Science Bigarella 1964

8/2/2019 Science Bigarella 1964

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M a r i n e G e o l o g y - E l s e v ie r P u b l is h in g C o m p a n y , A m s t e r d a m - P r i n t e d i n T h e N e t h e r l a n d s

S A N D - R I D G E S T R U C T U R E S F R O M P A R A N /~ C O A S T A L P L A IN 1JOAO JOSI~ BIGARELLAGeolog ical Ins t i tu te , Un ivers ity o f Paran d , Cur i t iba , Paran t i (B raz i l )( R e c e i v e d D e c e m b e r 8 , 1 96 4)

S U M M A R Y

The s t ruc tu re , t ex tu re and env i ronm enta l c ond i t ions o f ex tensive beach r idges fo rm edpara l l e l to the p re sen t shore l ine a re ana lysed in th i s pape r , w i th a b r i e f d i s cus s ion abo u tt h e p r o b a b l e m a r i n e e n v i r o n m e n t o f d e p o si t io n . T h e r e i s, h o w e v e r , s o m e l a c k o fce r t a in ty abo u t the spec if ic cond i t ions o f depos i t ion . Co nc lus ions a re based p r imar i lyon pa ra l l e lly s tra t if i ed and s l igh t ly s eaward- inc lined s ands , w h ich ma y ind ica te depos i -t ion in the shoreward env i ronment . Cros s -s t ra t i f i ed beds a re be l i eved to have o r ig i -n a t e d s h o r e w a r d o r s e a w a r d f r o m a b a r o r p o s s i b l y o f f s h o r e b y l o n g s h o r e c u r r en t s .

INTRODUCTI ON

The coas ta l p la in in the s t a te o f Pa ranf i, b e tween Paranagu~i and G uar a tu ba Bays , ism ade up o f ex tens ive beach ridges , pa ra ll e l to the p re sen t shore line . These s an dysed iments , c l ea r ly o f mar ine o r ig in , have bee n ca l led " re s t ing a" in the Braz il i an geo -log ic and geograp h ic l i te ra tu re . Be twe en the s and r idges the re a re r ive rs o f f re sh o rb r a c k i s h w a t er , o r s w a m p y n a r r o w b a n d s o f sa n d y c l a y e y s e d im e n t s. M a n g r o v e i scha rac te r i st i c o f the a rea in f luenced b y t ides .

S e d i m e n t a r y s t r u c t u r es o f b e a c h r i d g e s a r e d e s c r ib e d f r o m e x p o s u r e s a l o n gt h e M a c i e l R i v e r m a r g in , n o t v e r y f a r f r o m P a r a n a g u fi B a y ( F i g . l) . T h e s e s t r u c tu r e shave been expo sed in the c li ff s by f luv ia l e ros ion .

T h e o r i g i n a n d d e t ai ls o f t h e d e p o s i t io n a l e n v i r o n m e n t o f t h e s o - c al le d b e a c hr i d g e s a r e n o t v e r y w e ll u n d e r s t o o d . I n t h i s p a p e r s o m e d a t a a r e p r e s e n t e d a s a c o n -t r ibu t ion to the in te rp re ta t ion o f the c ros s -bed ded s t ruc tu re s in these beach r idges.

BEACH RIDGES

The beach r idges ( " fe ixe de re s t inga" ) a re a s e t o f ve ry low san dy h il ls , pa ra l l e l to the1 T h e w o r k o f t hi s p a p e r h a s b e e n f i na n c ia l ly su p p o r t e d b y t h e C o n s e l h o N a c i o n a l d e P e s q u is a s, R i ode Janeiro, Brazil.

