea and Ig1Eb; loc. 2 (figure 7) on the eastern shoreline of Lake Crowley. b. View west from 516605 into upper Owens Gorge showing two exposures of the same fall bed (part of F6) separating Ig1Ea from Ig1Eb; the location marked with an asterisk is shown in c. Note also the c. 30 m-high knoll of Ig2E in the background; the contact with Ig1Eb is toward the base of the knoll. The prominent traces on the left wall of the gorge are roads, which run obliquely across the stratigraphy of the Bishop deposits. Gorge is roughly 100 m deep. c. Fall material correlated with F6, incorporating three thin ignimbrite flow units (not correlated), between continuous Ig1Ea below and Ig1Eb above; loc. shown in b, at 509612. Hammer is ca. 30 cm long. d. Roadcut on US 395 (‘‘Big Pumice Cut’’; loc. 19, figure 7) where Ig1Ea is only represented by thin flow units and some surge deposits within the plinian fall deposit (figure 7); the base of the fall deposit is roughly at road level. Journal of Geology T H E B I S H O P T U F F 415

Table 2. Summary of the Ignimbrite Nomenclature Used in this Paper

Ignimbrite Sub- Hildreth (1979) package package Main Characteristicsa equivalent lobeb Sector

Ig1E Ig1Ea pf; nrl; non- to poorly welded Gorges (lp) E Ig1Eb pf; nrl; non- to densely welded Gorges (remainder) NE, E, SE Chidago Ig1SW pf; nrl; non- to densely welded San Joaquin (lp) SW Ig1NW pf; nrl; non-welded (Clark Canyon outcrop) c Ig2E Ig2Ea pb; srl; non- to poorly welded Tableland (lp) NE, E, SE Ig2Eb pb; arl; non- to poorly welded Tableland (mp) NE, E, SE Ig2Ec pb; arl; non- to poorly welded Tableland (up) E Ig2SW pb; nrl; moderate to dense welded San Joaquin (up) W Ig2NW Ig2NWa pb; nrl; non- to densely welded Mono (lp) NW Ig2NWb pb; nrl or srl; poorly to densely welded Mono (up) NW Ig2N Ig2Na pb; arl; non- to densely welded Adobe (in part) N Mono (in part) N Ig2Nb pb; arl; non-welded to sintered Adobe (in part) N Ig2Nc pb; srl; very low lithic Adobe (in part) N abundance; non- to poorly welded Mono (in part) N a pf: pyroxene-free; pb: pyroxene-bearing; nrl: ‘‘no’’ rhyolite lithics; srl: sparse rhyolite lithics; arl: abundant rhyolite lithics. b (lp) ϭ lowest part; (mp) ϭ middle part; (up) ϭ upper part. c 1 isolated outcrop. coeval fall deposits by Hildreth and Mahood (1986). tion of lithic-enriched segregation pipes and less of- (b) Low degrees of welding, being non-welded to ten by swarms of pumice lapilli and blocks. Such sintered. (c) Several included beds of fall material. clearly defined flow-unit boundaries are rare in The upper contact of Ig1Ea is defined by the thick- other thick, proximal to medial Bishop ignimbrite. est and most persistent of these fall beds, correlated At two sites (loc. 26 [figure 7] and 550607) in up- with part of fall unit F6 (figures 5 and 7) on the basis per Owens Gorge, Ig1Ea rests conformably (with no of grainsize and the characteristics of overlying and apparent scouring) on only 43 and 44 cm, respec- underlying ignimbrite. This fall bed represents the tively, of fall deposits (F1 and lower F2). The ab- longest hiatus in ignimbrite deposition in areas sence of upper F2 and F3–F5 under the earliest ig- close to the E margin of the caldera. (d) Numerous nimbrite (figure 7) and the distance of these sites flow unit boundaries, commonly defined by trunca- (ca. 14 km) from probable source suggests that pyroclastic flows began very early, if not right at the start of, the Bishop eruption. The pre-ignimbrite fall stratigraphy and lithic counts in the ignimbrite at loc. 25 (figure 2b) show that no Ig1Ea flows travelled Ͼ20 km along present-day Owens Gorge (figures 6 and 8) and, from the uninterrupted fall units F2 to mid-F6 at loc. 54 (figure 2d), also did not reach 23 km to the east. The contrast between the Ն80 m of Ig1Ea in upper Owens Gorge (figure 5b) and the total of ca. 45 cm only 3 km away at 15–100 m higher elevation (loc. 19, figures 5d and 7) suggests that the Ig1Ea flows were confined to a paleovalley coincident with or close to the line of the modern Owens Gorge, and that their limited distribution was primarily due to low emplacement velocities. Subpackage Ig1Eb is the greater part of the Gorges and Chidago lobes of Hildreth (1979). It con- Figure 6. Geographic distribution of Ig1 deposits. The tains no Glass Mountain lithics, except in the small outcrops marked at Clark Canyon (CC) and Sacra- northern Ig1Eb material west of Casa Diablo Moun- mento Mine (SM) represent outliers of Ig1 material dis- tain, some of the lowest ignimbrite in Chidago cussed in text. BH ϭ basement high. Canyon, and some thin flow units in Blind Spring Figure 7. Stratigraphic columns for five sections in upper Owens Gorge (OG) with inferred stratigraphic relationships between the thick ignimbrite of Ig1Ea in Owens Gorge and the nearby fall-dominated stratigraphy at loc. 19. The horizons marked ‘‘Ig1Ea in’’ and ‘‘Ig1Ea out’’ at loc. 19 represent inferred stratigraphic levels at which deposition of Ig1Ea began (cf. loc 26) and finished (cf. loc 521) nearby in Owens Gorge. Symbols and conventions as in figure 2; I ϭ ignimbrite; I/S ϭ mixed ignimbrite/surge material; ps ϭ paleosol. Grid references for localities are: 482630 (loc. 2), 537578 (19), 541607 (26), 511607 (444), and 540609 (521).

Figure 8. Scaled cross section (vertical exaggeration approx ϫ 20) along the line of Owens River, to show thicknesses of Bishop ignimbrite packages Ig1Ea, Ig1Eb, and Ig2E as seen on the walls of Owens Gorge, projected onto the true left side. Note the off-lapping relationships such that the thickest exposures of each package occur successively farther from the caldera. LVd ϭ Long Valley dam; PVd ϭ Pleasant Valley dam. Vertical scale is meters above sea level; horizontal scale from an arbitrary datum in the centre of Lake Crowley. Journal of Geology T H E B I S H O P T U F F 417

Valley (figure 1). These exceptional deposits con- The thickest Ig1Eb material was deposited along tain sparse (Ͻ5 count%) rhyolite lithics inferred to the paleo-valleys of the Owens River and Chidago have been picked up along with other recognizably Canyon drainages. The distal ends of these two ig- locally derived lithologies as early flows swept over nimbrite fans form separate lobes (figure 6), and the Glass Mountain volcaniclastic fan en route to sections in Chalfant Valley and Fish Slough demon- eastern and northeastern depositional areas. Ig1Eb strate that the Ig1Eb fans thin laterally and/or dis- shows a wide range of thicknesses and welding in- tally from Ͼ50 m of densely welded tuff to zero in tensities, from densely welded massive accumula- Ͻ3 km (figure 4). For example, the obviously in- tions up to at least 140 m thick to individual deci- traplinian flow unit exposed in the Chalfant Valley meter-thick flow units interbedded with fall pumice pit at loc. 21 (figures 4 and 9a) represents material (figure 9a,b). the non-welded extreme southern fringe of the Thick, densely welded Ig1Eb forms much of the main Chidago Ig1Eb fan. The limited control on the Bishop Tuff along Owens Gorge, Rock Creek shape of the thick, densely welded Ig1Eb deposits Gorge, and Chidago Canyon. Lithic abundances in suggest that they had geometries akin to those of the earliest material resting on the post-Ig1Ea fall alluvial fans, thinning evenly and rapidly toward bed (i.e., part of F6) are typically 50–100 clasts/m2 their margins, having apices at the two major low but diminish gradually upward (regardless of weld- points on the Benton Range–Casa Diablo Moun- ing intensity) to minima of 10–30 clasts/m2. Coun- tain basement high, and supporting an appreciable terbalancing this, maximum lithic sizes generally dip (1–5°) on the primary depositional surface increase upward by factors of 2–3, to reach 10–30 (though such dips have been subsequently modified cm at the top in proximal areas. Some proximal ex- by tilting associated with regional faulting: Pinter posures contain large (20–30 cm) lithic fragments 1995). Flow-unit boundaries are rare within the clustered together, but no lithic lag breccias have thick (Ն20 m) Ig1Eb accumulations, except in the been found in this sector (cf. package Ig2NW, be- distal parts of the fans, and fall intercalations are low), suggesting that the vents lay at least a few unknown. kilometer inboard of these localities. In contrast, the distal non-welded fringes of Ig1Eb shows the most intense welding of any ig- Ig1Eb were emplaced, at least initially, as individ- nimbrite east of the caldera, but the welding zona- ual decimeter-to-meter-thick flow units at time in- tion is complex. Lower Ig1Eb is moderately welded tervals often sufficient for centimeter-to-decimeter in its most proximal outcrop (at 496613), the weld- thick fall deposits to accumulate. For example, ing intensity diminishes upward to loose, uncon- from the fall chronology (p. 430, below; table 3), solidated material (e.g., at 473615), then back to these intervals ranged from a few minutes (for 1 cm sintered at the top. Welding intensity in the basal of F6 material) to as long as ca. 3 hrs (for 45 cm of zone is greatest in uppermost Owens Gorge, dimin- fall material) between the lower flow units at loc. ishes rapidly to the south and north as the deposits 95 (figures 2d and 9b). Subsequently, flows were thin away from the paleo-Owens Gorge, and more emplaced at shorter intervals, resulting in a contin- gradually to the east along the line of the modern uous, vaguely stratified ignimbrite deposit. The gorge toward the basement high (figure 8). In con- most far-travelled flows northeast of the caldera oc- trast, the upper Ig1Eb is merely sintered in the most cur in two batches, one toward the top of fall unit proximal areas, whereas partial welding with a F6, the other in upper F7 and lower F8; at Sacra- weak eutaxitic texture develops to the east toward mento Mine (figure 6) the distal fringe of the Chi- and across the top of the basement high. Down- dago fan was emplaced only during F7 (ϮF8) times. valley along Owens Gorge, these two zones of Thickness variations point to the flows having greater welding merge to give a single thick, mostly been preferentially channelled through and along densely welded unit that forms 100–150 m cliff sec- topographic lows in the basement rocks. The distal tions; this unit is also prominent in Rock Creek Ig1Eb flow units were energetically emplaced, lo- Gorge and along Chidago Canyon (coeval material cally showing shearing and incorporation of fall emplaced along a different route). Downstream be- material and incorporation of colluvial debris (fig- yond the basement high in Owens Gorge the de- ure 9c). Local heights climbed by the flows im- grees of welding in the uppermost few meters of ply that velocities of 20–30 m/s were maintained Ig1Eb are low (sintered to non-welded) in middle for significant distances from source (up to 30– Owens Gorge (e.g., at 608540), then increase to 35 km). dense welding (e.g., at 631454) as Ig1Eb becomes Both Ig1 subpackages have similar lithic propor- buried undersuccessivelygreaterthicknesses oflater tions, dominated by metapelite (Hildreth and Ma- Ig2E ignimbrite (Tableland lobe of Hildreth 1979). hood 1986). At many exposures, granitoid lithics Figure 9. Field relations in Ig1Eb. a. A single Ig1Eb flow unit representing the extreme southern fringe of the ‘‘Chidago lobe’’ interbedded with unit F8 at loc. 21, 792466. b. Thin Ig1Eb flow units (I) interbedded with part of F6; just north of loc. 95 (figure 2d) at 701754. c. Thin F7 fall deposits interbedded with Ig1Eb flow units at loc. 434 (686745); note the lenzoid bodies (L) rich in locally derived lithic fragments. Journal of Geology T H E B I S H O P T U F F 419

