THE LATHROP WELLS VOLCANIC CENTERi;,? - t, ,„ . STATUS OF HELD AND GEOCHRONOLOOy STUDIES ° WJ o B. Crowe and R. Morley, Los Alamos Naiional Laboratory. Las VegasSr, Nevad/ a S. Wells, University of , Riverside, Riverside, California 3. Geissroan, E. McDonald, L. McFaddcn, and. F. Perry, University of New Mesaco. Albuquerque, M. Murrell and J. Poths, Los Alamos National Laboratory, Los AJamos, New Mexico S. Forman, Ohio State University, , Ohio

INTRODUCTION passible age assignments for the Lalhrop Wells center.10-" The affects of Ihe different age assignments The Lathrop Wells volcanic center is located 20 km wer*. evaluated for calculations of the recurrence of of the poienusl Yucca Mountain site, at the south vdcanjc events for *-he Yucca Mountain region. The end of the Yucca Mountain range, [t has long been impact of a possible Late or Holocenc age recognized AS the youngesi center is the region.1 for all or parts of the center can be examined in terns However, determination of the age and eruptive of two hypotheses. If we assume the Lathrop Wells of the center has proven problematic The Lathrop center is young (< 50 ka) and (hat hypothesis is Wells center was interpreted originally as a incorrect, we have erred toward overesUmai ing volcanic monogenetk basalt center," Its age was inferred to be risk. Conversely if the hypothesis e tint the center is about 300 lea/-5 However, during the earliest stages of older (> 100 ka), and that ts incorrect, we have erred study of the Yucca Mountain area (1980-1985), two toward underestimating volcanic risk. The latter error perplexing questions emerged far the center. The first is not acceptable for evaluating the suitability of the is the unmodified geomarphic appearance of tfte mam Yucca Mountain site. Therefore we assume that parts scoria cone, which appeared inconsistent with a K-Ar of the center could have farmed during young age of 300 ka. The second is the markedly variable eruptions, (< 50 ka) until conclusive proof is obtained results of conventional K-Ar age determinations (whole to invalidate this assumption. The rationale for this is rock) obtained for and bomb samples from the to maintain a conservative perspective in site suitability center. Age determinations obtained at separate studies and because some data show that parts or much laboratories ranged from negative ages to > 700 ka.* of the volcanic center could be younger than 50 ka.

Preliminary geomorphic and sails studies were A detailedStudy Plan (&3.1.8.5.1 Characterization of completed at the Lathrop Wells center in 1986 to Volcanic Features)11 was prepared describing planned examine these questions.1 The concern that the cone geochronology and Held studies to assess the chronology could be significantly younger than 300 ka was of Ihe Lathrop Wells volcanic center and other substantiated by this work. A second phase of field and Quaternary volcanic centers in the region. A paper was laboratory studies were initiated at the center. Large published discussing the geomorphic and soil evidence scale (1:4000) color serial photographs were obtained for a late Pleistocene or age for the and the center was remapped.' A second group of cone of the center.11 The purpose of this paper was to samples was collected at selected sites that were judged expose the ideas concerning the age of the Lathrop to provide the best source material for K-Ar age Wells center lo scientific scrutiny. Additionally, field determinations. Conventional, whole rock K-Ar age evidence was described suggesting the Lathrop Wells detenninalioju were obtained for these samples. All center may have, formed from multiple eruptive events analysis were completed at a single laboratory* with significant intervals of no activity between events." Additional geomarphic and soils data were obtained for This interpretation breaks with established convention volcanic landfarms and soils at the Lathrop Wells in the volcanological literature that small volume basalt center and other Quaternary volcanic centers in Crater centers are monogenetic They arc inferred, based on Flat7 historic analogues,1" to have formed in a single eruptive event with a duration of months or years. The conclusion of multiple eruptions was based an the The results af volcanic risk assessment for the Yucca presence ofbasaWe fall deposits vd\h intecbedded saits. Mountain siit were reexamined, assuming a range of MASTER

DISTRIBUTION SF THIS 30CU.MENT IS UNLIMITED These deposits are exposed in luany cliffs on the soutli center is palycycljc; it formed du:i:i3 multiple, time Hank of the main cone." separate eruptive cvenis-

Controversy has .continiied rfmonnrng ifae enrprive The second part presents an overview of the history and chronology of the Lathrop Wells volcanic preliminary results of ongoing chronology studies and center. Turrin and Champion5 and Turrin et al.,s their constraints on the age and stratigraphy of the argued that conventional K-Ar and ^Ar/^Ar age Lathrop Wells volcanic center. Along with the determinations provide definitive age assignments of chronology data, the assumptions, strengths, and about 136 to 141 ka far the volcanic center with an limitations of each method are discussed. The current error of less than 10 ka. This conclusion was proffered statu of studies leads to the conclusion that the age of despite replicate age determinations that range over the Lachrop Wells volcanic center remains unresolved. almost three orders of magnitude. The basis for the Some chronology data support an age assignment for conclusion is compilation of the data using a weighted the center of > 100 ka. Other chronology and age- mean (l/o), a data reduction method that may not be calibrated mtihods indicate some eruptions at the applicable for this data set. Additionally, Champion1* center could be as young as Holocene. Mote work is suggested, based on palcomagnetic data, that the required to resolve this issue. ... Laihrop Wells volcanic center is a simple moragenetic center. This conclusion reverts to earlier GEOLOGY AND VOLCANIC STRATIGRAPHY interpretations of the volcanic center, made before recognition of the apparent compTexiry of the volcanic The Laihrop Wells volcanic center overlies alluvial stratigraphy.1^ These interpretations represent an deposits and faulted volcanic bedrock of the Paintbrush alternative and simplified approach to the chronology and Timber Mountain tuffs. It is located near the and stratigraphic problems of the center. However, a intersection of northwest-trending faults and the key point is that the K-Ar, ^Ar/^Ar and paleomagnetic northeast-trending Stagecoach Road fault (Fig. 1)." interpretaiiorts,,ils,w are based on an incomplete data The main cone of the center is elongate northwest. set. Not all volcanic units identified through detailed This elongation is probably controlled in part by the field studies' were sampled or analyzed. Additionally, direction of prevailing winds during the pyroclastic alternative sources of data in disagreement with the eruptions that formed the cone. Additionally, the radiometric and paleomagnetic data were not discussed feeder dikes for the center are probably oriented or considered in the analysis. *'"•'* northwest. This is indicated by two features. First, there is a zone of red scoria centered about the summit crater and extending to the southeast and northwest. The purpose of this paper is to describe the status The red scoria was probably formed by oxidization of of field and geochronolagy studies of the Lathrop Wells the scoria deposits by rising volcanic gases emitted from center. Our perspective is that it is critical to assess all an underlying northwest-irending . Second, there possible methods for obtaining cross-checking data to are two sets of northwest-trending, locally paired sets of resolve chronology and field problems. It is equally spatter cones and scoria mounds that demarcate important to consider application of the range of eruptive fissures. These are present along the cast base chronology methods available in Quaternary geologic of the main cone and at the northeast edge of the research.1' Such an approach seeks to increase the volcanic center. There is a third alignment of east- confidence in data interpretations through obtaining northeast trending spatter cones and scoria mounds that convergence among separate isolopic, radiogenic, and mark an additional fissure zone, north-northeast of the age-correlated methods." Finally, the assumptions, main cone, strengths, and weaknesses of each dating method need to be carefully describee] to facilitate an impartial evaluation of results. A detailed preliminary geologic map of the Lathrop Wells volcanic center was completed in 1988." The The paper is divided into two parts. The first part contacts of lithostraiigraphic units on the map have describes the status of continuing field studies for the remained largely unchanged. However, there have volcanic center. We are attempting to establish the been several additional units recognized and existing field and chronology relations of all the volcanic units. map units modified or assigned to different stratigraphic This work is supplemented by recently initiated intervals. Briefly, the significant new results include: trenching of contacts between lithostratigraphic units. The combined field and trenching studies represent one 1. A lava flaw, largely buried by colian sand and of the most detailed studies of a single small volume silt on the north side of the cone, has been basalt center. The initial results of (his work support excavated through trenching. The lava flow is the interpretation that the Lathrop Wells volcanic underlain and overlain by Fig. 1: CcologicsettingofihcLaihropWciisvolcanicccnier. Modified from Frizzell and Strollers.20 The center is located Pt (he south end of Yucca Mountain. It overlies Miocene hiff and alluvial deposits and is overlain by alluvium and eolian deposits. Ml: Miocene tuff undivided; Pb: Pliocene basalt of Crater Flat; Qb: Quaternary lava and scoria deposits of the Laihtop; Qbs: Main scoria cone of the center; Cross- hatched lines are eruptive fissures.

