Geochemical anomalies at the Cenomanian-Turonian boundary, northwest New Mexico

7/27/2019 Geochemical anomalies at the Cenomanian-Turonian boundary, northwest New Mexico

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Advances in Organ ic Geochemistry 1993O r g . G e o c h e m . Vol. 22, No. 3 -5, pp. 487-500, 1994Copyright 1994 ElsevierScienceLtdPrinted in Great Britain. All rights reserved0146-6380/94 $7.00 + 0.00

G e o c h e m i c a l a n o m a l i e s at t h e C e n o m a n i a n - T u r o n i a n b o u n d a r yn o r t h w e s t N e w M e x i c o

JOSEPH A. CURIALEUno ca l , Inc . , P .O. Box 76 , Brea , CA 92621, U.S.A.

A ~ t r a c t - - A high-resolu t ion sec t ion of 64 carbona tes an d ca lcareous sha les f rom northwest N ew Mexico ,sp a n n in g t h e C e n o m a n ia n -T u ro n ia n (C-T ) b o u n d a ry , i s e x a m in ed fo r sel ec te d e l e m e n ta l c o n c e n t ra ti o n sas wel l as ca rbon iso topic and molecula r d is t r ibut ions. Spa t ia l and tempora l sampl ing reso lu t ions forthe en t i re sample se t a re 1 sp l/19 cm and 1 sp l/33 ka , respec tively . In proximi ty to the C -T boun dary ,respec tive sp a t ia l and tempo ra l reso lu t ions increase to 1 sp l /10cm and 1 sp l/17 ka . Results show theoccurrence of severa l geochemica l ano mal ies , a l l wi th co inc ident maxim a a t ab out 0 .5 m be low the C Tboundary . Stable ca rbon iso tope ra t ios of the kerogen and the a l ipha t ic and a romat ic hydrocarbonfrac t ions increase by approxim ate ly 4%0 wi th in 20 m of the b oun dary; th is en t i re C-T bou nda ry eventiso tope anom aly occurred over approximate ly 2 .5 Ma. A 5-fold increase in the concentra t ions of U and T hoccurs sy nchro nous wi th the i so tope excursion , a l though th is [U + Th] sp ike occurs over a per iod o f only0 .2~) .3 M a. Eleva ted U a nd T h con centra t ions a t th is spike co-occur wi th (a ) the h ighest 20S/(20S + 20R)ste rane ep imer ra t ios , (b) C27 C27_30 s te ran e hom olog ra t ios and (c ) the lowest n-a lka ne y ie lds in kerogenpyrolyza tes . These observ a t ions suggest an e ffect of rad iogenic hea t p roduc t ion on the organic m at te r inthe sec tion . Th e or ig in of the ca rbon iso tope excursion i s a t t r ibuted , as in previous s tudies , to a tm ospher icCO 2 d ra w d o w n in l a t e s t Ce n o m a n ia n tim e, c a u se d b y r a p id b u r i a l o f o rg a n i c m a t t e r d u e t o a g lo b a l m a r in et ransgression . The U and T h sp ike i s prob ably secondary, perhaps resu l t ing f rom adsorp t ion of thesee lements onto c lays concentra ted a t the C-T boundary . Al though the occurrence of coeva l maxima forthese i so topic and e lementa l anomal ies suggests a com mo n cause , ex t rap ola t ion of the C O 2 drawd ownmod el to expla in the [U + Th] an om aly is unw arrante d a t th is time . Ong oing minera logica l ana lysis ofc lays and carb ona tes c lose to the C- T bou ndar y in the R ed W ash sec tion may u l t imate ly expla in theoccurrence of this U a nd T h spike.K e y w o r d s - - S t a b l e carbon iso topes, b iomarkers , rad iogenic hea t ing , uranium, thor ium, event geo-c h em ist ry , N e w Me x ic o , C e n o m a n ia n -T u ro n ia n b o u n d a ry , h ig h r e so lu ti o n sa m p l in g

INTRODUCTIONW i t h t h e d e v e l o p m e n t o f th e s e q u e n c e s t r a ti g r a p h i cp a r a d i g m o v e r t h e p a s t 2 d e c a d e s , m a n y s e d i m e n t o l -o g i s ts h a v e s h i f te d t h e i r a t t e n t i o n f r o m t h e a c c u m u l a -t i o n o f r o c k s ( l i t h o s t r a t i g r a p h y ) t o t h e m e a s u r e m e n to f t i m e ( c h r o n o s t r a t i g r a p h y ) . U s i n g h i g h - r e s o l u t i o ns a m p l i n g m e t h o d s , s o p h i s t i c a t e d b i o s t r a t ig r a p h i cz o n a t i o n s ( K a u f f m a n e t a l . 1 9 9 1 ) , a n d i n f e r e n c e sf r o m o r b i t a l f o r c i n g c o n s t r a i n t s ( B e r g e r, 1 9 88 ;S c h w a r z a c h e r , 1 9 91 ), s e d i m e n t o l o g i c a l m e a s u r e m e n to f t im e i n th e r o c k r e c o r d h a s b e e n d e m o n s t r a t e d a ts c al e s o f d e c a d e s a n d e v e n d a y s ( K v a l e e t a l . 1989).S e e E i n s e l e e t a l . ( 1 9 91 ) f o r a s u m m a r y o f r e c e n te f f o r t s i n c y c l e a n d e v e n t s t r a t i g r a p h y .

I n t e r p r e t a t i o n o f e v e n t s a n d c y c l e s i n t h e r o c kr e c o r d r e q u i r e s c o n f i r m a t i o n u s i n g b i o s t r a t i g r a p h i ca n d / o r g e o c h e m i c a l d a t a , a n d o f t e n t he s e d a t a c a n b ea s s i m p l e a s t o t a l o r g a n i c c a r b o n c o n t e n t m e a s u r e -m e n t s a t h i g h s a m p l i n g r e s o l u t i o n ( e . g . C a r p e n t i e re t a l . 1 99 1) . R e c e n t p o p u l a r i n t e r e s t i n g e o c h e m i c a la n o m a l i e s a s s o c i a t e d w i t h m a s s e x t i n c t i o n s h a sh e l p e d t o c r e a t e a s u b - d i s c i p l i n e r e f e r r e d t o a s

e v e n t g e o ch e m i s t r y . A m o n g t h e m o r e p r o m i n e n t

g e o c h e m i c a l e v e n t s a r e t h o s e a s s o c i a t e d w i t h t h es e c o n d l a r g e s t m a s s e x t i n c t i o n d u r i n g t h e C r e t a c e o u s ,t h a t o c c u r r i n g c o i n c i d e n t w i t h o r p r o x i m a t e t o t h eC e n o m a n i a n - T u r o n i a n b o u n d a r y ( E ld e r , 1 98 5; E l d e ra n d K i r k l a n d , 1 9 8 5 ; O r t h e t a l . 1 9 9 3 ) . W o r l d w i d e ,t h i s b o u n d a r y i s t h e l o c u s o f n u m e r o u s i s o t o p i ca n d m o l e c u l a r a n o m a l i e s ( S c h o ll e a n d A r t h u r , 1 98 0;P o m e r o l , 1 9 8 3 ; E l d e r , 1 9 8 5 ; Z e l t , 1 9 8 5 ; B r u m s a c ka n d T h u r o w , 1 9 86 ; H i l b r e c h t a n d H o e f s , 1 9 86 ;A r t h u r e t a l . 1 9 88 ; D e a n e t a l . 1 99 0; T h u r o w e t a l .1 9 8 8 ; O r th e t a l . 1 9 8 8 a , 1 9 9 3 ; P o p p e t a l . 1989;H a y e s e t a l . 1 9 8 9 ; P ra t t e t a l . 1 9 9 1 ; A r t h u r a n dD e a n , 1 99 1) . T h e s e f e a t u r e s , a s w e l l a s t h e b o u n d a r yo c c u r r e n c e o f o r g a n i c - r i c h s h a l e s a n d c a r b o n a t e s( e. g. B r u m s a c k a n d T h u r o w , 1 98 6) , h av e p r o m p t e dm a n y w o r k e r s t o s u g g e s t a w o r l d w i d e o c e a n i c a n o x i ce v e n t a t t h i s t i m e ( H a r t a n d L e a r y , 1 98 9) , a l t h o u g ht h i s a r g u m e n t i s n o t a c c e p t e d b y a l l ( P e d e r s e n a n dC a l v e r t , 1 99 0) . T h e o c c u r r e n c e o f o r g a n i c - r i c h s h a l e sa n d / o r i n d i c at o r s o f a n o x ia a r e n o t e d a t t h e C - Tb o u n d a r y i n E n g l a n d ( L e a r y e t a l . 1 9 8 9 ) , F ra n c e( C r u m i e r e e t a l . 1 9 9 0 ) , M o r o c c o ( B r u m s a c k a n dT h u r o w , 1 9 86 ), t h e w e s te r n M e d i t e r r a n e a n i n g e n e ra l( K u h n t e t a l . 1 9 9 0 ) , P o l a n d ( P e r y t a n d W y r w i c k a ,

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1 9 9 1 ) , t h e I n d i a n O c e a n ( T h u r o w e t a l . 1992) andn u m e r o u s l o c a t i o n s in N o r t h A m e r i c a a n d e l se w h e re( S c h l a n g e r e t a l . 1 9 8 7 ; A r t h u r e t a l . 1987).

