51 berthou & bengtson 1985 - ix congresso brasileiro de paleontologia

8/18/2019 51 Berthou & Bengtson 1985 - IX Congresso Brasileiro de Paleontologia

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CENOMANIAN-CONIACIAN MICROFACIES IN THE SERGIPE BASIN, BRASIL

P i e r r e - Yv e s B e r t h o u

P e t e r Bengtson

M i c r o f a c i e s s tudy o f o u t c r o p f rom t h e CenomanianCon iac i an C o t i n g u i b a L i m e s t o n e F o r m a t i o n o f t h e Serg ipe Bas in

i n n o r t h e a s t e r n B r a z i l were c a r r i e d o u t , i n o r d e r t o complement

t h e e x i s t i n g s t r a t i g r a p h i c a l f ramework b a s e d e s s e n t i a l l y on

ammoni tes . M a t e r i a l f rom 604 l oca l i t i e s was avai lable of which

278 had been d a t e d u s i n g ammoni tes a n d / o r i n o c e r a m i d s ) .

M i c r o f a c i e s a n a l y s i s was shown t o be u s e f u l i n most c a s e f o r

d a t i n g down t o s t a g e o r e v e n s u b s t a g e l e v e l . Four main g roups

o f m i c r o f a c i e s were r e c o g n i z e d , ~ o r r e s p o n d i n gt o t h e Cenomanian,l o w e r u r o n i a n ~uppe r Tu ron i an and C o n i a c i a n , r e s p e c t i v e l y . The

m i c r o f a c i e s t y p e s recogn ized a r e b r i e f l y d e s c r i b e d and f i g u r e d .

The p r e s e n c e i n most o f t h e samples of C a l c i s p h a e r u l i d a e ,

i n p a r t i c u l a r P i t h o n e l l a , i n d i c a t e s t h a t d e p o s i t i o n t ook p l a c e i n

t h e o u t e r s h e l f r e g i o n . Thi s i s i n acco rdance wi th c u r r e n t

concep t s o f t h e C o t i n g u i b a l i m e s t o n e s as open mar ine d e p o s i t s .

The m i c r o f a c i e s p o i n t t o a p o l a r i t y i n t h e b a s i n . Thus ,

ma cro fau n a l d e b r i s d e c r e a s e s f rom n o r t h e a s t t o s o u t h w e s t , f o r a l ls t r a t i g r a p h i c a l s t a g e s . The l i t t o r a l zone l i e s i n a n o r t h e a r l y

d i r e c t i o n , and a t a c e r t a i n d i s t a n c e f rom t h e c e n t r a l p a r t s o f

t h e Bas in .

The s e q u e n c e seems t o be c h a r a c t e r i z e d by c y c l i c

abundance o f macro fauna l d e b r i s b r o u g h t by c u r r e n t s f rom t h e

l i t t o r a l zone and p roduced by t h e r ework ing o f p r e v i o u s l y

d e p o s i t e d m a t e r i a l . Th i s i s t h e c a s e f o r t h e Cenomanian which

l o c a l l y c o n t a i n s A l b i a n o n c o l i t h s . Each s t a g e seems t o c o n t a i nc o a r s e m a t e r i a l a t t h e base and l ower- ene rgy d e p o s i t s above . A

co n s p i cuou s f e a t u r e i s t h e f r e q u e n t s i l i c i f i c a t i o n o f f o s s i l

f r agmen t s . Th i s i s p r o b a b l y r e l a t e d t o t h e p r o x i m i t y o f t h e

exp and i ng A t l a n t i c Ocean f l o o r d u r i n g t h i s g e o l o g i c a l i n t e r v a l .

Unive r s i±e P . M C u r i e , L a b o r a t o i r e de G e o l o g i e des B a s s i n s

S e d i m e n t a i r e s e t L.A .319 , t o u r s 14-15 , 4e e t a g e , 4 p l a c e

J u s s i e u , F-75230 P a r i s Cedex 5 , France

P a l e o n t o l o g i s k a musee t , Box 558 , S-751 22 Uppsa l a , Sweden

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51 berthou & bengtson 1985 - ix congresso brasileiro de paleontologia

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