1993alberdi_ lafargue_1993

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Org. G eochern.Vol. 20, No. 4, pp. 425--436, 1993 0146-6380/93 6.00 + 0.00Printed in Great Britain.All rights reserved Copyright 1993 PergamonPress Ltd

Vert ica l var iat ions o f organ ic matter content in Guayuta Groupupper Cretaceous ) , In ter ior Mounta in Be l t , Eas tern Venezue la

M A L B E R D I 1 and E. L A F A R G U E 2tlntevep S. A., Apdo 76343, Caracas 1070-A, Venezuelaand 2IFP, 1-4 Avenue Bois Preau, BP 311, 92506Rueil-Malmaison Cedex, France

Received 31 August 1992: accepted in revised form 4 December 1992)A b s t r a e t A geochemical study of the vertical variations of organic matter in the Guayuta Group(Querecual and San Antonio Formations) has been conducted on a series of samples taken from asedimentary sequence outcropping in the Interior Mountain Belt. The results indicate that despite veryhigh thermal maturity, the organic matter content in the samples is still very high and shows geochemicalcharacteristics related to anoxic paleoenvironmental conditions of sedimentation. Based on micropaleon-tological data, mineralogical data, and trace metal (V/Ni) ratio of kerogen, the analysis of the sequenceshows stronger anoxic depositional conditions toward the base of the section and an increase of clasticsediments toward the top. The total organic carbon of the sediments increases toward the base of thesequence. In addition, there are pseudocyclicpatterns and heterogeneities of the organic carbon contentdistribution at the meter-scale. The results indicate that the lower part of this sedimentary sequencerepresents a major anoxic event, that can be correlated with other sequences of middle and upperCretaceous age.K e y w or ds - - G uay u t a , Querecual, anoxia, fluctuations of organic content, mid-Cretaceous, EasternVenezuela

INTRODUCTIONThe organic-rich Cenomanian-Maas tricht ian Gua-yuta Group contains amorphous marine type IIorganic matter and is the main source rock in theMaturin Sub-basin, Eastern Venezuela (Talukdaret al . , 1988). It is subdivided int o two formations:the Querecual Formation (Cenomanian-Turonian)and the San Antonio Formation (Santonian-Maastrichtian). The Querecual Formati on consists offinely laminated black pelagic mudstones, rich inplanktonic foraminifers, nearly devoid of any ben-thonic foraminifers and has been described as de-posited in a predominantly anoxic environment(Hedberg, 1937). The San Ant oni o Format ion can bedistinguished from the underlyi ng Querecual for-mati on in certain areas of the Interior Mount ain Beltby the occurrence of sandstones and cherts. Never-theless in many cases, the scarcity of sandstonesmakes it difficult to establish the contact between thetwo formations. This is the case of the Cerro Negroarea we describe herein.

The Guayuta Group represents the maximum ofthe Cretaceous transgressive events in EasternVenezuela (Hedberg, 1937). It overlies the massivebiostromes of the E1 Cantil Formation (Aptian-Albian) and underlies the massive sandstone bodiesof the San Juan Formation, which corresponds tothe recurrence of a regressive cycle during the*To whom all correspondence should be addressed.

Maastrichtian. The lateral equivalent of the GuayutaGroup toward the Guayuta shield (South) is the TigreFormation which consists of glauconitic sandstones,limestones and phosphatic mudstones.

The thickness of the Guayuta Group can reacha maximum of 1200m in the Interior MountainBelt, and within this group, the Querecual Form-ation presents an average thickness of 700m(Hedberg, op cit, Lexico Estratigrafico de Venezuela,1970; Gonzalez de Juana et a l . , 1980). The very largethicknesses encountered illustrate the importantphase of deposition of organic-rich sedimentsthat took place in this area of South America duringthe mid and late Cretaceous. In this regard, thisevent is comparable to anoxic events reportedelsewhere in the world at the same period, as rep-resented by the La Luna (Venezuela-Colombia),Napo (Ecuador), or Agua Nueva (Mexico) for-mations, among others.

The Guayuta Group outcrops in the InteriorMountain Belt, which is part of the thrust fault beltsystem marking the northern end of the MaturinSub-basin (Figs 1 and 2). The Eastern VenezuelaBasin contains the giant oil fields of El Furrial andthe Orinoco Oil belt.

The very good quality of the outcrops in this areaallows a detailed investigation of the variations oforganic matter quantity within the Guayuta Group.This is the objective of the present work and, to ourknowledge, it is also the first detailed study conductedon the Guayuta Group in Venezuela.

