Gcological Society of Anrer icaSpecial Paper 3;16

2(XX)

Recognition of primary and diagenetic magnetizatinns to determinethe magnetic polarity record and timing of deposition of the

moat-fiIl rocks of the Oligocene Creede caldera, Colorado

Richard L. ReynoldsU.S. Geoloqitttl Sun'ev, Box 25046, M.S.980, Den'er, Coktratlo 80225 USA

Joseph G, RosenbaumU.S. Geolot!i.'.!l S ^'ev, Bo.r 25046, M.S. 980, Dent'er, Colorado 80225 USA

Donald S. SweetkindU.S. Geoloeitul Sun'er, Btt.x 25016, M.5.980, Dent'er Colorudo 80225 USA

Marvin A. Lanpher€IJ.S. Geokryitul Sun'ev. M.5.937, Menlo Purk, Ctrlilbntiu 94025 USA

Andrew P. RobertsI)(lr.trtnent oJ Ore(uutgruph; Univrsit,- ol Southampten, S(r h nptotl O.eetograplt| Ce tt".

Sonhunptott SO l4 3ZH, Unitetl KittgdontKenneth L. Yerosub

I)epurtnent ol Geologl', Unitersitt' of Cttliforniu, Dulis, Colilbntia 956l6 USA

ABSTRACT

Sedimentary and volcaniclastic rocks of the Oligocene Creede Formation fill themoat of the Cr€€de caldera, which formed at about 26.9 Ma during the eruption of the

Snowshoe Mountain Tuff. Paleomagnetic and rock magnetic studies of two cores (4lE

and 703 m long) that penetrated the lower half ofthe Cre€de Formation, in addition topaleomagnetic and isotopic dating studies ofstratigraphically bracketing volcanic units,provide inforrnation on the age and the tim€ span of s€dimentation of the caldera fiI..Normal polarity magnetizations are found in Snowshoe Mountain TUff beneath themoat sediments; in detrital-magnetitFbearing graded tuffs near the bottom ofthe moatfill; in an ash-fall deposit about 200 m stratigraphically above the top of core 2; and inpostcaldera lava flows of the Fisher Dacite that ov€rlie th€ Creede Formation. Normalpolarity also charact€rizes detrital-magnetite-bearing tuff and sandstone units withinthe caldera moat rocks that did not undergo severe sulfidic alteration. The combinationof initially low magnitude of remanent magnetization and the destructiv€ effects ofsub-sequent diagenetic sulfidization on detrital iron oxides results in a poor pal€omagnetic

record for the fine-grained sedim€ntary rocks of the Creede Formation. Thes€ fine'grained rocks have either normal or reversed polarity magn€tizations that ar€ carriedby magnetite and/or maghemite. Many more apparent reversals ar€ found than can beaccommodated by any geomagnetic polarity time scale over the interval spanned by theages of the bracketing extrusive rocks. Moreover, opposite polarity magnetizations arefound in specimens s€parated by only a few centimeters, without intervening hiatus€s,

and by specimens in several tuff beds, each of which represents a single d€positionalevent. These polarity changes cannot, th€refore, be attributed to detrital rtmanent mag-

Rc}nolds.R'L. .Roscnbaum.J.C..Swcelk ind.D's ' .L nphere.M.A' .R()berts.A'P.andVcf)sub� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �

Col,)r do. (;cololicrl Socictv (n Anreric Speci.rl Paper 346.

7tt R. L. Retrut lds et u l .

netization. Many polarity changes are apparently related to chemical remanent magne-tizations carried by postdepositional magnetite and maghemite that formed in rocks inwhich most or all detrital magnetic iron oxide was destroyed. Incipient oxidation ofearly diagenetic pyrite may have produced the secondary magnetic iron oxides. Ther0Ar/3eAr dates on the normal polarity Snowshoe Mountain Tuff (26.89 + 0.05 Ma, I o)and on the normal polarity postcaldera Fisher lava flows (as young as 26.23 + 0.05 Ma,I o) indicate that deposition of the Creede Formation spanned about 340-660 k.y. Theintermittently defined normal polarity magnetization for the caldera-flrll sequence, com-pared with the different versions of the geomagnetic polarity time scale, is consistentwith the shorter time span.

INTRODUCTION

Sedi rnentary and vo lcan ic las t ic depos i ts o f the o l igocenecreede Formation that f l l l the rnoat of the 26.9 Mri creedecaldera (Lipnian" this volr-rme) are the target of mult idiscipl inaryinvest ieat ions to tes t theor ies about the f lu id and geochemica ll inks between the rnoat f- i l l and the Creede rnineral ized vein sys-tern. Palconragnetic ancl rock magnetic str-rdies of the Creecle For-nration in Creede caldera moat cores (Fig. l) were unclertakenprir lar i ly to deterrnine the t irne span represented by the moat f i l land to enable correlat icln between cores. In this chapter. paleo-magnet ic resu l ts are sumrnar ized f rom a var ie ty o f moat- f l l llacust r ine and pyroc last ic rocks in the cores that penet ra tedapprox i rnate ly the lower ha l f o f the creede Format ion. Theresults are sr,rpplcrnented by paleomagnetic data f iom volcanicrocks t l ' rat underl ie or cap the creede Formation. An earl ier prog-rcss rcport ()n magnetic propert ies of the Creede Formationfocr , rsed on depos i t iona l nrechanisrns o f pyroc last ic rocks. onrelat ions between rock types and rnagnetic susceptibi l i ty, ancl onthe reco-unit ion of postdeposit ional alterat ion of the Creede For-rnation and i ts possible paleornagnetic cornpl icat ions (Reynoldset al. . 199-1). The geokr-uic sett ing and -uoals of the overal l projectwere sunt rnar ized by Bethke and L ipman ( 1987) . Campbel l(1993) . and Bethke (1994) . The core r ra ter ia l was descr ibec l byHr,r len ( I 992 ). In this chapter. we f ir l low the strat igraphic nomen-cluture of Larsen and Crclssey ( 1996) and Larsen and Nelson (thisvolurne). who discr,rss the general strat igraphy of the cures anclthc geophysical propert ies of the boreholes. The deposits of theCrecde Furrnation are described in detai l by Larsen and crossey(1996: th is vo lunre) , F inke ls te in e t a l . ( th is vo lu lne) . anc l Heikenand Kr ier ( th is vo lurne) .

Measured paleomagnetic polari t ies f iom the creede Forma-t ion yield a polari ty seqLrence that is geological ly ancl geomag-net ica l ly unreasonablc . Th is complex record apparent ly ar isesl ' rom both depos i t iona l remunent mi lgnet izat ion (DRM) andchemical rernanent magnetizi l t ion (cRM), apparently caLrsecl bythe proci l"rct ion of magnetic minerals from pyri te. These resultsthr-rs have broad relevance f i l r magnetic studies of sedirnentaryrocks in which postdeposit ional pyri te is susceptible to oxiclat iclnto producc even yoLurser iron oxide minerals. A skeletal polari tyrecurd that spans the deposit ional history of the intracaldera l l l lwas constructed using results f iom the DRM of a f-ew ash-fal l tuff

beds and f rom the thermal remanent l t ragnet izat ion (TRM. r t rpart ial PTRM) of unaltered pynrclast ic and extrusive rocks thatbracket the Creede Forntation.

Another cornpl icat ion of the palertrnagnetic results frorl dri l lcore may have arisen from the coring process. Core barre ls havean ax ia l magnet ic f ie ld that n tay induce an ax ia l ly d i rectedisothermii l relnanent lnagnetization ( lRM) to core material. Typ-ica l ly . IRM assoc ia ted wi th dr i l l ing resu l ts in s tnrng NRM (N =

natura l ) r lagn i tudes wi th near-ver t ica l inc l inat ions that arernostly dernagnetized by low alternating l iclds.

