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. <\,tHE JEMEZ MlOUNTAINS REGION ' .. ,.____ .... ,..

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EdJton

FRAS~OOFF BARRY S •. KUES

MARGARET:~ ROGERS LES D~ M~ADDEN

JAMIE N~ GARDNER

New Mexico Geological Society Forty-Seventh Annual Field Conference

·sepurrnber25-28, 1996

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1996 FIELD CONFERENCE SCHEDULE

w •• ...,.,, September~ Day 4;(1()..1:00 p.m. Rep.u.tiou at Sheralon UplOWil Hocel. aortbeut comer of Menaul and Louisiaua, Albuquerque 5:00-8:00 p.m. kebRabr IDd snack&, Sheraton Upcown

~September ~Day 7:00 Lm.· Assemble at Fust Day IWting point, north side of Bernalillo

12:00 DOOQ LuDCh provided 6:00 p.m. Arrive ill Las Alamol 6:30 p.m. Banquer bu1fet ar Fuller Lodge. Los Alamos; lOW'S of nearby Los Alamos Historical Museum and Los

A1amol Lab Museum 8:30 p.m. Speaker: Hal K. Rothman. Historian, '"The Pajarito Plateau Before the Bomb."

friday, September 27--Secoad Day 6:00 Lm. BIQkfast ar Sullivan Facld. Diamond Drive, Los Alamos 7:00 Lm. Oepert from Sullivan Field in 15-passenaer vans

12:00 noon Llmc:h provided S:30 p.m. Return 10 Los Alamos 6:00 p.m. Barbecue diDDer at North Mesa picnic grounds, Los Alamos

Satllrdny, September 28-Tblrd Day 6:00 Lm. Breakfast at Sullivan Fteld, Los Alamos 7:00 Lm. Oepert from Sullivan F'teld

12:00 ooon Lunch provide1 5:30 p.m. Field Coafermc:e ends at Tent Roc::ks. oear Cochiti Pueblo

: ~ - CREDITS

. j FNIIl Coftr: Geoc:ra1ized map of die Jemez MocmraiDs region. Flucr Goff, Los Alamos Nationall..aborauxy.

ill

A·:.· .. ~ EDil Sbeets: Satellite pboco of lbc 1emez MOUIIIIins rqioo. c:ouncsy of Los Alamos National Llboratory.

'I .:i · · ·. · · ;"-"' "'Allkl•rncr Loa Alamo~ CluaDber of Commerce; Tom Menoo. Loa Alamol CieoloJical Society; Capt. Ken Honoa. Loa Alamol Police i~F . •"-. . ~s-a Fe NIIioaal Forest; Jotm Corbin, Sulphur Sprinp; Loa Alamos First United Methodist Women.

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Es:baoordlury AllbtaDce: Cicnenllblob to Los Alamos National Laboratory, EES IDd CST Divisioas. for financial support. logistical usiJ. tlllee.IDd a:ceu to some LANL lniU.

~ ad Plpta: Finer pboco& IDd c:apcioas provided by Los Alamos Natiooal Laboratmy; figure anparatioll. Anrboay Gan:ia of EES-1 (LAN!.), .Sitllbryll GleseDcr. NX Bmaa of MUles IDd MiDeral Raources. '

w.d Pa IK ' 1111111 CGI8p8fer hll¥ ' ·~ l.yDne Hemenway IDd Terry Tellis. N.M. Bureau of MiDes IDd MiDcra1 Resoun:a.

...... ad Graphic Daip: Ullivenity at New MeUco Printillg cl Quick Copy Servic:es •

COPYJUGHTC 1996 by 1be New MC1Uco Geo!OiiQI Sociecy, IDI:.

................... dlio .,....,..._....,.... rotllle471b-' field......_, .... New Malco~ SocieiJr,lleldie ~New Maico .. ~zs.. a. lfM. NopMolllllllpllbllcnDae ..,. .. ,.........., ... _..... sy-.or-.aiaedill.., f-orll)'..,_dlclroeic.........., I* 1 111 .... ._...., •

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BURIED EARLY PLEISTOCENE LANDSCAPES BENEATH THE PAJARITO PLATEAU, NORTHERN NEW MEXICO

DAVID E. BROXTON and STEVEN L. RENEAU ory Group. MS ~ Lae ~ N~ l.abon~Gry. Lae Alamo&. NM 81S45

