1990 Ainsworth Etal 1990

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    O s t r a c o d c o l o u r c h a n g e b y t h e r m a l a l te r a t io no f fs h o r e Ir e la n d a n d W e s t e r n U KNigel R . A ins w or th and Richard D. Burn et tD e p a r t m e n t o f G e o l o g y T r i n i t y C o l l eg e D u b l i n 2 I r e la n dan d Me rv in Ko n t ro v i t zD e p a r t m e n t o f G e o s c i e n c e s N o r t h e a s t L o u i s i a n a U n i v e r s i t y M o n r o e L A 7 1 2 0 9- 0 55 0U S AReceived27 December 989Ost racod co lou r changes f rom wh i te to amb er to red -b row n to b lack a re p r inc ipa l ly in te rp re tedto be the resu l t o f therma l a l te ra t ion . The changes in co lour a re thought to be caused byp ro g re ss ive ca rb o n f i xa t io n o f h yd ro ca rb o n s d e r ive d f ro m th e su r ro u n d in g se d ime n ts w h ichinvaded the ost racod she l l . Excep t ions to th is scheme inc lude faunas f rom organ ica l ly poorl ime s to n e s w h ich re ma in w h i t e i r re sp e c t ive o f t h e a mo u n t o f h e a tin g a n d g re y - b ro w n o rye l lo w -b ro w n sp e c ime n s co lo u re d b y t h e o x id a t io n o f d i f f u se p y r i t e . Th i r t y - f o u r e xp lo ra t io nwe l ls have been exam ined f rom the I r ish and Weste rn UK secto rs and these have show n aprogress ive darken ing o f co lours wh ich is co r re la ted w i th bo th bur ia l dep th and d is tance f romigneous in t rus ions. The Fastne t Bas in and sou thern reg ion o f the Nor th Ce l tic Sea exh ib i t h igherco lour chan ge/dep th ra t ios to the Porcup ine Bas in and n or thern reg ion o f the Nor th Ce l tic SeaBasin im p ly ing g rea te r pa laeoheat f low in the fo rm er bas ins.Keywords ostracod; ther ma l a l terat ion; co lour

    n t r o d u c t i o nD u r i n g b i o s t r a t ig r a p h i c a l a n a l y s is o f t h e m i c r o f a u n a sf r o m t h e I r i s h O f f s h o r e , i t w a s n o t i c e d t h a t t h ec a l c a r e o u s mi c r o f o s s i i s ( o s t r a c o d s , f o r a mi n i f e r a ,mi c r o b i v a l v e s , m i c r o g a s t r o p o d s ) d i s p l a y e d g r a d u a l t or a p i d c h a n g e s i n c o l o u r ( A i n s w o r t h , 1 9 8 6 , 1 9 8 7 ) . Fo ro s t r a c o d c a r a p a c e s , t h e s e c o mp r i s e a p r o g r e s s i v ed o w n h o l e s e q u e n c e f r o m c o l o u r l e s s / w h i t e , t h r o u g ha m b e r t o r e d - b r o w n t o b l a c k . W h e r e t h e s a m p l ei n t e rv a l s a r e c l o s e t o i g n e o u s i n t r u s io n s , c o l o u r s r a p i d l yd a r k e n t o w a r d s t h e h e a t s o u r c e a n d l i g h te n a w a y f r o mi t . T h u s c o l o u r c h a n g e s a r e i n t e r p r e t e d a s t h e r e s u l t o ft h e r ma l a l t e r a t i o n t h r o u g h b u r i a l / o r g a n i c ma t u r a t i o n o rin t rus ive hea t inpu t .A w i d e r a n g e o f f os s il i n v e r t e b r a t e s ( c o n o d o n t s ,g r a p t o l i t e s , i t h y o l i t h s , p h o s p h a t i c b r a c h i o p o d s ,s c o l e c o d o n t s ) a r e k n o w n t o s h o w o p t i c a l e v i d e n c e o ft h e r ma l ma t u r a t i o n , e i t h e r b y c o l o u r o r r e f l e c ti v i tyc h a n g e s , a n d i t i s p r o p o s e d t h a t o s t r a c o d s b e a d d e d t oth i s l is t. The use o f c rus ta cea ns as poss ib le sourc es o fc o i o u r a l t e r a t io n d a t a h a s p r e v i o u s l y b e e n s u g g e s t e d b yD o m i n g ( 1 9 8 6 ). O s t r a c o d c o l o u r c h a n g e s , h o w e v e r ,a r e e f f e c t e d t h r o u g h me c h a n i s ms d i f f e r e n t t o t h o s e i nt h e o t h e r me n t i o n e d f o s s i l g r o u p s .T h e f o l l o w i n g a c c o u n t i s b a s e d o n t h e e x a m i n a t i o n o fo v e r 1 80 0 p r o d u c t i v e s a mp l e s a n d 2 3 0 0 0 i n d iv i d u a ls p e c i me n s f r o m 3 4 e x p l o r a t i o n w e l l s . T h e o s t r a c o dc o l o u r s s o f o u n d a r e a p p l i e d i n a p r e l i m i n a r y a p p r a i s a lo f o r g a n i c ma t u r i t y i n t h e I r is h a n d U K o f f s h o r e .

    Present address: Paleo Services, Unit 15, Paramount IndustrialEstate, Sandown Road, Watford WD2 4XA, UK0 2 6 4 8 1 7 2 / 9 0 / 0 3 0 2 8 8 1 0

    1 9 9 0 B u t t e r w o r t h H e i n e m a n n L t d

    O s t r a c o d c o l o r a t i o nT h e g r a d u a l d a r k e n i n g o f t h e f o s s i l o s t r a c o d s h e l l i si n t e r p r e t e d a s a r e s p o n s e t o h e a t i n g . T o s i mp l i f y t h ed e s c r i p t i o n , t h e c o l o u r r a n g e h a s b e e n d i v i d e d i n t os e v e n a l t e r a t i o n c o l o u r s o r l e v e l s P l a t e 1 ) . A l t h o u g hmo s t o f t h e o s t r a c o d i n d i v i d u a l s e x a mi n e d c a n b ep l a c e d w i t h i n t h i s s e q u e n c e , a s i g n i f i c a n t p r o p o r t i o ns h o w a n a n o m a l o u s d u l l g r e y - b r o w n c o l o u r o f v a r y in gin tens i ty (c f . I thyo l i ths ; Tway e t a l . , 1986). Thesei n d i v i d u a l s a r e c o n s i d e r e d t o b e p y r i t i z e d ( s e e b e l o w ) .T h e c o l o u r s e q u e n c eThe seven ind ices i l lu s t ra t ed in P l a t e 1 c o v e r t h e r a n g eo f o s t r a c o d c o l o u r s a t t r i b u t e d t o th e r m a l a l t e r a t i o n a sd e s c r i b e d b y t h e Mu n s e l l Sy s t e m ( Mu n s e l l Co l o u r ,1975):(1 ) Co lour l e ss /wh i t e (5 YR S/1 to 10 Y R 8 /1 ). M os tRe c e n t o s t r a c o d t a x a p o s s e s s a w h i t e t o c o l o u r l e s sv a l v e / c a r a p a c e . So me t a x a S e m i c y t h e r u r an i g r e s c e n s , b l a c k ; C y p r i d o p s i s v i d u a , g r e e n )p o s s e s s a c o l o u r e d s h e l l , b u t t h e s e a r e i n t h emi n o r i t y . U n a l t e r e d M e s o z o i c a n d T e r t i a r ye x a mp l e s a r e g e n e r a l l y w h i t e .( 2 ) V e r y p a l e y e l l o w - o r a n g e ( 2 .5 Y 8 /4 t o 1 0 Y R 8 /4 ).( 3 ) A mb e r t o b r o w n i s h y e l l o w ( 1 0 Y R 6 / 6 ) .( 4 ) D a r k y e l l o w i s h b r o w n ( 1 0 Y R 4 / 6 ) .( 5 ) D a r k y e l l o w i s h b r o w n ( 1 0 Y R 4 / 6 ) t o d a r k r e d d i s hb r o w n ( 5 Y R 3 / 4 ) .( 6 ) V e r y d a r k g r e y - b r o w n ( 2 .5 Y 3 /2 ) t o b la c k (1 0 Y R2.1).(7 ) Black (7 .5 YR N 2 /0 ) to b lack (2 .5 Y N 2 /0 ) wi thr o u g h e n e d s u r f a c e s ( c o r r o s i o n ) .

