In: Neotectonks and sea level voriations in the Gulf of California oreo, a Symposium Hermosillo, Son., April 21-23, 1984), Mol ico Cruz b. Celis-Gutiérrez, S., Guerrero-Gorcío, f &-Ortlihb, i. (eds.), Univ. Nol. Autón. México, Inst. Geología, M6xico, D. F.

7

A M I N O A C I D D A T I N G OF Q U A T E R N A R Y MARINE TERRACES A T B A H I A ASUNCION, B A J A CALIFORNIA SUR, M E X I C O

K E E N A N , E v e r l y M.

ORTLIEB; L u c

WEHMILLER, J o h n F.

S h e l l D e v e l o p m e n t Co. P.O. Box 481 H o u s t o n , T e x a s , 7 7 0 0 1 , U.S.A.

O f f i c e d e l a R e c h e r c h e S c i e n t i f i q u e e t T e c h n i q u e Out r e-Mer 2 4 , Rue B a y a r d 7 5 0 0 8 P a r i s , F r a n c e

D e p a r t m e n t o f G e o l o g y U n i v e r s i t y o f D e l a w a r e N e w a r k , D e l a w a r e , 1 9 7 1 1 , U.S.A.

ABSTRACT

In t h e a r e a of B a h i a A s u n c i o n , o n t h e P a c i f i c c o a s t o f t h e B a h i a C a l i f o r n i a p e n i n s u l a , a m i n o a c i d r a c e m i z a t i o n d a t i n g h a s b e e n u s e d t o e s t i m a t e a g e s o f m o l l u s k s f r o m Q u a t e r n a r y m a r i n e t e r r a c e s . E i g h t e e n m o l l u s k s s a m p l e s ( o f t h e g e n e r a T i v e l a , S a x i d o m u s , a n d C h i o n e ) f r o m t e n l o c a l i t i e s h a v e b e e n a n a l y z e d . The h i g h mean a n n u a l t e m p e r a t u r e f o r t h e r e g i o n ( g r e a t e r t h a n 2 0 d e g r e e s C) h a s r e s u l t e d i n e x t e n s i v e r a c e m i z a t i o n i n w h a t a r e c o n s i d e r e d t o b e l a t e P l e i s t o c e n e low t e r r a c e l o c a l i t i e s . R a c e m i z a t i o n f o r m o s t a m i n o a c i d s i s e f f e c t i v e l y c o m p l e t e by a b o u t 3 0 0 , 0 0 0 y e a r s . Two a m i n o a c i d s , l e u c i n e a n d v a l i n e , d e m o n s t r a t e e n o u g h " r e s o l v i n g p o w e r " t o b e u s e d t o d e l i n e a t e d i f f e r e n t a g e g r o u p s among t h e t e r r a c e s i t e s . Where t h e s e a p p a r e n t g r o u p s a r e t e s t a b l e w i t h s t r a t i g r a p h i c o r g e o m o r p h i c e v i d e n c e , t h e y a r e g e n e r a l l y c o n s i s t e n t w i t h t h e a v a i l a b l e g e o l o g i c c o n t r o l . The a g e s e s t i m a t e d f o r t h e t h r e e a m i n o s t r a t i g r a p h i c g r o u p s r e c o g n i z e d i n t h i s s t u d y a r e a p p r o x i m a t e l y 1 2 0 , 0 0 0 , 2 0 0 , 0 0 0 a n d ?300,000 y e a r s .

I N T R O D U C T I O N

A l o n g t h e V i z c a i n o P e n i n s u l a , Baja C a l i f o r n i a Sur . , a n d , i n p a r t i c u l a r , i n t h e r e g i o n o f B a h i a A s u n c i o n ( F i g u r e 11, a s e r i e s o f O u a t e r n a r v m a r i n e t e r r a c e s e x i s t s t h a t r a n z e i n e l e v a t i o n f r o m

_ - The l o w e s t t e r r a c e s h a v e b e e n t e n t a t i v e l y c o r r e l a t e d o n t h e b a s i s o f e l e v a t i o n a n d m o r p h o l o g y a n d a r e t h o u g h t t o r e p r e s e n t t h e l a s t m i d d l e t o l a t e Q u a t e r n a r y h i g h s e a - s t a n d s c o r r e s p o n d i n g . t o t h e

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F i g u r e 1: S a m p l e l o c a t i o n s , B a h i a A s u n c i o n , s o u t h e r n V i z c a i n o P e n i n s u l a ( ß a j a C a l i f o r n i a Sur, M e x i c o ) .