GEOCHRONOLOGIC I, IMITS ON THE AGE ANDDURATION OF DEPOSITION OF THE CREE,DI'FORMATION

A nra-unetic strat igraphy r lay contain a detai led tenrpuralrecord of geomagnetic freld behavior i f the rna-enetizations wereacquired at or very short ly af ier deposit ion and in the absence ofmajor depos i t iona l h ia tuses (e . -s . , Jacobs, l9 t t4) . Such a recordmust be compat ib le wi th ava i lab le geochmnornet r ic data; i r r th iss tudy these are prov ided by 10Ar / reAl ' iso top ic dates on theSnowshoe Moun ta in Tu f f ( Lanphe re . t h i s vo lun ie ) . wh i chunderl ies the Creede Forntat ion. and on lava f lows of the FisherDacite that erupted af ier caldera col lapse. Reliable ages f i l r theCreede Format ion could not be measured d i rcc t ly ; to ta l - fus ion'+OAr/leAr ages on biot i te fr"om l lve ash layers in thc twcl rnoatho les y i e l ded i ncons i s ten t r esu l t s , r e f l ec t i ng t he p resence o frewrrrked rnaterial (Lanphere. 1994, this volurr ie). The youngestdate obta ined t rom ash beds l l 'ont the dr i l l core is 26.13 +

0.19 Ma (Lanphere. th is vo lLr rne) .Lanphere (this volurne) obtainsd x attAt-/reAr date of 26.92 +

0.07 Ma for the Snowshoe Mountain Tuff in core 2 and an expo-sure of outfkrw tuff. Lanphere (this volulne) also cletermined anage of 26.26 + 0.04 Ma fbr Fisher Dacite that represents a pooledestimate of dates from lavas that overlie the Creede Fornration onFisher Mountain and Copper Mountain. Two local i t ies of olderFisher Dacite, which erupted early in the caldera-t l l l in-q history.have also been dated. The Fisher lava fbw at Wagon Wheel Gapyielded l0Ar/-reAr laser total fusion and increnrental-heatingplatear"r ages on biot i te of 26.59 t 0.09 Ma and 26.68 + 0. l4 Ma,respectively (Lanphere. this volume). The Fisher lava fbw nearMcCall Creek at the west side of the caldera (see Liprlan. this vol-

107'00 'w 106'45'

EXPLANATION

3J"45 'N

I I Moat sediments (Creede Formation)and col lapse breccias

I r *

, *

- l

EPostcaldera lava flows (Fisher Dacite) Intracaldera tuff (Snowshoe Mtn. Tuff)

Precaldera volcanic rocks

Figurc | . Cencralized gcoloSic map of Creede calderu, showing approximate k)cations of erGled topographic rim; present day extenl ol catderulilldcpositsl norntal lirults reldled to r€surgent doming of Creede caldera and mineralization of Creede district: drill corcs I and 2 (CCM- I und CCM 2.reslo-clively )l itnd oulcrop sanlpling sites fbr Fisher Dacite (C94 sites) and ash-tall tuf (CFA I site). Dots indicate sampled outcrops of volcanic rock.CIosed synrbols indicate lhilt relidblc paleomagnetic resulls werc obtrined liom site. and open symbols indicitte unreliable results. See 4lso Fig-ures I. l and l5- Modil lcd t ionr Lioman (this volume).

ti0 R. L. Rcynolds et ul.

r-rnre) yieldccl a r0Ar' /r ' )Ar date ol '26.63 + 0.09 Ma (Lanphere this

volunre). The agcs were deterrr i ined using standard biot i te (SB-3.

K-Ar age o l ' 162.9 + 1 .3 Ma) as the f lux r ron i tor ' . us ing techniques

described by Dalrymple and Lanphere (1911. 1971). Dalryrnple

et al. ( l9t j l ) . ancl Lanphere ( 1988). Considerin-s the isotopic dates,

inc lud in-s uncer ta in t ies ( in th is chapter . a l l i so top ic dates are

repoftecl at *l o e rror level) of the bracketing volcanic units, depo-

sit ion of thc Creede Fclrmation probably occured over a period of

about 0.3.1-0.66 nr.y. The 0.66 rn.y. period is represented by dif--

t 'erent patterns in three recent geornagnetic polari ty t i rr ie scales(GPTS): rnostly norrnal polari ty but rer, 'ersecl in the r.rpper part

(Har land et a l . . 1990) . rnost ly normal but reversed in the lower

part (Wei. 1995). and about evenly divided reversed ( lower half)

and nurnra l (upper ha l f ) po lar i ty (Cande and Kent , 1995: F ig .2) .

The Snowshoe Mountain Tr.rff and three previously sarnpled f-lows

of the Fisher Dacitc have normal pcl lar i ty magnetizations (Rosen-

binrrn ct al. . l9tt7: Tanaka and Kono. 1913. Beck et al. . l9l1).

N,IETHODS

Ma-snet ic suscept ib i l i ty tMS) was measured on whole-cc l re

scgments :-rnd on strndard paleclr lagnetic specitnens (volr"rme

about l0 crnr). Mcasurernents were made at 600 Hz in a freld clf

about 0. I rnT usirrg sLlsceptometers that were cal ibrated usin-e

MnO, standards. Hysteresis propert ies clf paleornagnetic samples

were deternrined using a vibrat ing sample rnagnetorneter. Paleo-

magnet ic nrc i rsLr renrents o f samples (5-15 g cy l inders) were

nrade using loLrr dif l -erent ma-gnetometers: two cryogenic t lagrte-

tor r rc ters und l r . ro sp inr tcr n)aqnct ( )n ' le ters . Core sumplcs were or i -

cntcd vcr t ica l ly but not az i rnutha l ly . D i rect ions o f remanent

rnagnct izat ion (see Reynolds e t a l . . 1994. Table l ) were deter -

nr ined by least -sqL lares l ine f i t t ing o f dernagnet iz i t t ic tn decay

paths on vector c l ia -uranrs (Z i . iderve ld , 196 l ; K i rschv ink, l9U0) .

Each sanrplc was clenra-unetized either in progressive alternating

flelds (AFs). typical ly in l0 steps f iom -5 mT to 80 mT, or by ther-

r la l techniqLrcs. typ ica l ly in 8 s teps f rorn 100 to 600 'C. Demag-

nctization results provide irnportant information on the identtty

of nra-unctic nrinerals in the rerni lnence decay as a function o1'

peak AF or tenrperature.

Ttl faci l i tate analysis and discr-rssion of paleornagnetic results

frorn the cores. wc assigned a quali ty rat in-u tcl the magnetization

decay path l i rr cach sarnple (Fig. 3). The highest rat ing of I wits

rcserved f irr results in which the dernagnetization path decayed

toward the or ic in in u nc 'ur s t ra ight l ine. A ra t in-g o f 2 was

lss igned to paths that were curved or k inked and that t rended

general ly toward the origin but that clearly deflned a posit ive or

ncgativc incl inatiun. A clr.ral i ty rat ing of 3 was assigned to paths

that typ ica l ly have a c lus ter o f po in ts a t e i ther inc l inat ion s ign but

without a clear decay to the origin. About 5c/c ctf the more than

360 specirnens analyzed gave corr ipletely errat ic results and were

e l inr inated f l 'unr f ' r ,rr ther analysis.

The rnagnitirde of NRM also fitctot'ed in the evaluation of the

paleornagnetic clata. Not surprisin-ely. the NRM rnagnitude and

clual i ty ol ' the clenragnetization path are comtlotr ly l inked. In the

26q-26.23 F i s h e r D a c i t e ( C o p p e r

M o u n t a i n a n d F i s h e r M o u n t a i n )

F i s h e r D a c i t e( W a g o n W h e e l G a p )

S n o w s h o e M o u n t a i n T u f f+- 26.89

A B C

Figure 2. Part of Oligocene -seolragnctic polari ty t i l l lc scit lcs (1ionl A.Cande and Kent. 199-5: B. Wei. lc)c)-5. ancl C. Harland et al. , I990) withloAr/-reAr isotopic dates f irr volcanic units that Lrndcrl ie. arc withirt . andoverlie sedintentary rocks of Crecde Forrnation (fiont Lattphcrc. this vttl-r-rnre ). Vert ical bars on r ight side indicate I o analyt ical l tncertainty t irreach l isted datc. Fi l led arca indicatcs nornral polari ty; olrctt urca i t tdi-cates revcrsed polarity.

lbl lowing we ret-er to nragnetizi t t ions as either strong (those hav-

in ,c NRM rnagni tude >10 r A i rn [0 5 er r iu /cmr l ) . I l loderate ly

s t rong (NRM magn i t ude be tween 2 x l 0 I and l 0 r A / r r l ) . o r

weak (NRM <2 x l 0 I A / rn ) . I n t he l b l l ow ing d i sc t t ss i o t r . we

make use of the quali ty rat ing and NRM rnaglt i tr-rde to assess the

rel iabi I i ty of the paleornagnetic direct ions.

Other rnethods for ident i fy ing rnagnct ic t t r i r tc - r l r ls inc lude

thermomagnet ic analys is and d i rect pet rographic observat i t ln .

Thermomagnetic analysi s, the nreasurct l tent t l f hi -uh-f ic Id rnagne-

tization as a function of temperaturc. wlts use d to test filr the pres-

ence of rnagnetic iron oxide and iron sult lde rninerals which have

diagnost ic Cur ie temperatures and/or character is t ic shapes of

their therntomagnetic curves. Iron oxide and irt t t t sul l lde tt i inerals

were examined using standard reflected-light rrlicroscopy at l.ll:.ls-

nif . icat ions to about -500x. We viewed 64 pol ished thin sL-ct i()ns

made frorn paleotnagnetic samples and l0 pol ished rr l t lunts of

grains that were separated nta-eneticl t l ly l 'ntrn br"r lk sart lples.

