Global and local controls influencing the deposition of the La Luna Formation (Cenomanian-Campanian), western Venezuela

CHEMICAL GEOLOGY rwcr_uLvNc ISOTOPE GEOSCIENCE

ELSEVIER

Chemical Geology

130 ( 1996) 271-288

Global and local controls influencing the deposition of the La Luna Formation ( Cenomanian-Carnpanian) , western Venezuela Julio Perez-Infante

a71,Paul Farrimond a7*, Max Furrer b

a Newcastle Research Group in Fossil Fuels and Environmental Geochemistry, Drummond Building, University ofNewcastle, Newcastle upon Tyne, NE1 7RlJ, UK b Lagouen, S.A., Filial Petroleos de Venezuela, Apartado 889, Caracas 1010 A, Venezuela Received

12 April 1995; accepted 30 January

1996

Abstract Bulk and molecular geochemical, micropalaeontological, and carbon-isotopic data are used to address the different local and global factors influencing the environment of sedimentation of the La Luna Formation (Cenomanian-Campanian, approximate palaeolatitude IS’N) in a single section in western Venezuela. Based on the constructed chronostratigraphic framework, oxygen-depleted bottom-water conditions and black-shale deposition started in western Venezuela well before the widespread occurrence of organic-rich sediments in higher palaeolatitude regions such as the Tethys and the North Atlantic near or at the Cenomanian-Turonian boundary. In the La Luna Formation, palaeoenvironmental conditions that allowed the preservation of organic matter (mainly of marine origin), prevailed until Santonian times in a distal platform facies with very low siliciclastic input. Changes in lithology appear to reflect the local response to eustatic sea-level variations and the presence of a migrating upwelling belt affecting the bioproductivity of silica and carbonate. A marked 8’3COrg isotopic excursion is recognised in the middle part of the section, and is apparently unrelated to local palaeoenvironmental changes in bioproductivity and oxygen depletion. Biological marker dam show no variations in association with the isotopic excursion, being mainly controlled by local fluctuations in organic-matter input and preservation.

1. Introduction The Mid-Late Cretaceous age represents one of distribution of the periods of more extensive organic-rich sediments in both deep- and shallowmarine environments, throughout the world. Stratigraphic data from the Deep Sea Drilling Project and

* Corresponding author. ’Present address: Venezuelan Intevep, apartado

76343, Caracas

0009.2541/96/$15.00 Copyright PII SOOOS-2541(96)00019-8

Petroleum Research 1070 A, Venezuela.

Institute,

Ocean Drilling Program, and their correlation with many exposed sections on land show that organiccarbon-rich sediments of this age are confined to particular stratigraphic horizons (for a review see Arthur et al., 1990), leading to the conclusion that, within relatively narrow time envelopes (‘Oceanic Anoxic Events’), global oceanic conditions allowed the preservation of large amounts of organic carbon (Schlanger and Jenkyns, 1976; Jenkyns, 1980). Scholle and Arthur (1980) have shown that temporal variations in 613C values correlate with these events. Positive excursions in 613C of both organic matter

0 1996 Elsevier Science B.V. All rights reserved.

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and primary carbonates have been widely observed in many North American and European sections at or near the Cenomanian/Turonian boundary (e.g. Arthur et al., 1987; Bralower, 1988; Gale et al., 1993; Jenkyns et al., 1994). Although this isotopic excursion is commonly related to the presence of black shales, at many localities the signal (measured on primary carbonate) is found within CenomaniarTuronian shallow-water limestones lacking black shales and deposited under oxic conditions (Schlanger et al., 1987). Therefore, this isotopic excursion is considered a global event that resulted from a change in the carbon-isotopic composition of the hydrosphere and atmosphere (Scholle and Arthur, 1980; Popp et al., 1989). This has been explained through widespread accumulation of organic matter (enriched in 12C) in oceanic sediments leaving the oceanic carbon pool enriched in 13C and causing subsequent carbonate or organic carbon deposits to be also enriched in 13C. This interval of extensive black shale deposition has been called the Cenomanian/Turonian Oceanic Anoxic Event (‘OAE-2’; Schlanger and Jenkyns, 1976; Schlanger et al., 1987) or the Cenomanian/Turonian Boundary Event (CTBE). It has been suggested that the anoxic event is related to a major global sea-level rise. However, the highest sea-level peak in the Late Cretaceous appears to occur some time later in the middle Turonian (between 91.5 and 90.3 million years ago; Haq et al., 19871, as is pointed out by Hancock (1993) based on the stratigraphical position of hardgrounds and ammonoid zones of the Tethyan region. In western Mediterranean and North Atlantic regions the development of black shales is often restricted to the CTBE, but in low-latitude shelf basins along the African continental margin, deposition of black shales took place from as early as middle Cenomanian to late Turonian (Einsele and Wiedmann, 1982; Kuhnt et al., 19901, suggesting that local mechanisms contributed to their origin. In this study, we consider another low-latitude occurrence of mid-Cretaceous black shales: the La Luna Formation of Venezuela (Cenomaniar-Campaniann; palaeolatitude of approximately 15”N). In ‘he western area of the Maracaibo Basin, the La Luna Formation mainly comprises alternating beds of marls and pelagic limestones, changing laterally to less carbon-

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ate-rich facies towards the Andes (southeast), Colombia (west), and to the east. The La Luna Formation has often been quoted as an example of C/T black shale deposition (i.e. Schlanger et al., 1987; Kuhnt et al., 1990). It is considered that dysaerobic to anoxic conditions prevailed, and that these were probably linked to a regional anoxic event and seasonal upwelling along the coast of South America (Tribovillard et al., 1991; Martinez and Hemandez, 1992). Macellari and De Vries (1987) proposed that the La Luna’s anoxic sediments in northwestern South America are distributed on a Trough Sub-province (Colombia and southwestern Venezuela), affected by dynamic upwelling, and a Platform Sub-province (northwestern Venezuela), ‘reflecting a global anoxic event’. Turonian-Coniacian organic-rich sediments are also found in Deep Sea Drilling Project sites in the Venezuela Basin (Hay, 19851, and James (1990) argues that it is ‘unreal’ to imagine that upwelling would have affected such a large area of northern South America and the southern Caribbean. In this paper we present bulk and molecular geochemical, micropalaeontological, and carbon-isotopic data to address the occurrence of different local and global events during the sedimentation of the La Luna Formation. In addition to defining the Cenomanian/Turonian Boundary Event in the La Luna, stratigraphic variability represented by lithofacies changes, biomarker composition and micropalaeonto logical assemblages were considered in detail at a single but representative location, to investigate temporal relationships between eustatic changes and local bioproductivity of silica, carbonate and organic matter.

2. Experimental 2.1. Geologic setting and sampling

information

Samples for this study were collected from Maraca Ravine (20 km southwest of Machiques City; Fig. l), in the Perija Foothills. The La Luna here lies within an area of rain forest and although well exposed during the dry season (November to March), some segments of the section are covered by dense vegetation or stone blocks, producing gaps in sampling.

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Geology 130 (1996) 271-288

The section was measured from the base of the La Luna Fm. This contact is abrupt and clearly distinctive as a lithological change between the massive and organic-lean limestone of the Maraca Formation and the first occurrence of organic-rich, laminated marls of the La Luna Formation. A set of 90 samples were collected from an estimated 160 m vertical interval. Micropalaeontological analyses were performed on thin sections of selected samples placing emphasis on radiolaria and planktic and benthic foraminifers.

atomic absorbtion spectrometry using a Varian AA300 spectrometer, following digestion for 12 h at 110°C with hydrochloric and hydrofluoric acids in closed Teflon liners placed in stainless-steel cases. Total carbon and organic carbon (after acid dissolution of carbonate minerals) were determined using a Leco CS-244 Carbon-Sulphur Analyser. Carbonate content was determined by the difference between the two measurements.