K e e n a n , O r t l i e b , a n d W e h m i l l e r

o x y g e n - i s o t o p e c u r v e o f S h a c k l e t o n a n d Opdyke ( 1 9 7 3 ) ( 0 r t l i e b , 1 9 7 8 , 1 9 7 9 b , a n d 1 9 7 9 ~ ) .

A l o n g t h e P a c i f i c c o a s t o f N o r t h A m e r i c a , s o l i t a r y c o r a l s h a v e b e e n d a t e d by U r a n i u m - s e r i e s m e t h o d s a t t h e f o l l o w i n g low t e r r a c e l o c a l i t i e s : C a y u c a s , C a l i f . ( V e e h a n d V a l e n t i n e , 1 9 6 7 ) ; N e s t o r T e r r a c e (Ku a n d K e r n , 1 9 7 4 ) a n d E e l P o i n t T e r r a c e , C a l i f . (Muhs a n d S z a b o , 1 9 8 2 ) ; a n d M a g d a l e n a Bay, B a j a C a l i f o r n i a S u r , M e x i c o (Omura e t al., 1 9 7 9 ) . T h e d a t e s o n t h e s e s a m p l e s a p p e a r t o r e p r e s e n t t h e 1 2 0 , 0 0 0 t o 1 3 0 , 0 0 0 y e a r h i g h s e a - s t a n d t h a t i s r e c o r d e d by m a r i n e i s o t o p e r e c o r d s ( S h a c k l e t o n a n d O p d y k e , 1 9 7 3 ) a n d n u m e r o u s t r o p i c a l i s l a n d c o r a l t e r r a c e r e c o r d s ( e g . , Bloom s-f

C a l i f o r n i a , s e v e r a l l a s t i n t e r g l a c i a l r a d i o m e t r i c d a t e s hav'e b e e n o b t a i n e d ( B e r n a t et d., 1 9 8 0 ) .

al., 1 9 7 4 ) . A l s o , o n t h e S o n o r a n c o a s t o f t h e G u l f o f 7,

I \ \ A b s o l u t e d , a t i n g o f m a r i n e d e p o s i t s by U r a n i u m - s e r i e s h a s nol: beeh

t o t a l l y s u c c e s s f u l i n t h e V i z c a i n o P e n i n s u l a ( O r t l i e b -%I., i n p r e p a r a t i o n ) . I n t h i s s t u d y , t h e m a j o r i t y o f t h e s a m p l e s a n a l y z e d were m o l l u s k s w h i c h a r e t h e m o s t common m a c r o i n v e r t e b r a t e s f o u n d i n Q u a t e r n a r y d e p o s i t s . H o w c v e r , m o l l u s k s d o n o t a p p e a r t o b e a s r e l i a b l e a s c o r a l f o r U - s e r i e s d a t i n g ( K a u f m a n et al., 1 9 7 1 ) . A l s o , O r t l i e b ?J. ( i n p r e p a r a t i o m ) f o u n d t h a t d i a g e n e t i c a l t e r a t i o n a n d c o n t a m i n a t i o n w a s w i d e s p r e a d i n t h e s a m p l e p o p u a l t i o n w h i c h d i d i n c l u d e a f e w c o r a l s .