A blend-Thss paper deicriba !he arty Ple!lfOCelle !eMvlpC' of lhc nonhem llld ccnrnl Pajari10 Plar.cau uu pnor ro die rnpuoa ot lhc Olowi c 1.61 Mal llld Tshirep (1.22 Mal Memben of the ~!ldelierTutr.llld lhc ef'fecu of dleiC enpciOIII 011 lhc laDdscapel. Borehole and surface ouzaop dlla - UIC4 10 ~ pmiminay prc-Oiowi and pre. Tlllnp IUIIalll'e COIIUIW' and bedrock 1coloJic maps. aDd ilopKtlllllpl for the Orowi IDdTibirqe Membcn. Tbc tallrll pet of Ibis ma wu occupied by 1 broed SSW-crendio& valley prior 10 aupcion of die OIOwi Member. This ..ty PlcUioccDe valley vtu flanked onlbe welt by docply diuected ~ hip!lnds of lbc Sierra de loa Valla .-1 011 die - by 1 low nonft.cn:ndiftl llaaltic ricfae dial aood approainwcly 500 ft hip dill! die valley f1ocx Tbe t.altk ridp - a "'CStCm ettealioll of die Care. del Rio wk:anic: field dial walarJely buried by the Banddicr Tuff. Grlvicy dlla and die lilholoay of die upper Sura Fe CinJup sugelllballbe early l'lcill.occfte valley 0\'Crlies a Teniuy JRben on die westm1 5ide of lbe EspUiola buin. in 111m SUgaUDI In inherited SWC· nn.l COIIIrOI 011 va!lef locaDoll. Deposition of bod! !he Otowi llld dieT~ Memben sipificutly dlanpd !he laDdacape; lllblequnt draiDaae cbiMeiJ ~ c:ommonly aroaaJy oblique 10 prior cblaneiJ llld locally -sapa impaled 011 prior bedroct hips. flllllre IIIOdiaiq wdb cva11111inJ lbe 11111111: 11111 wan of perched puund wucr I)'1ICIIII bellalh die Pajariso "-· and atJe1i111 apecitic 1eollydrolop:: borizoas UIOCiatcd willllbele palcocopoJr~Pftic surf-. should consider die now pallia comrulled by lhae buriecllllldacapcl.

INTRODUCTION

Tbi1 paper delcribes me early Pleisloa::oe landsapes of lbe nortbem &ad QCIIU"Il Pajatilo PWeau prior 10 lhe auptioa of 1be Otowi and Tshirege Mcmben d lbe Bandclicr Tuff. 11111 me effeas of lbesc erupcioos oa the lmtP •1•1 Data presented were compiled 10 guide geobydrologic iDvesti­pioas nazninin& lbe tWDre and ~lall or percbcd ground Wiler systemS bcaeattllbe Pajarilo P\arcau. The pre·8mdclict laDdscape is important ID

~ eaviroamenW restonrioo iiMstipljom It Los Alamos National UboniUxy because ground Wlfer is perdxd abo¥e dlis buried land sur· ~ ill some areas. within the Guaje PUmice Bed or ill basal ignimbrites of lbe Olowi Member.. and aboYe soil borizoos de\oeloped on top of the Plio­~ Pllye Fomwion. In acleall one loatioa, a perched wau:r body aJIDinl rJICISinblc conccntmiOIIS of llllhi~ tritium. demonslrat· inJ teeeut n:ctuqe 10 a depdl of It Ieist 300 ft (Broxton et al .• 199S). In additioQ. we suspect ~ tephras and volcaniclastic sediments of lhe Cerro Toledo inJcn'al (Broxtoo and Rcocau. 199S). occurring between lbe Otowi and T lhirqe Members. 1I111Y also locally perch W111Cr. Because Cerro To­ledo arxified beds would be the first sisnificaDt pCrc:mng layers encoun· r.ered by infillnb.n& waiCr benealb much o( lbe P1atau. tbe palcogeogra· pby us.ociated wilb this unit is also impcnaDt in lbe evaluation of potential s.ub:surfaa: conwninaru amsport. Placement of future ground Wiler c:bar­aa.erizatioa and IDOOitorin& wells will be pided in put by estimating lbe gradic:tlt md direction or pound Wiler flow ill pocauial pc:n:hed zones a..,Qiftd with these buried landscapea.

Dransneld and Gardner ( 198S) originally prepared pre-61111delier StrUC·

tun: CXlnf.OUr llld bedrock seologic maps for tbe Pljlrito Plllcall. based on borebole data ava.ibble ar tbe time and !he geologic map of Griggs ( 1964 ). Our 1mps revise lbeir work and extend ilto ewuate !be pre· Tsbin:Jc land.K:ape. i.ncorporaring more recent bon:hole data and ~ten­sive field check.ina of the base of the Otowi and Tsbirege Members. AJ. tbouJb our undetswlding of the buried early Plei.aoc:eue landscapes will improve a data from future core boles become IYiibble and following rncxe detailed eumination of outcrop5. we believe tballhe available data ~ many of !be signiflUilt feanues of lhele paleotopographic sur­faces and should~ be Uleful in the siting of future ground water moni­toriaa wells. In addition. this woric allows an evaluation of how !be land­$ape wu affected by emplacement of Otowi and Tshirege ignimbrites and sunests &orne relationships berween the early Pleistocene palcotopopphy and the suuctural C\'Oiution of the Pajarito Plateau area ..

METHODS

Tbe IO'\ICtllre contour maps and pre-Bandelier pologic map were pre­pared 111ing surface outcrop and borehole da1a. located as shown in Fig­urn I and 2. Surface dau were collected in the northern and ea.srem part

oftbe area where lbe base of the ~i arulTshirege Members are exten­sively exposed. Guided in pan by available FOiosic maps (Grigp. 1964; Smilh et al .. 1970; Goff, 199S; Reneau et al., 199!; Dethier. ill press). most sites (Fip. I. 2) were field cbeckal. and lhe altiiUdc of the conract wu determiDed u1iJ11 USGS or NAD83 topographic base maps. hand leveling. andlor an altimeter. A few c:orurol points were taken from Griggs (1964); lhele poinll w~ replotted on a NAD83 topOgrlpbic base after confirming the location of the contact on high resolution ortho­photograplts. In some piKes wbere tbe base of lbe Tshirqe Member is not exposed.lhe lowest exposures oo canyon walls were used to provide maximum-limiting elevational conttol.

