Winterer, E.L., Sager, W.W., Firth, J.V., and Sinton, J.M. (Eds.), 1995Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 143

12. ORGANIC GEOCHEMISTRY AND CLAY MINERALOGY OF LOWER CRETACEOUSSEDIMENTS FROM ALLISON AND RESOLUTION GUYOTS (SITES 865 AND 866),

MID-PACIFIC MOUNTAINS1

François Baudin,2 Jean-François Deconinck,3 Reinhard F. Sachsenhofer,4 Andre Strasser,5 and Hubert Arnaud6

ABSTRACT

Clay- and organic-rich facies were deposited in Early Cretaceous shallow-water environments from both Allison (Hole 865A)and Resolution (Hole 866A) guyots in the northwestern Pacific Ocean. Together with mineralogical, sedimentological, andpetrographical examinations, organic matter was characterized using Rock-Eval pyrolysis, vitrinite reflectance measurements,elemental analysis of selected kerogens, and chromatography of the saturated and unsaturated hydrocarbons.

Low to very low organic carbon content characterizes gray green pyritic clay stones and most of the bioturbated clayey wacke-stones and packstones in Hole 865A, whereas organic carbon contents up to 50% are recorded in black and brown claystones.Percentages of organic carbon in Hole 866A present high-amplitude variations that range from 0.1 % to 34.5%. As a general rule,total organic carbon contents higher than 3% are recorded in laminated limestones, algal mats, and claystones with plant debris,whereas gray limestones, sandy claystones, and green claystones have organic carbon contents lower than 0.5%.

According to the hydrogen indices, H/C ratios of kerogen concentrates, and the richness in vitrinite, the origin of organic matteris mainly terrestrial (type III) in Hole 865 A. At the base of this hole, two organic-rich samples with well-preserved algal structures,high hydrogen indices, and high H/C ratios contain type I or II organic matter. In Hole 866A, the organic matter is widely distributedbetween type I (algal or bacterial) and type IV (altered organic matter), according to the wide range of hydrogen indices. Laminatedlimestones and algal mats are related to type I or II organic matter, according to their high hydrogen indices and H/C ratios, as wellas their richness in lamalginite and bituminite. Black claystones rich in vitrinite are related to type III (terrestrial), whereas otherlithologies generally contain a type IV (oxidized) organic matter.

Tmax and vitrinite reflectance measurements, as well as biomarker abundance, suggest that the organic matter is too immaturefor hydrocarbon generation.

The composition of clay-mineral assemblages results from various early diagenetic, volcanic, and detrital influences thatreflect the evolution of the Lower Cretaceous sedimentary environments of Allison and Resolution guyots.

Most of the sulfur in the sediment is in the form of pyrite at Hole 865A, whereas the proportion of sulfur in organic form ismuch more important in sediments from Hole 866A, where cyanobacterial mats predominate.

Variations in the quantity and quality of clay minerals and organic matter are strongly dependent on the lithology, as well asthe paleoenvironmental evolution of the guyots. During the early history of Allison Guyot, soil-derived material, includingillite-smectite mixed-layers, kaolinite, and terrestrial organic matter, were probably supplied from residual volcanic islands. Thisland-derived influx became less abundant upward and progressively disappeared, suggesting that the volcanic islands weredefinitively submerged. The clay- and marine organic-rich facies were mainly deposited during lagoonal to peritidal phases of thehistory of Resolution Guyot. Nevertheless, terrestrial organic matter occurs here and there, implying the existence of nearbyvegetated islands.

Comparison of Aptian sediments from Hole 866A with those of DSDP Site 463 suggests that the late early Aptian OceanicAnoxic Event, corresponding to the Selli level, may be recorded at both sites.

INTRODUCTION AND GEOLOGICAL SETTING

Two deep holes drilled during Ocean Drilling Program (ODP) Leg143 in the Mid-Pacific Mountains (Fig. 1) penetrated the Cretaceouslagoonal facies of Allison (Hole 865A) and Resolution (Hole 866A)guyots. Both sections present thick, shallow-water carbonate capsthat recorded the histories of the guyots from the submergence of thevolcanic pedestal through the final drowning of the carbonate plat-form (Shipboard Scientific Party, 1993).