units and reworked scoria. The lava flow validity or the uncertainty of correlations using underlies a topographically high-standing lava paleomagnetjc data. Existing paleocnagneticdata flow (Ql ). A soil with horizon development has 4 must be applied with caution unUl mare been identified in the primary and reworked complete data sets hai-e been published and the section between the lava flows. The data verified. sequence requires a hiatus between eruption of the . The Lathrop Wells volcanic center formed fres 2. Studies of sail horizon development an the predominantly Hawaiian and Strorabolian pyroclastic eruptive units, now supplemented by trenching, eruptions associated witii outpouring of minor volumes continues to support a late Pleistocene or of lava. The Hawaiian eruptions produced spatter Holocene age of parts of the center. cones, scoria mounds and lava flows from vents along 3. Detailed petrographic, gee-chemical, and iwiopic several, fissure systems. The Strorabolian eruptions studies of the scoria and lava units have produced a relatively large scoria cone composed strengthened some unit assignments and revealed predominantly of non-agglutinated scoria. Pyroclastic new field units. surge deposits are iiitetbedded with the lava and scoria 4. We reassessed the results of palconiagnctic data, deposits. However, the vents for these hydrovolcanie which were emphasized in establishing previous eruptions cannot, in most cases, be directly identified. straligraphic units. The uncertainty in the secular Erupted lava flows were unusually viscous for basalt variation curve during the lime of the Laihcop . They formed blocky aa flows directly at their Wells eruptions has been neither established nor vents. Flow lengths are generally < 1 km and some are evaluated. Additionally, sufficient data have not no longer than a few tens of meters. been presentee? in publications to assess the Special caution must be used in interpreting the vent —pyroclastic surge deposits. The flow, in turn, is overlain configurations and geometry of basaltic volcanic centers. by primary and reworked surge and scoria-tall deposits The pulsating dynamics of eruption columns and the containing a soil with distinct horiron development. variability in magma effusion rates can produce This soil has been '.Limbed disecinianaiEly by conspicuous u neon form] tics and complexities in the extensive bioturbation. Excavation of the buried (low stratigraphic relationships of volcanic centers. These and overlying lephra/soil units showed thai this

complication! can develop from eruptions over periods sequence underlies outcrops of the Q\t lava. We thus of hours, days, months or yean. For example, have identified an older lava of the center that is

eruptions of (he Pu Oo \ent on the flank of Kilauea separated from the Ql4 lava sequence by a sail-bounded opened first with a fountaining eruption a long a fissure, unconfanniry. No age determinations or paleomagnetic then focused lo a central vent. During the 5m few data have been obtained for the flow. We recently years of eruptive activity, a symmetrical spatter cone drilled and sampled this unit and arc now processing was formed, followed in lime by an asymmetrical scoria the samples Tor paleomagnetic determinations. cone, shaped by the prevailing trade winds." The second lava unit of chronostratigraphic unit 3 What we have discovered, through detailed mulli- consists of blocky aa lava flows and local cone scoria. disciplinary studies, is that some volcanic units at the This unit was mapped originally as pan of Qlj, Lathrop Wells center are separated by soil-bounded However, detailed petrology, and geochemical studies*1 unconformities. The degree of horizon development in have shown that one pari of the Qlj unit is the soils requires time between eruptions that far pelrographically.geochcmically.andisotopicaUy distinct. exceed the cooling rimes of small volumes of basalt This unit has been labeled Qt« on the revised geologic magma in the shallow . It is these features that map (Fig. 2). Unmodified exposures of the unit are lead to the inference of eruption of spatially and exposed along a narrow band at the southwest edge of