Tbe elevation of the pre-Otowi ar.d pre· Tshireae surfxcs was assumed to comspond to lbe general present larK1 Sli.iace in amas underlain by basalt near White Rock or by Tscbicoma Formation dacite aear Los Ala· moa. excluding narrow canyons (such as Pajarito Canyoo dlrough White Rock) which are likely post-8811delier features. This asswnption is rea­sonable because lbe lava flows undertyins these areas are resistant to erosion and probably have been relatively little modified in rhe last 1.61 Ma. Fwtbennore. iarBe pans of rbese areas were covered by the Tshirege Member of !be Bandelier Tuff. wbicb prevented erosion of these sur· faces for much of the last 1.22 Ma.

Subsurface dm were lllkcn primarily from tbe c:omprehensive compi· lation of geologic logs of Los Alamos area boreholes by Purtymun ( 199!). Additional subsurface stratigraphic infonnalion was taken from Broxton et al. (199S) and Gardner et al. (1993), and from unpublished geologic logs of recent boreholes drilled for tbe Los Alamos National Laboratory Environmental Ratoralion Project.

Isopach maps oflbe Otowi and Tshirege Members were prepared from the same data sources as the stn&Ctun: contour maps. To minimize thick· ness variations resulting from post-Tshirege erosion. borehole or out· crop sites were eliminated from the Otowi isopach data set if the Tshirege Member was absent above the Otowi Member. Because the Otowi Mem­ber was exposed to erosion for 400 k.a before lbe Tsbirqe Member was erupted. the Otowi isopacll map represenll the estimated thickness of the Otowi Member prior to eruption of lhe Tshirege Member rather lhalllbe original thickness. bopachs for lbe Tsbirege Member are based on mea­surements at mesa tops, thus minimizinslhe effects of erosion on csti· mated thicltneucs. Nonetheless. the Tshitege Member bas been exposed since its emplacement. and uppermost tuffs on mesa topS have bren eroded 10 varying degrees over the last 1.22 Ma. Therefore. the original thick· nesses of the Tshirege Member were c:ertainly J!Uier than shi)WI\.

PRE-BANDELIER BEDROCK GEOLOGY The pre-Bandelier bedrock geology of lbe Pajarito Plateau study area

(Fig. 31 consists of three lithologic units IGriggs. 1964; Smith et al..

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BROX10N and RENEAU l.f

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P10URE 1.1.-Dae 1111p of die PajariiD Plea 111ooriDa die c1a1a poillll useci co COIIIII1ICl cbe ~ 111n1a11n: -llllp ill f"'l'ft 4. DriU bole lila .-e iadiclad .., eold Wd;cilda IDd OUICIOp lila arc*""' •lblded diuDoads. The IIIIIDbcn IICiliD lbe symbols iDdicare !be ele¥llioa (ill fat) or die prHlllldelier IUifKle. dill ... ~~ II'C UDdcrliDed. i

! 1970; DnmfieJd md Cilrdzler. 1985). from West ID east, lbey Ire the T~ fGnDition. the Puye FormarioG.IIId baallic roc:b of Carol clal ltio.. s-ltwelt of !be RUdy area, !be Sanla Fe Group md Paliza CallJoe ~ abo formed pR-Bandelicr bedrock exposures aear Sl. ...... n-:. i

lhe Tactlic:orDa Formation cocuilts ol dKitic to rhyolitic lavas dl.r wen~ cruped from latJe O¥CrlappiDa dome compk:xel iD 1bc Sicrra:dc lial \\Des ~y 3 10 7 Ma (Grigl. 1964: Doell et al. 1968: CilntDcr ..... 1986). Lava floon from lbc ctial pu1ll of lbcse dome ""''4" •• C!dald under me we~~aD p11t of lA Allmo&. wbenllbcy are ~wilt! me Puye Formalioa (Pmtymun. 1995). ;

'lllc "-Ye Formation (1.6-4 Ma; l'llrbeYille et aL. 1989: Spell et il .. 1990) is a ~ic llluvial faa deposit abed eastward from the ,........_ 1llllcaic: hiplanda Prior 10 clqatQoD of tbc Olotri Mtm­ber. 1M ...,. formation - priJDIIrily ezpoiCd iD a NE·Ir'elldi.aJ belt of OIIICIIIIp& f1allling lbe Sierra de lol Valla <fia. l ). alttJou&b it a1lo in­chided ~ bads of alluvium aJcea the pre-Orowi lllr'CIIIl channels. 11 ia a rw,m rare C0111i11iAJ a( poorly ICXtCd boaldm. c:obblea. and coal a.dl.ade ap of da:itic 10 dlyolllic debris. ad ~ DWDCI'•

«* ~ l.apilli tuff beds ad labaric deposiu (Origs. 1964; W..IStwcl;- 'I'wbevillc. 1990). In oaraops aJoaa tbe IUo Onnde. lbe Puye FaaDIIiDft also c:ootains baaltic debris ~ from c:omempora­aec. -.alc:aliaD and erosion oldie Cc:mls del Rio volciDic field.

n. t-al!aafCmol del Rio (2.3-2.1 Ma; Mmlcy, 1976, 1979; lbdlmln ad~ 1978; '9.bldeGabriel et al, 1995,11111 dlil voiuDIC) iDClude

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PUocene olivine tholeiites, basaltic andesites, buaDita, altali oUviDe bualla.IDd bawaiita erupted from w:DIS cas& aad ... of the Rio Ormde (Aubele, 1978. BaJdridae, 19'79: Delhier. in press). The main p11t of the vok:aDic: field lies east of tbe Rio Orande. but ouraop llld bcnbole dala iDdicla: dlat a major extcmioo of the wk:IDic: field is buriecl belalb the BIDdelier'lUtf OQ die PajariiO Plateau {FiJ. 3). Cenol del Rio basalrl formed surfKe OUia'OpS IS far west IS barebolc PM·5 (Ha. 1). IDd dJey were buried by Puye deposits eYeD farther 1D tbe west (Pia. 3).