Hole 865A (Allison Guyot) penetrated 731 m of upper Albianshallow-water limestones that become organic-rich and clayey within

1 Winterer, E.L., Sager, W.W., Firth, J.V., and Sinton, J.M. (Eds.), 1995. Proc. ODP,Sci. Results, 143: College Station, TX (Ocean Drilling Program).

2 CNRS-URA 1761 et Département de Géologie Sédimentaire, Université Pierre etMarie Curie, 4 place Jussieu, 75252 Paris Cédex 05, France.

3 Sédimentologie et Géodynamique and CNRS-URA 719, Université des Sciences etTechnologies de Lille, 59655 Villeneuve d'Ascq Cédex, France.

4 Forschungzentrum Jülichl, Institut fur Erdöl und Organische Geochemie (ICG-4)Postfach 1913, Jülich D-5170, Federal Republic of Germany. (Present address: Institut fürGeowissenschaften, Mining University, Leoben A-8700, Austria.)

5 Institut de Géologie, Université de Fribourg, Pérolles, CH-1700 Fribourg, Switzerland.6 Institut Dolomieu et CNRS-URA 69, 15 rue Maurice Gignoux, 38031 Grenoble

Cédex, France.

the bottom 100 m (Fig. 2). Basaltic sills interlayered with limestoneswere recovered in the last 34 m. Dissolution, mineralization, andfilling in of cavities by Upper Cretaceous sediments are evidence ofprolonged emergence at the top of the shallow-water carbonates.During the Paleogene, the guyot acquired a substantial pelagic cap(about 140 m) that consists mainly of upper Paleocene and lower tomid-Eocene nannofossil and foraminiferal oozes.

Hole 866A (Resolution Guyot) produced a section of 124 m ofbasalt overlain by 1620 m of shallow-water limestones (Fig. 2). Thecarbonate sequence ranges from Hauterivian to Albian in age and hasa predominance of dolomitized oolitic or oncoidal facies near thebottom and cyclic, often restricted and organic-rich shallow-waterfacies at the top. Evidence of karstification and mineralized surfacesalso is seen on the top of this carbonate edifice. The guyot is cappedby a thin veneer of winnowed and reworked pelagic sediments thatcontain both nannofossil and foraminiferal assemblages ranging fromMaastrichtian to Pliocene in age.

The purposes of this study are to describe the sedimentological,mineralogical, petrological, and geochemical characteristics of theLower Cretaceous clay- and organic-rich facies from Holes 865 A and866A and to discuss the origin of the organic matter and clay minerals,as well as to reconstruct the environmental conditions that allowed thepreservation of the organic matter.

173

F. BAUDIN ET AL.

25 β N

2OβN

15°N

175°W

Figure 1. Bathymetric chart of the Mid-Pacific Mountains and location of Sites865 and 866 and nearby DSDP Site 463. Bathymetric contours are shown in1000-m intervals. Heavy line shows track of JOIDES Resolution.

DESCRIPTIONS OF ORGANIC CARBON-RICHFACIES

The 131 samples studied were selected for analysis of both sedi-mentological and organic geochemical characteristics at the samescale. The set of samples may be considered as representative of thevarious dark and clayey facies encountered in the cores.

Hole 865A

All studied samples from Hole 865 A are from the lower part of therecovered Albian sediments (Unit IV, Fig. 2). This unit is characterizedby clayey bioclastic wackestones to packstones that contain finelydispersed carbonaceous debris. The limestones contain benthic fora-minifers, ostracodes, gastropods, bivalves, sponges, and calcareousalgae, and are intensely burrowed by Thalassinoides. The lowermostSubunit IVD is intruded by several basaltic sills and shows abundantpyrite associated with burrows and plant fragments. Clayey mudstoneshaving well-preserved plant-cell structures or roots in growth positionare a common minor lithology. Coaly fragments and clay layers be-come less abundant upward and disappear in the upper part of SubunitIVA. A coaly claystone bed occurs in Section 143-865A-90R-3 (Fig.3), and a dark brown to black finely laminated claystone interval occursin Section 143-865A-92R-2 (Shipboard Scientific Party, 1993). Wechose 37 samples from Cores 143-865A-7 IR to -94R in the differentlithologies, especially within dark carbonaceous facies.