temporally distinct batches of magma. We refer to the center. Here the Ql4 unit consists of aa lava Qows these eruptions as polycyclic and consider them to be n and cone scoria. However, much of the original subclass of polygene tic volcanoes. Polygene tic distribution of the unit has been either removed or volcanoes typically are large volume volcanoes, often covered by scoria eacavaiion at the commercial quarry associated with shallow magma chambers. By contrast, site. The basal contact of the QI« lava b elevated we use the term polycyclic to refer to intermit lent above the modern pavement surface and base level It eruptions at small volume volcanoes (< 1 kmJ) that are is the only volcanic unit in the Lathrop Wells volcanic normally classified as monogencttc.13 center that shows this relation. Additfonalry, exposure edges for this unit are not flow margins. There has been sufficient erosional stripping of the lava margin to New field and strat igraphic evidence described above expose the massive aa flow interior which a underlain suggests the original five Hthostratigraphic units can and overlain by Dow breccia and clinker. No K-Ar or now be separated into at least seven lilhostraiigraphic paleomagnetic data have been published for this unit. units (Fig. 2). These seven lithostratigraphic units can We have sampled the unit for isotopic dau'ng using the be grouped into three chronostratigraphic units. We U-Th disequilibrium method. Paleomagnetic studies of infer that there is time signiGcance to these units but this unil are in progress. cannot define their temporal boundaries with satisfactory confidence based on existing chronology data. We recognize that the chronostratigraphic units The third lava unit of chronostratigraphic unit 3 is may change as additional geochronology and field data Qlj, a series of lavas that crop out along the southern are obtained. The current (December, 1991) division part of the Lathrop Wells center (Fig 2). These lavas of chronostraiigraphic units for the Lathrop Wells were inferred to be derived from a fissure (Osj) that volcanic center are described below, from oldest to extends northwestward, along the east base of the main youngest. cone.* This interpretation was based on ihe reported uniformity of the field magnetic directions for these Chronostratigraphic Unit Three: The oldest units* Several new observations indicate that this identified chronostratigraphic unit at the Lathiop Wells conclusion may be only partly correct. First, careful center consists of three lavas, with minor vent scoria, examination of the paleomagnetic sample sites has exposed at separate localities. The first lava is buried shown that the Qlj lavas were not sampled or analyzed largely by eolian sand and silt on the north flank of the for the paleomagnetic studies/ The basis for their center (Fig. 2). It was correlated tentatively with the correlation with the northwest-trending fissure cannot 8 be confirmed. Second, petrologic studies suggest thai Qlj lava. However, geochemical studies have not 12 the QI5 lavas are distinct compositionally from most of confirmed this correlation. Construction of shallow the northwest-trendine fissure.11 Fin.illu. we have trenches revealed that the buried flow rests upon Fig. 2: Geologic map of the lithostratigraphic units of the Lathrop Wells volcanic center. Modified from Crowe et aL8 Shaded patients outline the scoria mounds and cones that are the primary eruptive sites of the center. Chronostraiigraphic unit 3 includes ta lava flow units (Ql^, Q^, Qy, vent spatter, agglutinate, and scoria-fall

deposits (Qs,)- Chronosiratjgraphic unit X includes aa lava flow units (Ql4, QI3), pyroclasltc-surge deposits and vent spatter, agglutinate, and scoria-fall deposits (Q3«,

Ch) and Qslp). At least two pyroclastic surge units may be associated with this unit. Cbronostratigraphic unit 1 includes cone scoria (0„). and tephra-fall deposits derived from the scoria cone (not shown on the map). Farts of the scoria-fall sheet are

included with 0s]a.

tentatively reassigned the southern parts of :he main vent spatter and agglutinate through time. We cannot

northwest-trending fissure system (Qsu on Fig. 2) to a evaluate with confidence whether the geomorphic

revised Qs5 unit. This is based on an increased degree differences are temporally significant. Second, of erosional modification of the spatter and scoria Champion,16 and Turrin and Champion," have reported mounds a( the south end of the fissure, proximate to paleomagnetic data for the southern part of Ihe fissure outcrop lobes of the Ql$ lavas. There are two concerns that match the rest of the fissure. However, the with this assignment. First, systematic data have not number of sites in the newly identified Qs, unit (Fig. 2) been obtained on patterns of decradation of are insufficient to evaluate all vent zones. Further, palcomagnetic data for individual sites were rot pyroilastic surge unit that underlies the tephta beds of published so the correlations could not be evaluated chronostratigraphic unit t. These deposits may be part of the third, or second cfanmcHSraijnaphic uniu. Chronostrttigraphte HJnit Two: The wcond Champion1" and Traxm and Champion* assigned this chronoslratigraphic unit forms the major volume of the unit to what would be equivalent to our second Lathrop Wells center. Thii sequence includes part of chronostratigraphic unit based on paleo magnetic data. ihc main cane (outer pans of the scoria-fall sheet), the However, they show only one sample site* and did not

Ql, and Qs4 lavas and scoria/fissure system, the Olj and publish the data for this site. Trace-element Qij lava and scoria/fissure system, and nsost of the geochemical data shows that at least part of this 11

main northwest-trending fissure system (QsuonFtg.2; sequence is unique. Currently this unit is unaligned

previously assigned to Qst). The following stratigraphie until further data are obtained. relations for these units have been established through field mapping and trenching studies. Pyrochstic surge GEOCHRONOLOGY STUDIES deposits that appear to have been derived from the

main cone (QpSj) underlie the QL, lavas. The Ql4 lavas A range of geochronology methods have been used arc overlain by the Ql3 lavas. Tephra from the. main to establish the age of the Lathrop Wells overlies the Ql^/Qs, and Qlj/Qi, units. This last center. We describe die results of these studies, below, observation is based primarily on field mapping and has by individual method. The nomenclature used to been verified by trenching at a few locations. Turrit et IJ describe the chronology methods is after Colman ct 11 aL lump the QL/Q54 and Qjj/Gsj units together based al. The methods used include the conventional K-Ar, on identification of a similar field magnetic direction for ^Ar^'Ar, the U-Th disequilibrium method using solid these units. They separate the main northwest-trending source mass spectrometry, surface exposure dating rift zone and the main cone from the OI+ZQs^ and 3 ; using cosmogenic He, and the ihermoluminescence Qlj/Qsj unit on the bas s of a 4.7 degree angular method. Additional age determinations using the MQ difference in measured field magnetic directions. This method have been reported by other workers. Cross• may be a valid observation and we are attempting 10 checking methods used to evaluate the isotopic and verify it through additional palcomagnetic studies. radiogenic data include field, gcomorphic, soils and Exiting strat(graphic and trenching data neither refutes' paleomagnetic studies, and major and trace element nor collaborates their assignments. Petrographic, geochemistry. gcochemicaL and tsotopic studies suggest the units could all be closely related.21 Some chronology data described below supports an age division between the Conventional K-Ar method: Potassium-argon age QI< and Ql. lavas. determinations of whole rock samples of basalt from the Lathrop Wells volcanic center have been dated by a variety of laboratories.*'1 The most comprehensive Qironostrati'raphlc Unit One: The youngest summary of the results of the whole rock dating is by chronoslratigraphic unit is not shown in Fig. 2 because Turrin and Champion.* They obtained K-Ar ages of

it consists of small volume tephra units and it occurs samples, some replicate, for separate sites from the QI5,

largely in the area of active quarrying This group Ql4, Glj, and Qs, units. They reported a weighted