Marine Geol . , 3 (1965) 269 - 278


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Fig.1. Geographic location o f the M aciel Riv er in the ParanaguA Bay on the coast of P ararui, Brazil.The arrow in the center of the circles represent the average dip direction of the cross-bedded strata.The n umbers represent elevation in meters of the sandy terraces above m ean sea level.

s h o r e li n e , w h i c h r e p r e s e n t t h e r e s u l ts o f a c o n t i n u o u s r e t r e a t o f t h e s e a . T h e y b e g i na t t h e l o c a l it y o f A l e x a n d r a w i t h a n e l e v a t i o n o f 10 m . T o w a r d t h e s e a t h e i r h e i g h td e c r e a s e s p r o g r e s s i v e l y to a b o u t 2 - 3 m . A m a r i n e r e g r e s s i o n i s r e s p o n s i b l e f o r t h isc o n s t a n t d e c r e a s e in e l e v a ti o n . H o w e v e r , b e t w e e n t i m e s o f r e t r e a t o f t h e s ea , t w o s m a l lt r a n s g r e s s i o n s t o o k p l a c e . T h e o l d e r o n e is n o t w e l l r e c o r d e d , b u t t h e m o r e r e c e n to n e o c c u r e d a b o u t 2 ,7 0 0 y e a r s a g o , r e a c h i n g a l e v el a b o u t 1 .5 m a b o v e t h e p r e s e n tsea l evel (BIGARELLAan d SANCHES, 1965) .

D u r i n g d e p o s i t io n o f t h e s a n d y t e r r a c e s o f A l e x a n d r a ( o ld b e a c h r i d ge s ), t h er e l a ti v e s e a le v e l h a s b e e n e s t i m a t e d t o b e a b o u t 7 .5 m a b o v e t h e p r e s e n t l e v e l. T h i sw a s d e t e r m i n e d b y u s i n g a m e t h o d b a s e d o n g r a i n s i ze a n al y s es a s d e s c ri b e d b yBmAI tELLA et a l . (1961) .

T h e M a c i e l R i v e r s a n d y te r r a c e is a t a n e l e v a t i o n o f 5 .2 m a t t h e t o p o f t h e r i d g ea n d 3 .7 m i n th e v a l l e y p a r t . T h i s r e p r e s e n t s a s e a l ev e l 2 .5 m h i g h e r t h a n t o d a y .A s s u m i n g t h a t t h e s a n d y t e r r a c e s h a v e b e e n b u i lt u p a s a s t o rm - w a v e t e r ra c e( b e rm ) , i n a m a n n e r s i m i la r t o t h e p r e s e n t d a y " b e a c h re v e r s e " ( P o r tu g u e s e : " r e v e r s o

M a r i n e G e o l . , 3 (1965) 269-278


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SAND-RIDGE STRUCTURESFROM PARANA COASTALPLAIN 271d a p r a i a " ) , t h e c o n s i d e r a t i o n o n s e a- le v e l p o s i t i o n s re l a ti v e t o t h e t e r r a c e s c a n b et a k e n a s c o r r e c t .

H o w e v e r , s t u d ie s i n b e a c h - r i d g e s t r u c tu r e s , i n d i c a te t h a t n o t a l l t h e s a n d y t e r -r a c e s h a v e b e e n f o r m e d a s s t o r m - w a v e t e r ra c e s . A l s o i s it n o t d e fi n it e ly k n o w n i nw h i c h p a r t o f t h e e n v i r o n m e n t th e s e s t r u c tu r e s w e r e fo r m e d . I n c a s e o f a n o f f s h o r eo r s h o r e t a c e e n v i r o n m e n t t h e s a n d y t e r r a c e s w o u l d a c t u a l l y r e p r e s e n t a s e a le v e ls o m e w h a t l o w e r t h a n t h e o n e a s s u m e d i f th e y w e r e " b e a c h r e v e r s e " l ev el s. F u r t h e rr e s e a r c h o n t h e s u b j e c t is st il l n e e d e d . F o r s i m i la r b e a c h r i d g e s f r o m C o s t a d e N a y a r i t(Mex ico ) C tZP ,R AY a n d M O O R E ( 1 9 64 , p . 8 0 ) p o s t u l a t e t h a t t h e s h o r e f a c e w a s b u i l ts e a w a r d t o p r o v i d e a b a s e f o r t h e f o r m a t i o n o f a n e w l o n g s h o r e b a r , w h i c h w a s b u i ltu p a b o v e s e a le v e l t o f o ~ m a n e w b e a c h r i d g e . T h i s se e m s t o b e v e r y r e a s o n a b l e . T h es t r u c t u r e s f o u n d i n t h e M a c i e l R i v e r a r e a c o n f i r m t h e o r i g i n o f t h e b a se . H o w e v e r , o v e rt h is b a s e n o t h i n g h a s b e e n f o u n d t h a t w o u l d e x p l a in t h e o r i g i n o f th e o v e r l y i n g s t ru c -t u r e l e s s s a n d y s e d i m e n t s .

