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The Bishop Tuff: New Insights from Eruptive Stratigraphy1

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The Bishop Tuff: New Insights from Eruptive Stratigraphy1 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 signi®cant 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 subpack- age and ca. 36 hrs for a later, mostly welded subpackage. Average accumulation rates range up to $1 mm/s of dense- welded massive ignimbrite, equivalent to $2.5 mm/s of non-welded material. Comparisons of internal strati®cation in Ig1 and northern Ig2 lobes suggest the thickest northern ignimbrite accumulated in #35 hrs. Identi®able vent positions migrated from an initial site previously proposed in the south-central part of the caldera (F1±8, Ig1) in com- plex 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 identi®ed. 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 (®gure Hildreth 1979). Numerous other studies have built 1). Long Valley has been a focus of rhyolitic mag- on, modi®ed 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; Duf®eld 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 Middle®eld 2. Based on the Fe-Ti oxide magmatic tempera- Road, Menlo Park, CA 94025. ture estimates of Hildreth (1979) and the lithic [The Journal of Geology, 1997, volume 105, p. 407±439] 1997 by The University of Chicago. All rights reserved. 0022-1376/10504-007$01.00 407 408 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 (modi®ed 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 ®rst 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 5 Birchim Canyon; BHS 5 Benton Hot Springs; BSH 5 Blind Spring Hill; BSV 5 Blind Spring Valley; CB 5 Chalk Bluffs; DC 5 Dexter Canyon; FS 5 Fish Slough; LRV 5 Little Round Valley; NC 5 North Canyon; PV 5 Pleasant Valley; RCG 5 Rock Creek Gorge; RRC 5 Red Rock Canyon; WC 5 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) em- placed 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 De®ne 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-®ne-ash, ®ner-grained than F2 F2 moderately bedded; three coarser bands merging into one distally; top 19 537578 350 and base de®ned 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 ®ner-grained, merging with normally graded top to middle bed .30±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 ®ner 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 ®ner-grained low-angle cross- bedded band; no systematic overall grading; maximum clast sizes simi- lar to F5 F7 base de®ned 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; coars- est unit in Bishop fall deposit F8 base de®ned 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-strati®ed; no systematic grading except for pronounced nor- mal grading in top 50±100 mm with admixed ®ne to very ®ne ash; level of incoming of Glass Mountain rhyolite lithics is 60±80% through F8 thickness F9 base de®ned 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-strati®ed; 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.
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