triassic rift-related sedimentary basins in northern chile (24°–29°s)_3

8/13/2019 Triassic rift-related sedimentary basins in northern Chile (2429S)_3

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Journal of South American Earth Sciences Vol. 6, No. 3, pp. 109-121, 1992Printed in Great Britain0895-9811/92 $5.00+.00 1993PergamonPress Ltd& E arth Sciences& Resources nstitute

Triass ic r i f t -related sedim entar y bas ins in northe rn Chi le 24-29S)M. SUAREZ1 and C. M. BELL2

1Servicio Nation al de G eologia y Minerla. Avda. Sta. Maria 0104, Santiago, Chile2Cheltenham & GloucesterCollege of Higher Education. Cheltenham,Glou., GLS0 3PP, England, UK(R ece i ved D ecem b er 1 9 91 , R ev i s io n A ccep ted N o vem b er 1 9 9 2 )

Abst ract - -Tr iass i c r o c k s i n n o r t h e r n C h i l e ( la t i tu d e 2 4 - 2 9 S ) i n c l u d e m a r i n e a n d c o n t i n e n t a l r i ft - r el a t e d s e d i m e n t a r y d e p o s i t s ,a s s o c i a t e d w i t h b a s a l t i c , a n d e s i t i c , a n d s i l i c ic v o l c a n i c r o c k s . F i v e m a i n s u c c e s s i o n s i n c l u d e t h e d e p o s i t s o f t w o m a r i n e b a s i n s( P r o f e t a a n d S a n F ~ l i x ) s e p a r a t e d b y t h r e e c o n t i n e n t a l b a s i n s ( C i f u n c h o , L a T e m e r a , a n d L a C o i p a ). T h e m a r i n e s t r a t a i n c l u d e t u r-b i d i t e s a n d d e b r i s f l o w d e p o s i t s i n t e r p r e t e d a s c o a r s e - g r a i n e d f a n - d e l t a s e d i m e n t s . T h e c o n t i n e n t a l s e d i m e n t s i n c l u d e la c u s t r i n ef a n d e l t a , o p e n l a k e , b r a i d e d r i v er , al l u v i a l f a n , a n d s a b k h a d e p o s i t s . T h e w i d e s p r e a d f a n - d e l t a s y s t e m s ( b o t h m a r i n e a n d l a c u s -t r i n e ) , t o g e t h e r w i t h a b r u p t l a t e ra l a n d v e r t i c a l f a c i e s d i s c o n t i n u i t i e s a n d l a r g e - s c a l e d e p o s i t i o n a l c y c l e s , a r e i n d i c a t i v e o f r i f t-c o n t r o l l e d s e d i m e n t a t i o n . T h e a s s o c i a t e d m a g m a t i c a c t iv i t y i n d i c a t es t h a t t h i s ri f l in g w a s t h e p r o d u c t o f s u b d u c t i o n - r e l a te d e x t e n -s i o n o r s t r i k e - s l ip m o v e m e n t o n t h e a c t i v e p l a t e m a r g i n . T r i a s s i c r i f ti n g w a s f o l l o w e d i n L a t e T r i a s s ic t o E a r l y J u r a s s i c t im e s b yw i d e s p r e a d t h e r m o t e c t o n i c s u b s i d e n c e .R e s u m e n D R o c a s t r i ~ i c a s e n e l n o r t e d e C h i l e (l a t i t u d e s 2 4 - 2 9 S ) i n c l u y e n d e p 6 s i t o s s e d i m e n t a r i o s m a r i n o s y c o n t i n e n t a l e s ,r e l a c i on a do s a t e c t on i s m o e x t e ns i o na l ( r i f t ) , y a s oc i a d os a roc a s vo lc ~ ln i ea s ba s d l t i c a s , a nde s i t i c a s y s i l i c e a s . S e ha n r e c on oc i doc i n c o s u c e s i o n e s s e d i m e n t a r i a s p r in c i p a l e s q u e i n c l u y e n l o s d e p 6 s i t o s d e d o s c u e n c a s m a r i n a s ( P r o f e t a y S a n F 6 1i x) s e p a r a d a s p o rt r e s c u e n c a s c o n t i n e n t a l e s ( C i f u n c h o , L a T e r n e r a y L a C o i p a ) . L o s e s t r a t o s m a r i n o s i n e l u y e n d e p 6 s i t o s d e t u r b i d i t a s y f l u j o s d ed e t r i to s i n t e r p r e t a d o s c o m o s e d i m e n t o s d e d e l t a s d e a b a n i c o d e g r a n o g r u e s o . L o s s e d i m e n t o s c o n t i n e n m l c s i n c l u y e n d e p 6 s i t n s d ed e l t a d e a b a n i c o l a c u s t r e , l a g o a b i e r to , r f o s t r e n z a d o s , a b a n i c o a l u v i a l y s a b k h a . L a a m p l i a d i s t r ib u c i 6 n d e s i s t e m a s d e d e l t a s d ea b a n i c o ( t a n t o m a r i n o s y l a c u s t r e s ), j u n t o a l a e x i s t e n c i a d e a b r u p t a s d i s c o n t i n u i d a d e s l a t e ra l e s y v e r t i c a le s d e f a c i e s y l a p r e s e n c i ad e c i c l o s d e p o s i t a c i o n a l e s m a y o r e s , e s t a r f a n i n d i c a n d o s e d i m e n t a c i 6 n c o n t r o l a d a p o t r i f t. L a a c t iv i d a d m a g r n d t i c a a s o c i a d ai n d i c a rf a q u e e s te r i f t in g h a b r f a s i d o p r o d u c t o d e t e c t o n i s m o e x t e n s i o n a l o d e r u m b o a a n c i a d o a s u b d u c c i 6 n e n u n m a r g e n a e t i v od e d e p l a t a . D u r a n t e e l T r i d s i c o S u p e r i o r - J u r d s i c o I n fe r i o r, e l r i f t i n g t r i d s ic o f u e s e g u i d o d e s u b s i d e n c i a t e r m o t e c t 6 n i c a d ea m p l i a d i s t r ib u c i 6 n .