These s tudies u t i l i ze for the i r conc lus ions botho r g a n i c a n d i n o r g a n i c g e o c h e m i c a l b u l k d a t a , s u c h a se l e m e n t a l o r i s o t o p i c c o m p o s i t i o n ( i. e. c o m p o s i t e s o fm i n e r a l s o r m o l e c u l e s ) . F e w c o m p r e h e n s i v e o r g a n i cg e o c h e m i c a l s t u d i e s , c o m p r i s i n g m o l e c u l a r g e o -chemica l a s we l l a s i so topic and e lementa l da ta , havea d d r e s s e d t h e q u e s t i o n . I n t h i s p a p e r , o u r p r e v i o u ss t u d ie s o f a la te C e n o m a n i a n / e a r l y T u r o n i a n s e c ti o ni n n o r t h w e s t e r n N e w M e x i c o ( C u r i a l e e t a l . 1992a;Cur ia l e , 1994) a re ex tended , and focussed spec i f i ca l lyo n a n o m a l o u s g e o c h e m i c al b e h a v i o r o c c u r r i n g w i th i n0 . 5 - 1 . 5 M a o f t h e C - T b o u n d a r y . T h e l o c a t io n o f th i sb o u n d a r y i s p i c k e d b a s e d u p o n b i o s t r a t i g r a p h i c d a t a(Cur ia l e e t a l . 1992a; Cur ia l e , 1994; E . Ka uf fm an,p e r s o n a l c o m m u n i c a t i o n ) . S p a t i a l a n d t e m p o r a ls a m p l i n g r e s o l u t i o n s t h r o u g h m o s t o f t h e s tu d yseque nce are , respect ively, 1 spl/19 cm a nd 1 spl /33 ka ;a t the C -T bou nda ry , these reso lu tions a re 1 sp l/10 cma n d l s p l / 1 7 k a . O r g a n i c c a r b o n i s o t o p e r a t io s ,e l e m e n t c o n c e n t r a t i o n s ( C u , F e , M n , N i , T h , U , V )a n d m o l e c u l a r r a t i o s a r e u s e d t o d e f i n e a s e t o fanomal ies

T h e s e c t i o n u n d e r e x a m i n a t i o n i s i n n o r t h w e s t e r nN e w M e x i c o , in a n a r e a k n o w n l o ca l ly as R e d W a s h( F i g . 1 ) . G e o g r a p h i c a l i n f o r m a t i o n a n d t h e r e g i o n a l /loca l geologica l pe rspec t ive have been d i scussed inde ta i l e l s ewhere (Cur ia l e e t a l . 1992a) . The fo cus he reis o n t h e B r i d ge C r e e k M e m b e r o f th e G r e e n h o r nF o r m a t i o n , w h o s e d e p o s i t i o n o c c u r r e d c o n c u r r e n t l yw i t h t h e m a x i m u m e a s t- w e s t e x t e n si o n o f t h eC r e t a c e o u s s ea w a y o f N o r t h A m e r i c a ( K a u f f m a n ,1985). Th e Br idge Cree k cons i s t s of sha les, ma r l s toneand l imes tones , p resent l a rge ly as l imes tone-sha le

couple t s (F ig . 2 ) a t t r ibu ted to va r i a t ion in e i the rp a l e o c l i m a t e o r p a l e o p r o d u c t i v i t y ( K a u f f m a n e t a l .1 9 8 5 ; E i t h e r a n d D i n e r , 1 9 8 9 ; A r t h u r a n d D e a n ,1991) . Fur the r de ta i l s a re rev iewed in Cur ia l e e t a L(1992a) and C ur ia l e (1994) . The pu rpos es of th iss t u d y a r e , fi rs t, t o e x t e n d k n o w l e d g e o f a n o m a l o u sC - T b o u n d a r y g e o c h e m i c a l b e h a v i o r f r o m t h e w e l l -s tudied Pueblo s i t e (P ra t t , 1984; P ra t t e t a l . 1985;rev iewed in Cur ia l e e t a l . 1 9 9 2 a ) s o u t h w e s t w a r d t ot h e m o r e s h o r e l i n e - p r o x i m a l s i t e a t R e d W a s h . As e c o n d a r y p u r p o s e i s t o e v a l u a t e t h e e f fe ct s o n m o l -e c u l a r c h a r a c t e ri s ti c s o f ( a ) c h a n g e s i n p a l e o c l i m a t ea n d / o r p a l e o p r o d u c t i v i t y , a n d ( b ) r a d i o g e n i c d a m a g ed u e t o U a n d T h l o c a l i z a t i o n a t t h e C - T b o u n d a r y .A n d f i n a ll y , t h r o u g h t h e u s e o f a f u ll s u i te o f o r g a n i ca n d i n o r g a n i c g e o c h e m i c a l d a t a , f u r t h e r p e r s p e c t i v eo n t h e c a u s e ( s) o f t h e C - T a n o m a l i e s is p r o v i d e d .

M E T H O D S

A l l s a m p l e s a r e f r o m s ev e r a l m i n i - s e c t i o n s a tt h e R e d W a s h o u t c r o p s . T h e s e m i n i - s e c t i o n s w e r ec o m p o s i t e d i n t o a s i n g l e s t u d y s e c t i o n t h r o u g h t h euse of s evera l d i s t inc tive mar ker hor i zon s (Cur ia l ee t a l . 1992a) . Ver t i ca l s ampl ing reso lu t ion ( i . e . e r rori n s a m p l i n g a t o u t c r o p ) is a p p r o x i m a t e l y + 0 . 0 5 m .S a m p l e s w e r e p r e p a r e d f o r a n a l y s i s a n d a n a l y z e da c c o r d i n g t o m e t h o d s o u t l i n e d b y C u r i a l e e t a l .(1992a ,b) and Cur ia l e (1994) . E lementa l ana lysesw e r e c o n d u c t e d o n p o w d e r e d w h o l e r o c k m a t e r i a l i na l l cases , us ing induc t ive ly coupled p lasma ( ICP)e m i s s i o n s p e c t r o m e t r y a c c o r d i n g t o O d e r m a t t a n dCur ia l e (1991) or , for ce r t a in s amples an d e lement s ,b y I C P - m a s s s p e c t r o m e t r y . E l e m e n t a l d a t a p r e c i s i o nvar i es, genera l ly be ing abo ut + 10% ( re la tive) ; wo rs tprec i s ions a re appro xim ate ly + 15- 20% ( re la tive) .

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Fig. 1. General location map of the study section at Red W ash , northwest New M exico.

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A nomal ies a t the C - T boundar y 489M o l e c u l a r d a t a p r e c i s i o n f o r t h e r a t i o s p r e s e n t e dh e r e i s ( a p p r o x i m a t e l y ) - I -6 ( r e la t i v e ), b a s e d o n 9r e p e t i t iv e e x p e r i m e n t s u s i n g a r o c k e x t r a c t f r o m t h es a m p l e s e t. Q u a n t i t a t i v e p y r o l y s i s - g a s c h r o m a t o g -r a p h y m e t h o d s u s e d h e r e a re d e s c r i b e d in L a r t e r a n dS e n f t le ( 19 8 5 ), C u r i a l e a n d S t o u t ( 19 9 3 ) a n d C u r i a l ea n d G i b l i n g (1 9 9 4) . C a r b o n i s o t o p e a n a l y s e s a r e

d e s c r i b e d i n C u r i a l e a n d G i b l i n g (1 9 94 ) a n d r e f er e n c e st h e r e in ; a l l i s o t o p e d a t a a r e r e p o r t e d r e l a t iv e t o t h eP D B s t a n d a r d . A l l o t h e r a n a l y t i c a l d e t a i l s a r e i nC u r i a l e ( 1 9 9 4 ) .

A l l B r i d g e C r e e k g e o c h e m i c a l d a t a d i s c u s s e d h e r ea r e p r e s e n t e d i n T a b l e s 1 ( e l e m e n t a l , i s o t o p i c ) a n d2 ( m o l e c u l a r ) . S i x t y - f o u r s a m p l e s f r o m t h e B r i d g e

m R

S NDSTONE CO L

C LC REOUS BENTONITESH LE

BURROW SMUDST ONESHELLSLIMESTONEROOTS

SH LEPYRITE

Fig. 2. Detai led s trat igraphic sect ion of the Br idge Creek Member of the Greenhorn Formation innor thwest New Mexico. Also shown are par ts of the over lying Fairp or t Mem ber of the Car li le Shale andunderly ing H ar t land Sha le Member of the G r eenhor n For m at ion . The Cen omanian- Tu r onian boundar yis shown a t 39.25 m in the section, corresponding to 92.63 Ma . G y = gypsum.


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4 9 0 JOSEPH A . CURIALEC r e e k a r e a n a l y z e d . A f e w s e l e c t e d i s o t o p i c d a t ap o i n t s f r o m t h e u n i t s o v e r l y i n g a n d u n d e r l y i n g t h eB r i d g e C r e e k a r e a l s o d i s c u s s e d ; th e s e d a t a a r e g i v e ni n T a b l e 4 o f C u r i a l e e t a l . (1992a) .RESULTSO r g a n i c c a r b o n i s o t o p i c e v e n t

S t a b l e c a r b o n i s o t o p e r a t i o s i n t h e D a k o t a -t h r o u g h - C a r l i l e s e c t i o n f o r k e r o g e n s , a l i p h a t i ch y d r o c a r b o n s a n d a r o m a t i c h y d r o c a r b o n s , s h o w ap r o n o u n c e d e x c u r s i o n t o w a r d ~ 3C e n r i c h m e n t n e a rt he C e n o m a n i a n - T u r o n i a n b o u n d a r y ( F ig . 3; d a t ai n T a b l e 4 o f C u r i a l e e t a l . 1992a; T ab le 1). ~ ]3Ck+v a l u es r a n g e f r o m b a c k g r o u n d l ev e ls o f a b o u t- 2 5 . 5 % 0 t o m a x i m u m ( ~ 3 C -e n r ic h e d) v a l u e s o f a b o u t- 2 1 .5 % o . T h e s e b a c k g r o u n d l ev e ls a r e r e a c h edw i t h in 2 0 m ( a b o v e a n d b e l ow ) o f t h e b o u n d a r y .A l t h o u g h s a m p l i n g r e s o l u t io n i s v a ri a b l e t h r o u g h o u tt h e s e c t i o n , h i g h t e m p o r a l a n d s p a t i a l r e s o l u t i o n s( s ee a b o v e ) w e r e u s e d w i t h i n 5 m o f t h e b o u n d a r y .T h e p r o n o u n c e d s i m i l a r i ty a m o n g ~ 1 3 C k e r ~ 13Calipa n d fi ~3C . . . v a l u e s i n d i c a t e s t h a t t h e o r g a n i c m a t t e rd i s t r i b u t i o n i n t h e s e s a m p l e s i s i n t e r n a l l y c o n s i s te n t ,a n d t h a t m i g r a t e d h y d r o c a r b o n s a r e n o t p r e s e n t(Curia le , 1994) .