BEACH AND ADJACENTENVIRONMENTSA c c o r d i n g t o JO HN SO N ( 1 9 19 , p . 1 60 ), t h e b e a c h i s t h e z o n e o v e r w h i c h t h e w a t e r l i n e ,o r t h e l in e b e tw e e n l a n d a n d s e a, m i g r a te s . I t m a y b e d i v i d e d in t o f o r e s h o r e a n d b a c k -s h o r e . S e a w a r d f r o m t h e b e a c h a r e t h e o f f s h o r e s e d i m e n t s .

S a n d b o d i es f o r m e d o f f s h o re t h r o u g h w a v e a c t i o n a t t h e p o i n t o f w a v e b r e a k ,a r e c a l l e d l o n g s h o r e b a r s . A c c o r d i n g t o M C K E E a n d S TE RR ET T 1 9 6 1 , p . 1 4) t h e y d e p e n d" o n d e p t h o f t h e w a t e r ( v a r i a ti o n i n s l o p e) , a m o u n t o f w a v e a c t i o n a n d a v a i la b i l it yo f s a n d . T h e s e s a n d b o d i e s m a y d e v e l o p a s: ( 1 ) s u b a q u e o u s m a s s e s t h a t d o n o t b u i l du p t o w a t e r l e v e l ; ( 2 ) m a s s e s t h a t b u i l d u p t o , b u t n o t a b o v e , w a t e r l e v e l, o r (3 ) m a s s e st h a t b u i l d t o m a x i m u m w a v e h e i g h t " .

L a b o r a t o r y e x p e r im e n t s m a d e b y t h e se a u t h o r s i n d i c at e t h a t p r o b a b l y " i n v e r ys h a ll o w w a t e r a n e m e r g e n t b a r c o m m o n l y f o rm s ; i n s o m e w h a t d e e p e r w a t e r a s u b-m a r i n e b a r is b u il t ; a n d i n s ti ll d e e p e r w a t e r n o b a r f o r m s . I n c r e a s e i n i n t e n s i ty o fw a v e s t e n d s t o b u i l d a b a r t o w a r d , a n d e v e n o n t o , t h e b e a c h . W e a k e r w a v e s b u i ld b a r su p w a r d t o f o r m b a r ri e rs , w i t h l a g o o n s t o s h o r ew a r d . A b u n d a n t s a n d f u rn i s h e d o nt h e s e a w a r d s id e o f a g r o w i n g b a r s i m u l a t e s c o n d i t i o n s c a u s e d b y s o m e l o n g s h o r e a n dr i p c u r r e n t s a n d c a u s e s g e n t l e s e a w a r d - d i p p i n g b e d s t o f o r m . I n c o n t r a s t , a - l i m i t e ds a n d s u p p l y r e s u lt s i n g r o w t h o f b a r s t h a t c h a r a c t e r is t i c a ll y h a v e s h o r e w a r d - d i p p i n gs t r a t a o f s t eepe r an g le " (MCKEE a nd STEP, Ea- r, 1961 , p . 13 ).

