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T h e s t r a t a that a r e e x p o s e d in the w e s t e r n A m a z o n B a s i n h a v e p o s e d a c o n s i d e r a b l e p r o b l e m for g e o l o g i s t s w h o h a v e a t t e m p t e d to d e t e r m i n e t h e i r t e m p o r a l a n d g e o g r a p h i c e x t e n t . T h e u n i f o r m i t y o f s e d i m e n t s a n d t h e v i r t u a l a b s e n c e o f f o s s i l s that c a n be r e -lated to p a r t i c u l a r s t r a t a h a s l e d to a g e a s s i g m e n t s o f A m a z o n i a n s t r a t a that necessar_i_ ly r e f l e c t e x a s p e r a t i o n a n d g e n e r a l i z a t i o n . B e c a u s e so l i t t l e h a s b e e n w r i t t e n o f t h e g e o l o g y o f this a r e a , a n y p u b l i s h e d g e o l o g i c a l o b s e r v a t i o n o n t h e s t r a t a o r f o s s i l s h a s pivotal s i g n i f i c a n c e o n f u r t h e r g e o l o g i c a l i n t e r p r e t a t i o n . A s o f t h e p r e s e n t , no fewer than s e v e n t e e n f o r m a t i o n a l n a m e s c o u l d b e a p p l i e d t o s t r a t a in t h e v i c i n i t y of R i o B r a n -co w i t h a g e a s s i g m e n t s r a n g i n g f r o m C r e t a c e o u s t o H o l o c e n e . If t h e s e n a m e s a p p l y to d i s c r e t e s t r a t i g r a p h i c u n i t s , o n e w o u l d h a v e to s a y that the d e p o s i t i o n a l h i s t o r y o f w e s t e r n A m a z o n i a is i n d e e d c o m p l e x . It is o u r o p i n i o n that (*) D e p a r t m e n t o f C e o l o g y , M i d l a n d C o l l e g e , M i d l a n d , T e x a s 7 9 7 0 5 U S A . (**) U n i v e r s i d a d e F e d e r a L d o R i o G r a n d e d o S u l , C e n t r o d e I n v e s t i g a ç õ e s d o G o n d w a n a , P o r t o A l e g r e , R S , B r a z i l . (***) U n i v e r s i d a d e F e d e r a l d o A c r e , L a b o r a t ó r i o d e P e s q u i s a s P a l e o n t o l ó g i c a s , R i o B r a n -co - A C , B r a z i l . A C T A A M A Z Ô N I C A , 1 8 ( 3 - 4 ) : 1 1 9 - 1 4 3 . 1 9 8 8 . 119
t h e r e is s u b s t a n c i a l d u p l i c a t i o n in the f o n r a t i o n a l n a m e s that n a v e bet.' a p p l i e d to w e s t e r n A m a z o n i a a n d that the a g e r a n g e s u g g e s t e d by these f o r m a t i o n s is e x t r e m e a n d l i k e l y i n c o r r e c t . F o l l o w i n g a nuirfaer of r e c e n t p a p e r s o n the g e o l o g y o f this a r e a , w e s u g g e s t tnat there a r e o n l y two f o r m a t i o n s that a r e e x p o s e d o v e r m u c h of the upper A m a z o n B a s i n . H o w e v e r , r a t h e r t h a n m e r e l y sr nullifying the p r e v i o u s i n t e r p r e t a t i o n s of d e p o s i t i o n a l h i s t o r y , o u r s t u d i e s r e d i r e c t e d o u r t h i n k i n g t o w a r d the i mpl i cat iens of t h i c k , e x t e n s i v e , a n d a p p a r e n t l y v e r y y o u n g s t r a t a . If, a s w e s u g g e s t , t h e s * s e d i m e n t i a c c u m u l a t e d in a h u g e l a k e , t n e n n o t o n l y is the Late P l e i s t o c e n e antf H o l o c e n e g e o l o g i c h i s t o r y of the u p p e r A m a z o n m u c h d i f f e r e n t t h a n w h a t ίΐ n o w a c c e p t e d , b u t the c ü s t r i -ÍJutinn of m o d e r n p l a n t s a n d a n i m a l s , a n d p e r h a p s h u m a n s , r e f l e c t s the p r e s e n c e o f this r e c e n t t o p o g r a p h i c f e a t u r e .
D i s c u s s i o n of P r e v i o u s G e o l o g i c a l W o r k
The s t r a t a that a r e e x p o s e d in the v i c i n i t y o f R i o B r ^ o c o a n d a l o n g the Rio A c r e in Brazil a r e r e f e r r e d to as r. heíol imoes f o r m a t ion in the inost r e c e n t a n d e x t e n s i v e r e f e -rence to the g e o l o g y o f this a r e a Í R A D A M Ê R A S I L , 1 9 7 6 ) . T h e S o l i d e s h a s b e e n d e s c r i b e d as b e i n g c o m p o s e d o f c l a y , silt a n d fipe s a n d . T h e c l a y u n i t s a r e m a s s i v e to t h i n , -with c a l c i f i c a n d g y p i i f e r o u s c o n c r e t i o n s a n d v e i n s , a n d l a r g e - s c a l e c r o s s s t r a t i f i c a t i o n o r s i m p l y v a r i e g a t e d w i t h f i n e l a m i n a t i o n s . T h e s a n d s a r e Fine to c o a r s e , m a s s i v e , f r e q u e n t l y i n t e r f i n g e r w i t h silt o r c l a y , p o o r l y t o well c o m p a c t e d , h a v e ca 1 c a r e o u s c e m e n t , a n d m a y c o n t a i n t h i n , o r g a n i c - r i c h l e n s e s . C r o s s - b e d d i n g c a n be of small o r l a r g e a m p l i t u d e . A l s o a s c r i b e d for this f o r m a t i o n n,r*i lin^onitic lenses a/id w e l l - r o u n d e d c o n g 1 o m e r a t e s . T h i s d e s c r i p t i o n a c c u r a t e l y p o r t r a y s the s u r f a c e g e o l o g y n e a r R i o B r a n c o . F r o m p e r s o n a l o b s e r v a t i o n s o f the a u t h o r s , it a l s o d e s c r i b e s the s u r f a c e g e o I o g y i η s o u t h e a s t e r n P e r u( n o r t h e r n S o l i v i a , a n d a s far n o r t h a s the R i o J u r u ã . It u n d o u b t e d l y is a n a d e q u a t e d e -s c r i p t i o n o f e x p o -s e d -s t r a t a a -s far n o r t h a e the type -s e c t i o n of the Soli/i^e-s F o r m a t i o n o n the Rio Sol inrões (central t r i b u t a r y of the R i o A m a z o n ; R e g o , a n d p r o b a b J y n o r t h of this river a s w e l l . T h i s d e s c r i p t i o n t h e r e f o r e d e p i c t s the s u r f a c e g e o l o g y of thíi ira j a r p o r t i o n of the w e s t e r n A m a z o n B a s i n d e s p i t e the p l e t h o r a o f na^nes a s s i g n e d in the g e o l o g i c a l l i t e r a t u r e .