E,T. 'FECT OF LITHOLOGIC VARIATION ANI)

DIAGENETIC ALTERATION ON PALEOMAGNI'TIC

RESULTS FROM THE CREEDE, FORMATION

Both I i tho log ic z .onat ion and postdepos i t io t r - t l a l tera t io t r

exer t a s t rong in f luence on the rock t t tagnct ic proper t ies and.

consequently, on the paleornagnetic record of the Creede Ftlr-

mat ion. The lower par t o f the Creede Ft l r r r ra t io t r d t l r l inant ly

cons is ts o f tu f f brecc ia . conglomerate. anc l l ' luv ia l s i t r lc ls t t lne.

aoCI

c

= 2 7=

=tti3:33

N (up)

2 0

Qua l i t y 1

Rccogrt i t iort o.f 'pr i tr turv urrd t l iagettet i t ' ntu,qrtct i l .uf ions, Oli ,qoccttc Crcctla culdaru, Colrtrtulo

N (up)

(Horizontal) 2-7 8-8 .8

3 x 1 0 31 - 6 7 - 1 . 7

til

FigLrrc 3. Ortho-sonal r , 'cctor- d ia-erants(see Z i . j de rve ld . 1967) i l l us t ra t i ng th reecategor ies of qLral i t i , ' o f ' a l ternat ing t ie ld(AF) de r lasne t i za t i on pa th . f ron t h igh( l ) t o poor (3 ) . Two d ias ran rs i l l us t ra tequa l i t y I resu l t : spec in ten l -67 - 1 .7 . ha r ' -i r - r -9 nurrnal polar i tv nragnet izat ion. andspecimen 2-78-8.8. having reverse polar-i t y rna -enc t i za t i on . (Den tagne t i za t i on

steps are in rnT). Natural rentanent mag-ne t i za t i on (NRM) inc l i na t ion po in ts a reo f f sca le i n th ree d iag ra rns i l l us t ra t i ngnurrnal polar i ty . Closed synrbols repre-scnt pr t l . jcct i r lns 0 l ' nr i lUnct ic Vecl( ) r ont()ho r i zon ta l p lane . Open syn tbo ls rep rc -sc l l t p r ( ) . i ce t i ( )ns r l l ' r r raunc t i c Vce t ( ) r on t ( )r c r l i c l l p lane . (Rc r r r l r r r cn t l l t i . t ! nc t i , / ; - l -t ions arc g iven in A/nt) .

N (up)

Qual i ty 1

E (Horizontal)

N R M

(Horizontal)

E (Horizontal)

2 x 1 0 3

1 0 0

6 0

20 1 -67 -T .2

5 m T

represent ing h igh-energy, coarse-gra ined fac ies. In cont rast , theupper par t donr inant ly cons is ts o f laminated beds c l f s i l ty ,reworked tu f f w i th in tervening ash- fa l l tu f f beds, represcnr inglow-energy. f ine-gra ined depos i t iona l fac ies. The coarse-

-u ra ined beds gene ra l l y have h igh NRM magn i t ude and MS( t yp i ca l l y >10 r vo lun re S I t F i g .4 ) . as expec ted because o f t heabundan t magne t i t e i n t he heavy -m ine ra l l a rn i na t i ons o f t hesandstone as wel l as in the vo lcan ic- rock c las ts o f the brecc iaand congk lmerate. The f ine-gra ined beds have much lowerNRM magn i t ude and MS ( t yp i ca l l y <2 .5 x l 0 a vo lume S I ) . aswould be expected on the basis of lower energies of secl inrenttransport to the lake bottrtrn.

The boundary between the high- and low-MS zones does notexactly coincide with the l i thologic boundary between the twof'acies, but rather is present within the coarse--erained rocks(Fig. 4). This break thus corresponds more closely to a geochem-ical boundary than to a deposit ional boundary. In core l , the MSbreak occLrrs between about 305 and 320 m within an interval ofin terbedded pebble to cobble conglonterate and pebbly sand-

1-67 -5 .55

Qua l i t y 3

stone. whereas in core 2 the break occurs between aboirt 505 and

530 rn within a thick rnonornict ic breccia.

The geochemical bor"rndary separates a less altered zone con-

taining sparse pyri te in the krwer third of thc crlres f l 'onr an over-

lying. Inore altered upper zone that is characteriz.ed by abundantpyr i te . The pyr i te is a d iagenet ic product o f bacter ia l su l fa te

reduct ior t (McKibben et a l . . 1993; Rye et a l . , th is vo lurne. Chap-

ter I l ) . Because alterat ion strongly af l 'ected the nragnetic proper-

t ies of niuch of thc Creede Forrnation. i t is irnportant to consider

thc paleornagnetic results in l ight of sulf irr geochcnrical data andpetrographic observations of su l f ld ic alterat irxr.

The rel i .r t ions between sulfur contcnt and MS i l l r ,rstrate the

efl 'ects of sulfrdization on rnagnetic propert ies (Fig. 5). MS valr.res

have a strong inverse relat ion to total sulfur content regardless o1'

l i thology. In this sett ing, total sulfur is a proxy l i rr i ron sr,r l f ldc

nr inera ls because of the absence o l ' su l t i r te rn inera ls . such as

barite or gypslul. in the Creede Fornratiorr. F-urlher evidence f irr

iron sulf ide carne f iorn MS measurerrents of sarnples al ier ther-

tnal demagnetization steps. An r lr iginul presence of iron sulf ide in

N (up)

Qua l i t y 2

u2 R. L. Reynolds et ul.

1 0-5 1f 1 0-3 1O-2 1o-s 1 f 1o-3 ' ro2

M a g n e t i c s u s c e p t i b i l i t y ( S l v o l u m e )

FigLrre -1. Magnet ic sLrscept ib i l i ty .1ror t t wholc-corc lneasl l rcntel t ts .p lot ted against c lcpth in cores Iancl 2. WT is welclcrl tLrfl.: GT isgradcd tLr f f l CGI- is conslonter-i t t c . D l r s l t c d l i n c l r p p l o r i n u r t c sboundary bctu 'een f lLrr ia l f i rc iesbekru and lacLrstr ine fhcies above.Dcp th i s shown in rnc tc rs ( i t a l i cs )

and l 'cct .

1 5 0 0

rnany sarnples is incl icated by strong increases in MS, after heat-ing above about .100--1-50 oC. temperatures at which pyrite mayalter to rr iagneti te.

Petrouraphic observations dernonstrate that sulf ldizationaf tected det r i ta l t i tan iurn-bear ing magnet i te . Pyr i te commonlyrims rnagncti te (Fig. 6A) or f trrnter t i taniunt-bearing magneti te, inwhich the rr iagneti te was destroyed by complete leaching ctf i ron(Fi-s. 6B). In thesc latter cases. the clr iginal presence of Ti-mag-neti te is indicated by rernnants of i l rnenite, clr by TiO., forrnedfrorn i l rncnite. as larnel lae in the { I I I } crystal lographic orienta-tiorr. Pyrite rints around lbrr-ner magnetite, now leached of its iron.are a conrnon expression of sulf ldization in which H.,S causes dif ' -fusion of iron f innt the ircln oxide (Canfield and Berner. 1987:Canf ie ld e t a l . . 1992) .

No rnagnet i te was seen in po l ished sect ions o f some sam-ples. even thr tugh thernta l demagnet izat ion resu l ts ind icatedr-rrrblocking tenrperatures that are diagnostic for rnagneti te. as dis-cussec l in the fo l lowin{ r . In o ther po l ished sect ions, genera l ly

100

; 0 0

200

3000 0 0

5 0 0500

6002 0 0 0

from coarse-grained rocks, sn-ral l rel icts o1' lbnnerly large mag-

neti te grains remain inside thick r ims of pyri te. In sorne of the

samples that are devoid of visible magneti te, l 'err irnagnetic f 'er-

r ian i lmeni te is present . Ferr ian i l rnen i te was ident i f ied in po l -

ished grain mclunts clf magnetic-mineral separates and by

thermornagnetic analysis. The resistance of f 'err ian i l lnenite, and

other var ie t ies o f the i l rnen i te-hemat i te so l id-so lu t ion ser ies . to

su l f ld izat ion has been wide ly documented (e .g . , D imanche and

Birrtholorn6., 19i6. Reynolds, 19821 Roberts and Turner, 1993:

Roberts and Pil lans, 1993). These grains are optical ly horno-ue-

neous, with rapid cool ing of the volcanic rrrcks havirrg prevented

exsolut ion intcl separate phases.

It is perhaps important to this study that t 'err ian i lntenite of ir

ce r t a i n co rnpos i t i ona l r ange (0 .45 < x < 0 .60 , f o r Fe I l l l , , ^

Fe[l ] Ti* Or) has an unusual capacity f i rr sel l ' reversal cl f magne-

t izat ion; i .e.. the minerals may acquire a stable remanent tnagne-

t izat ion when cooled lrom elevated ter-nperatlrres (>-200 .C) in a

d i rect ion oppos i te to that o f the appl ied f ie ld (see Hof fnran.