A c c o r d i n g t o M C K E E a n d S rE R ~E T T ( 1 9 61 , p .2 6 ) t h e s t r a t i f ic a t i o n i n l o n g s h o r eb a r s a t B im i n i i n t h e B a h a m a I s l a n d s s h o w e d " t h a t l a r g e p a r t s o f t h e b a r s w e i ef o r m e d m a i n l y o f s t r a t a d i p p i n g s h o r e w a r d a t 1 6 - 2 0 , b u t i n s o m e p l a c es , s e a w a r do f t h e b a r c r e s t, u p p e r s t r a t a d i p p e d s e a w a r d a t 4 - 5 " .

A s u r v e y o f s e d i m e n t a r y s t r u c tu r e s o n b e a c h e s w a s m a d e o n t h e P a r a n h c o a s tby BXGARELLAe t al . ( 19 6 5 ). B e s id e s t h e n o r m a l t y p e o f fo r e s h o r e a n d b a c k s h o r e a n dl o w - a n g l e c r o s s - s t r a ta s e ts , t h e r e a r e s e v e r al o t h e r t y p e s o f m i n o r s t r u c t u r e s r e p r e s e n t -

Marine Geol., 3 (1965) 269-278


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27 2 ~ j. BIGARELI ~,e d i n c l u d i n g b e a c h - c h a n n e l f il li n g s ( s m a l l - s c a le t r o u g h c r o s s - s t r a t if i c a t io n ) , c o n t o r t e db e d d i n g , r i p p l e m a r k s a n d o t h e r s . I n s o m e ca s e s, e s p e c ia l l y i n b a c k s h o r e d e p o s i t s ,a f a i r ly t h i c k s e t o f p a r a l l e l s t r a t a m a y b e p r e s e n t . T h e y d i p in l a n d a t 2 - 3 ' , w h i l e t h ef o r e s h o r e s e ts c o m m o n l y d i p s e a w a r d 3 - 6 ~' i n f in e to m e d i u m - s i z e d s a n d b e a c h e s a n dm o r e t h a n 6 i n c o a r s e r d e p o s i t s . C h a r a c t e r i s t ic o f th e b e a c h e n v i r o n m e n t , e s p e c ia l lyo f t h e f o r e s h o r e , is th e r h y t h m i c a l t e r n a t i o n o f li g h t a n d h e a v y m i n e r a l la m i n a ewi th in s e t s .

TEXTURE

T e n s a n d s a m p l e s w e r e c o l l e c te d f r o m b e a c h - r i d g e c l if fs i n t h e M a c i e l R i v e r . M o s ts a m p l e s a r e c o m p o s e d o f f i n e - g ra i n e d sa n d . S i z e d i s t r ib u t i o n w a s d e t e r m i n e d b ys ie v in g a n d c a l c u l a ti n g t h e m e d i a n d i a m e t e r o f t h e p a r t i c le s i ze d i s t r ib u t i o n e x p r e s s e di n m i l i m e t e r s ( F i g . 2 , 3 ) .

T h e m o s t c o m m o n s e d i m e n t h as a m e d i a n d i a m e t e r o f a b o u t 0 .1 6 0 m m , w h i let h e m e d i a n s i ze o f a l l s a m p l e s r a n g e s f r o m 0 .1 2 8 m m t o 0 .5 11 m m . T h e m e a n d i a m e t e ra v e r a g e s 0 .1 9 5 m m . C o a r s e s a n d o c c u r s le ss t r e q u e n t l y i n s o m e l o c a l s e ts o f s t r a t a .

T h e s a n d s s h o w g o o d T R A SK 'S ( 1 9 32 ) s o r t i n g c o e f f i c ie n t ( S o = 1 .13 -1 .34 ) . T hes m a l l es t s o r t i n g v a l u e o b s e r v e d c o r r e s p o n d s to t h e c o a r s e s t s a m p l e . T h e a s y m m e t r yi s g e n e r a l l y n e g a t iv e , m e a n i n g t h a t t h e d i s t r i b u t io n h a s a s k e w n e s s t o w a r d t h e c o a r s e rs id e . T h e k u r t o s i s i n d i c a t e s t h a t t h e g r a i n - s i z e d i s t r i b u t i o n c u r v e s a r e l es s p e a k e d t h a nt h e n o r m a l d i s t r i b u t i o n .