    O v e r a l l , t h e r e i s a g e n e r a l d e c r e a s e i n v a l u e a n dc h r o m a d o w n t h e s e r i e s ( K o n t r o v i t z e t a l . , 1988).

    2 8 8 M a r i n e a n d P e t r o l e u m G e o l o g y 1 9 90 V o l 7 A u g u s t

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    O s t r a c o d c o l o u r c h a n g e s . IV . Ft. A i n s w o r t h e t a l .a b l e 1 C o m p l e t e d d a t a s h e e t f o r d o w n h o l e e v a l u a t io n o fo s t r a c o d c o l o u r s G u l f 5 6 / 1 8 -1 ; s e l e c t e d le v e l s o n l y )D e p t h P y r i t i z e d / O s t r a c o d a l t e r a t i o n l e v e l M e a n N )m ) G r e y - b r o w n 1 2 3 4 5 6 71 7 6 8 - 6 1 2 7 )1 8 1 3 2 6 2 2 8 )1 9 0 8 3 2 5 2 . 5 7 )1 9 6 9 5 2 2 2 1 3 3 )2 1 7 0 2 1 9 5 3 . 5 2 4 )2 2 0 1 5 8 3 8 4 4 6 )2 2 7 4 1 0 1 5 5 4 . 5 2 0 )2 4 4 1 - 1 1 5 .5 2 )3 0 0 5 - 1 1 7 . 5 2 )3 1 5 5 1 1 1 6 . 5 2 )3 2 4 3 - 5 1 5 6 )3 5 9 3 1 3 9 2 5 1 4 )

    A l t h o u g h w i t h i n a n y g i v e n s a m p l e t h e c o l o u r r a n g em a y va r y s l i gh t l y , t he a l t e r a t i on i nde x o f a n i nd iv idua ls a m p l e is b a s e d o n a m e a n v a l u e . A n e x a m p l e o f a d a t aa r r a y i s shown in T a b l e 1 .

    r ig in o f co lou r var ia t ionT h e i n o r g a n i c c h e m i c a l i n p u t t o t h e c o l o u r i n d e x P l a t e1) i s no t t hough t t o be s i gn i f i c a n t ; t he c ha nge s i no s t r a c o d c o l o u r a p p e a r t o b e l i n k e d to g r a d u a l c h a n g e si n o r g a n i c m a t u r a t i o n a n d h e a t i n p u t .Or ga n i c m a te r i a l i s i r r e gu l a r ly d i s t r i bu t e d w i th in t heost racod va lves /carapaces (Wil lga l l i s , 1969) . I t can bep r e s e n t a s o r g a n i c m e m b r a n e s e n c l o s i n g c o n s t i t u e n tc r ys t a l un i t s o f t he c a r a pa c e , a s d i sc on t inuous she e t sw i th in t he c a l c if i e d she l ls ( Jo r ge n se n , 1970) a nd i n we l lp r e s e r v e d f o r m s a s a p r o t e i n a c e o u s e p i - a n de ndo c u t i c l e ( Ba t e a nd E a s t , 1972). T h e de c a l c i fi c a t iono f t h e o s t r a c o d s h e l l l e a v e s a c o h e s i v e s p o n g e y m a t r i xof c h i t in f i b ri ls ( L a n ge r , 1973) in R e c e n t a nd /o r we l lp r e s e r v e d e x a m p l e s ( S o h n , 1 9 5 8 ; J o r g e n s e n , 1 97 0) ,t yp i c a l ly t he r e m a ins o f a c h i t i n - in f i ll e d por e c a na lsys t e m ( Ba te a n d E a s t , 1972). I n o lde r fos s il f o r m s th i sr e s idue i s a f o r m le ss non- c o he s ive m a te r i a l ( Sohn ,1958).A l t h o u g h c h e m i c a l l y o r g a n i c s t r u c tu r e s m a y t o t a l1 5 - 4 5 o f c r u s t a c e a n s k e l e t o n s , t h is f ig u r e m a y b e asl o w a s 2 - 1 5 f o r o s t r a c o d s ( W e i n e r e t a l . , 1983).Ne a r ly a ll o f t h is m a te r i a l i s c h i t i nous , a l t ho ugh a sm a l lp r o p o r t i o n i s p r o t e i n a c e o u s ( W e i n e r e t a l . , 1983;M c Co y, 1988) . A l a r ge pe r c e n t a ge o f t he o r ga n i cm a t e r i a l i s t h o u g h t t o b e l o s t d u r i n g b i o d e g r a d a t i o n( S c h i m m e l m a n n e t a l . , 1988) a nd , a l t hough th i s m a yle a ve a sm a l l pe r c e n t a ge o f ke r oge nous m a te r i a l , i t i sd i f f i c u l t t o e nv i sa ge how suc h a sm a l l a nd va r i a b l ec o m p o n e n t c o u l d p r o d u c e t h e r a n g e o f r e c o g n i z e dc o l o u r v a r i a ti o n s s h o w n i n P l a t e 1 . T hi s is sup por t e d byt h e r e s u l ts o f e x p e r i m e n t a l w o r k f r o m h e a t i n g t e s ts w i t ho p e n - a i r a n d e n c l o s e d v e s se l s a t e l e v a t e d p r e s s u r e s o nR e c e n t w h i t e a n d J u r a s s i c a m b e r o s t r a c o d s . W h e nh e a t e d t h e o s t r a c o d s p e c i m e n s s t a y e d o r t u r n e d w h i t er a t h e r t h a n d a r k b r o w n o r b l a c k .I n s t e a d , o s t r a c o d c o l o u r c h a n g e s a r e t h o u g h t t or e f l e c t o r g a n i c i n p u t f r o m t h e s u r r o u n d i n g r o c k s .M o b i l e h y d r o c a r b o n s a r e e x p e l l e d f r o m t h e h o s ts e d i m e n t s a n d p e r h a p s f r o m s e d i m e n t i n f i l l i n g t h ec a r a pa c e , i nva d ing t he os t r a c od s t r uc tu r e l e f tp e r m e a b l e b y t h e v a r i a b l e r e m o v a l o f t h e c h i t i n o u s