B e c a u s e a b u n d a n t f o s s i l m o l l u s c a n m a t e r i a l i s o f t e n a v a i l a b l e i n Q u a t e r n a r y d e p o s i t s , s a m p l e s o f t h i s t y p e h a v e b e e n u s e d by n u m e r o u s w o r k e r s i n e v a l u a t i o n a n d a p p l i c a t i o n o f t h e a m i n o a c i d r a c e m i z a t i o n ( A A R ) d a t i n g m e t h o d . T h i s r e c e n t m e t h o d r e l i e s u p o n t h e c o n v e r s i o n ( r a c e m i z a t i o n ) o f L-amino a c i d s ( " l e f t - h a n d e d " ) p r e s e n t i n l i v i n g m o l l u s k s h e l l s t o a n e q u i l i b r i u m m i x t u r e of 50% L - f o r m s a n d 50% D-forms ( " r i g h t - h a n d e d " ) . D / L v a l u e s i n c r e a s e f r o m 0 .0 t o 1 . 0 w i t h i n c r e a s i n g a g e o f t h e s a m p l e , a n d i n d i v i d u a l c l u s t e r s o f D / L v a l u e s a r e of t e n r e c o g n i z e d a s " a m i n o s t r a t i . g r a p h i c u n i t s " o r " a m i n o z o n e s " w i t h i n a s p e c ' i f i c g e o g r a p h i c r e g i o n ( M i l l e r a n d H a r e , 1 9 8 0 ) . T h e s e a m i n o a c i d s r e p r e s e n t t h e r e s i d u u m o f t h e o r i g i n a l c a l c i f i c a t i o n p r o t e i n , a n d t h e r a c e m i z a t i o n of t h e a m i n o a c i d s i s o n e o f s e v e r a l d i a g e n e t i c r e a c t i o n s t h a t o c c u r . W e h m i l l e r ( 1 9 8 2 ) h a s r e c e n t l y r e v i e w e d many of t h e a p p l i c a t i o n s o f a m i n o a c i d r a c e m i z a t i o n d a t i n g t o Q u a t e r n a r y c o a s t a l c h r o n o l o g i c p r o b l e m s .

Amino a c i d r a c e m i z a t i o n d a t i n g h a s b e e n e m p l o y e d e x t e n s i v e l y i n s t u d i e s o f Q u a t e r n a r y d e p o s i t s of t h e P a c i f i c c o a s t o f t h e U n i t e d S t a t e s ( W e h m i l l e r d., 1 9 7 7 ; L a j o i e e., 1 9 7 9 ; K a r r o w a n d B a d a , 1 9 8 0 ; Kennedy d., 1 9 8 2 ; a n d Muhs, 1 9 8 3 ) . R e s u l t s f r o m Woods ( 1 9 8 0 ) , W e h m i l l e r a n d E m e r s o n ( 1 9 8 0 1 , a n d E m e r s o n et aJ. ( 1 9 8 1 1 , p e r m i t t h e e x t e n s i o n o f t h e U n i t e d S t a t e s

1 a m i n o s t r a t i g r a p h i c s e q u e n c e s t o l o w e r l a t t i t u d e s ( W e h m i l l e r ,

152

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153

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1982: Figure 6 ) . These studies have' demonstrated that D/L ratios ; : 4 increase in samples of increasing stratigraphic age, and in 4 .

- samples of equal age at lower lattitudes (higher temperatures). ' In addition, D/L analysis of different amino acids and different genera have permitted relative intrageneric and intergeneric racemization rates to be established. This allows for the conversion of the results for one genus into equivalent results for another genus thus enabling direct aminostratigraphic comparison (Lajoie et al., 1980; Wehmiller, 1982).

m Calibration of the racemization kinetics to the local

"effective temperature" can be made by using amino acid analyses of samples from localities that have been independently dated. Because several of these localities now exist for the mid-lattitude Pacific coast of North America, aminostratigraphic age estimates can be made by simple interpolation between the D/L data points for the 120,000 year dated sites (Wehmiller, 1982). Kinetic models of racemization are needed to extrapolate to age . m estimates for otherwise un-dated localities, and can be tested d with data from calibration localities and information on the climatic history Qf the region. The mid-lattitude Pacific coast of North America has been an excellent region in which to study the combined effects of age and temperature o n racemization. This is primarily due t b the relative abundance of calibration sites and the moderate late Pleistocene temperature change recorded in this region.

The present investigation was undertaken to evaluate the utility of amino acid racemization methods in resolving discrete time intervals of terrace.formation in a high temperature environment (current m.ean annual temperature is about 20 degrees Cl. Since samples from terraces of different elevations are available, the consistency of the aminostratigraphy with various 3eologic criteria could be evaluated, and the effective age limits at these temperatures could be determined. Three apparent aminostratigraphic units have been found for the Bahia Asuncion 'region and they generally conform to local stratigraphic control. Based upon kinetic modeling, it appears that the amino acid racemization methods employed in this area reach their limit of iisefullness in approximately 300,000 years.