I N T R O D U C T I O NTRIASSIC STRATA in C hi le a re represented by num erousgeog raphica lly isolated outcrops (Charrier, 1979). M ost ofw h a t l i t t le s t u d y t h e y h a v e r e c e i v e d h a s b e e n d u r i n gregiona l geolo gica l mapp ing projec ts . Hence , the i r s trat i-graph y, sed lmen tolog y, and tec tonic si~,nificance are sti llpoor ly know n. This a r t ic le summar izes cur rent work doneby the authors in nor thern and cent ra l Chi le (26 -29 S,Fig. 1), r eview s publ i shed work, and proposes pa leogeo-graph ic and tectonic interpretations of the Triassic deposi-t ioual systems.

Th e Triassic geologica l evo lution o f Chile is diff icult toreconst ruc t in de ta i l due to lack of chronologic da ta and tothe un c e r t a in t e c ton i c r e l a ti onsh ips o f t he d i f f e r e n t ou t -crops. G uide mac rofossils are rare, and l i t tle micropaleo n-to log ic a l wor k ha s ye t be e n d one . E x te ns ive f o ld ing a ndth r us t i ng p r oduc e d subs t a n t i a l shor t e n ing . Ro ta t i on o fcontinen tal blocks an d strike-sl ip displacem ents hav e beenpropo sed for Tr iass ic s t ra ta wi thin the Coasta l Cordi l le raat lat i tudes 25*40' and 30*30' (Forsythe e t a l . , 1987) . Theinte rpre ta tions give n in this a r t ic le a re based o n present -

day locations o f the northerly Triassic outcrops, wh ich arenot ne cessar i ly where they w ere during the Triassic.

G E O L O G I C S E T T I N GRi f t ba sins , m a ny w i th a s soc i a t e d m a gm a t i sm , we r e

f o r m e d i n S o u t h A m e r i c a d u r i n g w i d e s p r e a d T r i a s s i cextensiona l tec tonism tha t preceded the breakup o f Gond -wa na (Char r ie r , 1979; St ipan ic ic , 1983; Su~irez e t a l . ,1985; Dalziel e t a l . , 1987; Ul iana and Biddle , 1988; Kok o-gian and M anci l la, 1989; Ram os an d Kay, 1991; Su/t rezand Bell, 1991a,b). M ost Triassic sy n-rift deposits in Ch ilea r e c o n t i n e n t a l . I n m a n y p l a c e s t h e y a r e o v e r l a i n b ymarine se dimen ts (Cho ng an d I-I il lebrandt, 1985; Bell andSu~rez, 1991b). By contrast , the Triassic strata of Argen -tina a re entirely nonm arine (Stipanicic, 1983).