A t t e m p t s a t c o r r e l a t i o n h a v e b e e n b e d e v i l e d by the n a t u r e of A m a z o n i a n g e o l o g y . S m a l l , u s u a l l y un f o s s i I i f e row s e x p o s u r e s t h a t a r e n e a r l y u n i f o r m in l i t h n l o g y a n d w h i c h w a y be s e p a r a t e d by h u n d r e d s o f m i l e s h a v ç b e e n d e s c r i b e d i n d e p e n d e n t i y o v e r the last c e n t u r y , H o one r e s e a r c h e r H a s s e e n the e x p o s u r e s t h r o u g h o u t the A m a z o n B a s i n a n d the g e o l o g i c a l l i t e r a t u r e i l l u s t r a t e s the limits of i n d i v i d u a l p e r s p e c t i v e s in so v a s t a n e n t e r p r i s e . W i t h the a d v e n t of large s c a l e p r o g r a m s such, a s RADAMBfiAS I L , the p r o b l e m s have b e e n b r o u g h t into s h a r p e r f o c u s a l t h o u g h the d e f i n i t i v e s y n t h e s i s o f A m a z o n i a n g e -o l -o g y is y e t in the f u t u r e . G e -o l -o g i c a l e x p i -o r j t i -o n in the w e s t e r n p a r t -of the A m a z -o n B a s i n h a s p r o v i d e d s e v e r a l n a m e d f o r m a t i o n s w h o s e r e l a t i o n s h i p s to e a c h o t h e r a r e still
in d o u b t ( R A D A M B R A S I L , 1 3 7 7 ) . The most f r e q u e n t l y e n c o u n t e r e d n a n e s a r e the s u p p o s e d l y t e m p o r a l l y e q u i v a l e n t R e d B e d s of S r n g e w a l d ( 1 9 2 7 ) ; the " P u c a F o r m a t i o n " o f S t e i m r a r ( 1 3 2 3 ; B r a n i s a 1 9 6 8 , r e p l a c e d the n a m e P u c a w i t h Ρ i 1 cofnayo a n d c o n s i d e r e d this a s a strat_i_
g r a p h i c g r o u p a l t h o u g h " P u c a " o r " P u c a - t y p e " c o n t i n u e s to b e u s e d for A n d e a n r e d b e d s a n d t h e i r s u p p o s e d A m a z o n i a n e q u i v a l e n t s , e . g . , S i m p s o n & P a u l a C o u t o , 1 9 8 1 ) ; a n d t h e C r u z e i r o F o r m a t i o n o f O p p e n h e i m ( 1 9 3 7 ) · T h e s e a r e g e n e r a l l y t h o u g h t to be C r e t a c e o u s o r E a r l y T e r t i a r y . Y o u n g e r b e d s h a v e b e e n g i v e n a v a r i e t y o f n a m e s s u c h a s B r o w n B e d s ( S i n g e w a l d , 1 9 2 8 ) , C o n t a m a n a G r o u p (Kummel , 1 9 ^ 8 ) , a n d C o r r i e n t e s F o r m a t i o n ( P a r r a , 1971*). A n e x t r e m e p o s i t i o n y e t o n e t h a t d e m o n s t r a t e s t h e p r o b l e m s o f T e r t iary A m a z o n i a n g e o l o g y w a s that p r o p o s e d b y C a p u t o , R o d r i g u e s a n d V a s c o n c e l o s ( 1 9 7 1 ) . T h e y p r o p o s e d that all T e r t i a r y f o r m a t i o n s w e r e in fact o n l y o n e f o r m a t i o n a n d that o n e s h o u l d be c a l l e d the S o l i m õ e s . In 198*f, C a m p b e l l δ F r a i l e y i d e n t i f i e d t w o f o r m a t i o n s in the v i c i n i t y o f I n a p a r i , P e r u , a n d r e s t r i c t e d t h e Inapari F o r m a t i o n o f O N E R N ( 1 9 7 7 ) to t h e u p p e r m o s t s t r a t u m ( F i g . I ) . T h a t f o r m a t i o n a l n a m e h a s n o w b e e n s u p p r e s s e d in f a v o r o f t h e M a d r e d e D i o s F o r m a t i o n ( O p p e n h e i m , 19^6; C a m p b e l l £ R o m e r o P i t m a n n , in m a n u s c r i p t ) . O n the b a s i s o f r a d i o c a r b o n d a t e s , t h e M a d r e d e D i o s F o r m a t i o n (or Inapari F o r m a t i o n , a s r e s t r i c t e d ) w a s t h o u g h t to B e H o l o c e n e in a g e . S u b s e q u e n t d a t e s f r o m w o o d s a m p l e s taken f r o m t h e lower u n i t s o f t h i s f o r m a t i o n i n d i c a t e t h a t d e p o s i t i o n o f M e m b e r A o f t h e M a d r e de D i o s F o r m a t i o n s ( = l n a p a r i F o r m a t i o n ) b e g a n a p p r o x i m a t e l y 3 5 , 0 0 0 y e a r s a g o ( m o d i f i e d to 3 6, 5 0 0 y e a r s B . P . in C a m p b e l l £ R o m e r o P i t t m a n , in m a n u s c r i p t ) . In C a m p b e l1 £ F r a i l e y (198V) a n d e l s e w h e r e (Campbell e t a l . , 1 9 8 5 ; F r a i l e y , 1986) , t h e u n d e r l y i n g s t r a t u m w a s not r e f e r r e d t o a n a m e d f o r m a t i o n a n d i d e n t i f i e d o n l y a s t h e " T e r t i a r y r e d b e d s " . V e r t e b r a t e f o s s i l s f o u n d in t h e T e r t i a r y r e d b e d s d a t e t h i s f o r m a t ion to t h e H u a y q u e r i a n ( L a t e M i o c e n e ) S o u t h A m e r i c a n L a n d M a m m a l A g e ( F r a i l e y , 1986) . C l e a r l y , t w o f o r m a t i o n s a r e e x p o s e d a l o n g t h e r i v e r s o f t h e u p p e r A m a z o n B a s i n , b u t r e f e r e n c e to p r e v i o u s l y n a m e d f o r m a t i o n s is n o t e a s y . In t h i s c o n t e x t , t h e S o l i m õ e s F o r m a t i o n in t h e v i c i n i t y o f t h e Rio A c r e is e i t h e r t h e s a m e a s t h e M a d r e d e D i o s F o r m a t i o n ( I n a p a r i F o r m a t i o n , a s r e s t r i c t e d ) , t h e s a m e a s t h e " T e r t i a r y r e d b e d s " , o r e q u i v a l e n t to b o t h In o u r u n d e r -s t a n d i n g o f R A D A M B R A S I L ( 1 9 7 6 ) , a n d in ν i e w o f t h e ini t ial d if f icu 1 ty iη -s e p a r a t i n g t h e -s e two u n i t s a l o n g t h e R i o A c r e (a d i f f i c u l t y that w a s o n l y c l e a r l y s u r m o u n t e d w h e n m a m m a l i a n f o s s i l s o f v e r y d i f f e r e n t a g e s w e r e f o u n d in t h e u n i t s ) , w e s u s p e c t t h a t t h e S o l i m õ e s F o r m a t i o n a s n o w d e f i n e d in t h e S t a t e o f A c r e e n c o m p a s s e s b o t h u n i t s a l o n g the A l t o R i o A c r e . T h e M a d r e d e D i o s F o r m a t i o n s (= Inapari F o r m a t i o n ) is d i v i s i b l e into f o u r m e m b e r s (Campbell 6 F r a i l e y , 1 9 8 4 ) . T h e l o w e s t m e m b e r is a n a r r o w , d a r k , o f t e n i n d u r a t e d , c l a y -p e b b l e c o n g l o m e r a t e t h a t w a s d e s i g n a t e d t h e A c r e C o n g l o m e r a t e M e m b e r (Cam-pbell e t a l . , 1 9 8 5 ) . T h e u p p e r t h r e e m e m b e r s a r e p r i m a r i t y t h i c k u n i t s ( 5 - 2 0 m ) o f v a r i e g a t e d c l a y o r sandy c l a y a n d o c c a s i o n a l p a l e o c h a n n e l s . O n l y t h e l o w e s t o f t h e s e t h r e e ( M e m b e r A o f Campbell & F r a i l e y , 1984) d i s p l a y s g r e a t v a r i a t i o n in s e d i m e n t t y p e a n d is a p p a r e n t l y a c o m p l e x s e r i e s o f c h a n n e l d e p o s i t s w i t h c r o s s - b e d d e d s a n d s t o n e s a n d l e n s e s o f m a t u r e s a n d s , v a r i e g a t e d c l a y , a n d l i g n i t i c c l a y . T h i s c o m p l e x l e n s i n g s e q u e n c e o c c a s i o n a l l y is r e p l a c e d l a t e r a l l y b y a v a r i e g a t e d , b l o c k y c l a y l i k e t h a t s e e n i η t h e u p p e r t w o u n i t s . The e n v i r o m e n t s o f d e p o s i t i o n f o r t h e t h r e e u p p e r u n i t s a p p e a r t o b e f l o o d p l a i n f o r t h e m o s t p a r t w i t h p e r h a p s d e l t a i c d e p o s i t i o n f o r p a r t o f M e m b e r A . P u b l i s h e d i n f o r m a t i o n a b o u t t h e R i o J u r u á ( P a u l a C o u t o , 1 9 7 8 ; S i m p s o n a n d P a u l a
C o u t o , I 9 8 I ) and f i e l d i n s p e c t i o n b y one us (AR) i n d i c a t e s t h a t t h e A c r e C o n g l o m e r a t e M e m b e r o f t h e M a d r e d e D i o s F o r m a t i o n (= Inapari F o r m a t i o n ) is p r e s e n t a l o n g t h e R i o J u
ruá a s w e l f . A r e c e n t p u b l i c a t i o n i d e n t i f i e d t h e t h r e e c l a y m e m b e r s o f the M a d r e d e D i o s F o r m a t i o n s (= Inapari F o r m a t i o n ) a l o n g t h e R i o Beni ( C a m p b e l1, F r a i l e y & A r e l1 a n o , 1985). S u b s e q u e n t f i e l d w o r k in 1985 a n d 1986 l o c a t e d all the m e m b e r s o f t h e M a d r e d e D i o s F o r -T i a t i o n (= Inapari F o r m a t i o n ) a n d t h e ' T e r t i a r y r e d b e d s " a l o n g t h e R i o M a d r e d e D i o s f r o m R i b e r a l t a , B o l i v i a , to t h e b o r d e r o f P e r u ( C . D . F . , A . R . , a n d d e s c r i b e d in C a m p b e l l & R o m e r o P i t t m a n n , in m a n u s c r i p t ) . O l d e r p u b l i c a t i o n s h a v e d e s c r i b e d c o n g l o m e r a t i c l e n s e s o v e r l a i n b y h o r i z o n t a l c l a y u n i t s t h r o u g h o u t the u p p e r A m a z o n ( B r o w n , 1 8 7 9 a ; 1 8 7 9 b ; Kurnmel , 1 9 4 8 ; R ü e g g & R o s e n z w e l g , 1 9 4 9 ; M o u s i n h o d e M e i s , 1 9 7 ' ) · I η o n e publ i c a t ion (De n e v a n , I 9 8 O ) , a p h o t o g r a p h t a k e n in 1 9 1 0 s h o w s w h a t a p p e a r s t o be t h e c o n t a c t b e t w e e n M e m b e r s Β a n d C o f t h e M a d r e d e D i o s (= Inapari F o r m a t i o n ) a l o n g t h e R i o H e a t h . T h e s e p u b l i c a t i o n s i n d i c a t e that t h e s t r a t i g r a p h y o f t h e R i o A c r e a s d e s c r i b e d by C a m p b e l l & F r a i l e y (1984) e x t e n d s t h r o u g h o u t t h e r e g i o n . U p p e r R i o A c r e L o w e r Rio A c r e F i g . 1. A c o m p a r i s o n b e t w e e n g e n e r a l i z e d g e o l o g i c s e c t i o n s in t h e S t a t e of A c r e , B r a ? i l . I n t e r p r e t a t i o n s o f t h e d e p o s i t i o n a l e n v i r o n m e n t s f o r e a c h u n i t a r e i n d i c a t e d . A. A l o n g the R i o A c r e , w e s t of A s s i s B r a s i l ( r e d r a w n f r o m C a m p b e l l a n d F r a i l e y , 1 9 8 4 ) . B . N e a r t h e c i t y of R i o B r a n c o . L i t h o l o g i c d e s c r i p t i o n s of the u n i t s a r e p r o v i d e d in t h e t e x t . In F i g u r e A , t h e l o w e s t m e m b e r of the I n a p a ri / S o l i m õ e s F o r m a t i o n is t h e A c r e C o n g l o m e r a t e .