A l t h o u g h s e v e ra l w o r k e r s h a v e o b s e r v e d c a r b o n a t ec a r b o n i s o t o p i c e x c u r s io n e v e n ts a t t h e C - T b o u n d -a r y i n C r e t a c e o u s s e a w a y s e c t i o n s a n d e l s e w h e r e( P o m e r o l , 1 9 8 3 ; S c h l a n g e r e t a l . 1987 ; Pra t t e t a L1 99 1) , o r g a n i c c a r b o n i s o t o p i c e v e n ts a r e n o tc o m m o n , a n d s a m p l i n g r e s o l u t i o n a n d e x c u r s i o np r o m i n e n c e i s r a r e l y a s s h a r p a s s h o w n i n F i g . 3. A6 1 3 C o r g e v e n t , o b s e r v e d b y T h u r o w e t a l . (1988) andK u h n t e t a l . ( 1 9 9 0 ) i n c a r b o n a t e s a c r o s s t h e C - T

b o u n d a r y i n o f fs h o r e M o r o c c o , r e c o r d s a n e x c u rs i o nf r o m a b o u t - 2 7 . 8 t o - 2 4 . 3 % o . S p a t i a l r e s o l u t io na c r o s s t h e b o u n d a r y i n t h e i r w o r k i s a p p r o x i m a t e l y1 s p l /3 7 5 c m a n d b a c k g r o u n d l e v el s a r e r e a c h e d4 0 m b e l o w a n d 9 0 m a b o v e t h e b o u n d a r y . P r a t ta n d T h r e l k e l d ( 1 9 8 4 ) a n d P r a t t e t a l . (1991) p re sen t6 1 3 C o r g d a t a f o r a C r e t a c e o u s s e a w a y s e c t i o n f r o mP u e b l o , C O , U . S . A . , t h a t s h o w a n e x c u r s i o n f r o m

b a c k g r o u n d l e v el s o f - 2 7 . 5 t o - 27 .0 % o0 t o am a x i m u m o f ab o u t - 2 4 . 0 t o - 2 3 . 5 % o . S pa t ia ls a m p l i n g r e s o l u t i o n a c r o s s t h e b o u n d a r y i n t e r v a l i sa b o u t 1 s p l / 4 0 c m , a n d b a c k g r o u n d l ev e ls a r ea c h i e v e d w i th i n 5 m a b o v e a n d b e l o w t h e b o u n d a r y .A N e b r a s k a C r e t a c eo u s s e a w a y s e c ti o n e x a m i n e d b yP r a t t e t a l . (1991) show ed a weak ly -de f ine d even ts l i g h t ly b e l o w t h e b o u n d a r y , w i t h a n e x c u r s i o n f r o ma b o u t - 2 7 t o a b o u t - 2 3 % o .

A r t h u r e t a l . ( 1 9 8 8 ) e x a m i n e d a t m u c h h i g h e rs a m p l i n g r e s o l u t i o n ( a b o u t 1 s p l / 1 3 c m ) a P u e b l o ,C O , s e c t i o n s i m i l a r t o t h a t o f P r a t t a n d T h r e l k e l d( 1 9 8 4 ) a n d P r a t t e t a l . (1991) . T he h ighe r - r e so lu t i ons t u d y s h o w e d a s i m i l a r e x t e n t o f o v e r a l l e x c u r s i o n ,a l t h o u g h b a c k g r o u n d ~ t3Corg levels a re a lsoo b s e r v e d w i th i n t h e C - T e v e nt . D a t a c o m p i l e d b yA r t h u r e t a l . ( 19 88 ) f o r v a r i o u s o t h e r C - T s e c t i o n ss h o w t h a t C - T ~ ~3Cor a n o m a l i e s a r e n o t u n i v e r s a l ,b u t , w h e r e p r e s e n t , e x c u r s i o n s a r e u s u a l l y a b o u t4 - 6 % o , a l w a y s t o w a r d t h e h e a v i e r i s o t o p e ( D e a ne t a l . 1990) . I t i s no t ed t ha t t he re i s som e ques t i ona b o u t t h e s y n c h r o n e i ty o f t h e C - T 6 1 3 E k e r e v e n t( B r a l o w e r , 1 98 8) . H o w e v e r , r e c e n t w o r k s u g g e s t s t h a tt h e e v e n t i t s e l f m a y a c t u a l l y r e f le c t m u l t i p l e c a r b o ni s o t o p e s p i k e s ( G a l e e t a l . 1 9 9 3 ) t h a t m a y o r m a yn o t b e e v i d e n t a t t h e r e s o l u t i o n s u s e d b y p r e v i o u sw o r k e r s .

9 0 18 58 O7 S7 O6 56 0E

202s33s4 ~ ~is1 0 I i I I I

- 2 6 - 2 5 - 2 4 - 2 3 - 2 25 1 a C K E RI I I I /

2 9 2 8 2 7 2 6 2 e8 1 3 C A L

o . 0 s~ . A B S O L U T E )

2 7 ' ' 2 42 6 2 5 2 38 1 3 C A R

3 1 3 1 3Fig. 3. Distribu tion, with depth, of 8 Ck~r left), 8 Caj h (middle) and 8 C . . . (right) for the entire sectionat Red Wash. Data are l isted in Table 1 for the Bridge Creek Member samples, and in Curiale (1994)for sam ples abov e and below the Bridge Creek. Precision is appro ximately _+0.05%o. Dep th is measuredfrom an arbitrary datum at the base of the composite section.


5/14

A n o m a l i e s a t th e C - T b o u n d a r yT a b l e 1 . C a r b o n i s o t o p e r a t i o a n d w h o l e - r o c k e l e m e n t a l d a t a f o r t h e B r i d g e C r e e k M b r s a m p l e s et

Al iph A lum ~ J(m ) % o / o o o / o o p p rn p p m p p m p p m p ~ n p p m p p m ~ p m

=62~~ 7~ 8~629~630~632F633F634F635F636F ~ 7F638F639F640F ~ IF ~ 2F643F644F ~ 5F ~ 8F ~ 7F648F ~ 9F650F ~ IF ~ 2F653F854F855F656F ~ 7F658F6,5gF660F ~ IF662F663F664F665F866F ~ 7F668F669F670F671F672F573F674F675F675F679F680F ~ tF ~ 2F683F684F685F686F ~ 7F688F689F6~OF ~ I

4~.0 0.42140.0 0.38148.7 0.541~ . 5 5 . 0 0 148.4 0.8114 ~ ( ] 2 . 8 3 147.8 0.9414 7 . ~ 0 . 5 24 7 . ~ 0 . 7 647.C 0 .Sg46.7 0.3548.7 0.8144,4 0 .3544.: 0.5844,1 0 .7844.C 0.3843.1; 0.6243.7 0.6643.8 0.3543.~ 1 .304 3 . 3 0 . 4 843. ; 0 .g64 3 . 1 0 . 2 342.1~ 0.5242.~ 0 .1942.~ 0 .384 2 . ~ 0 . 7 242.~ 0.7142.~ 0 .9842.~ 0 .9342.1~ 1.1242A 1 . (X)42.," 0.7642.,~ 1.2142.1 0 .3842.C 0 .1641. , ; 0 .5440.~ 0 .153g.~ 0 .0039,; 0.2739,~ 0.2139., 0.183g.Z 0 .963g.," 0.9039.~ 0 ,173g.; 0.303g,1 0 .353g,C 0 .2738.{ 0.4238.~ 0.3438. ,= 0 .288 A 1.24

38.," 0.8338.1 0 .8937.~ 0 .6137.~ 0 .6737.~ 0 .8537.E 0 .8637.E 0 .7937.:" 0,9337..~ 0.9837.C 0 ,763 8 . ~ 0.8136.E 0 .73

-28 .7 -26 ,71-28.~-2g..., -2~.51- 2 8 . E - 2 7 . 1 1-28.t -2~.51-28.7 -2( .71-28.E - 2 6 . 7 1-28.'~ -27.0-28.~ -27.0-28.( -26.9-26.( -26.5-28.7 -26.9.28., -26.7-25., -26.5.28.. -26.8-28.~ -26.4-28.~ -26.4-28.~ -26.6-28.~ -26.5-28.," -26.2-28.1 -26.4-28.t -26.3-28 . : -25,5-28.; -2~.4.~ r / .{ -25,5-28.1 -25.8-28.( -26.3-27.~ -26.0-28 .~ -2~.1-28.( -26.1-27.; -26.1-27,( -26.0-27,{ -26.1-27.| -26.0*27 . { -26 .0-27.; -25.8-27.~ -25.7-27,(-27.~-25.(-25 .~ -23.5-25.( -23.4

-25.: -23.7-25.| -24.3-25.1 -24.3-25.~ -24.3-25.~ -24.2-25.t -24.2-25.~ -24.3

-24.2-25., t -24.1-25., -23.9-25.1 -24.0-25 .~ -24 .1-25. i -24.1

-25.7-25.e-24.7- 2 6 . ;-24.~- 2 6 . ;-25.E-25.~"-26.C-25.~-24.+-26.1-25.:-25.;-25A-25.1-24.{-25.~-25.~-25.+-25.~-25.(-24.(-25.3-24.(-24.:-24.{-24.{-25.1-25.1-25.(-25.1-25.(-25.;-25.~-24.~-24.~-23.1-24.~-22.;-22.4-22.=-22.(-21 ;-21 ;-2 1 .,~-21 ~-21 ,~-21 ,~-21.1-23.(-23.(-23.~-23.,"-23.(.23.(-22.|-23.Co22.~-22.t-22.;-22:-22.(

7.717.71 10000

20.01 1850026.01 1680022.01 2100015.01 1700022-01 3g

9.91 1O3O08.21 120003.11 10800

20.01 158OO3.01 9900

8OOOt3.01 1170016.01 1170(12 2 . 0 1 1 5 9 0 015.01 1210015.01 1220014.01 1570017.01 1500C26.01 1720C22.01 1700G

6 , 1 1 1 0 6 0 ~24.01 19300

5.21 1310015.01 1340( ]11 .01 1410~10.01 1370~15,01 1580~13.01 1550C14.01 1460~15.01 1440( ;t3.01 1400C14.01 12g0C

8.51 940(:560C

15.01 1260C11.01 1550~

4 . 4 1 1 7 7 0 ~971 1050( :

15.01 22301:15 .01 1890~15.01 1960~1 3 . 0 1 1 9 4 ~13.01 1960C00.01 1740(1 2 . 0 1 I ~ 0 C

8.01 1460C15.01 1530~" 1 1 9 0 ~

8 . 5 1 1 7 2 0 ~12.01 1750~15.01 2010( :15 .01 2050( :14 .01 2240( :11.01 1970C11.01 1980C13.01 2080~11.01 1940~14.01 2270( :14 .01 2200( :11 .01 1810( :t2.01 1870(:12 .01 1841~