consists of tephra units separated by soils that arc mean of 1115+13 ka for the Qla unit and 133+10 ka for 11,1 exposed in quarry cliffs south of the main cone. * the QI3 unit (a combined GVQs, and Q\JQsi unit). These tephra units are correlated with a distinctive The arithmetic mean is 185+.190 for their Ql, unit and sequence of plane-paraliel bfdded scoria-fall ar.d 214+86 for their QI, unit; the arithmetic mean for all hydrovolcanic deposits identified in the south quarry samples is 220±362 ka. Turrin and Champion* site wall of the main cone. These deposits can be traced slraiigraphic constraints and U-Th ages on alluvium continuously in the upper quarry walls to the summit of deposits that contain primary and reworked cinders 4 the main cone. Because of the presence of the soil km northwest of the Lathrop Wells center. They deposits beneath and between the , they are suggest the tcphra was deposited between 240+30 and separated as a distinct chronostratigraphic unit. 145+25 ka. This is based on an inferred correlation of tephra with Quaternary stratigraphi'c units dated by the U-trend method. We have examined these ash beds Discussion: One unit of the Lathrop Wells center and note the following uncertainties with the suggested remains problematic. A cluster of vent agglutinate and correlation. Firs:, neither pctrographic nor geochemical related scoria deposits forming latelliiic scoria cones data have been obtained for the tcphra. While the fall arc exposed on the south flank of the main scoria cone. deposits probably were derived from the Laihrop Weils These cones have been extensively modified by center, no correlation has been established with specific commercial quarrying. They are overlain by a eruptive units. The most likely correlation for the lithologically and gcochemically disiinel scoria-fall and tephra arc wiih one or several of the pyroclasiic surge ages arc not in agreement with die weighted averages units at the ccniex. However, because there are of the *°Ar/"Ar and conventional K-Ar age muItiplC-SUIge unllS, '<"• ^fratiff-pphir^igniFirnnc-i- Qf this determinations reported in their earlier paper.7 correlation is unorP3'*" Second, i""'^ripni TqajTryng and stratigraphy studies, have been completed GO the The limitations of the *,Ar/'*Ar data set are several. alluvial units associated with the tephra ta establish Most important, analysis of fine-grained basalt using a cither the number of tephra units, or the stratlgraphic laser fusion method is recognized to yield anomalously relations with the alluvial deposits dated by the G-trend aid ages because of recoil effects of wAr during neutron method. activation.11 This limitation was not discussed.13 Second, the extreme spread in the age determinations The strengths of the conventional K-Ar data set are cannot readily be explained as analytical error. It may that this is the best tested and proven of chronology jesull from a systematic error term that hes not been methods applied to the Laihrop Wells center. The ages identified. Third, the ^Ar/^Ar ages of individual units are derived from a large number of age determinations. are systematically older than the conventional K-Ar age Additionally, the. ages are consistent with the age determinations. Averaging of both data sets may not obtained using the U-Th disequilibrium method (see be appropriate. At the least, justification needs to be discussion below). presented for the treatment of the data. Finally, examination of the isochron and inverse-isochron plots 1 The weaknesses of the conventional K-Ar age (Turrin el aL *; their figure 2), show that the slope is determinations are several First, the range of controlled strongly by two points that are divergent measured ages for the conventional K-Ar age from the main data set These points are not determinations is from 37i29 to 57I+.36Q ka - an identified, [f removed from the data set, the unaceeptably large range to attempt to use the data to regression parameters and slope of the plots would be resolve the chronology of volcanic units with a precision dramatically changed. of about 10 to 100 ka. This range is also too large to explain as analytical error. Second, the age Summary of K-Ar Methods: Evaluation of the determinations are based on whole rock analysis, not conventional K-Ar and "Ar/^Ar age determinations for mineral separates- Third, the age determinations do the Lathrop Wells center show the measurements are not show a normal distribution.15 Thus, averages of the of high quality and provide one approximation of the data set may not be meaningful Finally, averaging or age of the center. Controversy concerning the age of the data using a weighted mean is not valid if the the center is with the method of averagiag the age sources of error arc not analytical. determinations, not the analytical methods or results. If the data sets are compiled using conventional "Ar/'^Ar Age Determinations: Tunin and arithmetic means, large errors (one o) are obtained. Champion' cite weighted mean: of 183±21 ka for unit These errors overlap wiih and are consistent with ihe QI3, 138*54 ka for unit Ql, and 149+45 ka for unit Qs results of virtually all other methods used to assess the t ^gc of the Lathrop Wells center. Moreover, the errors using the '"Ai/^Ar method. The age determinations suggest lhat the K-Ar methods, as applied to ihc range from 42+1B5 to 947±24 ka. The arithmetic mean basaltic rocks of the Lathrop Wells center, may have of the Ql samples is 171+87 if samples identified as 3 insufficient precision to successfully re:orvc the contaminated are removed from the data set. chronology problems. However, no criteria are presented for identifying contaminated samples,* The authors appear to have rejected selectively, any age determinations > 400 ka-' The use of a weighted mean leads to age If the rejected samples are included in calculations, the assignments with small errors. These data do conflict arithmetic mean for the sample set is 252*220 ka. The with results of other chronology methods and soils and 12 age determinations range from -20*263 to 368*644 for isomorphic studies. Three questions must be asked. First, is a weighted mean a proper statistical method to the combined Qls and Qsj data set. The arithmetic mean of these samples is 150*90 ka. use with* data sets where the range in replicate analysis exceeds considerably the expected analytical errors? Turrin ct al15 use the same data set but average Second, can cross-checking data be obtained to test the validity of applying a weighted mean for replicate age (weighted mean) the conventional K-Ar data set with determinations with poor reproducibility? Are there the ^Ar/^Ar age determinations. They obtain an age other explanations for the large age range of replicate of 136±8 ka for the Qlj/Qsj data set and 141*9 ka for analyses such as excess Ar, or contamination of the the QIJ/QSJ units. They do not list the conventional K- basalt with Cenozoic tuff? These questions can only be Ar data used in their calculations so an arithmetic mean answered by further chronology studies. cannot be calculated from their papsr. These weighted Uranii n-Serics Darini? 4. Pure Mineral Separates. An efficient separation of minerals and phases containing the U and Th Magcratic "processes, -such as inching or is required in order to differentiate between a crystallization, can produce Tudioactivc tJiscqmJibrrom Tnicisoi^uaaiidttetascofaToningfincT»hicfa between members at the uranium decay series.2* Such could result from a nurture of minerals from disequilibria can be used to estimate the time since different sources and/or of different ages. eruption for a volcanic event. To da so, however, requires that certain conditions be met. The fallowing Samples from three flow units at Lathrop Wells have requirements and assumptions must be verified before been analyzed for ^U-250!^ disequilibrium. The pair any meaning should be attached 10 such a date: 23 ^(J- *!!! was selected because it is well-suited fQr providing chronology data over the age range of 1. Closed System Behavior. This is a necessary interest (< 200 Jca). initially a whole roelc isoehron condition boih for magma chamber processes as method was attempted. This approach sometimes can well as during and after eruption. be applied to a sequence of volcanic flows that axe 2. Short Residence Time. The residence lime of closely spaced in lime and have measurable "U-13*!!! minerals in the magma chamber after disequilibrium and different mUp&m ratios.25 crystallization or in ascent to the surface must be However, this approach was unsuccessful at the short relative to the radioactive half-lives of Lathrop Well center because the whole rock samples interest appear to be jn radioactive equilibrium (wimin £0J%). 3. Measurable parent-daughter fractionation. This Mineral phases were then separated from a sample of condition has became somewhat easier to meet the Ql4 lavas in order to obtain an internet bochron with the application of mass spectrometry to (Fig. 3). The apparent isochron age of this sample is these measurements. lSG±40ka. Theprecisionaftheisochronislunitedby