1be west aide ofWbire Rock was a basaltie IDpOp8pbic: bilh prior ID auption of tbe Otowi Member. West of White Rock. die uppauw ba­salt flows. iDCJuding those buried by Puye formatioD ranJlomerasa. sys­laDIJic:aiJy decrease in elevllioa from east 10 wesL Tbe uppeunoll basalis clccRuc in elevuion from 6500 ft aear White Rock 10 6320 a in bore· bole PM-4, beDCalb tbe D0111H:eDaaJ put of tbe Pajarilo Platau. 11111 1D '977 ft iD borehole DT-.5A (Punyuum. l ~ iD the IOUdH:enual pmt of pla1au. Basalt~ dip eastWard from lhe crest or lhe bualtic: hilh towards lhe Rio Gnmde in !be eastan pan of White Roc:k.

PRE-OTOWI LANDSCAPE Sttuc:ture coatoun for cbe bue of tbe Bandelier Tutr lhow the ~:f of die Pajlrito Plafau area ll about 1.61 ~Ia (aaes of Baodelier 1Utf f'rom lzea and Obndovicb, 1994),just prior ro enqllion of lbe Otowi Mt~mber. and reveal a landscape dlat was sipific:aDtly dif. rerat trvm tbat of today (Fia. 4). Wbereu tbe modem Pajarito Plalau is a JUab E- and SE- dippinz tableland disaec:ted by numerous deep e&'t-

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f1CiUR.E 2. LocCoa 1111p c r 111e PajmT.o P'*-lbowiq lbe dD poims used to COIIIInlellbe pre-Tllllilqe IINCIIIre caatoar 111111 ill fiaure 6. Drill bole lill:sll'e inllil:allld by totid l*kcin:ics -s 0111aop lila .re llblllnl alriqlel. Siii:S used to COCISII'Iin lbe liiUimum deYarioft of lbeTIIIirep baa! c:oatKt (wbln lbe I.e iJ 1101 apllllld) .. ..ad by "X"&. The •lllben aeato-of lbe symbols illdiale lbe elevuioa (ill filet) of lbe pre-Tsllirqe llllfxe aa dill site: oaly lelecled dD .relbown rot­...a a hiP demlly or--- Bordlok aamcs ll'e UDderllDed.

yoor.. audl of tbe sam: area was occupic:d by a broad SSW-draining valley prior to eruption of tbc BIDdelierTuft' Cfi&. 4). A rouahJy S-drain­iftl ~Baadelier valJe:l - also ilaferrcd by Drmsfidd and Gardner (1915) ill Ibis &reL 11Jt, wlley waa t1lllbd 0111be west by lbe deeply disac=d volc:anic biJhllnds of tbe Sierra de lol Valles whicb rose -4300 fl: llbove !be valley flocx. Tbe east side of tbe valley was leplr.lled from tbe IJiaiiD'I1 Rio GraDce by lbe bualtic bighlmd west of White Rock tba dOOd ipp'OXimar.ely 400 fl: lqbcr diaD tbe valley floor.

The ovenl1 shape aad orieutaboa of cbc ~ 'IIlley is fairly well c:onsm.iDcd by IUdlc:e and sublurface elm. Tbe valley was about 4 IDi wide. bad a miaimum leqth of 6.2 mi <Fll· 4),1Dd em:uded from the paeat l.o5 Alamos Canyoa 011 cbc IIOrlb allculiO Frijoles Canyon on die~ AJJ E-10 SE-dippiq surface of low reliefbowlds lbe oonh end of the wiley. Tbis aurfac:c is UDdertaiD by tbe Pllye Fomwioa. and prob­tlbly aepcescDII the upper~ limit of lllllluvial fan complex mod frcm Tac:bicoma Fonnllion da:Des in the Sierra de len Valles (e.g., Wlftllbock and Tlatle¥i!le. 1990).

The JOUihwal fllllk is poorly c:bancrerized compan:d 10 lbe resl of die 'f&Uey. Few deep wells pnMde c:oattol in tbil area. and critical field llllllioM ~covered by tbe Tlbirep Member. However. dllla from bore­hok SHB-3 (Gardner et al.. 1993) and from tbe east slope of tbe Sierra de a Valla provide IOIDe c:onuraintl oa the locatWa of tbe weu side of 1be Y&IJey lild i11c1ic:e dw lbe YIDcy uis IDIISl be near tbe location *'" ill Fipre 4.

Early Pleislocene canyons of tbe Sierra de los Valles as far north as cbc beadwarers of modern Raldija CanyoD appan:ndy ctr.med easnnrd aDd soutbeastward into this pre-Bandelier Yl1ley <FII· 4). SUQml CDII:riDJ lbe valley probably mespd into a mas= draiDqe lblt flowed IOUlb to­wards lbe Rio Cirallde. UDdovblrldly, smaller west-flowiq dminaps cmplied iDto tbe Yl1ley from die low N-tralding basalt ridae 1D die east as well. Howewr. this inferred fiDer-sc:ale IOpOpapby can DDt be JaO!ved with anilable data.