425


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426 M . ALBERDI an d E. LAFARGUEMETHODOLOGY NDS MPLNG

1 31 o u t c r o p s a m p l e s h a v e b e e n c o l l e c t e d in t h eC e r r o N e g r o c r e e k w h ic h c u ts t h e G u a y u t a G r o u p i nt h e s o u t h e r n f l a n k o f C e r r o N e g r o a n t i c l i n e F i g . 1 ).T h e a n a l y s e d s e c t io n F i g . 3 ) c o n s i s ts m a i n l y o f

l a m i n a t e d p e l a g i c m u d s t o n e s w i t h s o m e b e d s o fh o m o g e n e o u s m u d s t o n e s a n d s il ic i c m u d s t o n e s d i s -t r i b u t e d t h r o u g h o u t t h e s e c ti o tL T o w a r d t h e t o p ,c l os e t o t h e S a n J u a n F o r m a t i o n - G u a y u t a G r o u pc o n t a c t , s a n d s t o n e v e i n s c u t t i n g t h e p a r a l l e l t o t h es t r a t if i c a t io n w e r e o b s e r v e d . T o w a r d t h e b o t t o m , t h e

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Fig. 1. Location of the Cerro Negro anticline and geological setting.


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Vertical variations of organic matter content in Guayuta Group 427

PASSIVE MAR61N- ~ FORELAND BAS IN N [INTERIOR M OUNTAINBELT

ARAOUA DE MATURIN GUMANA

q

Fig. 2. Schematic geological cross-section of the Maturin Sub-basin (Roure et al. 1993).

millimeter-scale laminations increase and calcareousconcretions are also present.The Cerro Negro creek is a good sampling site inthe Inter ior Mount ain Belt since it cuts a thick section

of the Guayuta Group and, there is no evidence offaulting or folding (Creole, 1969) that might repeatthe sequence. Furthermore, the samples collectedshow no physical sign of weathering. There areseveral gaps in the sequence due to vegetation cover(


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428 M. ALBERDIand E. LAFARGUE

L i t h o l o g y 1 : 2 5 T o t a lO r g a n i c C a r b o n C a r b o n a t e so . . : o o o o 2

~ a m p l eS N J U N ~ n u m b e r S a m p l i n gCN1 s i t e

|1 ~ C N 1 8 ~ b ~i ~ C ~ SITE3

~ C ) ~ C N 5 2S ITE S ~ , \ S

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~ ~ jCN77--SITE8

~ ~ 1C,N31EL C N T I L L I I I I Ix~[ [ ]

J , L a m i n a t e d l i m e s t o n e . - . ;~ == = = ~ H o m o g e n e o u s l im e s t o n e ~ S a n d s t o n eFig. 3. Litholog yof the Ce rro Negro section showing he sampling sites and the distribution of TOC andcarbonate conten ts along the series [ . . . ) indicates a pseudocyclic rend of variation].


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Vert ical variat ions of organic mat ter con tent in Guay uta Gro upTable 1 Tota l Organic Carbon and carbonate contents

429

Sample TOC C a r b o n a t e S a m p l e TOC C a r b o n a t e S a m p l e TOC CarbonateCN- I 0.97 27 CN-53 2.92 39CN-2 I. I 1 40 CN-54 3.14 40CN-3 0.2 31 CN-55 4.12 41CN-4 1.22 39 CN-56 3.73 39CN-5 0.99 36 CN-57 3.94 43CN-6 1.07 57 CN-58 0.71 44CN-7 1.12 36 CN-59 3.26 43CN-8 1.6 41 CN-60.5 3.48 32CN-9 1.01 42 CN-61 3.21 71CN-10 1.73 26 CN-62 2.14 41CN-I I 2.19 30 CN-63 2.18 32CN-12 0.63 12 CN-64 3.43 71CN-13 2.31 41 CN-65 2.97 65CN- 14 2.35 26 CN-66 3.31 44CN- 15 1.99 21 CN-67 3.32 30CN-16 1.34 28 CN-68 4.72 33CN-17 1.7 38 CN-69 3.5 30CN- 18 2.73 42 CN-70 3.28 48CN- 19 2.43 49 CN-71 2.39 20CN-20 3.2 41 CN-72 2.36 37CN-21 2.98 45 CN-73 2.91 31CN-22 2.5 28 CN-74 1.84 30CN-23 2.81 40 CN-75 1.4 23CN-24 2.77 41 CN-76 2.58 36CN25 2.26 42 CN-77 3.9 51CN-26 3.04 41 CN-78 2.48 56CN-27 2.53 42 CN-79 3.68 47CN28 2.98 47 CN-80 3.52 50CN-29 2.89 48 CN-81 3.6 51CN-30 3.91 44 CN-82 3.82 43CN-31 3.49 47 CN-83 3.28 44CN-32 3 52 CN-84 1.21 5CN-33 3.25 56 CN-85 1.16 16CN-34 2.34 52 CN-86 1.99 49CN-35 1.85 44 CN-87 3.13 51CN-36 2.52 39 CN-88 1.63 18CN-37 2.79 39 CN-89 1.97 60CN-38 2.13 53 CN-90 1.6 49CN-39 2.78 47 CN-91 3.32 64CN-40 2.54 40 CN-92 2.89 62CN-41 2.42 40 CN-93 1.34 51CN-42 2.31 40 CN-94 0.68 26CN-43 2.22 51 CN-95 0.57 77CN-44 1.62 50 CN-96 1.06 50CN-45 1.34 52 CN-97 1.72 75CN-46 2.24 38 CN-98 0.67 62.CN-47 2 54 CN-99 1.31 43CN-48 1.93 35 CN-100 4.88 53CN-49 1.86 28 CN-101 3.75 63CN-50 2.43 25 CN- 102 4.79CN-51 2.21 32 CN-103 4.04 59CN-52 2.66 16 CN-104 4.72 20