Hole 866A

The organic carbon-rich facies at Hole 866A occur from Units IVto VII within the thick Barremian to middle Aptian carbonate sectionthat caps Resolution Guyot (Shipboard Scientific Party, 1993). These

consist mainly of clay- and organic-rich bioturbated mudstones (Fig.4) and algal laminites (Fig. 5). These facies are largely dolomitized inUnit VII.

Although recovery was low (15% in average), the cored mate-rial shows well-developed meter-scale sequences in the lagoonal-peritidal environment (Strasser et al., this volume). The ideal se-quence starts with dark gray layers and grades up into peloidal pack-stones, then into algal or cyanobacterial mats. Boundaries of smallsequences are commonly underlined by bird's eyes, tepee structures,and desiccation fissures. Algal laminites and clay- and organic-richmudstones locally contain ostracodes and black peloids. Packstonesand wackestones contain marine gastropods, bivalves, benthic fora-minifers, and dasycladacean algae, and are commonly burrowed, sug-gesting an open-marine environment. Lignitic fragments, indicatingterrestrial influences, occur in the section.

Ninety-four samples were collected from Cores 143-866A-50R toCore -148R. A small selection was made in oolitic grainstones fromUnit V and in dolomitic grainstones from Subunit VIIC.

METHODS

Petrology and Mineralogy

The lithofacies were classified after optical descriptions of thesamples, and when available, of thin sections observed in transmitted-light. More detailed investigations were also done on rock fragmentswith the scanning electron microscope (SEM) in two different modes,using a JEOL 840A. Mineral structures were observed by the second-ary electron (SE) mode, and organo-mineral textures were examinedin a back-scattered electron (BSE) mode. BSE imaging is particularlyuseful for visualizing organic matter, because a good contrast existsbetween organic products and the mineral matrix, owing to the lowmean atomic number of organic matter (Belin, 1992).

All samples were crushed in an agate-mortar, and the carbonatecontent was measured using a calcimetric bomb.

The clay-mineral assemblages of the 131 samples were studiedusing X-ray diffraction (XRD) on oriented pastes. Clays were defloc-culated by successive washing with distilled water after decarbonationof the crushed rock with 0.2N HC1. The clay fraction (particles of lessthan 2 µm) was separated by sedimentation and centrifugation (Brownand Brindley, 1980; Holtzapffel, 1985). XRD patterns were obtainedusing a Philips PW1730 diffractometer with CuKα radiations and a Nifilter. A tube voltage of 40 kV and a tube current of 25 mA were used.Three X-ray diagrams were obtained: after air drying, after ethylene-glycol solvation, and after heating at 490°C for 2 hr. The goniometerscanned from 2.5° to 28.5° 2 θ for air-dried and glycol-solvated con-ditions, and from 2.5° to 14.5° 2 0 for heating conditions. Clay miner-als were identified according to the position of the (001) series of basalreflections on the three X-ray diagrams (Brown and Brindley, 1980;Moore and Reynolds, 1989). Semiquantitative estimations are basedon intensity and area of the main diffraction peak of each clay-mineral(Holtzapffel, 1985). The percentages of smectite layers in illite-smectitemixed-layers (I/S) are estimated by measuring the "saddle index"(Inoue et al., 1989). The chemistry of clay minerals belonging to theillite-smectite mixed-layers group was analyzed using a CAMEBAXmicroprobe. This was done on selected samples that were charac-terized by an almost monomineral clay fraction. Differential thermalanalyses (DTA) were performed on some clay fractions under Ar-gaswith a SETARAM TAG 24 thermo-analyzer at a heating rate of 10°C/min. The observations by transmission electron microscopy (TEM)were performed using a JEOL 100CX.