0.925 - /•

0.900 150 + -40 Ka /mag

0.875 - " olv i ^plag

0.85Q -

0.B25 • " / ^equiline

0.800 i ' • , , 0.600 0.825 0-850 0.875 0.300 C.925 0.950 MU/*»Tri

Figure 3: Plot of ""Th/^Th versus ^U/3*!^ for the Ql« lava of the Lathrop Wells center. the small degree of Th/U fractionation observed for the COSMOGENIC 511H AGE DCTERM1NATIONS separated phases (Table 1). The accuracy of the isochron for defining an eruption age is dependent on We have estimated &c ages ol Jilhostratigraphic how vnV the criteria listed above-have been -met. units at thcLathrop Wells volcanic center by measuring While the extremely fine-grained size of the basalt the accumulation of cosmogenic He in volcanic rocks sample arguet against prolonged magma storage, it exposed at the surface. The calculation of 3He surface makes clean mineral separates difficult to obtain. exposure ages follow standard techniques.14 After field Without pure mineral separates, there is always the collection of samples, an fraction separated from possibility that the apparent isochron far the Q\t unit is a sample is first crushed in vacuum, releasing the produced by mixing of components of different age. roagmatic noble gases. These gases arc analyzed by With additional work, it should be possible to better 21 110 static noble gas mass spectrometry. The crushed evaluate the accuracy of the ^tJ. !!! dates by sample is then melted, releasing cosmogenic helium analyzing other volcanic units from the center. Samples plus any residua] magmatic helium. The concentration oF the Ql* lava are currently being processed. 3 J of cosmogenic He b equal to total He released in the melt step minus residual magroabc 3Hc We assume The uranium-series method provides high that magma lie 3Hc is equal to the 'He component precision in the age range of interest Given that the released by melting, times the 3He/*He ratio released in assumptions of an analysis arc met, the measured age the crushing step- should represent the crystallization age of the lava Table II lists the age estimates from the measured flows. Wc have no reason to discount the U-Th age concentrations of cosmogenic 'He from surface samples other than noting the small degree of Th/U of the Lalhrop Wells center. Tie data have been fractionation of the analyzed phases and the pcribiliry corrected for mass discrimination, processing blank, and of a mixing age. Because this is a development method, we evaluate the results with caution until sample thickness. Toallowcomparisonofrelatrvcages, additional age determinations using the uranium-series the uncertainties in the ages only include analytical method are obtained. uncertainties (one a). The production rates used in

Table U Thorium and Uranium Concentrations Tar Lathrop Wells Samples

Sample/Fraction Thorium (ppm) Uranium (ppm) 232Th/238U

LWS9-3-2J-1

whole rock 6.89 2.11 3.38+0.01 non-magnetic 7.06 2.16 337±0.02 magnetic 5.50 1.71 3.33±0.01 LW-89-3-21-2 fG'J whole rock 7.97 Z26 3.63±0.02 olivine 0.32 0.09 3-67*0.02 plag/glass 7.36 2.13, 3.57+0.02 magnetite 3.45 1.03 3.47+0.02 LW-89-3-21-3

non-magnetic 7.20 2.02 3.69+0.02 magnetic 6\50 1.84 3.65+0.02 Tabid II arc from Cerling.23 They arc adjusted for There arc two concerns with the cosmugonic: 3llc latitude and altitude using the corrections of LaL"' method. First, is the surface exposure history of the These production rates are correlated to ibc "C time rint>-tl <:iirf:ir*- Minimum ages will be obtained if ihere scales. If tncy are correlated to the Uranium series has been erosional removal of material or previous time scale using the data of Bard et al.3* the ages would cover of the surface. Second, the calibration of the 3He increase by about 17%. The production rates far the surface production rates are uncertain and vary with helium data used in Table II are probably accurate to altitude and latitude. Most of the cited helium ages in ±30% or better. the western United States are calibrated to one locality.14 The samples from Qs, were collected at the summit of the main cone of the Lathrop Wells. The samples Zerda et al.31 have attempted to date the volcanic were selected from a flat part (several meters wide) of units from the Lathrop Wells volcanic center using the the cone rim to avoid erosional loss of overlying scoria cosmogenic **C] method. They initially reported dates

down the sleep cone slopes or the crater interior. AM of about 75 ka far the Qls unit and (he main cone rim samples collected are volcanic bombs. Selection criteria and 110 ka for a surface bomb of unknown included parallelism of the direction of aerodynamic straligraphic position. These ages have recently been elongation cf the bombs to the layering of the scoria revised (downward) because of surface xCl

depusits, flattening (impact-induced) of the bomb contamination. Their age for the Ql3 lava now appears parallel to layering of the scoria deposits, preservation consistent with the JHe age. However, their revised of a well developed rock varnish upper surface, and the ages have not been reported in the literature. presence of calcite-silica coatings an (he bottom surface Moreover, cosmogenic production rates for the MC1 of bombs indicating no movement of (he clast The and 3Hc methods have not been cross-calibrated. spread in ages of (he bomb samples may indicate that (hey do not all share a similar surface exposure history.

A pap'cular process of concern is the eolian erosion of 3 ash from the exposed rim of the crater. There is also Table II: Cosmogenic He Ages for the Lathrop Wells the possibility of an inherit 3ti exposure history for same Cenler bombs if the Lathrop Wells center is polycyclic. This possibility is discounted, however, because cf the careful selection criteria used to collect the samples. Unit 1>pe Rate (ka) We are relatively confident all collected bombs showed evidence or aerodynamic shaping indicating aerial jl H ejection in a molten state. The cosmogenic JHe ages Qs, 288 •14+5 are minimum surface-exposure ages. Therefore the Qs, 288 28*4 oldest 3ge of the bomb from the summit may piovide Qs, 288 23±4 the best approximation of the minimum age of the cone («±13 ka). Ql. Lava 257 48i5