M a r ine se d im e nt s ove r ly ing t he T r i a s s i c c on t ine n t a ls trata were deposi ted by a diachrono us t ransgress ion tha tc ove r e d l a r ge a re a s o f Chi l e dur ing t he M idd le a nd pe r -ha ps E a r ly T r i a s s i c ( Ce c ion i a nd We s t e r m a nn , 1968 ;Reut te r , 1974; Mundaca e t a l . , 1979; Bell an d Su/trez, inAddress all correspondenceand reprint requests o Dr. Man uel Su~ ez: fax [56] (2) 737-2026.

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110 M. SUAREZ and C. M. BELL

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L o c o l i te s m e n t i o n e d in t h e t e x t( ~ C e r ro Pascu a(~ Cerro R i r lcones( ~ C e r r o B a y o(~ Cerro Ch inch i lla(~ Quebrada C i funcho(~ Q u e b ra d a d e l Sa l i t r e(~ ) Quebrada Do~a Ine 's Ch ic( ](~ ) Quebrodo Marance les and QuebradaPastas La rgosm(~ Q u e b ra d a L a Pe lod a(~ Q u e b ra d a E l T a ro(~) Quebroda de l Carbo 'n(~ Quebrado Po ipo te

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press) , the La te Tr iass ic (Corva l (m, 1976; Moscoso andCov acevich , 1982; Ch ang and I-Ii l lebrandt, 1982; Bell andSu/ t re z , i n p r e s s ) , a nd t he E a r ly Ju r a s s i c ( N a r a n jo a ndPuig, 1984; Bel l and Su4rez , in press) . Tr iass ic s tra ta incent ra l and nor thern Chi le a re p redominant ly e las tic sedi-men tary and volcanic rocks , wi th loca l l imestone interca-l a t i ons . T he y a r e c ha r a c t e r i z e d by l a t e r a l a nd ve r t i c a lf a c i e s c ha nge s , one r e su l t o f whic h i s a p r o l i f e r a t ion o fstrat igraphic nom enclatu re. At mo st locali ties the Triassicsuccess ions uncon formably ov er l ie u pper P a leozoic s i licicvo l c a n i c r oc ks a nd g r a n i to ids ( Se p d lve da a nd Na r a n jo ,1982; Mercado , 1982; Naranjo and Puig, 1984), equiva-l e n t to t h e C h o i y o i G r o u p o f A r g e n t i n a ( C o i r a an dKouldaarsky, 1976). A long the C oasta l Cordi l le ra , how -e ve r , t he y unc on f or m a bly ove r l i e Pa l eoz o i c se d im e nta r ya nd m e ta s~ im e nta r y s t ra ta , som e o f t he m be long ing t o asubduct ion complex o f the same ag e (Bell , 1984).

M JOR P LEOGEOGR PHIC DOM INS

The Tr iass ic rocks in nor thern Chi le (be tween 24 S and29 S) inc lud e ma r ine and cont inen ta l deposi t s assoc ia tedwi th basa l t ic , andesi t ic , and s i l ic ic volcanic rocks . F ivedepo sit ional basins have b een reco gnize d (Table 1, Fig. 2) .Th e tw o marin e basins (Profeta and San F,61ix) we re ei therse pa r a te d by a NW - t re nd ing c on t ine n t a l r i dge wh e r e t heL a C oipa Be ds a nd t he C i func ho and L a T e m e r a f o r m a -

tions ac cum ulated or al ternatively, they w ere conn ected bya w aterway be tw een two cont inenta l a reas.

P r o f e ta M a r i n e B a s i n

A Late Tr iassic to Neocom ian mar ine bas in deve lop edbetw een la ti tudes 24S and 2615 'S (Fig. 2; Ch ang a ndHi l le br a nd t, 1985 ; Bogda n ic a nd Cha ng , 1985 ; Fue uz a -l ida, 1986 ; Bell and S u~'ez, in press; M arinov ic e t a l . i npr e p .) . T he L a t e T r i a s s ic t o K im m e r idg i a n se d im e nta r yin f il l o f t h i s ba s in i s g r oupe d in t he P r of e t a For m a t ion( M a r i n o v i c e t a l . i n p r e p . , a f t e r Cha n g , 1973) , wh ic hc om pr i se s a l a r ge ly c l a st i c m e m be r ( i nc lud ing t he Qne -br a da de l Sa l i tr e For m a t ion o f N a r a n jo a nd Pu ig , 1984)that overl ies and laterally interfmgers with a largely calc-a r e o u s m e m b e r ( in c l u d i n g t h e P r o f e t a F o r m a t i o n o fC h a n g , 1 97 3). T h e P r o f e t a F o r m a t i o n u n c o n f o r m a b l yover l ies Pa leozoic volcanic and plutonic rocks (La TablaFor m a t ion) c or r e l a te d w i th t he Choiyo i Gr oup o f Ca i r nand Ko-kharsk3' (1976).