CN-105 4.77 50CN-106 4.52 19CN-107 2.82 70CN-108 5.84 51CN- 109 4.4 62CN-110 5.01 52CN-l I I 5.42 48CN-112 3.15 77CN-I 13 2.73 80CN-114 4.58 64CN- ll 5 5.56 51CN-116 3.08 86CN- I17 3.52 75CN-118 4.47 55CN- II 9 3.17 69CN-120 3.82 62CN-121 3.87 70CN-122 3.4 71CN-123 3.19 69CN-124 2.96 62CN-125 1.89 67CN- 126 0.67 84CN- 127 2.56 46CN- 128 2 76CN-129 0.89 68CN- 130 0.42 89CN- 131 0.46 65

s e d i m e n t s f r o m N o r t h A t l a n t i c O c e a n d e p o s i t e dd u r i n g m i d a n d l a t e C r e t a c e o u s ( D e G r a c i a n s k y eta L 1984) .

F o r t h e w h o l e G u a y u t a G r o u p , i t i s p o s s i b l e t od i s ti n g u is h a n i n c r e a s e o f T O C c o n t e n t f r o m t h e t o po f t h e s e r i e s t o t h e b o t t o m , w i t h m a x i m u m v a l u e s a ts a m p l i n g si t e n u m b e r 9 ( 5 - 6 ) . O n a s m a l l e r s c a l e ,w e t e n t a t i v e l y r e p r e s e n t e d o n F i g . 3 p s e u d o - c y c l i cv a r i a ti o n o f t h e T O C c o n t e n t i n t h e G u a y u t a G r o u p .N e v e r t h e l e s s t a k i n g i n t o a c c o u n t t h e l ac k o f s a m p l e sf r o m c o v e r e d i n t e r v a l s , t h e i n t e r p r e t a t i o n o f t h es e c t i o n i n t e r m s o f o r g a n i c m a t t e r r i c h n e s s c y c l e sn e e d s f u r t h e r c o n t r o l s i n o t h e r s i m i l a r s i t e s . O n t h es a m e f i g u re , a t a m e t e r - s c a l e w i t h i n t h e s a m es a m p l i n g s i t e , t h e r e s u l t s a l s o i n d i c a t e i m p o r t a n tf l u c tu a t i on s o f t h e T O C c o n t e n t f r o m o n e s a m p l e t oa n o t h e r .

T h e m a i n v a l u e s f o r c a r b o n a t e c o n t e n t o f t h es a m p l e s ( c a l c i te ) a r e i n t h e 4 0 - 6 0 r a n g e w i t h am i n i m u m o f 5 a n d a m a x i m u m o f 8 9 ( T a bl e 2a n d F i g . 3 ) . C a r b o n a t e c o n t e n t s h o w s i m p o r t a n tv a r i a t i o n s w i t h i n t h e s e c t i o n , b u t t h e r e i s a t e n d e n c yo f i n c r e as i n g c a r b o n a t e t o w a r d t h e b o t t o m o f t h es e c t i o n . C a l c i t e i s d is t r i b u t e d i n t h e s a m p l e s a s m i c r i t ew i t h i n t h e m a t r i x , a s l a t e v e i n s c u t t i n g t h e l a m i -n a t i o n s a n d p r i n c i p a l l y , f i l l i n g u p t h e f o r a m i n i f e r a lw a l l s . P a l e o n t o l o g i c a l o b s e r v a t i o n s s h o w t h a t t h ef o r a m i n i f e r a l c o n t e n t o f t h e s a m p l e s i n c r e a s e s t o w a r dt h e b o t t o m ( i n c r e a s e b i o p r o d u c t i v i t y ? ) , p o s s i b l y e x -p l a i n i n g t h e g e n e r a l t e n d e n c y f o r i n c r e a s i n g c a r b o n -a t e c o n c e n t r a t i o n i n t h e s a m e d i r e c t i o n .

T h e r e l a t i o n s h i p b e t w e e n o r g a n i c c a r b o n a n d m i n -e r a l c a r b o n i s n o t v e r y c l e a r b u t , i n s o m e s a m p l i n gs i t e s , a t a m eter - sca le , th ere i s a c l ea r in v erse re la t io n


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430 M . ALBERDI an d E. LAFARGUETable 2. M ineralogical contents

Sample % Qz % Clays % CalciteCN 1 44 9 45CN3 61 5 34CN 7 30 12 58CN20 26 9 65CN27 24 8 64CN34 22 5 73CN 44 31 7 61CN53 26 I I 63CN60,5 36 6 58CN66 24 9 67CN80 20 6 74CN88 61 3 36CN 92 11 7 82CN94 46 3 5 ICN95 8 2 90CN99 23 7 67CN I00 17 6 77CNI01 11 7 82CN I03 13 5 82CN I05 19 5 76CN I08 20 6 73CN II3 4 2 94CN II5 18 4 78CNI20 8 3 87CN 123 8 6 87CN 128 9 2 89CN131 81 0 10