Ql, Lava 257 Mi6 The lava samples were collected from well preserved, primary flow tops of the aa lava Oows. Because there has been a history of iniermiiieni

deposition of eolian sands on the lobes of the Q]t lava, TH ERMOLU M F.N ESCENCE AGE wc attempted to collect samples from aa spines that DETERMINATIONS projected above the height of active sand deposition. Eolian sands are thinner at the O'j locality and were Seven analysts of four different samples have been judged not to be a problem- At both lava flow obtained using the thermoluminescence (TL) method localities, we collected spiny, vesicular flow material (Table lip. These results are judged to be analytically (hat appears to represent undisturbed clinker from the reliable and reproducible, but preliminary. The TL top of an aa flaw. The preliminary data suggest that method has not been used previously in attempting to there may be a difference in age between the 01, and date soil units for a volcanic center. All soil samples Qls lavas, consistent with a polycyclic interpretation of were collected from the Av horizon. The TL age the center. However, we interpret these results estimates were determined by the total and partial cautiously until experiments are completed on the bleach methods. All samples were preheated at 100 reproducibility of analysis of multiple samples from the degrees C for two days to isolate the most time same lava flow surface. Such an experiment is in sensitive TL signal and to obviate any effects of progress. anomalous fading. These samples were tested for anomalous fading for over 60 days.and exhibited no •"Cntcr is no older than 20 ka. The weaknesses of this fading within 5% analytical resolution. Three samples correlation are primarily twofold. First, the age were collected from buried ioils *"*pented with lepha relations lor the Cima senna cone are only partially deposits in the quarry wall somn ctf lie main cone constrained. Second, the correlation is dependent on The soil samples are ihterbedded with basaltic fall the assumption of the similar operation of rates of deposits from chronostratigraphie unit one. The processes of erosion and soil formation. The first resulting age estimates are in ihcir correct stratjgraphje weakness has been addressed by Wells el al.n They sequence. Sample F89-LA3 provides the best have obtained new cosmogenic 3He, and approximation of the age of the lower tephra deposits thermolumincscene ages for the Black Tank center in (9 ka; Table HI). The upper two samples date the age the Cima . These data support an age of of the upper tephra units (4 ka; Table III). between 9 and 14 ka for the main cone sequences at the Black Tank center. In addition, recent trenching The sample from the sediment sequence was has demonstrated that the base of the main scoria cone collected several tens of centimeters from the basal at Lathrop Wells is flanked by a sand ramp which contact of an overlying aa flow lobe. Preliminary displays little evidence of mass wasting or colluviation modeling af the temperature rise of the sample from from the cone slopes. This supports the original emplacement of the overlying lava flow, shows it to be inference of Wells el al" that the cone slope is well above 300 degrees C These temperatures are virtually unmodified by erosional processes. The sufficient, as shown through laboratory experiments, to second weakness is discussed in (he following section on remove any previously acquired TL signal within 15 soil processes. minutes. The indicated TL age estimate of the Ql, lava is 24J+2J lea. We are in the process of obtaining TL SOIL STUDIES ages for lava flows overlying sediments that have been dated by the "C-method for multiple localities in the Studies of soils on volcanic landfonns associated with Snake River Plains. These results will be used to test Ihe Lathrop Wells center show that weakly developed the reproducibility of TL ages of lava-baked sediments. calcic soils have formed in scoria deposits that Dank the north and south side of the main cone and on the cone slope.11 The surfaces of the lavas Bows arc almost Table III: Thermoluminescence Age Estimates For the completely mantled by eo!ian deposits Dr by pyroclasuc Lathrap Wells Volcanic Center deposits. These deposits have only incipient soil development in the upper several decimeters. The Utb*. Sample Unit Field Number ubNuober TLAfc primary pedogenic features exhibited by the soils 11 U«l graphic (ka) include weakly developed "vesicular A" (Avk) horizons and weakly developed B horizons in which fine sand, silt clay, calcium carbonate (exhibiting stage I Quany Buried Av F89-LA3 m,Yl 8.9*0.7 morphology of Gile ct at-)3* did trace amounts of Qi, Soil I* 93*0.7 &7±l,0 soluble salts have accumulated. The presence of substantial amounts of quartz and other pedogenSc Qwny Buried Av FB9-LA4 rTUYfr 3.7*0.4 materials (calcium carbonate and sulfates or chloride' Qi, Soil 2 salts) thai are rare or absent in the basaltic tephra unequivocally demonstrate thu calian origin of most of Ou»ny Buried Av F»LA6 rTY-Y4 3.710.4 these pedogenic materials. QJ, Sou" A * StfLA At the Lalhrop Wells volcanic center, ihc most strongly developed soils have been observed on Ihe ' SoQ description and unii assignments bora Welti et iV* summit of a scoria mound (Qs,) located northeast of the main cone and in the scoria-rail sheet of the main cone.*' These soils have the thickest, most well GEOMORPHIC STUDIES developed vesicular A horizons in soils observed to date, ranging from 5 to 8 centimeters thick and Geomorphic studies of the Latttiop Wells volcanic possessing strong, coarse platy structure with center have been described previously by Wells et aL" subordinate subangular blocky to prismatic structure. They equated the geomorphic and pedogenic features The subjacent Bwk horizon* ate approximately 6 to 17 of the Lathrop Wells center with a is to 20 ka cone in centimeters thick, with subangular to blocky structure. the . Tin. close comparison of the These horizons do not, howeve;, exhibit color hues or centers sujy>ested, by inference, thai the Lathrop Weils chromas substantially redder than those of the Icasl altered loamy sandy parent materials or the most of sand. Subsequent to demolition oi \]\$ sand, recently accumulated materials above the vesicular A geamorphic conditions were presumable sufficiently horizon. Pedogenesis in the lowest 1 meter of the different to eoaMe the development ot curculic soils. profile' exposed in pits ts characterized by the IThese sous incorporated "much -finer grained desert \ accumulation of moderately thick to thin, largely loess and formed aciteuouaiy cambtc B ana" vesicular • discontinuous coatings of carbonate, gypsum and A horizons. soluble, salts. A small amount of pedogenic silica may also have accumulated, although this requires Soil profile de—^lopmcnt similar to ire Lathrop confirmation by chemical analysis. The content of these Wells center is a^soriated with landfbrms and dcuuiiu materials diminishes progressively with depth. Only the or late Pleistocene age in the Cima volcanic field r^-14 most incipient coatings can be observed at depths of U Soils that possess Avlc, weakly reddened Bw or weak Bt to 1.5 ro in the parent scoria materials. A soil observed horizons and Bly horizons with stage I morphology, are on the steeper pan of the cone slope has a similarly associated with flows with K-Arage determinations" of thick, calcareous B horizon, but lacks the well about 60 to 140 ka. Similarly developed soils are alio developed vesicular A horizon. present on lava Hows in the Cima volcanic field, which may be as young as -.5 ka.u Soils with these 31 Soils observed in the sequence of buried scoria units characteristics arc referred to as phase I soils. The exposed in the quarry on the south side of the center,11 most detailed studies of soils in the Cima volcanic field c?e more weakly developed. They exhibit 2 to A cm have focused on soils developed on the lava Qow tuick vesicular horizons and very incipient, calcareous surfaces. Here, soils have formed primarily in eolian cambic B horizons. The scoria parent materials have deposits that locally the surface. Soil carbonates, salts and perhaps silica accumulated development also extends into flow rubble and primarily on tiic bottoms of scoria fragments. As noted commonly into fractures that penetrate the dense, above, pedogenic materials are observed to depths that interior parts of the flows. The predominant process exceed 1 m. Fresh scoria lacking any pedogenic that influences the ievelopment of these soils is materials arc only present at depths of 1-5 m or more. cumulic. That is, eolian materials (desert loess) entrapped on the flow surface are continuously translocated t.