The e las tic me mb er of the Profe ta Fo~axation, expo sedi n C o r d i ll e ra D o m e y k o b e t w e e n C e r ro R i n c o n e s a n dCer ro Chinchi l la (Fig. 1) , i s about 350 to 500 m thick(Fuenza l ida , 1986, 1987; Mar inovic e t a l . i n prep. ) . I tinc ludes a basa l uni t of mat r ix- suppor ted conglom era teswith granit ic clasts, w ith interbedd ed acid tuf ts and l ime-s tone s w i th c he r t nodu le s a nd m a r ine f os s il s , i nc lud ingr e wo r ke d c or a ls . T he c ong lom e r a t e s a r e i n t e r p r e t e d a sdebris-flow deposits a nd th e l imestone s as she lf deposits.T h i s s e d i m e n t a r y s u c c e s s i o n i s o v e r l a i n b y m e d i u m -g r a i n e d t u r b i d i t e s w i t h i n t e r c a l a t e d l i m e s t o n e s , a n dmatr ix-supported debris-flow cong lomerates. These are, intu r n . ove r l a in by c l a s t - suppor t e d c ong lom e r a t e s , s a nd-s t ( r u e s , a n d l i m e s t o n e s w i t h a b u n d a n t a m m o n i t e s .b iva lve s , a nd ga s tr opods . T h e l im e s tone s i nc lude c or a l


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~o4. JOz~ o~ _o ~b 100 m thick) , expo sed bynew eng ineer ing works in the a rea of the La Coipa m ine ,c om p r i se s a t h i c k suc c e ss ion o f fi ne ly l a m ina t e d b l a c kshales . T hese lacust r ine sediments were deposi ted f romsuspension an d as thin, fine- to m edin m- grain ed turbiditesin a n ope n- l a ke e nv i r onm e nt . T he y a r e a s soc i a t e d w i ththin lake-margin on colitic limestones.

The second fac ies assoc ia tion (>200 m thick) expo sedin the Quebrada L a Pelada (Su~rez and Bell , 1991b) and inQue br a da s M a r a nc e l e s a nd Pa s tos L a r gos ( Z a m or a a ndVila, 1991), com prises repeated successions of coarse- an dfine-grained clast ic sediments. Sedim entary structures arei n d i c a ti v e o f a w i d e r a n g e o f d e p o s i t i o n a l p r o c e s s e s ,including suspension, turbidity currents, streamflow , andmass f low. Deposition probably occu rred in the subaquaticpa r t o f l a c us t r i ne f a n de l t a s ( Su~r e z a nd B e l l , 199 l b ;Nem ec and Steel , 1988).The third facies association (~100 m thick) , expo sed inQnebrada El Toro, com prises crossbe dded sandstones andpe bb ly sa nds tone s , c ong lom e r a t e s , a nd sha l e s p r oba b lydeposited in a braided riv er environment.The fourth facies association, possibly representing theinitial stages of lacustrine sedimentation, comprises a thin(-2 0 m thick) but very distinctive su ccession in an isolatede xposur e t o t he NNE of t he L a Coipa m ine . T he t h in lyl a m i n a t e d r e d m u d s t o n e s a n d f i n e s a n d s to n e s c o n t a i na bunda n t sm a l l gypsum nodule s . De pos i t i on wa s f r omsuspension and ~rbid i ty cu r rents und er evapor i tic condi -tions.

T he w ide va r i e ty o f f a c i e s a nd f a c i e s a s soc i a t i ons ,forming a thick success ion o f lacust r ine and f luvia l sedi -ments , i s indica t ive of an ac t ive ly subsiding bas in. Fan-de l t a suc c e ss ions , dom ina t e d by c l a s t ic de br i s de r ive df r om s i l i c ic m a g m a t i c r oc ks , sugge s t t ha t t he l a ke w a sadjacent to a reas of high re l ie f domina ted by upper Pa leo-zoic vo lcanic rocks and granitoids.