Table 3. Estimation of the Duration of Fall Phases of the Bishop Tuff Eruption (see p. 430 for details)

Accumulation Inferred Cumulative Bishop Thickness rate from times, times in fall Comparative Locality at locality published e.g.s seconds seconds unit Isopleth Usedb (mm) (mm/s) (hrs) (hrs)

F1 ML5a ϭ 10 19 300 .006 50000 50000 (14) (14) F2 ML5a ϭ 10 25 230 .02 12000 62000 (3) (17) F3 (Unit 2 used as model) 25 70 (.02) 4000 66000 (1) (18) F4 (Unit 2 used as model) 25 370 (.02) 19000 85000 (5) (23) F5 ML5a ϭ 40 19 1700 .035 49000 134000 (14) (37) F6 (Unit 5 used as model) (94) (1840) (.022)c 82000 216000 (23) (60) F7 ML5a ϭ 50 436 3900 .04d 98000 314000 (27) (87) F8 (Unit 5 used as model) 24 430 .035 12000 326000 (3) (90) F9 (Unit 5 used as model) 436 930 .035 27000 e (8) a ML5 ϭ average maximum lengths of the 5 largest lithic fragments, units in millimeters. b Localities used are at grid references 537578 (loc. 19), 661413 (24), 594564 (25), 789465 (94), and 659814 (436). c The full thickness of fall unit F6 is not exposed at loc. 19 where unit F5 is complete. As the relative thickness proportions of F5 and F6 are very similar at all sites where both fall units are complete, the accumulation rate for F6 here is estimated from loc. 94 where the accumulation rate for F5 is calculated pro-rata to have been (940 mm/1700 mm)*0.035 mm/s, i.e., 0.022 mm/s. d 0.04 cm/s represents a best estimate, based on the single historical example of Tambora (table 4). Prehistoric examples are inferred to significantly overestimate the accumulation rates. e Cumulative figure uncertain because of the possibility of a time break between F8 and F9. are the next most common lithology, but they are through the Ig1/Ig2 change. However, the wide- often sub- to well-rounded and fragile, suggesting spread occurrence of a fall bed in medial to distal derivation from pre-existing weathered glacial till. areas of the ignimbrite demonstrates that flow gen- Recycled co-eruptive tuff occurs as a minor compo- eration was less voluminous or less energetic dur- nent (Ͻ5 count %) in Ig1E and the coeval fall depos- ing the Ig1/Ig2 transition. its; this tuff varies from sintered ignimbrite similar Package Ig1SW. This package is represented by in appearance to Ig1Ea (rich in metapelite frag- several glacially eroded remnants in the Middle ments) to intensely welded black vitrophyre, often Fork of the San Joaquin River (figure 1). Hildreth breadcrusted and revesiculated. (1979) reported two cooling units in this area; the The upper contact of Ig1E is unequivocal where lower (Ig1SW) lacks pyroxene phenocrysts and the ignimbrite is overlain by F8 and F9 fall material yields magmatic temperature estimates of 725– that contains the horizon where rhyolite lithics 728°C. Preserved material ranges from non-welded first appear. This fall in turn is overlain by Ig2E to densely welded. Lithic abundances are compara- (figure 4). However, at loc. 481 (figure 4) the incom- ble to those of Ig1Eb (Ͻ5 to 71 clasts/m2). There ing of rhyolite lithics occurs in intensely vapor- are greater amounts of basalt and granite, possibly phase altered material ca. 5 m below the top of ig- picked up while the flows crossed the steep divide nimbrite vertically continuous with Ig1Eb. Above between Long Valley and the San Joaquin headwa- this is a rhyolite-lithic-bearing, cross-bedded, fines- ters, rather than reflecting a different vent area to poor surge or wind-affected fall bed, correlated with Ig1Eb. No fall material has been found with the part of F9 seen between ignimbrite packages Ig1E Ig1SW package, but flow-unit boundaries defined and Ig2E in lower Owens Gorge (figure 10). Closer by swarms of fiamme are locally well defined (e.g., to the caldera than loc. 481, the incoming of rhyo- at 148633), implying episodic deposition. lite lithics and pyroxene-bearing pumices—diag- Package Ig1NW. Represented by an isolated ex- nostic of the Ig2E ignimbrite—is not accompanied posure near the northern caldera wall at Clark Can- by any fall or surge intercalation or parting, sug- yon (figure 6), this ignimbrite lies topographically gesting flow deposition may have continued below the earliest of the pyroxene-bearing material 420 C. J. N. WILSON AND W. HILDRETH

Figure 10. Upper part of the east wall of Owens Gorge at 632469, showing the F9 fall parting between dense welded Ig1Eb below (‘‘lower cooling unit’’ of Sheridan 1968) and Ig2E above. In Ig2E, the paler base is poorly to moderately welded, then the darker top is dense-welded, forming the ‘‘upper cooling unit.’’ Ig2E is roughly 60 m thick. The F9 fall parting is ca. 35 cm thick, is poorly to moderately welded, poorly bedded, and composed of coarse ash to medium lapilli. Hammer is 32 cm long. of the northern lobes. The true stratigraphic rela- lobes by Hildreth (1979), one east (Adobe) and the tionship is not exposed, but Ig1NW probably lies other west (Mono) of the N-S trending basement paraconformably below Ig2NW. The lithic abun- ridge of Sagehen Peak (ﬁgure 1). From lithic studies dances (27–37 clasts/m2), sizes (3–4 cm), and lithol- and Fe-Ti oxide temperatures, the Adobe and Mono ogies (metapelite dominant, rhyolite absent) are lobes were interpreted to represent partly overlap- similar to those of IgIEb. For this material to have ping but progressively higher-temperature magma reached this site we infer that it must have wholly batches erupted as a ring fracture opened up anti- predated vent opening or collapse along the north- clockwise around the northern margin of the cal- ern caldera rim. dera (Hildreth 1979; Hildreth and Mahood 1986). Although pumice compositions and Fe-Ti oxide temperatures overlap to a great extent between Ignimbrite Ig2 Ig2E (‘‘Tableland’’) and northern Ig2 (‘‘Adobe’’, The term Ig2 is applied to ignimbrite erupted later ‘‘Mono’’) the operative assumption since Hildreth than Ig1 and characterized by common to abundant (1979) and Hildreth and Mahood (1986) has been crystal-rich pumices that include two pyroxenes as that northern Ig2 in large part postdates Ig2E. In phenocryst phases. As with Ig1, Ig2 is divided geo- contrast, we infer that the northern Ig2 packages graphically into four packages (Ig2E, Ig2SW, Ig2NW were erupted largely synchronously with Ig2E, con- and Ig2N, some of which are further divided into strained by the following evidence. subpackages) that largely correspond to the Table- a) Nearly all pumices (Ͼ99%) from the northern land, upper San Joaquin, Mono and Adobe lobes, re- lobes are pyroxene-bearing, but pyroxene is absent spectively, of Hildreth (1979). or vanishingly rare in pumices from Ig1E and coeval North of the caldera Ig2 was divided into two fall deposits (F1 to early F8). Eruption of northerly Journal of Geology T H E B I S H O P T U F F 421