SEDIMENTARY STRUCTURES

T h e s a n d y t e rr a c e s ( s t r a n d p l a in ) a r e n o r m a l l y s tr a ti fi e d . T h e s e t s o f s t r a t a a r e g e n e r -a l l y p a r a l le l a n d s u b h o r i z o n t a l , d i p p i n g a s m u c h a s 1 - 2 . I n s o m e p l ac e s , h o w e v e r ,b o t h c r o s s - st r a ti f ic a t io n a n d r i p p l e l a m i n a t i o n a r e p r e s e n t.

C r o s s , s t r a ti f ic a t i o n s u c h a s u s u a l ly d e v e l o p s in f o r e s h o r e s w a s n o t f o u n d . T h eb a c k s h o r e s t r a t a a r e h a r d t o i d e n t i f y w h e r e th e y a r e n o t a s s o c i a te d w i t h t h e f o r e s h o r es e ts . S o m e o f t h e s et s i n t h e b e a c h r i d g e m i g h t h a v e b e e n d e p o s i t e d o n t h e b a c k s h o r e .

T h e l o n g s h o r e b a r c r o s s - s t r a t a o b t a i n e d i n l a b o r a t o r y e x p e r i m e n t s (M C K E E a n dS TE R R ET r, 1 96 1) , a s w e l l a s t h o s e o b s e r v e d i n t h e f ie l d , d i p 1 0 - 3 0 s h o r e w a r d o r u p t o6 s e aw a r d . T h e y r e p r e s e n t t w o d i f fe r e n t a tt i tu d e s o f s t r a ta , d i p p i n g i n o p p o s i t ed i r e c ti o n s . H o w e v e r , b o t h s t ri k e r o u g h l y p a r a ll e l t o t h e s h o r e li n e .

L a b o r a t o r y e x p e r im e n t s o n b e a c h f o r m s a n d s t ru c t u re s m a d e b y M c K E E a n dSX ER mZ Tr ( 1 9 6 1, p .1 3 ) s h o w t h a t f o r e s h o r e b e a c h s t r a t a n o r m a l l y d i p s e a w a r d a t l o wa n g l e s f r o m t h e c r e s t o r b e r m t o a p o i n t b e l o w w a t e r le v e l. O f f s h o r e f r o m t h i s p o i n tt h e r e a r e f o r e s e t b e d s w i t h r e l at i v e ly h i g h a n g l e s w h i c h f o r m a s h o r e f a c e t e r r a c e . T h e s ef o r e s e t b e d s s t r i k e p a r a l le l t o t h e s h o r e li n e a n d d i p 2 7 - 3 1 s ea w a r d . T h e f o r e s h o r eb e a c h s t r a t a d i p 7 - 1 5 a s s h o w n b y th e e x p e r i m e n t s .

M arine G eol. , 3 (1965) 269-278


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F ig . 2 . C u m u la t ive c u r ve s o f g r a in - s i z e d i s t r i bu t ion in s a nd - r idge sa m ple s a t t he M a c ie l R i ve r cl if fs . Th es a m p l e s w e r e c o l l e c t e d f r o m t h r e e d i f f e r e n t b e a c h r i d g e s f r o m t h e l a s t 5 k m o f t h e r i v e r c u t b a n k s . T h e yb e l o n g t o t h e c r e s t o r t r o u g h o f t h e s a n d r i d g es a c c o r d i n g t o t h e e le v a t i o n s r e f e r re d t o i n th e p r o fi l e s o fF i g .3 . O n a n a v e r a g e t h e y r e p r e s e n t al l t h e si d es f r o m t h e ri d g e s. T h e c u r v e s w e r e d r a w n o n a r i t h m e t i cp r o b a b i l i t y p a p e r .M a r i n e G e o l. , 3 (1965) 269-278