San F g l i x M ar ine Bas inA M iddle a nd poss ib ly L ow e r T r ia s s i c m a r ine ba s in

d e v e l o p e d s o u t h o f t h e c o n t in e n t a l C i f u n c h o a n d L aTem era basins. Outcrops occ ur on the present-da y CoastalCordillera and about 90 km to the east , nea r the vil lages of


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Trias sic rift-related sedimentary basins in northern Chile 117San F~l ix and Alto del Carmen (Fig. 1 . The sed imen taryin f i l l o f t h i s bas in i s rep resen ted in the eas t by the SanF~l ix Format ion (Reut ter , 1974; Ribba, 1985; Schooner1985 ; Bel l and Su~rez , i n p rep . ) and in the wes t by theC a n t o d el A g u a F o r m a t i o n ( M o s c o s o et al. 1 9 8 2 ;M oscos o and Covacev ich , 1982).

San F~lix Formation. The San F~lix Format ion com-p r i s e s a t h i c k s e q u e n c e ( p o s s i b l y o v e r 4 0 0 0 m ) o f c o n -g l o m e r a t e s , s a n d s t o n e s , a n d s h a le s ( R e u t t e r , 1 9 7 4;Schoener, 1985; Ribba, 1985). I t i s unconformably under-l a i n b y P a l e o z o i c b a s e m e n t r o c k s a n d i s c o n f o r m a b l yo v e r l ai n b y 7 0 0 - 1 0 0 0 m o f c o n t in e n t al v o l c a n i c r o c k s o fthe La To to ra Format ion (Reu t t e r , 1974) . The San F~l ixFo r m a t i o n t h i n s r a p i d ly t o w a r d t h e e a s t, f r o m > 4 0 0 0 malong the E1 Carmen River to a fe w hundred m eters in theTr~ns ito R iver , sou th o f La s P i rcas and abou t 15 km to theeast (Fig. 1) . App roxim ately 12 kin farther SSE, at Cord6nAmar i l lo, t he absence o f the fo rmat ion be tween the Pa leo-zo ic basement and the La To to ra Format ion (Ribba , 1985)s u g g e s t s t h e e x i s t e n c e o f a s t e e p , p r o b a b l y f a u l t - c o n -trolled, b asin mar gin in the intervening area.

The San F61 ix Form at ion has b een subd iv ided in to f ivein fo rmal mem bers (M 1-M5, F ig . 5 ; Ribba , 1985) . Mar inef o s s i l s i n t h e l o w e s t f o u r me m b e r s ( M 1 - M4 ) i n d ic a t e a ne a r l y t o m i d d l e A n i s i a n a g e ( M i d d l e T r ia s s ic ; R i b b a ,1985). The form at ion is predominant ly clas t ic , com pris ingmu d s t o n e s , p e b b l y s a n d s t o n e s , s a n d s to n e s , a n d c o n g l o -m e r a t e s d e r i v e d f r o m m e t a s e d i m e n t a r y b a s e m e n t r o c k sand s i l icic mag ma t ic rocks (Ribb a, 1985) equivalent to theupper Pa leozo ic C ho iyo i Group .

T h e M 1 s e c t i o n in Q u e b r a d a T a b a c o i s o v e r 1 0 0 mt h i c k . I t i n c l u d e s i g n i mb r i t e s a t t h e b a s e , d e b r i s - f l o wdepos i t s , s ands tones depos i t ed by tu rb id i ty f lows , tu f t s ,l e n t ic u l a r f i n e l y l a m i n a t e d l i me s t o n e s , a n d s a n d s t o n e s ,wi th foss i l t ree minks in growth posi t ion. I t i s in terpretedas the deposits of a lacustrine (or floodplain) environment.These s t ra ta a re over l a in by coarse and th i ck cong lomer-ates with tuff intercalations. A 20-m -thick crinoid-bearingl imestone s everal hundred m eters farther to the north over-l i e s m a t r i x - a n d c l a s t - s u p p o r t e d c o n g l o m e r a t e s . T h i soccurre nce o f coarse-graine d m ass-f low deposi ts interbed-ded wi th ca l careous sha l low-mar ine sed imen t s ind ica test h a t t h e s u c c e s s i o n i s t h e s u b ma r i n e p a r t o f a c o a r s e -g ra ined de l t a , p robab ly a subaqueous fan de l t a . The M1s u c c e s s i o n t h e r e f o r e s t a r t e d w i t h s i l i c i c s u b a e r i a l v o l -c a n i s m a n d c l a s t i c d e p o s i t i o n i n a la c u s t r in e ( o r f lo o d -p la in ) env i ronm ent . A subseq uen t ma r ine transgress ionprodu ced a subm arine fan delta .