G e o l o g y o f t h e R i o B r a n c o A r e a
A l o n g the Rio A c r e at Rio B r a n c o , a p p r o x i m a t e 1 ν I &Q mi I es f rom the sect ion d e s c r ibed by Campbell S F r a i l e y ( 1 9 8 ^ ) , the h o r i z o n t a l " T e r t i a r y red b e d s " a n d the four m e m b e r s of the M a d r e de D i o s F o r m a t i o n (= Inapari F o r m a t i o n ) c a n be s e e n a l o n g the Rio A c r e in s e d i m e n t s that a r e r e f e r r e d to as the S o l i m õ e s F o r m a t i o n in c u r r e n t g e o l o g i c l i t e r a t u r e ( R A D A M B R A S I L , I 9 7 & ; F i g . I ) . T h e q u e s t i o n t h e n bec.orr.es: W h i c h of the two n a m e s i s correct ? The q u e s t i o n is ir.ore d i f f i c u l t t h a n s i m p l e g e o g r a p h i c e x t e n s ion of o n e of these f o r m a t i o n s
in that y e t a n o t h e r n a m e that h a s b e e n a p p 1 i e d to s o m e parl of th i s υπ i t m a y b e the p r o p e r r e f e r e n c e . Until s o m e o n e c a n v i s i t all the a p p l i c a b l e type s e c t i o n s , d e t e r m i n e the e x tent of the f o r m a t i o n s , a n d u n r a v e l the p l e t h o r a of f o r m a t i o n a l n a m e s , the m a t t e r is not likely to be r e s o l v e d . For the m o m e n t , the q u e s t i o n is n e c e s s a r i l y left o p e n . In this p a p e r , the f o r m a t i o n is i d e n t i f i e d a s a c o m b i n a t i o n of the two narr.es, the M a d r e de D i o s / Sol inoes F o r m a t i o n . O n e of t h e s e is p r o b a b l y the c o r r e c t n a m e for the s e d i m e n t s in the v i c i n i t y of R i o D r a n c o . If t h e s e two forrrational n a m e s p r o v e to r e f e r to a c o n t i n u o u s s t r a t u m , then the n a m e "Sol i m o e s " ( R e g o , 1930) w o u l d t a k e p r e c e d e n c e . If it c a n be d e t e r m i n e d that the S o l i m o e s F o r m a t i o n d o e s not e x t e n d to the R i o A c r e , then the s e d i m e n t s are r e f e r a b l e to the M a d r e de D i o s F o r m a t i o n .
fhe u p p e r part of the M a d r e de D i o s / S o I i m o e s s e q u e n c e , M e m b e r s Α , β, a n d C , are v i s i b l e in r o a d c u t s a n d a l o n g small s t r e a m s s o u t h of R i o B r a n c o a n d n o r t h to the R i o Puru's ( F i g s . 1 -1* ) . M e m b e r s A a n d Β a r e v a r i e g a t e d c l a y s t h r o u g h o u t the a r e a . M e m b e r A
is light g r e y to g r e e n in c o l o r w i t h n o d u l a r iron c o n c r e t i o n s a n d c a l c a r e o u s s t r i n g e r s . Member Β is a red a n d g r e e n s i 1 ty c l a y . O n a n e x p o s e d s u r f a c e , b o t h m e m b e r s w e a t h e r into a b l o c k y t e x t u r e that is light red in c o l o r .
M e m b e r C is the u p p e r m o s t s t r a t u m of the M a d r e de D i o s / S o l i<n5e-s F o r m a t i o n a n d can be s e e n o n l y o n the c r e s t s of the low h i l l s near R i o B r a n c o . M e m b e r C is a c o m p l e x siratum in w h i c h the l i t h o l o g y c h a n g e s a l m o s t w i t h e a c h h i l l t o p ( F i g . 3, w i t h o u r i n t e r p r e t a t i o n s of d e p o s i t i o n a l e n v i r o n m e n t ) . In p l a c e s , t h i s u n i t is c o m p o s e d of fine s a n d a n d silt that is o v e r lain by a t h i n clay unit w i t h b u r r o w s o r m u d c r a c k s in it that is in turn o v e r l a i n by a t h i c k , v a r i e g a t e d c l a y . E l s e w h e r e , M e m b e r C c o n s i s t s of c r o s s - b e d d e d , f i n e , b r o w n s a n d w i t h r i p p l e m a r k s that is o v e r l a i n by v a r i e g a t e d r e d / g r e e n c Uiy. M e m b e r C a l s o has t h i n n l y l a m i n a t i o n s of c l a y a n d s a n d o v e r l a i n by n o d u l a r clay c o n g l o m e r a t e that is in turn c o v e r e d by b r o w n , f i n e - g r a i n e d s a n d c a p p e d wi t h a c 1 ay un i t. R a p i d late_r al l i t h o l o g i c c h a n g e s of this t y p e a r e c h a r a c t e r i s t i c of d e l t a i c s e d i m e n t a t i o n a n d i rtdj^ cates the m a n y s u b e n v i r o n m e n t s f o u n d in a d e l t a . O c c a s i o n a l l y , o n e c a n see an o b v i o u s d e l t a i c s e q u e n c e s u c h a s that at k/n 15 ( F i g s . 3» ^) · T h i s c l e a r e x a m p l e of p r o g r a d a t i o n s t r o n g l y s u p p o r t s the i n t e r p r e t a t i o n of o t h e r s e d i m e n t s a s h a v i n g r e s u l t e d f r o m the s h i f t i n g of v a r i o u s l o c a l i z e d d e l t a i c s u b e n v i r o n m e n t s . At km 1 5 , the f i n e l y l a m i n a t e d clay is o v e r l a i n by silt that is in turn o v e r l a i n by t r o u g h c r o s s - b e d d e d fine s a n d that is t o p p e d by a thin clay u n i t . A t k m A 5 , the c r o s s - b e d d e d c l a y a n d s a n d o v e r l a i n by u n i f o r m c l a y i n d i c a t e s r e p e a t e d ( p r o b a b l y s e a s o n a l ) c h a n g e s of s e d i m e n t a s w o u l d r e s u l t from a d e l t a front that w a s s u p e r c e d e d by q u i e t - w a t e r d e p o s i t i o n , A c r e v a s s e - s p l a y is indicated at km 1 Ό by t h i c k , c r o s s - b e d d e d s a n d an<> c l a y u n i t s . T h e d i v e r g e n t d i p s of
t h e s e u n i t s w o u l d b e d u e to the f a n - s h a p e o f t h e s p l a y . Sand lenses in thick c l a y u n i t s (km 8 5 , k m 144) c o u l d be i n t e r p r e t e d a s e v i d e n c e for fluvial d e p o s i t i o n w e r e t h e r e n o t c o m p e l l i n g e v i d e n c e f o r d e l t a i c d e p o s i t i o n in t h e n e a r v i c i n i t y . A s s u c h , a n d in v i e w o f the n u m b e r a n d r a n g e o f s i z e s that c a n b e s e e n a t o n e e x p o s u r e , t h e s e s a n d u n i t s a r e i n t e r p r e t e d a s h a v i n g b e e n p a r t o f t h e d e l t a p l a i n o r , a s a t k m 1 4 4 , p a r t o f the s e r i e s o f d i s t r i b u t a r y c h a n n e l s . A t k m 4 3 , t h e s e q u e n c e i n d i c a t e s a p r o g r a d i n g d e l t a i c e n v i r o n m e n t t h a t w a s a d v a n c i n g t o t h e n o t h w e s t ( a p p r o x i m a t e l y 315 d e g r e s s , F i g . 2 , 3 ) . T o t h e s o u t h o f t h e R i o A c r e , a n o t h e r p r o g r a d i n g s e q u e n c e is p r e s e n t b u t t h e d i r e c t i o n o f d e p o s i t i o n is to t h e s o u t h e a s t ( b e t w e e n 100 a n d 160 d e g r e s s , F i g . 