Organic petrological studies were performed on two whole-rocksamples and seven kerogen concentrates from Hole 865A and 10kerogen concentrates from Hole 866A. Because of the small amountof rock material, it was not possible to study whole-rock and kerogenconcentrates of the same sample. Quantitative maceral evaluationwas performed in white light and in fluorescence mode. Vitrinite and

174

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ORGANIC GEOCHEMISTRY AND CLAY MINERALOGY

866A

200-

300-

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700 _

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1200-

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Figure 2. Schematic lithostratigraphic successions of Holes 865A and 866A with the studied intervals.

inertinite particles were counted using reflected light. The followingliptinite macerals were recognized using a fluorescence mode:

1. Higher plant liptinites: sporinite, suberinite, cutinite, and resinite;2. Telalginite: alginite derived from large thick-walled, unicellu-

lar or colonial algae;3. Amalginite: alginite derived from thin-walled lamellar algae;4. Bituminite: liptinite maceral without definite shape and having

a strong affinity for degraded algae (e.g., Taylor et al., 1991). Theirfluorescence intensities vary in a wide range, but are low in compari-son to other liptinite macerals; and

5. Liptodetrinite: fluorescent fragments that cannot be assignedto a specific primary maceral.

For maturity assessments, vitrinite reflectance was measured un-der oil immersion (see Stach et al., 1982).

Organic Geochemistry

Free and potential hydrocarbons were measured by Rock-Evalpyrolysis (Espitalié et al, 1985a, 1985b, 1986) on the crushed cruderock samples. Standard notations have been used: S; and S2 in milli-grams of hydrocarbons (HC) per gram of rock; Tmax expressed in °C,and the total organic carbon (TOC) content in weight percent. Thehydrogen index (HI = S/TOC × 100) and oxygen index (OI = S3/TOC× 100) are expressed in milligrams HC per gram of TOC and milli-grams CO2 per gram of TOC, respectively. To give a wider statistical

175

F. BAUDIN ET AL.

cm

8O-1

8 5 -

9 0 -

9 5 -

100-

105-

1 10-

1 15-

120-

125J

Figure 3. Photograph of Interval 143-865 A-90R-3, 80-125 cm, showing coalypackstone and black claystone.

geochemical characterization of the organic material, all sampleswere analyzed twice.

Kerogen concentrates were extracted from 21 rock samples byHC1-HF treatment according to the Institut Français du Pétrole stand-ard procedure (Durand and Nicaise, 1980). Kerogen concentrateswere analyzed by Rock-Eval, and their elemental compositions (C, H,O, N, S, and Fe) were measured using the method summarized byDurand and Monin (1980). Pyrite quantity and organic-sulfur valueswere calculated assuming that all iron was in the form of pyrite. Otherminerals, called here "ash," were calculated taking the differencebetween the sum of all elements and 100. Of course, this value isapproximate and includes several elements that may be present in

cm70-

71 -

7 2 -

7 3 -

7 4 -

Figure 4. Bioturbated peloidal wackestone with clay- and organic-rich flakesand undulating laminites from Unit IV (Interval 143-866A-56R-1,70-74 cm).

kerogen. Kerogen types were recognized using the classical vanKrevelen diagram, which plots H/C vs. O/C (atomic ratios).

Kerogen concentrates were extracted with dichloromethane at60°C for 1 hr. The extracts, called "bitumen," were concentrated in arotary evaporator and dried in open air. The relative proportions ofbitumen are expressed as Ext/TOC in percent. All the bitumens wereseparated by thin-layer chromatography into saturates+unsaturates,aromatics, and N,S,O-compounds. Capillary gas-chromatographicseparation of the C15+ aliphatic fraction was accomplished on aVARIAN 3500 gas-chromatograph fitted with a quartz column DBI(Φ 0.32 mm × 30 m).

RESULTS AND DISCUSSION

Petrography

Hole 865A

Five thin sections and two rock fragments examined with SEMallow us to roughly characterize the microfacies and nannofacies ofcarbon-rich facies from Subunits IVC and IVD.

The main microfacies are wackestones, more or less rich in detritalquartz and clay minerals. Ostracodes, bivalve fragments, and gastro-pods are the main bioclasts. Pyrite is ubiquitous and locally abundant(Sample 143-865A-94R-3, 33-34 cm). Organic particles are presentin all samples and show two different populations. One correspondsto black particles that reach a length of 0.5 cm; these are related tohigher plant debris. Some of the fragments are partly pyritic. Micro-cracks are frequent in these plant fragments, which suggests a strongalteration, probably subaerial, before burial (PI. 1, Fig. 1). The otherpopulation is generally smaller (less than 100 µm) and is light brownto intense yellow or dark red. Samples from Section 143-865A-92R-2contain a high proportion of yellow particles that reach a width of 300µm, which suggests a concentration of algae (PL 1, Fig. 2).