+rl

oo

tts

ttc(g

oL

o{-,oEe-r 1TJ.

oo

100

5 0 0

200

3000 0 0

ooAn

Reworked tuff and sandstone

Sandstone

TuffCoarse sandstone-breccia

o).V

co

E

=-oo-o()af,a.9c)co)(E

1 0 - 5

10-6

1o'7

0 0 2 0 4 0 6 0 8 1 1 2Sulfur (wt%)

FigLrrc -5 . P lo t o l ' Inagnet ic susccpt ib i l i tv agu inst to ta l su l l 'L r r ( i r r weightpcrct: nt ) in incl ir idLurl sunrplcs. coclecl f i rr l i thic t1,pe .

1915). One therrnortragnctic analysis clf a ntagnctic extract of f-er-r ian i l r len i tc ( - l -52. -+ rn in core l )gavc a Cr- r r ie tcmperature c l fI l0 "C (F ig . 7) . cor responc l ing to a cornpos i t ion o l 'x :0 .5 . whichis wel l w i th in thc se l t - rcvers ing r i rnse.

PALI'OMA(; NI'TI C RtrSULTS

Paleomagnetism o.f ash-flow tuff, core 2

Paleonragnetic behavior of the ash-fktw tr_rff in crtre 2 is eas-i ly in terprc ted and c le l ' ines a nc l rnra l po lar i ty lnagnet iz i r t ion(F ig . l tA) . A l l l3 s1- rcc i r r iens havc ntoder l l rc coerc iv i ty . s ing le-component n tagnet i / .a t ions that together wi th pet rographicexanr inat ion i r r ip ly a TRM carr ied by pr inrary nragnct i te . The ins i t r " r nrearr inc l inat ion. ca lcu la ted by the rnethoc l o f McFaddenand Rc id ( l 9 t t 2 ) . i s 7 -1 .2 " ( c [ , ) : = 2 .8 " ) .The i nc l i na t i on was co r -rcc tec l or r thc bas is o f thc r r ieasured borehole a t t i tu rde (az i -rr iuth 306". of l 'sct -5" l l 'orr vert ical: Nclson and Kibler ' . l99zl): thecorrected ntcan inc l inat ion thus rcpresents a r i tnse c l f poss ib leinc l inat ions. i rs i l lus t ra ted in F ig . 9 . The pa leornagnet ic resu l tsare cons is tent wi th chern ica l and pet rographic rcsu l ts that con-f l rnr the basal ash- l ]ow Lrn i t in cure r as Snowshoc Mounta inTufl ' (Lipntan ancl Wcston. | 99.1).

Pa I e o m ag n e t i s rtt o.f' h r e c c ia a n d c o n g lo m e rat e

Thc tLrl l i rceoLrs. rnutr ix-supported brccciu in the bclt torn l tJ r-no l 'core I r r r ight reprcscr . r t e i ther a debr is - l low depos i t or a l r th ic -r ich. nonwelc lec l ash- f low tLr f f ' (Hulen. 1992: Larsen and Nelsnn.t h i s vo lun re ) . Thc l a rge sca t t e r i n pa leon tagne t i c i nc l i na t i ons

Rt'L'o,qrt i l iott o. l ' l t r i r t tur.t ' t tr t t l t l iu,gancti t ' r)ru,qn(,t i : .( t t i () tr . \ . 0l i ,qtx.utt , (-r(t ' r l t ' ctt ldt,rrt , Crt lontr lo l t3

(rartging fhrr l 3-5" to 130": Rcynolds et al. . 199-l) preclr-rdes anypclssibi l i ty that this unit was entplaccd at tentpe-ratLlfcs to ucrprirea TRM or PTRM (e. -s . . Hobl i t t and Kel logg. l ( ) l ( ) ) . The scut tcr inrernanent inc l inat ions probably reprcse l l ts ins teud thc depos i t iono l 'c las ts in a co lc l dcbr is l low. S i r r r i la r ly . po lyr t r ic t ic tu f l ' b recc ia(lkrrn depths o1'622-(r-19 nt ancl -567 -594 nt)as well as ntoltolt l -ict ic tLrf l 'bleccia (-t t t t t--5;19 nt) in core ? havc shal low to steep.posit ive und ne-gativc rcl lanent incl inations that rcl lcct apparentlyrandonr or ienta t ion o f ind iv idr " ra l c las ts . No cons is te n t po lar i typattenr was cvident when cotlparing nearly ad.iaccnt santples ol 'l i th ic c las t anc l nra t r ix . These resu l ts ind icate a lack o l ' t ' cn l rsr r r . t i -z. i t t i t tn. wl 'rethcr lkrrn visct)Lrs overprint ing. lhrnt clr i l l ing-inclLrcedIRM. t t r lk t t r t CIRM. Thc abunc la t tcc o1 'unal tc rcc l . or i rnurv t i tano-

I r i su rc ( r . I i . e t ' l cc t cd - l i gh t pho to r t t i cxx r raphs o l ' po l i shcc l t h in scc t ion .S i tn rp le ( l h rn t co re ? .71 .1 n r ) l i as h ig l t na tu ra l r cn t lu l cn t n rasnc t i z l r t i onntagl t i tuc le ( I .3 x l0 r A/nr ) . A: Dunrbbel l -shapccl t i tunontasnct i tc thut ispart ly rcplaced and surrounded by ' l tvr i tc (whi tc) . Magnct i tc (dark gray)i s cL r t by i l n t cn i t c l a t re l l ac ( l i gh t -u ray ) . T i tunonr lg t )e l i t c s lu in i s l -10 ; . rn rin Iong c l i r t tensiot t . B: Mugnet i tc h l rs bcerr ren.rov 'cr l lh lnt l i r r rncr t i tano-t t t i tgnet i te. lc i tv ins re l ic t i lnrcni te lanrel luc: grain is surrol rnclcc l by th inr in r o1 'py r i t c . Gra in i s I l 0 p rn t i n l ons d in tcns ion .

84 R. L. Ret'noltl,s et ul.

a.=cf

o.=-o(E

c.9(g

.Nc)c(')o

1 R-57 A-0. 8 B

c o o l i n g

h e a t i n g

0 200 400 600Tempera tu re ( oC )

Figure 7. Thermomagnetic curve showing change in normalized magne-tization as function of heating and cooling in air of n-ragnetic separatefrom sandstone that contains f 'errian i lmenite (sample lR-57A-0.88;-152.4 m [50 fi] in core l). Curie temperature is about 110'C. Arrowsindicate heating (down) or cooling (up) path.

magnetite in the clasts of these coarse-grained units precludes thepossibil i ty of randomizing CRM components acquired over along time period relative to polarity reversals.

Near the top of core l, we sampled three conglomerate units(39-51 m, 55-56 m, and 17-86 m) which we refer to informallyas conglomerates l, 2, and 3, respectively (see Reynolds et al.,1994, Fig.2). MS is high ( typical ly >6 x l0a volume SI) , andspikes of relatively high MS near the upper and lower bound-aries of conglomerate 1 (Fig. a) resemble MS patterns related tocooling near boundaries in other volcanic rocks that may pro-duce variations in the magnetic-mineral grain size (see Rosen-baum, 1993). The upper two conglomerates possess mixtures ofpositive- and negative-inclination magnetizations and thus showno evidence fbr the acquisition of coherent remanence, whetherby TRM, PTRM, CRM, or dri l l ing-induced IRM. In conglomer-ate and coarse-grained sandstone of the conglomerate-3 unit, 7of 12 specimens possess moderate-angle (-35"-70') posi t iveinclinations (Fig. 8B). Of the remaining specimens, one had ashallow ( 18") positive inclination, two specimens had nearly flatinclinations, and two others had negative inclinations. The pre-dominance of specimens having positive inclinations is consis-tent with the acquisit ion of TRM or perhaps PTRM, but thispossibil i ty has not been tested using thermal demagnetization. Apervasive normal IRM can be ruled out because of the lack ofuniform behavior in nearby specimens and the lack of very steepincl inat ions.

Paleomagnetism of ash-fall tuff and lacustrine beds

Within the lower parts of both cores. reworked, graded

ash-fall tuff beds (tuff A) possess a normal polarity ntagnetiza-

tion. Of l7 samples from the graded tutf doublet in core I (GT

interval in Fig. 4) . l6 have wel l -def ined moderate to steep (as

much as 88 ' ) remanent inc l ina t ions (F ig . t3C and l0 ) . The

inclination trend, front steep at the bottclm to shallow toward

the top of each set of tuf fs, suggests a dr i l l ing- induced mag-

net ic overpr int (probably a low-f ie ld IRM) on the coarse-grained tuff at the base. The sirnplest explanation fbr the sole

negat ive inc l ina t ion is tha t a core segment o r spec imen was

inadvertent ly f l ipped dur ing handl ing and marking. The same

is not observed for the graded tuff in ctlre 2; the average incli-

nat ion calculated from the rnethod of McFadden and Reid( 1 9 8 2 ) i s 6 2 . 9 " ( c x e s = 4 . 1 " ) .