2.2. Bulk geochemistry

Approximately 25 g of powdered rock were extracted in a soxhlet apparatus using cellulose thimbles for 48 h with dichloromethane (DCM)/methanol (230 ml/20 ml) as solvent. Activated copper was added to the extraction flask to remove any elemental suiphur. After extraction, excess solvent was removed by rotary evaporation and the dried extracts were weighed in order to calculate the total extractable organic matter (EOM). The extracts were fractionated by column chromatography using pre-

Samples were crushed to powder in a rotary disc mill (Terna@) and aliquots analysed by Rock-Eva1 pyrolysis, and for their total organic carbon (TOC) and carbonate contents. Free bitumen (S l), kerogen pyrolysis yields (S2) and maximum temperature of pyrolysis CT,,,) were determined using a Leco THA200 Thermolytic Hydrocarbon Analyser. The elements silicon and aluminium were determined by

2.3. Sediment extraction

and fractionation

I

! \ I i ‘, lo”,

0 I

! \ \ /’

15km

I

? ?Field

/

i

section studied i

I;

9”4O’N

72”H

I

Fig.

I. Sampling site of the La Luna Formation in the western Maracaibo Basin.

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extracted silica and alumina. Aliphatic hydrocarbons were obtained by eluting with petroleum ether. An aromatic hydrocarbon fraction was obtained by elution with 1: 1 petroleum ether:DCM, and the final fraction (polar compounds) with 1: 1 DCM:methanol. 2.4. Analysis

of aliphatic hydrocarbons

Capillary gas chromatography was performed using a Carlo Erba Mega Series 5160, fitted with an OV-1 fused silica capillary column (30 m X 0.32 mm id.; 0.25 km film thickness) and an on-column injector. Analyses were performed using hydrogen as the carrier gas, with an oven temperature of 50°C for 2 min then ramped up at 4”C/min to a final temperature of 3OO“C, which was held for 20 min. Data were acquired and processed using a VG Multichrom laboratory data system. Subsequent analyses by gas chromatography-mass spectrometry (GC-MS) were performed using a Hewlett Packard 5890 gas chromatograph fitted with an HP-5 fused silica capillary column (25 m X 0.2 mm i.d.; 0.11 p_m film thickness> and linked to a Hewlett Packard mass selective detector (electron energy 70 eV; filament current 220 p,A; source temperature 220°C). Helium was used as carrier gas. The initial oven temperature was 5O”C, ramped to 300°C at 6”C/min, and held isothermally at this temperature for 20 min. Compounds were identified by comparing their mass spectra (or ion responses) and relative retention times with either those of reference compounds or with literature data. For quantification of GC-MS data, a deuterated sterane standard, (20R)-5o, 14o, 17a(H)-[2,2,4,4D,]-cholestane, was added to the aliphatic hydrocar bon fractions (molecular ion m/z 376 and a base peak of m/z 221). No correction was made for the differences in mass spectral response of various hydrocarbons relative to the internal standard. 2.5. Carbon isotopes S13C ratios of organic matter were measured from carbon dioxide gas generated by the combustion of decarbqnated samples. An aliquot (1 to 5 g> of bulk powdered sample, was treated with 1 N hydrochloric acid solution for 24 to 36 h, filtered with precombusted glass fibre filters (Whatman GF/C) and dried. The carbon dioxide from the organic fraction was

Geology

I30 (1996) 271-288

obtained following the original method described by Craig ( 1957) and modified by Hollander (1989). The C-isotope composition of the obtained pure carbon dioxide gas was measured on a triple collector VG Micromass 903 Mass Spectrometer (ETH, Zurich), using NBS-22 Hydrocarbon Oil Standard (613C = -29.63%0), and the 613C values are reported as per mil (o/00)relative to the PDB isotopic standard. Precision of the isotopic analyses (+0.20%0) was calculated using values obtained from duplicate analyses of samples.

3. Results and discussion 3.1. Organic-carbon

isotopic variation

In this study, isotopic-composition measurements made on inorganic carbon in the samples of the La Luna Formation, indicate diagenetic alteration, particularly by secondary carbonate precipitation in the sulphate reduction zone, giving considerable scatter and unreliable negative values. Similar results have been reported by Schlanger et al. (1987) for the Cenomanian-Turonian Bridge Creek Limestone at Rock Canyon, Colorado. These authors associated the scatter in the isotopic results of biogenic carbonate to cyclically varying amounts of organic matter in the section which caused diagenetic alteration of the primary signal of the skeletal calcite. Therefore, only organic carbon isotopic measurements made on isolated kerogen residues are considered in this study of the La Luna Formation. These S13Corg values display a marked positive excursion from between - 27.6 to - 28.0%0 and - 25.0 to - 25.5%0 in the 55 to 92 m interval of the section (Fig. 2). Visual kerogen analysis and biomarker composition (see later) show no significant changes at the boundaries of this interval, indicating that the isotopic excursion cannot be directly correlated to a change in organic matter composition. In sediments of similar age from Deep Sea Drilling Project sites and in several onshore sections in high latitudes, a similar isotopic shift occurs, often accompanied by a lithological change, an increase in the amount of TOC (e.g. Schlanger et al., 1987; Farrimond et al., 1990) and in the hydrocarbon generating potential of kerogen (HI). However, in the La Luna Formation the isotopic

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excursion is not associated with any marked change in lithology, TOC or HI, and it occurs within a thicker interval with good hydrocarbon source rock potential (Fig. 2), suggesting that dysoxic-anoxic bottom waters were already locally established in the basin. Fluctuations in TOC and carbonate contents within the interval of the isotopic excursion (Fig. 2) indicate that local palaeoenvironmental conditions changed. However, isotopic values show no apparent correlation with these changes. Bulk parameters such as TOC and carbonate contents are directly influenced by intra-basin processes affecting productivity and preservation of biogenic carbonate, opal and organic matter (Hut, 1988; Morse and Mackenzie, 1990; Ricken and Eder, 1991). Local biogenic productivity can vary according to the relative proximity of a specific site to upwelling systems. Modem coastal upwelling areas have a ‘core region’ of very high productivity and high nutrient abundance that is commonly lo-20 km wide (Hilbrecht et al., 1992). Within these zones, the plankton is strongly dominated by siliceous organisms (Bremner, 1983; Suess and Thiede, 1983; Molina Cruz, 1984). With increasing distance from this region the carbonate-producing organisms become more abundant, and their

Geology 130 (1996) 271-288

275

remains contribute increasingly to the sediment (Hilbrecht et al., 1992). On the other hand, total organic matter accumulated in sediments may not only be controlled by the primary productivity of the overlying water column but also by local redox conditions. In an extensive review of the literature related to the ‘productivity versus preservation debate’, Tyson (1995) pointed out that “in detail there is rather poor spatial correlation between areas of high marine productivity and areas of organic-rich sediment deposition ( . . . ) due to the combined influence of other key factors including water depth, bottom water oxygenation, sediment grain size, down slope redeposition and dilution and auto dilution effects.” The lateral and stratigraphic heterogeneity of the La Luna Formation (e.g. Macellari and De Vries, 1987; Martinez and Hemandez, 1992) suggests the existence of temporal and area1 variations in nutrient-rich upwelling currents during deposition. However, primary productivity may have had only a minor influence on the final TOC signal in the site considered in this study since this parameter does not change markedly in those intervals where an increase in primary productivity can be expected (i.e marlchert alternations of Unit 2). We interpret that TOC in the La Luna Formation was mainly controlled by

Unit

-

.

5 -

. . .

4

??

??

3 -

. .

??? ?

a

.

-

.

8 8 . . L=

2

.

-

.O .

1

. -

0 -28

-25

0

-24

%CaCQ

%TOC Carbonate

free

H.I.

I

I

5

10

q

15

%SiOr/O/.Alz~

Fig. 2. S’3C(org) = carbon isotopic composition of organic matter (%o PDB); % carbonates (as CaCO,); % TOC (expressed on a carbonate-free basis); H.I. = hydrogen index (from pyrolysis Rock-Eval, mg HC/g TOC); silica/alumina data of the La Luna Formation at Quebrada Maraca. Lithological units are those proposed in this work. (Only values for marls and shales are plotted.)