1bc early Pleisr.oc:cne tupo&S apby of die eiSl fllllk of die Sierra de los Valles highilllds 'tiS probably little c1ifl'emll from dtar of die middle and upper pans of lbe r111ge IOday. Erosiooaii'CID!IIMS of Barw:ielier 1\df are fowld in many of die major cauyons and 011 die lower slopes of die Sierra de los Valles. and die COIIliCt betweeD tbe BaDdelic:r Tuff llld die d.acilic lnu of cbe hi&hlands preserves a record of the pre-Bandelier paieotopoppby. Tbe distribution of dlese l'eiiUWitS on c:anyoa floors and walls u well u on eul-sloping surflces iDdicar.e dlat die localions. li.ze$. and shapes of canyoas ll tbe time of die Otowi eruption were prob­ably similar 1D thole of IOday.

A aonh-lrelldingltNCIUI'al hip aonbwea oftbe valley is an eucan outlier of !hick Tschicoma Fomwion lava flows exposed on tbe aonh wall of modem Pueblo Canyoa Cfi&. 4). These lava flows form a narrow ridge bounded by the Rendija Canyon IDd Guaje Moumain f'lulrs <F11. 3 ), sugestiq a cectonic innuence on the paleolOJIOifaphy. Allhouafl tbe apparent relief or tbil buried ridge was enhanc:ecl by post-Otowi faultiq,

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32! BROXTON and RENt :AU

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FJGUU l. a-.li:Jed bGoct J11111o1ic 1111p f/1 ..t.a ..._..., dae ~ "-a. COIIIrol pot1a- iDdialed by opeD .:ildel for borebob IDd .,.. ....... tor-.. Qeolasic .... lilllll'bii~(Qrias, 1964; SaUdi el &1. 19'10) -IIIIo u.d II:IIUfl dae ~fllbedloc:k llllica. The~ at -.;or ,..... (1llict IIUick .-., .odiilld tro. Olldler 1917) _.,.. for COIIIplritoD. The dahed bl8dt liDe aboM lhe ~ wa~en~ amot ol Cerro& del Jllo t..1ts buied beiiiCIIIIt l'u)<e ~ • die Plltaa. POiirioe of-'>' PleiJIOCelle RW Onnde (heavy daabed lille) il from Ralcau ud DeUiier (IIIia volume).

J b:ci1b1 rclmYc to polt· Tlbirqe f.auhiDa .P.,apbic hilb borh before a: derlbe OID'Iri !enmtiall..

\ Tbc baaltic: L.iafUaad almoe 6SOO Rock. which ~s--d die arty PklltorlCIIC viDey &:xa e11e Rio Grande <Fia. ~}. txpCICIID I WeGI:nl Cltllr:DSicla of tbe Plioa:De Cerrol del Rio ¥Olea· aic tidd.. !DOlt of wbidl ocaiXI ec at IDe IUo Gmadc. Thillow N-cread­ilia ridp: is problbly a line of bualtic Willi IbM aJalcl from die west *lc of WbiiG Rock 10 Itt: vicillity of t.lan-w Cauyoa md pedlaps flrtbc!r IOI'Ih. \lnl t.::ias I ..aiDe: c:iDdrr IDd qpri!!l1f! depolits are ex­pC.ad wat ol Whi1e Roc'' ad were I1DIIdy ~ in boreholes ¥-10 1~ aod S4-IO 16 (U:-fl. &riroaeaal Raldratioo Project. UDpUbl. dMa. 199S) (Pi&. 1). Smit:1 et al. (19'70) ~bowed~ _. on !mid ~ !be ridF u lac DDI1Il as Bayo c.r,.. ne · iidae wu com­JI\IDidY buried by !be Tsb.itqe Member. ..S_lt J1!1a DO ropoppbjc ex-prcraKJe toc11y. I l AD iloUr.cd pUeotopopphic lqb IC!Udrwell ofWhite -Rock. between

Chtqtzboi lad ADcho Caay~ ia a PlioceDc baaltic ~ cone (Fig_ 4). The c:iDdcr moe.. wiUctl llOOd aboul 500 f\ ~ die valley 10 tbe aoclibwcst and 900 t't libo"Ve die earty Pleilcoc:ale Rio Orude to the south­ust.. - almoit CO!IIIpktely buried by 1bc Tlbirqe Member of the B~ Tutf aDd bas lillie IUtface apralion IOdly.

The early P\eiJtocr.:ue landlape wu problbiy e:xpoK'd for a long time prior 10 bemc covered by die Ofowi Member. a1Joowina lbe dnelopment of str'DII& toil borW- ia mall)'~ ~ly beoeath the Otowi Member ia Ptaebw CAayon. - !be jUDCCioa wirb Loc A1aiDOI Canyon (fiJ_ I). 20 em of undy soil O¥erfiea 3 c:m otscqe 41amiur cartlonales {soil Cll'b::.4latt termiDOiogy of GUo « al.. 1966). aDd carbona1e-coa1e clasU Ot:CUI Ut rhc upper I m of the andtt!yinc Puye Fannation. Clay­rich s.olb occur on top of the Puye Formation ift Los Alamos Canyon in . I

borehole LADP-3 (Broxton« al., l99S) aad in outcropS in upper Rendija Can) on. Laminar carbonlres are abo c:ommoaat !be top of bualu in !be White Rock area. Because ol rbeir apparau widespread occwrena: aod rellli"Vely impermeable Dllure,. rbae buried soiJI may be be responsible for perthinl of around Wl.ler iu tbe Guaje Pumice Bed where it overlies the Puye Fonnadon in Los Alamol Canyon (Broxtoa et al .• 199~) aod in otbet· areu.