    i n f r a s t r u c t u r e a n d o t h e r c o n s t i t u e n t s . E x p e r i m e n t a ls t u d i e s o n R e c e n t t r a n s l u c e n t / w h i t e s p e c i m e n s s h o wt h a t w h e n t h e y a r e h e a t e d w i t h i n c r u d e o i l , t h eind iv idua l va lve s / c a r a pa c e s be c om e pr ogr e s s ive lyd a r k e r u n t i l a p p r o x i m a t e l y 3 7 5 C , w h e r e u p o n t h e ybe g in t o l i gh t e n . T he c o lour s d i sp l a ye d a r e no t j us tc a use d by t he supe r f i c i a l c oa t i ngs o f t he a dhe r e n to r ga n i c m a te r i a l , bu t a r e pe r va s ive a nd d i r e c t l yc om pa r a b l e t o t hose i l l us t ra t e d in Pla te 1 . Addi t i ona l l y ,o p e n - a i r h e a t i n g o f a m b e r , r e d - b r o w n a n d b l a c kind iv idua l s p r oduc e s spe c im e ns o f a un i f o r m whi t ea p p e a r a n c e ; a t 3 75 C a n d a b o v e t h e y b e c o m e l i g h te r ;a m b e r f o r m s , i n w h i c h t h e c o l o u r i s p a rt i cu l a r ly d e e pa r o u n d t h e m a r g i n s o f t h e v a l v e s , d e v e l o p a d a r kb r o w n - b l a c k c a r b o n f i l m a l o n g t h a t a r e a o n r a p i dhe a t i ng .I f t he os t r a c od c o lour i s i nhe r i t e d f r om thes u r r o u n d i n g h o s t r o c k s , i t m i g h t b e e x p e c t e d t h a t t h ec o lour se que nc e i s c on t r o l l e d d i r e c t l y by t he o r ga n i cc o n t e n t o f t h e l a t t e r r a t h e r t h a n d i r e c t l y b y t h e r m a le f f e c t s , t he o r ga n i c c on t e n t be ing a m a jor c on t r o l o fs e d i m e n t c o l o r a t io n ( H o s t e r m a n a n d W h i tl o w , 1 98 1).T h i s i s c e r t a in ly t r ue f o r spe c im e ns e x t r a c t e d f r omwhi t e o r ve r y pa l e c a l c a r e ous l i t ho log i e s , pa r t i c u l a r lyc ha lks , m ic r i t i c , oo ida l a nd c oa r se ly c r ys t a l l i nel im e s tone s , a l l o f whic h posse s s a ve r y l ow or ga n i cc o n t e n t . H o w e v e r , a b o v e a t o ta l o r g a n ic c a rb o n ( T O C )c o n t e n t o f 0 . 2 - 0 . 5 , a n d e v e n g i ve n re l a ti v e ly l o wl e ve ls o f g e n e r a t e d h y d r o c a r b o n s ( P y r o l y s is d a t a f r o mge oc h e m ic a l s t ud i e s ) , t he r e se e m s to be l i tt le c on t r o l ono s t r a c o d c o l o u r b y t h e o r g a n i c c o n d i t i o n o f t h es e d i m e n t . T h e o s t r a c o d c o l o u r a l t e r a t i o n s e q u e n c eP l a t e 1 ) a ppa r e n t ly owe s i t s o r ig in t o t he e a syi m p r e g n a t i o n o f t h e o p e n o s t r a c o d s t r u c t u r e b y t h ee a r l i e s t p h a s e s o f t h e r m o g e n i c l i q u i d h y d r o c a r b o n s .T h e l a t te r a r e p o s t u l a t e d t o b e g r a d u a l ly c o n v e r t e d t of ixed c a r bon w i th i nc r e a se d he a t i ng . I nd ige n ouso r g a n i c m a t e r ia l m a y a l s o c o n t r i b u t e t o c o l o u r c h a n g e ,by i t s g r a dua l c a r bon iz a t i on o r e ve n by c onve r s ion t o

    7 M EAN = 2 .2 5 . (n) = .2 . . . . . . . . . . . . . . . . . . . .M E A N = 1 6 7 ;n) = 4

    MEA N = 1 .18;( 5 . 5 ) ( n ) = 7 ~ 1M E A N = 1 .0 3 ; ~15 (n) = 6 m 0 _-MEA N = 86; mBm BBI 5 :( 4 . 8 ) n) = 7

    M EAN = 6 8 ; BBL m FR EQU EN C Y(n) ~ 7( 3 . 5 ) M E A N = . 58 ; _ ~ L ( n ) = 5 D A T A : O S T R A C O D C O L O U R V S V I T R I N I T E R E F L E C T A N C EM E A N = 4 9 ; R A N G E S { O F F S H O R E W E L L S , I R E L A N D A N D U . K . )

    3 EAC H QU OTED R EFL EC TAN C E VAL U E OR R AN GE IS FR OMA DIFFEREN T WELLO S T R A C O D V I T R IN I T E R E F L E C T A N C EC O L O U R V A L U E S / R A N G E SM E A N = 4 5~ b l ~ 6 1 .32~ 3 2, 2 0- 24 , < .3 5, 31 .3 2, 2 5. 27 , 3 61 5 .28, .32, .24, .27, .36(2. 5) 2 .35-.41, .47-.60, .30..34, .40, 30, .32..45, .43.47. 25.5 0,.40, .35-A3, .37-.39, .34-40, 27- 34, 27.32 3 7 -4 0 ,38-40, .37-.49, 40-412,5 ,41=60, .41-.47, 30-6 0, 37-50, 40 43-55, 45-.48,.45-.53, .25-65, .34- 48, .38-.46, 39-.57, 39-.41, .34-,51m l l l k ~ E = . ; 4 4 4 7 1 4 ~ ~ ~ 5 02 .40 3 48-.56, .40, 55.6 0, .45-.48, 51-5 4, .60- 75, .41.39 -55.32-.53, .45..53-66, .43=52, 34-.5133 60, 33=57, .50-.60, .66, 52-.574 .70, .55-.60, ,56- 63, 53-1 2, .52-814.5 .75..76-,1 3 , ,63, 35, .B6, 36=81 ,1.7

    _ . _ _ MEAN=O 8 8 0 , 6 4 - 7 0 , . 4 6 - 6 0 , 5 1 - 1 2 , 1 5 , 16{1 m ~ l i b 53 1 .0-1 .2 , 1 .0-13, .60-.24, 1 .3 2 .0 , 61-12, 17 - 2 1 , 2 0(n) =5 6 1.4, 1,08-1.99, 1.3-20 1.9-2.3M EAN =( .3 ; 6 .5 2 .31 m m S~ (n )= 7 7 1 .6,2 734 ' . . . . . . .5 .6 .7 .6 .~ 1 ,0 1 .1 1 .2 1 .3 14 1 .5 116 1 .[7 1 . 6 1 .9 210 2 . 1 2 2