% Error in C lays determinations > 15%% Error in Qz and calcite determinations = 15%

b e t w e e n t h e a m o u n t s o f o r g a n i c a n d m i n e r a l c a r b o n sa s i l l u s t r a t e d , f o r e x a m p l e , i n s a m p l e s i t e s 5 , 1 0 a n ds o m e s a m p l e s o f 9 . T h i s i n v e r s e t r e n d i s m o r e c l e a r l ys h o w n o n a c a r b o n a t e v e r s u s T O C d i a g r a m ( F i g . 6 ).S u c h t r e n d h a s b e e n r e p o r t e d b y B r o s s e e t a l ( 1 9 8 8 ,1 9 9 0 ) i n c a r b o n a t e s e r i e s f r o m I t a l y a n d i n d i c a t e s a

p r e f e r e n t i a l a s s o c i a t i o n o f t h e o r g a n i c m a t t e r w i t h t h ec l a ys r a t h e r t h a n c a r b o n a t e s i n t h e r o c k . P e t r o g r a p h i co b s e r v a t i o n s o n t h i n s e c t io n s s u p p o r t t h i s h y p o t h e s is .I n p a r t i c u l a r , w e o b s e r v e d i n v a r i o u s t h i n s e c t i o n s ,t h e m u l t i p l e l a m i n a e c o n t a i n i n g c a l c a r e o u s p l a n k -t o n i c f o r a m i n i f e r s w i t h i n t h e m i n e r a l m a t r i x r i c h i nc l a y s a n d o r g a n i c m a t t e r ( P l a t e I ) . T h e s e r e g u l a rb e d d i n g f e a t u r e s c a n b e r e l a te d t o d e p o s i t i o n a l e n -v i r o n m e n t ( K e n n e d y , 1 9 7 6, c i t. S c h o l l e , 1 9 7 7 ), b u tt h e i r o r i g i n i s n o t e n t i r e l y c le a r . I t h a s b e e n p r o p o s e dt h a t t h e s m a l l cy c l e s r e f le c t f l u c t u a t i o n s i n t h e p r o -d u c t iv i t y o f p l a n k t o n i c o r g a n i s m s a n d / o r r h y t h m i cf l u c t u a t i o n s i n t h e i n te n s i t y o f c a r b o n a t e d i s s o l u t i o nc o u p l e d w i t h p e r i o d i c v a r i a t i o n s i n t e r r i g e n o u si n f l u x ( A r t h u r a n d R o g g e n t h e n , 1 9 76 ). H o w e v e r ,t h e f r e q u e n t o c c u r r e n c e o f r e c r y s t a ll i z a ti o n w e o b -s e r v e d b o t h i n t h e f i e l d a n d i n t h i n s e c t i o n s , a n d s m a l lc a r b o n a t e d i s s o l u t i o n s ( le a c h i n g ) m a y o b s c u r e a c l e a rr e l a ti o n b e t w e e n o r g a n ic c a r b o n a n d m i n e r a l c a r b o nt h a t c o u l d h a v e e x i st e d o r i g i n a l l y .

A t t h e t o p o f t h e s e c t i o n ( e s p e c i a l l y a t s a m p l i n g s i t en u m b e r 1), th e l o w e r c o n t e n t o f o r g a n ic c a r b o n c a nb e a s c r i b e d t o a d i l u t i o n e f f e c t c a u s e d b y h i g h e rc l a s t i c i n p u t a t t h e t r a n s i t i o n z o n e b e t w e e n t h eG u a y u t a G r o u p a n d t h e Sa n J u a n F o r m a t i o n , w h i chis k n o w n t o c o r r e s p o n d t o a m a j o r r e g r es s i ve e v e n t i nt h i s a r e a . T h i s i s s u p p o r t e d b y t h e g e n e r a l i n c r e a s e o ft o t a l c la y c o n t e n t a n d a s l i g h t t e n d e n c y o f i n c r e a s in gq u a r t z c o n t e n t , e x c e p t f o r s a m p l e s C N - 8 8 , C N - 9 4a n d C N - 1 3 1 .

I n s a m p l i n g s i te n u m b e r 9 , a t t h e b o t t o m o f t h es e c t io n , p e t r o g r a p h i c o b s e r v a t i o n s s h o w t h e ex i s te n c e

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V e r t ic a l v a r i a t i o n s o f o r g a n i c m a t t e r c o n t e n t i n G u a y u t a G r o u p 4 31

