t^6 O_ LA-8845-MS Geology and Petrology of the Basalts of Crater Flat: Applications to Volcanic Risk Assessment for the Nevada Nuclear Waste Storage Investigations * _ o %._- co.. 0 ._ aj) ._= LOS ALAMOS SCIENTIFIC LABORATORY Post Office B0x 1663 Los Alamos. New Mexico 87545 I,; I~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ I I An Affrmative Action/Equal Opportunity Employer This work was supported by the US Depart. ment of Energy, Nevada Nuclear Waste Storage Investigations. Tis rort w*. Pp~jd asap i oun ofl3wk 4q'..nmtr byv n acwy f the Vasde S's WC" aiwni. V4ahlwi ti tiiNd Sljae 4. rnagnccl Oiu JaixfaYn htvf. fla ay o th'i v nagploytws. inut. a atrafty. %pvg w ripik. .g assmons any lepal iability oir vestainslaligy Mewtha aaar. .WY. V0PICUMNILaaa aa.1AaNIOCUof JnY M~ofilalkI. pg'aatu1S. plaa~d. iM g'MMc aduital. o tP ccnt hat ats uaw wual n ahow prw.atly oiwned i th. Itclemawe hti t any s.stW aoam- nmer.aI pruajuat. prisu~.. air vkv y r aine. radakmik. nnua..iax . o uherwiu.. Sas nut nC6Waxusayvitstltic a nly its nurwinen. raxumaienajatiu. a favtiny y tit U~nited Sics (aiwrann oir any .Arecnayglvocuf. The views anad pnions of avihuts eresased hundj ala nt nov- amiudy a r ilea h, attho;i atedtaStalts (.avaanlr air any .apty heuI. UNITED STATES DEPARTMENT OF ENERGY ) CONTRACT W7405-FENG. I LA-8845-MS UC 70 Issued: June 1981 Geology and Petrology of the Basalts of Crater Flat: Applications to Volcanic Risk Assessment for the Nevada Nuclear Waste Storage Investigations D. Vaniman B. Crowe I- I ,I - . GEOLOGY AND PETROLOGY OF THE BASALTS OF CRATER FLAT: APPLICATIONS TO VOLCANIC RISK ASSESSMENT FOR THE NEVADA NUCLEAR WASTE STORAGE INVESTIGATIONS by D. Vaniman and B. Crowe ABSTRACT Volcanic hazard studies of the south-central Great Basin, Nevada, are being conducted for the Nevada Nuclear Waste Storage Investigations. This report presents the results of field and petrologic studies of the basalts of Crater Flat, a sequence of Pliocene to Quaternary-age volcanic centers located near the southwestern part of the Nevada Test Site. Crater Flat is one of several basaltic fields constituting a north-northeast-trending volcanic belt of Late Cenozoic age extending from southern Death Valley, California, through the Nevada Test Site region to central Nevada. The basalts of Crater Flat are divided into three distinct volcanic cycles (3.7, 1.1, and 0.3 Myr) based upon geologic mapping, potassium-argon (K-Ar) dating, and magnetic polarity determina- tions. The cycles are characterized by eruption of basalt-magma of hawaiite composition that formed cinder cone clusters and associated lava flows. Total volume of erupted magma for respective cycles is about 0.5 to 4.0 x 10 1 km'; volumes of indi- vidual cinder cone and lava flow centers are about 0.3 to 1.5 km3. The basalts of Crater Flat are sparsely to moderately porphyritic; the major phenocryst phase is olivine, with lesser amounts of plagioclase, clinopyroxene, and rare amphibole. Basalts of the 3.7-Myr cycle contain glomeroporphyritic clots of bytownite and augite typical of hawaiite basalts in the southwestern United States. Major and trace- element differences between cycles, as well as the variations within cycles (in particular the 11-Myr cycle), cannot be explained simply by crystal- liquid fractionation. However, the consistent recurrence of evolved hawailte magmas in all three cycles points to- crystal fractionation from more primitive magmas at depth. A possible major i transition in mantle source regions through time may be indicated by a transition from normal to Rb- depleted, Sr-enriched hawaiites in the younger basaltic cycles. The recurrence of small volumes of hawaiite magma at Crater Flat supports assump- tions required for probability modeling of future volcanic activity and provides a basis for esti- mating the effects of volcanic disruption of a repository site in the southwestern Nevada Test Site region. Preliminary data suggest that succes- sive basalt cycles at Crater Flat may be of de- creasing volume but recurring more frequently. I. INTRO DUCTION The Nevada Nuclear Waste Storage Investigations (NNWSI) are evaluating the suitability of the Nevada Test Site (NTS) for location of a high-level radioactive waste repository. Current geologic exploration studies within the NTS are focused on Yucca Mountain (Dixon et al. 1980), a large fault block composed of multiple sequences of ash-flow tuff erupted from the Timber Mountain-Oasis Valley cauldron complex (Byers et al. 1976). Yucca Mountain is located within the south-central Great Basin, a physio- ) graphic subprovince of the larger Basin and Range Province, which includes much of the western United States. The Great Basin is a tectonically active re- gion. Its geologic history is characterized by extensional block faulting that produced linear mountain ranges separated by broad alluvial basins (Nolan 