Ig2 is therefore inferred to have not commenced tribution area for this material, here labeled Ig2E, earlier than the close of F8 or the onset of I2E flows. and divide it into three subpackages. All Ig2E mate- b) Evidence presented below shows that vents rial contains some rhyolite lithics and pyroxene- along the northern caldera margin opened up from bearing pumices. In proximal areas where no fall west to east, the opposite of the sequence inferred material separates Ig1Eb and Ig2E, we use the in- by Hildreth and Mahood (1986). Eruption of north- coming of rhyolite lithics to define the basal con- ern Ig2 is thus not constrained to postdate the latest tact of Ig2E. Further work is required to delimit the Ig2E. precise onsets of rhyolite lithics and pyroxene-bear- c) Only one ignimbrite outcrop matching north- ing pumices, but from existing data they do closely, ern Ig2 has been found east of the caldera. Field evi- if not exactly, coincide in both ignimbrite and fall dence shows that the northern lobes traversed a deposit. hilly topography on and beyond the caldera rim Ig2E is more widely and evenly distributed than (heights climbed indicate local minimum veloci- Ig1E, although thinner and less welded. Apart from ties of 50 m/s) to at least 20–25 km from the cal- five knolls in the upper Owens Gorge–Little Round dera. We suggest that had northern Ig2 occurred Valley area, Ig2E has been eroded from proximal ar- wholly later than Ig2E, more evidence for the for- eas and has the overall geometry of an overlapping mer should have been preserved east of the caldera wedge (e.g., figure 8). The most-proximal continu- where the topographic relief along potential flow ous Ig2E occurs immediately southeast of the base- paths is lower. ment high in Owens Gorge, where it is Ͻ10 m thick d) Dense, crystal-rich pumices with a character- but includes all three subpackages. The thickest istic crimped, fibrous vesicle texture and numerous Ig2E reaches 130 m (in Pleasant Valley), but con- schlieren of crystal fragments, characteristic of the tains only the lowest two subpackages. Borehole northern lobes, occur in minor quantities through- data (Bateman 1965) and isolated exposures along out the Ig2E ignimbrite package, most abundantly the east margin of Owens Valley show Ig2E origi- in Ig2Ec. nally extended Ͼ15 km farther south than the pres- e) Distinctive ignimbrite lithic fragments occur ent margin of continuous ignimbrite outcrop along in the very top of subpackages Ig2NWb and Ig2Na, Chalk Bluffs. throughout Ig2Nb, sparsely in uppermost Ig2Eb, Three lines of evidence imply that Ig2E deposi- and in Ig2Ec. These fragments are sub- to well- tion followed on from Ig1E, any time break, if pres- rounded, mostly poorly welded, glassy, and contain ent, being brief and constrained to accompany or a lithic suite with moderate proportions of Glass- follow deposition of the fine-ash-bearing marker Mountain-rhyolite lithics. The fragments are inter- horizon that caps fall unit F8. preted as recycled Bishop ignimbrite, torn away as a) There is a marked contrast between the mod- vent enlargement or migration disrupted ignim- erate welding in the basal part of Ig2E at 631454, brite that had accumulated within the caldera. The where it overlies moderately to densely welded lithic suites within these blocks are compatible Ig1Eb, and the totally nonwelded basal Ig2E, where with intracaldera equivalents to ignimbrite sub- it overlies fall deposits nearby in Pleasant Valley packages Ig2Eb and Ig2Na. These recycled lithics (at 661413). This implies that Ig1Eb was still hot are taken to indicate that later stages of Ig2Eb and enough when buried to induce welding in the overall of the Ig2Ec ignimbrite was erupted synchro- lying Ig2E ignimbrite under similar load stresses nously with the latest-Ig2NWb, late-Ig2Na, and that were inadequate to cause welding in basal Ig2E Ig2Nb subpackages. where it overlay a cold substrate. Our interpretation is that Ig2 flows to the east b) Virtually no evidence of penecontempora- and north of the developing caldera were largely neous erosion or re-working has been seen in the synchronous, but from a number of vents. Based on ignimbrite. The clearest example is meter-scale existing chemical data, lithic proportions, and the gullying at loc. 481 (figure 4) at the top of ca. 5 m presence of crystal-rich pumices, we infer that of rhyolite-lithic-bearing material continuous with some limited interchange of material took place be- Ͼ140 m of underlying Ig1Eb. An overlying thin tween flows heading north and east from sources surge or wind-affected fall deposit is overlain by the that intersected the Glass Mountain edifice and its rest of Ig2E. Any time break thus must coincide surrounding volcaniclastic fans. closely with the contact between fall units F8 and Package Ig2E. The term ‘‘Tableland’’ was ap- F9 (i.e., slightly later than the incoming of rhyolite plied (Hildreth 1979) to pyroxene-bearing ignim- lithics) and is not considered significant. brite on top of Ig1E east and southeast of the c) Ig2E emplaced above thick, densely-welded caldera. Based on correlations with the fall Ig1Eb in Rock Creek Gorge, Owens Gorge, and Chi- stratigraphy, we recognize a significantly larger dis- dago Canyon is often vapor-phase-altered and re- 422 C. J. N. WILSON AND W. HILDRETH crystallized, regardless of the welding intensity in Ig2E. In contrast, Ig2E emplaced above thin, moderately- to non-welded Ig1Eb, or fall deposits, generally remains glassy. These relationships imply that degassing of the thick, welded Ig1Eb fans during their cooling and devitrification also altered the blanketing Ig2E. Zones of vapor-phase alteration within Ig2E thus vary laterally (sometimes over distances of only tens of meters) even within demonstrably coeval emplacement units. Occasional fall intercalations within Ig2E (including the youngest subpackage, Ig2Ec, at 705661), coupled with thin Ig2E flows interbedded with fall deposits in Blind Spring Valley, demonstrates that a buoyant ‘‘plinian’’ column was sustained through all ignimbrite generation and deposition east of the caldera. From the various thicknesses of fall unit F9 exposed below Ig2E, and the local occurrence of decimeter-to-meter-thick flow units within F9 (e.g., at 706547), we infer that the onset of Ig2E deposition is slightly diachronous. Occasionally (e.g., at 725453) we have also found marked meter-scale vertical fluctuations in the rhyolite lithic proportions within material with average rhyolite proportions appropriate to subpackage Ig2Eb (see below), suggesting that mixing of flows with different characteristics, if not source areas, also occurred at some sites. Further work is required to better establish the stratigraphic relationships between individual fall beds in F9 and flow units forming the coeval and subsequent Ig2E Figure 11. Main Ig2E outcrop area, with scaled vertical material. sections from selected localities to show the vertical dis- Three Ig2E subpackages (a, b, and c) are recog- tribution of count % of rhyolite in the Ͼ5 mm lithic frac- nized, based on differing rhyolite proportions in the tion. The levels of rapid change in rhyolite % are used lithic fraction. These proportions fluctuate within to subdivide Ig2E into three subunits (a, b, c) which have certain definable limits throughout 15–80 m of ma- distinctive areal and vertical distributions (figures 12,13). terial, then to change over smaller vertical intervals The columns are composites from several numbered lo- (Ͻ1 m to ca. 10 m) to values characteristic of an- calities; grid references are: 477587 (site A), 505606 (B), other subpackage. Although these subpackages are 609554 (C), 608540 (D), 630453 (E), 660417 (F), 763457 not sharply demarcated, they individually repre- (G), 72E52N (H), 772567 (I), 675637 (J), 718647 (K), sent batches of material with broadly similar prop- 706690 (L), 717707 (M), and 659880 (N). Line x-x is the erties but contrasting geographic distributions section shown in figure 13. (figures 11 and 12). At most sites, rhyolite proportions in the lithic fraction are either relatively uni- Bluffs, where Ig2Ea overlies only fall deposits, this form or increase upward. subpackage is glassy and non-welded to sintered. Subpackage Ig2Ea contains Յ25 count % rhyo- Over a broad area between Casa Diablo Mountain lite in the lithic fraction. It forms the lower third and north Fish Slough, Ig2Ea is intensely vapor- to half of the preserved Ig2E in the middle to south- phase-altered and recrystallized, although so highly ern part of the Ig2E distribution but is virtually ab- porous that we infer it to have also been non- sent farther north (figures 11 and 13). The top of welded to sintered; we attribute the extreme alter- Ig2Ea is locally expressed in the Pleasant Valley– ation to gases from the underlying Ig1Eb. The only Chalk Bluffs area by a horizon that weathers out as welded Ig2Ea occurs west of Rock Creek Gorge and a discontinuous ledge (figure 14a). No parting or fall along lower Owens Gorge, i.e., where it buried a intercalation is visible on fresh surfaces that tra- still-hot Ig1Eb substrate. Lithic abundances are no- verse the ledge. Along Pleasant Valley and Chalk ticeably higher in Ig2Ea than in Ig1Eb, reaching 30– Journal of Geology T H E B I S H O P T U F F 423

Figure 12. Outcrop areas and pathways of the thickest fan-forming Ig2E; a. Ig2Ea; b. Ig2Eb; c. Ig2Ec. BHS ϭ Benton Hot Springs; BSV ϭ Blind Spring Valley; CDM ϭ Casa Diablo Mountain; FS ϭ Fish Slough; n.d. ϭ not deposited.