EMPLACEMENT OF THE O'IOWI MEMBER Elllplac:ement of tbe Otowi ipimbrite ~beet latply reshaped lbe early

Pleistocene landscape of !be Pajarilo Pl-.., . fanimbrites from lbe Jemez Mountaina 10 the wat. movin&IUider lbe iuflueoce of pvil)', nYdled dowo tbe 5lopes md c:&D)'011'1 of lbe Sian de loa Vanes before 111aJin1 at lbe mountain front and ..,._tina acroa the adjlaut lowlmd areas. The ignimbrites largely filled in topOpphic lows suc:b u tbe ~ early Pleiaoc:ene valJey and l!liOOCbed out the land swfla:. formiJI& a broed ignimbrite sheet that dippe\l FttiY 10 lbe eut. IO'IIbcwr, ltDd lo­cally south_

The isopach map (Fig. '> shows the diaa'ibulion and rbicbesl of Olowi deposits prior 10 emplac:euoan of rbe T~ Member. llr. map may reflect 1be ori1iaal thiclcneu of 1be Otowi Member in eome • reu. How­ever, because of its poorly iDdumtd llllUre and iU e:xpccun at tbe sur­faa: for about 400 Ita. the Otowi Me:nber WI! ~ eaumi ~ely eroded befon: its burial by the Tshitqe Member, and iu original d\io.:kncu on­not be estimaced.

11Uclr. Otowi deposits are fo.Jnd in rbe IIOithem JWf of the ~;W-draift. ing pre-Bandelier valley. n- deJIO'its are up to46S fi thicl; in bon:· hoi<: EGH-LA-1 (Fia. ~).and may represent topOgraphically 1:ontrolled poading of the ignimbrites near the moutll of the early PleisDXene Los.

329

FlGURE 4. S«naaurc contOUr awp (coaroar ~ 100ft) for lbe pre-Omwi surfxe br:.-11 lbe Pajlrito PII!Cal&. Control poiiiU an: showo for boreboles (solid blld c:irda) and OUiaVp IileS ( diamoada).

Alamos Canyon. Thick depOOts ( 41 ~ ft) of Otowi ignimbriteS were also peuenaled in borehole SHB-3 oa !be SOU!bwest flank of !be valley. The SHB-3 deposits are adjaceot!O !he area wbl:re Cdoa de Valle exits !he Sic:rra de los Vallc5. mel !hey may n:praent an apron of Otowi ignim­brites piled up arlbe mouth of this early Pleiaoc:eoe canyon. Los Alamos Caoyoa mel CaAoo de Valle were major dnillages tbll headed in broad pascs between the dacitic dome9 d tbe Tsc:hicoma Fomwioa prior to lbe Otowi erupcioa, mel may bave acltd as funnels during passage of i~Jlimbritea from the caldera source to lbe west.

The Otowi ipimbrires thin can-ards apinst lbe basaltic ridge, and lbey we abseut in !be area arauod White Rock (Fig. ~). These ignim· br.r.e:s may have overtopped !be bua!bc ridge near White Rock. but. if so. lbe resulting deposiu were relal:ively dUD IDd were eroded before dc'SJO'Itioo of the Tahirege Member. Ipimbritea skirted the basaltic ridge 10 saonh. resulting in thicker deposits of lbe OWwi Member in thal area.

PRE-TSHIIlEGE LANDSCAPE

Srrucnae contoc1r1 for the base of !be Tshirq:e Member show the esti­mated paleocopocBpby of the Pajariro PlataJJ area ar about 1.22 Ma (q:e from b:dt mel O!ndovich. 1994}, and indJcarc lianificant differ­ences from both the pre...()toowi ud lbe modem lmdsape {fig. 6). Al­thouJb aome of the major pre...()toowi feanm:s remained, suc:h as lbe ba­llltic bilb oearWhite Roclt and the SSW-tte:ndiDa valley in !he southern pet of lbe nap area. their relie( bad deaeased asbstantial1y and the north­eru pan of lbe c:arly P\cistocene valley bad been completely buried.

The pre-T lhireJe landvape included ourcrops of TIIChicoma Forma­DOG dacite 10 !be west and Cenu& del Rio bualts 10 !he east. with exten­sive areas of relarively low relief where dninaaes hid eroded into the Otowi Mercbet. Widespread areas ~ !lpPU'elltly occupied by stream terna:~ thlt formed between 1.61 and 1.22 Ma. indicated by alluvial deposits !hal we typically ovcr1&in by pumice beds auociated with the

Cerro Toledo Rhyolite (Heiken et al .• 1986) IDd interveuiDg buried soils. In other areas, illferred 10 be paleotopograpbie drainqe divides or hillslopes, pte· Tlhilqe adluvial deposill are abseat and lbe Otowi Mem­ber is overlain eilber by a sequeoc:e of Cerro Toledo pumice beds and buried soils or by thin colluvial deposits. Loc:a1 relief was apparently mucb less tban today. IDd. where well expoled 10 lbe DOr!h, !he pre­TibircJe IIIQm valleys were typic:ally about 50-100ft deep in areas wbere modem canyons loc:ally ellCI:red 300 ft in depth (FiJ. 6). Nocably, be­cause the Otowi Member is entirely nonwelded in tbis area. die pre­Tshirqe laod5C'I(le lacked the sbarp mesa-c:auyoa !DpOJnpby thll il so prominent today.