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local environmental factors related to the preservation/degradation of organic matter. In contrast, the similarity of the La Luna Formation organic-carbon isotopic excursion at Maraca Ravine with other sections of North America, Africa and Europe suggest that these isotopic records reflect variation in the global carbon cycle which responds to widespread geochemical changes in the hydrosphere and atmosphere around the Cenomanian-Turonian Boundary (Arthur et al., 1985; Hayes et al., 1989; Popp et al., 1989). The timing and cause of the Cenomanian/ Turonian isotopic event and its relationship with microfossil assemblages has been a subject of discussion for many years. Bralower (1988; U.S. Westem Interior Basin) and Hilbrecht and Hoefs (1986; NW Germany) found differences in the dating of the isotopic excursion based on nannofossils, suggesting that it was diachronous even on an intra-basin scale. However, more recently, Gale et al. (1993) have shown convincing evidence that the timing and structure of carbon-isotope curves of expanded Cenomanian-Turonian boundary sections in England (Eastboume, Sussex) and North America (Pueblo, Colorado) are identical with reference to first appearances and disappearances of macroand micropalaeontological markers. Foraminifera and macrofossil assemblages appear to be strongly influenced by palaeoenvironmental factors, and it seems likely that the fauna1 changes observed in the La Luna Formation were to some extent controlled by the local development of environmental conditions that allowed the accumulation of organic matter before the CTBE. Gale et al. (1993) have pointed out that not all taxa of the Cenomanian-Turonian interval have identical ranges at Pueblo (Colorado) and in Europe. However, a detailed correlation of the C/T isotopic event to planktonic foraminiferal zonation has been proposed by Kuhnt et al. (1990) for sections (e.g. Tarfaya coastal basin, Morocco) deposited in similar palaeolatitudes (15”N) and water depths (200-300 m) to the La Luna Formation. They correlated the onset of the isotopic shift in the Tarfaya to the top of the R. cushmani biozone by the presence of important ‘events’ that are also clearly recognisable at the base of the isotopic excursion in the La Luna section studied here (Fig. 3). These events include a marked

Geology

130 (1996)

271-288

increase in radiolarian abundance, the absence of Rotalipora cushmani and the occurrence of heterohelicid/hedbergellid-dominated planktonic foraminiferal assemblages. The position of this boundary between the R. cushmani and W. archaeocretaceu biozones in the La Luna Formation (55 m) is additionally supported by a significant change in microfossil assemblage, including the first appearance of Whiteinella balh’ca and Heterohelix reussi. The latter species is considered a morphotype of H. globulosa that, according to Nederbragt (199 1), records the Cenomanian/Turonian boundary fauna1 turnover. However, the placing of the CenomanianTuronian boundary depends upon which fossil groups are applied. The datums considered here are based mostly on appearances and disappearances of specific foraminifera. The biostratigraphy of Gale et al. (1993) largely employs macrofossils, and a comparison of our C-isotope curve with theirs indicates that the Cenomanian-Turonian boundary in the La Luna Formation may actually be somewhat higher in the section, nearer the top of Unit 3 (cf. Fig. 3). Unfortunately, our sampling resolution over this part of the section does not allow us to identify the detailed C-isotope curve characteristics which appear laterally correlatable (Gale et al., 1993; Jenkyns et al., 1994). Future macro- and micropalaeontological studies and more detailed isotope stratigraphy of the section need to be performed to support or correct the interpretation proposed here. At the top of the section, 6’“C values again shift gradually towards heavier values (Fig. 2), possibly corresponding to a third anoxic event of the Cretaceous (OAE-3; Coniacian-Santonian period; Jenkyns, 1980; Arthur et al., 1990). This isotopic shift at the top of the La Luna Formation is consistent with the C-isotope curves for the Coniacian of the English Chalk and the Coniacian/Santonian of the Italian Scaglia in the Bottaccione Gorge, Gubbio (Jenkyns et al., 1994). 3.2. Lithostratigraphic changes

variation

and

sea-level

In the La Luna Formation, two major scales of lithological variation can be distinguished both in the field and from geochemical data. Marl-limestone and marl-chert alternations are easily recognisable

J. Perez-Infanie et al./Chemical

?trE in ctic -

271

Planktonic foraminifera zonation

Unit 5

140

Geology 130 (1996) 271-288

-

120

4

Foraminifera relative abundance increase *

........ .......... ........... ............ ............ ............. ............ ............. ............ ............. D.ccmcavata 5 ............ ............ ............. ............ ............. ............ ............. ............ ............ ............. ............ ............ ............ ............. ............ f ............. D. primtiim 4 ............ 3 ............. ............ ............. ............ -----_ ......................

............ g-z-- ............. ............ ............. ............

n.y.O.

-

08

-

89

-

91

-

92

............

3

100

I

H. helvetica

3

80 -

W. atchaeocretacea 2

...................................... ............ ............. ............ ............ ........... ........... ......... ......... ........ ........ ...... i

60 2

- 92.:

40

R. cushmani 20

-

Fig. 3. Chronostratigraphic framework and biostratigraphic correlation used. ’Time scale of Haq et al. (1987). ’ Defined by first 1991); strong increase of radiolarian abundance, appearance (F.A.) of Whiteinella baltica along with Heterohelix reussi (Ncderbragt, positive 613C (organic) excursion, assemblages dominated by heterohelicid/hedbergellid, absence of Rotalipora cushmani (Kuhnt et al., 1990); and decrease of planktonic foraminifera abundance around the C/T boundary (Kauffman, 1984). 3 Lower boundary defined by F.A. of Marginotruncana coronata. 4 Lower boundary defined by F.A. of Dicarinella primitiva. ’ Lower boundary defined by F.A. of Dicarinella concavata.

on a bed to bed scale (commonly < 1 m). Considering the average sedimentation rates suggested for the La Luna Formation (Martinez and Hemandez, 1992; and the present study; Fig. 4), the duration of such bed-scale couplets in the La Luna Formation corresponds to 10 to 50 Ka, apparently within the Milankovitch frequency band. The bed-scale variations are superimposed onto records of longer-term processes which affected the basin; it is these longer-term variations which are addressed in this paper. The La Luna Formation at Quebrada Maraca has

been subdivided into five informal units in the present study, based upon lithological changes (Table 1, Figs. 4 and 5): Unit I (the lowermost unit) is the most calcareous, consisting of limestone layers (0.3 to 1 m thick) interbedded with approximately 30% of thinner laminated marls containing ellipsoidal carbonate concretions. As in the rest of the section studied, the carbonate fraction is composed mainly of planktonic foraminifera and calcareous nannoplankton. Unit 2 consists mainly of alternating layers of

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Chemicd

Geology

130 (1996)

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t3.M. Accumulatiof 1 Sedimentation rate (cm/l OOOy)2 rate (g/m2.y)3

Unit

dion 5

n.d.

n.d

4

1.5 (1.3)

1.1

3

1.7 (1.5)

1.3

2

5.5 (5.2)

5.1

1

n.d

n.d

--_140

120-

lOO-

ao-

60-

40-

20

m

-

Shale

pzJ

Marlstone

lzz

i%z”.a

El

Chetimarl alternation

Fig. 4. Sedimentation and organic matter accumulation rates for the units of the La Luna Formation at Quebrada Maraca. Time zonation is proposed on the basis of planktonic forammifera assemblages, scdimentological features and carbon isotope stratigraphy (see Fig. 3). ’ The Haq et al. (1987) time scale has been used to calculate sedimentation rates. Differences of average compaction between units were considered negligible. 3 The following formula has been used: O.M. AC = 0.1 X TOC X SR X D. D = 2.3 g/cm3 (average dry density estimated for fine-grained, organic-rich rocks; Kuhnt et al., 1990).