60 clasts/m2, and as the abundances of non-rhyolite and 13). Lithic abundances, however, remain com- as well as rhyolite lithologies are elevated in Ig2Ea, parable to those in the earlier Ig2Ea subpackage the rate of vent-wall erosion is inferred to have in- (typically 30–60 clasts/m2). creased. The southern Ig2Eb fan is non-welded at its base Subpackage Ig2Eb contains 35–60 count % rhy- and grades up over Ͻ30 m to become poorly to olite in the lithic fraction. This subpackage forms densely welded in its upper quarter to half. This three substantial fans, each 40–80 m thick, sepa- welding zone forms the upper cooling unit of Sheri- rated by areas where material with this range of dan (1968) and is a conspicuous cap rock along the rhyolite lithic proportions is Ͻ5 m thick (ﬁgures 11 southern limit of continuous ignimbrite outcrop

Figure 13. Line of section marked in figure 11, showing approximate vertical and lateral dimensions of Ig2E accumulation fans perpendicular to their flow paths. BSV ϭ Blind Spring Valley; BV ϭ Benton Valley; CB ϭ Chalk Bluffs; LCC ϭ lower Chidago Canyon; NFS ϭ north Fish Slough; PV ϭ Pleasant Valley; RRC ϭ Red Rock Canyon. Figure 14. a. South wall of lower Owens Gorge just downstream from Pleasant Valley dam. The base of the Bishop sequence is exposed on the left just above valley floor level (loc. 24, figure 2c) with 4.7 m of fall deposit (units F1– F9). Ig2E is roughly 100 m thick and non-welded, except in the top 10–15 m where it is poorly to moderately welded, with a weak eutaxitic texture. The top of the discontinuous ledge about one-third of the way through the ignimbrite marks the contact between Ig2Ea and Ig2Eb subpackages (figure 13). b. Thin fall (F9) and/or surge unit (at base of hammer) interbedded with Ig2Eb flow units at 701675. Hammer is 32 cm long. Journal of Geology T H E B I S H O P T U F F 425

(figure 14a). The densest welding occurs in lower fragments of fresh rhyolite lava (prominent in Owens Gorge near Birchim Canyon (figure 1); both Ig2Nb; see below) are rare. This implies that Ig2E the maximum welding intensity and original thick- vents did not intersect the Glass Mountain lava ed- ness of Ig2Eb diminish up-gorge, until at 609554, ifice but were restricted to surrounding volca- Ig2Eb material is only ca. 4 m thick, sandwiched niclastic fans. between Ig2Ea and Ig2Ec. Ig2Eb is eroded from most Package Ig2SW. This is the upper of the two areas closer to source than this but forms the five cooling units in the San Joaquin River canyon. knolls of Ig2E near Crowley Lake, which exhibit a Ig2SW is separated from Ig1SW by a thin vitro- welding zonation similar to that of Ig2Eb at Pleas- phyre: it has pyroxene and higher Fe-Ti oxide tem- ant Valley. A significant (years) time break in depo- peratures (Hildreth 1979). Preserved material is sition of Ig2Eb was postulated from studies of non-welded to densely welded, with lithic counts cryptoperthite lamellae in feldspars by Snow and 40–65 clasts/m2, and similar lithic proportions and Yund (1985, 1988), but we found no field evidence sizes to Ig1SW in the same area, but without rhyo- for breaks. lite lithics. Similar source vents and flow paths for The middle Ig2Eb fan resulted from spill-over of Ig1SW and Ig2SW are thus inferred, despite the material through the broad gap between Casa Dia- change in magma composition. blo Mountain and the Benton Range. Ig2Eb is Package Ig2NW. This package and its compan- poorly represented along modern middle and lower ion north of the caldera (Ig2N) correspond largely Chidago Canyon, and it appears that the Chidago to the Mono and Adobe lobes, respectively, of Hil- paleovalley had been filled by Ig1Eb and Ig2Ea, thus dreth (1979). Hildreth and Mahood (1986) reported diverting Ig2Eb flows to the north down the line of rhyolite lithics to be sparse to absent in the Mono modern Red Rock Canyon. No full section through lobe and abundant in the Adobe lobe. Our data have the middle fan is known, but the welding intensity yielded three key pieces of information that clarify increases downward, though at most a weak eutaxi- the relationships between them. First, the earliest tic texture is developed. Ig2 (Ig2NWa) between Crestview and Bald Moun- The northern Ig2Eb fan represents flows that tain contains no rhyolite lithics and is best devel- travelled north along the west margin of the Benton oped to the west (figure 15). This constrains the ini- Range, then spilled east through a conspicuous top- tial opening of the northern vents to have occurred ographic notch near Black Lake (figures 1 and 12b). from west to east, the opposite of what has been This fan is Ͼ40 m thick north of Benton Hot previously inferred (Hildreth and Mahood 1986). Springs but is only weakly sintered near the base of Second, ignimbrite similar to the bulk of the the exposed material; otherwise it is entirely non- Adobe lobe does occur west of the basement high, welded. Between the middle and northern fans, sec- forming part of the geographic Mono lobe defined tions in Blind Spring Valley show that Ͻ1 m of Ig2E by Hildreth (1979). In sections northwest of Bald ignimbrite was deposited in the intervening area. Mountain, ignimbrite containing sparse or no rhyo- Subpackage Ig2Ec contains Ͼ80 count % rhyolite lithics incorporates two intercalations of mate- lite in the lithic fraction. Although nowhere Ͼ15 m rial with an appreciable rhyolite lithic content, the thick, it appears to have been widely but unevenly intercalations wedging out to the west (figure 15). distributed from northern Fish Slough to Benton In a section northeast of Bald Mountain, ignimbrite Hot Springs, although absent from much of Blind containing 27–55 count % rhyolite in the lithic Spring Valley (figure 12c). Ig2Ec appears to have fraction forms most of the thickness, but three in- been entirely non-welded and is mostly preserved tercalations of rhyolite-poor material (1–4 count as mounds of indurated, vapor-phase altered mate- %) occur (figure 15). We interpret these relation- rial especially prominent around lower Chidago ships to mean that flows lacking or poor in rhyolite Canyon (Sheridan 1970). Ig2Ec is notable for its lithics (Ig2NW) and flows richer in them (Ig2N) greater lithic abundance (40–100 clasts/m2) as well were erupted contemporaneously from sources as for its high proportion of rhyolite lithics. The ap- west and east, respectively, of Bald Mountain. parent absence of Ig2Ec farther down Owens Gorge Batches of material carrying one or the other char- than the solitary 1-m thick capping at 609554 (fig- acteristic lithic suite moved dominantly north- ure 11) suggests (along with the high rhyolite-lithic ward, outward from the caldera, with only limited content) that the source vent(s) for Ig2Ec flows was slopping over the basement ridge extending north well northeast of the initial vent area suggested by from Bald Mountain. Farther outboard than a few Hildreth and Mahood (1986). Rhyolite lithics in kilometers, lithic counts suggest that mixing of the Ig2E subpackages are similar in their subangular to contrasting materials from the two source areas subrounded shapes and weathering states to clasts took place only within a narrow zone (figure 16). in the Glass Mountain volcaniclastic fans; angular Third, ignimbrite containing sparse rhyolite 426 C. J. N. WILSON AND W. HILDRETH

Figure 15. The Bishop ignimbrite near the northern caldera margin (see figure 17 for location), with stratigraphic columns to show the interfingering of material richer and poorer in rhyolite lithics on the flanks of Bald Mountain. Elevation in meters above sea level. The two data columns represent: R ϭ count % of rhyolite in the lithic fraction, and C ϭ number of Ͼ5 mm lithic fragments/m2. Lines of ϩ symbols denote zones of welding with a good eutaxitic texture. Where ‘‘inferred base’’ is marked, the contact is not exposed but controlled by exposures of pre-Bishop lithologies; bns ϭ base not seen, with no evidence available to estimate its position. Contours on the index map are at 400 foot (ca. 122 m) intervals. lithics (Ig2Nc, below) caps both the eastern Mono crops out almost entirely west of Bald Mountain and western Adobe lobes, implying that the latest (figure 18a). No fall material has been found below northerly-directed material was erupted from it, except for ca. 1 cm of fine vitric ash in sheltered source(s) roughly midway between the eastern and pockets at loc. 208 (figure 17) and tentatively inter- western limits of vent sites along the northern ring- preted as a fall deposit accompanying emplace- fault zone. ment of Ig1NW nearby at Clark Canyon. Exposed Package Ig2NW crops out along or near the topo- Ig2NWa is up to Ͼ70 m thick (at 305834) and is graphic caldera margin from near Bald Mountain to characterized by a high lithic abundance (160–630 the Sierran front (figure 1), and extends northward clasts/m2). Basal Ig2NWa is non-welded, but sin- toward Mono Lake. It is divided into two parts sep- tering and a eutaxitic texture develop progressively arated by zones, either of lesser welding intensity upward over 10–15 m (e.g., 241812), grading to lo- or of more-intense vapor-phase alteration, within cally ca. 70 m of densely welded material (e.g., uniformly densely welded material (figures 15 and 305834) before the transition into Ig2NWb. Lithics 17). are dominated on a count percent basis by meta- Subpackage Ig2NWa is the earliest ignimbrite morphics (60–80%), but mafic lavas dominate on a erupted along the northern caldera margin and weight-percent basis. Both lithic lithologies crop Journal of Geology T H E B I S H O P T U F F 427

