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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 100, NO. B12, PAGES 24,593-24,609, DECEMBER 10, 1995 Correlation and emplacement of a large, zoned, discontinuously exposed ash flow sheet: The 40ArP9Archronology, paleomagnetism, and petrology of the Pahranagat Formation, Nevada Myron G. Best,' Eric H. Christiansen,' Alan L. Deino,' C. Sherman Gr0mm6,~ and David G. Tingeyl Abstract. Many single-cryslal 4oAr/3gArages and thermoremanent magnetization directions have resolved the problematic stratigraphic correlation of the laterally and vertically zoned rhyolite ash flow sheet of the Pahranagat Formation in the southern Great Basin. This outflow sheet was previously designated by four different stratigraphic names in different locations over its highly discontinuous exposure area of 33,000 kmz.We show that it is a single cooling unit emplaced at 22.639+0.009 Ma around its source, the Kawich caldera. The volume of the outflow sheet was about 1600 h3after compensation for 50% post volcanic east-west extension. A comparablc volume of tuff likely accumulated inside the Kawich caldera. Modal and chemical compositions of bulk tuff and cognate pumice fragments, together with compositions of phenocrysts, show the preemption magma body was zoned from high-silica rhyolite (two feldspars, quartz, biotite, and titanomagnetite) to underlying, silica-poor, more mafic rhyolite and trachydacite (plagioclase, minor biotite, titanomagnetite, amphibole, and clinopyroxene). Initial cvacuation of the uppermost evolved zone produced proximal outflow hundreds of meters thick of relatively densely welded, pumice-poor, high-silica rhyolite tuff. As eruption pro- gressed, tens of meters of more mafic ejecta were deposited in distal areas and locally near the caldera and consist of less welded, pumice-rich ash flow tuff derived by physical mixing of pyroclasts from all zones of the magma chamber. This mixing during eruption invalidates direct comuariso~lof the eom~ositionof tuff and a particular part of the magma chambcr. Thc ~ahrana~atash flow sh'cet provides a rigorous test case for application of high-precision correlation tools because of the zonal emplacement of ejecta from the compositionally stratified magma chamber together with the subsequent tectonic dismemberment and erosion of the sheet that created widely scattered exposures. Introduction is llall~perednot only by their sheer size, but also by widespread dismemberment due to subsequent faulting, erosion, and conceal- Unambiguous correlation of an ash flow deposit can be a clial- ment beneath extensive valley fill. Not only is it impossible to lenging exercise in geologic field work and laboratory analysis. physically correlate an individl~alcontinuous sheet, but added 'This is particularly the case where the ejecta were broadcast in a impediments to correlation are posed by lateral and vertical complex manner around the vent(s) from a compositionally zoned composirional variations within some sheets and the multitude of magma chamber, where exposures of the deposit are poor or compositionally similar cooling units in any one sequence. The widely scattcrcd, and whcrc othcr similar dcpasits occur in thc number of cooling units in a single section can he as many as 20; volcanic succession [Hildreth and Mahood, 19851. Many such 10 is common. deposits typify the middle-Tertiary Great Basin ash flow province We have made a multidisciplinary study of the ash flow tuff in thc northcrn Basin and Range province of the wcstcrn Unitcd sheet of the Miocene Pahranagat Formation that is exposed over an States. This vast ash flow province contains well over 1M) ash area of 33,000 km2 in the southern Great Basin of Nevada (Figure flow tuff cooling units related to about 70 sources, mostly malked by calderas [Stewart and Carlson, 1976; Best et al., 1989b; Best 1). Because it is one of the younger deposits found in the fault- bounded, uplifted, and variably eroded mountain ranges of this part and Christiansen, 19911. Some outflow sheets span as many as ten ,mountain ranges covering an area as great as 40.W km': pat.ts of of the Basin and Range Province, erosional remnants are highly discontinuous and widely scattered. Substantial lateral compos- a single caldera !nay be exposed in four ranges [Best et al., 1989al. Smdy of outflow sheets and associated calderas in the Great Basil' itional variations west to east in the sheet compound the problem of correlation by conventional stratigraphic. methods. It is our intent to (1) confirm the correlation of thc outflow shcct, which 1 ~~~~~t~~~tof ~~i~h~~younc~~i~~~~i~~. prove. ~~~1,.was initially based on stratigraphic position and composition, by ~~~l~~~.-. , . ~ ' Bcrkcl:) tic~~.'I~rdn~~l.q!r<owr, Ikrkclc), C~,l~r<>rot~. prcitsr. ~l~r.r~~~orc~~~ancntI~:~IIC[IZ~II~II JI~C;LIIIII> .mJ .in:le-cr)5taI St.rvc), ' C.S. tic.!l~~~~c.tl \lcnls9 Park, ('.,III.,~II.:I "" Ar." Ar 3gr.b. We ~L.IIIOIISIIIIICllril ill< tuft >11cc1.w111il1 112% Copyright 1995 American Geophysical Unio~~ been designated by four stratigraphic names (Pahranagat Lakes Tuff, granite-weathering tuff, tuff of Saulsbury Wash, and upper Paper number 95JB01690. tuff of White Blotch Spring) in different areas by different 0148-0227195195JB-01690$05.00 geologists, is in fact just one cooling unit, compound in places. 