    F i g u r e 1 C o r r e l a t io n b e t w e e n o s t r a c o d c o l o u r a n d v i t r i n i ter e f l e c t a n c e d a t a . H o r i z o n t a l d o t t e d l i n e s i n d i c a t e m e d i a no s t r a c o d c o l o u r a n d s p e c i f i c v i t r i n i t e r e f l e c t a n c e e q u i v a l e n t

    2 9 0 M a r i n e a n d P e t r o l e u m G e o l o g y , 1 9 90 , V o l 7 , A u g u s t

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    Ostracod colour changes: N R Ainsworth et a l .hyd roca rbons Thom pson and C rea th , 1966 ), bu t suche f f e c t s a r e p r o b a b l y s ma l l . D o w n h o l e , t h e r e i s ag e n e r a l d a r k e n i n g i n s e d i me n t c o l o u r f r o m w h i t e t ob u f f C r e t a c e o u s c h a lk s ) , g r e y t o g r e y - g r e e nG a u l t - G r e e n s a n d s ) , g re y s M i d d l e - U p p e r J u ra s si c)a n d l i g h t - d a r k g r e y s a n d b l a c k s L o w e r J u r a s s ic ) in ar e s p o n s e t o a ra n g e o f f a c t o rs c f . H o s t e r m a n a n dW h i t l o w , 1 9 8 1 ) . H o w e v e r , t h i s i s n o t t h o u g h t t o h a v eh a d a d i r e c t p r e - ma t u r a t i o n c o n t r o l o n o s t r a c o d c o l o u ra s a n y c o l o u r - i n d u c i n g o r g a n i c p h a s e w i t h i n t h es e d i m e n t w o u l d h a v e t o h a v e b e e n m o b i l e to c o l o u r t h eo s t r a c o d s . T h e r e s p o n s e s o f o s t r a c o d c o l o u r s t oi n t r u s i o n s d o n o t e q u a t e w i t h s e d i me n t c o l o u r s , i na d d i t i o n t o w h i c h t h e d o w n h o l e d a r k e n i n g o f o s t r a c o d si n ma n y i n s t a n c e s d o e s n o t c o i n c i d e w i t h c h a n g e s a n df l u c t u a t i o n s i n s e d i me n t c o l o u r s .C o r r e l a t i o n o f o s t r a c o d c o l o u r w i t h v i t r in i ter e f l e c t a n c eBy c o mp a r i n g o s t r a c o d c o l o u r s w i t h e x t e n s i v e s u i t e s o f

    l a t e 3 S cann i ng e l ec t ron m i c rog raph s o f a l te red os t racods . 1 ) A l te ra t i on l eve l 3 , ca rapace . 2 ) A l te ra t i on l eve l 3 , ca rapace su r face , 3 )A l te ra t ion leve l 6 , carapace. 4 ) A l te ra t io n leve l 6 , carapace. 5 ) A l te ra t ion leve l 7 , carapace. 6 ) A l te ra t ion leve l 7 , carapace sur face. 7 )A n om a l ou s g re y - b r ow n os t racod , i n f i l l i ng ma te r i a l . 8 ) S ec t i oned os t racod ; shee t - li ke ca l c ite s truc tu re o f ca rapace enc l os i ng i n f i ll o fm i c roc rys ta l l ine g ran u l a r ca lc i te . 9 ) E D A X F e , S ) i mage o f 7 , sugges t i ng the p resence o f bo th d i sc re te and d i f fuse py r i te

    M a r i n e a nd P e t r o l e u m G e o l o g y , 1 99 0, V o l 7 , A u g u s t 2 91

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    O s t r a c o d c o ~ ou r c h a n g e s : N . R. A i n s w o r t h e t a l .v i t r i n i t e r e f l e c t a nc e da t a ( we l l c om ple t i on r e por t s f o r1 5 o f f s h o r e w e ll s ), a b r o a d c o r r e l a t io n b e t w e e n t h e t w oc a n b e m a d e Figure 1) . Ha l f un i t s a r e a l so shown,c a l c u l a t e d f r o m t h e s y s t e m o u t l i n e d i n Table 1,a l t h o u g h t h e y c a n n o t b e r e a d i l y d i s c e r n e d i n p r a c t i c ea n d a r e o n l y i n c l u d e d h e r e t o f a c il i ta t e c o r re l a t io n . T h ea c t u a l d a t a o n w h i c h Fi gur e 1 i s ba se d show ac o m p a r a t i v e ly w i d e r a n g e , i m p l y i n g o n l y a l i m i t e dva lue f o r t h i s c o r r e l a t i on Figure 2) . Som e a t l e a s t o ft he po or d e f in i t i on i s p r ob a b ly d ue t o t h e va r i a t i ons i nt h e q u a l i t y o f t h e r e f l e c ta n c e d a ta . I n t e r - o p e r a t o rva r i a t i ons i n t he ge n e r a t i on o f v i t r i n it e r e f l e c t a nc ev a l u e s a r e n o t o r i o u s ( D e m b i c k i , 1 98 4) a n d t h e w o r k o fs ix c onsu l t a nc i e s i s r e pr e se n t e d i n Figure 2 . S o m e o f th ed i f f e re n c e s e n c o u n t e r e d b e t w e e n o s t r a c o d a n d v i t r in i tes a m p l e s m a y b e d u e t o m o d i f i c a t i o n s o f c o l o u r b yinor ga n i c i n f lue nc e s , pa r t i c u l a r ly pyr i t i z a t i on , i na d d i t i o n t o t h e s e c o n d a r y r o l e o f o r g a n ic - p o o rc a l c a r e ous l i tho log i e s .

    X-RAY O - 20 keVL i v e : 1 s e c s P r e s e t : 1 s e c s R e m a i n i n g : s e c sReal: 115 secs 13 Dead

    r

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    0 j ~ ~ ~l ~o< .0 5.120 keV 10.2 >Ful l Scale = 2 K c h a n n e l 2 6 5 = 7 9 c o u n t s

    Figure 4 S E M / E D A X e le m e n t p r o f il e . A n o m a l o u s g r e y - b r o w nostracod. High Fe and S content implying pyrit iz t ion