Ii3OeOl3 3 o o 1 3 5 0 0

114001 8000108001 81oo114001 81100107001 8000

5 l m 1 4 24 0 0 0 1 4 3 0 08 7 0 0 1 8 0 0 05 6 0 0 1 5 2 0 O

111001 89005 1 0 0 1 4 9 G 036001 47008GO01 68009 7 0 0 1 8 0 0 0

101001 570070001 651108 7 0 0 1 7 2 0 092001 7200g g o 0 1 8 0 0 0

112001 8000lOgO01 8000

4,go01 5200122001 8700

8 8 0 0 1 5 8 0 O102001 8000100001 7600

95OO1 7700118001 9300106001 7600102001 8700101001 8400

94001 790086001 770046001 480018201 295094001 84008 2 O0 1 e ~ (X )

189001 1710073001 6200

126001 g600135001 9600136001 10200151001 10800140001 10200128001 10000107001 10800

77001 112009 7 0 0 1 1 2 2 0 012" /001 13000

122001 11500129001 5700158001 10100185001 10000188001 10800130001 9000135001 0000144OO1 9700119001 8400158001 10300151001 10400133001 g1001 3 4 0 0 1 ~ 0 01 3 0 0 0 1 [ 1 8 0 0

27~ 11012 ~ 12.011 4 1 2 6 . 0 1131 2 5 . 0 1I ~ 2 5 . 0 1137 33.01

3 51.01251 12.0116~ 24.01174 18.0113C 41.01178 18.0125E 17.020~ 17.01 4 / 17.0157 23.020~ 15.011~ 15 .01 1 ~ 15.01 4 1 1 5 . 013G 38.014: : 30 .031~ 8.51 ~ 3 5 . 027~ 11.01 8 ( t 4 . 0151 19.016E 22.01 ~ 2 0 . 014( 22.013~ 82.015( 18.015G 18.018~ 20.038( 11,0130( 13.014~ 13.037{ 13.0

g~ 1.531: 6 .9121 12.011~ 1t,012E 11.010( 10.010~ 11.010~ 10.014~ 7.525( 5 .5251 5.31 ~ 4.121E 5.819E 14.01 ~ 16.0

I~ 17.0101 17.022; 14.025( 18.0211 15.028~ 15.015( 18.018~ 15.041( 1 4 . 0

1 6 . 01 1 E 1 4 . 0

1 7 1 1 . 41 . 8 1 1 . 47.21 4.16 .41 3 .08 . 5 1 4 . 77 1 1 3 . 26 . 8 1 3 . 52 . 0 1 1 . 33 . 9 1 2 . 53 . 5 1 2 . 58 . 8 1 4 . 03 . 4 1 3 . 02 . 5 1 1 . 34 . 4 1 2 . 78 . 4 1 3 . 28 . 1 1 5 , 44 . 8 1 2 , 05.91 2.47.31 2.58.81 3.49 . 0 1 3 . 58 . 1 1 2 . g4 . 1 1 1 . 48.91 3,6571 1.89.51 2.88 . 1 1 2 . g8 . 4 1 2 , g

I 1 . 0 1 5 , 4I 0 . 0 1 3 . 81 1 . 0 1 3 , 6I 1 . 0 1 3 , 30 . 8 1 3 . 79.81 3,55.71 2,53.41 1 A5 . 8 1 2 . 82 . 9 1 1 . 5

i ~ . O l Z O . O5 . 5 1 5 . 4

1 3 . 0 1 2 . 514,01 2.516.01 3.119.01 3.51 8 . 0 1 2 . 7~ 1 . 0 1 4 . 1D . O I 5.3~7.01 7,2Ig.OI 9.g15.01 13.0~.01 I0.09.71 2.4

11.01 2.212.01 2.215,01 2.19 . 6 1 1 . 59 . 3 1 1 . 8

~ 0 . 0 1 1 . 85 . 0 1 1 . 5

1 1 . 0 1 1 . gI0.01 2.Ce . 4 1 1.88,51 1 . 4

15.01 2.7] 3C d a t a a r e p r e s e n te d f o r t h e o r g a n i c c a r b o n i n t he a l i p h a t ic h y d r o c a r b o n , a r o m a t i c h y d r o c a r b o n a n d k e r o g e nf r a c t i o n s . T o t a l o r g a n i c c a r b o n v a l u e s a n d a l l e le m e n t a l c o n c e n t r a t i o n s a r e c a l c u l a t e d o n a w h o l e - r o c k b a s i s .M i s s i n g i s o t o p e r a t i o d a t a i n d i c a t e t h a t i n s u f f ic i e n t t o t a l o r g a n i c m a t t e r w a s a v a i l a b l e f o r a n a l y s i s i n t h ec a s e o f 6 ] 3 C k ,) o r t h a t i n s u f f i ci e n t e x t r a c t a b l e o r g a n i c m a t t e r w a s a v a i l a b l e f o r c o m p o u n d c l a s s s e p a r a t i o n13 13i n t h e ca s e s o f 6 C ~]~ p an d 6 C = ro m . M i s s i n g d a t a f o r co p p e r co n ce n t r a t i o n i n ce r t a i n s am p l e s i s d u e t or e s u l t s b e l o w d e t e c t i o n l i m i t s . N o t e t h a t e l e m e n t a l d a t a f o r s a m p l e F 6 3 2 a r e c o n s i d e r e d s u s p e c t . D e p t h i sm e a s u r e d f r o m a n a r b i t r a r y d a t u m a t t h e b a s e o f t h e c o m p o s i te s e c ti o n.

4 9 1


6/14

492 JOSEPH A . CURIALEA m o d e l f o r t h e w o r l d w i d e 6 t 3Ck e e x c u r s i o n a t t h e

C - T b o u n d a r y h a s b e e n p r o p o s e d b y A r t h u r e t a l .( 1 9 8 8 ) a n d D e a n e t a l . ( 1 9 9 0 ) , a n d a s u m m a r y o fc a u s e s a n d e f fe c ts i s p r e s e n t e d b y K a u f f m a n ( 1 9 88 )a n d E i c h e r a n d D i n e r ( 19 9 1 ). T h i s m o d e l e n t a il s r a p i db u r i a l o f t 3 C - d e p l et e d o r g a n i c c a r b o n d u e t o a m a j o rm a r i n e t r a n s g r e s s i o n a t th e e n d o f t h e C e n o m a n i a n ,a n d p a r a l l el s a m o d e l r e c e n t ly p r o p o s e d t o e x p l a i ns i m i la r e x c ur s io n s a t t h e F r a s n i a n - F a m e n n i a n b o u n d -a r y ( ~ 3 6 7 M a ; J o a c h i m s k i a n d B u g g i s c h , 1 9 9 3 ) . T h er e s u l t i n g C O 2 d e f ic i t i n t h e a t m o s p h e r e - o c e a n s y s t e m ,i f n o t e l i m i n a t e d b y c o m p e n s a t o r y a d d i t i o n s o f C Of r o m ( f o r e x a m p l e ) v o l c a n i c e x h a l a t i o n s , w o u l d h a v es t r i p p e d t h e s y s t e m o f i t s C O 2 w i t h i n a f e w h u n d r e dt h o u s a n d y e a r s ( g i v e n p C O 2 b a s e l i n e l e v e ls g r e a t e rt h a n t h o s e a t p r e s e n t ; B e r n e r , 1 9 9 0 ; H o l l a n d e ra n d M a c k e n z i e , 1 9 9 1 ; F r e e m a n a n d H a y e s , 1 9 9 2 ) .K a u f f m a n ( 1 9 8 8 ) a n d A r t h u r e t a l . ( 1 98 8 ), a n d o t h e rw o r k e r s r e f e r e n c e d t h e r e i n , s u g g e s t t h a t v o l c a n i ce x h a l a t i o n s a t C - T t i m e d i d n o t s u p p l e m e n t t h e C O 2 ,resu l t ing in a p osi t ive 3 13Cor excu rsio n o f 6-7 0 ,

a s o b se r v ed a t th e R e d W a s h C - T b o u n d a r y a n de l s e w h e r e . T h e e n t i r e e v e n t a t R e d W a s h a p p a r e n t l yo c c u r r e d o v e r a p e r i o d o f 2. 5 M a , w i t h a n o n s e t a ta b o u t 9 4 . 25 M a ( K a u f f m a n , 1 98 8) , c o r r e s p o n d i n g toa b o u t 2 7 m a t R e d W a s h ( C u r i a le e t a l . 1992a).E l e m e n t a l e v e n t s

C o n c e n t r a t i o n s o f s e l e c t e d e l e m e n t s ( C u , F e , K ,M s , M n , N i , T h , U a n d V ) s h o w v a r io u s t y p es o fd e p t h p r o f i le s a t R e d W a s h , r a n g i n g f r o m n o d e p t hr e l a t i o n s h i p t o d i s c r e t e d i s c o n t i n u i t i e s . T h e t r a n s i t i o ne l e m e n t s ( C u , F e , M n , N i , V ) s h o w n o a p p a r e n tr e l a t i o n s h i p w i t h s t r a t i g r a p h i c l o c a t i o n i n t h e B r i d g eC r e e k M e m b e r , w h e r e a s p o t a s s i u m a n d m a g n e s i u me x h i b i t g e n e r a l l y d e c r e a s i n g c o n c e n t r a t i o n s u p s e c t i o n( T a b l e 1 ). T h i s is r e a s o n a b l e i n t h e c a s e o f K , w h e r et h e d e c r e a s e i s c o n s i s t e n t w i t h a d e c r e a s e i n c l a s t i ci n p u t u p s e c t i o n ( C u r i a l e , 1 9 9 4 ) . W i t h o n e e x c e p t i o n( s ee F i g . 4 ) , U a n d T h c o n c e n t r a t i o n s e a c h r e a c hm a x i m a a t t h e s a m e p o i n t n e a r t h e C - T b o u n d a r y .T h e s u m m e d [ U + T h ] c o n c e n t r a t i o n i s g r e at e s t

5O

4 9

4 8

4 7

4 6

4 5

4 4

4 2

4 1

4 O

3 9

383 7

3 e

3 50

C e n o m a n l a n

I I I I I I I I I I I I I I I I5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 5 5 0 5 5 6 0 6 5 7 0 7 5 8 0 8 5[ U T h ] p p m

Fig. 4. Distribu tion of total [U + Th] concentration with depth through the Bridge Creek M ember at R edWash. D ata ar e l isted in Table 1. No te that unusually high U a nd Th concentrations are present in asamp le at 39.9 m (sample F664; Table 1). This is attributed to the occurrence o f bentonite in this sample(Curiale e t a l . 1992a). TO C measurem ent in this sample resulted in values below detection limit (0.05 ).Dep th in this figure is measured from an arb itrary datum at the base o f the composite section.