2 , s e e a r r o w s ) . If t h e d i r e c t i o n s o f t h e s e p r o g r a d i n g d e l t a s a r e e v e n a p p r o x i m a t e l y a c c u r a t e , t h e n o n e of t w o e x p l a n a t i o n s i s p o s s j ^ b l e . E i t h e r t w o d r a i n a g e s y s t e m s w e r e i n v o l v e d a n d the d e l t a s bear n o r e l a t i o n to e a c h o t h e r , o r they a r e p a r t o f o n e v e r y l a r g e d e l t a in w h i c h t h e s e d i m e n t s w e r e f a n n i n g - o u t o v e r a large d e p o s i t i o n a l b a s i n . It h a s b e e n s u g g e s t e d that r i v e r s in t h e U p p e r A m a z o n o c c u p y t h e d r a i n a g e c h a n n e l s o f e a r l i e r , larger r i v e r s ( A l m e i d a , 1 9 7 5 , in R A D A M B R A S I L , I 9 7 7 ) . If t h e p r e s e n t - d a y R i o A c r e f o l l o w s t h e c o u r s e o f a n e a r l i e r R i o A c r e , t h e n it a l s o lies b e t w e e n t h e d i r e c t i o n a l c h a n g e o f d e l t a i c s e d i m e n t s a s w o u l d h a v e t h e a n c i e n t r i v e r . B e c a u s e o f this a n d t h e w i d e s p r e a d u n i f o r m i t y a n d t h e r e f o r e p r o b a b l e l a c u s t r i n e o r i g i n o f M e m b e r s A a n d B , w e h a v e b u i l t o u r model o n t h e i n t e r p r e t a t i o n o f a s i n g l e , large d e l t a . W e p r o p o s e t h a t t h e u p p e r m o s t s e d i m e n t s o f t h e w e s t e r n p a r t o f t h e A m a z o n B a s i n a r e d u e to l a c u s t r i n e d e p o s i t i o n . F u r t h e r m o r e , t h i s d e p o s i t i o n a l e n v i r o n m e n t i n c l u d e d all o r a m a j o r p o r t i o n o f t h e A m a z o n B a s i n . T h i s e n o r m o u s l a k e , h e r e d e s i g n a t e d L a g o A m a z o n a s , is c o n s i d e r e d to h a v e b e e n a s i g n i f i c a n t f a c t o r in t h e g e o m o r p h o l o g i c a1 a n d b i o l o g i c a l d e v e l o p m e n t o f t h e A m a z o n B a s i n d u r i n g t h e L a t e Ρ1e i s t o c e n e a n d H o i o c e n e . T h e t e r m i n a t i o n o f t h e o l d e r river w o u l d h a v e b e e n a t R i o B r a n c o d u r i n g t h e t i m e o f d e p o sit ion o f M e m b e r C . M e m b e r C h a s n e i t h e r a b s o l u t e n o r r e l a t i v e a g e d a t e s a s s o c i a t e d w i t h i t b e y o n d t h e fact t h a t it is y o u n g e r t h a n M e m b e r s A a n d B . W o o d s a m p l e s t a k e n f r o m M e m b e r A a l o n g the R i o M a d r e d e D i o s n e a r C a n d e l á r i a , B o l i v i a , a n d o n t h e R i o A c r e n e a r B é l g i c a , P e r u , a n d near R i o B r a n c o , B r a z i l , h a v e y i e l d e d d a t e s b e t w e e n
1 0
,000 a n d
36, 5 0 0 y e a r s ( C a m p bell & F r a i l e y , 1 9 8 4 ; C a m p b e l l & R o m e r o P i t m a n n , in m a n u s c r i p t ) . T h e p r o p o s e d c y c l e s o f d e p o s i t i o n a n d e r o s i o n t h a t c r e a t e d m e m b e r s o f the Inapari F o r m a t i o n in t h e A l t o A c r e (Campbell & F r a i l e y , 1984) a r e refl e c t e d i η s e d iment c h a n g e s t h a t m a r k t h e c o n t a c t s b e t w e e n the u n i t s n e a r R i o B r a n c o . N o c h a n n e l s w e r e s e e n in t h e s e l o w e r u n i t s a t R i o B r a n c o . T h e y w e r e p r e s u m a b l y n o t e x p o s e d t o e x t e n s i v e e r o s i o n t h i s f a r into t h e b a s i n . F o l l o w i n g t h e h y p o t h e s i s o f K u t z b a c h (1 9 8 1) , C a m p b e l l £ F r a i l e y (1984) p r o p o s e d that t h e u p p e r t h r e e m e m b e r s o f t h e M a d r e d e D i o s (= Inapari) F o r m a t i o n a r e c o r r e l a t i v e to w o r l d c l i m a t i c c o n d i t i o n s d u r i n g t h e L a t e P l e i s t o c e n e a n d H o l o c e n e . If this is t r u e , than the d e p o s i t i o n o f M e m b e r C m a y h a v e b e g u n a s r e c e n t l y a s 2800 y e a r s a g o . T h e c u r r e n t t i p o g r a p h i c e x p r e s s i o n o f t h i s r e g i o n , a n d by e x t e n s i o n n e a r l y all o f t h e u p p e r A m a z o n , m a y t h e r e f o r e b e v e r y y o u n g . T h e r a m i f i c a t i o n s o f t h i s t e m p o r a l r e - o r d e r i ng a r e stagger^ ing a n d e x t e n d into e v e r y f i e l d o f A m a z o n i a n s c i e n c e . H o w e v e r , w h i l e c e r t a i η ν i e w s m u s te i t h e r be m o d i f i e d or a b a n d o n e d , o t h e r q u e s t i o n s t h a t h a v e p e r s i s t e n t l y e v a d e d e x p l a n a -n a t i o -n -n o w h a v e a t h e o r e t i c a l a -n s w e r . F i g . 2. T h e r i v e r s a n d p r i n c i p l e c i t i e s of the e a s t e r n h a l f of the S t a t e of A c r e . T h e T r a n s A m a z o n H i g h w a y b e t w e e n R i o B r a n c o a n d S e n a M a d u r e i r a (BR-364) is m a r k e d . N u m b e r s i n d i c a t e K i l o m e t e r s n o r t h of the b r i d g e at I g a r a p é São F r a n c i s c o at t h e n o r t h e r n e d g e of R i o B r a n c o a n d c o r r e s p o n d to s e c t i o n s i l l u s t r a t e d in F i g u r e s 3 a n d 4. K i l o m e t e r 1 5 5 is the F a z e n d a L u l a L o c a l i t y . T h e two h o l l o w a r r o w s n e a r R i o B r a n -co i n d i c a t e the g e n e r a l d i r e c t i o n of p r o g r a d a t i o n in d e l t a i c s e d i m e n t s of M e m b e r C , I n a p a r i / S o l i m õ e s F o r m a t i o n .
F i g . 3. C o m p a r a t i v e litholofiies a n d i n t e r p r e t a t i o n s of d e p o s i t i o n a l e n v i r o n m e n t s f o u n d in M e m b e r C of the I n a p a r i / S o l i m õ e s F o r m a t i o n in the v i c i n i t y o í R i o B r a n c o a n d S e n a M a d u r e i r a . K i l o m e t e r d e s i g n a t i o n s r e f e r to d i s t a n c e s n o r t h of I g a r a -pé São F r a n c i s c o ( F i g . 2) o n the T r a n s A m a z o n H i g h w a y . V e r t i c a l s c a l e of the d r a w i n g s is v a r i a b l e a n d g i v e n in p a r e n t h e s e s a f t e r e a c h K i l o m e t e r d e s i g n a t i o n , Λ . N e a r B u j a r i , K m 15 (4 m ) ; Β. k m A 3 (2 m ) ; C. K m 45 (1.5 m ) ; D. K m 4 7 (2 m ) ; E . K m 8 5 0 0 m) ; F . K m 103 (10 m ) ; G . K m 143 (4 ™ ) ; Η , K m 144 (4 m ) , a p p r o x i m a t e l y 1 k m s o u t h e a s t o f the K i o C a e t e .
45 km
47 km
F i g . 4. P h o t o g r a p h s of M e m b e r C, lttap;<ri/Sal imõe.s F o r m a t i o n . A . N e a r Bu.inri, K m 15 B, K m 4 3 . C , K m 4 5 . D. K m 4 7 . Th-e s t r s u i f n s p h y o f ihii-s? o u t c r o p s is de s c r i b e d in F i g u r e 3.
I m p l i c a t i o n s o f L a g o A m a z o n a s : G e o l o g y T h e p r e s e n c e o f a l a r g e T e r t i a r y lake o r l a k e s , a n e s t u a r y , o r e v e n a m a r i n e e m -b a y m e n t in the w e s t e r n p a r t o f t h e A m a z o n B a s i n is n o t a n u n u s u a l c o n c e p t a n d h a s -b e e n f r e q u e n t l y m e n t i o n e d in t h e s c i e n t i f i c l i t e r a t u r e ( R ü e q q & R o s e n z w e i g , 1 9 4 9 ; O l i v e i r a , 1 9 5 6 ; P a u l a C o u t o , 1 9 5 6 ; S o m b r o e k , 1 9 6 6 ; S i o l i , 1968). T h i s i s , in f a c t , the usual e x p l a n a t i o n f o r t h e e x p o s e d , h o r i z o n t a l c l a y u n i t s a l t h o u g h a p p a r e n t l y b a s e d o n o l d e r , h i g h l y s p e c u l a t i v e w o r k . O u r o b s e r v a t i o n s o v e r t h e u p p e r A m a z o n B a s i n f r o m t h e R i o Beni , Bol_i_ v i a , t o t h e R i o P u r u s , B r a s i l , h a v e c o n v i n c e d u s that c o r r e l a t i o n o f c l e a r l y d e f i n e d , 1 ithological 1y u n i f o r m s t r a t a is p o s s i b l e o v e r c o n s i d e r a b l e d i s t a n c e s . T h a t w o u l d b e ex p e c t e d a s a r e s u l t o f l a r g e - s c a l e l a c u s t r i n e d e p o s i t i o n a n d n o t f r o m a s e r i e s o f c u t - o f f m e a n d e r s . F u r t h e r m o r e , t h i s l a c u s t r i n e d e p o s i t i o n if o f L a t e P l e i s t o c e n e / H o l o c e n e a g e . Irion ( 1 9 7 6 , 1984) s u g g e s t s a d e p o s i t i o n a l p a t t e r n f o r t h e l o w e r A m a z o n o f r e l a t i v e l y l a r g e , t e m p o r a r y l a k e s d u r i n g t h e P l e i s t o c e n e a n d H o l o c e