    M o d i f i c a t i o n s o f c o l o u r b y p y r i t i z a t i o nS o m e o f t h e e x a m i n e d s p e c i m e n s p o s s e s s c o l o u r s n o td i r e c t l y c om pa r a b l e t o t hose i l l us t r a t e d in Plate 1. F o re x a m p l e , o s t ra c o d s r e c o v e r e d fr o m p a l e y e l l o w - b r o w nU p p e r J u r a ss i c l i m e s t o n e s h a v e i n h e r i t e d t h e i r c o l o u r sf r o m t h e p e r v a s i v e m i n e r a l p i g m e n t a t i o n ( F e , M n ,M g ? ) o f t h e i r h o s t l i th o l o g y Plate 2, Figure 2).M o r e c o m m o n a r e a n o m a l o u s v a r y i n g - in t e n s i ty g r e yb r o w n - b l a c k o s t r a c o d s , w h i c h o c c u r i n s a m p l e si r r e spe c t i ve o f t he i r o r ga n i c m a tur i t y . T h e se c o lour s a r ec a u s e d b y t h e p r e s e n c e o f p y r it e . O c c a s io n a l s p e c im e n sPlate 2 , Figures 7 -12 ) s h o w c o m p l e t e p y r i t iz a t io n w i thi ts a s soc i a t e d m e ta l l i c l us t r e , b u t m os t py r i t iz a t i on i s o fa d i f f use na tu r e Pl a t e 2 , F i gur es 13 -1 5 ) . I nc ide n t a l l y ,a n o m a l o u s l y d a r k c o l o u r s a r e p r o d u c e d b y t h eox ida t i on o f pyr i t e ( c f . C l a r k a n d L u tz , 1980). O e r t l i( 197 l ) t ho ugh t t ha t t h is l a t t e r p r oc e ss a c c ou n te d f o r t hec om ple t e r a nge o f f os s i l os t r a c od c o lour s . I n Plate 3,F i gur es 5 -9 , t he p r e se nc e o f d i f f use pyr i t e w i th in t hein f i l i o f a nom a lous ly c o lour e d os t r a c ods i s i l l us t r a t e d ;Figures 3 a n d 4 c o n t r a s t t h e e l e m e n t a l c o m p o s i t i o n o fa m b e r a n d a n o m a l o u s g r e y - b r o w n o s t r a c o d s .T he s ign i f i c a nc e o f d i f f use pyr i t i z a t i on a nd i t sda r k e n in g e f f e c t s a r e suc h t ha t f o r os t r a c ods a t o ra b o v e a l t e r a t i o n l e v e l 5 , t h e d a r k b r o w n s d u e t opyr i t i z a t i on c a nno t be r e a d i ly d i s t i ngu i she d f r om the

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    Figure 3 S E M / E D A X e l e m e n t profi le . Exter ior of carapace,amber a l te ra t ion l e ve l 3 os t racod . D om i nan t C a peak ,subs idia ry Mg, A I , S i , C I , K , Mn and Fe

    br owns o f o r ga n i c o r ig in . T h i s i s r e f l e c t e d i n t hec ons t i t ue n t s i n Table 1. Above a l t e r a t i on l e ve l 5 ,how e ve r , a d i f f e r e n t t e c hn ique c a n be use d , v i z. su r f a c et e x t u r e . A l t h o u g h f a c i e s - r e l a t e d , a n d t h e r e f o r es t r a t i g r a ph i c a l l y c on t r o l l e d , va r i a t i on i n pyr i t i z a t i onin t e ns i t y i s known f r om the Ju r a s s i c se c t i ons ( F i sc he ra n d H u d s o n , 1 98 7), t h e u n i f o r m r e s p o n s e o f o s t r a c o dc o l o u r s t o b u r i a l d e p t h a n d d i s t a n c e f r o m i g n e o u sin t r us ions i n t he we l l s s t ud i e d im pl i e s t ha t pyr i t i z a t ioni s no t t h e m a jor c on t r o l o f t he de f ine d os t r a c od c o lour .S u r f a c e t e x t u r e a l t e r a t i o n sI t ha s l ong be e n r e c ogn iz e d t ha t c a l c a r e ous m ic r of oss il se xh ib i t t e x tu r a l a l t e r a t i on t h r ough r e c r ys t a l l i z a t i on o rc or r os ion i n r e sponse t o d i a ge ne s i s a nd t he r m a l i npu t( Sc h l a nge r et al., 1 9 7 1 ; A d e l s e c k et al., 1973).Kont r ov i t z ( 1980 , 1987) a nd Kont r ov i t z et al. (1983)d i sc usse d suc h f e a tu r e s i n os t r a c ods , c onc lud ing t ha tve r y e x t e ns ive su r f ic i a l a l t e r a t i on o f os t r a c od c a r a pa c ef i r s t o c c u r r e d a t ( e x p e r i m e n t a l ) t e m p e r a t u r e s o f150- 180C, bu t c a n oc c ur ove r a r a nge up t o 360C.T hi s a l t e r a t i on i s i l l us t r a t e d i n b l a c k os t r a c ods(a l te ra t ion leve l 7) Plate 3, F igures 6 a n d 8). A t l o w e ra l t e r a t i on l e ve ls Plate 3 , Figures 1-4 ) , c o r r o s io n a n dr e c r ys t a l l i z a t i on a r e m uc h l e s s a ppa r e n t , a l t hougho s t r a c o d s r e c o v e r e d f r o m t h e U p p e r C r e t a c e o u sCh a lk a nd L o we r Ju r a s s i c L i a s s ic L im e s ton e i n t heI r ish o f f shor e ha ve a c ha lky du l l su r f a c e l us t r e Plate2, Figures 4 a n d 5). T hi s i s s im i l a r to t ha t de sc r ibe d byS c h l a n g e r et al. ( 197 l ) f o r t he hya l i ne f o r a m in i f e r aw h i c h h a v e u n d e r g o n e e x t e n s iv e d i a g e n e si s .

    O s t r a c o d c o l o r a t i o n i n t h e M e s o z o i c a n d T e r ti a r yo f f sh o r e r e l a n d a n d t h e W e s t e r n U KFr om the 34 we l l s s t ud i e d Figure 5), 27 c on ta ine dsuf f i c i e n t os t r a c od sa m ple s t o f o r m c o lour p r o f i l e s f o re a c h we l l . T h i s a l l owe d us t o i n i t i a te r e g iona l s t ud i e s o fo s t r a c o d - d e f i n e d p a l a e o g e o t h e r m a l g r a d i e n t s f o r t h eM e so z o ic a nd T e r t i a r y se c t ions i n the I ri sh a nd we s t e r nUK of f shor e . Som e of t he m or e c ha r a c t e r i s t i c p r o f i l e sare i l lus t ra ted in Fi gur es 6 -14 .P o r c u p i n e B a s i nT he BP 26 /28-1 we ll Figure 6) di sp l a ys a ve r y g r a dua linc r e a se i n os t r a c od c o lo ur dow nho le . T h i s i s t yp i c a l o f

    292 Mar ine and Pet ro leum Geo logy 1990 Vo l 7 Augu s t

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    6 4 / 1 : ' 13 / 5 - 1 /6 3 / 1 0 - 1 9 o

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    Figure 5 Geographical location of w ells investigated offshore Ireland and Western UK

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    V I T R I N I T E R E F L E C T A N C EF i g u re 6 0 s t r a c o d a l t e r a ti o n l e v e l an d v i t r in i t e r e f le c t a nc epro f i l es fo r the BP 26/28-1 we l l . (O) Ostracod ; ( ) V i t r in i te F i g ur e 7 0 s t r a c o d a l t e r a ti o n l e v el a n d v i t r in i t e r e f le c t an c ep ro f i l e s fo r the A rco 106/24 -1 w e l l . (O) Os t racod ; ( ) V i t r i n ite