tends farther from the caldera rim. Ig2NWb is moderately to densely welded in all but the basal few meters; lithic abundance decreases upward from 100–150 to Ͻ20 clasts/m2, with most material having 40–20 clasts/m2. Where basal sections occur at Aeolian Buttes (e.g., 175925), Ig2NWa is missing and Ig2NWb has only a relatively thin (Ͻ5 m) non- welded base. Subpackage Ig2NWb was usually sampled by previous workers as ‘‘Mono lobe’’ material (largely from sites at Aeolian Buttes). A noteworthy aspect is that, in the area of Alpers and Clark Canyons, the lithic counts and clast sizes imply that poorly welded Ig2NWb material forms some of the topographically lowest outcrops inboard of the caldera’s topographic rim. The geometry of these outcrops suggests that they form an accumulation at least 15 m thick plastered onto a slope dipping at ca. 6° into the caldera from a crest coincident with the modern topographic rim of the caldera. Package Ig2N. This includes three subpackages (figures 17 and 18), together broadly equivalent to the Adobe lobe (Hildreth 1979); all contain rhyolite lithics, but they are distinguished by variations in lithic abundance, rhyolite count percentages, and overall grainsize characteristics. The earliest Ig2N deposits demonstrably postdate the earliest Ig2NW Figure 16. Part of Bishop ignimbrite outcrop area along around Bald Mountain (figure 15), but much of and north of the caldera perimeter (see figure 17 for loca- Ig2N and Ig2NW deposition proceeded synchro- tion), showing values of the count % of Glass Mountain nously (figures 15 and 16). Evidence from distal ex- rhyolite in the lithic suites of subpackages Ig2NW and posures shows, however, that later parts of Ig2Na Ig2Na. Apart from the five localities in figure 15 (filled and all of Ig2Nc (and thus by inference Ig2Nb) post- squares), material at each locality can be assigned to ei- date the latest Ig2NW ignimbrite preserved along ther Ig2NW (R% Յ 2; open circles; tr ϭϽ1%), or Ig2Na their mutual contact. (R% Ն 25; filled circles), with a limited zone where mix- Subpackage Ig2Na forms most of Ig2N. It is up ing of flows from the two contrasting sources appears to to ca. 120 m thick in canyons northwest of Glass have occurred (3 Ͻ R% Յ 10; half-filled circles). See text Mountain, but its base is not exposed there. Lithic for discussion of faulting at Big Sand Flat. abundances range from 350–600 clasts/m2 in its basal non-welded parts to 40–100 clasts/m2 in the out nearby on the caldera rim and underlie the in- most intensely welded zone, and 20–30 clasts/m2 tracaldera Bishop Tuff in the NW sector of the cal- in its poorly welded top. Rhyolite lithics are typi- dera (Bailey 1989). Lithic-rich clast-supported ma- cally 60–80 count %, locally Ͼ90 %, and remain terial, containing fragments up to 70–100 cm and similar into Ig2Nb but decrease to Ͻ30 % across interpreted as lag breccia, occurs at several sites the contact into Ig2Nc (figure 17). Sections Ͼ50– (figure 18a); we thus infer that vent(s) for Ig2NWa 100 m thick in Wet and Dexter Canyons contain an were just inboard of the adjacent caldera rim (cf. the upper densely welded zone, but welding intensities lack of comparable breccias in proximal Ig1E east diminish toward distal exposures on the west side of the caldera). The scarcity of granitoid lithics con- of Adobe Valley. The poorly welded upper distal strains any putative vent areas for Ig2NWa to lie parts of Ig2Na are faulted against Ig2NWb at Big east of the granite that crops out extensively imme- Sand Flat (figure 16), and the fault offset (Bailey diately west of the Hartley Springs Fault (Bailey 1989) indicates that upper Ig2Na overlies the latest, 1989; figure 18a). densely-welded Ig2NWb. This area is the local dis- Subpackage Ig2NWb rests on Ig2NWa with a tal limit of Ig2Na, as Ig2Nc overlies Ig2NWb in ex- gradational contact, is thinner (45 m max. exposed posures farther north (figure 17). thickness at 305834) but more widespread, and ex- Subpackage Ig2Nb overlies Ig2Na in the head- 428 C. J. N. WILSON AND W. HILDRETH

Figure 17. Bishop ignimbrite outcrop area north of caldera, with selected stratigraphic columns to show relationships between the northern ignimbrite subpackages. Stippled lines represent contact zones between subpackages, while at localities 303/334, 135/136/350, and 296/362 the gaps in the stratigraphic columns denote that the stratigraphic relationships of the subpackages are unequivocal, but the contacts between them are not exposed. Columns R and C are rhyolite-lithic percentages and lithic counts/m2 respectively, as in figure 15; bns ϭ base not seen. Grid references are: 153915 (loc. 104), 305833 (108), 455944 (135), 452945 (136), 383946 (181), 241812 (208), 217856 (209), 426884 (296), 452905 (301), 327972 (304), 451907 (312), 320963 (334), 451950 (350), and 429890 (362). waters of Wet Canyon and forms most or all of the Glass Mountain lavas. Ig2Nb is notable for con- exposed thickness of Ig2N on the northwestern taining up to 5 count % of 5–40 cm blocks of co- flanks of the Glass Mountain edifice. Ig2Nb is non- eruptive Bishop ignimbrite (p. 421, above). The geo- welded to sintered, is unusually coarse, and rich in graphic distribution of Ig2Nb and the gradational pumice lapilli and blocks commonly 40–70 cm nature of its contact with Ig2Na indicate that the long. The lithic abundance is moderate (30–170 abundance of these blocks in Ig2Nb cannot simply clasts/m2), decreasing upward, and the lithic suite reflect plucking of extracaldera Ig2Na material dominated by rhyolite lava (55–85 count %). Many from the ground surface, but instead reflects recycl- rhyolite lithics are fresh and angular, indicating ing of intracaldera ignimbrite during vent enlarge- that flow paths traversed and/or the vent(s) cut ment or migration. Journal of Geology T H E B I S H O P T U F F 429

Figure 18. Generalized areas covered by the different subpackages forming the northern outcrops of the Bishop ignimbrite. V pattern marks the segment of the caldera perimeter from which the subpackages were vented (though distance inside the caldera margin is arbitrary). Major flow pathways are shown by arrows, where available field information is adequate to define them. a. Ig2NWa: filled squares mark localities with lithic lag breccias. The paucity of granitoid lithics in this material and Ig2NWb implies that venting did not extend west of the Hartley Springs Fault (HSF) where large amounts of granite occur, particularly in the caldera wall. b. Ig2NWb: note the distinction between rhyolite- lithic-poor to -free material (Յ2 count %; stipple), and material bearing sparse amounts of rhyolite lithics (3–10 count %; ϩ symbol) formed by mixing with Ig2Na (figure 16). c. Ig2Na; DC ϭ Dexter Canyon; NC ϭ North Canyon; WC ϭ Wet Canyon. d. Ig2Nb. e. Ig2Nc; BSF ϭ Big Sand Flat; CM ϭ Cowtrack Mountain; GM ϭ Granite Mountain; SP ϭ Sagehen Peak.

Subpackage Ig2Nc largely rests on Ig2Na (with Ig2Nc is characterized by low lithic abundances (3– a gradational contact) but also extends farther north 20 clasts/m2), which decrease upward, and by typi- and west to overlie Ig2NWb north of Big Sand Flat. cally 15–30 count % rhyolite lithics. Ig2Nc is Ig2Nc flows were more mobile than those of Ig2Na mostly sintered to poorly welded, but its matrix is and Ig2Nb. Surviving outcrops of Ig2Nc lap up on often mildly vapor-phase-altered to vivid pink or to higher areas in the Sagehen Peak-Cowtrack purple and heavily indurated by cementation and Mountain-Granite Mountain area (figure 18) and weathering. there rest either directly on basement (e.g., 353003) The stratigraphic relationship of Ig2Nc and or on Ͻ5 m of earlier ignimbrite (e.g., 383946). Ig2Nb is not unequivocally established. On the 430 C. J. N. WILSON AND W. HILDRETH west side of Adobe Valley, Ig2Nc ignimbrite over- Using accumulation rates from observed historic lies thick sections of Ig2N ignimbrite, the topmost eruptions (table 4) we infer that the eruption up to parts of which locally (e.g., at 500975) contain large the top of F8 lasted about 90 hrs. The fine-ash-bear- pumice clasts of a size (50–70 cm) common in ing bed at the F8/F9 boundary may mark a short Ig2Nb but virtually absent elsewhere in Ig2Na. On time break, with local traces of erosion (at loc. 481, this basis, Ig2Nc is inferred to largely post-date figure 4); fall and flow activity subsequently re- Ig2Nb and is the youngest ignimbrite unit of the sumed for at least 8 hrs, producing fall unit F9. Bishop eruption. Ignimbrite. A major challenge in studying prehistoric ignimbrites is determining how long they took to be emplaced. We introduce two approaches Eruption chronometry here. The first uses recognition of the intraplinian Fall Deposit. Modeling plinian fall deposits has nature of the ignimbrite coupled with the fall chro- produced a framework from which eruptive vol- nometry to bracket the times of ignimbrite deposi- umes (Pyle 1989), column heights (Carey and tion for Ig1E. The second arises from the first and Sparks 1986), discharge rates (Sparks 1986), and du- uses the development of flow-unit boundaries in in- rations of prehistoric eruptions (Carey and Sigurds- traplinian ignimbrite, where accumulation rates son 1989) can be estimated. Gardner et al. (1991) can be estimated, to provide crude estimates for the applied this to the Bishop plinian fall deposit, but accumulation rates of other ignimbrite where there this study was flawed by incorrect stratigraphic is no fall chronology. correlations (p. 433, below). Given the limited ex- Using the correlations in figure 7, the onset of posure (especially the lack of sufficient information Ig1Ea deposition in upper Owens Gorge is inferred to infer fall deposit volumes), we here combine our to have begun roughly 35% through unit F2 and compiled field data with the available models to in- ceased roughly 20% through the inferred original fer the timing of the Bishop fall deposits, thus. thickness of unit F6. On the basis of the timings in 1) Isopleths of maximum lithic sizes (unpub. table 3, this would imply a total deposition time data) for Bishop fall units 1, 2, 5, and 7 (for which for Ig1Ea of ca. 25 hrs. Ig1Ea reaches 80 m thick, we have the most data) provide crude estimates of giving an average accumulation rate of ca. 0.9 the eruption column heights (table 4), using the mm/s of tuff. The presence of several fall beds in method of Carey and Sparks (1986). Most dispersal Ig1Ea means that this average is lower than the axes are not constrained, but distances between iso- mean accumulation rate of the ignimbrite itself, pleths constrain the rate of decrease of lithic sizes. but not by more than 10–20% (i.e., the total thick- (2) The inferred eruption column heights were then ness of intra-Ig1Ea fall partings divided by the total compared to young plinian fall deposits for which thickness of fall material coeval with Ig1Ea exposed volumes are known and discharge rates and erup- at loc. 19; figure 7). tion durations are known or can be estimated Given that the base of Ig1Eb occurs ca. 75% of (Carey and Sigurdsson 1989; table 4). (3) With pub- the way through F6 at loc. 25 (figure 2b), and that lished data for young plinian deposits, the accumu- the top of Ig1Eb in the same area coincides roughly lation rate at any distance from vent can be esti- with the F8/F9 contact, the chronology in table 3 mated by dividing the relevant thickness by the constrains the Ig1Eb accumulation to a period of ca. eruption duration. A particular accumulation rate 36 hrs. This timing also applies to the thinner, non- was then selected by taking each deposit along its welded deposits emplaced around upper Chidago dispersal axis at the distance from vent where its Canyon and Blind Spring Valley, as the base of maximum-lithic size matched that of the relevant Ig1Eb is at comparable (e.g., at 636638) or slightly Bishop fall unit at its thickest known locality. higher stratigraphic levels (e.g., at 701753) there. In (4) The accumulation rate selected from the model middle Owens Gorge, where Ig1Eb is Ͼ140 m thick deposit was then assumed to apply to the relevant and mostly densely-welded, this timing corre- Bishop fall unit and, from the fall thicknesses, an sponds to an average accumulation rate of Ͼ1 elapsed time for accumulation of the Bishop fall mm/s of welded tuff (roughly equivalent to Ն2.0– unit could be estimated. By doing this for a repre- 2.5 mm/s for tuff of comparable density to Ig1Ea in sentative suite of Bishop fall units we can (albeit upper Owens Gorge). crudely) infer the overall timing of the eruption (ta- The maximum thickness of F9 is in Blind Spring ble 3). Having identified coeval fall and ignimbrite Valley, with a corresponding accumulation time of activity (figure 4, and below), we can now use the Ն8 hrs (table 3). This also represents a minimum inferred accumulation rates to estimate when ma- accumulation time for Ig2E. However, the levels of jor bodies of intraplinian ignimbrite accumulated. onset and termination of individual Ig2E subpack- Table 4. Information from Published Plinian Fall Deposit Studies Used for Estimating Accumulation Rates

col. isopleth usedb th.c timed, acc. ratee, Model deposit ht.a, km (mm) mm s mm/s Sourcef