n e d u e to c h a n g i n g sea l e v e l . W e are b a s i c a l l y in a g r e e m e n t w i t h h i s e v i d e n c e y e t s u g g e s t a m u c h l a r g e r b o d y o f w a t e r and, at l e a s t in the u p p e r A m a z o n , t e c t o n i c c o n t r o l o f t h e d e p o s i t i o n a l b a s i n . Other e v i d e n c e for late H o l o c e n e f l o o d i n g , a n d p e r h a p s a c o n t i n u o u s l a k e , c a m e s f r o m a s e r i e s o f l a k e s in the u p p e r A m a z o n b a s i n o f E c u a d o r at e l e v a t i o n s b e t w e e n 3 0 0 - 7 0 0 m ( C o l i n v a u x , e t a l . , I 9 8 5 ) . C e r t a i n l y , r a d i c a l l y d i f f e r e n t e n v i r o n m e n t s o f d e p o s i t i o n a r e i n d i c a t e d f o r a g r e a t p o r t i o n o f t h e A m a z o n B a s i n in t h e r e c e n t p a s t . A c c o r d i n g to c u r r e n t t h i n k i n g , t h e r i s i n g A n d e s b l o c k e d t h e d r a i n a g e s t s y e m s that e x t e n d e d w e s t w a r d f r o m t h e G u i a n a a n d B r a z i l i a n h i g h l a n d s ( B e u r l e n , 1 9 7 0 ; P u t z e r , 1 9 8 4 , and o t h e r s ) . D u r i n g t h e M i o c e n e a n d P l i o c e n e , s e d i m e n t s f r o m t h e s e s y s t e m s a n d f r o m t h e A n d e s a c c u m a l a t e d in t h e r e g i o n o f t h e U p p e r A m a z o n . A t s o m e t i m e , a n o u t let t o t h e s e a formed in t h e p r e s e n t l o c a t i o n o f t h e l o w e r A m a z o n R i v e r . W i t h d o w n c u t t i n g t h r o u g h t h e e x p o s e d P r e c a m b r i a n r o c k s , t h e l a k e s o f t h e u p p e r A m a z o n w e r e e m p t i e d a n d t h e p r e s e n t -day A m a z o n d r a i n a g e w a s f o r m e d . It is o u r f e e l i n g t h a t t h e s t r u c t u r a l f r a m e w o r k b e h i n d the d e p o s i t i o n o f t h e l a t e P l e i s t o c e n e a n d H o l o c e n e s e d i m e n t s o f t h e M a d r e d e Dios/SolJ^ m õ e s F o r m a t i o n is s i m i l a r to t h a t p r o p o s e d f o r t h e s a m e s t r a t a w h e n it w a s t h o u g h t they w e r e L a t e T e r t i a r y in a g e . T h e s t r a t a s i m p l y a r e m u c h y o u n g e r a n d t h e r e f o r e t h e t e c t o n i c e v e n t s a r e m o r e r e c e n t . I n t e r i o r b a s i n f o r m a t i o n a s a n e x t e n s i o n o f t e c t o n i s m c a u s e d by c o n v e r g i n g p l a t e s has b e e n s u g g e s t e d f o r L a t e C r e t a c e o u s a n d C e n o z o i c b a s i n s o f w e s t e r n N o r t h A m e r i c a
(Scholz e t a l . , 1 9 7 1 ; D i c k i n s o n , 1 9 7 6 ; C r o s s & P l l g e r J r . , 1 9 7 8 ) . Subsi d e n c e a n d s e d irrent a c c u m u l a t i o n d u r i n g t h e C r e t a c e o u s , f o r e x a m p l e , o c c u r r e d in s t a g e s a n d e x c e e d the r e s -p o n s e -p r e d i c t e d f r o m s e d i m e n t l o a d i n g a l o n e ( C r o s s ε P í l g e r J r . , 1 9 7 8 ) . C r o s s &am-p; P i l g e r s u g g e s t that s h a l l o w p l a t e s u b d u c t i o n ( F i g . 5A) r a t h e r t h a n s e d i m e n t l o a d i n g w a s t h e c a u s e o f s u b s i d e n c e in t h e i n t e r i o r o f N o r t h A m e r i c a d u r i n g t h e C r e t a c e o u s . F u r t h e r m o r e , they s u g g e s t that t h e u p p e r A m a z o n B a s i n is a m o d e r n a n a l o g u e o f s h a l l o w p l a t e s u b d u c t i o n . T h e r e c e n t i d e n t i f i c a t i o n of v o l c a n i c l a k e s in t h e A m a z o n B a s i n o f E c u a d o r ( C o l i n v a u x et a l . , 1 9 8 5 ) , is a n a r g u m e n t f o r t h i s h y p o t h e s i s . A n o t h e r p o s s i b l e t e c t o n i c m e c h a n i s m is i s o s t a t i c a d j u s t m e n t t o t h e a d d i t i o n a l l o a d o f t e l e s c o p e d t h r u s t s h e e t s in
an " A n d e a n - t y p e " c o n v e r g i n g b o u n d a r y ( F i g . 5 B ; P r i c e , 1^73; D i c k i n s o n , 1 976) . T h i s meclma π i s m is in a c c o r d w i t h basin l o c a t i o n , w e d g e - s h a p e of the s e d i m e n t s , a n d i n t e r m i t t e n t d e p o s i t i o n a l a c t i v i t y in the N i o b r a r a S e a a n d the A m a z o n B a s i n . H o w e v e r , a l t h o u g h this m e c h a n i s m is p o s t u l a t e d for d e p r e s s i o n in the u p p e r A m a z o n B a s i n , the e x t e n t of basin d e p r e s s i o n is not t h o u g h t to have p r o d u c e d an inland s e a w a y e q u i v a l e n t to the N i o b r a r a S e a , For c o n c o r d a n c e w i t h the h y p o t h e s i s of L a g o A m a z o n a s , the o n ) y n e e d e d m o d i f i c a t i o n
is a n i n c r e a s e d rate of d e p r e s s i o n that at t imé s e x c e e d e d s e d i m e n tat i o n , T h e resu I t v.oul d be a f r e s h w a t e r body o f w a t e r that o c c u p i e d the c e n t r a l A n d e a n f o r e l a n d b a s i n at v a r i o u s times d u r i n g the L a t e C e n o z o i c . E i t h e r h y p o t h e s i s , s h a l l o w p l a t e s u b d u c t i o n or thrust sheet d o w n w a r p i n g , is a c c e p t a b l e frcm o u r v i e w p o i n t . T h e c r i t i c a l f a c t o r is a r e a s o n a b l e t e c t o n i c m e c h a n i s m that c o u l d h a v e c r e a t e d a d o w n - w a r p e d b a s i n in a n a r e a that w a s a n e x t r e m e l y low g r a d i e n t a n d r e c e i v e s large v o l u m e s of p r e c i p i t a t i o n a n d r u n - o f f .
A p p l y i n g the c o n c e p t of a f o r e l a n d b a s i n , d e p o s i t i o n in the An'azon Bas i π i s 1 i n k e d to the u p l i f t of the A n d e s . T h e p r o c e s s o f rr.oun ta i n - b u i 1 d i ng in the A n d e s h a s not p r o c e e d e d s t e a d i l y but r a t h e r o c c u r r e d in s t a g e s w i t h long p e r i o d s of q u i e s e n c e b e t w e e n p e r i o d s of a c t i v e t e c t o n i s m ( N o b l e e t al ,, 1971!). T h e last of t h e s e p o s t u l a t e d e p i s o d e s o f t e c t o n i s m o c c u r r e d f r o m the M i d d l e M i o c e n e t h r o u g h the P l i o c e n e . It is this e p i s o d e that w e c o r r e l a t e w i t h the d e p o s i t i o n of the T e r t i a r y red b e d s . T h e lack of a c o m p l e t e temporal sequenci.- is t h e r e f o r e a result of n o n - d e p o s i t i o n d u r i n g a time of r e l a t i v e qui e s c e n c e (as f o r e x a m p l e , tfie a b s e n c e of s t r a t a oetvteen the L a t e M i o c e n e " r e d b e d s " a n d the Late P l e i s t o c e n e M a d r e de D i o s / S o 1 i m ò e s F o r m a t i o n ) . T h e o v e r l y i n g M a d r e d e D i o s / S o 1 i m o e s F o r m a t i o n w o u l d r e p r e s e n t a later p e r i o d of a c t i v i t y (Late P l e i s t o c e n e a n d H o l o -c e n e ) . D u r i n g this last p e r i o d of u p l i f t , w e p r o p o s e that s e d i m e n t s fiI 1ed t he fore I a n d basin of the u p p e r A m a z o n f r o m large r i v e r s that lay in the u p p e r v a l l e y s o f the prç.sçnt day A m a z o n t r i b u t a r i e s . T h e c u r r e n t p e r i o d o f r e l a t i v e q u i e s c e n c e a n d e r o s i o n r a t h e r than d e p o s i t i o n in the A m a z o n B a s i n , m a y not be the a d v e n t of a p o s t - t e c t o n ι c p h a s e , but m a y i n s t e a d m a r k a q u i e t i n t e r l u d e in a larger e p i s o d e .