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432 M . ALBERDI an d E. LAFARGUEo f d e t r i t a l q u a r t z a n d a u t h i g e n i c q u a r t z o f s ev e r a ls i ze s . S m a l l q u a r t z g r a i n s ( < 0 .0 1 m m ) a r e m i x e d w i t hc lays a n d k e r o g e n i n t h e m a t r i x o f t h e s a m p l e s .amples C N - 8 8 , 9 4 a n d 1 31 s h o w r a d i o l a r i a a n dforaminifera f i ll e d w i t h c h a l c e d o n y , w h i c h i s in a g r e e -

m e n t w i t h M a r c u c c i ' s o b s e r v a t i o n s (1 97 9) t h a t e m -p h a s i z e d t h e b i o l o g i c a l o r i g i n o f t h i s m i n e r a l i n t h eS a n A n t o n i o F o r m a t i o n c h e r t s . O n t h e o t h e r h a n d ,o n l y t h e s a m p l e C N - 1 2 2 w h i c h I n o c e r a m u s s h e l l s ,presents d e t r i t a l , a n g u l a r q u a r t z . T h e q u a r t z c o n t e n to f t h e s e s a m p l e s a n d i t s r e l a t i o n w i t h T O C i s s h o w ni n F i g . 6 w h e r e t w o g r o u p s o f s a m p l e s c a n b ed i f f e r e n t ia t e d . T h e f i rs t g r o u p s h o w s h i g h q u a r t z a n dl o w T O C ( C N - 8 8 , 9 4 , 1 3 1 ), a n d t h e s e c o n d g r o u ps h o w s a d i r e c t r e l a t i o n s h i p b e t w e e n q u a r t z ( m a i n l yless t o 0 .0 1 m m ) a n d T O C . T h e s e r e s u l t s m u s t b es t u d i e d w i t h m o r e d e t a i l t o e m p h a s i z e t h e r e l a t i o n -s h i p t h a t c o u l d e x i s t b e t w e e n t h e o r i g i n o f q u a r t zg r a i n s a n d T O C . H o w e v e r , p o s i t i v e r e l a t i o n s b e t w e e na u t h i g e n i c q u a r t z f r o m b i o l o g i c a l s o u r c e s ( M a r c u c c i ,1 97 9) a n d T O C c o u l d s u p p o r t b i o l o g i c a l p r o d u c t i v i t ya s a c o n t r o l o v e r t h e T O C d i s t r i b u t i o n . I f t h e q u a r t zi s a s e c o n d a r y p r o d u c t o f c l a y r e a c t i o n s th i s r e l a t i o n -s h i p c o u l d s u p p o r t t h e a s s o c i a t i o n b e t w e e n c l a y s a n do r g a n i c m a t t e r . A l s o o f i n t e r e s t w o u l d b e t h e u s e o fs t a b l e c a r b o n i s o t o p e a n a l y s e s t o s h e d l i g h t o n t h eq u e s t io n o f p r o d u c t i v i t y a s a c o n t r o l o v e r t h e T O Cd i s t r i b u t i o n . U n f o r t u n a t e l y i n t h e s e r i e s s t u d i e d ,t h e f r e q u e n t o c c u r r e n c e o f r e c r y s t a l li z a t i o n m e n -t i o n e d e a r l i e r m a k e s i t d if f ic u l t t o o b t a i n a d e q u a t ec a r b o n a t e .T r a c e e l e m e n t s d e t e r m i n a t i o n s i n k e r o g e n s a m p l e sa n o x i c c o n d i ti o n s a n d t h e i r r e l a ti o n s h ip s w i t h T O Cc o n t e n t .

Q u a l i t a t i v e t r a c e e l e m e n t a n a l y s i s o f 1 9 k e r o g e n sa l o n g t h e s e q u e n c e s h o w a g r e a t d i v e r s i t y o f e l e m e n t sa s s o c i a t e d w i t h o r g a n i c m a t t e r ( Z n , V , N i , C u , P b ,F e , a m o n g o t h er s ). Q u a n t i t a ti v e d e t e r m i n a t i o n o f Va n d N i o n k e r o g e n s w a s c o n d u c t e d i n o r d e r t o f i n dp o s s i b l e g e o c h e m i c a l i n d i c a t o r s o f a n o x i a i n t h i so v e r m a t u r e s e q u e n c e , w h e r e o t h e r g e o c h e m i c a lp a r a m e t e r s f a i l e d .I t i s p o s s i b l e t o o b s e r v e a n i n c r e a s e i n V / N ir a t i o f r o m t o p t o b o t t o m w i t h r a p i d c h a n g e o f t h i sr a t i o f r o m s a m p l e s C N - 6 6 ( V / N i = 1 .7 9) to C N - 7 9a n d C N - 8 0 w i t h V / N i r a t i o s o f 3 .7 a n d 4 2 . r e s p ec t -i ve ly (F ig . 7 ) . T hese r e su l t s i nd i ca t e m ore r educ ingc o n d i t i o n s ( a n o x i c ) t o w a r d t h e b o t t o m , w h i c ha l l o w s t h e p r e f e r e n t i a l f i x a t i o n o f v a n a d i u m o v e rn i c k e l ( L e w a n , 1 98 4; B a k e r a n d L o u d a , 1 9 8 3 ; F i l b yand V a n B e r k e l , 1 98 7) . P a l e o n t o l o g i c a l o b s e r v a t i o n sc o r r o b o r a t e t h i s r e s u l t i n s a m p l i n g s i t e s 9 - 1 0 - 1 1 ,f r o m C N ~ -8 4 t o C N - 1 3 1 , w h e r e b a s i c a l ly o n l yp l a n k t o n i c f o r a m i n i f e r s a r e p r e s e n t . L e s s r e d u c i n gc o n d i t i o n s c a n b e in f e r r e d f o r th e u p p e r p a r t o ft h e s e d i m e n t a r y s e c t i o n , w h e r e k e r o g e n s h o w s aV / N i r a t i o o f 1 . 7 ( F i g . 7 ) a n d t h e p a l e o n t o l o g i c a lo b s e r v a t i o n s s h o w g r e a t e r q u a n t i t i e s o f b e n t h o n i cf o r a m i n i f e r s .