SEM images show clear higher-plant cell structures and pyritizedcasts of tracheids, as well as fossilized bordered pits. Such impregna-tions of iron sulfides generally disturb the original structures, butthese latter are here easily recognizable.

Hole 866A

Most studied samples appear as clayey mudstones, with ostra-codes, foraminifers, and small organic particles or organic laminae.Such microfacies mainly characterize the early to middle Aptian UnitIV. Porous wackestone and packstone with small ooids, peloids, for-

176

ORGANIC GEOCHEMISTRY AND CLAY MINERALOGY

aminifers, and ostracodes predominate in Unit VI. These generallycontain yellow to pale brown laminae.

Thin sections of organic carbon-rich samples display rare ligneousdebris and two populations of algae. One is fine, dispersed, and darkred to brown (PI. 1, Fig. 3). The other is elongate, yellow, and consti-tutes a more or less dense network of laminae (PI. 1, Fig. 4). Clay andorganic matter, which are closely associated, appear dark in transmit-ted light. In the absence of clay minerals, organic matter is darkyellow to yellow. Pyrite is dispersed as very fine grains within theyellow laminae, but locally presents 250-µm-sized euhedral habits(Sample 143-866A-71R-1, 102-104 cm). Cuticles and coaly frag-ments with microcracks are locally abundant (Sample 143-866A-86R-3, 4 7 ^ 9 cm).

SEM images provide a detailed picture of the sediment and particu-larly enabled us to distinguish between organic matter and mineralsthrough use of the BSE mode. Many organic laminae have wavyshapes and present a 10- to 100-µm thickness, with a mean value of 30µm. Single laminae in thin section under SEM display a succession offine (5-10 µm) and continuous laminae (PI. 1, Fig. 5). Some isolatedcrystals mark the limits between elemental laminae and may corre-spond to an interruption in the growth of the algal or bacterial mat. Thecontact between carbonate and organic lamina is generally sharp, butalteration of organic material is evident in some samples (PI. 1, 6).Isolated coccoliths are frequent in Unit IV in the carbonate layersbetween algal laminae, but their occurrence cannot be considered as anevidence of pelagic environment (Noel et al., 1993). Rather, theseimply oxygenated surface water with repeated access to an open-marine environment.

Carbonates and Clay Minerals

Hole 865A

Carbonate content of Unit IV is characterized by high-amplitudevariations that range from 0% to 93% (Table 1), with a general trendfrom 0% at the bottom to 70%, on average, at the top (Fig. 6). Thegray wackestone and clayey packstone facies, both intensely biotur-bated, have 60% to 90% carbonate content vs. 25% to 60% for facieswithout bioturbation. The carbonate content for the coaly claystonefacies of Subunits IVC and IVD is less than 20%, and many samplesshow zero values.

The clay fraction of the 39 studied samples from Unit IV iscomposed of variable amounts of I/S, illite, kaolinite, and berthierine.Each subunit exhibits a distinct clay mineralogy (Fig. 7).

Sediments from Subunit IVD, which consist of clayey bioclasticlimestones intruded with basaltic sills, display a clay fraction com-posed almost entirely of random I/S that occurs together with traces ofkaolinite (Fig. 7). According to the position of the (001) reflections andby comparison with XRD patterns published by Reynolds (1980), theI/S layers contain about 70% smectite. This is confirmed by the mea-surement of the saddle index, which is from 0.3 to 0.5. DTA curvesfrom the clay fraction of two samples (Fig. 8) indicate a well-expressedfirst endotherm between 100° and 150°C that corresponds to dehydra-tion. The second endotherm (dehydroxylation) occurs on both curvesat 500°C and the third reaction, corresponding to the endo-exothermicreaction (destabilization/recrystallization), occurs between 850° and95O°C. The shape of this latter curve is typical of Cheto (Mg-rich)montmorillonite (Chantret et al., 1971; Paterson and Swaffield, 1987).TEM indicates a fleecy shape to the I/S particles. The clay fraction ofsediments from Subunit IVC consists of a mixture of illite, I/S, kaolin-ite, and berthierine. The identification of berthierine is difficult becausethe 001 reflection of this mineral (7.04-7.05 Å; Bailey, 1980) is nearthe 001 reflection of kaolinite (7.14 Å). The relatively important widthof the peak at 7.1 Å (Fig. 9) is unusual for sedimentary kaolinite, whichgenerally is well-crystallized and displays sharp peaks. This suggeststhe presence of another 1:1 layer silicate (Fig. 9). More convincing isthe occurrence of the reflection between 3.5 and 3.55 Å (Sample143-865A-86R-3,51-52 cm), which is also commonly expressed by a