S e d i m e n t s a l s o e n t e r e d t h e b a s i n f r o m the B r a z i l i a n h i g h l a n d s to the e a s t . At E s p e r a n z a F a l l s , a p p r o x i m a t e l y 30 km b e l o w R i b e r a l t a , Boi iνia, t h e R i ο Beni p a s s e s t h r o u g h a s e r i e s of falls c r e a t e d by e x p o s e d P r e c a m b r i a n rocks of the Β raz i 1 i a n Shie1 d . Here t o o , a n d e x t e n d i n g w e s t w a r d t o w a r d R i b e r a l t a , js a t h i c k c o n g l o m e r a t e at the b a s e of the M a d r e de D i os/Sol linoes F o r m a t i o n . Dy its s t ra t i g r a p h i c p o s i t i o n , th i s c o n g 1 o m e r a t e w o n 1 d seem to be the s a m e a s the A c r e C o n g l o m e r a t e M e m b e r , b u t that is n o t s o . U n l i k e the A c r e Con_ g l o m e r a t e , this conn. 1 o p e r a t e cornsists of f r a g m e n t s of the s h i e l d , C l a s t size r e a c h e s 30 cm at Esperan2a F a l l s a n d d i m i n i s h e s to the e a s t . A p p r o x i m a te 1y 1 5 km e a s t of the f a l l s ,
the c o n g l o m e r a t e is no l o n g e r s e p a r a b l e f r o m f i n e r s e d i m e n t s of the M a d r e de D i o s / S o l i -rnões F o r m a t i o n , A p p a r e n t l y , d u r i n g the Late P l e i s t o c e n e , the f o r e l a n d b a s i π of the upper A m a z o n w a s s i g n i f i c a n t l y d o w n w a r p e d a n d s e d i m e n t s e n t e r e d b o t h f r o m the e a s t a n d Lhe w e s t ,
T h e r e p r e s u m a b l y w a s a n e a s t e r n m a r g i n to this g r e a t i n l a n d lake a l t h o u g h w e h a v e seen no d i r e c t e v i d e n c e o t h e r than the c o n g 1 o p e r a t e that o r ΐ g i n a t e d near E e p e r a n z a Fal 1 s . The e l e v a t i o n o f the e a s t e r n m a r g i n of the u p p e r A m a z o n b a s i n m a y not h a v e b e e n h i g h e r than it is t o d a y , but n e e d o n l y have b e e n h i g h e r than the d o w n w a r p e d u p p e r A m a z o n , T h e
e a s t e r n m a r g i n m a y h a v e b e e n l o c a t e d a t t h e I q u i t o s A r c h , the P u r u s A r c h , o r t h e G u r u p á H o r s t . In t h a t s a m e o r d e r , t h e d e p o s i t i o n a l b a s i n d u r i n g t h e L a t e C e n o z o i c c o u l d h a v e i n c l u d e d o n l y t h e A c r e S u b b a s i n , t h e A c r e S u b b a s i n a n d the U p p e r A m a z o n S u b b a s i n , o r t h e s e t w o a n d t h e M i d d l e A m a z o n S u b b a s i n ( F i g . 6, n a m e s of s t r u c t u r a l a r c h e s a n d s u b -b a s i n s f o l l o w s B i g g a r e l l a , 1 9 7 3 ) . B a s e d o n t h e t h i c k w e d g e o f s e d i m e n t s p r o p o s e d b y Βiçj g a r e l l a (1973) a n d t h e r e l a t i v e l y t h i n a n d h o r i z o n t a l u p p e r s t r a t a iη t h e mi d d l e a n d lower A m a z o n i a n s u b b a s i n s , w e s u g g e s t that L a g o A m a z o n a s e x i s t e d p r i m a r i l y in t h e u p p e r t w o s u b b a s i n s . T h e a p p a r e n t c o n t i n u i t y o f C e n o z o i c s t r a t a t h r o u g h o u t t h e s e t h r e e s u b b a s i n s ( F i g . 7) s u g g e s t s that at t i m e s t h e e a s t e r n m a r g i n o f t h e A m a z o n i a n f o r e l a n d m a y h a v e e x t e n d e d m u c h f a r t h e r e a s t a n d p e r h a p s a s f a r a s t h e G u r u p á H o r s t . If t h i s is t r u e , t h e n all o f the p r e s e n t - d a y A m a z o n B a s i n w a s a s i n g l e d e p o s i t i o n a l e n v i r o n m e n t ( l a c u s t r i n e ) a n d t h e M a d r e d e D i o s / S o l i m o e s F o r m a t i o n m a y e x t e n d f r o m t h e b a s e o f t h e A n d e s to t h e m o u t h o f t h e A m a z o n ( F i g . 8 ) . T h e c o r r e l a t i o n o f o s c i l l a t o r y f l o o d p l a i n s in the lower b a s i n w i t h g l a c i a l - e u s t a t i c f l o o d p l a i n s f o u n d o n p a s s i v e c o a s t s iη o t h e r c o n t i n e n t s (Klam m e r , 1 9 8 4 ) , i n d i c a t e s t h a t t h e l o w e r b a s i n w a s r e s p o n d i n g to w o r l d w i d e s e a - l e v e l c h a n g e s . A l t h o u g h the level o f h y p o t h e s i z e d L a g o A m a z o n a s w o u l d h a v e f l u c t u a t e d w i t h c h a n g e s in w o r l d g l a c i a t i o n , it c a n n o t b e c l e a r l y s h o w n w h e t h e r t r r r a c i n g in the lower b a s i n is r e s p o n d i n g t o o s c i1 I a t i n g s e a - l e v e l ( u l t i m a t e b a s e Ievel) o r o s c i1 1 a t i n g 1 a k e 1evel (tempo r a r y b a s e l e v e l ) . In o u r m o d e l , w e p r o p o s e a l a r g e , c o n t i n u o u s d e p o s i t i o n a l b a s i n f r o m t h e b a s e o f t h e A n d e s to t h e G u r u p á H o r s t . A c c o r d i n g l y , m u c h o f t h i s a r e a (but n o t n e c e ^ s a r i l y a l l , o r all a t o n e time) w a s c o v e r e d b y a v a s t l a k e w i t h l a r g e o s c i l l a t i o n s in lake level i n d i c a t e d by t h e d i f f e r e n t l i t h o l o g i e s o f M e m b e r s A , B , a n d C iη the M a d r e d e D i o s / S o l i m õ e s F o r m a t i o n (recent o b s e r v a t i o n s by P r o f3 R o s a l i e B e n c h i m o 1 o f t h e U n i v e r s i d a d e Federal d o A m a z o n a s i n d i c a t e that t h e s e m e m b e r s , a n d t h e r e f o r e t h e lake o s c i l l a t i o n s that w e s u g g e s t c a u s e d t h e m , d i d n o t e x t e n d f a r into t h e u p p e r A m a z o n B a s i n , p e r s o n a l c o m m u n i c a t i o n , 1 1 . 2 9 . 8 7 ) . T h e m a j o r d o w n w a r p i n g o c c u r r e d a t the b a s e o f t h e A n d e s . H e r e , t o o , o c c u r r e d t h e g r e a t e s t a c c u m u l a t i o n o f s e d i m e n t s a n d p e r h a p s t h e m o s t s t a b l e lake c o n d i t i o n s . E a s t e r n b a s i n a l d e p o s i t i o n is l i k e l y to h a v e b e e n m o r e i n f l u e n c e d by s e d i -m e n t s that o r i g i n a t e d f r o -m t h e e x p o s e d s h i e l d t h a n b y e v e n t s a l o n g t h e w e s t e r n c o a s t o f S o u t h A m e r i c a . T h e r e f o r e , s e d i m e n t type a s well a s t h i c k n e s s o f t h e u n i t s c a n be e x -D e c t e d tft c h a n g e as the s t r a t a a r e t r a c e d e a s t w a r d a c r o s s the A m a z o n i a n s u b b a s i n s . U n d e r t h e L a g o A m a z o n a s model , t h e e n d o f the m o s t r e c e n t p h a s e o f a c t i v e t e c t o n i s m b r o u g h t a n e n d to d o w n w a r p i n g . S e d i m e n t s c e a s e d t o p o u r into t h e f o r e l a n d b a s i n o f the U p p e r A m a z o n , a n d i s o s t a t i c a d j u s t m e n t l i f t e d t h e b a s i n a l s e dim e n t s to t h e i r c u r r e n t e l e v a t i o n s ( b e t w e e n 2 2 0 a n d 2 6 0 m ) . T h e n e w l y d e v e l o p e d d r a i n a g e s y s t e m s c a r v e d the u n d u -l a t i n g t o p o g r a p h y that is s e e n o v e r t h e u p p e r A m a z o n B a s i n . O n e f u r t h e r , a n d c o n s i d e r a b l e , d i f f e r e n c e b e t w e e n t h e p r o p o s e d A m a z o n B a s i n o f the r e c e n t p a s t a n d that o f t o d a y , is t h e l o c a t i o n o f t h e b a s i n ' s o u t l e t to the s e a . T h e e x t r a o r d i n a r y s i z e o f t h e lower A m a z o n R i v e r a n d the g r e a t vai I e y t hi s r i v e r h a s s e e m i n g l y c a r v e d t h r o u g h the s h i e l d r o c k s c a u s e o n e to t h i n k t h a t the p r e s e n t l o c a ti o n o f the lower A m a z o n R i v e r is a s a n c i e n t a s t h e r i v e r i t s e l f . H o w e v e r , several o t h e r f a c t o r s , b o t h g e o m o r p h i c a n d b i o l o g i c , i n d i c a t e a n o r t h e r n o u t l e t t h r o u g h t h e O r i n o c o V a l l e y that h a s
o n l y r e c e n t l y c e a s e d to be t h e p r i m a r y o u t l e t . I n d e e d , t h i s c o n n e c t i o n in not y e t fully s e v e r e d . T h e O r i n o c o a n d the R i o N e g r o a r e c o n n e c t e d t h r o u g h o u t the y e a r a c r o s s the C a s l q u i a r e C a n a l . D u r i n g e a c h y e a r , s e a s o n a l d i f f e r e n c e s in rainfall c a u s e s w a t e r flow from the O r i n o c o to the R i o N e g r o o r j u s t the r e v e r s e ( L o w e - M c C o n n e l , 1975, p . 73)· T h i s u n u s u a l , if n o t u n i q u e , c o n n e c t i o n b e t w e e n two m a j o r w a t e r s y s t e m s m a y h a v e f o r m e d by the normal p r o c e s s o f h e a d w a r d e r o s i o n in b o t h the O r i n o c o a n d R i o N e g r o . H o w e v e r , it s e e m s u n l i k e l y that s u c h a c o n n e c t i o n w o u l d long r e m a i n a s a g e o m o r p h i c f e a t u r e e v e n c o n s i d e r i n g the t r e m e n d o u s r a i n f a l l this area r e c e i v e s . T h e C a s i q u i a re C a n a 1 , in the c o n t e x t of a d o w n w a r p i n g w e s t e r n e d g e o f the A m a z o n B a s i n , m a y be s e e n a s a l i n g e r i n q c o n n e c t i o n b e t w e e n a n o r t h e r n o u t l e t that h a s c e a s e d to be t h e p r i m a r y o u t l e t a s it t o o a d j u s t s to i s o s t a t i c u p l i f t (a h y p o t h e s i s f u r t h e r s u p p o r t e d by b i o g e o g r a p h y ) , H o w e v e r , the p r e s e n c e of a large d e e p - s e a f a n o f f the m o u t h o f the A m a z o n R i v e r , the A m a z o n C o n e , i n d i c a t e s that a n O r i n o c o o u t l e t w a s n o t t h e o n l y d r a i n a g e o u t l e t d u r i n g the L a t e C e n o z o i c ( D a m u t h E. K u m a r , 1975; K r o n b e r g e t al . , 1 9 8 6 ) . W e s u g g e s t that the d r a i n a g e o u t l e t o s c i l l a t e d b e t w e e n the O r i n o c o a n d lower A m a z o n v a l l e y s in c o r r e s p o n d e n c e w i t h a c t i v e d o w n w a r p i n g of the u p p e r A m a z o n B a s i n , D u r i n g p h a s e s o f a c t i v e t e c t o n i s m in the c e n t r a l A n d e s , the
f o r e l a n d b a s i n of the u p p e r A m a z o n w o u l d h a v e b e e n f o r n « d a n d L a g o A m a z o n a s w o u l d h a v e d r a i n e d t h r o u g h a l i k e w i s e l o w e r e d O r i n o c o v a l l e y . D u r i n g p h a s e s o f r e l a t i v e t e c t o n i c q u i e s c e n c e , a s at p r e s e n t , i s o s t a c y m a y h a v e t i l t e d t h e bowl o f t h e A m a z o n B a s i n to the east until the e a s t e r n o u t l e t w a s b r e e c h e d a n d L a g o A m a z o n a s c o l l a p s e d .