7/14

Anom al ies a t the( e x c l u d i n g t h e b e n t o n i t e s a m p l e n o t e d i n t h e c a p t i o nt o F i g . 4 ) a t 3 8 .8 m , a b o u t 0 . 5 m b e l o w t h e C - Tb o u n d a r y ( T a b l e 1 ; t h e s u m o f U a n d T h i s p l o t t e di n F i g . 4 ) . T h e m a x i m u m v a l u e a t t h i s p o i n t ,a m p l i f i e d f r o m b a c k g r o u n d l e v el s b y a f a c t o r o f 5 , i sw e l l d e f i n e d b y a 1 3 - s a m p l e p e a k i n a h i g h - r e s o l u t i o nin t e rva l .

A l t h o u g h a n o m a l o u s e l e m e n t a l c o n c e n t r a t io n s i nC e n o m a n i a n - T u r o n i a n s e c ti o n s h a v e b ee n o b s e r v e dp r e v i o u s l y ( B r u m s a c k a n d T h u r o w , 1 98 6; O r t h e t a l .1 9 9 3 ) , m o s t o c c u r i n b l a c k s h a l e s e q u e n c e s a n da r e a t t r i b u t e d t o s t a g n a n t ( a n o x i c ) w a t e r - c o l u m nc o n d i t i o n s . P r a t t e t a l . ( 1 9 9 1 ) o b s e r v e d d i s c r e t em a x i m a i n ( f o r e x a m p l e ) M n c o n c e n t r a t i o n s a t t h eC - T b o u n d a r y i n F r a n c e a n d i n a P u e b l o c o r e f r o mt h e C r e t a c e o u s s e a w a y ( s ee a l s o P o m e r o l , 1 98 3) . O r t he t a l . ( 1 9 9 3 ) o b s e r v e d e l e m e n t a l a n o m a l i e s a t o rs l ig h t ly b e l o w t h e C - T b o u n d a r y i n s e ve r al N o r t hA m e r i c a n s e c t io n s , a l t h o u g h a n o m a l i e s w e r e g e n e r a l lya b s e n t i n E u r o p e a n s e c t i o n s . I n c o n t r a s t , a N e b r a s k as e c ti o n o f th e s e a w a y C - T i n t er v a l s h o w e d n o s u c ha n o m a l y ( P r a t t e t a l . 1991) , no r d id a Pueb lo sec t i onm e a s u r e d b y O r t h e t a l . (1988a ) . I t i s no t ed t ha tt h e s e s t u d i e s a r e c h a r a c t e r i z e d b y w i d e v a r i a t i o n s i na n a l y t i ca l m e t h o d s a n d s a m p l i n g r e s o l u ti o n s .

U a n d T h c o n c e n t r a t i o n s o f u p t o 1 2 a n d 8 p p m ,r e s p ec t iv e l y, w e r e m e a s u r e d i n G r e e n h o r n F o r m a t i o nm a r l s t o n e s a n d l i m e s t o n e s a t P u e b l o , C O , u s i n g v e r yc o a r s e s a m p l i n g r e s o l u t i o n s ( Z e l t , 1 9 8 5) . T h e s e v a l u e sa r e s i g n i fi c a n tl y l es s th a n t h o s e o b s e r v e d a t R e d W a s h

C-T bounda ry 493( T a b l e 1 ). A l t h o u g h n o d i s c re t e U o r T h m a x i m aw e r e e v i d e n t a t t h e C - T b o u n d a r y a t P u e b l o ( Z e l t ,1 9 8 5 ) , s u c h m a x i m a h a v e b e e n o b s e r v e d a t o t h e rm a j o r g e o l o g ic a l b o u n d a r i e s ( e. g. t h e M i s s i s s i p p i a n -P e n n s y l v a n i a n b o u n d a r y i n O k l a h o m a a n d T e x a s ;O r t h e t a l . 1 98 6) . E l e v a t e d e l e m e n t a l c o n c e n t r a t i o n s( i n c lu d i n g U a n d T h ) a s s o c i a te d w it h b o u n d a r ye v e n t s a r e o f t e n c o n s i d e r e d t o b e e v i d e n c e f o r p a l e o -i m p a c t s ( A l v a r e z e t a l . 1980) , a l t hough seve ra la n o m a l i e s o c c u r w h i c h a r e u n a s s o c i a t e d w i t h b o u n d -a r i e s ( O r t h e t a l . 1 98 8b ), a n d n o n - c o s m o g e n i c o r i g i n sm u s t b e c o n s i d e r e d ( O r t h e t a l . 1988a, 1993).

G E O C H E M I C A L E V I D E N C E F O R A C-T BOUNDARYEVENT--LOCA L MATURITY EFFECTS?R a d i o g e n i c n u c l ei , i n c l u d i n g t h o s e o f U a n d T h ,

w i l l g e n e r a t e h e a t i n t h e s u b s u r f a c e i n p r o p o r t i o n t ot h e i r c o n c e n t r a t i o n , a n d t h e c a l o r i c i n p u t p e r v o l u m ep e r t i m e t o r o c k s i n t h e e a r t h ' s c r u s t i s c a l c u l a b l e(Bi rch , 1954 ; Roy e t a l . 1968; Rybach, 1986) .G i v e n a d e q u a t e a m o u n t s o f r a d io g e n i c m a t e ri a l, a n ds u f f ic i e n tl y s e n s i ti v e m e a s u r i n g t o o l s , i t m a y b e p o s s -i b l e t o i d e n t i f y c h a n g e s i n t h e t h e r m a l m a t u r i t y o fo r g a n i c m a t t e r r e s u lt in g f r o m e x p o s u r e t o r a d i o g e n ich e a t . U s i n g d a t a f o r U , T h a n d K c o n c e n t r a t i o n s i nT a b l e 1 , a n a v e r a g e s e c t i o n d e n s i t y o f 2 5 0 0 k g / m 3,a n d t h e r e l a t i o n s h i p s p r e s e n t e d b y R y b a c h ( 1 9 8 6 ) ,h e a t p r o d u c t i o n i n t h e R e d W a s h s e c t i o n m a y b ec a l c u la t e d ; v a lu e s o f 5 - 1 0 # W / m 3 a r e r e a c h e d i n t h e

0. ,500

0 . 4 7 5

0 . 4 5 0

0 . 4 2 5I0 . 4 0 0

I 0 . 3 7 5

0 .3SO

0.~.50.300

0 2 7 5

a s ~ =

: o 3 M . ~ m

. . . ' . . .

E R R O R

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a J n l I I I I I I II 10 11 20 25 30 S4 0 4S 90 55 60 66[ U + T h l p p m

5 5

p

~

E R R O Re I

I I I I I I I I I U t

5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 5 5 0 5 5 6 0 6 S[U+Th ]ppm

Fig. 5. Plots show ing the relationship between the to tal [U + Th] concentration (abscissa) and the20S/(20S + 20R)-5~ , 14~, 17~-24-ethylcholestane rat io (left) an d the am ou nt of 5~, 14~, 17~,20R -cholestane(right; relative to tota l C27-30 5~,14~,17~ steranes). N ote tha t the samples w ith the highes t [U + T h ]concentrations are also amo ng the highest in these two molecular ratios.


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49 4 JOSEPH A . CURIALET a b l e 2 . M o l e c u l a r p a r a m e t e r s d e r i v e d f r o m p y r o l y s i s- g a s c h r o m a -t o g r a p h y , m e t a s t a b l e r e ac t i o n m o n i t o r i n g - G C M S a n d c o n v e n t i o n a lG C M S

D e p t h C 2 9 C27m S/S+R %

F625 4g.0 0.41 38.4F827 49,0 0.41 35.~F828 48.7 0.34 39.CF529 48.5 0.31 32..mF630 46.4 0.22 42.~Ff131 48.0 0.32 37.."F632 47.8F633 47.5 0.37 40.(F634 47.2F635 47.0 0.38 40.,F636 46.7 0.32 37.~F637 46.7 0.38 36.;F638 44.4 0.38 40.~Ff139 44.2 0.39 42.4F640 44.1 0.38 37.1F641 ,44.(~ 0.38 41."F642 43.g 0.35 42.~F643 43.7 0.40 43.:F544 43.6 0,37 39.1F645 43.5 0.35 38.~F646 43.3 0.37 40.~F647 43.2 0,40 42.'IF648 43.1 0.40 43A:649 42.9 0.30 45.:=650 42.9 0.38 39.~=651 42.9 0.37 44A=652 42.8 0.37 42.,~653 42.8 0.37 42.1P654 42.7 0.38 40:~855 42.6 0.35 40.'~656 42.6 0.35 38.'.F657 42.4 0.37 40.'F658 42.3 0.37 42J~65~ 42.2 0.33 39.:F660 42.1 0.38 38:F661 0.41 41.:F662 0.33 39:F663 0.3~F664F665 0.3~F666 0.38F667 0.38 45.F668 0.38 38.1F669 0.37 44.,F870 0.37 50.,F571 0.37 50.,F672 0.39 51.,F673 0.42 53.:F674 0.47F675 0.41 54.F676 0.38 52.F879 0.37 44:F880 0.34 46.F681 0.35 46.F682 0.36 49.F683 0.37 46.F664 0.37 49.F685 0.36 47.F686 0.37 48.F687 0.38 51.F688 0.37 49.F689 0.35 44.F690 0.36 4 5F691 0.36 4 9

~

LbS.3 28.725.0 30.627.9 26.621.9 39.023.8 26.325.9 31.421.623,123.925.021.222.125.626,223.527,524.024.825.425.519.523.019.g19.820.822.e24.423.C24.e2524.8:25.926.524.221.5

17.115.317.818.114.3t5.315.015.916.717.514.715.214.715.214,114.815.514.416.715.114.514.3

C 3 0

5.88.56.45.65.35,5

32.0 5.731.7 5.032.4 8 532.1 5.333.3 4.830.8 4.331.8 4.728.4 4.230.5 3.226.5 2.830.4 5.330.8 5.E27.7 6.425.8 8.230.g 5.727.1 4.e34.3 5.130.7 4.731 E 4.E29.3 5.22g.7 5.631.2 5.13 1 . 5 5 . 530.8 4.429.0 4.229.7 5.129.1 5.728.8 6.035.1 3.7