T h e M 2 s u c c e s s i o n ( F i g . 5 ) c o mp r i s e s a mo n o t o n o u sseq uen ce of shales, sil tstones, and fine-grained sandstones.There i s an overa l l upward coarsen ing and th i cken ing o fthe beds . The bas e o f the success ion inc ludes abou t 70muds tone , b u t toward the top th is p ropor t ion decreases toabou t 35 . Som e b io tu rba t ion i s apparen t , and a sparsefauna includes sma l l th in-shel led aImnonites and bivalves .Small , wel l -p reserved p lan t f ragment s a re re l a tive ly com-mon. Sedimentary s t ructures indicate deposi t ion from tur-bidity currents.

The M 3 succ ess ion fo rms a th i ck band o f e ros iona l lyresis tant p ebbly sandstone and sandstones wi th mino r con-g lomera tes and m uds tones . Rare th in -bedded tu f t s rep re-s e n t a s h - f a l l d e p o s i t s . T h e u p p e r a n d l o w e r s e c t i o n sc o mp r i s e me d i u m- t o c o a r se - g r a i n e d p e b b l y s a n d s t o n e ,and the central sect ion consis ts of p ebb ly sandstoaaes, con-g lomera tes , and pebb ly muds tones . These sed imen t s a reinterpreted as coarse-grained proxim al turbidites.

The M 4 succ ess ion is in sharp con tact wi th the underly-ing M3. I t com prises about 90 mu dstone and about 10fine- to medium-grained sandstones wi th min or conglom -era tes . Th in ly bedded f ine-g ra ined sands tones and s i l t -stones, interpreted as thin distal turbidites, are intercalatedwith the shales . Bioturbation (com mo n chondri tes and rarehe lmin tho ids) i s comm on, and p lan t foss i l s inc lude de l i -ca t e ly p reserved leaves and hor i zons o f abundan t m acer -ated material. Sm all thin-shelled biva lves , ech inoid s, andammonites axe rare. The M4 sediments are interpreted asmar ine depos i t s accumula ted be low wave ba se by turb id -ity currents and by hem ipelagic settl ing.

The upp er parts of the M5 sequenc e comprise parallel-bedded and c rossbedded sands tones , pebb ly sands tones ,conglom erates , and shales , so me with abundant plant fos-s ils and probable ro ot t races . The sediments resem ble f lu-v i a l a n d s w a mp d e p o s i t s . Be n e a t h t h e s e s e d i me n t s a r ea b o u t 1 0 0 m o f p e b b l y s a n d s to n e s , c o n g l o me r a t e s , a n dsha les which a re in t e rp re t ed as the subaq ueous par t o f acoa rse-g rained delta. This interpretation is ba sed on theirs t rat igraplaic posi t ion between underlying marine turbid-ites and ov erlyin g fluvial deposits.

M o s t o f t h e Sa n F61 ix Fo r m a t i o n w a s d e p o s i t e d b ymass flow and turbidity currents. The sparse fauna of thin-shelled but wel l -preserved marine bivalves and am mon itesis probably pelagic in origin . Extensive bioturbat ion indi-c a t e s a n o x y g e n a t e d s e a b e d . A b u n d a n t a n d w e l l - p r e -served p lan t mater ia l sugg es t s a nearby shore l ine w i th ahum id climate.

T h e mu d s t o n e s o f t h e M2 a n d M4 s u c c e s s i o n s w e r emain ly depos i t ed f rom suspens ion and low-dens i ty tu r -b id i ty cu r ren t s . Pebb ly sands tones and cong lom era tes inM1 and M3 represen t the p rox imal depos i t s o f h igh den-s i ty turbidi ty currents . The M5 seque nce is in terpreted asf a n - d e l t a d e p o s i t s a n d , a c c o r d i n g l y , u n d e r l y i n g f a c i e sassoc ia tions (M1-M 4) m ay a l so be in t e rp re ted as hav ingaccumula ted in the submar ine par t s o f a coarse-g ra ineddel ta, proba bly a fan del ta . T his suggests act ive tectonismduring their deposition.