well-laminated marls bearing abundant radiolarian tests and thin bands of chert (up to 15 cm; approximately 15% of the unit). Unit 3 lacks chert bands and the Si/Al ratio drops to a minimum (Fig. 2). This unit also has the lowest carbonate contents and the highest occurrence of shale beds in the section (approx. 15% of this unit). Unit 4 sees an increase in carbonate content with an increase in the number of limestone beds. Rare thin arenaceous layers with abundant bivalve shells

are found in this unit, probably formed by storm action. Unit 5 displays a gradual decrease in carbonate content, and a few thin chert bands ( < 2 cm) are observed. These changes in lithology appear to reflect the local response to eustatic sea-level variations and changes in the productivity of biogenic silica, probably controlled by variations in the relative proximity of an upwelling belt to the site studied. Whilst local subsidence (in combination with sediment accumula-

219

J. Perez-lnfanre et al./ Chemical Geology 130 (1996) 271-288

Table 1 Average study

values (mean f standard

deviation)

of bulk geochemical

parameters

Unit

Metres

n

% CaCO,

% TOC

1 2 3 4 5

O-42 42-70 70-91 91-140 140- 157

18 25 14 19 14

78.4 50.3 31.3 70.1 49.2

2.84 3.98 3.29 3.02 3.75

+ + 5 + *

12.3 24.5 23.0 17.2 19.0

+ f + f f

%TOC 1.31 1.84 1.29 1.53 1.86

n = number of samples in each unit. Organic carbon content is additionally index.

tion rate) could be an additional influence, Macellari (1988) and Erikson and Pindell (1993) argue for little regional tectonic activity in northern South

Unit 5 100

.

80

Unit 3 .

80 60

Unit 2

100 80 II

.

.

Unit 1

% TOC Fig. 5. Plots of % TOC against % carbonates (as CaCO,) units of the La Luna Formation at Quebrada Maraca.

for the

for the units of the La Luna Formation

13.96 9.59 5.94 10.89 8.07

4.17 4.40 3.99 4.32 3.81

expressed on a carbonate-free

in this

H.I.

* f * + + f

differentiated

438 F 6 44Ok-3 439,l 439 f 2 438 * 2

191 214 210 274 338

+ 61 rf: 62 + 33 + 83 * 93

basis (% TOC ’ ). HI. = hydrogen

America at this time. The middle Cenomanian-early Turonian comprises a transgressive period (Haq et al., 1987), followed by high-stand deposits corresponding to the peak of maximum eustatic rise (91.5-90.75 m.y. ago). Late Turonian to Santonian times are characterised by a succession of transgressive and low-stand wedge systems tracts within a long-term regressive period (Haq et al., 1987). The following discussion examines geochemical and micropalaeontological records of sea-level changes in the Maracaibo Basin, affecting the lithofacies pattern of the La Luna Formation. All the facies changes described above are consistent with a pelagic belt in a distal pericratonic basin as suggested for northwestern South America during the Late Cretaceous by several authors (Zambrano et al., 1971; Macellari, 1988; Martinez and Hemandez, 1992). Except for Unit 3, the La Luna Formation is low in siliciclastic components (screening tests of XRD analyses indicate clay content lower than 5% in marl samples), supporting the hypothesis of very distant sources of elastic sediment. Therefore, we consider a low and fairly steady supply of detrital input, with local changes in production and preservation of carbonate and opal, as being the most important factors controlling sedimentation of the La Luna Formation. Carbonate sedimentation is strongly affected by water depth, reaching its highest rates in clear, warm and shallow waters. Consequently, the mass of biogenie carbonate grows most rapidly along preferred positions on the upper part of any seaward slope, within the zone of maximum biological productivity (see for example Wilson, 1975, and Morse and Mackenzie, 1990). At the site studied, the La Luna

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et al./ Chemical Geology 130 (1996) 271-288

Fm. overlies platform deposits predominantly composed of shallow-marine carbonates (Cogollo Group). The uppermost part of these shallow deposits (Maraca Formation) consists of coarse bioclastic grainstones with large, partly leached bivalves and echinoderm debris that were deposited during rapid deepening of a shallow platform environment (Vahrenkamp et al., 1993). Considering that elastic dilution by runoff was locally minimal (since positive areas were distant) during the Late Cretaceous, we interpret the units of La Luna with higher carbonate content (Units 1 and 4) to have been deposited on shallow outer-shelf/slope areas, while the carbonate-poor Units (2 and 3) represent deeper environments and maximum sea-level periods. Although changes in nutrient supply can bring about profound changes in sedimentation rates independently of sea level (Gawthorpe et al., 1994), foraminifera and radiolarian assemblages (see later), and the correlation of the units with global eustatic curves (Haq et al., 1987) further support this interpretation. In the deeper-water units (2 and 3), foraminiferal shells are commonly well preserved, suggesting that the water depth never became sufficiently great to cause extensive dissolution of these organisms. (Berger (19701, based on suspended tests of foraminifera from the East Pacific Rise, found slight dissolution of the foraminifera from 300 m.) In general, the lithofacies of the La Luna Formation remarkably resemble coeval pelagic sequences of the North African Margin (e.g. Tarfaya coastal basin, Morocco; Einsele and Wiedmann, 1982) where maximum palaeowater-depths of 300 m have been reported by Kuhnt et al. (1990). As carbonate dissolution is unlikely in the studied section, the lower carbonate contents of Units 2 and 5 suggest a relative decrease in primary carbonate productivity and/or dilution by a relative increase in biogenic silica production. Opal bioproductivity is strongly controlled by the availability of nutrients, and obviously, of dissolved silica in marine waters. In Unit 2, radiolarian tests and chert bands along with small-sized foraminifera (Het&ohelix, Globotruncana), high fauna1 density of Hedbergella and scarcity of PraeSP* and Globigerinoides globotruncana, strongly suggest the presence of nutrient-rich upwelling currents (Einsele and Wiedmann, 1982). Although the occurrence of a vigorous

upwelling system in the area could have increased both carbonate and biogenic silica productivity, the palaeolocation of the maximum carbonate and opal productivity belts need not have coincided since radio&a and calcareous planktonic organisms appear to be highly tuned to specific oceanographic environments (see Welling et al., 1992). Spumellarian and Nassellarian radiolarians, present in Unit 2, suggest open-water conditions with a peak in abundance in middle-outer shelf or deeper upper-slope environments (Koutsoukos and Hart, 1990). In Unit 3, the absence of chert bands, preserved radiolarian tests and the lowest Si/Al ratios of the section (< 5; Fig. 2) shown for some beds of this unit, suggest that the inferred belt of high opal bioproductivity had shifted away before its deposition. A striking decrease in the density of planktonic foraminifera and the occurrence of dwarfed specimens of unkeeled foraminifera (Herdbergella and Heterohelix) in Unit 3 (cf. Ford and Houbolt, 19631, together with the fact that this unit shows the lowest average carbonate content of the section (31.3%; Table l>, indicates that calcareous productivity also decreased. The linear sedimentation rates for the La Luna Formation units suggested in this study (Fig. 4) must be considered as rough estimates since no correction has been made for compaction of the sediments. However, assuming similar average compaction factors for the different units, the linear sedimentation rate appears to decrease by a factor of 3 between Units 2 and 3. Whilst a migrating upwelling system (moving in response to sea-level changes) could explain the changes in productivity, an alternative explanation could be the possible influence of an expanded oxygen-minimum zone upon shallower parts of the water column, affecting the development of intermediate planktic foraminifera (Martinez and Hernandez, 1992) and radiolaria. Whatever its origin, the observed decrease in planktonic productivity during the accumulation of Unit 3 would decrease the sedimentation rate, resulting in the deposition of sediments passively enriched with clays (Fig. 2); such sediments are recognised as characteristics of maximum-flooding black shales (Wignall and Maynard, 1993). In Units 4 and 5, the facies patterns of Units 1 and 2 return, and may be associated with a rapid