METHODS AND RESULTS

Ten localities (Figures 1 and 2 ) were sampled by one of u s tL.0.1 in 1977. Eighteen mollusk samples (of the genera Tivela. Saxidomus, and Chione) were analyzed from these sites. Ideally, more samples should be studied to improve the statistics of each sminostratigraphic group. Amino acid enantiomeric (DILI ratio a'nalyses were made by gas chromatographic methods described by Kvenvolden d. (19721, Frank &. (19771, Wehmiller et al. (19771, and Wehmiller and Emerson (1980). Using different derivitazation procedures, it has been possible to determine the

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D / L v a l u e s f o r a s many a s s e v e n a m i n o a c i d s i n m o s t o f t h e s a m p l e s s t u d i e d h e r e . D / L d a t a a r e r e p o r t e d i n T a b l e 1. G e n e r a l l y , t h e r e i s g o o d a g r e e m e n t b e t w e e n t h e two d e r i v a t i v c p r o c e d u r e s u s e d ( T a b l e 1) , a l t h o u g h a s l i g h t c o n v e r s i o n h a s b e e n u s e d ( s e e W e h m i l l e r a n d E m e r s o n , 1 9 8 0 ) f o r t h e c o m p a r i s o n o f l e u c i n e D / L v a l u e s o b t a i n e d by t h e d i f f e r e n t p r o c e s s e s . The r e s u l t s r e p o r t e d i n T a b l e 1 r e p r e s e n t t h e mean v a l u e s of p e a k h e i g h t r a t i o d e t e r m i n a t i o n s f r o m a t l e a s t two c h r o m a t o g r a m s o f e a c h s a m p l e d e r i v a t i v e . O v e r a l l p r e c i s i o n o f m u l t i p l e s a m p l e a n a l y s e s of w e l l - p r e s e r v e d s h e l l s i s u s u a l l y b e t w e e n 5% and d e p e n d i n g o n t h e a m i n o a c i d .

D I'S C U s s I O N

The m o l l u s k s e x a m i n e d i n t h i s s t u d y g e n e r a l l y h a d r e l a t i n t r a g e n e r i c r a c e m i z a t i o n r a t e s s i m i l a r t o t h o s e o b s e r v e d by

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L a i o i e g . ( 1 9 8 0 ) : p h e n y l a l a n i n e > a l a n i n e z p r o l i n e > l e u c i n e > g l u t a m i c a c i d b v a l i n e . A s t h e D / L v a l u e s i n c r e a s e d i n t h e s a m p l e s s t u d i e d , t h e s e i n t r a g e n e r i c t r e n d s b e c a m e l e s s p r o n o u n c e d a n d s o m e t i m e s were i n v e r t e d i n t h e m o s t e x t e n s i v e l y r a c e m i z e d s a m p l e s . V a l i n e , l e u c i n e , a n d g l u t a m i c a c i d w e r e t l i Q o n l y a m i n o a c i d s t h a t were n o t r a c e m i c i n t h e o l d e s t s a m p l e s . V a l i n e a n d l e u c i n e f o r m t h r e e a p p a r e n t c l u s t e r s o f D / L d a t a ( F i g u r e 3 ) . G l u t a m i c a c i d d a t a a r e g e n e r a l l y c o n s i s t e n t w i t h t h e s e c l u s t e r s b u t a r e m o r e v a r i a b l e b e c a u s e o f t h e d i f f e r e n t a n a l y t i c a l m e t h o d s u s e d ( T a b l e 1). A l a n i n e , p r o l i n e , p h e n y l a l a n i n e , a n d a s p a r t i c a c i d a r e a l l s o e x t e n s i v e l y r a c e m i z e d ( o r a n a l y t i c a l l y v a r i a b l e , a s i n t h e c a s e o f a s p a r t i c a c i d ) t h a t t h e y a r e o f l i t t l e v a l u e i n a m i n o s t r a t i g r a p h i c a p p l i c a t i o n s a t t h e s e t e m p e r a t u r e s .