    M a r i n e a n d P e t r o l e u m G e o l o g y 1 99 0 V o l 7 A u g u s t 2 9 3

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    Figure 80 st ra co d al terat ion level and vi t r in i te ref lectanceprof iles for the Gu l f 49/9-2 wel l . (0) Ostracod; (& ) V i t r in i te

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    V I T R I N I T E R E F L E C T A N C EF i g u r e 90s t r aco d a l te r a t i o n l eve l an d v i t r i n i te r e f l e c t anc eprofiles for the Arco 47/29-1 w ell. (0) Ostraco d; (&) Vitrinite

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    ~ ~ , ; ~ ~ ; ' 1 1 1 2 , '4 1 ' 5 1 ' 5V I T R IN I T E R E F L E C T A N C E 2 1 5Figure 10 Ostracod al terat ion level and v i t r in i te ref lectanceprof iles for the Total 57/2-1 we l l . (0) O stracod; (& ) V i t r in i te

    t h e r e g i o n ( a n d o f a d j a c e n t w e l l s i n c l u d i n g B P 2 6 / 2 8 - 2 ,B P 2 6 / 2 8 - 3 , B P 2 6 / 2 8 - 4 , B r i t o i l 3 5 / 1 9 - 1 a n d C h e v r o n3 6 / 1 6 - 1 ) . A t t h e b o u n d a r y b e t w e e n t h e J u r a ss ic a n d t h eC a r b o n i f e r o u s , a r a p i d i n c r e a s e i n a l t e r a t i o n l e v e lo c c u r s , f o r e x a m p l e i n t h e C h e v r o n 3 6 / 1 6 - 1 w e l l( o s t r a c o d c o l o u r 6 , v i t r i n i te r e f l e c t a n c e 2 . 5 R o a t2 6 0 0 m i n a D i n a n t i a n s e c t i o n ) . S u c h r e s u l t s s u g g e s t ah i g h p r e - M e s o z o i c m a t u r i t y l e v e l i n t h e C a r b o n i f e r o u so f th e P o r c u p i n e B a si n ( R o b e s o n e t a l . 1 9 8 8 ) . I n m a n yo f t h e w e l l s s i t u a t e d i n t h e P o r c u p i n e B a s i n ,C a r b o n i f e r o u s r e w o r k i n g i s p r e v a l e n t .W h e r e r e c o v e r e d , t h e r e w o r k e d C a r b o n i f e r o u so s t r a c o d f a u n a s p o s s e s s d a r k c o l o u r s ( 6 - 7 a l t e r a t i o nl e v e l s ) , w h e r e a s t h e s u r r o u n d i n g i n s i t u M e s o z o i cf a u n a s a r e a m b e r ( a l t e r a t i o n l e v e l 2 - 3 ) .C a r d ig a n B a y - N o r t h C e lt ic S e a - S o u t h C e lt ic S e aB a s i n sF r o m n o r t h e a s t t o s o u t h w e s t , a n i n c r e a s e in m a t u r a t i o ng r a d i e n t s c a n b e o b s e r v e d . I n t h e m o r e n o r t h e r l y wel l s ,f o r e x a m p l e Arco 106 /24-1 Figure 7) a n d M a r a t h o n50/3-1, v e r y g r a d u a l d e p t h - r e l a t e d i n c r e a s e s in o s t r a c o dc o l o u r a r e s e e n . S l i g h t l y m o r e r a p i d , d o w n h o l ei n c r e a s e s o c c u r i n t h e B P 5 0 / 1 2 - 2 A a n d G u l f 49/9-2w e l l s Figure 8) . The Arco 47 /29-1 w e l l Figure 9)in i t ia l ly s h o w s s l o w i n c r e a s e s , b u t r a p i d d a r k e n i n g o ft h e o s t r a c o d s p e c i m e n s o c c u r t o w a r d s t h e b a s e o f th ew e l l , i n f e r r i n g a p r o x i m a l h e a t s o u r c e i n t rus ion?) . A

    2 9 4 M a r i n e a n d P e t r o l e u m G e o l o g y , 1 9 9 0 , V o l 7 , A u g u s t

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  • 8/12/2019 1990 Ainsworth Etal 1990

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    l2 . 3 . 4 ~ . 6 . 7 , 8 . 9 1 2 2 1 51 , 2 1 , 4 1 . 6 1 . 8M I T R I N I T E R E F L E C T A N C EF i g u r e 1 1 O s t r a co d a l t e r a t i o n l e v e l a n d v i t r i n i t e r e f l ec t anc ep r o f i l e s f o r t h e G u l f 5 6 / 1 8 - 1 w e l l . ( 0 ) O s t r a c o d ; ( & ) V i tr i n it e

    very rapid increase in ostracod colour with depth isshown by the Total 57/2-1 well, with very darkostracods at the base of the well Figure 10) ; the sourceof heating at present is unknown. A similar rapidincrease occurs in the Esso 56/14-1 and Gulf 56/18-1wells Figure 11) . In the latter this is attributed toMesozoic basic intrusions in the mid- and lowersections. Below the main intrusion, the ostracodindividuals lighten in colour.F a s t n e t B a s i nIn the Fastnet Basin, all eleven wells have beenexamined. The Demin ex 56/21-1 well displays amoderate rate of downhole increase in ostracod colour.Greater increases occur in the adjacent Deminex56/21-1 well. Although no intrusion was encounteredduring drilling, sulphide mineralization towards thebase of the section implies thermal activity. In theTexaco 56/22-1 well a rapid darkening in colour occurswith depth, whereas in the Elf 55/30-1 well the increaseis very slow. In both the BP 56/26-1 and Elf 64/2-1wells, the steep profiles are largely due to intrusive sillsFigures 12 and 13). In the BP 56/26-2 well, anothersteep profile is observed above an intrusive sill,however, below which ostracod colours lighten F igure14). In all three wells, which contain thick igneousintrusions, both ostracod colour and vitrinitereflectance exhibit strong influences of the sills well

    Ostracod co our changes: N. R. Ainsworth e t a l1 0 0 0 - |

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    Figure 12 Ostracod a l t e r a t i o n l e v e l p r o f i l e s f o r t h e B P 56 /26 -1w e l l . ( 0 ) O s t r a c o , 4

    away from the contacts. Organic alteration in hostsediments typically occurs up to 2- 3 times the thicknessof the intrusion (Burgess et al . , 1977; Dennis et al. ,1982). In the Fastnet Basin and also within the NorthCeltic Sea, the extent of alteration is significantlygreater, up to 4-6 times the sill thickness. Such highvalues are not without precedent since Jones andCooper (1970) have recorded a contact organicalteration zone up to six times sill thickness.In the eastern region of the Fastnet Basin, moderatedarkening of the ostracod colour occurs in the CitiesService 63/4-1 and Occidental 64/1-1 wells. On the welllog of the latter, an olivine basalt 'intrusive sill' wasrecorded. However, the ostracod colours do not alterabove or below the 'sill'. Thin sections of the 'sill' havebeen analysed by the present authors. The igneous rockconsisted of a fine-grained serpentinised basalt, which