S m a l l i n t e r r u p t i o n s i n t h e s e g e n e r a l a n o x i cc o n d i t i o n s ( s a m p l i n g s i t e 9 ) a r e s h o w n b y t h e p r e s -e n c e o f I n o c e r a m u s t a n a t h o c e n o s e l e v e l ( C N - 1 2 2 ;T O C = 3 . 2 % ) b e t w e e n b e d d i n g p l an e s . T h i s m o d e o fo c c u r r e n c e i s c h a r a c t e r i s t i c o f o p p o r t u n i s t i c s p e c i e st r a n s i e n t l y o c c u p y i n g a h i g h s t r e s s e n v i r o n m e n tb u t w i th t e m p o r a r y a l le v i a ti o n o f b o t t o m w a t e ra n o x i a ( H a l l a m , 1 98 7) . V / N i r a t i o s i n s a m p l i n g s i te 9( C N - 9 9 a n d C N - I 1 3 ) c o r r o b o r a t e t h i s o b s e r v a t i o na n d s u p p o r t t h e b r i e f i n t e r r u p t i o n s o f a n o x i cc o n d i t i o n s , p r o b a b l y a s s o c i a t e d w i t h o x y g e n a t e dcur ren t s .

T h e f r e q u e n t a p p e a r a n c e o f E p h r o l i t u s f l o r a l i sa n d M i c u r a S t a u r o p o r a i n sam pl ing s i t e 9 , a l l ows ust o a s s i g n th i s p a r t o f t h e s e q u e n c e t o t h e L o w e rT u r o n i a n a g e ( M u l l e r , p e r s o n a l c o m m u n i c a t i o n ,1991) and t h i s da t e cou ld be r e l a t ed t o t he ex i s t enceo f a w o r l d w i d e a n o x i c e v e n t i n t h i s p a r t o f t h es t u d ie d s eq u e nc e . T h e C e n o m a n i a n - T u r o n i a n s t r at i -g r a p h i c b o u n d a r y h a s b e e n r e p o r t e d b y d i f f e r e n ta u t h o r s a s t h e p e r i o d o f o c c u r r en c e o f m a j o r w o r l d -w i d e a n o x i c e v e n t s ( S c h l a n g e r a n d J e n k y n s , 1 9 7 6 ;D e m a i s o n a n d M o o r e , 1 98 0; P a r i s h a n d C u r t i s , 1 98 2;A r t h u r e t a l . 1 9 8 7 ; D e G r a c i a n s k y e t a l . 1987;S c h l a n g e r e t a l . 1987) and cou ld be r e l a t ed t oo u r o b s e r v a t i o n s . T h e g e n e r a l in c r e a s e o f T O C c o n -t e n t f r o m t o p t o b o t t o m c a n b e c o r r e l a t e d w i t h ag e n e r a l i n c r e a s e o f a n o x i c c o n d i t i o n s i n t h i s s a m ed i r e c t i o n .

A l t h o u g h t h e s e d i m e n t s w e r e d e p o s i t e d u n d e ra n o x i c c o n d i t i o n s , th e r e i s a n i m p o r t a n t r e d u c t i o n o fT O C c o n t e n t a t t h e t o p o f s a m p l i n g s i t e 9 ( C N -8 8 / C N - 9 9 ) , 1 0 a n d 1 1 . T h i s p s e u d o c y c l i c t r e n d i si n d e p e n d e n t o f a n o x i c c o n d i t i o n s a n d c o u l d b e e x -p l a i n e d b y c h a n g e s i n r a t e o f s e d i m e n t a t i o n a n d / o rc h a n g e s i n p a l e o p r o d u c t i v i t y .

ON LUSIONS

T h e d e t a i l e d g e o c h e m i c a l s t u d y c o n d u c t e d h e r e i nt h e I n t e r i o r M o u n t a i n B e l t g i v e s i n s i g h t s i n t o t h ev e r t i c a l v a r i a t i o n s o f o r g a n i c m a t t e r c o n t e n t w i t h i nt h e G u a y u t a G r o u p . D e s p i t e th e l a c k o f a v a i la b l ei m m a t u r e G u a y u t a G r o u p s a m p l es , i t w a s p o s s i b le t oo b t a i n v a l u a b l e d a t a f r o m a s e r ie s o f m a t u r e s a m p l e s .T o t a l o r g a n i c c a r b o n , t r a c e e l e m e n t d e t e r m i n a t i o n so n k e r o g e n , a l o n g w i t h t h e m i n e r a l o g i c a l a n d p a l e o n -t o l o g i c a l d a t a c a n b e g o o d t o o l s t o a s s e s s o v e r m a t u r es e q u e n c e s w h e r e m o r e t r a d i t i o n a l g e o c h e m i c a l t o o l sc a n n o t b e a p p l i e d .