J. Perez-lnfante et al./Chemical

regressive pulse during the middle Turonian (Haq et al., 19871, and the restoration of pelagic sedimentation on the outer platform. Normal-sized foraminifera are again observed, along with fragments of small bivalves, particularly Znocerumus. The upper part of Unit 5 may also then represent the onset of a new transgressive phase. The presence of an upwelling system (less vigourous than that of Unit 2) is suggested by a similar foraminiferal assemblage and the occurrence of a few chert bands. 3.3. Organic Formation

matter

enrichment

in the La Luna

Although the La Luna Fm. displays area1 and stratigraphic changes in lithology throughout the Maracaibo Basin, its organic matter content remains relatively high both in exposed sections at the edges of the basin (Perija Foothills and northern flank of the Venezuelan Andes) and in cores from wells drilled all over the basin (Gonzalez de Juana et al., 1980; Talukdar et al., 1985). Contemporary organicrich facies are also found in eastern Colombia (La Luna and Villeta Formations; Zumberge, 1984) and eastern Venezuela (Guayuta Group; Talukdar et al., 1985) suggesting regionally widespread anoxic bottom waters during the Late Cretaceous in northwestem South America. At Quebrada Maraca, the conformable contact between the La Luna Formation and the underlying, shelly organic-lean limestones of the Maraca Formation, displays a striking increase in organic-matter content, suggesting a rapid decrease in oxicity. Total organic carbon (TOC) contents of the marl samples vary from 1.50% to 6.85%, but the hydrocarbon-generating potential (HI) shows less variation (Fig. 2). Although maturation of organic matter greatly contributes to diminish original fluctuations in HI, the data are consistent with relatively constant dysaerobit to anoxic conditions throughout most of La Luna’s sedimentation. This interpretation is further supported by the near absence of benthic foraminifera, with only a few occurrences towards the top of the formation. Fluctuations in TOC can be produced by the combined effect of the flux of organic matter from the water column, its degree of preservation in sediments, and dilution by changes in the accumulation

Geology 130 (1996) 271-288

281

of inorganic material (biogenic and elastic). Although average TOC values are similar for all units of the La Luna Fm. (Table 11, plots of carbonate content versus organic matter distinguish between the units (Fig. 5). Unit 1 and (to a lesser extent) Unit 4 show inverse carbonate-organic carbon relationships typical of deposition mainly dominated by changes in the influx of carbonate components (Ricken, 1994). Units 2 and 5 show more complex patterns due to greater dilution by biogenic silica in relatively deeper environments. Unit 3 shows a pattern largely independent of the carbonate concentration, suggesting (as discussed earlier) that contribution of biogenic siliceous and calcareous components remained low during the deposition of this unit. It is interesting to note that this apparent change in bulk geochemistry, probably related to palaeoproductivity, occurs within the positive carbon-isotope excursion. Despite variations in lithology, microscopic analysis of kerogens from the studied section show little variation in the proportion of structured and amorphous organic matter. All the samples examined are ,dominated by amorphous kerogen (95% or more> which has been widely ascribed to marine planktic and/or bacterial material (Tissot and Welte, 1984). There is no palynological evidence for fluctuations in the relative proportions of marine and terrestrial organic input during the sedimentation of the La Luna Formation.

3.4. Biomarker

composition

Signals preserved in the distributions of biomarker compounds in a heterogeneous section such as the La Luna Formation are often sensitive to both environmental (source of organic matter, ecological assemblages, redox conditions, salinity) and diagenetic factors (maturation, mineral matrix catalysis). In order to minimise the influence of lithological variation on the organic matter in the section studied, only marl samples will be considered here. Furthermore, organic matter maturity differences are negligible between the top and the bottom of the section; neither T,,, values (440-445°C) nor vitrinite (very scarce particles; reflectance 0.7-0.8% R,) display any gradient. Therefore, variations in the biomarker data are considered to be largely dependent upon

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changes in the primary input of organic matter and other palaeoenvironmental factors. As noted earlier, and reported for other sections of the La Luna Formation in the northwest of the Maracaibo Basin (Talukdar et al., 1985), molecular geochemistry and microscopic analyses indicate that the bulk of the organic matter is algal and bacterial in origin. Saturated hydrocarbons (between 20% and 30% of EOM) display little apparent molecular variation through the section. Normal and branched alkanes show a dominance of medium molecular weight components (with a maximum around n-C,,) and the pristane/phytane ratio is always lower than 0.8. (Fig. 6). The most abundant cyclic compounds

25

Ill 19

~

Retention time -*

Fig. 6. Typical gas chromatograms of the La Luna Formation at Quebrada Maraca. Pr = pristane; Ph = phytane; selected n-alkanes are labelled with their carbon numbers.

Table 2 Range of concentration (ppm of EOM) for selected groups in the La Luna Formation UP Hopanes (C,,-C,,) aI3 Hopanes CC,, -C,J Total hopanes Tricyclics (C ,9 -C,,) Tricyclics (C,,-C,,) Total tricyclics cxp@ Steranes CC,, -C,,)

compound

240-730 145-725 385-1450 180-315 330-1340 50.5-1880 120-360

throughout the section are tricyclic terpanes and hopanes, whilst steranes are relatively low (Table 2). The sterane distributions (C2,-C2s) display very little variation, being always dominated by C,, steranes, consistent with a marine source of organic matter (Volkman, 1988), and suggesting a relatively constant biotic input. Principal component analysis (PCA) of biomarker data from GC-MS analyses was used to aid interpretation of molecular differences between samples. This type of statistical analysis allows a complex multivariate data set to be simplified by identifying the covariance of individual variables (biomarkers) and discriminating or grouping samples on the basis of the direction and amplitude to which the distributions of the variables deviate from that of the average (see Davis, 1986). This method allows a large data set to be expressed in terms of a limited number of components (PCl, PC2, PC3, etc.) which are linear combinations of the individual variables. In this study, GC-MS peak areas of tricyclic terpanes, hopanes and steranes (Table 3) were used for the analysis, making a total of 42 variables for each of the 27 samples selected. Biomarkers were only selected if they could be accurately integrated for each sample and suffered no co-elution problems. The original raw data were normalised (to make each sample the same ‘size’), autoscaled (i.e. dividing each variable by its standard deviation, in order to give them all comparable weight in the analysis) and centred prior to PCA. A ‘loadings plot’ can be used to show the relationships between variables (biomarkers) and the degree to which each variable contributes to the PC’s (Fig. 7). The first and second principal components account for 70% of the total variance within the

J. Perez-lnfante

et al./Chemical

scaled data set so they are the only ones discussed here. Long-chain tricyclic terpanes (> C,,) display high positive loadings on PCl, identifying a strong importance of these compounds which vary largely independently from the other biomarkers considered (< C,, tricyclics, hopanes and steranes). In confirmation, concentrations of long-chain tricyclic terpanes (Fig. 8) closely follow the trend of PC1 variation through the section, with an important increase in absolute abundance between 35 and 110 m of the section. Tricyclic terpanes containing up to at least 35 carbon atoms are clearly observed within Units 2, 3 and 4 (Fig. 9). Peters and Moldowan (1993) have proposed that tricyclic terpanes may be source indicators relating to a group of bacterial or algal lipids, but their specific origin is still poorly understood. Several authors (Volkman et al., 1989; Simoneit et al., 1990; Aquino Neto et al., 1992; McCaffrey et al., 1994) have pointed out that at least one source of

0.25

Geology 130 (1996) 271-288

283

tricyclic terpane@Fs may be Tasmanites, a genus of extinct unicellular green algae, frequently associated with anoxic black shales (Tyson, 1995). Specifically, extended tricyclic terpane hydrocarbons have been identified as the major biomarkers in the bitumen of the Tasmanian tasmanite (Simoneit et al., 1990). However, remains of Tasmanites were not observed in the samples studied of the La Luna Formation. Furthermore, in the La Luna Formation the increase in abundance of tricyclic terpanes ( > Cz6) is not correlated to marked changes in any potentially redox-sensitive parameters (i.e. HI, pristane/phytane). Hopanes and steranes are important variables in PC2 (Fig. 7). The observed depth trend of PC2 essentially records fluctuations in steranes vs. hopanes, a parameter commonly associated with changes in organic matter supply (e.g. Farrimond et al., 1994). With the exception of the sample at the bottom of the section which displays a relatively