The dis tinctive co ntacts betwe en the different mem bersof the San F61 ix Form at ion m ark m ajo r d i scon t inu i t i es .The M 2-M 3 contact indicates the progradat ion o f coarse-grained, more proximal , turbidi tes . I t may represent tec-tonic upl i f t of the source area. The M 3-M 4 contact repre-sents a sudden tectonic subside nce (or a r ise in sea level) .The M 4-M 5 contact marks the progradation of a fan del ta ,probab ly due to tect tmic upl i f t of the source area.

Canto Del Agua Formation. Sands tones , cong lomer-ates. and shales with limeston e intercalations of the Can tod e l A g u a Fo r ma t i o n . e x p o s e d i n t h e a r e a n e a r H u a s c o(Fig. 1) , have an es t imated thickness of 2100 m (M osco so


10/13

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Triassic r if t- related sedim entary basins in northern Chile 119e t a l . 1 9 82 ). T h e f o r m a t i o n u n c o n f o r m a b l y o v e r li e sPa l e oz o i c m e ta se d im e nta r y r oc ks a nd i s unc onf or m a blyo v e r l a i n b y N e o c o m i a n s t r a ta o f t h e B a n d u r r ia s G r o u p( M o s c o s o e t a l ., 1982). L ow e r hor i z ons o f t he f o r m a t ionc on ta in f os s i l s o f M idd le T r i a s s i c a ge ( M osc oso e t a l .1982). Pub l i shed descr iptions of the C anto de l A gua For -m a r ion ( M osc oso e t a l . 1982 ) do no t al low a precise iden-t i fi c a ti on o f t he se d im e n ta r y p r oc e sse s a nd de pos i t iona le nv i r onm e nt s . Howe ve r , c ong lom e r a t e s a nd sa nds tone s(some showing eviden ce of a m rbidi te or igin) inte rbeddedwi th she l f limestones sugg est accumula t ion in the sub ma-rine part o f a coarse-gra ined fan delta .

D I S C U S S I O N A N D C O N C L U S I O N ST r i a s s i c se d im e nta r y ba s ins i n Sou th Am e r i c a ha vebee n interpreted as r if t basins (Charr ier , 1979; Stipanicic.

1983; Su~rez e t a l . . 1985; Dalz ie l e t a l . 1987; Ul iana andBidd le, 1988). based on: t h i r geo graph ical distr ibution; the abundance of the coarse-gra ined fan-de l ta

depo sits character ist ic of oth er lacustrine andma rine r if ts (Gaw thorpe and Colella . 1990);

the absence of evidenc e for re la tedcom pressio nal tectonics; and the presence of coeva l volcanic rocks ,including alkali basalts (Ram os and Kay,1991).

E x te ns ive r i f t i ng i n nor the r n Ch i l e be ga n dur ing t heP e r m i a n a n d w a s c h a r a c t e r i z e d b y a m a j o r e p i s o d e o fs i l ic ic magma t ism, wi th magm as der ived f rom cont inenta lc r us t a l a na t e x i s ( Ka y e t a l . . 1989; M podoz i s a nd Ka y ,1990; Smirez e t a l . 1990). Ri f t ing cont inu ed through theE a r ly t o L a t e T r i a s s i c . p r oduc ing a w ide ly d i s t r i bu t e ds e r i e s o f s e d i m e n t a r y s u c c e s s i o n s , i n c l u d i n g t h o s edes crib ed here . Triass ic andesitic, basaltic, an d silicic vol-c a n i sm , p r oba b ly r e l a t e d to subduc t ion ( Su~ e z a nd Ve r-gara. in prep.), suggests a relat ionship betwe en rif t ing andsubduct ion processes .

Th e r i f t phase was fol lowed , dur ing the ear l ies t Juras-s i c. by a m a r ine t r a nsgr e s s ion e x t e nd ing we l l be yond thea r e a o f s y n - r i f t d e p o s i t i o n ( S e p ~ l v e d a a n d N a r a n a j o ,1982). Such a widesp read ma rine transgression above con-tinental r if t sedimen ts suggests sub sidence related to ther-mal and s edimen t loading. Loca l ized hiatuses , suggest inglong pe r iods o f nonde pos i t i on bu t m a r ke d by no obv iouseros iona l unco nformi t ies , appear to separa te the r i f t sedi-m e nt s f r om the ove r ly ing m a r ine s t ra t a. T he se i nc lude aM idd le T r ia s s i c to L i a s s ic h i a tus be twe e n the L a CoipaBe d s a nd t he L a u ta r o For m a t ion a nd a L a t e T r i a s si c toHet tangian hia tus b e tween the C lfuncho and Pan de Azt i-car F ormat ions .