T h e r e s i d u a l T O C d i s t r i b u t i o n s h o w s v a r i a t i o n st h a t c a n b e r e l a t e d t o t h e p a l e o e n v i r o n m e n t a l c o n -d i t i o n s o f s e d i m e n t a t i o n . T h i s s t u d y c o n f i r m s t h e v e r yg o o d s o u r c e r o c k q u a li t y o f t h e G u a y u t a G r o u p a n di t s i m p o r t a n c e i n t h e g e n e r a t i o n o f h y d r o c a r b o n s i nt h e M a t u r i n S u b - b a s i n . T h e v e r y h i g h o r g a n i c c a r b o nc o n t e n t , t o g e t h e r w i t h t h e s i g n i fi c a n t t h i c k n e s s o f t h i sg r o u p , m a k e i t v e r y i n t e r e s t i n g w i t h r e g a r d t o i t sp o s s i b l e c o r r e l a t i o n t o s i m i l a r o r g a n i c - r ic h d e p o s i t s a ta w o r l d s c a l e d u r i n g t h e C r e t a c e o u s .


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Fi g . 7 . V / Ni p r o f i l e i n t he Gua yu t a Gr oup o f t he Ce r r oNe gr o s e c t i on .

T h i s p a p e r r e p r e s e n t s t h e f i rs t d a t a o f t h is n a t u r eo n t h e G u a y u t a G r o u p i n V e n e z u e l a a n d c a n b eh e l pf u l f o r o u r u n d e r s t a n d i n g o f t h e d e p o s i ti o n o fo r g a n i c - ri c h s e d i m e n t s in t h e N o r t h e r n p a r t o f t h eS o u t h A m e r i c a m a r g i n . I t i s a l s o c l e a r th a t a d d i t i o n a lw o r k i s n e c e s s a r y t o r e l a t e t h e p r e s e n t r e s u l ts w i t ho t h e r s tu d i es o f G u a y u t a G r o u p s e q u e n c e s o u t cr o p -p i n g i n t h e I n t e r i o r M o u n t a i n B e l t . I n p a r t i c u l a r , i tis i m p o r t a n t t o u n d e r s t a n d t h e f a c to r s c o n t r o l l i n gT O C a c c u m u l a t i o n s i n o rd e r t o d i s c ri m i n a t e th ep r i n c i p a l c o n t r o l s o n t h e o c c u r r e n c e o f th i s g o o dC r e t a c e o u s s o u r c e r o c k . I n f u t u r e w o r k , t h i s in f o r -m a t i o n m u s t b e a d d e d t o t h e s t u d y o f o t h e r a n o x i ce p i so d e s su c h a s L a L u n a V e n e z u e la a n d C o l o m b i a )a n d N a p a r i m a H i l l /G a u t i e r T r i n id a d ) F o r m a t i o n s t od e v e l o p a c o h e r e n t b l a c k - s h a l e m o d e l o f s e d i m e n -t a t i o n w h i c h c o u l d e x p l a i n t h e s i m i l a r it i e s a n dd i f f e r e n c e s b e t w e e n o r g a n i c m a t t e r g e o c h e m i c a lc h a r a c t e r i s t i c s .A c k n o w l e d g e m e n t s - - S p e c i a l t ha nks a r e due t o J . D i Cr oc ef o r h i s he lp du r i n g t he f ie ld t r i p a n d t o N . J o r da n , I . Be r r io s ,F . Le B i ha n , C M ul l e r , M. Fu r r e r , G . P i c ha ud , B . Agu a do ,I . T r ukow s k i , P . D i a z a nd C . Sc hw a r t z f o r t e c hn i c a l a s s i st -ance . We are a lso gra teful to E. Brosse , G, Deroo,O . Ga l l a ngo , J . P . He r b i n , A . Huc , L . t e n Ha ve n a nd

W. Sc he r e r f o r t he c a r e f u l r e v ie w o f t he m a nus c r i p t . C r i t ic a lr e v i ews by R . J a f f e a nd K . Pe t e r s we r e a l s o ve r y u s e f u l t oi mpr ove t he f i r s t ve r s i on o f t h i s pa pe r .