cm

38 -

40 -

42 -

44 -

46

48 -

50

52

Figure 5. Common lithologies in Unit VI: finely laminated algal or cyanobac-terial mats at the bottom of the photograph (Interval 143-866A-85R-2, 50-53cm; age: Aptian) pass upward into greenish-gray to dark gray millimeter-lami-nated clay mudstone with lignitic debris (Sample 143-866A-85R-2,47-49 cm)and then to light-colored wackestone (Interval 143-866A-85R-2, 38^6 cm).

177

F. BAUDIN ET AL.

Table 1. Rock-Eval data of samples from Holes 865A and 866A.

Core, section,interval (cm)

Depth(mbsf)

CaCO, TOCLithology HI Ol

143-865A-71R-CC, 16-1773R-1,23-2474R-1,110-11282R-1, 19-2082R-2, 41-4385R-1, 123-12485R-2, 3-485R-2, 4 3 ^ 486R-3, 51-5286R-3, 70-7186R-3, 72-7387R-1,89-9287R-2, 81-8287R-2, 134-13788R-1, 57-5988R-2, 120-12389R-2, 2-489R-2, 110-11589R-3, 33-3689R-3, 65-7089R-6, 11-1590R-1, 110-11190R-2, 33-3490R-3, 12-1390R-3, 37-389OR-3, 77-7890R-3, 95-9690R-3, 110-11190R-3, 112-11390R-3, 131-13291R-5, 130-13292R-2, 44^792R-2, 58-6092R-2, 62-6392R-2, 68-7092R-2, 75-7794R-3, 72-73

143-866A-50R-CC, 31-3255R-CC, 1-256R-1, 20-2156R-1,42-4356R-1,45-4656R-1,73-7457R-1, 33-3658R-1, 91-9358R-1, 98-9960R-1, 63-6462R-1, 116-11862R-2, 37-4062R-2, 40-4163R-2, 43-4563R-2, 93-9463R-2, 109-11165R-1, 143-14466R-1, 18-2070R-1, 19-2071R-2, 67-6871R-2, 74-7571R-2, 100-10172R-1,60-6173R-1,42-4374R-2, 74-7575R-1,67-6885R-3, 4 7 ^ 985R-3, 50-5286R-1, 143-14586R-2, 111-11388R-1,29-3188R-1, 56-5888R-1, 140-14289R-1,92-9389R-1,94-9589R-1,96-9789R-1, 97-9889R-1,98-9991R-1, 10-1191R-1, 11-1391R-1, 13-1491R-1, 30-3196R-1,26-2898R-1,96-9898R-2, 37-3899R-1, 124-125100R-1,49-50102R-2,13-14

650.96670.33680.80757.29758.44787.43787.47787.87799.27799.46799.48806.29807.61808.14815.67817.75826.19827.27827.87828.19831.48832.90833.27834.36834.61835.01835.19835.34835.36835.55848.06849.07849.21849.25849.31849.38866.62

454.01501.91511.80512.02512.05512.33521.63531.91531.98550.93570.76571.47571.50581.23581.73581.89600.03608.48647.09658.45658.52658.78666.80676.22687.52695.77795.22795.25802.93804.11821.19821.46822.30831.52831.54831.56831.57831.58849.60849.61849.63849.80897.96917.96918.87925.14933.89954.33