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SAND -RIDGE STRUCTURESFROM PARAN]~ COASTALPLAIN 275F o l l o w i n g t h e c l a s s i f ic a t i o n o f M C K E E a n d W E IR ( 1 95 3, p . 3 8 7 ) , t h e M a c i e l R i v e r

s a n d y t e r ra c e s s h o w p l a n a r a n d t r o u g h t y p e s o f c r o s s - s tr a t if i c a ti o n . T h e p l a n a r t . v ~ ,w i t h p a r a l l e l b o u n d i n g s u r fa c e s o f e r o s i o n b e t w e e n t h e s e t s o f s t ra t a , i s t h e m o s tf re q u e n t . T h e t r o u g h t y p e is c o m m o n l y a s so c i a te d w i th c o - s et s o f t h e p l a n a r t y p e , f o r m -i n g c o m p o s i t e s e ts o f c r o s s - s t ra t a .

T h e m a g n i t u d e o f t h e c r o s s -s t r a ti f ic a t i o n is c la s si fi ed a c c o r d i n g t o M C K E E a n dW E IR ( 1 9 5 3, p . 3 8 8 ) a s b e i n g g e n e r a l l y o f m e d i u m - s c a l e a n d , l e ss f r e q u e n t l y , s m a l l - s c a l e .S o m e e x p o s u r e s s k o w r i p p l e l a m i n a t i o n a n d a n i m a l b u r r o w s . F i g . 3 i l l u s t r a t e s t h ev a r i o u s f o r m s o f c r o s s - s t ra t if i c a ti o n .

I n s o m e e r o s i o n c u t s , r i p p l e - l a a - n i n a t e d s a n d i s a s s o c i a t e d w i t h s e q u e n c e s o fp l a n a r - t y p e c r o s s - s t ra t i fi c a t io n ( F ig . 4 ). T h e t r a n s p o r t a t i o n t r e n d s s h o w n b y t h e r ip p l e sc o i n c i d e w i t h th o s e d e d u c e d f r o m c r o s s - s t r a ta m e a s u r e m e n t s . B e s id e s r i p p l e la m i -n a t i o n , a m i n o r - s c a l e s t ru c t u r e , p o s s i b l y d u e t o a n i m a l b u r r o w i n g f o l l o w e d b y s o m ef il li ng a n d l a t e r c o m p l e t e d b y c o l l a p s e o f t h e a b o v e s t r a t a , i s s h o w n i n F i g .5 .

Fig.4. Planar type o f cross-stratification associated w ith ripple-lam inated sand. The drawing is orient-ed n orth-s ou th (left to r ight side). The cross-strata d ip northward.

Fig.3. Cross-be dded sand-ridge structures. H istograms illustrating the distribution o f size grades insamples from the cutbanks of the last 5 km of the Maciel River. Eight profiles were m ade downstreamin the river cutbanks. Th ey refer t o : c r e s t (I), trough (VII, VIII) and intermediate p osition b etween theridges (H-VI). The cr0ss-bedding drawings are relative to the cutbanks referred to in the profiles. Thel o w e r pa rt o f the structure drawing is roughly located c l o s e t o "se a level". Two drawings were madefo r profile HI. B etween them exists a vertical distance of 0.5 m. "Se a level" in the i llustration is maxi-mu m high- tide level , abou t 1 m abov e mean sea level.

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27 6 J.J . BIGARELLA

Fig.5. Planar type of cross-stratification associated with ripple-lamination and minor-scale structurepossibly due to animal burrowing followed by some fi ll ing and subsequent collapse of the abovestrata. T he drawing is oriented north-south (left to right side). The cross-strata dip northwa rd.