1943; Stewart 1978; Christiansen and McKee 1978). This faulting was closely associated in time and space with silicic volcanic activity at major cauldron complexes. Silicic volcanic rocks as old as 40 to 45 Myr are present in the central Great Basin. Younger volcanic rocks occur within broadly arcuate belts that are successively younger to the south and toward the margins of the Great Basin (Armstrong et al. 1969; Scott et al. 1971; Noble 1972; Stewart and Carlson 1978). Silicic rocks of late Miocene age are most abundant within an east-west-trending belt of the south-central Great Basin. This belt extends from southeastern Nevada through the NTS region and may bend to the northwest along the Walker Lane structural trend (Stewart and Carlson 1978). Since about 14 Myr ago, two major changes in the patterns of tectonic and volcanic activ- ity have occurred. First, there has been a progressive concentration of tec- tonic activity toward the margins of the Great Basin (Scholtz et al. 1971; 2 Christiansen et al. 1978). Second, silicic volcanic activity has been re- placed by basaltic volcanism including minor amounts of bimodal basalt- rhyolite volcanism (Christiansen and Lipman 1972; Christiansen and McKee 1978). This basaltic activity occurs within distinct belts along the eastern and western margins of the Great Basin (Stewart and Carlson 1978; Best and Hamblin 1978) and within a less prominent northeast-trending belt in the south- central Great Basin that extends through the NTS region (Fig. 1; Crowe and Carr 1980). Volcanic hazard studies, being conducted as a part of the NNWSI, are attempting to assess the risk of disruption of a waste repository within the NTS by future volcanic activity. Crowe and Sargent (1979) compared the geol- ogy and geochemistry of the Silent Canyon and Black Mountain peralkaline vol- canic centers, the latter representing the youngest major silicic volcanism within the NTS region. They concluded that the Black Mountain cycle repre- sents a renewed phase of silicic volcanism following the Timber Mountain- Silent Canyon volcanic cycle. This suggests that there is a small but finite possibility of recurrence of silicic volcanism within the NTS area. Crowe and Carr (1980) provided a preliminary assessment of the risk of basaltic volcan- ism within the southern Great Basin. They briefly described the Late Cenozoic volcanic geology of the southwestern NTS region, calculated the probability of disruption, and examined the disruption effects due to intrusion of a reposi- tory by basaltic magma. In this report, a continuation of previous work, we describe the detailed geology, geochronology, and petrology of the basalts of Crater Flat. This basaltic field is located within and adjacent to Crater Flat, an alluvial basin west and southwest of Yucca Mountain (Fig. 1). The basalts of Crater Flat record three small volume magma pulses that are spa- tially and temporally distinct (3.7, 1.1, and 0.3 Myr). Each pulse erupted basalt that may be classified as hawaiite following the definition by Best and Brimhall (1974). II. GEOLOGY AND PETROGRAPHY OF THE BASALTS OF CRATER FLAT Crater Flat contains over 15 small basaltic volcanic centers that consist of cinder cones and associated lava flows. The distribution and tectonic set- ting of the volcanic rocks has been described by Crowe and CUrr (1980). The rocks are divided into three distinct cycles or magma pulses based on geologic I I - - - - - - ) {Min eral N~ye * % \ 8 '- - -'- } - 3§ A;Lan oE.1,,*'*. og a u~~~~~~ 1~~380 4P C~* Inyo \~~N.,S~~~~l ~~Clark- a~~~ ~**;. I ~~~~ Mon *:s:. 4 1X I r. 01aO 0 00 l Kilometers 1180 1170 Ilse 115w 1140 Fig. 1. Distribution of Late Cenozoic basaltic volcanism in the south-central Great Basin. Modified from Stewart and Carlson 1978, and unpublished work by W. J. Carr. Gravity symmetry axis is the line of bilateral symmetry of the observed Bouguer gravity field of the Great Basin (after Eaton et al. 1978). LC, Lunar Crater volcanic field; RR, Basaltic rocks of the Reveille Range; QC, Basaltic rocks of the Quinn Canyon Range; BR, Basalt of Basalt Ridge; SC, Basaltic rocks of the Silent Canyon cauldron; SB, Basaltic rocks of the Sleeping Butte cauldron; BB, Basalt of Buckboard Mesa; PR, Basaltic rocks of Paiute Ridge; NC, Basaltic rocks of Nye Canyon; CF, Basaltic rocks of Crater Flat; 114, Basalt of Dome Mountain; SM, Basalt of Skull Mountain and Kiwi Mesa; GM, Basaltic rocks of the Greenwater Mountains; DV, Basaltic rocks of southern Death Valley; WL, Walker Lane; LV, Las Vegas shear zone; DV-FC, Death Valley- Furnace Creek fault. 4 N - field relations, potassium-argon ages (Table I), and magnetic polarity deter- minations (Fig. 2). * 3.7-Myr cycle (Tb, Tbp) Rocks of the older cycle-consist of deeply dissected cones and flows with locally exposed feeder dikes. ' They crop out in the central and southeastern part of Crater Flat (Fig.