The t h r e e w e l l - r e s o l v e d g z o u p s o f l e u c i n e a n d v a l i n e D / L v a l u e s a r e p l o t t e d i n F i g u r e 3 . The mean v a l u e s a n d s t a n d a r d d e v i a t i o n s f o r t h e s e t h r e e g r o u p s a r e a l s o p l o t t e d i n F i g u r e 3 . T h e s e mean D/L v a l u e s , a n d t h e c o r r e s p o n d i n g g r o u p d e s i g n a t i o n s , a r e a s f o l l o w s : G r o u p I , l e u c i n e , 0 . 6 6 0 ; v a l i n e , 0.500; G r o u p I I , l e u c i n e 0 . 7 4 6 ; v a l i n e , 0 . 6 7 1 ; G r o u p I I I , l e u c i n e , 0 . 8 7 8 ; v a l i n e , 0 . 8 6 0 . S a x i d o m u s D / L v a l u e s h a v e b e e n c o n v e r t e d ( b y r e g r e s s i o n e q u a t i o n s i n L a i o i e e t a l . , 1 9 8 0 ) t o r a t i o s t h a t w o u l d b e d i r e c t l y c o m p a r a b l e t o T i v e l a o r C h i o n e r e s u l t s . V a l i n e d i f f e r e n t i a t e s t h e t h r e e p o p u l a t i o n s m o r e c l e a r l y b e c a u s e o f i t s s l o w e r r a t e o f r a c e m i z a t i o n .

T h e a g e s r e p r e s e n t e d by t h e t h r e e l e u c i n e - v a l i n e g r o u p s o f F i g u r e 3 a r e d e r i v e d f r o m t h e i s o c h r o n s shown i n F i g u r e 4 ( r e d r a w n f r o m W e h m i l l e r , 1 9 8 2 ) . T h e s e i s o c h r o n s a r e d r a w n f o r o n e a m i n o a c i d , l e u c i n e , a n d o n e g e n u s , ( P r o f o t h a c a . w h i c h i s k i n e t i c a l l y e q u i v a l e n t t o C h i o n e a n d T i v e l a ) The i s o c h r o n f o r " e a r l y S t a g e 5" r e p r e s e n t s t h e D/L d a t a f o r 1 2 0 , 0 0 0 y e a r c a l i b r a t i o n l o c a l i t i e s b e t w e e n c e n t r a l C a l i f o r n i a a n d M a g d a l e n a Bay. A l l o t h e r i s o c h r o n s d r a w n i n F i g u r e 4 a r c d e r i v e d f r o m t h e l e u c i n e k i n e t i c m o d e l o f W e h m i l l e r a n d B e l k n a p ( 1 9 7 8 1 , u s i n g t h e

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D / L l e u c i n e a n d v a l i n e r a t i o s f o r a l l m o l l u s k s e x a m i n e d i n t h i s s t u d y . A v e r a g e v a l u e s a r e r e p o r t e d i

f o r m u l t i p l e s a m p l e s o f t h e s a m e s p e c i e s f r o m t h e same l o c a l i t y . D/L l e u c i n e r a t i o s f o r S a x i d o m u s , a f a s t r a c e m i z i n g g e n u s , h a v e b e e n c o n v e r t e d t o e q u i v e l e n t v a l u e s f o r s l o w r a c e m i z i n g g e n e r a by t h e r e g r e s s i o n e q u a t i o n s o f L a j o i e e t a l . ( 1 9 8 0 ) . T h i s was d o n e t o e n a b l e t h e c o m p a r i s o n o f T i v e l a a n d C h i o n e ( s l o w r a c e m i z i n g g e n e r a ) r a t i o s w i t h t h o s e of S a x i d o m u s . r e p r e s e n t e d o n t h i s d i a g r a m . Symbols a r e as fol lows: T i v e l a , one sample (a), two samples, average (0); Saxidomus, one sample, ( m 1, two samples, average (0) ; Chione, 2-3 samples, average (A).

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California I Bajo Califorcia Washington

Oregon I

PRESENT MEAN ANNUAL TEMPERATURE, *C. - I I 13.3 16 21.5

8 I O 12 14 16 18 20 22 I I I I I I I I

E F F E C T I V E QUATERNARY TEMP. , * C .