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    ,, 5 '~ . . . . . . ~ ' , ~ 6 ' 5, 5 3 3 . 5 4 4 . 5 5O S T R A C O D C O L O U RI I I I I I I I I I I3 . 4 . 5 . 6 . 7 8 . 9 1 1 . 2 1 1 4 1 . 6 1 . 8 2

    V I T R I N I T E R E F L E C T A N C EFigure 13 Ostracod a l te ra t ion leve l and v i t r in i te re flec tanceprof i les for the E l f 64 /2 -1 we l l . (0 ) Os tracod; ( ) V i t r in i te

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    Figure 4 Os trac od alteration level and vitrinite reflectanceprofiles for the BP 56/26-2 well. O) Ostracod ; A) V itrinitem a y r e p r e s e n t a n e x t r u s i v e b as a lt . O f th e t w or e ma i n i n g w e l l s , Ra n g e r 6 3/8 -1 s h o w s a v e r y sl o wi n c r e a s e i n d a r k e n i n g o f o s t r a c o d c o l o u r a n d t h e C i t i e sServ ice 63 /10-1 wel l exh ib i t s a modera te increase inc o l o u r w i t h d e p t h .A s u mma r y o f t h e a l t e r a t i o n l e v e l s f o r a s i n g l es t r a t ig r a p h i c a l i n t e r v a l i l l u st r a t e s t h a t o s t r a c o d c o l o u r i sl a r g e l y i n d e p e n d e n t o f l i t h o l o g y a n d i s t h e p r o d u c t o fs u b s e q u e n t t h e r m a l i n f l u e n c e s . T h e u p p e r m o s tH e t t a n g i a n - l o w e r m o s t S i n e m u r i a n i n t e r v a l i n t h eF a s t n e t B a s i n c o n t a i n s a n a b u n d a n t o s t r a c o d f a u n a ,w i t h u p to 2 0 0 + s p e c i m e n s p e r s a m p l e . T h e s e o s t r a c o d ss h o w c o n s i d e r a b l e l a te r a l v a r i a ti o n s i n c o l o u r b e t w e e nwel l s P l a t e 1 , T a b l e 2 ) , i n p a r t d u e t o l o c a l h e a t i n g b yi g n e o u s i n t r u s i o n s , b u t o v e r a l l t o h i g h e r t e mp e r a t u r e sa f f e c t i n g t h e i n t e r v a l i n t h e e a s t o f t h e b a s i n .InterpretationRe l a t i v e l y lo w g r a d i e n t s o f i n c r e a s i n g o s t r a c o d c o l o u ra r e s e e n i n t h e P o r c u p i n e B a s i n , C a r d i g a n B a y , a n d i nt h e n o r t h e a s t r e g i o n o f t h e N o r t h C e l t ic Se a Ba s i n . I nt h e s o u t h w e s t p a r t o f t h e l a t te r a n d m o r e e s p e c i a l l y i nt h e Fa s t n e t Ba s i n , g r a d i e n t s a r e t y p i c a l l y h i g h e r . I nab l e 2 0 s t r a c o d a l t e r a t io n l e v e ls i n t h e l o w e r m o s tS i n e m u r i a n - u p p e r m o s t H e t t a n g i a n o f t h e F a s t n e t B a s i n

    Exploration well Ostracod alteration levelB P 5 6 /2 6 - 2 6 - 7C i t ie s S e r v i c e 6 3 /4 - 1 2 - 2 . 5C i ti e s S e r v i c e 6 3 /1 0 -1 4 . 5 - 5D e m i n ex 56 /21 -1 4D e m i n e x 5 6 /2 1 -2 5 - 6El f 55/30-1 2.5E l f 62 /4 -1 3 . 5 - 5Oc c i de n t a l 64 /1 -1 2 . 5 -3 . 5R an ger 63 /8 -1 2 5T e x a c o 5 6/2 2 -1 3 . 5 - 5

    s o me c a s e s , t h e s e h i g h e r g r a d i e n t s c a n b e d i r e c t l ya t t r i b u t e d t o p r o x i ma l i g n e o u s i n t r u s i o n s ; h o w e v e r , t h eg e n e r a l c o n c l u s i o n i s h i g h e r p a l a e o h e a t f l u x i n t h es o u t h .T h e o r i g i n o f s u c h a f l u x p r o b a b l y r e l a t e d t o t h ea b u n d a n t Mi d d l e J u r a s s i c a n d E a r l y T e r t i a r y s i l l se n c o u n t e r e d i n t h e F a s t n e t B a s i n ( C a s t o n e t a l . , 1981).T h e s e i n t r u s io n s a r e a s s o c i a t e d w i t h A t l a n t i c r i f t in g a n dl a te r T h u l e a n m a g m a t i s m . A l t h o u g h a n u m b e r o fc o m p a r a b l e i n t r u s i o n s o c c u r i n t h e P o r c u p i n e B a s i n( T a t e a n d D o b s o n , 1 9 88 ), t h e y a p p e a r l es s w i d e s p r e a dt h a n i n t h e Fa s t n e t a r e a , a n d a p p e a r t o h a v e h a d l i t t l eo r n o i n f lu e n c e o n p a l a e o g e o t h e r m a l g r a d ie n t s. I t m a ya l s o b e t h a t t h e v o l u me t r i c a l l y l a r g e r a n d mo r ea b u n d a n t i n t r u s i o n s i n t h e Fa s t n e t Ba s i n a r es y m p t o m a t i c o f g r e a t e r a n d p e r h a p s m o r e r a p id c r u s ta lt h i n n i n g d u r i n g a p r e - s u b s i d e n c e e x t e n s i o n a l p h a s e i nt h e b a s i n s e v o l u t i o n . T h i s c o u l d h a v e e l e v a t e d t h e h e a tf l o w ( M c K e n z i e , 1 98 1) a b o v e t h a t o f t h e ma j o r p a r t o ft h e N o r t h Ce l t i c Se a .