The ash-fall tuff.s. reworked tuffs. siltstone, and sandstone in

the high-sulfur, low-MS zone have predominately moderate to

steep, posi t ive or negat ive incl inat ions. No obvious polar i ty

zonation is evident: nclnnal and reversed polarity salnples alter-

nate through both cores I and 2 (Fig. I l). Many polarity changes

occur over short stratigraphic intervals or within individual beds.

Diff-erent polarities are fclund in specimens separated stratigraph-

ically by only a f 'ew centimeters and within the same core seg-

rnent. Dual polarity magnetizations are found within some

individual tuff beds. each of which represents a single deposi-

t ional event (Fig. l2) .The broad polarity pattern is poorly deflned, and it coffe-

sponds poorly between the two cores, both in an overall sense

ancl in detai led comparison of presurnably equivalent tuf fs(Fig. I I ). Many samples in core l. frorn tuff G down to the top

of the high-MS zone, have normal polarity, whereas most sam-

ples in core 2 over this interval have reversed polarity. Further-

more, more normal than reversed polarity directions are found

in tuffs E and F in core l, but the oppitsite is tound in these tufl i

in core 2 (Fig. I l ) .The polar i ty patterns and relat ions are independent of

demagnet izat ion technique. Al though many fewer samples

were demagnet ized by thermal than by AF demagnet izat ion,the proport ion of normal and reversed polar i ty direct ions is

sirnilar afier the diftbrent treatments.Screening the resul ts on the basis of NRM rnagni tude

reveals a predominance of normal polanty directions (Fig. l3)'

Such a screen, arbitrari ly accepting samples with NRM magni-

tudes > l0-2 A/m. eliminates about 90c/r, of all results from core I

and about 8O7o of those fiom core 2, with nearly all reversed

polarity directions being removed from core l. Of the quality I

and -2 results in core 2, only three reversed polarity directions

are retained, compared to I I normal polarity directions.

Paleomagnetism of ash-fall tuff in outcrop (site CFA-I)

The elevation of ash-tall tuff at site CFA- I (Fig. l) is about

2790 m, 160 m above the top of the core 2. Ntl known struc-

1 . 0

0 . 8

0 . 6

0 . 4

0 . 2

0

l l t 'co,grt i t iort t l f pr irnurt ' utrt l t l iuganeti t ' t t tugnati : .ut iotr, \ , Ol igocerte Crccde culdcru, Colorudtt 85

t r rnr l o f ' l ' se ts cross the esscnt ia l ly l ' la t - ly ing Creede Fornrat ion

b c t r v c c n c o r c I a n c l s i t e C F A - 1 . T h i s t u f l ' t h u s a l l o w s 1 o r a

polur i ty detcrnr in l t ion u t a hur izon rou-uh ly 200 rn s t ra t igraph-

icu l ly abor ,c the Lrppcrnrost sanrp led in terva l in dr i l lho le 2 .' l ' he

resu l t s o l 'AF den ragne t i za t i on ( l - i g . 1 .1 ) y i e l d a we l l -

r lc l ' incd nornra l po lar i ty nragnet izat ion wi th u nrean d i rect inn o t '

r l c c l i n a t i o n . [ ) = - ] - l 9 . l J " : i n c l i n a t i o n . I = 6 t 3 . 2 ' ( c t e : = 1 5 . 7 ' ;

/ i = 19.2) . T 'he norn.ur l po lar i ty renranence is in terpreted to be a

I ) l tM . bccause : ( l ) t he den ragne t i z . a t i on pa ths (Reyno lds e t a l . .

199- l ) are cor . rs is tent wi th r la-unet i te behav ior : (2) det r i ta l rnag-

r rc t i tc anc l i ln ren i te (probably the f 'e r r ian type) are the on ly rnag-

nct ic nr incra ls ident i f ied by pet rographic exanr i r .ur t ion: and (3)

r ru . rcr 'oscopic ev idcncc f i l r dc t r i nrenta l a l tc ra t ion. rc la ted e i ther

to su l l ' i d i za t i on o r ox ida t i on . i s absen t .

A N (up)

Ash-flow tuff

N (up)E (Horizontal)

w

40

30

201 5

1 0

NRM

c N (up)

Paleontagnetism of the Fisher Dacite (postcollapse lavas of

the Creede caldera)

We also measured rernanent directions of lava flows fiom six

local i t ies (Fig. I ) of the Fisher Dacite that postdate col lapse of the

Creede caldera (Steven and Ratt6, 1913: Steven and Liprnan. 1973;

L iprnan. th is vo lurne) . Of the s ix loca l i t ies , on ly three y ie ld

straightlorward paleomagnetic results with well-defrned normal

polari ty rnagnetizations (Fig. l5): ( I ) the isotopical ly dated Wagon

Wheel Gap lava dome (site C94- l) . which erupted during sedi-

nrentation ol' the Creede Formation . (2) a lava flow at Mill Park

that overl ies this dome (C9r1-2)t and (3) the McCall Creek f low on

the west side of Snowshoe Mountain (C9,1--5) that erupted befbre

caldera resur-gence and then was unconfbrmably overlain by cur-

E (Horizontal)

Conglomerate 3

B

20

Fi-eure t3. Orthogonal vector d iagranrs

i l lustrat ing behavior o1' three l i th ic types:

A. ash-f low tuf l ' . core l : B, volcanic con-

-ulonterate. core I ; C. -graded tuff, core 2.

Syrnbols as in Figure 3. (Demagnet izat ion

steps are in mT. Natural remanent lt lagne-

t i za t i ons tNRMI a re i n A /m. )

.1 6

1 0

Lower gradedtuff

E (Horizontal)

0.06

S (Down)

6040

3020

1 0

S (Down)

S (Down)

86 R. L. Reynoltl.s ct ul.

3 5 0

360

3 7 0

FigLrre 9. EclLral -urea pro. ject ior . r o l ' r . r . rcan incl inat ion of remanent r -n: igne-t i za t i on f l -o rn Snowshoc Moun ta in TL r f f a t bo t to rn o t ' cu rc 2 . B lackclashccl l i r tc rcprcse nts possib lc r lngc in azi r l r , r th uf me: ln inc l inat ion thatresul ts l l 'onr unccrta intv in azi r r ruthal or ientat ic ln o l ' thc core cor-nbincdu i th -5 'o l ' l ' sc t ( th r rn \e r t i ca l ) i n bo rcho lc a t t i t r - rde . as shown hy o r ien tu -t ion of corc ur is . Dashed l ines rcprcscnt unccrta inty at t )57r conf ldencelc rc l abou t n rcun inc l i na t ion . Norma l po la r i t y d i r cc t i ons o l -SnowshocMur"rntain Tul l ' t l 'onr oLrtcrop s i tcs at Ci t t t le Mountain and Spar Ci ty arercDrcsclr ted br ' 9-5( , ' l c()ncs of 'conf ldence.

tairrside. postcool ing rotat ion off the front of : , t newly fbrrned

flow. or the acquisit iorr of therrnorernanence in an intermediate

georna-gnet ic f ie ld or ient i l t ion coLr ld exp la in th is unusual d i rec-

t ion. Two other exposures o f F isher Dac i tc y ie lded ur r re l iab le

cl irect ions (Fig. l ) : one was apparently atfbctcd by large-scale

lands l ic l ing and the other by l ightn ing- induced lRM.

POLARITY STRATIGRAPHY OI.- THE CREEDE

FORMATION

Causes of apparent magnetic reversals

The rnost re l iab le pa lec l r r ragnet ic resu l ts reveal nor tna l

polari ty nri ignetizations in volcanic rocks below and above the

Creede Furmation and intermittently in dif l 'ercnt l i thologic z-() l tL's

wi th in the Creede Fr t r rnat ion. Graded tu f l 'A in the low-su l t 'u r .

high-MS zone of thc lower part ol ' the Creede Formation has i t

pr imary nornra l po lar i ty . as does the undcr ly ing Snowshoc

Mountain Tul j ' (Fig. I I ) . A conglorneratc bcd in the upper part ol '

core I has a rnostly coherent norrnal polari ty direct ion carr ied

by pr i rnary t i tanomagnet i te that is perhaps a TRM or PTRM

acquired during cool ing of a debris t low that was emplaced at

elevated temperatures. Results frorn the ash-l i t l l bed in outcrop

indicate nornral polari ty at a hrlr izort that is about one-third of

the way up the interval between the top of core 2 and the top of

the Creede Forrnation. Well-deflned normal polari ty trasnetiza-

t ions arc f irund in stably ma-gnetized lavas f lows that overl ie the

Creede Forrnation.

Frlr scveral reasons, the rr iany polari ty chartgcs in the high-

sulf irr . low-MS zone cannot be attr ibutcd to a DRM that t i r i thful ly

recorded the geomagnetic polari ty at or near the t irne ol 'deposi-

tion. First. In:iny more reversals are tirr"rnd than ciur be accolntno-

dated by any geornagnetic polari ty t i rne scale over thc interval

between the ages of the bounding ext rus ive un i ts (F igs.2 and I l ) .