>claw an evolving stone pavement and progressively altered by pedogenic processes. This In contrast, soils formed in sand ramps that flank the results tn the development of ar increasingly thick, cone are very weakly developed Pedogenesis is predominantly stone-free Av and B horizon above the indicated primarily by the slight increases in How surface/rubble.11** disseminated carbonate with depth and the accumulation of very thin, discontinuous coatings of carbonates and perhaps salts on the bottoms of many Morphologically and tewually, these soils strongly of the larger coarse fragments. Scoria fragments in resemble soils that have formed on alluvial fan and such deposits commonly exhibit thicker and nearly eolian deposit, that post-date the highest lake shoreline continuous coatings of carbonate. However, the observed above the playa of Silver Lake, located 15 km nonsystemaiic spatial location of the coatings on the north rf the Cima volcanic field. This shoreline, fragments as a function or depth show that only a formed during the highest stand of plun'al Lake minor volume of material has been derived from higher , approximately 13 to 16 |».w,,0,4Ml The soils positions on the cone slope by gravitational forces. on the latest Pleistocene and Holoccne alluvial "vis and Soils on (he cone slope, as noted previously, have Bk eolian deposits in this area have been the subject of horizons with carbonatecoated fra^Ticnu.They provide several studies.*0-0,4*'*1-" These studies, t ©«• that these a source For most of the fragments with thick carbonate soils have also formed by processe; •. -ifar to the coatings observed in the sand ramp deposits and distal described soils in the Cima volcanic fitkl. Soib on cone slope sediments and soils. deposits in the Silver Lake area that are considerably older than the latest Pleistocene fan deposits are associated with strongly developed desen pavements The medium-and fine-grained sandy deposits of and have reddened, relatively thick clayey Bt horizons eolian origin are inferred to bury previously formed and strongly developed calcic with stage II morphology. vesicular horizons of soils in the scoria deposits. Thcjc These soils resemble phase ?. soils that occur on older deposits range from 2 cm to over )J m thick in the Pleistocene flows in the Cinia volcanic field which alio sand ramps. They are present below the weakly have strongly developed desert payments and thick. developed scoria pavement and have little or no soil reddened Bt horizons. Most importantly, the age development. These soil-straiigraphic relations suggest constraints for latest Pleistocene and Holoccne deposits an increase in eolian activity during the fate Hofocene, and soils in the Silver Lake area enable calibration of resulting in the deposition of locally thick accumulations rates of soil-forming processes during the latest those at the Cima volcanic field. However, ihcrc arc Pleistocene and Holocene in the arid regions of the no indications that the rates or processes of soil Majave .Desert. This in tura provides a basis for ioimatiou are substantially different. If eofcui iofinx estima tion cf ages of soils is this icgion, where so other raies .have been higher at the Lathiop WcHs center, age information is available. thick deposits of desert loess would nramle stable Pleistocene and Holocene landforms and soils Sails formed in scoria deposits or in aprons that throughout the area. This has not been observed. flank flows at the Cima volcarjc field have also been Equally, eolian activity in the Yucca Mountain region examined.14,47-** These soils are generally similar to cannot be substantially lower than the Cima volcanic soils on the flow surface. However, the soils formed in field because of the presence in the former, of active scoria deposits associated with the youngest scoria sand dunes on flows and the nearby Armagosa dune cones in the Cima volcanic field pos-ess relatively weak field. Abundant sources of eolian materials, including development of Avk horizons. They do not have B desert loess are provided by the numerous basins, many horizons that are as red or thick as those typically of which contain large playas. Accordingly, we observed in the phase 1 soils on associated lavas. This conclude that the weakly developed soiis of the Lathrop indicates that much of the eolian material entrapped on Wells center closely resemble the Holocene soils in the surfaces associated with scoria is readily translocated Silver Lake area arid the Cima volcanic field. We infer through the highJy permeable, open framework scoria thac (he soils must have farmed over a similar time to depths of more than a meter by infiltrating soil span. We conclude that the Lathrop Wells soils have water. In the lower part of the soil profiles, the initially formed largely during the late Pleistocene and fragile, glass-coated irregularities and edges of scoria Holocene. They probably cannot be older than 20 ka fragments are altered by infiltrating water, as shown by and almost certainly cannot be older than 50 ka. the presence of reddish, brown coatings on the tops of the fragment!, the destruction of vesicle edges and spines, and the chemical alteration of glass. Pedogenic PALEOMAGNr-TIC STUDIES accumulation of calcium carbonate, salts and perhaps some amorphous silica primarily on the bottoms of the Considerable paleomagnettc data have been fragments h a major attribute of these soils. Soil obtained for the Lathrop Wells volcanic center in order to discriminate among different eruptive events, Turrin development in cone aprons resembles that observed 15 on flows. Equally, sou development on sJoria-cone et a!. report that the patcomagnctic data from the aprois of older cones is closely similar tu observed eruptive units fall into two statistically distinguishable phase 2 soils, demonstrating the primary role of cumulic populations, correlated to their revised definition of pedogenesis on Ibis volcanic landform. units Qs3 and Qlj. They use the angular difference between the means of the two populations to infer an age difference between the two events of no more than Soils on flows and volcanic aprons associated with 100 years. At some localities in the westers United the Red and Black cones at Crater Flat exhibit the Slates and Hawaii, a sufficient amount of morphologic and textural features of phase 2 soils in paleomagnetic data from recent, radiocarbon dated the Cima volcanic fieii.13 Soils on vokanic units of the lavas suggests that the average rate of directional secular variation of the geomagnetic field is on the Lathrop Wells center are similar to phase 1 soils of the 5 Cima volcanic field. These consistencies suggest that order of 4 to 6 degrees per century. * Use of the similar sofl-fonmng factors influence pedogenesis in angular distance between the estimated mean directions of two populations of magnetization data to constrain these areas. Similar bndforms, closely analogous f climate and similar biota have been conducive to the the amount o time separating the eruptive events, is operation of similar pedogenie processes which formed not well founded. The angular difference between two morphologicalry and tcxtualry similar profiles. These paleomagnetic data sets can at best constrain the gencralizatia.-s are probably especially applicable for minimum age difference between the rocks sampled, latest Pleistocene and Hofocene soils of much of the nor the absolute difference, as only two, foully Great Basin and other parts of the arid southwestern independent records of the field have been obtained. United States. This is controlled primarily by the overwhelming influence of acceleraled eolian activity coupled with aridification on pedogenesis that has Additional palcomagnelic data are presently being characterized the geologic record over the last IS obtained from three volcanic flows at Laihrop Wells