A
Pie-
5 C o n t r a s t i n g t h e o r i e s o n the f o r m a t i o n of a f o r e l a n d b a s i n . Λ. F o r e l a n d b.isni c a u s e d b y a s h a l l o v l y s u b d u c t e d p l a t e ( r e d r a w n fruto C r o E H jnd P i l f e r , lc)R2) . Β . F o r e l a n d b a s i n c a u s e d b y i s o s t a t i c a d j u s t m e n t u> a b a c k a r c f o l d - t b r u s t b e lt( r e d r a w n f r o m D i c k i n s o n , Ϊ97-6). A r r o w s inflirate ;;ui^ i d e n c o i n the f o r e l a n d b a s i n .
F i g . 6. T h e l o c a t i o n s ο ϊ s i g n i f i c a n t a r c h e s a n d s u b - b a s i n s i n t h e A m a z o n B a s i n ( m o d i f i e d f r o m B i g a r e l l a , 1 9 7 3 ) . S u b - b a s i n s ( f r o m w e s t to e a s t ) : A , A c r e ; U , U p p e r , M , M i d d l e ; M B , M a r a j ó . S t r u c t u r a l a r c h e s ( f r o m w e s t to e a s t ) : I, I q u i t o s , P , P u -r u s ; G , G u -r u p a . I m p l i c a t i o n s o f L a g o A m a z o n a s : B i o g e o g r a p h y C e r t a i n b i o g e o g r a p h i c a s p e c t s of the O r i n o c o a n d A m a z o n b a s i n s al so s u p p o r t a prevj_ o u s c o n n e c t i o n b e t w e e n t h e s e t w o v a l l e y s . For e x a m p l e , the o e t e o o 1 o s s i d f i sh a p p e a r u n a b l e to e x t e n d t h e i r r a n g e s u p r i v e r b e y o n d r a p i d s ( G o u l d i n g , 1 9 8 0 . ) . Y e t the o s t e o g l o s s i d s o c c u r in b o t h t h e t r i b u t a r i e s o f t h e A m a z o n ( b e l o w the r a p i d s ) a n d in the O r i n o c o R i v e r (above t h e r a p i d s ) . In the f r a m e w o r k of a n o r t h w a r d d r a i n a g e of t h e A m a z o n B a s i n , t h i s w o u l d n o t v i o l a t e G o u l d i n g ' s h y p o t h e t i c a l b a r r i e r to o s t e o g l o s s i d r a n g e e x t e n s i o n . T h o s e o s t e o g l o s s i d s a b o v e t h e r a p i d s o f t h e O r i n o c o m a y b e p a r t of a o n c e c o n t i n u o u s r a n g e t h a t e x t e n d e d n o r t h w a r d f r o m t h e A m a z o n B a s i n . M o r e d i r e c t e v i d e n c e c o m e s f r o m a r e c e n t l y c o m p l e t e d s t u d y of t h e fossil m o l l u s k s of the A m a z o n B a s i n ( N u t t a l , p e r s o n a l c o m m u n i c a t i o n ) . T h e f o s s i l m o H u s k s o f t h e A m a z o n a r e a p p a r e n t l y m o r e c l o s e l y r e l a t e d to t h o s e o f the C a r i b b e a n t h a n to t h o s e the P a c i f i c O c e a n . T h i s t o o i n d i c a t e s a n e x t e n s i v e - f o r m e r c o n n e c t i o n b e t w e e n the A m a z o n a n d O r i n o c o d r a i n a g e s y s t e m s a n d a C a r i b b e a n d r a i n a g e o u t l e t . O t h e r z o o g e o g r a p h i c a s p e c t s o f A m a z o n i a n f i s h r e l a t e to the h y p o t h e s i s o f L a g o A m a z o n a s . S e v e r a l s p e c i e s of small A m a z o n i a n f i s h a r e c o n f i n e d to the u p p e r e x t e n t s of t r i b u t a r y s t r e a m s o n the p e r i p h e r y o f t h e b a s i n a n d a r e n o t f o u n d in the A m a z o n R i v e r itself ( G é r y , 1 9 6 4 , 1 9 8 4 ) . U n d e r t h e e x i s t i n g r i v e r p a t t e r n , t h e e x i s t e n c e of the s a m e s p e c i e s in small r i v e r s o n o p p o s i t e s i d e s o f t h e m a i n c h a n n e l r e q u i r e e i t h e r m i g r a t i o n a c r o s s the m o u t h o f the A m a z o n R i v e r o r c h a n n e l - h o p p i n g ( l e a v i n g no i n t e r v e n i n g p o p u l a t i o n s ) a l o n g t h e b a s e o f the A n d e s (Fig. 9 A ) . B o t h r o u t e s a r e u n l i k e l y f o r small f i s h , a s G e r y , r e a l i z e d , but no a l t e r n a t i v e is p o s s i b l e if the m o d e r n d r a i n a g e w a s e s t a b l i s h e d
in the E a r l y C e n o z o i c . A t h i r d p o s s i b i l i t y is that t h e s e p o p u l a t i o n s r e p r e s e n t a r e l i c tual d i s t r i b u t i o n s f r o m r e c e n t l y w i d e s p r e a d l a k e p o p u l a t i o n s o f t h e s e s p e c i e s (Fig. 9 B ) .
west east U Μ MB — • i v y ^ 0 ^ ^ : ; ¾ I Ρ G Fig. 7. G e o l o g i c a l c r o s s - s e c t i o n of the A m a z o n B a s i n ( m o d i f i e d f r o m B i g a r e l l a , 1 9 7 3 ) . S t i p p l i n g i n d i c a t e s the y o u n g e s t u n i t t h a t is g e n e r a l l y c o n s i d e r e d to b e Terti_ a r y . A t h i n v e n e e r of h o r i z o n t a l , Q u a t e r n a r y s e d i m e n t s o v e r l i e s t h e s e T e r t i a r y s e d i m e n t s . S u b - b a s i n s ( f r o m w e s t to e a s t ) : A , A c r e ; U , U p p e r ; M . M i d d l e ; M B , M a r a j ó . S t r u c t u r a l a r c h e s ( f r o m w e s t to e a s t ) : I , I q u i t o s ; P , P u r u » G , G u r u p á . Fig. 8. A r e c o n s t r u c t i o n of L a g o A m a z o n a s w i t h a n o r t h e r n o u t l e t t h r o u g h the O r i n o c o V a l l e y . L a k e m a r g i n is d r a w n o n t h e 5 0 0 f t . ( 1 5 2 . 4 m ) c o n t o u r l i n e . Fig. 9. A . H y p o t h e s i z e d p a t h s of r a n g e e x t e n s i o n s in s o m e A m a z o n i a n f i s h e s ( f r o m G é r y , 1 9 6 4 ) . Β. A r e d u c e d L a g o A m a z o n a s , d r a w n o n the 1 0 0 m c o n t o u r l i n e , w i t h h y p o t h e s i z e d r e l i c t u a l d i s t r i b u t i o n o f l a k e f i s h in '.he n e w l y f o r m e d r i v e r c h a n n e l s ,
Normal vicariance biogeography can then be called upon to explain the disjunct distri
butions of small Amazonian fish.
The diversity of Amazonian fishes provides additional support for the former ex
istence of a vast freshwater lake over the whole of the Amazon Basin. The great numberof
species (over 2400) is far greater than typical for a river system and inv ite comparison
to the diversity of cichlid fishes in the great lakes of East Africa (Lowe-McConnel
1,
1975).
The opportunity for resource partitioning ίη anenduring, stable environment such
as a lake is a ready explanation for species diversity and has been documented in the
fossil record (Smith, 1975)· Also in Smith (1
975)
> the return to river conditions was
followed by a radical drop in species diversity. Species abundance among Amazon
i
an f i sh
may represent lacustrine resource partitioning that has not adjusted to riverine condi
tions due to the recency of this change. Perhaps this explanation has not been applied
to the question of Amazonian diversity because of the extraordinary dimensions of the
hypothetical lake. It seems as probable an explanation as those that propose speciation
under current conditions of Amazonian drainage (Lowe-McConnel1, 1
975
; Sal o e t a l . , 1 9 8 6 ) .
Species diversity of fishes in the Orinoco and Amazon basins contrasts sharply with
diversity in the nearby Magdalena River of Colombia (Lundberg e t a l . ,
1
986). Although
previous taxonomic similarity followed by extirpation in the Magdalena River may be a
factor as Lundberg e t a l . ,
1986,
suggested, this lack of diversity may a I so be exp
1
ained
if the Magdalena River never developed diversity equal to that of the larger systems due
to different geologic histories. This second explanation seems more in accord with the
information presented by Weitzman & Fink
(1985)
on endemism of xenurobryconiη fish in
the Magdalena and Amazon basins.