Est. column height 15 km, equivalent to Bishop fall unit F1 Mayogatai 19 ML5 ϭ 10 120 (4700) .025 (1) Oguni 16 ML5 ϭ 10 500 (22000) .022 (1) Shingo 12 ML5 ϭ 10 700 (158000) .0045 (1) Maita 1 13 ML5 ϭ 10 130 (30000) .004 (1) Fogo (1563) 19 ML3 ϭ 13 500 (55000) .009 (2) Mt. St. Helens 5/18/80, A.M. 19 MP5 ϭ 40 80 11700 .007 (3)(4) Hekla 1991 12 MS ϭ 60 (60) 10800 .0055 (5)(6) average .011 all examples (.013) prehistoric .006 historic Est. column height 28 km, equivalent to Bishop fall units F2, F3 and F4 Apoyo A 27 ML5 ϭ 10 1500 (85000) .02 (7) Apoyo C 29 ML5 ϭ 10 2500 (72000) .03 (7) Primavera D 26 ML3 ϭ 13 300 (20000) .015 (8) Primavera E 24 ML3 ϭ 13 1000 (33000) .03 (8) Chuseri 29 ML5 ϭ 10 1300 (42000) .03 (1) Nambu 25 ML5 ϭ 10 1000 (16000) .06 (1) Toluca upper 30 ML3 ϭ 13 1200 (90000) .01 (9) Toluca lower 28 MP3 ϭ 30 600 (11500) .025 (9) Fogo A 30 ML3 ϭ 13 500 (43000) .01 (2) Taupo B4 27 ML5 ϭ 10 200 (8000) .025 (10) Vesuvius 79 white 26 ML5 ϭ 10 1000 (25000) .04 (11) Llao Rock 28 ML5 ϭ 10 1000 (43000) .023 (12) Cleetwood Յ30 ML5 ϭ 10 500 (33000) .015 (12) El Chichon A 1982 27 ML5 ϭ 10 100 9400 .01 (13) El Chichon C 29 ML5 ϭ 10 40 11900 .003 (13) Novarupta A 1912 26 ML5 ϭ 10 1000 40000 .025 (14) Novarupta CD 25 ML5 ϭ 10 1000 94000 .01 (14) Novarupta FG 23 ML5 ϭ 10 1000 43000 .03 (14) average .023 all examples (.026) prehistoric .020 historic Est. column height 35 km, equivalent to Bishop fall units F5, F6, F8 and F9 Hatepe 33 ML3 ϭ 50 1800 (21000) .09 (15) Primavera B 36 ML3 ϭ 50 8000 (182000) .05 (8) Primavera J 33 ML3 ϭ 50 4000 (53000) .08 (8) Hachinohe 6 33 ML3 ϭ 50 900 (12000) .07 (1) Hachinohe 4 37 ML3 ϭ 50 500 (4000) .12 (1) Taupo E3 36 ML5 ϭ 50 520 (7000) .07 (10) Vesuvius 79 grey 32 ML5 ϭ 50 1500 38000 .04 (11) Tarawera 1886 34 ML3 ϭ 50 750 16000 .05 (16) S. Maria 1902 34 ML5 ϭ 40 1250 65000 .02 (17) average .066 all examples (.083) prehistoric .035 historic Est. column height 45 km, equivalent to Bishop fall unit F7 Waimihia 42 ML3 ϭ 50 7000 48000 (.15) (15) Osumi 44 ML3 ϭ 50 15000 190000 (.08) (18) Taupo 51 ML3 ϭ 50 1500 14000 (.10) (19) Mazama 42–53 ML3 ϭ 50 5000 35000 (Ͻ.12) (12) Tambora F4 44 ML5 ϭ 40 Ͼ250 8000 Ͼ.03 (20) average Ͼ.09 all examples .11 prehistoric Ͼ.03 historic

Sources. (1) Hayakawa 1985; (2) Walker and Croasdale 1971; (3) Waitt and Dzurisin 1981; (4) Criswell 1987; (5) Larsen et al. 1992; (6) T. Thordarson pers. comm.; (7) Sussman 1985; (8) Walker et al. 1981; (9) Bloomﬁeld et al. 1977; (10) Wilson 1993; (11) Carey and Sigurdsson 1987; (12) Young 1990; (13) Carey and Sigurdsson 1986; (14) Fierstein and Hildreth 1992; (15) Walker 1981a; (16) Walker et al. 1984; (17) Williams and Self 1983; (18) Kobayashi et al. 1983; (19) Walker 1980; (20) Sigurdsson and Carey 1989. a Column height estimates are from coarse-clast dispersal patterns; data mostly from Carey and Sigurdsson (1989), with some more- recent additions. b Isopleth used as a basis for comparison with our unpublished Bishop data. ML3, ML5 denote the average length of the 3 or 5 largest lithic clasts (in mm), respectively; P ϭ pumice; S ϭ scoria. c Thickness on or near the dispersal axis at the distance from source appropriate for the corresponding isopleth. d Time calculated by dividing deposit thickness by inferred discharge rate appropriate to the inferred column height; actual elapsed time for deposition during historic events. e Average accumulation rate. f Primary source of ﬁeld data; most column heights and elapsed times from Carey and Sigurdsson (1989). 432 C. J. N. WILSON AND W. HILDRETH