Second. several polari ty changes are recordecl by specit 'nens sep-

arated by on ly a I 'ew cent i r le ters and wi thoLr t in tervening h ia-

t uses . Th i rd . seve ra l i nd i v i dua l t u f f beds . each o f wh i ch

represents a sin-ule deposit ional event. contuin r.rppi lrelt t revcrsals.

Final ly. opposite polari ty dorninatcs ror-rghly eqr-r ivalcnt parts of

the two corcs-normal polari ty dorninates corc I and reversed

polari ty donrinates core 2. For reasons detai led in the f ir l lowing,

we conclude that the cornplex polari ty recrtrd in thc lacustr ine

l ircies al iscs nlost ly t iom dit l 'erent degrees of cherrical alterat iort

and the acqr"r isi t ion of CRM over i ln cxtendecl t i rne interval that

spannecl geomagnetic reversals. long after deposit ion o1'the moat-

f- i l l sediments. I t is alscl possible that sotre results i trc l i 'ont salr-

plcs takcn 1'rom l l ipped core segrnents.

We have tr ied to refrne the polari ty record by using ntagttet ic

characterist ics to separate severely altered rocks frorn less altered

rocks that rnight retain a rel iablc DRM. Clues to thc paleomag-

net ic re l iab i l i ty o1 ' the sed imentary rocks. exc luc l ing brecc ia and

con-ulomerate. are f irund in relat ions anron.g NRM niagnitude.

bu lk gra in s izc . and po lar i ty . Rela t ive ly h igh NRM nragni tudes(>10 r A/ni) correspond dorninantly to norn.r i t l polari ty Inagneti-

E

0)c)

-co-c)o

1120

1140

1 1 6 0

1 1 8 0

1200

1220

o

o^ O O

Jo

oO

O

a0)oE-o-c)o

-90 -60 -30 0 30 60 90

l n c l i n a t i o n ( d e g r e e s )

Fisurc 10. I ' � lo t o f 'c lcpth against rentanent inc l inat ion f i r r graded tLr l ' l 'Ai n c r l r c | .

rently r-rrrtiltecl Creecle Forntation beds (see Steven, 1961 . Stevenarrd Ratte. 1973: Stevcn and Liprnarr. 1973: Liprnan, this volunie).

Poorly cxposecl and isolated. blocky led-ses of Fisher Dacite

wcre also sarnplcd on Copper Mountain at the s.rme site fron-r

w 'h ich an isotop ica l ly dated san-rp le was obta ined (Lanphere.

199.1: th is l 'o lLr rne) . The f 'k rw at Copper Mctunta in c tver l ies

Clreede Forrnlt t ion sedirnentary bcds at their highest known ele-vation and has a l0Ar/reAr date of 26.30 + 0.0U Ma. The meanpaleornaenctic direct ion. har, ' ins an unusual westerly decl inationanc l re la t ive ly sha lkrw inc l i r ra t ion (31") . is ne i ther c lear ly nr l r rna lnor revcrscd tFig. I -5: si te C9-l-6). Recent slurnping on the r loun-

6 0200

3001 0 0 0

1240

C o r eN

1 2 3 ' I

2R

C o r e 1

oo

oU

oL

o+,oE

+,o.oo

200

4 0 0

6 0

200

3001 0 0 0

1 2 0 0

4 0 0

c 1C 2

c 3

H

G

FED

c

B 400

Lacus t r ine 1 4 o

F l u v i a

1 6 0 0A 500

1 8 0

600200

220

700

Lacus t r i ne

F l u v i a l

FED

Figure I l . Sunrnrary ot ' a l l palcor lagnet ic resLr l ts lhrrncores I and 2 af ier a l ternat ing t le ld and therrnal dentag-net izat ion. Sol id c i rc les reprcscnt norntal (N) ur rcversed(R) polar i ty nragnet izat ions. p lot ted in colunrns that rep-resen t qua l i t y o f demagne t i za t ion pa th -1 . 2 . o r 3 ( sce

text) . Ash-t i r l l tutT beds are dcnoted by let ters (A-K)t

C l . C2 . and C3 rep rescn t cong lon te ra tes 1 . 2 . anc l 3 ,rcspect ively. Depth is in nteters ( i ta l ics) and I 'eet .c

1 2 3Q u

1 2 3l i t ya

A s hF l o wTu f f

2 3i t yQ u a l

8 8 R. L. Ravnolds at ul.

MS (xl o-3 St vol . )5 1 0 1 5

5 1 0

NRM (x10-3 A/m)

Stro n gA i l da ta (NRM >

N R N

sam p le0 . 0 1 A / m )

R

FigLrre l l . Vrr iat iot ts in t r tagnet ic suscept ib i l -

i t r t MS ) l tnd ni t lu l ' ; . t l rc t t l l . t l tcnl ml tgt tet iz l t t ion(NRM) in tL r l ' l ' bcd (F tL r f f ) . and 111 s l l c l l r s i t l - u

sandstone and s i l ts tone in corc L Changes in

po la r i t l ' (N . no rn ta l : R . r c rc rsed) \ \ ' i t h in tu f f

and e 'nclosing bcds i l lLrstrate cot t tp l icat iot ts i r t

cstabl i sh i ng unantbi -s i t tot r s pol ar i ty ' s t rat i s t ' ; - t -

phy . Dcp th i s i n rne te rs ( i t a l i cs )and f ' ee t .

o)c)

!c(!aooE

o-o)o

175

580

180

siltstone

F tuff

sandstone

siltstone

za t i ons (Rcyno lds c t a l . . 199 -1 . Tab le l ) . Nea r l y a l l spec i t r l ens

having high-rnagnitucle NRMs f iorn the low-MS z-t lne hltve rela-

t ive ly largc bu lk sed iment gra in s ize. typ ica l ly larger than f ine

sand (Reynolds e t a l . . 199.1. Table l ) . An example o f these re la-

t ions inc ludes an ash- t i r l l tu f f bed anc l immediate ly unc ler ly ing

f ine- to medi r - r rn-gra inec l sandstones ( tu f f G in cgre l . aboLr t

114_n154 rn dcpth: F ig . l3 ) In th is case. the under ly ing sandstone

has a strong nctrmal polari ty ntagnetization. The coarse-grained

lower part of the graded tuff also has ngrmal polari ty ancl higher

NRM n-ragnitude than does the interior part o1'the tLrff . Sarrlples

frorn this interval having relat ively strong magnetizations contain

rel ict detr i tal magneti te. A further petro-uraphic test that a rel iable

DRM was re ta inec l by det r i ta l rnagnet i te in re la t ive ly c t larse

-grained beds was carr iecl oLlt On sandstone specitnct.ts f l 'oln the

uppcr 3(X) nt in cgrc 2. Eight qual i ty I and 2 resr-r l ts in this inter-

vi i l have NRM rnagnitucles >10 r A/rn, and of these. seven have

norrnal polari t l , ' . Petrographic observit t ions revealed cletr i tal Fe-

Ti oxide rninerals in each specinten and abundant rel ict Ini. tgneti te

ir-r sorne specirnens.

These observations sLlggest that detr i tal rnagneti te in coltrset '

grained rocks wi. ls l l tore l ikely to have sttrvived sLrl f ldizati t ln than

in f ine-grained rocks. Coarse-grained beds ct lnseqLlently are lr lore

l ikely to yicld a rel iable polari ty record. This observi, l t ion is pr.t t ' :" t-

doxical. because t ine-grained sedirnents. in the absence t l f i t l ter-

a t ion. typ ica l ly g ivc tnore re l iab le pa leomagnet ic resu l ts than

coitrse-grained sedintents. A corrtbination of small part icles and

the genera l ly qu ie t -water cnv i ronment that character izc n lud-

stone s. l i t r example. is mttre conducive to eff lcient rnagnetic al ign-

ntent than is the case fitr lar-ger grains that are tclund in siindstones

that rnay have bcen deposited in hi-gh-enel 'gy erlvirontttcnts. In the

hi,Uhly altered Creede Formation. httwever, we sLlggest that srnall

cletr i tal rnagneti te grains in f ine-grainecl rocks were completely

re Inoved. or nearly so. by sult ldization. Very snlal l ct lncentr i t t ions

of secondar)' ma-gnetic minerals probably accoLtllt filr the meilsLlr-

ab le , br - r t weak. CRM magnet izat i t tns . In cont rast . rnany large

rnagneti tc grains in the coarse-grained nrks had str l l lc ie l l t volt t t l ' tc

to wi ths tand conrp le te d isso l r - r t ion. Thei r i l r i t ia l a l ign l t icn t in thc

seolnugnetic f leld t t iay have been ir lcff lcient hecaltse t l l ' their sizc.