(Ql*, Tour sites, LWH; Q!s, four sites, LWS-S, and Qlj, ita_»Mja.Mf4j1*t,4».j« two sites, LW9-10). Directions of natural remanent magnetization (NRM) Ji3ve been measured for all We do not currently propose that pedogenic samples from the 10 sites. In addition, anisotropy of processes at the Lathrop Wells center arc identical to magnetic susceptibility measurements have been obtained for alJ samples prior . lo progressive volcanic center. The uncertainties and assumptions of deraagnetcarion of the NRM. Either alternating Geld palcomagz'cuc studies arc just cow being examined. or thermal demagnetization uii] be xaaiai oui on all More data are required IO a«i-

SUMMARY REFERS: XES

Field studies of the Lathrop Wells volcanic center 1. D. T. VANIMAN, anu R M. CROWE, •Geology are in progress. Revisions of the lithosirctfgraphie units and petrology of the of Crater Flat: have been greatly enhanced by systematic trenching, Applications to volcanic risk assessment far the which is in the initial phases. Current data provide nuclear waste storage investigations," Los compelling trodence (soil-bounded unconformities) that Alamos National Laboratory Rept. LA-8845-MS, the volcanic center is polycyclic Eruptive deposits arc 53 p. (1981). divided into three soil-bounded chronosli-itigraphic units. The chronology of these units has not been 2. B. CROWE, S. SELF, D. VANIMAN, R. AMOS, established firmly. The results of conventional K-Ar and F. PERRY, "Aspects of potential magmatic and "Ar^'Ar studies appear to have insufficient disruption of a high-level radioactive waste precision to constrain the age of thechronostratigraphic units. The results of U-Th series, cosmogenic JHe and repository in southern Nevada," Jour. CeoL,91, pp. thermoluminescence age determinations are somewhat 259-276 (1983). inconsistent. Individual methods give ages that vary from older (> 100 ka; U-Th) to intermediate (25 to 65 3. B. M. CROWE, "Volcanic hazard assessment for la; 3He) to younger (5 to 25 ka; ihermolumincsccnce). disposal or high-level radioactive waste," in Active These methods are development and we are using new Tectonics. National Academy Press, Washington, applications of the methods. Therefore it is not D.C, pp. 247-260 (1986). surprising that data are inconsistent at an carry stage of research. Resolution of the chronology data will 4. B.'M. CROWE, M. E. JOHNSON, and RJ. require mare data at the Lathrop Wells center. BECKMAN, 'Calculation of the probability of Additionally, it may be prudent to test the chronology volcanic disruption of a high-level radioactive methods at other localities in the western United States waste repository within southern Nevada, U.SA,' where there are independent controls on the age of Radioactive Waste Management, 3, pp. 167-190 volcanic units. Data from geomorphic and soils studies, (1982). supplemented by trenching, continue to support a late Pleistocene or Holocene age assignment for the 5. D. T. VANIMAN, B. M. CROWE, and E. S. GLADNEY, "Petrology and geochemistry of nawaiite lavas from Crater Flat, Nevada," Conmb. geamorphic assessment of late Quaternary Min. Petrol, 80, pp. 431-b57 (19ft:). volcanisia in the Yucca Mountain area, southern Nevada: Implications for the proposed high-level 6. S. STNNOCX, and JL G. JEASTERUNG, ia dioactive waste lepositmy" Geology, Tip. 661-^62 "Empirically determined uncertainty in potassium- (1991). argon ages for Plio-PIeistocene basalts (rom Crater Flat, Nye County, Nevada," Sandia National 15. B. D. TURRIN, 1>. CHAMPION, and R. J. Laboratories, SAND82-2441, 17 p. (1982). FLECK, -"Ar/^Ar age of the Lathrop Wells volcanic center, Yucca Mountain, Nevada," 7, S. G- WELLS, L. D., MCFADDEN, C E. Science, 253, pp. 654-657 (1991). RENAULT, B. D. TURRIN, and B. M. CROWE, "A geomorphic assessment of Quaternary 16. D. E. CHAMPION, "Volcanic episodes near volcanism in the Yucca Mountain area, Nevada Yucca Mountain as determined by paleoraagnetic Test Site, southern Nevada," GeoL Soc. Amet studies at Lathrop Wells, Crater Flat, and Abst. with Programs, 20, p. 242 (1988). Sleeping Butte, Nevada," in Proceedings, High Level Radioactive Wast: Management Conference, a B. CROWE, C. HARRINGTON, L. America i NucL Society, La Grange Park, Illinois, MCFADDEN, K PERRY, S. WELLS. B. pp. 61-67, (1990). TURRIN, and D. CHAMPION, "Preliminary geologic map of the Lathrop Wells volcanic 17. K. I_ PIERCE, "Chapter 13: Dating methods," in center," Los Alamos National Laboratpry RepL Active Tectonics, National Academy Press, LA-UR-88-4155, 7 p. (1983). Washington, D.C, pp. 195-214 (1986).

9. B. D. TURRIN, and D. E. CHAMPION, 18. S. M. COLMAN, K. L. PIERCE, and P. W. "*qAr/wAr laser fusion and K-Ar ages from BIRKELAND, "Suggested terminology for Lathrop Wells Nevada, and Gma, California: The Quaternary dating methods," QuaL Res., 28, pp. age of the latest volcanic activity in the Yucca 314-319(1987). Mountain area," in Proceedings, High Level Radioactive Waste Management Conference, 19. J. W. WHITNEY, and D. R. MUHS, "Quaternary American NucL Society, La Grange Park, Illinois, movement on the Paintbrush -Stagecoach pp. 63-75, (1990). Road fault system, Yucca Mountain, Nevada,"

Abst. GeoL Soc. Amern 23, No. 5, p. : .'9 (1991). 10. B. M. CROWE, C D. HARRINGTON, B. D. TURRIN, D. E. CHAMPION, S. G. WELLS. F. 20. V. A. FKIZZELL, AND J. SHULTERS. V. PERRY, L. D. MCFADDEN, and C R "Geologic map of the Nevada Test Site," U.S. RENAULT, "Volcanic hazard studies for the GeoL Survey, Misc. Invest. Series, Map 1-2046. Yucca Mountain project," in Waste Management (1990). •89, 1, pp. 485J91 (1989). 21. E. W. WOLFE, M. O. GARCIA, D. B. 11. "Study Plan8.3.1.8J.I Characterization of volcanic JACKSON, R- Y-, KOYANAGL, A. T. features' VS. Department of Energy, 52 p. OKAMURA, M. L. SUMMERS, and WILFRED (1990). R. TANIGAWA, "The Puti Oo eruption of Kilauea volcano, episodes 1-20, January. 3, 1983, (2. S. G. WELLS, l_ D. MCFADDEN. C E. to June 8; 1984," US GeoL Survey Prof. Paper RENAULT, and B. M. CROWE, "Geamorphic 1350. pp. 471-508, (1987). assessment of late Quaternary volcanism in the Yucca Mountain area, southern Nevada: 22. F. V. PERRY, and B. M. CROWE, "Geoehemical

Implications for the proposed high-level evidence fortwaning magmatism and polycyclic radioactive waste repository," Geology, IS, pp. 549- volcanism at Crater Flat, Nevada," (this volume) 553 (1990). (1992).

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