T PC2 C2s-C2s Tricyclics

023 24.26,27 b 8262936 e&t

C&II&

Tricyclics

025

035 I

($1 -C23 Tricyclics

05

C&-C=

Hopanes

013

/

Hopanes

Fig. 7. Loadings plot from the principal two principal components.

components

analysis,

showing the relationship

between the biomarker

variables

in terms of the first

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et al./Chemicd

Geology 130 (1996) 271-288

greater content of steranes, the sterane/hopane ratio displays a smooth trend throughout the section. This is consistent with other geochemical results of this study (Rock-Eva1 pyrolysis, GC analyses and optical microscopy) that show no evidence of changes in the proportion of terrestrial/marine organic matter supply during La Luna’s sedimentation. The abundance of 28,30-bisnorhopane (BNH) also shows a clear trend through the section, similar to that shown by the long-chain tricyclic terpanes (Fig. 8). After a marked increase, the absolute concentration of BNH falls from a maximum of 70 ppm (EOM) at the top of Unit 1 and the lower part of Unit 2 to 10 ppm of the EOM in the upper part of Unit 4. The origin and significance of this compound remain unclear, although kerogen pyrolysis studies indicate that it occurs in sediments as the free hydrocarbon and is not present within the kerogen (Noble et al., 1984). High abundances of BNH have been associated with severely oxygen-deficient conditions (Mello et al., 1989, and references within). In the La Luna Formation maximum concentrations of BNH occur in Unit 2, where we interpret the highest productivity within the studied section, perhaps consistent with productivity-induced anoxia. Whatever its origin, the presence of BNH in the La Luna Formation clearly records changing local environmental conditions which precede, but are apparently

Tdcyclii retpanes’

PC1

not genetically associated with, the isotopic shift recorded in the organic matter. In general, none of the biomarker trends correlate with the interval of the isotopic shift in the section. Since the variation in biomarker composition is known to be largely controlled by local environmental factors (e.g. palaeowater depth, productivity, local anoxia and salinity), the lack of parity with the organic-C isotopic event is consistent with the latter being mainly independent of local conditions but rather a result of global depletion in ‘*C in the oceans in response to widespread organic matter sequestration into sediments around the Cenomanian/Turonian and Coniacian/Santonian boundaries.

4. Conclusions Oxygen-depleted sedimentation of the La Luna Formation in the western Maracaibo Basin started well before the CTBE, during the middle-late Cenomanian in an open-marine environment, with a palaeowater depth of a few hundred metres and very low elastic input. We consider this early deposition of black shales to be the local response to a combination of rapid sea-level rise and the presence of upwelling currents. Geochemical evidence suggests

Sleranesil-fopanes”

28.30 bisnorhopane

160

140 120 100

eo so 40 20

i

n

“0

loo0

pprn of EOM

-10

0

10

0.15 0.35

0.55

10

0

10

0

50

100

pprn of EOM

Fig. 8. Depth plots of the scores of the first two principal components identified from the biomarker data compared to selected molecular profiles. The PC1 plot essentially records fluctuations in relative concentrations of long-chain tricyclic terpanes, while PC2 records fluctuations in steranes vs. hopanes. The distinctive trend of absolute concentration of 28,30-bisnorhopane is also shown. ’ Css-Css. * * Steranes/hopanes ratio = Speak areas aPP steranes (C,,-C,,)/Xpeak areas oP hopanes (C,,-C,,).

J. Perez-Infante

et d/Chemical

that high carbonate and silica bioproductivity continued until the early Turonian. The CTBE is, nevertheless, recorded in the middle part of the La Luna Formation as a marked isotopic excursion. This isotopic shift is 613C appg&tly unrelated to locally controlled changes in bioproductivity and intensity of oxygen depletion. In the early Turonian, before the end of the isotopic excursion, biogenic productivity decreased

285

Geology 130 (1996) 271-288

at the site studied probably due to migration of upwelling currents as is inferred from geochemical and micropalaeontological changes. However, preservation of organic matter remained relatively high due to the expansion of the oxygen-minimum zone during the peak of the sea-level rise, as indicated from foraminiferal assemblages. Sedimentological and micropalaeontological evidence suggest that the upper part of the La Luna

7

a

15 6

b 4

6

16

JLL Fig. 9. Hopanes Table 3.

and tricyclic

terpanes (m/z

191) in typical samples

from Units 2-3 (b) and Units 4-5 (a). Peak identities

are given in

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et al./Chemical

Table 3 Biological marker compounds used in the principal components analysis (the numbers are used as peak labels in Figs. 7 and 9) Hopunes:

I. 18a(H)-nisnomeohopane 2. 17o(H)-trisnorhopane 3. 28,30-bisnorhopane 4. C,, 17o (H), 21E (H) 5. C,, 17o (H), 21B (H) 6. C,, 17~ (HI, 21p (H) 7. C,, 17o. (H), 21f3 (H) 8. C,, 17~ (H), 21p (HI 9. C,, 17a (H), 2lB (HI IO. C,, 17a (H), 2lp (H) 11. C,, 17a (H), 2lB (H) 12. C,, 171~(H), 2lp (H) 13. C 34 I7a (H), 2 1p (HI

(Ts) (Tm) hopane hopane hopane hopane hopane hopane hopane hopane hopane

(22s) (22R) (22s) (22R) (22s) (22R) (22s)

hopane (22R)

Tricyclic terpanes: 14. C ,9 13P(H),l4cx(H)

15. C,, 16. C,, 17. C,, 18. Cz4 19. C,, 20. C,, 21. C,, 22. C,, 23. C,, 24. C,, 25. C,, 26. C,, 27. C,, 28. C,, 29. C,, 30. C,, 3 1. C,, 32. C,, 33. C,, 34. C,, 35. C,,

13P(H), I4a(H) 13B(H),14a(H) 13P(H),14o(H) 13@(H), 14a(H) 13P(H),l4u(H) 13f3(H),l4cx(H) 13B(H),14a(H) 13P(H),l4o(H) 13E(H),14u(H) 13E(H),14cx(H) 13B(H),l4a(H) 13P(H), 141x(H) 13P(H),14a(H) 13P(H),14u(H) 13P(H),l4o(H) 13E(H),14a(H) 13@(H), 14o(H) 13B(H),14a(H) 13B(H),14o(H) 13B(H),14u(H) 13B(H), 141x(H)

Tetracyclic

tricyclic tricyclic tricyclic tricyclic tricyclic tricyclic tricyclic tricyclic tricyclic tricyclic tricyclic tricyclic tricyclic tricyclic tricyclic tricyclic tricyclic tricyclic tricyclic tricyclic tricyclic tricyclic

terpane terpane terpane terpane terpane terpane terpane terpane terpane terpane terpane terpane terpane terpane terpane terpane terpane terpane terpane terpane terpane terpane

(22s) (22R) (22s) (22R) (22% (22R) (22s) (22R) (22s) (22R) (22s) (22R) (22s) (22R) (22s) (22R)

terpane:

36. C,, tetracyclic

terpane

Geology

130 (1996)

271-288

Formation (middle-Turonian to Santonian) experienced a rapid regressive pulse followed by a second period of milder upwelling influence. Isotopic stratigraphy at the top of the La Luna Fm. indicates the onset of a positive carbon isotopic excursion related to OAE-3 (Coniacian-Santonian). Biological marker data appear generally similar for all samples upon initial inspection, although application of principal components analysis identified some significant variability, particularly in the amount of extended tricyclic terpanes, 28,30-bisnorhopane and the sterane/hopane ratio. However, these biomarker variations do not occur in association with the isotopic excursion and are probably controlled by local fluctuations in organic matter input and preservation. PCA identifies the abundance of extended ( > C 27) tricyclic terpanes as the major source of biomarker variability. The significance of these compounds remains unclear, but they are apparently unrelated to the shorter-chain tricyclics, and appear to have a different biological source.