Figure 19. Timings of fall and ignimbrite units of the Bishop Tuff. (a) Left: a hypothetical west to east section along an arc from 310° to 050° (relative to grid north, and as projected from a hypothetical origin in the center of Long Valley caldera; see inset map) with the northern ignimbrite subpackages drawn to represent their projected dispersal arcs (horizontal scale) and relative stratigraphic positions (vertical scale). The blank zone (between a and b) represents the gap in information northeast of Glass Mountain. (b) Right: a section from proximal to distal areas, roughly along the line of Owens Gorge. The vertical axis is scaled for the earlier (Ig1 equivalent) part of the eruption (derived from the fall deposit chronometry), and schematic for later times (Ig2 equivalent). The horizontal axis is distance from the initial vent area (Hildreth and Mahood 1986). Fall deposits are left unornamented but labeled with the unit numbers. a and b are linked by two inferred time-marker planes; A marks the level above which Glass Mountain-derived rhyolite lithics (eastern ignimbrite and Ig2N) and pyroxene-bearing pumices (both sides) occur; B marks the level above which clasts of recycled intracaldera Bishop ignimbrite with rhyolite lithics occur. ages with respect to the fall deposit stratigraphy are time sequence of fall deposits and the inferred rela- not yet adequately defined to allow timings within tive timing of ignimbrite subpackages yield a first- Ig2E to be inferred. order, overall time-space diagram for the Bishop Comparisons among flow-unit boundaries in eruption (figure 19). Key elements are: (a) The time Ig1E deposits of various thicknesses (and hence ac- sequence of the fall deposits (table 3). (b) The ab- cumulation rates) suggest that structureless ignim- sence of field evidence for any significant breaks in brite accumulates at average rates of Ն1 mm/s the eruption, except possibly a minor hiatus be- (densely welded) or Ն2.5 mm/s (non-welded). The tween fall units F8 and F9 when plinian activity structureless nature of almost all the northern ig- ceased. (c) The absence of pyroxene-bearing pum- nimbrite packages therefore suggests that such ac- ices with Fe-Ti oxide geothermometer tempera- cumulation rates applied there. Given the maxi- tures Ͼ737°C from any Ig1 material or its coeval mum thickness of 120–140 m of densely welded fall deposits (Hildreth 1979). Coupled with identi- tuff in the northern lobes (in Ig2NW at loc. 108; fication of Ig1NW ignimbrite at Clark Canyon be- figure 17), this approach yields a crude upper esti- low the pyroxene-bearing Ig2NWa ignimbrite (i.e., mate on accumulation times of the northern lobes the earliest eruptive unit known from sources of ca. 35 hrs. along the northern caldera rim), this restricts the Whole Eruption. The constraints imposed by the onset of Ig2NW and Ig2N ignimbrite deposition to Journal of Geology T H E B I S H O P T U F F 433 any putative F8/F9 hiatus or the earlier part of fall ian and co-ignimbrite components, and this applies unit F9. (d) The stratigraphic level at which recy- also to other eruptions where coeval plinian fall cled clasts of rhyolite-lithic-bearing Bishop ignim- and pyroclastic flows have occurred (e.g., Mount St. brite occur. These clasts are numerically most Helens 1980; Novarupta 1912; Taupo 1800 yr B.P.). abundant in Ig2Nb, but also occur sparsely in the The distinction between fall- and flow-derived ma- uppermost parts of Ig2Na, Ig2Eb, Ig2NWb, and terial in a distal ash-fall deposit is a prerequisite if throughout Ig2Ec. This information is used to link accurate assessments of eruptive volumes for each chronologically the northern and eastern sectors of phase are to be made, but our results suggest that the ignimbrite, but present data are inadequate to such a distinction may never be possible. Further- establish any linkage to the fall-deposit chronome- more, the wide range in phenocryst contents in try. (e) Subpackage Ig2Nc being the latest-erupted Bishop pumices precludes applying the crystal con- ignimbrite. centration technique of Walker (1980) to estimating volumes of distal fall material. Ignimbrite Facies. At the same time from the Eruption Products same inferred vent, Ig1Eb pyroclastic flows gener- Fall Deposit. The fall deposit stratigraphy (table ated both low-energy, welded material and higher 1, figure 2) and our observations imply that there energy, non-welded material. Such a contrasting are no discernible breaks in the eruptive sequence, pair of facies is also present (though much smaller except possibly on the F8/F9 contact. The absence in volume) in the Taupo 1800 yr B.P. (early ignim- of any plinian fall material at a favourable site at brite units of Wilson and Walker 1985) and Nova- Crestview (figure 1) implies that no F1–F8 fall ma- rupta, 1912 AD (Fierstein and Hildreth 1992) depos- terial was dispersed to the northwest. The wide- its. We speculate that comparable associations may spread distribution of distal Bishop fall material to be present in other deposits where the vent lay the east and southeast is well established (e.g., Izett within a pre-existing basin, promoting choking et al. 1988), and the chemistry and petrography of and/or ‘gargling’ of the eruption column. At ash in this major fall lobe are consistent with it be- Bishop, the densely welded Ig1Eb deposits clearly ing distal F1 to F8 material. Distal Bishop fall mate- did not mix much with the atmosphere, as the em- rial also occurs west and southwest of Long Valley placement temperatures suggested by the degree of (Izett et al. 1988); at a location visited by us ca. 110 welding (Ͼ630–650°C; Riehle 1973; Cas and km southwest of the caldera there is plinian Wright 1987, p. 251–258; Riehle et al. 1995) are at pumice fall material, but we have not yet correlated most ca. 100°C below the inferred magmatic tem- it within the overall eruptive sequence. peratures (Hildreth 1979). In contrast, the inferred Gardner et al. (1991) attempted to reconstruct higher emplacement velocities and non-welded plume heights and eruption rates from maximum- character of the distal thin intraplinian Ig1Eb flow clast sizes measured at various stratigraphic levels units suggest they were cooled by atmospheric in the plinian deposit, with heights in the fall de- mixing in the eruption column and may have posit normalized to its thickness at each site. They gained extra momentum by collapse from higher in also correlated a number of thin intraplinian flow the column. However, occurrence of these flows in and surge deposits but assumed that the onset of a relatively narrow easterly (downwind) sector and continuous ignimbrite deposition at their six sites the absence of significant bedding or grainsize was synchronous. This assumption is demonstra- changes in the fall deposits (indicative of any major bly incorrect (cf. figure 2), and their maximum-clast reduction in power of the plinian plume) imply that size isopleths are largely invalid because most of any column collapse was partial, or short-lived in their datum levels in fact represent a variety of lev- comparison to the characteristic response period of els in the fall deposit at different localities. the eruption plume. Since the area around the vent A tendency has arisen to treat eruption plumes was covered by ignimbrite and because the dis- as either ‘‘stable’’ (plinian) or ‘‘collapsing’’ (co-ig- persal axis of the coeval fall units appears to have nimbrite) (e.g., Woods 1995, p. 508–513) with gen- been to the east, we infer that the thin non-welded eration of characteristic products; either a lapilli- flows were portions of the plume shed off on the rich pumice fall deposit (e.g., Walker 1981b)ora downwind side where atmospheric interaction was fine-grained ash-fall deposit (e.g., Sparks and restricted. Walker 1977), respectively. The Bishop fall deposit Ignimbrite Distribution Patterns. A feature noted is of archetypal plinian style, yet is demonstrably by Hildreth (1979) and emphasized by lithic studies co-ignimbrite in nature. In the distal Bishop fall de- (Hildreth and Mahood 1986; this paper) is that in posit it will thus be impossible to disentangle plin- any given area around the northern or eastern side 434 C. J. N. WILSON AND W. HILDRETH of the caldera, the ignimbrite tends to have unique aar 1992). Packages Ig1Ea and Ig2E commonly do characteristics that tie it to a particular restricted show numerous flow-unit boundaries; in the for- segment of the caldera margin; e.g., the boundary mer case, a lower mean accumulation rate of ca. 0.9 between packages Ig2NW and Ig2Na is radial to the mm/s of non-welded tuff is inferred. The degree of caldera rim (figure 8). This is inferred to show that development of flow-unit boundaries in the Bishop the relevant flows had sources along the caldera ignimbrite is thus primarily a function of the time ring fracture and flowed radially outward within a between successive batches of material reaching a confined sector (much of each package undoubt- given distance from source. edly also accumulated within the caldera). Sectori- In Ig1Ea, several flow-unit boundaries are spaced ally distinctive ignimbrite from multiple vents at 1–10 m intervals (demarcated by intercalations along ring faults appears to be common, although of fall material, or sharp breaks in grainsize and/or not shown by the Taupo and Oruanui ignimbrites truncation of segregation pipes), implying accumu- in New Zealand (Wilson 1985, 1991). For these, lation as individual batches of material with lithic studies (Wilson unpub. data) imply that most, enough time between them for fall-deposit accu- if not all, of each ignimbrite came from a single cen- mulation, or for the earlier unit to have partly tral source rather than ring fracture vents, even consolidated and formed a firm substrate for the though lithic fragments from surfaces around the subsequent flow. Since Ig1Ea is at most sintered, caldera rim were incorporated into coarse, proxi- penecontemporaneous welding cannot realistically mal breccias. Relationships among asymmetries have been a factor in the consolidation process. The shown by ignimbrites and the dynamics and timing time breaks indicated by the thicknesses of interca- of caldera collapse represent fruitful lines for fur- lated fall material are only of the order of minutes ther study. to tens of minutes (except for the F6 material sepa- Ignimbrite Flow Units. The Bishop ignimbrite is rating Ig1Ea from Ig1Eb, which may represent the cumulative product of tens of hours of activity hours). The inferred low emplacement velocities of (figure 19), based on comparative eruption and ac- Ig1Ea flows, coupled with the presence of fall inter- cumulation rates for the fall deposits, but the way calations, suggest that the flow units represent sep- in which the ignimbrite accumulated varied among arate batches of material generated at the vent. packages. Packages Ig1E and Ig2E show a wide In Ig2E, flow-unit boundaries are commonly con- range of flow-unit structures. In Blind Spring Val- spicuous toward the top or distal edges of the ig- ley, several thin flow units of ignimbrite are sand- nimbrite, where they are typically expressed by wiched within sections dominated by fall deposits contrasts between pumice-lapilli-rich tops of un- (e.g., figure 9b). The accumulation-rate estimates derlying flow units and lapilli-poor bases of the fol- (table 3) thus imply flow emplacement here at lowing units. The scarcity of fall intercalations in hours to tens of hours intervals. Toward the axes distal areas suggests that the flow units accumu- of the ignimbrite fans, the fall intercalations first lated rapidly and that secondary slumping and re- become more numerous (but thinner) as the num- mobilization of the ignimbrite may have generated ber and aggregate thickness of the ignimbrite flow some units. units increases (e.g., loc 95; figure 2d), then become The northern ignimbrite packages are notewor- fewer in number (e.g., figure 9c). Gradually the clar- thy for the virtual absence of any flow-unit bound- ity with which the flow unit boundaries can be de- aries; at best, coarser lithics or pumices concen- fined diminishes and fall material is unrecogniz- trated in stringers across an exposure indicate some able in continuous thick ignimbrite sections. irregularity in deposition. If inferences regarding These sections are sometimes vaguely stratified or accumulation rates from Ig1E are correct, we infer contain discontinuous trains of pumice or lithic that the northern packages may have accumulated clasts, but where densely welded, the tuff typically rapidly, at a rate comparable to that for Ig1Eb. shows no discernible internal boundaries. The Ignimbrite Cooling Units. The Bishop ignimbrite stratigraphic relationships presented here imply shows a number of zones of welding, devitrification that thick structureless ignimbrite accumulations and vapor-phase alteration characteristic of large such as Ig1Eb in middle Owens Gorge accumulated ignimbrites (e.g., Smith 1960; Christiansen 1979). progressively, at an average rate of Ն1mm/s Previous studies inferred the Bishop to be a com- (equivalent to 2–2.5 mm/s of non-welded tuff), posite sheet (Sheridan 1968; Sheridan and Ragan with individual batches of material aggregating ei- 1972; Hildreth 1979), and although the observa- ther continuously or at such short time intervals tions remain valid, our stratigraphic studies show that each batch merged across its lower contact by that the zonation is more complex. The change shearing (Fisher 1966; see also Branney and Kokel- from a single cooling unit in middle Owens Gorge Journal of Geology T H E B I S H O P T U F F 435 to two cooling units in lower Owens Gorge (Sheri- sustained from at least close to the beginning of the dan 1968) is not merely a simple to compound tran- eruption. sition but represents the end result of depositional The fall deposit shows an overall upward in- shingling of two entirely separate ignimbrite pack- crease in mean and maximum grainsize up to and ages, each with its own welding zonation. More- including F7 (Bateman 1965; Gardner et al. 1991; over, the major Ig1Eb cooling unit that forms the figures 2 and 3). The coarsest fall material (i.e., from cliff sections dominating middle Owens Gorge also the highest eruption column) was deposited in the divides proximally into two zones of welding sepa- time interval during which emplacement of the vo- rated by a non-welded interval in upper Owens luminous Ig1Eb ignimbrite occurred (figure 4). Co- Gorge. Complexity in welding patterns develops eval eruption products during this time interval in- because the welding state of the ignimbrite is ev- cluded (a) widely dispersed plinian fall deposits (fall erywhere a complex function of temperature and units F6-F8); (b) low-energy ignimbrite (thick, load stress, and such factors do not change uni- densely-welded Ig1Eb); and (c) high-energy ignim- formly from proximal to distal areas of the Bishop brite (thin, non-welded distal Ig1Eb). Such concur- ignimbrite. rence is not explained by available models for ex- The welding of the lowest parts of Ig2Ea where plosive eruptions (e.g., Woods 1995, for review). it was deposited on the still-hot Ig1Eb deposits Caldera Development. Our expanded lithic data shows (as does all available stratigraphic evidence) and better time-stratigraphic framework modify that any time break between F8 and F9 was brief. the previous picture of caldera development (Hil- Field relationships between ignimbrite and fall de- dreth and Mahood 1986) whereby vents for fall unit posits on the eastern side show that the Ig1E and F9 and the Ig2E, Ig2N and Ig2NW packages propa- Ig2E subpackages accumulated continuously, con- gated in a counter-clockwise direction around the tradicting the 1–2 year time break within the ‘‘Ta- caldera ring fracture from an initial source in the bleland’’ material proposed by Snow and Yund south-central part of the caldera. Our new data are (1985, 1988). compatible with that initial vent position for fall deposits F1–F7, most of F8, and ignimbrite Ig1E (figure 20), but the subsequent development of Eruption Styles vents is more complex than previously supposed. Fall versus Flow Activity. Models for eruption Evidence from Ig2E shows that the vent(s) propa- columns emphasize a contrast between ‘‘stable’’ gated into areas where increasing amounts of Glass buoyant plumes that give rise to plinian pumice fall Mountain volcaniclastic debris were incorporated deposits and ‘‘unstable’’ collapsing columns that but not the lavas of the Glass Mountain edifice it- give rise to pyroclastic flows together with co-ig- self. The high proportion of rhyolite fragments (80– nimbrite ash-fall deposits (e.g., Sparks and Wilson 95%) in the latest east-side ignimbrite (Ig2Ec) sug- 1976; Wilson et al. 1980; Woods 1995; Bursik and gests that by then vents had propagated northward Woods 1996). This ‘‘either/or’’ situation is increas- and eastward toward Glass Mountain (figure 20). ingly turning out to be untrue for eruptions investi- Evidence from northern Ig2 is that the initial gated in detail; e.g., Novarupta, 1912 (Hildreth vent opening was from west to east, that a number 1983; Fierstein and Hildreth 1992), Mount St. Hel- of vents were active simultaneously, and that the ens, 1980 (Criswell 1987), Taupo, 1800 yr B.P. (Wil- last-active vent area (for Ig2Nc) was between Glass son and Walker 1985), and the Bishop eruption (this and Bald Mountains (figure 20). The coarse lithic paper). For each, substantial volumes of ignimbrite breccias in Ig2NWa (figure 18) and the inter-wedg- were generated synchronously with and from the ing of ignimbrite from contrasting sources on the same vent system that sustained a high plinian col- east and west sides of Bald Mountain (figure 15) umn, and thus estimates of eruption rates for fall show that the vents lay along the northern caldera versus flow episodes are specious (cf. Bursik and ring fracture, rather than being more toward the Woods 1996). At Novarupta and Mount St. Helens caldera center with flows incorporating their char- the vent is unambiguously defined as a single point acteristic lithic suites as they traversed the caldera source, and for Taupo a single source is inferred. For margin. The source(s) for the two packages in the the Bishop Tuff, similarities of lithic proportions San Joaquin River valley are not well constrained, in early plinian fall deposits and coeval ignimbrite but the conclusions of Hildreth and Mahood (1986) imply that they shared the same initial vent site remain valid, viz., that they were fed from a source (Hildreth and Mahood 1986). The relationships be- or sources within the southwestern part of the cal- tween ignimbrite and fall material in upper Owens dera. Gorge imply that both fall and flow activity were The migration of vent sites with stratigraphic 436 C. J. N. WILSON AND W. HILDRETH