35.2 2.740.5 3.333.7 3.830.1 3.131.3 3.830.5 2.628.2 3.326.3 3.226.5 3.932.2 5.535.5 3.534.4 3.832.1 4.234.3 3.933.6 2.734.3 3.332.8 3.730.7 3.330.5 3.737.3 3.436.0 3.533.3 3.4

3eP-GC

0.500.410.150.800.180.430.480,490,380A00.310.210.280.250,350.290.330.300.410.3~0.340.3(;0.190.250.130.2:0.3,~0.2~0.3510,2E0.290.410.260.260.260.400.310.150.230.150.140.180.380.110.120.080.080.000.040.040.310.320.280.250.270.190.180.270.380.370.280.180.32

The 20S/(20S + 20R) ra t io is der ived f r om the 5 :( (H) , 14= (H) ,17c( (H)-2 4-ethy lchole s tanes . C27, C28, C29 and C~0 repres ent then o r ma l i zed d i s t r i b u t io n o f 5 ~ ( H) , 1 4 = ( H) , 1 7 : ( ( H) , 2 0 R - 2 4 - n -a lk y lch o le s t an es ( mo n i to r ed a s t h e p a ren t- -- * m z 2 1 7 t r an s i t i o n ) .n - C~ 2 /s td i s th e am o u n t o f n - C t2 in t h e k e r o g en p y r o ly za t e ,r e l a ti v e to a n i n t e r n a l s t a n d a rd . M i s s i n g d a t a i n d i c a t e i n c o m p l e t ea n a l y se s . D e p t h i s m e a s u r e d f r o m a n a r b i t r a r y d a t u m a t t h e b a s eo f t h e co mp o s i t e s ec t io n .

C - T b o u n d a r y i n t e r v a l . I t i s n o t e d t h a t h e a t p r o d u c -t i o n o n a p e r s a m p l e b a s i s c o r r e l a t e s a l m o s t e x a c t l y(r = 0.99) with [LT + Th], as exp ected, d ue to relat ivelym i n o r i n p u t f r o m 4 K . T h e r e m a i n d e r o f t h is s e c t io ndi scusses, for the R ed W ash d a ta s e t, t he empi r i ca lr e l a ti o n s h i p s b e t w e e n t h e c o n c e n t r a t i o n o f [ U + T h ]a n d t h e v a lu e s o f v a r i o u s o r g a n i c g e o c h e m i c a lp a r a m e t e r s .

C e r t a i n m o l e c u l a r r a t i o s f r o m e x t r a c t a b l e o r g a n i cm a t t e r a n d k e r o g e n p y r o l y z a t e d i st r i b u ti o n s a c r o s st h e C - T b o u n d a r y a t R e d W a s h a p p e a r c o - v a r i a n tw i t h U a n d T h c o n c e n t r a t i o n s , a n d m a y r e f l e c t l o c a linc reases in the rmal matur i ty l eve l s due to rad iogenich e a t i n g o v e r g e o l o g i c ti m e . H i g h e s t v a l u e s f o r c e r t a i nm o l e c u l a r r a ti o s o c c u r c o n s i s te n t l y w h e r e U a n dT h c o n c e n t r a ti o n s a r e m a x i m a ( u s ua l ly [ U + T h ]> 25 ppm ) . T hi s i s the case for the 20S / (20S + 20R )-5~,14c(,17~-24-ethylcholestane rat io (Fig . 5, left;Table 2) , which i s known to inc rease wi th inc reas ingt h e r m a l m a t u r i t y ( S e i f e r t a n d M o l d o w a n , 1 9 8 6 )A t R e d W a s h , t h e v a r i a t i o n i n t h i s r a t i o f o r a l l b u tthree s amples i s smal l (0 .32-0 .42) , a s expec ted int h i s c o n s t a n t - m a t u r i t y s e c t i o n ( T a b l e 2 ) . H o w e v e r ,the three s amples wi th e l eva ted ra t ios (0 .41-0 .47) a l sohave the h ighes t [U + Th] (> 30 ppm ; F ig . 5 ) .

T h e d i s t r i b u t io n o f s te r a n e s a c c o r d i n g t o c a r b o nn u m b e r a l s o s h o w s a s l i g h t r e l a t i o n s h i p w i t h U a n dT h c o n c e n t r a t i o n s ; t h o s e s a m p l e s h a v i n g t h e h i g h e s t[ U + T h ] a l s o h a v e t h e h i g h e s t p r o p o r t i o n o f t h e C27s t e ra n e h o m o l o g u e ( T a b l e 2; F ig . 5 , r ig h t ). A l t h o u g ht h e s t e r a n e c a r b o n n u m b e r d i s t r i b u t i o n i s d o m i n -ant ly fac ies re l a t ed (Pe te rs and M otd ow an , 1993) ,a few s tudies have sugges ted a re l a t ive inc rease int h e C 27 h o m o l o g u e w i t h i n c r e a s in g t h e r m a l m a t u r i t y(Mac kenz ie , 1984; Cur ia l e , 1986, 1992; van G raas ,1 9 90 ; G a l l a n g o a n d C a s s a n i , 1 9 9 2 ) . A s i n t h e c a s e o fthe 20S / (20S + 20R) ra t io noted ea r l i e r , t he s amplesw i t h a n o m a l o u s s t e r a n e c a r b o n n u m b e r d i s t r i b u t i o n sa ll o c c u r w i t h in 0 . 7 m b e l o w t h e C - T b o u n d a r y .

D e c r e a s e d y i e ld s o f n - a l k a n e s i n k e r o g e n p y r o -l y z a te s o f s a m p l e s ri c h in U a n d T h m a y p r o v i d ef u r t h e r m o l e c u l a r e v i d e n c e o f a r e l a ti o n s h i p b e t w e e n[ U + T h ] a n d m a t u r i t y . T y p i c a l p y r o l y s i s - g a sc h r o m a t o g r a m s ( P - G C ) f o r k e r o g e n s f r o m s a m p l e s3 .0 m a b o v e , 0 . 5 m b e l o w a n d 1 .8 m b e l o w t h e C - Tb o u n d a r y a r e s h o w n i n F i g . 6 ; t h e s e s a m p l e s c o n t a i n13 .5 , 58 .0 an d 9 .5 pp m [U + Th] , re spec t ive ly . Th el o ss o f n - a l k a n e s i n t h e s a m p l e r i c h in U a n d T h ,r e la t iv e t o t h e s a m p l e s l o c a t e d m o r e t h a n 1 .0 m a w a yf r o m t h e C - T b o u n d a r y , i s e v i d e n t w h e n c o m p a r e dt o t h e i n t e rn a l s t a n d a r d . T h e r a t i o o f n - C ~2 t o t h ein te rna l s t andard ( ' n -C~2/s td ' i n Table 2) shows ad i s t i n c t d e c r e a s e i m m e d i a t e l y b e l o w t h e b o u n d a r y(F ig . 7- -note tha t the absc i s sa i s logar i thmic ) . Theinse t to F ig . 7 show s tha t those samp les wi th [U + Th]c o n c e n t r a t i o n s g r e a t e r t h a n 2 5 p p m a ll h a v e n C ~ z /s td ra t ios l e s s than 0 .10 . Recent work by Venka tesanet a l . (1993) shows an ini t ia l increase in the n-C~2/s tdr a t i o o f c o a l p y r o l y z a t e s as m a t u r i t y i n c r ea s e s f r o m0.4 to 0 .7 Ro, fo l lowed by a rap id los s of n-C l2 as


9/14

Anomalies a t the C -T boundary

1 2 o [ - I [ U + T h ] = 1 3 . 5 p p m_E n - e l Is o , ( 2 1 \ 1

-~z . ~ j n . C 1 8

o6 0 S F 6 7 5

[ U + T h ] = 5 8 . 0 p p m0 . 5 m b e l o w C / T

s o n . C 1 2Z ,oo 4,\il

o+ 4 0 t S F 6 8 6+ 2 o I [ U + T h ] = 9 . 5 p p ms o I 1 . a m b e l o w e / r

i s o 0 . 2 7 1s on C18

o 18 . 0 18 . 0 24 . 0 36 . 0 42 . 0 48 . 0 S 4 . 0 60 . 0T IM E M IN U T E S )Fig. 6. Typical kerogen py rogram s from samples 3.5 m abov e (top), 0.5 m below (middle) and 1.8 rn below(bottom) the C enom anian-T uronian b oundary . The in ternal s tandard is designated 'S ' . The C~2 n-alkaneis identified in each chromatogram; the rati o of n-C~2 to the st and ard is shown in parentheses. Also shownis the concentration of [U + Th] in each whole rock sample.

495

V R v a l u e s r e a c h a n d e x c e e d 1 . 0 % R o . T h e r e l a t i o n -s h i p b e t w e e n [ U + T h ] c o n c e n t r a t i o n a n d n - a l k a n ep y r o l y s i s y i e l d a t R e d W a s h i s a l s o c o n s i s t e n t w it ho t h e r p u b l i s h e d P - G C r e s u l t s , w h i c h s h o w t h a tn - a l k a n e s a r e a b s e n t i n p y r o l y z a t e s o f u r a n i u m - r i c ho r g a n i c n o d u l e s ( C u r i a l e , 1 9 8 6 ; C u r i a l e e t a l . 1983).A d d i t i o n a l e v i d e n c e f o r a r e l a t i o n s h i p b e t w e e n[ U + T h ] a n d t h e r m a l m a t u r i t y m a y b e p r e s e n t i n t h eR o c k - E v a l d a t a f o r t h e B r i d g e C r e e k a t R e d W a s h .T h e s e d a t a , l i s t e d i n C u r i a l e ( 1 9 9 4 ) , s h o w t h a t t h o s es a m p l e s h a v i n g [ U + T h ] c o n c e n t r a t i o n s g r e a t e r t h a n

2 5 p p m ( 3 8 . 5 - 3 9 .3 m ) a l s o h a v e 2 y i e l d s t h a t a r ec o n s i s t e n t l y l e s s t h a n 0 . 2 m g / g . I t i s a l s o i n t e r e s t i n g t on o t e t h a t , a l t h o u g h T i n , v a l u e s , p r o d u c t i o n i n d i c e s( P I ) a n d h y d r o g e n i n d ic e s ( H I ) a r e g e n e r a l l y s u s pe c tw h e n 2 y i e l d s a r e b e l o w 0 . 2 m g / g ( P e t e r s , 1 98 6) ,a n o m a l o u s l y h i g h T i n , a n d P I v a l u e s , a n d l o w H Iv a l u e s , o c c u r i n t h o s e s a m p l e s w i t h h i g h [ U + T h ]c o n c e n t r at i o n s , im m e d i a t e ly b e lo w t h e C - T b o u n d a r y .