but i t was presLtnrably was etl lcietrt etrough to rcct lrd thc pt l l i t r i t ,r '

in rnany samples. The suggest ior l that so lne s i lndstone in thc

Creede Formatittn adcqr-rately recclrcled a DRM ptllarity raiscs tltc

problcni of dri l l in-g irrdr,rcecl rettragtlet izi t t i t l t l via IRM. I l" t t t l l t l . tv

sandstone satnples. sLrch IRM is expressed as i t Very sl.rong attcl

steep posit ive incl ination NRM direct i t ln. Neverthelcss. 1'rcrr" l tsivc

d l ' i l l i l lg - indt recd l 'c l l l i t !1nct i / ' l t t io t t t r t cxp la i r l lhc t lo t t t i t t l l l l ce ( ) l ' l l ( )1"

r la l po lar i ty r r ia-ur te t izat ior - rs is t rn l ike ly [ - rccLt t tse ( l ) ncar l ) ' a l l

breccia and conglolnerate becls. which ct lntait l relat ivcly coLl l 'sc

grained rnagneti te. arc r l t t tdt l l r ly r l lagnetized: arld (2) rt l t lst t t t tr-

rna l po lar i ty c l i rcc t ions s tab i l izc a f ier l t lw AF dct l lagnet iz r t l io l t

with rnoclcrate incl inati t t trs ir l t l ie r l l l lgc o1"10"-70". rci lsol. t l lblc

va lues f i t r the erpected Ol igocene inc l inat i t ln (ahtx t t -56") l t1 thc

sarr ip l ing la t i tude (Reynolds e t a l . . 199; l ) .

Tl ' re role of f-err irnagnetic l 'err ian i l rrer-r i te, with ct lr t tposi l iotts

in the self-reversing range-. is urrecrtuin as a cl luse f irr solt ie i l l l l ) l l r-

ent reversa ls . L ike magnet i te b t r t w i th a s tna l ler l l l i lg l lc t i f

l lo lnent . i t coLr ld a l ign as an ind iv idua l par t ic lc in thc antb ic t t t

t leld cl irect i t tn and rccorcl a DRM. There is otre cornbinatiott ol '

cleposit ional ancl chcrnical lactors iurcl evettts. ht lwcver. that coLtl t l

have leci to l t lnirsnetization carr iecl bv cletr i tal l 'err ian i l r l icnitc

oppos i te to that o1 ' thc geol r lag l le t ic l le lc l a t the t i t . t l c o l 'dc1 ' rost -

t ion: ( l ) f 'en ' ian i l r len i tc occurred in snta l l (sat ic l s iz .c t l r s t r l l t l l c t ' )

igneous rock fragrlents i t long with clt l tr l inant nlagneti te: (2) t l lc

or ig ina l TRM c l i rec t ions o l ' the magnet i te anc l l ' c r r iar l i l t t l c l t i tc

were opposite: (3) the fragments wcre al i-ened along the tt lagttcl ic

frelcl dircct ion upon deposit ion accorcl irrg to the tnagttet izat iott ol

the rrtagneti te: and" (: l) sulf ldiz. i t t i t ln retr l t lved al l the tt lLtgttct i tc '

leaving a renulnence carr iecl by I 'err iarr i l rnenitc in thc cl ircct iort

oppos i te to the appl ied f le lc l d i rec t ion a t the t in tc o l 'c lcpos i t io r l . I t t

niagnctic tr t ineral sepi lr i l tes. wc have t lbsen'cd rt lck clasts that

conta i t i i l t rer " r i te . presut t lab ly thc f 'c r r i l l lagnct ic var ie ty . l t t l t l

C o r e 1

Core 2N t R

1 2 3 ' 1 2 3

N

c)0)

oU

a

o

c)E

-c

o-q)

o

2 0 0

100

4 0 0

6 0 0

200

8 0 0

3001 0 0 0

c 1c 2

c 3

H

2 0 0

100

4 0 0

6 0 0

200

8 0 0

400

1400

F

ED

Figure 13. Summary of paleomagneticresults fiom cores I and 2 fbr stnrngly mag-net ized (natur r l remanent magnet izat ionmagnitude >10 2 Aim) specimens afieralternating freld and thermal demagnetiza-t ion. Sol id circles represent normal (N) orreversed (R) polari ty, plotted in columnsthat represent qual i ty of demagnetizationpath-l ,2, ot '3 (see text). Letter symbols asin Figure I L Depth is in meters ( i tal ics) andfeet.

Q u a l i t y

Q u a l i t y

90 I l . L. Reyrrold,s el ul.

Fisher Dacite

ce4-1 [6] ce4-2 t51w"dLilj.;:rb9*l;,Ti

+ * Snowshoec94:6 [8]Copper Mountain

N (up) N (up)

'10040

E (Horizontal)

mTNRM

S (Down)

FisLrt 'c l-1. , \ : [ :r l tut l-urcu plo. jcct iott of ' r 'crt .rrrr. tclr t nrLrgnc't izut ion cl irec-t ions t ir t ' s1-rccit t tcns lhrnr ush-l l l l tul ' f 'ut si tc Cl-A- I aftcr altcrnating f iclcldcnt r . tgnct izut ion (so l i i l squarcs) . N lcan d i rect ion (so l id d iantonc l ) is sur -rounrlct l br ' 9-5r,, corrc ol 'contlclcncc: r is crpcctcd ntocleru dipole f ieldt l i rcct iort: + is prcscnt l lc lr l cl i rcct ion. B: Orthogonul vector cl iagranr ofA l r r lc tnuunct izut io t t bc l tuv ior (s tcps in nrT. d iv is ions are in A/nt ) . Syrn-bt t ls us in I r igu lc - i .

gru t r r ts o t 'a l tc rec l t i tanonr lgnct i tc in which a l l nragnet i tc rc l ic tsapparent lv l tavc becn rcr t rovec l . Th is sccnar io is thus p laus ib le onthc basis of a'uai lablc pett l)-gralthic ancl therrnorlragnctic analyscs.

Thc or ig in o f thc in l 'c r rcc l secondary n t i " lg l te t i tc o t 'n ras l t cn t i t c t ha t i s r cspons ib l e t o r t hc i n t c rp re tec l CRM in t heIow-MS zor . rc is unccr la in . The presencc of n tagnet ic i ron su l f rdenrinerals. pyrrhoti tc ancl grcigitc. has not been suggcstccl by thcr-rna l c lcnursnct izat ion rcsu l ts r t r by thcr rnont i t -unet ic and p le t ro-g laphic anal l ,s is o f ' rnagnct ic anc l cherr i ica l scparates. Hcntat i tc iss inr i la r ly prcc luc lcc l us a u ' ic lcsprrcac l car r ier o f CRM. The inc ip i -

Wagon Wheel Gaplava dome

c94-1-2E (Horizontal)

8060

5 m T

S (Down)Figure l-5. A: Eclr-url-area project iort ot 'renrancnt nt i t-r lnetiz.at iort direc-t ions surroundcd by 9-57 cones o1'conl ldence t irr si tes f l-orn postcalclcralava l lows of Fisher Dacite. Number in brackets aftcr si tc clcsignation isnr.rnrber o1'speci l lens uscd in calcLrlat ion ol 'si te mean. B: Representa-t ive orthosonal vector- cl iagranr showing alternating l icld de nrrrgnetizu-t ion behav ior (s teps in mT) o l -spcc inrcn lhrnr s i tc Cc) ; l -1 . I ) i r is ion is inA/rn. Syrnbols as in [ j i - ture 3.

cnt oxidation of pyri te may have prodr-rced secondary iron oxidcs.

Petrographic exarnination of sonre sarnples indicatecl the pres-

ence of 'snta l l reg ic lns (genera l ly <L l p tn t in any d in ter ts ion) in

which pyri te has been replacecl by a grayish inrn oxide. These

areas i r re too smal l to conf l rnr the presence of nragnct i tc or

maghemite by optical propert ies" but are largc enough to discctuttt

the presence of hematite . SLrch iron oxide rr i ight be responsible

firr the inl'errecl CRM.

W

RM

Rc cr t,q rr i t i r t n r tf' p r i ttt tt ry u rttl tl i u ge uct i c ttttt gne t i : ut i ort s, Oli.gocene C reetle culde ru, Colorudo 9 l

Hystercs is nreasLrre tnents were rnade on n ine spec imens.which were sclectecl on the basis of pct lari ty and strength of mag-nctization. in an attcrnpt to characterize the domain states of therna-unetic rnincmls. l t was thor-rght that f ine-grained minerals. per-haps CRM-carrying inrn oxides converted frorn pyri te and havingrelat ively t 'cw rnagnetic dornains. wcluld be dist inct from lar_eer,perhaps rnult iclomain. detr i tal part icles. Hysteresis rat ios (Fig. l6)f l 'onr weaklv rnagnetized samples (both nclrmal and reversedpolar i ty ) are i r r the midd le o f the pseudos ing le-dorna in f ie ld .whcreas the s t rongly magnet ized samples (a l l normal po lar i ty )

are ckrser to. or alrnost in. the mult idontain f leld. This dist inct ion.howevcr. docs not confirnt the possibi l i ty that the weakly magne-t ized rocks arc dorn inated by CRM components . The poorknowlcclge of hysteresis propert ies fbr al l potential ly importarrtrninerals. sr-rch as detr i tal f 'err ian i lntenite. prevents a cclmpleteinterpretat ion. Nevertheless. the resr,r l ts are consistent with weak

CRM carr ied by srna l l pseudos ing le-dorna in s izes re la t ive tocletr i tal rnagneti te that carr ies a normal DRM.