M E AS URE M E NT S AND T HE I R I NT E RP RE T AT I ON

T h r e e d i ff e r e n t lo c a l it ie s in t h e M a c i e l R i v e r a r e a h a v e b e e n s u r v e y e d , a n d 1 62 c r o s s -b e d d i n g o r i e n t a ti o n m e a s u r e m e n t s t ak e n . T h e s e s h o w a n a v e r a g e d ip d i re c t io n o fN 6 E , w h i c h is c l o s e ly p a r a l l e l t o t h e o r i e n t a t i o n o f th e s a n d r i d g e s . T h e a v e r a g e d ipd i r e c t io n ( 0) a n d t h e c o n s i s t e n c y r a t i o ( c .r .) w e r e o b t a i n e d b y t h e f o l lo w i n g f o r m u l a s :

Z s i n 0t a n 0 . . . .E c o s 0s i n 0 ) 2 o ) 2

c . r . ~ . . . . . . . . . . . . . . . . . . . .n u m b e r o f re a d in g sT h e r e s u l t s a r e i l l u s t r a t e d i n T a b l e I a n d i n F i g . 1 .

T h e t h re e l o c al it ie s i n t h e M a c i e l R i v e r h a v e v e r y s i m i la r a v e r a g e d i p d i r e c t io n sw i t h 9 d e v i a ti o n , a s w e l l a s f a i rl y u n i f o r m h i g h - a n g l e d i p s o n a l l s tr a t a . A l t h o u g ht h e h i g h - a n g l e d i p o f t h e s e s t r a t a is s im i l a r t o t h o s e o f t h e s h o r e w a r d s id e o f l o n g s h o r eb a r s , t h e s e s t r a t a d i f fe r f r o m t h o s e o f t h e b a r s b y 9 0 o r i e n t a t i o n i n t h e d i p d i r e c ti o n .T h e s t r u c tu r e s h e r e d i s c u s s e d s e e m t o h a v e b e e n p r o d u c e d i n t r o u g h s p a r a l l e l to t h e

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S A N D - R I D G E S T R U C T U R E S F R O M P A R A N / ~ C O A S T A L P L A I N

T A B L E IC R O S S -B E D D IN G M E A S U R E M E N T S I N T H E M A C I E L R IV E R S A N D - R I D G E T E R R A CE S ( P A R A N A G U ~ , B R A Z I L )

277

Local it y Number f Average dip Consi s tency M axim um Averagemeasurem ents direct ion rat io dip dipA 6 0 N 3 E 0 . 6 0 2 7 1 6 . 6 B 5 3 N 1 2 E 0 . 7 9 2 9 2 1 . 0 C 4 9 N 9 E 0 . 6 2 2 8 1 8 . 8

T o t a l 1 6 2 N 6 E 0 . 8 3 2 9 1 8 . 7

coast situated between the shoreline and the longshore bar. The argument for thisinterpretation is the cross-bedding dip-direction vector, which is not in agreement withthe known structures either of longshore bars or of beaches. In the same way, otherstructures in the sand-ridge terraces, mainly the sub-horizontal sets of parallel strata,do not seem to be characteristic of the beach but were formed either outside of thelongshore bars, as offshore deposits, or in the troughs between the longshore barsand the shoreline. In the first case, the nature o f bottom-sediment structures is knownto some extent, but in the latter case very little is known.

C O N C L U S I O N S

The beach ridges show:~a) Parallel stratification dipping slightly seaward t 1-3), almost at right angles

to the strike of the ridges. These structures might have been formed in the shorefaceenvironment (nearshore) with sand introduced by longshore currents. They seem tobe characteristic of these ridges, and the slight dipping is very significant. They arealso present in the lime-cemented "beach rocks" ("arrecifes") of northeastern Brazil.

The "beach rock" is exposed at low tide and emerges from the sea as a straightline separating a lagoon shoreward. The "beach rock" structure is mostly composedof parallel strata. Only in a few places are typical nearshore cross-strata or beachstructures present. The term "beach rock" is not properly applied to these sedimentssince they were not deposited in a beach environment, but rather in an offshoreenvironment.

The rectilinear distribution pattern of "beach rock" actually represents the strikeof the dipping set o f parallel strata against the horizontal plane of the sea. Shorewardbehind this line, the beaches are disposed in large archs.