S e v e r a l w o r k e r s h a v e a d d r e s s e d t h e q u e s t i o n o fr a d i o g e n i c h e a t p r o d u c t i o n a n d i t s e f f e c t o n l o c a lt h e r m a l m a t u r i t y o f o r g a n i c m a t t e r i n s e d i m e n t a r y

OG22/3 5 K


10/14

496 JOSEPH A CURIALE

Qa

SO4 9

4 8

4 7

4 6

4 5

4 4

4 3

4 2

41

4 0

lOO

IO

0 01 IlO

Br idge Creek | U p p e r M i d d bLow e r

I I I I20 30 40 50 60

[ U + T h l p ~ .

uronlanCenoman ian3 9

3 8

373 6

3 50.01 I I I I I I I 1 I I I | I I I I I0 1 0 1 0 0n c 1 2 / s t d

Fig. 7. Distribution with depth of the ratio of n-C~2 o the standard, for kerogen pyrograms of the entiresample set. Note that abscissa is logarithmic. The significant decrease immediately below the C-Tboundary is coincident with other anomalous geochemical parameters in this section. Depth is measuredfrom an arbitrary datum at the base of the composite section. The inset (upper left) shows the relationshipbetween the n-Ci2/standard ratio and total [U + Th]. These data show the correspondence between thesetwo parameters for the entire data set (shown graphically in Fig. 6 for three representative samples). Datapoint symbols indicate assignment to (arbitrary) upper, middle and lower parts of the Bridge CreekMember at Red Wash, and show that the [U + Th]-rich samples occur in the lower one-third of theMember.

rocks (Sheppard, 1944; Middleton, 1979; Keen andLewis, 1982; Curiale e t a l . 1983 ; Rybach , 1986).Positive correlations between vitrinite reflectance anduran ium concentration have been observed in Jurassicfossil woods of New Mexico (Sassen, 1983, 1984) andCretaceous kerogens of Alaska (Curiale, unpublisheddata). Studies of uraniferous Cambrian shale ofSweden show that radioactive decay of U modifiesseveral thermal matu rity parameters, including Rock-Eval s Tmax and produ ctio n index values, methylphen-anthrene indices, triaromatic steroid hydrocarbonside-chain-length distribution and H/C atomic ratiosof kerogen (Dahl e t a l . 1988, 1989; Lewan andBuchardt, 1989). Other data document moleculareffects on organic matte r that are not consistent with

those expected from increased temperature (Lewanand Buchardt, 1989; Sund ara raman and Dahl, 1993)

Conflicting results are available on the effect ofradiogenic damage on isotope ratios. AlthoughLewan and Buchardt (1989) did not observe a changein carbon isotope ratios, Dahl e t a l . (1989) and Eakin(1989) noted increases in ~3C in organic matt er associ-ated with higher levels of uranium. This frac tionat ioneffect is supported by earlier work of Leventhal andThrelkeld (1978) on uranium ores in Jurassic hostrocks of New Mexico. For high-grade uranium ores,these workers observed 13Corg values tha t are heavierthan those in low-grade ores by 5-8/oo.

Altho ugh the data suggest that [U + Th] concen-trations in the Red Wash carbonates control, in part,


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Anomalies a t the C- T boundary 497s e l e c t e d t h e r m a l m a t u r i t y p a r a m e t e r s , a t l e a s t t w oc o n c e r n s a r e p r e s e n t . F i r s t , t h e U a n d T h c o n c e n -t r a t i o n s a t R e d W a s h , a s w e ll a s th e a g e o f t h e h o s tr o c k , a r e m u c h l es s t h a n t h o s e i n th e C a m b r i a n s h a le so f S w e d e n ( D a h l e t a l . 1988, 1989; Lewan andB u c h a r d t , 1 9 8 9 ) . T h e r e f o r e , t h e t o t a l r a d i o g e n i cd a m a g e t o t h e o r g a n i c m a t t e r i n t h e R e d W a s hs a m p l e s s h o u l d b e s u b s t a n t i a l l y l e s s t h a n t h a tobse rved for the Swedi sh sha les . A second concern i st h e p o s s i b i l i t y t h a t t h e g e o c h e m i c a l p a r a m e t e r su n d e r d i s c u s s i o n ( i . e . U a n d T h c o n c e n t r a t i o n s a n dm o l e c u l a r m a t u r i t y r a t i o s ) a r e n o t f u n c t i o n a l l yre la t ed , bu t fac ies re l a t ed . Al though no ev idence i sapparent for a fac ies or ig in for these re l a t ionshipsa t R e d W a s h , n e i t h e r c a n t h e p o s s i b il i ty b e r u l e d o u t .F u r t h e r u n d e r s t a n d i n g o f th e n a t u r e o f o r i g in a le m p l a c e m e n t o f th e U a n d T h ( d i s c u ss e d b e lo w )would be he lpfu l in th i s regard .

CAUSE AND EFFECT: ISOTOPIC AND [U Th]ANOMALIEST h e i s o t o p i c a n d e l e m e n t a l a n o m a l i e s o f t h e R e d

W a s h s e c t io n a l l o c c u r w i th i n 0 . 7 m b e l o w t h e C - Tb o u n d a r y , a l t h o u g h t h e y c o v e r s i g n i fi c a n tl y d if f e re n tt ime scales (cf . Orth e t a l . 1 9 9 3 ) . U s i n g s e d i m e n t a t i o nr a t e d a t a d e r i v e d f r o m b e n t o n i t e d a t e s ( C u r i a l e e t a l .1992a) as we l l a s b ios t ra t igraphic informat ion (E .K a u f f m a n , p e r s o n a l c o m m u n i c a t i o n ) , i t i s c a l c u l a t e dt h a t t h e i s o t o p i c e x c u r s io n r a n g e s o v e r a p p r o x i m a t e l y2 .5 M a, wh ereas the [U + Th] sp ike o ccurs wi th ino n l y 0 . 2 - 0 . 3 M a . A l s o o f in t e re s t i s t h e o b s e r v a t i o nt h a t b o t h a n o m a l i e s a r e g e n e r a l l y d e p t h - s y m m e t r i c a l( F i g s 3 a n d 4 ) , i n c o n t r a s t t o , f o r e x a m p l e , C - Ti s o t o p i c e x c u r s i o n s e l s e w h e r e ( K u h n t e t a l . 1990).T h e f a c t t h a t e x c u r s i o n m a x i m a a r e c o e v a l a n d s y m -met r i ca l sugges t tha t a s ingle event i s re spons ib le fort h e g e o c h e m i ca l p e r t u r b a t io n s a t R e d W a s h . H o w -e v e r , th e u n e v e n i n f lu e n c e o f t h e e v e n t o n ~ 3 C o r g a n d[U + Th] sugg es t s tha t the s ens it iv i ti es of these tw oparamete rs to th i s event d i f fe r .

S e v e ra l o r ig i n s f o r t h e [ U + T h ] m a x i m u m m a y b eproposed , and a l l may be c l as s i f i ed as e i the r pr imary( i . e . t hese e l ement s were depos i t ed wi th the or ig ina lm i n e r a l / o r g a n i c d e t r i t u s , a n d a r e f a c i e s / d e p o s i t i o n a li n o r i g i n ) o r s e c o n d a r y ( i . e . t h e y e n t e r e d t h e C - Tz o n e o f t h e s e d i m e n t a r y c o l u m n a t s o m e ti m e a f t e rd e p o s i t i o n o f t h e m i n e r a l s a n d o r g a n i c m a t t e r ) . A tl eas t th ree poss ib i l i t i e s ex i s t for a pr imary or ig ino f [ U + T h ] . ( 1 ) U a n d T h m a y o c c u r in a s h f al l f r o mu n u s u a l l y p r o m i n e n t v o l c a n i c a c t i v i t y a t t h e p r e c i s et i m e o f t h e C - T b o u n d a r y . T h i s e ff ec t c o u l d b eh e i g h t e n e d i f n o n - a s h s e d i m e n t a t i o n r a t e s w e r e u n -u s u a l l y l o w d u r i n g t h is p e r i o d . ( 2 ) T h e e l e m e n t s m a ybe as soc ia ted wi th cosm oge nic pa rt i c les , re l a t ed to ab o l i d e i m p a c t . ( 3 ) A n u n u s u a l o r g a n i c m a t t e r t y p em a y i n c o r p o r a t e ( d u r i n g b i o s y n t h e s i s ) U a n d T h , o rh a v e a d s o r b e d U a n d T h d u r i n g p a s s ag e t h r o u g h t h ewate r co lum n. C om bin a t io ns of these possib il it ie scould a l so occu r (e .g . adsorp t io n o f unusu a l quant i t i e s

o f c o s m o g e n i c c a t i o n i c U a n d T h s pe ci es o n t o c l a y sdur ing se t t l ing in the wa te r co lumn) . In genera l , t hesethree poss ib i l i t i e s a re inadequa te . (1) and (2) may bes e t a s i d e , b e c a u s e t h e U a n d T h s p i k e s a t R e d W a s ha r e n o t a c c o m p a n i e d b y s p i k e s i n o t h e r c o s m o g e n i can d/o r volca nog enic elements (e .g. i r idium). Poss ibi l i ty( 3) i s s e t as i d e b e c a u s e [ U + T h ] c o n c e n t r a t i o n s s h o wn o r e l a t i o n s h i p w i t h a n y f a c i e s - r e l a t e d m o l e c u l a rp a r a m e t e r s i n t h e s a m p l e s e t, s u g g e s t i n g t h a t o r g a n i cm a t t e r t y p e d o e s n o t v a r y d i s ti n c ti v e ly w i t h i n a m e t e ro f th e C - T b o u n d a r y .