24,593 24,594 BEST ET AL.: THE ZONED PAHRANAGAT ASH FLOW SHELL Figure 1. Kawich caldera and outflow tuff sheet of tlte Paliranagat Formation. Thicknesses and contours in Imeters. Zero indicates sites where the outflow sheet is absent between older and yuunger deposits. Co~rtours drawn by Techbase computer program which grids sheet into cells 5 x 5 km and determines best fit contours by iterative extrapolation between data points. Sites of samples dated by the *uAr/i"At. method shown by triangle, paleomagnetic santple sites by circle. Unlabeled txaleumaenetic sa11111lesite suutheast of the Cactus .. as foliows: CR, Cactus; FICR, 1,Iut Creek; KR, Kawlch; MR. oill lor; PAR, Pancake; I'R, Pahranag;; RR, Reveille. Inset [nap shuws Grcat Basin (dotted line) in northern part uf Basin and Range \)rovi~~ce encompassing most of Nevada and western Utah. Rectangular area is main diagran~. CNCC, central Nevada caldera complex: KC. Kawich caldera: R, Reno; LV, Las Vegas: SIC, Salt I.;lke City. The Pahranagat Formation thus serves as an example of how caldera sources were stacked cullformably olie upon anotlier [Besr correlation of widely scattered rrosional remnants of an outflow a,& Cltri,~tia~~,,lcetr.1991J. After about 22 Ma, ash flow volcanism sheet may necessitate upward adjustments in eruptive volu~lteand slowly declined over the next several milli~inyears, extrusiolts of reduction in number of eruptive events, thus affecting models of canpositionally diverse lava flows became Itlure prominent, and continental magma genesis and eruption processes. (2) Document widesprrad crustal exlensiorr created tlw bxin and range struc1ul.r. the chemical and mineralogical compositions of tuff and cognate A major part of tlle as11 tlou, volunie ill the province was derived pumice fragments to understand how the magma chamher was from two especially large, long-sustained lnaglna centers: the zoned and how it was systematically evacuated lo produce a Indian Peak [Best el nl., 1989aI and the central Nevada [Brsr et vertically and laterally zoned outflow deposit around the Kawich 01.. 19931. Tlte latter is marked by a cluster of as rllally as 12 caldera. nested calderas and two indefinite source areas which altogether generally are younger t~rthe southwest. Tl~evolumc uf as11 flow Geologic Setting of the Pahranagat Formation lull associated with this central Nevada caldera colnplcx, including iiitracaldcra deposils and surrounding outflow shects, is at lcast The ignimbrite flareup, which created the Great Basin ash flow 20,000 knl'. Large rhyolite outflow tuff sl~eetswere en~placed pruviu~e,wah acconrpa~tied, at least in its early stages, by hetwecn 35.3 and 18.3 Ma and witllirl this time interval, rliorc vol~tminauseruptions at 31.3 Ma yip1dr.d zo~~edrl~yolite to dacite " and at 27.3 Ma dacite. The rhyolilic I';~l~ranagatFormation was from about 34 to 22 Ma, the crust was tectonically (nore or less emplaced a1 22.6 Ma. Formally defined by ScoN d 01. 119951, this quiescent so that ash flow sheets outside their complexly evolving formation cansists of two facies: intracaldesa and outflow. BEST ET AL.: THE ZONED PAHRANAGAT ASH FLOW SHEET 24,595 Althougb intracaldera deposits within tbe Kawich caldera source tion. If indeed Pahranagat Formation, we are uncertain whetller are briefly considered below, tlie main focus of this report is the the considerable thickness (500i- '! m) ol the tutt in tlie Cactus correlative outflow facies slleet (Figure 1). Range reflects ponding of outflow facies in an uldcr caldera or We now describe the general geology of the Kawich caldera and con~titutesa remnant of tlie intracaldera facies. then the surrounding outtlow sheet of the Paliranagat Formation. More field work invulvir~gdrtailed sampling and mapping will 1:ollowing sections present pertinent 'oAr/39Ar chronologic and be necessary to fully chara~.terizctllc Kawich caldera with regerd paleotnagnetic data essential to the correlation of the intracaldera to its history of collapse, resurgence [Ekren er al., 1971, p. 341, and outflow tacies. Sample sltes, denoted by capital letters, such and the magmatic evululion of the intracaldera rocks in relation to as ST1 and MU, are shown in Figure 1. tlie outtlow facies rocks. Even though accurate knowledge rrf the struclurr and cvululion of thc caldera is not available. its mareill is fairly well cnnstraiztcd, allowing us to ehtimate its area at about Kawich Caldera 1550 ktn2. The original area of tlie caldera prior to east-west The Kawich caldera [Stewart and Carlsnn. 1976; Sargenf and crustal extension would be 1040 km2 (For this estimate we used Roggemack, 1984; Besr er al, 19931 that marks the source of the a uniform extension of 50%; the amount of strain differs lo Pahranagat Formati011 lies near the southern margin of the central different parts of the Great Basin but the 50% value is compatible Nevada caldera complex.