Comparison of paleomagnetic results from the CreedeFonnatiort to the geomagnetic polarity time scule

Thc t'ew reliable paleontagnetic results frorn the Creede For-mation are of nornral polarity. The normal polarity magnetizationfiorn graded tuffA in the krwer parts of both cores and fiom the ash-fall tutl'bed in oLrtcrop in the upper pafi of the Creede Fonnationprcvide the nrost reliable polarity data obtained. Mclst results fiornstronuly nragneti/.ed saniples frtrl the lacustrine fircies also indicatea nornral polari ty carr ied by detr i tal rnagneti te that has survivedextensive sr.rlfidization. The nornral polarity r-nagnetizations of theSnowshm Mountain Tufi'and of the f-ew sites in the Fisher Daciteprovide addit ional nraglretostr l t igraphic cclnstraints. None of thenLl rneroLr s reversecl clirections i nterspersed with nonnal directions i nthe krw-MS zone of extensive sulfrdic alterat ion can be demon-strated to bc :-r prirnary DRM. We conclude that the entire CreedeFonnution was cleposited during a nomal polarity chron.

Isotopic clating of thc volcanic rocks that bracket the CreedeFornration sLlggests that caldera filling occured over an interval ofno nrore than about 600-700 k.y. The normal polarity rnagnetiza-t ions of the Creeclc Frlrntat ion and the caldera-t i l l ing extrusiveunits arc consistent with the datin-e results. For each clf the threeGPTSs shown in F ig . 2 . the longest normal po lar i ty in terva lbetween 26 ancl 27 Ma is <600 k.y. duration. Considered together.the results frr lrn paleonti lgnetisrn and isotopic dating al lctw thepossibi l i ty that sedir lentat ion in the Creede caldera moat occurredover an interval as shctrt as about 300 k.y.

These conibincd palerlrnagnetic and geochronologic resultscan bc used to re f r r re the GPTS (F i_s.2) . The in terva l betweenabout 26.9 ancl 26.6 Ma (thc ages of the Snowshoe Mountain Tlrf fand the Fishcr f low at Wagon Wheel Gap) rnust be an interval ofnormal po lar i ty . Th is conc lus ion is s t rengthened by resu l ts o fother ash-fkrw tuffs ot ' thc central San Juan volcanic freld. Threesr-rch tuffs (frorn cl lclest to youngest. the Rat Creek. CebollaCreek. ancl Nelson Mcluntain tr.rtfs). that are inf-erred ctn the basis

0.6

0 .5

0.4

0 .3

0 .2

0 . 1

0 .0

B". /B.Figurc 16. Plot of hysteresis rat ios (see Day et al. . 1977). Sol id tr ianglesdenote weak-nragnetization. nurmal polari ty samples: invertcd. open tr i-angles denote weak-rragnetrz.rt ion. r 'eversed polari ty samplest sol id cir-cles denote strong-nragnetization. norr-nal polari ty samples. Mr,.saturation remanent magnetization: Mr satur; l t ion nragnctization: B.,.coercivity of remanence: B., coercive fbrce. Magnetic dornairr areus: SD.single domain: PSD. pseLrdosingle donrain. MD. rnLrl t idorttain.

of geologic relat ions to predate the Snowshoe Mountain Tr,rf f but

are analyt ical ly indist inguishable by att{17rvAr dating, also have

normal magnet izat ions (see L ipman, th is vo lurne: Rc lsenbaum

et a l . . 1987 ) . The21 . l Ma Wason Pa rk Tu f f , an ash - f l ow tu f f

direct ly underlying the Rat Creek Tuff. is reversely rnagnetized(T i rnaka and Kono, 1913. Beck et a l . . l9 l7) . These resu l ts ind i -

cate a reversed tcl nonnal polarity bor-rndary cklse to 27 Ma.

SUMMARY

In both Creede caldera moat cores. volcanic breccia. graded

tr-rf fs. and f luvial deposits having high rnagnetic susceptibi l i ty

1MS: t yp i ca l l y >10 - r vo lume S I ) a re ove r l a i n by mos t l y f i ne -

gra ined lacust r ine beds and tu f fs hav ing senera l ly lower MS(typ ica l ly <2. -5 x l0 i ) . The t rans i t ion between h igh and low MS

occurs over an interval of <30 m and corresponds more closely

to a geochemica l boundary than to a fac ies chan-ue. The geo-

chemica l boundary separates a deep zone wi th sparse pyr i te

f iom a shal low sulf ldized zone with local ly abundant pyri te. In

the sulfrdized zone. much of the pyri te furrned at the expense of

de t r i t a l magne t i t e .

This sLrl f- idic alterat ion, combined with apparent acquisit ion

of CRM components. has obscured the rnagnetic polari ty strut ig-

raphy. The measured paleomagnetic incl inaticlns, rnost having

rnoderate angles expected for the age and paleolatitude. seern tcr

arise fiom the presence of both primary and secondary remanent

o

EoL

=

654320

SD

PSD

- $

A 9n

o

o

M P ,

9 l R. 1,. Ret'rroltls et ul.

rrr lgrrct izat ions. Rocks that contain detr i tal ntagneti te rnay have irdonr i runt DRM. but nrcks in rvh ich n tost o l 'a l l such rnasnet i tewas clcstroyed ntay havc a dontinant CRM. Thernral dernagneti-zut ion and s tuc l ics o f rnLrsnet ic minera logy sL lggcst that theinl 'cr lccl CRM is carr iccl by rnagneti tc and/or rnagherri te. Suchrnincrals rnay hai, 'e f i rrr led r, ' ia incipient oxidation of pyri te. Pet-rr luruphic exarnination has not clearly revealecl such CRM carri-e r s . w 'h i ch . i l ' t hey ' cx i s t as pos tu la ted . r nus t occu r i n ve ry l owconccnt r i l t ions (<0.003 wt%). basec l on NRM ntagni tuc le .

Wc l-ral 'e l i runcl no convincing evidence f irr a trLlc polari tyrevcrsul within the Crccclc Formution. When cclmpared to dif l 'er-cnt sconrasnet ic po lar i ty t in rc sca les. the combined resu l ts o fpraleorttarrrct isrn and isotopic dating of thc volcanic rclcks brack-ct ins thc Crccclc Forrnation sltggest that sedirnentation occurredovcr iul interl 'al o1'no ntorc than about 600-7(X) k.y. and perhaps

as lcw as about 3(X) k .v .

ACKNOWI,EDGNIENTS

Wc arc gratc lu l to Phi l Bethke. Petc L iprnan. Tr t rn Steven,lur tc l Wayrrc Carnphel l l i r r the i r suppor t in ln i - rny ways. toD. Lurscn. D. Finklestein. D. Yager. and P. Nclson fbr usefir l dis-cussions. and to M. Hr"rclson. S. Harlan. and D. Yager l i tr excel lenttcchrr ica l rc ' , ' i cws o l ' th is ar t ic lc . We apprec ia te the or - r ts tandingtcchrr ica l uss is tancc of F . Gay. J . Hulcn. D. Hayba. A. Hopk ins,R. St reLr t 'e r t . W. Rivcrs . J . Kahn. and P. F i tz rnaur ice. We a lsogratcl ir l ly acknowlecl.uc thc use of ' the vibrat ing sarnple rnugne-tonrc ter a t t l rc Ins t i tu te l i l r Rock Masnet is r r i . which is lLrnded bythe W.M. Kcck For- rndat ion. the Nat iona l Sc ience Fr l r " rndat i< tn(NSF) . anc l t hc Un i ve rs i t y o f M inneso ta . Th i s wo rk was sL rp -por tec l by the U.S. Geolog ica l Survey anc l NSF grant EAR-91-15962 ( to VcrosLrb) .

RF] I.' I' R IiN C I'S C IT ET)

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l i t r thc S i t t t . lu i t t t vo le l t t t i c l l c l c l o l ' sou the rn ( 'o lo rar lo : [ ru r t l t und P lu r rc ' ta rv

S c i c n c c l - c t t c r s . r . - 3 7 . p . l l l l - 1 0 .

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i n g s o l . t h c V l l t h I n t c r n u t i o n a l S y r r r p o s i u r n r l n t h e O b s c r v a t i o n o l ' t l r c C o n -

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L ' r t i o r t ) . r ' . ( r f i . p . 1 7 7 . l t t T - 1 u 9 .

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rcservo i r ' ' o l ' thc ( ' rccc lc

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