    C o n c l u s i o n sO s t r a c o d c o l o u r i s s e e n a s a n i n d i c a t o r o f o r g a n i cm a t u r i ty . T h e m e c h a n i s m o f c o l o u r ch a n g e i s b ya l t e r a t i o n o f a n o r g a n i c p h a s e i n h e r i t e d f r o m e n c l o s i n gs e d i me n t s . A s a t o o l , o s t r a c o d c o l o u r c a n d e f i n e t h ee f f e c t s o f i g n e o u s i n t r u s io n s a n d o f b u r i a l d e p t h . T h ep r o b l e m a t i c n a t u r e o f re c o g n iz i n g c o l o u r s e q u e n c e s i nc e r t a i n l i t h o l o g i e s ( o r g a n i c a l l y p o o r l i me s t o n e s ) a n dt h e c o m p l i c a t i o n s o f p y r i t i z a ti o n c a u t i o n a g a i n s t t h e u s eo f o s t r a c o d s a s t h e o n l y i n d e x i n o r g a n i c m a t u r a t i o ns t u d i e s . H o w e v e r , i t i s e n v i s a g e d t h a t v a l u a b l es u p p l e m e n t a r y i n f o r m a t i o n c a n b e o b t a i n e d f r o mo s t r a c o d s . T h e t e c h n i q u e o u t l i n e d i s f a r c h e a p e r a n df a s t e r t h a n t h a t o f v i t r in i t e r e f l e c t a n c e , a n d t h e p r o c e s sc a n b e u s e d a t t h e w e l l s i t e f o r o r g a n i c ma t u r i t yscreen ing .Fu r t h e r s t u d i e s o n a l a r g e r n u m b e r o f t h e w e l l s i nn o r t h w e s t E u r o p e , i n c lu d i n g t h e Ir is h a n d U K o f f s h o r eb y t h e p r e s e n t a u t h o r s ( in p r o g r e s s ) , w i l l r e f i n e th es c h e m e a n d p r o v i d e f u r t h e r d a t a o n r e g i o n a lp a l a e o h e a t f l o w p a t t e r n s . I t is a l s o i n t e n d e d t h a t t h er e l a t i o n s h i p b e t w e e n s e d i m e n t c o l o u r a n d o s t r a c o dc o l o u r , p o o r l y c o n s t r a i n e d i n t h i s s t u d y , w i l l b e b e t t e ra p p r a i s e d .

    cknowledgementsT h e a u t h o r s e x p r e s s t h e i r s i n c e r e t h a n k s t o D r K e i t hR o b i n s o n a n d M r N o e l M u r p h y o f t he P e t r o l e u mA f f a i r s D i v i si o n , D e p a r t m e n t o f E n e r g y , D u b l i n , f o rt h e i r a d v i c e a n d c o m m e n t s o n t h e m a n u s c r i p t . M a n yt h a n k s a r e a l s o d u e t o D r J o h n A . A . M c C a l l u m a n dM r J i m B r o w n o f B P E x p l o r a t i o n ( G l a s g o w ) , t o B PP e t r o l e u m C o m p a n y , R e s e a r c h C e n t r e ,S u n b u r y - o n - T h a m e s , t o D r D a v i d W . L e w i s o fC h e v r o n U K L t d , L o n d o n , t o D r G r a h a m H o p k i n s ,A r c o Br i t i s h L t d , L o n d o n , t o Mr D u s t y Mi l l e r , T o t a lO i l M a r i n e , D u b l i n , f o r t h e l o a n o f s a mp l e s ,ma t u r a t i o n d a t a a n d t h e r e l e a s e o f c o n f i d e n t ia li n f o rm a t i o n a n d t o M r C o l i n R e i d o f t h e E l e c t r o nMi c r o s c o p y U n i t , T r i n i t y Co l l e g e , D u b l i n f o r t h e X - r a ys p e c t r o m e t e r w o r k . R i c h a r d B u r n e t t i s i n r e c e i p t o f aD e p a r t m e n t o f E d u c a t i o n p o s t - d o c t o r a l fe l lo w s h ip .

    2 9 6 M a r i n e a n d P e t r o l e u m G e o l o g y , 1 9 90 , V o l 7 , A u g u s t

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    eferencesA de l s ec k , C . G ., Geeh an , G . W. and R o t h , P . H . 1973 )E x p e r i m e n t a l e v i d e n c e f o r t h e s e l e c t i v e d e s o l u t i o n a n do v e r g r o w t h o f c a lc a r e o u s n a n n o f o s s i l s d u r i n g d i a g e n e s isGeoL Soc. Am. Bu l l . 8 4 , 2 7 5 5 - 2 7 6 2A i n s w o r t h , N . R . 1 9 86 ) T o a r c i a n a n d A a l e n i a n O s tr a c o d a f r o mt h e F a s t n e t B a s i n , o f f s h o r e s o u t h w e s t I r e l a n d Bul l . GeoLSurv . I re land 3 , 2 7 7 - 3 3 6A i n s w o r t h , N . R. 1 9 8 7) P l i e n s b a c h i a n O s t r a co d a f r o m t h e

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    Ostracod colour changes: N R Ainsworth e t a lK o n t rov i t z , M . , D ehon , R . A . and M y e rs , J. H. 1983 ) S i m u l a t edbu r i a l o f os t rac ode s he l l s : p rog res s i v e a l t e ra t i on . I n :A p p l i c a ti o n s o f O s t r a c o d a E d . R . F . M ad doc k s ) . U n i v e rs i t y o fH o u s t o n G e o s c i e n c e s , p p . 9 6 - 1 0 3K on t rov i t z , M . , M an n , J . B . and B a rne t t , S . D . 1988 ) Os t rac odes h e l l c o l o u r : a p o t e n t i a l p a l e o t e m p e r a t u r e i n d i c a t o r GeoLS oc A m A bs t r . w i th P rog ram s 20 221L a n g e r , W . 1 9 73 ) Z u r U l tr a s t r u k t u r , M i k r o m o r p h o l o g i e u n dT a p h o n o m i e d e s O s t r a c o d a - C a r a p a x P a l aeon tog raph i ca .A b t . A 1 4 4 , 1 - 5 4M c C o y , W . D . 1 9 88 ) A m i n o - a c i d r a c e m i z a t i o n i n f o s s iln o n - m a r i n e o s t r a c o d s h e l l s : a p o t e n t i a l t o o l f o r t h e s t u d y o fQ u a t e r n a r y s t r a t i g r a p h y , c h r o n o l o g y a n d p a l a e o -t e m p e r a t u r e . I n : Ostracoda in the Ear th Sc iences Eds. P. DeD ec k k e r , J . P . C o l i n and J . P . P ey pouque t ) A m s t e rdam ,E l s e v ie r , p p. 2 1 9 - 2 2 9M c K e n z i e , D . P . 1 9 8 1) T h e v a r i a t io n o f te m p e r a t u r e w i t h t i m ea n d h y d r o c a r b o n m a t u r a t i o n i n s e d i m e n t a r y b a s i n s f o r m e db y e x t e n s io n Earth Planet. Sci. Lett. 5 5, 8 7 - 9 8M u n s e l l C o l o r 1 9 75 ) Munse l l S o i l C o l o r C ha r ts . B a l t i m o r e ,M a c b e t h D i v i s i o n , K o l l m o r g e n C o r p o r a t i o n , 8 c h a r t sO e r t l i, H. J . 1 9 71 ) T h e a s p e c t o f o s tr a c o d e f a u n a s - a p o s s i b l en e w to o l i n p e t r o l e u m s e d i m e n t o l o g y . I n : P a l eoeco l og i eOstracodes Pau 1970 Ed. H . J . Oer t l i ) . Bul l . Ce ntre Rech. Pau- - S N P A S u p p l . 5 , 1 3 7 - 1 5 1R obe s on , D . , B u rn e t t , R . D . and C l ay t on , G . 1988 ) T he U ppe r

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