(b) Cross-stratification is mostly of the planar type and less frequently of thetrough type. The average dip direction (N 6 E) of cross-strata do not differ greatlyfrom the average strike o f the beach ridges (N 35 E). These structures might havebeen formed shoreward or seaward from a bar or even offshore (?) by longshorecurrents.

M a r i ne Ge oL, 3 (1 9 6 5 ) 2 6 9 - 2 7 8


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278 J . J . B tGARE LL AT h e c r o s s - s t r a t a a t t i t u d e , i n w h i c h t h e a v e r a g e d i p d i r e c t i o n r o u g h l y f o l l o w s t h e

a l i g n m e n t o f b e a c h r i d g e s a n d o f t h e p r e s e n t s h o r e l i n e , is d i f fe r e n t f r o m t h e b a r s t r u c -t u r e s d e s c r i b e d b y M C K E E a n d S TER RET T ( 19 6 1) . T h e s t r a t a i n o t h e r l o n g s h o r e b a r sd i p i n t w o o p p o s i t e d i r e c t i o n s : s h o r e w a r d ( 1 6 - 2 0 a n d s e a w a r d ( 4 - 5 ) .( c) T h e s t r u c t u r e s s h o w t h e e f fe c t i v e ne s s o f c u r r e n t s i n t r a n s p o r t i n g s e d i m e n t a r ym a t e r i a l p a r a l le l to t h e c o a s t a n d t h e y a l l o w d e t e r m i n a t i o n o f t h e t r e n d o f d r if t .

ACKNOWLEDGEMENT

T h e a u t h o r i s g r a t e f u l t o E d w i n D . M c K e e f o r h is k i n d ne s s in r e v i s in g a n d i m p r o v i n gt h e m a n u s c r i p t .

REFERENCESBIGARELLA,J. J . , SALAMUNI,R . e MARQUES,F . P . L . , 1961. M 6todo p ara ava l iac~o do n ive l oce~n ico6poca da fo rm ac~o do s t e r r aqos de cons trugfio m ar inha . Bol. Paranaense Geografia, 1961,(4/5) : 111-115.BIGARELLA,J. J . e SANCHES, J. , 1965 . Con t r ibu ig f io ao es tudo dos s ed im en tos p ra ia i s r ecen tes. V .P ra ia suspensa do Saco da T am baru taca , m unic ip io de ParanaguA. In p res s .BIGARELLA, ]'. J. , FREIR E,S., SALAMUNI,R. e VIANA,R. , 1965 . Con t r ibu ig fio ao e s tudo dos s ed im en tospra ia i s r ecen tes . I I . P ra ias de M at inho Ca iob t i Paranfi . In p res s .CURRAV, J . R . a nd MOORE, D. C. , 1964. Ho loce ne regress ive l i t to ral sand, C os ta de Na yar i t , M exico.

In : L . M . J. U . VAN STRAATEN Ed ito r), Deltaic and Shallow Marine Deposi ts . E lsev ie r, Am -s te rdam , pp . 76-82 .JOHNSON, D . W ., 1919. Shore Processes and Shore-Line Development. Wiley , New Y ork , N . Y . , 584 pp .McKE E , E . D . and ST FaU~T r, T . S . , 1961 . L ab ora to ry exper im en ts on fo rm and s t ruc tu re o f longsho rebar s and beache s . In : J . A . I~TERSON and J . C. OSMOND (Editors ) , Geometry of SandstoneBodies. Am . Assoc . Pe t ro l . G eo log is t s , T u lsa , Ok la . , pp . l 3 -28 .McKE E , E . D . and W~IR, G . W . , 1953 . T erm ino logy fo r s t r a t i f ica t ion and c ros s - s t ra t i f i ca t ion n s ed i -m e n t a r y r o c k s . Bull. G eol . Soc. Am ., 64 : 381-390.TRASK, P. D ., 1932. Origin and Environment o f Source Sediments o f Petroleum. G u l f P u b l i s h in g C o . ,Hou s ton , T exas , 323 pp .

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Science Bigarella 1964

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