RE FE RE N E S

A r t h u r M . a n d R o g g e n t h e n 1 97 6 ) P r i m a r y b e d d in gr hy t h ms i n pe l a g ic c a l c a r e ous s e d i me n t s a bs t r a c t ) . A m .Assoc. Pet. Geol. Bull . 60, 46.Ar t hu r M. , Sc h l a nge r S . a nd J e nk yns H . 1987 ) The Ce no -m a n i a n - T u r o n i a n o c e a n i c a n o x i c e v e n t - - l I . P a l e o c e a n o -g r a p h i c c o n t r o l s o n o r g a n i c m a t t e r p r o d u c t i o n a n dp r e s e r va t i on . I n Mar i ne Pe t r o l e um Sour c e - Roc k s , Edi t e dby Br ooks J . a nd F l e e t A . ) . Geol. Soc. Spec. Publ. 26,401 - 421 .Ba ke r E . W. a nd L oud a J . W. 1983 ) The r m a l a s pe c ts inc h l o r ophy l l ge oc he mi s t r y . I n Advance in Organic Geo-c he mi s t r y 1981 Edi t e d by B j o r oy et al.), pp. 401-421.Wi l e y , C h i c he s t e r .Br os s e E . Lo r e a u J . P ., H uc A . , F r i xa A . , Ma r t e l l i n i L . a ndRi va A . 1988) The o r ga n i c ma t t e r o f i n t e r l a ye r e d c a r bo n -a tes and c lays sediments , Tr ias /Lias , S ic i ly . In Adv anc e sin Organic Geo chemis try 198 7 Edi ted by Ma t a ve l l i L . a ndNovelli L.) . Org. Geochem. 13 , 433 - 443 . Pe r ga mon P r e s sOxf o r d .Brosse E. , Riva A. , Santucci S . , Bernon M. , Loreau J . P . ,F r i xa A . a nd La ggo un- D e f a r ge F 1990) Some s e d i me n t o -l og i c a l a nd ge oc he mi c a l c ha r a c t e r s o f t he l a t e T r i a s s i cN o t o F o r m a t i o n , s o u r c e - r o c k i n t h e R a g u s a B a s i nSicily). In Advances in Organic Geochemis try 1989,Edi t e d by Dur a nd B . a nd Be ha r F . ) . Org. Geochem. 16,714 - 734 .CR EO LE 1969) Ma pa ge o l og i c o de la Se r r a n i a de l I n t e r i o r ,Esca la 1 : 500000.De G r a c i a ns k y P ., De r oo G . , He r b i n J . P. , M on t a de r t L .,Mu l l e r C . , Sha a f A . a nd S i ga l J . 1984 ) Oc e a n wi des t a gna t i on e p i s ode i n t he l a t e Cr e t a c e ous . Nat ur e 308, 22,346-349.De Gr a c i a ns ky P . , B r os s e E . , De r oo G . He r b i n J . P . ,M on t a de r t L . , Mu l l e r C . , Si ga l J. a nd Sc ha a f A . 1987 )O r g a n i c - r i c h s e d i m e n t s a n d p a l e o e n v i r o n m e n t a l r e c o n -s t r uc t i ons o f t he Cr e t a c e ous Nor t h A t l a n t i c . I n Mar i nePe t r o l e um Sour c e - Roc k s , Edi t e d by Br ooks J . a nd F l e e tA . ) Geol. Soc. Spec. Publ. 26, 317-345.D e m a i s o n G . a n d M o o r e G . 1 98 0) A n o x i c e n v i r o n m e n t san d o i l source bed genes is, Am. Assoc. Pet. Geol. Bull . 64,1179-1209.Es p i t a li 6 J . , De r o o G . a nd Ma r qu i s F . 1985) La py r o l y s eRoc k - Eva l e t se s a pp l i c a t i ons de ux i 6me pa r ti e ) . Rev. Inst .Fr. Pbt. 40, 755-784.

F i l by R . H . a nd Va n B e r ke l G . J . 1987) Ge o c he m i s t r y o fme t a l c ompl e xe s i n pe t r o l e um, s ou r c e r oc ks a n d c oa l s : a noverview. In M eta l Com plexes in Foss i l Fuels , Edi t e d byF i l by R . H . a nd Br a n t ha ve n J . F .) . ACS Sy mp . Se r . 344 ,2 39.Gon z a l e z de J ua na C . , I t u r r a l de M. J. a nd P i c a r d X . 1980 )Ge o l og i a de Ve ne z ue l a y s u s c ue nc a s pe t r o l i f e r a s , TomoI . F O N I N V E S .Ha l l a m A. 1987) M e s oz o i c ma r i ne o r ga n i c - r i c h s ha l e s.In Mar i ne Pe t r o l e um Sour c e - Roc k s , Edi t e d by B r ooks J .and Flee t A. ) . Geol. Soc. Spec. Publ. 26, 251-261.He d be r g H 1937 ) Es t r a t i g r a f i a de l a se c c i on de l R i oQue r e c ua l e n e l Nor e s t e de Anz oa t e gu i , Ve ne z ue l a . Bol.Geol . Min. Mi n i s t e r i o de Fome n t o . Tomo I . 2 , 3 , 4 ,253 265Le wa n M 1984) Fa c t o r s c on t r o l l i ng t he p r opo r t i ona l i t y o fva na d i um t o n i c ke l i n c r ude o i l s , Geochim. Cosmochim.Ac t a 48 , 2231-2238.Ma r c uc c i E . 1979 ) Los f t a n i t a s de l Cr e t a c i c o Supe r i o r deVe ne z ue l a Oc c i de n t a l y Or i e n t a l : t ypo , o r i ge n , a mb i e n t ede f o r ma t i on y c a mb i os d i a ge ne t i c os . H Congresso Lat i -noamericano de Geologia, 1277-1293.


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