Acknowledgements The authors are grateful to INTEVEP for financial support and a Ph.D. studentship (J.P.1). We also thank W. Scherer, M. Alberdi and A. Pilloud (INTEVEP) for their assistance during collection of outcrop samples. GC and GC-MS technical assistance was provided by I. Harrison and P. Donahoe, and the artwork produced by Ms. C. Jeans (NRG). D. Ariztegui, S. Bernasconi and J. McKenzie (ETH, Zurich) are thanked for their help in obtaining the stable isotopic data. We are also grateful to R. Tyson (NRG) for invaluable discussions regarding this work, and to the reviewers of the manuscript for their constructive comments. (RA)

Sterunes:

37. 38. 39. 40. 41. 42.

C,, Ci, C,, C,, C,, C,,

k(H), 14E(H), 17P(H) 5a(H), 14B(H),17P(H) ~IX(H),~~B(H),~~B(H) 5a(H),14P(H),l7P(H) 5c~(H),14B(H),l7B(H) 5a(H),l4B(H),17B(H)

sterane sterane sterane sterane sterane sterane

(20R) (20s) (20R) (20s) (20R) (20s)

References Aquino Neto, F.R., Triguis, D.A., Azevedo, D.A., Rodrigues, R. and Simoneit, B.R.T., 1992. Organic geochemistry of geographically unrelated Tamunites. Org. Geochem., 18: 791803. Arthur, M.A., Dean, W.E. and Claypool, GE., 1985. Anomalous

J. Perez-Infante et d/Chemical 13C enrichment in modem marine organic carbon. Nature (London), 315: 216-218. Arthur, M.A., Schlanger, SO. and Jenkyns H.C., 1987. The Cenomanian-Turonian Oceanic Anoxic Event, II. Paleoceanographic controls on organic matter production and preservation. In: J. Brooks and A.J. Fleet (Editors), Marine Petroleum Source Rocks. Geol. Sot. London, Spec. Publ., 26: 401-420. Arthur, M.A., Jenkyns, H.C., Brumsack, H.J. and Schlanger, S.O., 1990. Stratigraphy, geochemistry and paleoceanography of organic carbon-rich Cretaceous sequences. In: R.N. Ginsburg and B. Beaudin (Editors), Cretaceous Resources, Events and Rhythms. Kluwer, Dordrecht, pp. 75-l 19. Berger, W.H., 1970. Planktonic foraminifera: selective solution and the lysocline. Mar. Geol., 8: 111-138. Bralower, T.J., 1988. Calcareous nannofossil biostratigraphy and assemblages of the Cenomanian-Turonian boundary interval. Paleoceanography, 3: 275-3 16. Bremner, J.M., 1983. Biogenic sediments in the South West African (Namibian) continental margin. In: J. Thiede and E. Suess (Editors), Coastal Upwelling: Its Sediment Record, Part B: Sedimentary Records of Ancient Coastal Upwellin. NATO (N. Atlantic Treaty Org.) Conf. Ser. IV, lob, Plenum, New York, N.Y., pp. 73-103. Craig, H., 1957. isotopic standards r carbon and oxygen and correction factors for mass-spectrometric analysis of carbon dioxide. Geochim. Cosmochim. Acta, 12: 133-149. Davis, J.C., 1986. Statistics and Data Analysis in Geology. Wiley, New York, N.Y., 2nd ed., 646 pp. Einsele, G. and Wiedmann, J., 1982. Turonian black shales in the Moroccan coastal basins: first upwelling in the Atlantic Ocean. In: U. von Rad, K. Hinz, M. Sarntheim and E. Seibold (Editors), Geology of the Northwest African Continental Margin. Springer, Berlin, pp. 396-414. Erikson, J.P. and Pindell, J.L., 1993. Analysis of subsidence in northeastern Venezuela as a discriminator of tectonic models for northern South America. Geology, 21: 945-948. Farrimond, P., Eglinton, G., Brassell, S.C. and Jenkyns, H.C., 1990. The Cenomanian/Turonian anoxic event in Europe: an organic geochemical study. Mar. Pet. Geol., 7: 75-89. Farrimond, P., Stoddart, D.P. and Jenkyns, H.C., 1994. An organic geochemical profile of the Toarcian anoxic event in notthem Italy. Chem. Geol., 111: 17-33. Ford, A. and Houbolt, J.J., 1963. The Microfacies of the Cretaceous of Western Venezuela. Brill, Leiden, 109 pp. Gale, A.S., Jenkyns, H.C., Kennedy, W.J. and Cortield, R.M., 1993. Chemostratigraphy versus biostratigraphy: data from around the Cenomaniar-Turonian boundary. J. Geol. Sot., London, 150: 29-32. Gawthorpe, R., Hunt, D., Taylor, A. and Underhill, J., 1994. Course Notes of NERC Sequence Stratigraphy Workshop. Dep. Geol., Univ. of Manchester, Manchester. Gonzalez de Juana, C., Iturrilde de Arocena, J. and Picard, X., 1980. Geologia de Venezuela y de sus Cuencas Petroliferas. Foninves, Caracas, 103 1 pp. Hancock, J.M., 1993. Sea-level changes around the Cenomanian-Turonian boundary. Cretaceous Res., 14: 553562.

Geology 130 (1996) 271-288

287

Haq, B.U., Hardenbol, J. and Vail, P.R., 1987. Chronology of fluctuating sea levels since the Triassic. Science, 235: 11561166. Hay, W.W., 1985. Paleoceanography of the Venezuelan Basin. 1st Geol. Conf., Geol. Sot. Trinidad-Tobago, Proc., pp. 302-307. Hayes, J.M., Popp, B.N., Takigiku, R. and Johnson, M.W., 1989. An isotopic study of biogecchemical relationships between carbonates and organic carbon in the Greenhorn Formation. Geochim. Cosmochim. Acta, 53: 2961-2972. Hilbrccht, H. and Hoefs, J., 1986. Geochemical and paleontological studies of the 613C anomaly in boreal and north Tethyan Cenomanian-Turonian sediments in Germany and adjacent areas. Palaeogeogr., Palaeoclimatol., Palaeoecol., 53: 169- 189. Hilbrecht, H., Hubberten, H. and Oberhansli, H., 1992. Biogeography of planktonic foraminifera and regional carbon isotope variations: productivity and water masses in Late Cretaceous Europe. Palaeogeogr., Palaeoclimatol., Palaeoecol., 92: 407421. Hollander, D., 1989. Carbon and isotopic cycling and organic geochemistry of eutrophic Lake Greifen: Implications for preservation and accumulation of ancient organic carbon rich sediments. Ph.D. Thesis, ETH (Eidgeniissische Technische Hochschule), Zurich. Hut, A.Y., 1988. Aspects of depositional processes of organic matter in sedimentary basins. Org. Geochem., 13: 263-272. James, K.H. 1990. The Venezuelan hydrocarbon habitat. In: J. Brooks (Editor), Classic Petroleum Provinces. Geol. Sot. London, Spec. Publ., 50: 9-35. Jenkyns, H.C., 1980. Cretaceous anoxic events: from continents to oceans. J. Geol. Sot. London, 137: 171-88. Jenkyns, H.C., Gale, A.S. and Corfield, R.M., 1994. Carbon- and oxygen-isotope stratigraphy of the English Chalk and Italian Scaglia and its palaeoclimatic significance. Geol. Mag., 131: l-34. Kauffman, E.G., 1984. The fabric of Cretaceous marine extinctions. In: W.A. Berggren and J.A. Van Couvering (Editors), Catastrophes and Earth History: the New Uniformitarianism. Princeton University Press, Princeton, N.J., pp. 151-246. Koutsoukos, E.A.M. and Hart, M.B., 1990. Radiolarians and diatoms from the mid-Cretaceous successions of the Sergipe Basin, northeastern Brazil: palaeoceanographic assessment. J. Micropalaeontol., 9: 45-64. Kuhnt, W., Herbin, J.P., Thurow, J. and Wicdmann, J., 1990. Distribution of Cenomanian-Turonian organic facies in the Western Mediterranean and along the adjacent Atlantic margin. In: A.Y. Hut (Editor), Deposition of Organic facies. Am. Assoc. Pet. Geol. Stud. Geol., 30: 133-160. Macellati, C.E. 1988. Cretaceous paleogeography and depositional cycles of western South America. J. S. Am. Earth Sci., 1: 373-418. Macellari, C.E. and De Vries, T.J., 1987. Late Cretaceous upwelling and anoxic sedimentation in northwestern South America. Palaeogeogr., Palaeoclimatol., Palaeoecol., 59: 279292. Martinez, J.I. and Hemandez, R., 1992. Evolution and drowning of the Late Cretaceous Venezuelan carbonate platform. J. S. Am. Earth Sci., 5: 197-210.