At l east fou r s econ dary -or ig in poss ib i li ti es ex i s tf o r th e [ U + T h ] . ( 1) T h es e e le m e n ts c o u l d b ec o n c e n t r a t e d a t th e C - T b o u n d a r y b e c a u se o f th ep r e s e n c e o f u n u s u a l r e d o x c o n d i t i o n s a t t h is b o u n d -a ry (e .g . , o rganic mat t e r concent ra t ion) , re su l t ing int h e s tr i p p i n g o f s o lu b l e f o r m s o f U a n d T h f r o mm o v i n g s u b s u r f a c e w a t e r s. T h i s i s c o n s i s t e n t w i t h t h ek n o w n e f fe ct o f v a r y i n g E h a n d p H c o n d i t i o n s o n Uan d Th species s tabi li ties (e .g. Lan gm uir , 197 8). Th isposs ibi li ty is re jected because (a) TO C values are low era t the bou nd ary than abo ve or be low i t (Curial e, 1994);a n d ( b ) a l t h o u g h U m a y b e r e d u c e d f r o m i t s h e x a -v a l e n t s o l u b l e f o r m t o a t e t r a v a l e n t i n s o l u b l e f o r m ,T h s o l u b i l it y i s g e n e r a ll y n o t v a l e n c e - c o n t r o l l e d(e .g . , Ke leper t s i s , 1981; S . B loch , pe rsona l commun-i c a t i o n ) . ( 2 ) U a n d T h m a y b e s e c o n d a r i l y c o n c e n -t r a t e d a t t h e C - T b o u n d a r y d u e t o r e w o r k i n g o fp r e - C - T s e d i m e n t s d u r i n g t h e s l o w s e d i m e n t a t i o nra tes charac te r i z ing the C-T event pe r iod ( J . Pa rne l l ,p e r s o n a l c o m m u n i c a t i o n ) . T h i s p o s s i b i l i t y i s i n c o n -s is t en t w i t h k n o w n s e d i m e n t a t i o n r a t e s ( C u r ia l e e t a l .1992a) , which do not dec rease suf f i c i en t ly in pro-p o r t i o n t o t h e o b s e r v e d i n c r ea s e in U a n d T h c o n c e n -t r a t i o n s . ( 3 ) T h e i r c o n c e n t r a t i o n c o u l d b e d u e t oa d s o r p t i o n o f U a n d T h o n t o c l a y s p r e s e n t i n e l e v a te dc o n c e n t r a t i o n a t t h e C - T b o u n d a r y . B o t h U a n d T hare readi ly adsorbed onto c l ays (both i l l i t e s andm o n t m o r i l l o n i t e s ) f r o m a q u e o u s s o l u t i o n s a t v a r i o u scondi t ions (S ika l id i s e t a l . 1989; Bonot to , 1989;T s u n a s h i m a e t a l . 1981) . (4) E leva ted [U + T h] con-c e n t r a t i o n s c o u l d b e d u e t o c o p r e c i p i t a t i o n o f th e s ee l e m e n t s d u r i n g d o l o m i t i z a t i o n ( M r l l e r e t a l . 1976).Thi s poss ib i l i ty i s cons i s t en t wi th the covar i anceo b s e r v ed b e t w e e n [ U + T h ] a n d m a g n e s i u m c o n c e n -t r a t i o n i n t h e R e d W a s h s a m p l e s ( T a b l e 1 ) - - [ U + T h ]c o n c e n t r a ti o n s g r e a te r t h a n 2 0 p p m c o - o c c u r w it hM g c o n c e n t r a t i o n s g r e a t e r t h a n 1 % . T h i s p o s s i b i l i t ywould imply se lec t ive dolomi t i za t ion in the Red Washs e c t i o n p r e c i s e l y a t t h e C - T b o u n d a r y .

O n l y s e c o n d a r y a c q u i s i ti o n p r o c e ss e s, e i t h e r d u r i n gd o l o m i t i z a t i o n o r r e s u l t i n g f r o m a d s o r p t i o n o n t oc l a y s , a p p e a r t o b e f e a s i b l e m e c h a n i s m s f o r t h ee n r i c h m e n t s o b s e r v e d . T h e c o r r e s p o n d e n c e b e t w e e nM g a n d [ U + T h ] m a y s u g g e s t d o l o m i t i z a t io n a s t h el ink , however th i s conc lus ion i s unsa t i s fy ing becausei t le av e s u n a n s w e r e d t h e q u e s t i o n o f w h y d o l o m i t -i za t ion should occur a t g rea te r ra t es prec i se ly a t theC - T b o u n d a r y . F u r t h e r m o r e , M g / C a r a t i o s ( c a l c u -l at e d f r o m M g d a t a i n T a b l e 1 a n d C a d a t a f r o m


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98 JOSEPH A . CURIALEC u r i a l e , 1 9 94 ) a t t h e [ U + T h ] s p i k e r e a ch m a x i m ao f o n l y a b o u t 0 . 1 . O n t h i s b a s i s, i t m a y b e c o n c l u d e dt e n t a ti v e l y t h a t U a n d T h a d s o r p t i o n o n t o a d i s t i n ct i v ec l a y c o m p o s i t i o n ( e i t h e r q u a n t i t a t i v e o r q u a l i t a t i v e )a t t h e C - T b o u n d a r y a c c o u n t s f o r t h e [ U + T h ] sp i k e.C o n f i r m a t i o n o f t h i s c o n c l u s i o n a w a i t s t h e r e s u l t s o fa d e t a i l e d m i n e r a l o g i c a l e x a m i n a t i o n o f cl a y a n dc a r b o n a t e c o m p o s i t i o n s a c ro s s th i s b o u n d a r y a t R e dW a s h . S u c h a p r o j e c t is c u r r e n t l y u n d e r w a y .

CONCLUSIONST h i s s t u d y h a s s h o w n t h a t e l e m e n t a l , i s o t o p i c

a n d m o l e c u l a r a n o m a l i e s o c c u r a t t h e C - T b o u n d a r yi n c a r b o n a t e s a n d c a l c a r e o u s s h a l e s o f t h e B r i d g eC r e ek M e m b e r o f th e G r e e n h o r n F o r m a t i o n a t R e dW a s h , N e w M e x i co . A l t h o u g h a n o m a l o u s m o l e c u l a rp a r a m e t e r s m a y b e a t t r i b u t e d t o r a d io g e n i c h e a t i n gc a u s e d b y t h e o r i g i n a l d e p o s i t i o n o r s e c o n d a r ya d d i t i o n o f U a n d T h t o t h e s e d i m e n t s , t h e ~ ~3Co~ge x c u r s io n a n d t h e [ U + T h ] c o n c e n t r a t i o n a n o m a l ya r e r e s u l t s o f , r e s p e c t i v e l y , g l o b a l p h e n o m e n a ( e . g .l at e C e n o m a n i a n - e a r l y T u r o n i a n t r a n s g r es s io n ) a n ds e c o n d a r y e n r i c h m e n t .

I t i s p o s s i b l e t h a t a s i n g l e e v e n t i s r e s p o n s i b l e f o rb o t h t h e i s o t o p e e x c u r s i o n a n d t h e [ U + T h ] s p ik e .A s n o t e d p r e v i o u s l y , t h e 1 3 - p o i n t [ U + T h ] e x c u r s i o ni s c o e v a l w i t h t h e i s o t o p i c a l l y h e a v i e s t p o r t i o n o fthe 13C org exc urs ion ( - - 21 .8 to - -21 .4 o ; Ta b le 1 ) ,a l t h o u g h t h e d u r a t i o n o f t h e [ U + T h ] s p i k e w a sc o n s i d e r a b l y le s s t h a n t h a t o f t h e i s o t o p e e x c u r s io n .N e v e r t h e l e s s , i t i s t e m p t i n g t o t i e t h e t w o o b s e r v -a t i o n s t o g e t h e r w i t h i n a s i n g l e m o d e l c f O r t h et a l . ,1 9 9 3) , a n d f o r t h i s t o b e f e a s i b l e a l i n k m u s t b ee s ta b l is h e d b e t w e e n t h e d r a w d o w n o f a t m o s p h e r i cC O 2 a t C - T t i m e a n d t h e p r e f e r e n t i a l d e p o s i t i o n o fa d s o r p t i v e c l ay s . S u c h a l i n k i s u n l i k e l y f r o m at r a n s g r e s s i v e m o d e l s t a n d p o i n t , b e c a u s e m a x i m u mC O 2 d r a w d o w n s h o u l d b e a s s o ci a te d w i t h m a x i m u mt r a n s g r e s s i o n , a n d t h e r e f o r e m a x i m u m w e s t w a r dm o v e m e n t o f t h e l a te C e n o m a n i a n s h o r e l in e i n th eC r e t a c e o u s s e a w a y ( C u r i a l e et a l . , 1992). But th ism a x i m u m d i s t a n c e f r o m s h o r e ( a n d t h e p r o b a b l ei n c r e a s e in w a t e r d e p t h ) s h o u l d m i n i m i z e c l a y d e tr i t a li n p u t . T h u s , t h e i s o to p e e x c u r s i o n m o d e l a p p e a r s a to d d s w i t h t h e p r o p o s a l t h a t c l a y a d s o r p t i o n a c c o u n t sf o r t h e [ U + T h ] s p i ke . T h e a c t u a l s o u r c e o f T h ( w a t e rc o l u m n ? h e a v y m i n e r a l s ?) a ls o r e m a i n s u n d e t e r m i n e d .O n g o i n g e f fo r ts a t m i n e r a l o g i c a l d e f i n i t io n o f t h es e ct i on w i t h i n a m e t e r o f t h e C - T b o u n d a r y a t R e dW a s h m a y e v e n t u a l l y p r o v i d e e v i d e n c e t o l i n k t h e set w o p h e n o m e n a i n t o a s in g l e m o d e l .Ac k now l e dge m e n t s - - I apprec iate samplin g assistance fromR. Cole , R. Witmer and H. Leff ingwel l . Samples wereob ta ined on the N ava jo In d ian R es e rva tion , and the au tho rswould l ike to thank the N avajo N at ion for grant in g per-miss ion for the f ie ld work. Sample prepa ra t ion and analyseswere condu cted by R. Becke s , M. Desai, Z. W ilk andR. Lukas iewicz. Th ank s go to S. Bloch, R. Ha ddad, B.Bromley, and J. Parnell for useful discussions of the data

and the ir i n t e rp re ta t ion , and to G . I s aks en and G . Khava r iKhorasani for helpful reviews of an earlier version. I alsoappreciate supp ort from U noca l in cond ucting and publishingthis research.

R E F E R E N E S

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