F i g u r e 4: A m i n o s t r a t i g r a p h y a n d k i n e t i c m o d e l i n t e r p r e t a t i o n o f P a c i f i c c o a s t e n a n t i o m e r i c d a t a , a l l d e p i c t e d a s P r o t o t h a c a D/L l e u c i n e v a l u e s . M o d i f i e d f r o m W e h m i l l e r ( 1 9 8 2 : F i g . 4 ) . The h e a v y l i n e r e p r e s e n t s c a l i b r a t e d 1 2 0 , 0 0 0 y e a r D/L d a t a . O t h e r i s o c h r o n s r e p r e s e n t D / L v s . e f f e c t i v e t e m p e r a t u r e ( i n v e r s e l y p r o p o r t i o n a l t o l a t t i t u d e ) a s d e t e r m i n e d f o r D/L d a t a f o r i n d e p e n d e n t l y d a t e d s i t e s , e x t r a p o l a t i n g t o h i g h e r D / L v a l u e s w i t h t h e k i n e t i c m o d e l o f W e h m i l l e r a n d B e l k n a p ( 1 9 7 8 ) . B a n d s l a b e l e d " S t a g e 5 " , S t a g e 7" , e t c . , r e p r e s e n t r a n g e s o f D/L v a l u e s e x p e c t e d f o r t e r r a c e d e p o s i t s f o r m e d d u r i n g S t a g e 5 , S t a g e 7 , e t c . S i t e s w i t h U - s e r i e s s o l i t a r y c o r a l c a l i b r a t i o n s i t e s a t e i n d i c a t e d by (+I. B a j a s i t e s a r e a l s o l a b e l e d . G r o u p s I , I I , a n d I I I ( 1, f a l l i n r e g i o n s o f e a r l y S t a g e 5 , l a t e S t a g e 7 , a n d S t a g e 9 ( o r o l d e r ) , r e s p e c t i v e l y .

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.!20,000 year isochron to establish the kinetics (or Effective ,Temperature, as discussed in Wehmiller, 1982 and elsewhere) for a ziven lattitude-temperature region. These isochrons demonstrate that D/L values in age-equivalent samples increase with decreasing lattitude and increasing temperature. Pairs of isochrons are drawn to represent the probable range of D/L data that could be observed for terraces deposited during Stages 3 , 5, I, 9, etc. for the marine isotope record, using the ages proposed b y Shackleton and Opdyke (1973) for these stagesr Above D/L values of 0.80, the isochrons become less steep as they converge t o values of 1.0. Thus, the resolving power of the method is slightly decreased in this numerical range. Figure 4 can be used to estimate ages for D/L ratio groups such as those presented here by using the assumptions: 1) lattitudinal gradients of effective temperature can be smoothly interpolated between calibration sites; and 2) effective temperatures for a local area czn be considered equal (or nearly s o ) for samples of different ages but with the same present temperature.

The Group I leucine data fall in the early Stage 5 portion o f the isochron band shovn in Figure 4. Therefore, those data are interpreted to re.present a time of deposition approximately 120,000 years before present. Other Baja sites that fall within the same region of the 120,000 year isochron are those at CamalÚ (Valentine, 1980), Punta Santa Rosalillita (Woods, 19801, Bahia Tortugas (Emerson et a., 19801, and Magdalena Bay (Wehmiller and Emerson, 1980).

The Group II leucine data plot above all the Baja results thought to represent any portion of Stage 5. Because of the lack of calibration sites older than 120,000 years and the difficulty of extrapolating the kinetic model to D/L leucine values as high as those in either Group II or Group III, we can only tentatively propose an age of approximately 200,000 years (late Stage 7) for the samples representing Group II. Elevation and morphologic characteristics of the Group II terrace suggest that it is older than the Group I terrace (Ortlieb, 1978, 1979a, 1979b). Because the valine and glutamic acid D/L values for the Group II samples are significantly greater than those observed in nearby "Stage 5 " samples (Wehmiller and Emerson, 1980; Emerson et al., 198l>, we feel that the Group II terrace must represent a pre-120,000 event rather than Groups I and II representing a late and early Stage 5 terrace complex.

Group III plots within the extrapolated isochron band for either Stage 9 or #Stage 11 of the marine isotope record (Figure 4). Therefore, wc propose that the age of the samples in this group is at least 3 0 0 , 0 0 0 years. In most cases the aminostratigraphic groups recognized here are consistent with the relative ages that could be inferred from local elevation differences between terrace sites. However, it is clear that elevation differences alone do not correspond to the I

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aminostratigraphic groups when comparisons are made over ' distances of a few kilometers.