The approximate locatims of four major pre· Tsbirqe drainqes have been identified based bocb oo the SU1ICtUI'C eoarour 11111p (fiJ. 6) IDd on tbe preseDCC of pre-Tahirqc alluvial deposits dial c:oat.ain c:obblcs aod boulders ofTIIChicoma Formalioa dacite de:rMd from lbe Siena de los Valles. Ooe large draina&c basin laded in tbeTsc:bicoma bigblaods north and northwest of Los Alamos and included the beadW~~CR of !he mod­em Rendija Canyon basin. The axial clwmcl of this large dn:nqe aeoded southeast ICrOSS what are now Bayo. Pueblo. Los Alamos. md Saodia Cauyoos before crnpcyiq into the ancestral Rio Grande oonb of lbe White Rod: bualt high. Another stream. whole beadwaten appcently included !be modem Los Alamos Canyon beadwllers. lmldod IOUtheaa across PajariiO, Potrillo. and Fc:noc Canyons; alluvial deposits from this sttearn are exposed along lower Waller Cayoa oo !he SOUihwest side of the White Rock basalt!OpOITaphic high. Pre-Tlhirqe alluvial depolits uooalower Anclao Canyon record an addiciooal ~ DOitbeast of tile Pliocene cinder cone. and !he active SU'Cam c:twme1 at !he time of die initial Tshirqc enaplions wu exposed at one location. sbown by fluvial erosion of !he basal Tsanbwi pumice bed and buaJ Tlhilqe ipimbrite prior 10 em­placement of !he main Tshirf.ae i111imbri11es. The course of Ibis stream beneath the Plateau is poorly defined due to the sc:arcity or subsurface

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FIGUU 6. Scrucae coa&our 1D1P (COIIIOIIr ~ 100ft) tor die pre-Tlllirqe -'-:e beDalb die r.jarUD ~ Coalrol poiDa- lbowll tor borebo1cs (IIOiid bid: c:irda) .S Ola:rop Iiies (D'WiaJa). SiMI ...S 10 C08IInlilllbe IIIUiiDunl devaioD oflbeTIIIirqe baaJ C1111DC1 (wbere lbe bae ia110t cqaed) e m.bd by "x"s. Hole ttmtloc:al ~ CF lljpijAiic relief lad loalioD ofbaried clnillqel is kDoWII wilhar-mnf!dcnce ia-where lbe llae oflbeTJbireae Member is apcad in ou=up (Jc:8milly 10 !he - lllld DOnb). lad ttml -*r IIIIICb of die l'lll.eau. dlla - iDiufticiaiiiO ddiDaze dniaqes. In lbele - (Jalenlly 10 die west 111111 IOUdl). the ~ is uadoublcdly more cli-=aal m. dcpiceed.

piSiiDJ cbrough low areas on the caldera rim and cravelling down the pre-Tabilqe l..ol Al.amo5 Canyon and CaAoa de Valk.

IMPUCATIONS FOR STRUCTtJR.U. EVOUrriON OF THE PAJARITO PLATEAU .uEA

Gravity data &Dd lhe lirboio&Y of lbc upper Santa Fe Group suggw lblt lbc ssw~ pre-Ocowi valley at me PajariJo PWeau is at least in p1n a JtlUCUnlJy cooaolled baiD. a oppoeed 10 limply an erosional valley Cbat formed bcfween two~ YOk:laic biJh)ands. A se­ties of en cdleloll &nMty lowl oa:ur oa tbe west lide of !be Pajari10 Pl.-~ (BuddiJI&. 1978; Cordell, 1979; Fapsoa et IL. 199S). These pavily lows t.ve amplitude~ of ma" 4 mpla .S are dcfiDed by N- and NNE-craldi.DJ moc:nalies. One of lbae paWy 1oM oa:urs in tbe vicin­ity of Lo& Alamos. It baa a dumbbelllblpe with tbe llrJer, IOUdlem pan ~ me wea.em flanlt of me arty P1eiatoceDe valley CFi&- 8). Tbe WCil Ade of die IDOmllly c:oiDcidcs wirh die Pajarilo fault zooc. aDCi die Clll side appcan 10 coincide wirh rhc west f1llllt of lbc Ccnol del Rio ~field <Fil· 8). Fergu.soa et IL ( 199S) iJicapRted lbe pavity lows a rhidc requenoes of low~ ....Si!!lfi!'Wy deposita tbat ~CC~Jmu­l&!ld in aarrow pbenJ on the wesa:m llidc ol tbe Espdola basin of tbe IUo Grade rift.

It MallS likely tlw tbe location of tbe early Plcialoceoc: valley wu al

least plltly controlled by subsideoce of this paben. Tbe Pajari10 fault

zone bas been active from 11 least 16 Ma co tbe praeot (Gardner and Goff, 1984; Garduer and House. 1987> and ""'imr:niJ or Saasa Fe IDd Puye qe may baYe ICCUI1lulared in this pabeD clurin& much of this time. Tbe axis of tbe valley lies eul of tbe axis of tbe pmry low (fiJ. 1). 111gestiDa that prosrwliftl alluvial fiDI of tbe Puye Formatioa iDfiUed tbe sraben from tbe west. forcinJ tbe dcposiQoDal axis 10 mipate 10 the eullide of tbe pabc:n.