The Triassic sedim entary successions o f northern Chilec a n be g r oupe d in to f i ve de pos i t i ona l ba s ins ( F ig . 2 ) .These inc lude two mar ine bas ins (Profe ta and S an F~lix).which were e i ther conn ec ted by a nar row m ar ine passage-way be tween two c ont inenta l a reas or separa ted by a s in-g l e c o n t i n e n t a l a r e a t h a t c o n t a i n e d t h e C i f u n c h o . L aCoipa. an d La Ternera Basins (Fig. 6) .

T he c oa r se- g r a ine d t ex tu r e o f m a n y o f t he m a r ine a ndcontinental sediments, together with relat ively abrupt ver-t ical and lateral facies chang es, suggests active tecton ismdur ing deposi t ion, wi th e roding uplands adjacent to sub-s iding basins. S t ra tigraphic and sed imentologic evide nceindica tes tha t the eas te rn boundar ies of both the La Tern-e r a a nd Sa n F6 l ix Ba s ins c o ns i s t e d o f w e s t - f a c ing f a u l tscarps. Direc t evide nce for the na ture o f the other bas inm a r g ins i s l a c k ing , bu t t he se d im e n ta r y c ha r a c t e r is t i c s(Gawthorpe and Col le la , 1990) , in assoc ia t ion wi th theregio nal tectonic setting (C harrier. 19 79; Stipanicic. 1983;S u a r e z e t a l . 1985; Dalz ie l e t a l . . 1987; U l i a na a ndBiddie . 1988), provide com pel l ing evidence for r i f t -bas insedimentation. T he distr ibution of exposu res (Fig. 2) andfac ies , toge ther wi th pa leocur rent da ta , suggests tha t thebasins had a N-S or NNW -SSE elongation, approxim atelyparallel to the plate margin. The basins w ere the produ ct ofr if t ing prod uced b y subduction-related exten sion or strike-sl ip movement.The m ar ine sediments a re charac te r ized by m ass- f lowe la s t i c de pos i t s i nc lud ing t u r b id i t e s a nd de b r i s f l ows .These s trata a re inte rpre ted as the deposi t s o f the sub ma-r ine par t s of coarse-gra ined fan d e l tas . Large-sca le sedi -m e n t a r y c y c l e s a n d m a j o r f a c i es d i s c o n t i n u i ti e s a r epr oba b ly t e c ton i c i n o rig in . T he y r e su l t e d f r om m ove -m e nt s on t he m a jor f a u lt sys t e m s bounding t he se d im e n-tary basins.

The c ont inenta l sedimen ts inc lude lacust r ine , fluvia l ,al luvial, an d sabkha deposits. L acustr ine sedim ents of theLa Coipa Bed s com pr ise debr is- f low deposi t s and tur -

bidi tes deposi ted on the subaqu eous par ts of fan de l tas ,and open-lake turbidites and black shales. Other contine n-t a l e nv i r onm e nt s i nc lude b r a ide d r i ve r s , s a bkha s , a ndephem era l lakes (Cifuncho Format ion) and a l luvia l f ansand braided floo d plains (La Ternera Formation). T he con -tinental sedim entary facies provide apparen tly con flict inge v ide nc e f o r a r i d . s e m i - a rid , t e m pe r a t e , a nd h um id c l i -m a t i c c ond i t ions . T he a ge c on t r o l on t he se se d im e nt s i si na de qua t e t o de t e r m ine whe the r t he se va r ia t i ons w e r etemporal or ge ographical in or igin.

Aelo low ledsmen ts Th is study forms part of a research program fundedby FONDECYT Grant No. 90-511, Servicio Nacional de Geologfa yMine~a, Cheltenham & Gloucester College of Higher Education, and theRoyal Society. We appreciate the valuable comments of Reynaldo Char-tier and Randall Forsythe.

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triassic rift-related sedimentary basins in northern chile (24°–29°s)_3

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