The most rigorous test o f the relation between aminostratigraphic data and the morphostratigraphy of the terrace sites is found at sites LR464, LR470, and LR468 (Figures 1 and 2). A sample from the 11 meter terrace (LR464) falls into Group II (.Figure 3; Table 1) while a sample from the 3 0 nieter terrace (LR47O)(Figure 3 ; Table 1) falls into Group III, thereby

* demonstrating consistancy between relative aminostratigraphic snc! geomorphic ages. However, two samples from LR468, a depositional. surface at 6 meters on the flank of the 11 meter terrace, also fall into aminostratigraphic Group II (Figure 3; Table 11, even though geomorphic relationships imply a younger age (Stage 5 ) for the' LR468 site (Ortlieb, 1978, 1979a). Either the shells at LR468 have been reworked from the higher deposit, or both LR464 and LR468 are penecontemporaneous and the aminostratigraphic data cannot resolve small age differences between what appear (from D/L data alone) to be Group II, 200,000 year terrace deposits.

Localities LR458 and LR461 (Figures 1 and 2) both fall into aminostratigraphic Group III (Figure 3; Table I ) , yet their field relationships (Figure 2 ) suggest that they are stratigraphically separated by fluvial and eolian sediments that could represent one full glacial stage or substage. It appears that the leucine and valine enantiomeric ratios simply can not resolve the age difference (which could be as much as 100,000 years) between these samples. There is a suggestion that the glutamic acid data do conform to these known age differences. Clearly, the high effective temperatures at these lattitudes limit the utility of aminostratigraphic methods in resolving age differences to about the last 250,000 years.

In this area of the Pacific coadCof Baja California, amino acid racemization dating gives better geochronological results than U-series methods (Ortlieb, 1982). The available fossil material in marine deposits of this area are essentially mollusks which are suitable for amino acid analysis but are of controversial use for U-series dating. Diagenetic alterations of the fossil tests are such that many U/Th age determinations cannot be done (Ortlieb et al., in preparation). However, amino acid racemization dating provides internally consistent results which, in most cases, are in agreement with morphostratigraphic interpretations. A l s o , amino acid racemization dating can distinguish samples of the last two interglacials (Stages 5 and 7)'from older samples (Stage 9 and older), while U-series methods are not reliable enough to distinguish between Stage 7 and older fossils.

Comparison between apparent amino acid and U - s c r i e s ages of samples at the same localities on the Vizcaino Peninsula as this

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study are discussed elsewhere (Ortlieb et al., in preparation). Four of the samples examined in this study have been submitted for radiometric dating. Two of these (LR459=458 and LR530) had an excess of Ionium while the other two yielded apparent ages of 235,000 (+40,000; -30,000) years (LR522) and greater than or equal to 400,000 years (LR470) (Ortlieb et al., in preparation). Radiometric ‘ages cannot be calculated for the first two localities because of the excess of Io due to contamination. However, these two localities (LR458 and LR530) fall within aminostratigraphic Group III (Figure 3; Table 1). The U/Th apparent age of LR522 suggests that this locality corresponds to the Stage 7 high sea stand but both the morphostratigraphic interpretation and the aminostratigraphy of locality LR522 favor a Stage 5 age (120,000 years).

CONCLUSIONS

Amino acid racemization dating methods have been applied to Quaternary marine terrace localities (with elevations between 3 and 3 0 meters) at Bahia Asuncion, Baja California Sur. Three aminostratigraphic age groups have been recognized, the youngest of which appears correlative with U-series dated 120,000-year terrace localities at higher and lower lattitudes. The older aminostratigraphic groups appear to represent ages of about 200,000 years and 300,000 years or older. These apparent ages are generally consistent with geomorphic criteria for relative terrace ages. The high effective temperature for the central and southern Baja region limits the utility of amino acid racemization dating but it can be used to define ages representing either Stage 5 (early and late) or pre-Stage 5 depositional events without much difficulty, given adequate quantities of well-preserved samples. The long-term average uplift rates inferred from the age-elevation relationships reported here are all less than O.lm/1000 years, at the low end of the range of uplift rates reported for coastal regions undergoing strike-slip deformation (Lajoie K t d., 1979; Muhs, 1983).

ACKNOWLEDGEMENTS

The amino acid dating of mollusks in this study was funded by several grants from the United States Geological Survey. The field work and collection of samples have been completed through a cooperative program (GEOCORTEZ) of O.R.S.T.O.M.(France) and the Instituto de Geologia, Universidad Nacional Autonoma de Mexico. Part of the field studies has been completed in collaboration with V. Malpica (Inst. Geol., U.N.A.M.)

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