288

.I. Perez-infante

et ul./Chemicul

McCaffrey, M.A., Simoneit, B.R.T., Aquino Neto, F.R. and Moldowan, J.M., 1994. Functionalized biological precursors of tricyclic terpanes: information from sulfur-bound biomarkers in a Permian Tasmanite. Org. Geochem., 21: 481-488. Mello, M.R., Koutsoukos, E.A.M., Hart, M.B., Brassell, SC. and Maxwell, J.R., 1989. Late Cretaceous anoxic events in the Brazilian continental margin. Org. Geochem., 14: 529-542. Molina Cruz, A., 1984. Radiolaria as indicators of upwelling processes: the Peruvian connection. Mar. Micropaleontol., 9: 53-75. Morse, J.W. and Mackenzie, F.T., 1990. Geochemistry of Sedimentary Carbonates. Elsevier, Amsterdam, 707 pp. Nederbragt, A., 1991. Late Cretaceous biostratigraphy and development of Heterohelicidae (planktic foraminifera). Micropaleontology, 37: 329-372. Noble, R., Alexander, R. and Kagi, RI., 1984. The Occurrence of bisnorhopane, trisnorhopane and 25.norhopanes as free hydrocarbons in some Australian shales. Org. Geochem., 8: 171~ 176. Peters, K.E. and Moldowan, J.M., 1993. The Biomarker Guide: Interpreting Molecular Fossils in Petroleum and Ancient Sediments. Prentice Hall, London, 346 pp. Popp, B.N., Takigiku, R., Hayes, J.M., Louda, J.W. and Baker, E.W., 1989. The post-Paleozoic chronology and mechanism of “C depletion in primary marine organic matter. Am. J. Sci., 289: 436-454. Ricken, W., 1994. Complex rhythmic sedimentation related to third-order sea level variations. In: P.L. Boer and D.G. Smith (Editors), Orbital Forcing and Cyclic Sequences. Spec. Publ. hit. Assoc. Sedimentol., 19: 167-193. Ricken, W. and Eder, W., 1991. Diagenetic modification of calcareous beds - an overview. In: G. Einsele, W. Ricken and A. Seilacher (Editors), Cycles and Events in Stratigraphy. Springer, Berlin, pp. 430-449. Schlanger, S.O. and Jenkyns, H.C., 1976. Cretaceous anoxic events: causes and consequences. Geol. Mijnbouw, 55: 1799 184. Schlanger, S.O., Arthur, M.A., Jenkyns, H.C. and Scholle, P.A., 1987. The Cenomaniat-Turonian oceanic anoxic event, 1. Stratigraphy and distribution of organic-rich beds and the marine 813C excursion. In: J. Brooks and J. Fleet (Editors), Marine Petroleum Source Rocks. Geol. Sot. London, Spec. PubI., 26: 371-399. Scholle, P.A. and Arthur, M.A., 1980. Carbon isotope fluctuations in Cretaceous pelagic limestones: potential stratigraphic and petroleum exploration tool. Am. Assoc. Pet. Geol. Bull., 64: 67-87. Simoneit, B.R.T., Leif, R.N., Aquino Neto, F.R., Azevedo, D.A., Pinto, A.C. and Albrecht, P., 1990. On the presence of trcyclic terpane hydrocarbons in Permian tasmanite algae. Naturwissenschaften, 77: 380-383. Suess, E. and Thiede, J., 1983. Introduction. In: E. Suess and J. Thiede (Editors), Coastal Upwelling: Its Sedimentary Record,

Geology I30 (1996) 271-288

Part A: Responses of the Sedimentary regime to present Coastal Upwelling. NATO (N. Atlantic Treaty Org.) Conf. Ser., IV, IOa, Plenum, New York, N.Y., pp. l-10. Talukdar, S., Gallango, 0. and Chin-A-Lien, M., 1985. Generation and migration of hydrocarbons in the Maracaibo Basin, Venezuela: an integrated basin study. Org. Geochem., 10: 261-279. Tissot, B.P. and Welte, D.H.. 1984. Petroleum Formation and Occurrence. Springer, Berlin, 699 pp. Tribovillard, N.P., Stephan, J.F., Manivit, H., Reyre, Y., Cotillon, P. and Jautee, E., 1991. Cretaceous black shales of Venezuelan Andes: preliminary results on stratigraphy and paleoenvironmental interpretations. Palaeogeogr., PaIaeocIimatol., Palaeoecol., 81: 313-321. Tyson, R.V., 1995. Sedimentary Organic Matter. Chapman and Hall, London, 615 pp. Vahrenkamp, V.C., Franssen, R.C., Grotsch, J. and Muiioz, P.J., 1993. Maracaibo Platform (Aptian-Albian), Northwestern Venezuela. In: J.A. Toni, R. Scott and J.P. Masse (Editors), Cretaceous Carbonate Platforms. Am. Assoc. Pet. Geol. Mem., 56: 25-33. Volkman, J.K., 1988. Biological marker compounds as indicators of the depositional environments of petroleum source rocks. In: A.J. Fleet, K. Kelts and M.R. Talbot (Editors), Lacustrine Petroleum Source Rocks. Geol. Sot. London, Spec. Publ., 40: 103-122. Volkman, J.K., Banks, M.R., Denwer, K. and Aquino Neto, F.R., 1989. Biomarker composition and depositional setting of tasmanite oil shale from Northern Tasmania, Australia. EAOG (Eur. Assoc. Org. Geochem.) 14th Int. Meet., Paris, Abstr. 168. Welling, L.A., Pisias, N.G. and Roelofs, A.K., 1992. Radiolarian microfauna in the northern California Current System: indicators of multiple processes controlling productivity. In: C.P. Summerhayes, W.L. Prell and K.C. Emeis (Editors), Upwelling Systems: Evolution Since the Early Miocene. Geol. Sot. London, Spec. PubI., 64: 177- 195. Wignall, P.B. and Maynard, J.R., 1993. The sequence stratigraphy of transgressive black shales. In: B.J. Katz and L. Pratt (Editors), Source Rocks in a Sequence Stratigraphy Framework. Am. Assoc. Pet. Geol., Studies in Geology, 37: 35-47. Wilson, J.L., 1975. Carbonate Facies in Geologic History. Springer, New York, N.Y., 471 pp. Zambrano, E., Vazquez, E., Duval, B., Latreille, M. and Coffmieres, B., 1971. Sintesis paleogeografica y petrolera del occidente de Venezuela. Mem. IV Congr. GeoI. Venezolano, Caracas, pp. 483-552. Zumberge, J.E. 1984. Source rocks of the La Luna Formation (Upper Cretaceous) in the Middle Magdalena valley, Colombia. In: J.G. Palacas (Editor), Petroleum Geochemistry and Source Rock Potential of Carbonate Rocks. Am. Assoc. Pet. GeoI. Stud. Geol., 18: 127-133.