Tbe lithology of tbe upper Santa Fe Group supporu die inlapn:Wion of a gnbeD Wider die Pljarito Plareau. Punymun ( 199S) described alniiiJb of couse-paiDed sedimenl!l at die 10p of tbe Slllla Fe Group f'Cb8quebui Fomwionj tbat would allow tbe deftloplolg of biJb-yield.low-draw­down warer supply wcUs. Tbe !rOUp is lale MioccDe in aae (baed on c:a. 9 Ma baalt flows intedayered wirh tbe ledimems in weU Otowi-4; .., AIP'Ar qes (A. W. ,..,...,.lin. unpubl. report for Lol Allmos Naboaal Labcntory. 1993); and italoc:atioo c:oinc:ides widllbe arty Pleistocene vaUey described in chis paper Cfia. 8). Tbia MioceDc trouah il about 2.5 mi wide and cxlil:nds II least 7 ..5 mi from die oonbcut 10 tbe IOUtbwat. al1bouah 5Ubsurface dlla are not aftilable 10 dccamine ita fullleaJrh. It is fiUed wirh up to 1~ t\ of come paveUy sediments. illc:ludiq volc:a­aic, 111CW110rp1Uc. IDd sedimentary data possibly derived from IUJb­land.s 10 tbe nonh. east. and west. Tbe appareat persistalce of Ibis depo­aitiOGil basin from the Miocene into lbe early Pleistoceoe SUJJCSll tbat tbe p-aben benellh the wcstem pan of the Pajarico Plaaeau wu a lon&·

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0Jta5.R. L..19M. OeolosY ...Spowd =--ol'die Loa~ na. ,.... Milko: u.s. OllolciPII Slnw)'9Ma ..... , Plplr 17.53. 107 p.

He111a. 0.. aa.r., 1~ Scia. J~ ,._,.. S.. Slw"qqt'h' M.. o.da. S.llld HlpD. LC..lll6.he *'" ~IICIMtJ, 1\IWDc:aldenllldaabay..._ ~ H'"*' ,_ Maic:o: JolnllotO.• tsliclla-dl. "· 91, p.t799-1115.

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BROXTON aDd RENEAU

~ w. D~ l995,Giaiopc_,ltl*alackNCIIIIdloi..,,Mioa wells. -llolea.-...U..IIIflll7...a.,.,n.p._,..t.ae._illdleLIIIA!Ma .na:IM,.._ MIDoaJ Ld -1· a.,_ Llrl211S-MS.. Utp.

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lleDelu. S. L. Dldlia; D. P. IIIII c..y, J. S. 1995, I _,.Mee llld adler -IIICMIIICID -~Ar.ll,IAAIMDDI Nldoall ~ I.AIIAJa. 11101 Naliaaal..........., Rlpan LA·Im5-MS, 41 p.

Smldl. It L. BaDey. R. A. IIIII a... C. S .. 1970, 0eo1oJk 1111P ot die Jemez McJu111iD1. Hnl MlalicD: U.S. 01a1oJiC11 SarwJ, Mjtc:cJI.....,. Cieoloak ID­..UplkD ... J-571, tc*l:125.000.

Spdl. T.L..Hirrilcll, T.M..rWJtl.A.,I,_ •Ati"Atclllilllc:ldleBIIIddier 1\df Uld Sill DkfO C..,..lpialbilla. .lema~ New Maico: cem­pcnl ......... - maa-ic cvohmoa; JCIIIftll olVolc:laoloo _, Geodler· _.--=. v.•3, p.l7S.I93. -

n.tJcviiJe. B. N., ~ D. B. IIIII Self. S.. 1919, L1nHaa1e ,_mad aploiM valciDIIIIl ilt die.._.. .. ..... Maiar. ..;dace flam die Plio-Pkilllaoe1'117ellluYIII'-JCIIIIIIII~ ~~~~~~ ae. -=. •. 36. p. 267·291.

W.at.ct. D. B. _, 'lbrbeYIIIc. B. N~ 19110. E"'llaaioe ot a .... JIIeilloc:em ~ * IDirvlal faa: die Pll7e FarmldaD.J-Momuias. New Mexico: Geo&oaiCII Soeirly ol' A-a BulleliJI. "- Url. p. 291-314.

WoldeGatJriel. O .. l..allllliD.A. W~ Dedlicr, D. P~ Mid lkoaoa. D. E.. 1995. Epi­lodic \ICIIcalalllld... t •my .... Plljlrtro ...... .._ wolclllit fidd. New> Muklcr. OcoloJICII Sadcly ol Amcric:a, AIJIIracll wldll'lopams. "· 77. DO. 6, p. " .....

'Woldi:Oibrid. G.,I.-PJia..A. w~ D.:!1iier. D.P._, Hci2Jer. M.. t9961aapanl - .cod . ., .... ollavll ill Wllile Rod~ C.,U..IIId die Pljlrtlo ,. tau..JaiiClYalciDic field. New McUco. U.S.A.: New MaicoGeoloJic:l! Soci­a,, Guidebook 1,7.

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