Figure 20. Inferred vent areas active during emplacement of the northern and eastern Bishop ignimbrite (and coeval fall deposits). Widths of vents is schematic, but centered over the inferred position of the main ring fracture(s) (Bailey 1989). Two ornamentations in the same segment implies subpackages of different ages shared the same sector of venting. Vertical lining represents the initial vent site proposed by Hildreth and Mahood (1986), active during eruption of fall units F1 to lower parts of F8 and ignimbrite package Ig1E. Subsequent vent sites are ornamented according to their corresponding ignimbrite subpackage (as in figure 19). The two markers are as in figure 19, viz.: A, the incoming of Glass Mountain rhyolite lithics and/or pyroxene-bearing pumices and B, the incoming of recycled Bishop ignimbrite clasts. The extra-caldera extents of outcropping Glass Mountain volcaniclastics and lavas are shown, as are their approximate inferred original extents over the caldera. (N.B. Metz and Bailey [1993] show the Glass Mountain volcaniclastics extending farther west and south, but the total lack of rhyolite lithics in the earliest Ig1Ea ignimbrite in upper Owens Gorge and around Crowley Lake indicates that pathways from vent to these exposures did not traverse the loose volcaniclastics.) position is shown in figure 20. We infer that vents Conclusions propagated toward Glass Mountain both from southern and the northwestern parts of the caldera. 1. Fall and flow activity proceeded together from It is not known whether the northern and south- the earliest stages of the eruption to near the latest eastern vents ever formed a continuous arc; no stages, and a high plinian plume was sustained Bishop deposits have been found by us on the throughout emplacement of hundreds of cubic ki- northeastern flank of Glass Mountain, but this may lometers of tuff. The virtual absence of a discrete simply be due to subsequent erosion of thin, non- co-ignimbrite ash fall associated with fall deposits welded deposits, or to blockage of flows from that below or immediately around the ignimbrite out- sector by the high caldera wall. Reasons for the crop area is attributed to entrainment and dispersal northern caldera margin initially collapsing at its of the fine ash generated by flow emplacement into western end are unclear, but the early vent area pro- the co-existing powerful plinian column. Such syn- posed by Hildreth and Mahood (1986) and the cal- eruptive production of plinian fall deposits and ig- dera segment from which Ig2NWa was erupted are nimbrite have been demonstrated, e.g., Novarupta, connected today by a complex strand of NW-SE- 1912 (Hildreth 1983; Fierstein and Hildreth 1992), trending faults, associated as well with the post- Mount St. Helens, 1980 (Criswell 1987), Taupo, Bishop resurgent dome (Bailey 1989), that may re- 1800 B.P. (Wilson and Walker 1985) and may prove flect some deep-seated line of weakness. to be common, although not accounted for in theo- Journal of Geology T H E B I S H O P T U F F 437 retical models for large eruption plumes (e.g., addition, stratigraphic evidence is unequivocal that Woods 1995). Ig1E (ϭGorges and Chidago lobes of Hildreth 1979) 2. The new stratigraphic framework shows that erupted coevally with the fall deposit, despite hav- the pyroxene-bearing (Ig2) ignimbrite packages ing yielded Fe-Ti oxide temperatures higher than north and east of the caldera largely overlapped in any derived from plinian pumice. The coincidence time (figures 19 and 20). The eastern Ig2 material in time between the onset of vent migration and was erupted continuously on from Ig1E, although changing pumice mineralogy implies that pyrox- the area of the earliest Ig2E flows (synchronous ene-free and pyroxene-bearing magma might not with upper F8) is smaller than that of earlier or later have been in simple vertical succession in the flows. Based on the absence of pyroxene-bearing chamber but could have been in part laterally juxta- pumices in fall deposits through to near the top of posed. Moreover, the stratigraphic evidence from F8, onset of the pyroxene-bearing northern Ig2 the northern ignimbrite packages shows that em- packages is inferred to have taken place either in placement of the Mono lobe started before and was any F8/F9 time break or at the onset of F9. The very in part concurrent with emplacement of the adja- latest flows were to the north (subpackage Ig2Nc). cent Adobe lobe; neither the stratigraphy nor the 3. Patterns of venting and caldera collapse are overlapping temperature estimates for these lobes more complex than hitherto realized. Our work supports the oversimplified notion of an orderly supports the idea of an initial single vent in the drawdown of a geometrically simple, thermally southern part of the caldera. However, instead of stratified magma reservoir. We are now investigat- this vent area propagating anticlockwise around ing in detail the field distribution of the pumice the caldera ring fracture (Hildreth and Mahood compositions erupted and implications for the pre- 1986), vents propagated from the initial vent into eruptive compositional distribution in the reser- the Glass Mountain debris fan (though not reaching voir. All studies that utilized Hildreth’s tempera- the Glass Mountain edifice itself); simultaneously ture data as a stratigraphic guide have to be reevalu- a series of vents opened up from west to east along ated. the northern rim of the caldera. The timing and development of the remaining segments of the cal- ACKNOWLEDGMENTS dera margin to the west and southwest remain uncertain due to poor ignimbrite preservation in this CJNW acknowledges financial support from U.K. sector. Natural Environment Research Council grant 4. Withdrawal of magma from the reservoir was GR3/7032, the Royal Society of London, and the likewise more complex than previously realized. New Zealand Foundation for Research, Science and Hildreth (1979) used inferred magmatic tempera- Technology. WH is supported by the Volcano Haz- tures to erect a generalized eruptive sequence for ards Program of the U.S. Geological Survey. We are the geographically defined ignimbrite lobes and the grateful to P. E. Bruggman, J. E. Fierstein and B. plinian fall deposit exposed beneath them. We can Nichols for technical support, K. J. Wilson and now show that pumice types of different composi- G. A. Mahood for forbearance, and A. T. Anderson, tion (and thus magmatic temperature) erupted con- R. L. Christiansen, B. F. Houghton, P. W. Lipman, currently and in varying proportions during em- S. Self, Th. Thordarson, G. P. L. Walker and P. Wal- placement of each ignimbrite subpackage. In lace for helpful and exhaustive reviews.

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