The zoogeographic implications of a large lake in the Amazon Basin during the Late
Pleistocene and much of the Holocene are of course profound. The seemi ngl y d i sjunct ranges
of the same or closely allied species that are difficult to explain due to present-day
geomorphic features have been noted and commen ted upon by numerous researchers . For example,
the centripetal migration of primates and flora from the peripheral highlands into the
interior of the Amazon Basin (and the recency of that event) are discussed by Camp
(1952)
and Hershkovitz
( 1 9 7 2 ) .
The extension of ranges and subsequent isolation due to the
development of a large drainage system following the collapse of Lago Amazonas is a new
interpretation for these observations.
In recent years, the Tropical Forest Refugia Model of Haffer
(I969>
1974) and Van
zolini δ Williams
(1970)
has been a mode! for biological studies in the Amazon
(Duell-man, 1979; and Prance, 1
982
). Until the discovery of the young age of the Amazon
i
an sedi
ments, it was the only comprehensive and plausible explanation for zoogeograph
i
c distri
butions. Under this model, as the forest retreated and fragmented before the expanding
plains (presumably during the dry periods that corresponded to the glacial advances in
the northern hemisphere), the forest taxa were isolated. The resultant a
11
opatric speci
ation lingers as contiguous range boundaries among species of a now reunited forest.
The f o r e s t rebuff i o «ooel is e l e g a n t in i t* l o g i c y e t d o c s leavr soir.c n....-.c ioiv. u n a n s w e r e d . For e x a m p l e , it s e e m s most a p p l i c a b l e to t e r r e s t r i a l a n i m a l s a n d p l a n t s . V e l t z m s n £ W e i t z m a n (19^2) s p e c i F i c a 1 I y a d d r e s s e d t h e q u e s t i o n o f the re fug i urn m o d e 1 U i e A m a z o n i a n fish d i s t r i b u t i o n . In v:hiL t h e y s t r e s s e d w a s a n aa r 1 y ΐ t udy o f t h i s fc i n d , they c o u l d f i n d n o c l e a r r e l a t i o n s h i p b e t w e e n h y p o t h e s i z e d f o r e s t r e f u g i a a n d d i s t r i b u t i o n s of F o r e s t - r e s i r i c t e d f i s h . N o r d o e s the model a l l o w f o r t h t l i k e l y o c c u r r e n c e , of c o n i-dor f o r e s t s a l o n g t h e m a r g i n s of the p r e s u m e d s t a b l e d r a i n a g e p a t t e r n . In f a c t , a l t h o u g h the h y p o t h e t i c a l r e f u g i a a r e v a r i o u s l y p l a c e d by d i f f e r e n t a u t h o r s , t h * n'ajorl ty are. s i t u a t e d in t h e u p p e r r e a c h e s o f the t r i b u t a r y s t r e a m s a t e I üvat ions b e t w e e n $ ϋ ΰ a n d I ODD feet (Fit], I I ) . That is the the l o c a t i o n a n d e l e v a t i o n of rhe h y p o t h e s i J e d d e l t a i c s e d i m e n t s o f 2 o n e C in the M a d r e de Dins/£oI imoes F o r m a t i o n a s seen in the v i c i n i t y o f filo B r a n c o , U s i n g the l o w e r f i g u r e (500 f e e t , a p p r o x i m a t e l y 1 5 • m ) to r e c o n s t r u c t t h e m a r g i n s of a grual lai;e, m o s t o f the p r o p o s e d r e f u g i a c a n b e a c c o m m o d a t e d a l o n g the lake b o r d e r (Fig. 1 1 ) .
The: f o r e s t r e f u g i a model is f u r t h e r c h a l l e n g e d I r, that p a lyno logical s a m p l i n g o f o n e p r e s u m e d re fugi urn in E c u a d o r gavi; e v i d e n c e for a m o n t a n e , A n d e a n forest, b e t w e e n
26,00.(1 y e a r s b , p . ( K a m - b i u £ Col i n v a u x , 1 9 ^ 5 ) - In this c a s e , h y p o t h e s i z e d lago A m a z o n a s a n d a l t i t u d í n a l d e p r e s s i o n of t h e A n d e a n f l o r a , t o g e t h e r , m a y h a v e di s r u p t e d t h e rain forest- in t h e w e s t e r n A m a z o n B a s i n ,
R e c e n t p a p e r s have s u g g e s t e d that h a b i t a t d i v e r s i t y a n d d i s t u r b a n c e d u e to f l o o d -p l a i n c o n d i t i o n s -p r o v i d e a m o r e -p a r s i m o n i o u s e x -p l a n a t i o n of Amazonian- d i v e r s i t y a n d dis_ tri but ions ( R a ^ s a ^ n e n , S a l a ε K a l l i o l s , 1907; Col i n v a u x , 1967). A l t h o u g h f l o o d p l a i n d i s
t u r b a n c e in a m a t u r e l a n d s c a p e c a n n o t be d e n i e d u n d e r p r e s e n t c o n d i t i o n s a n d is p l a u s i b l e for t h e recent p a s t , t h e s e p a p e r s d o not a d d r e s s t h e r e m a r k a b l e p a t t e r n s o f Amazonian d i s
t r i b u t i o n s that p r o v o k e d t h e f o r m u l a t i o n a n d r e a d y a c c e p t a n c e o f the rafugihim inodel. T h e t h e o r i e s o f Forest r e f u g i a a n d L a g o A m a z o n a s a r e n o t i n c o m p a t i b l e . A n integral p a r t o f t h e r e f u g i s model is t h e w i d e s p r e a d p r e s e n c e of a p l a i n s b i o t a in t h e A m a z o n B a s i n (and s u p p o r t e d by pa l y n o S o g i c e l s t u d i e s such a s that o f A b s y ε v a n d e r Haimnen, 1 9 7 6 ) . G r a s s l a n d s c o u l d h a v e btr.n p r e s e n t d u r i n g lake level o s c i l l a t i o n s , o r , if t h e c o l l a p s e o f L a g o A m a z o n a s w a * a s r a p i d as s u g g e s t e d by the r e c e n t a g e o f the s e d i m e n t s , normal e c o l o g i c a l s u c c e s s i o n w o u l d h a v s p r o d u c e d a vast g r a s s l a n d that then q u i c k l y s u e cuiraned to the c l i m a x f o r e s t .
A n I n t e r e s t i n g a d d i t i o n t o t h i s d i s c u s s i o n is that A m a z o n i a n 1 i n g u i s 1 i c home 1 a n d s , like f o r e s t r e f u g i a , a r e p l a c e d o n the p e r i p h e r y o f the A m a z o n B a s i n a n d a p p e a r to s u p port the refugi uni model (Heggar^., 1979; H i g l i a z z a , 19&2) . M i g r a t i o n o f language t y p e s
(and p e o p i e ) into t h e interior o f the A m a z o n is h y p o t h e s i z e d to be v e r y r e c e n t , b e t w e e n ZOUO a n d Ί500 y e a r s a g o (Fig, I D ) . F u r t h e r m o r e , h y p o t h e s i z e d p a t h s o f m i g r a t i o n in t h e central b a s i n a f t e r 2 0 0 0 y e a r s a g o m o r e c l o s e l y f o l l o w s t h e m a i n d r a i n a g e o f the A m a z o n River a s v » u l d be e x p e c t e d if the d r a i n a g e b e c a m e e s t a b l i s h e d o n l y a f t e r this t i m e . T h e v e r y y o u n g d a t e s f o r t h e l i n g u i s t i c m i g r a t i o n s c o r r e s p o n d to h y p o i h e s i z e d a g e s y i v e n to Zone C o f the H a d r e de Díos/Sol imões F o r m a t i o n (Campbell ε F r a i l e y , l 9 o M . T h a t d a t e w a s ; r e a t e d to c o n f o r m wi th Ifolocene c l i m a t i c e v e n t s ( d e s c r i b e d by V!e n d l a n d 6 B r y s c n , \3~!b\
-F a i r b r i d g e , 1 9 7 6 ) . T h e l o n g a b s e n c e o f l a n g u a g e in the c e n t r a l p o r t i o n of the A m a z o n B a s i n is less e a s i l y e x p l a i n e d by the r e f u g i a m o d e l t h a n by c u r lake m o d e l . T h e s u g g e s t e d a g e s for the e n t r a n c e o f c e r t a i n l i n g u i s t i c p a t t e r n s into the A m a z o n B a s i n t h e r e f o r e s e r v e a s a n i n d e p e n d e n t test of p r o p o s e d g e o l o g i c a l e v e n t s in the H o l o c e n e a n d for the e x t r e m e l y y o u n g a g e of Z o n e C. F i g . 10. A . A r e a s p o s t u l a t e d b y V a n z o l i n i ( s o l i d b l a c k l i n e s ) a n d H a f f e r ( d a s h e d l i n e s ) as h a v i n g b e e n f o r e s t r e f u g e s d u r i n g t h e P l e i s t o c e n e ( f r o m V u i l l e u m i e r , 1 9 7 1 . B. L o c a t i o n of L a g o A m a z o n a s r e l a t i v e to the f o r e s t r e f u g e s . T h e h y p o t h e s i z e d l a k e o r m a r s h in s o u t h e a s t e r n B o l i v i a is d r a w n h e r e f o l l o w i n g W o l f f , e t al. (in m a n u s c r i p t ) . F i g . 1 1 . L a n g u a g e c e n t e r s a n d h y p o t h e t i c a l d i s p e r s a l r o u t e s of A m a z o n i a n l a n g u a g e s ( f r o n M i g l i a z z a , 1 9 8 2 ) . A . D i s p e r s a l r o u t e s p r i o r to 2 0 0 0 y e a r s a g o w i t h L a g o A m a z o n a s p r e s e n t . Β. D i s p e r s a l r o u t e s a f t e r 2 0 0 0 y e a r s a g o .