Gray wackestoneClayey wackestoneDark gray clayey wackestoneGrayish clayey wackestoneGray wackestoneBioturbated wackestoneBioturbated wackestoneBioturbated wackestoneBioturbated wackestoneClayey packstoneClayey packstoneClayey bioturbated packstoneClayey bioturbated packstoneClayey bioturbated packstoneClayey bioturbated packstoneClayey bioturbated packstoneDark brown wackestoneDark brown wackestoneCoaly claystoneDark brown wackestoneBlack carbonaceous mudstoneDark gray clayey packstoneClayey bioturbated packstoneDark gray clayey packstoneBlack claystonePackstone with wood fragmentsCoaly clayey packstoneCoaly claystoneCoaly claystoneCoaly claystoneGray bioturbated wackestoneBrown claystoneBlack claystoneBlack claystone with pyriteGray greenish claystone with pyriteGray greenish claystone with pyriteBioturbated clayey limestone

White limestone with black laminaeGray limestoneGray limestone with black laminaeWhite limestone with black laminaeGray limestone with laminaeGray limestone with black laminaeAlgal mat with black laminaeGray limestone with laminaeAlgal matGray limestone with laminaeWhite limestoneDark gray argillaceous limestoneDark gray clayey limestoneAlgal mat with black laminaeSandy limestone with plant debrisSandy limestone with plant debrisAlgal mat with black laminaeBeige algal matGray limestone with laminaeDark green claystoneGreen claystoneAlgal mat with black laminaeGray limestone with green flakesLight gray limestoneDark sandy grainstoneDark sandy grainstoneAlgal mat with black laminaeAlgal mat with black laminaeGray limestone with black levelGray limestone with black levelAlgal mat with black laminaeAlgal mat with black laminaeBlack claystoneDark gray claystoneDark gray claystone with plant debrisDark gray claystone with plant debrisBlack claystone with plant debrisAlgal mat with black laminaeDark green laminated limestoneBlack laminated limestoneBlack laminated limestoneAlgal mat with black laminaeAlgal mat with black laminaeAlgal mat with black laminaeAlgal mat with black laminaeAlgal mat with black laminaeBlack argillaceous laminated limestoneAlgal mat with black laminae

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0.01

0.090.07

0.020.930.012.281.600.280.250.550.272.49

2.26

0.352.050.181.820.561.050.940.062.540.250.010.610.111.040.03

0.01

1.32

0.03

5.392.291.975.930.010.840.01

0.040.040.390.57

0.030.010.080.130.050.000.16

0.180.882.280.010.760.230.360.090.010.090.410.510.060.09

0.490.011.873.320.780.847.140.47

19.3421.346.121.04

51.6220.12

184.03

0.09

33.53

7.2523.574.39

18.785.17

17.747.621.61

22.475.660.345.262.829.682.09

0.060.06

12.84

0.99

50.3622.2235.8358.170.33

14.180.050.050.961.59

18.7214.97

0.770.253.413.584.410.259.07

6788

2006*

101343422

3*15*

143120

10*21*

544*

299168372157

144424121

730234592

22

400

498578317600619425635274725458121615317213263

23*14*

676

174

146686720784

88390

1*10*9268

130318

8859

275187

31328

207118128259*163151139206327*103*184180156*214*

159635*

6874

1606465

160835965

125295325

160

47

6940

13147686965

2004991

46379

11274

142

311*314*

49

175

53354236

2654819*31*

177693951

164163102S3

10715566

178

Table 1 (continued).

ORGANIC GEOCHEMISTRY AND CLAY MINERALOGY

Core, section,interval (cm)

Depth(mbsf)

CaCO3 TOCLithology HI OI

104R-1,26-27105R-1, 104-106106R-1,75-77108R-1, 33-35109R-l,6-7109R-1,62-63110R-1,89-90110R-2,36-37110R-2, 88-90110R-2,105-106110R-2,113-115111R-1, 86-87111R-1,118-119lllR-2,8-9112R-1,48-50113R-CC,44^5114R-1,27-28116R-1,33-35117R-1,9-11117R-2,56-57118R-2,97-98119R-1,89-91120R-l,8-10124R-1,114-116125R-3,47-49128R-1,62-63128R-2, 31-32129R-1,14-15129R-1,44^5130R-1, 85-86132R-1,15-17144R-1,53-54144R-1, 58-59144^1,64-65145R-1,147-149145R-2,24-26146R-1,107-108146R-2,22-24147R-1,45^6147R-1, 89-90147R-1,91-92148R-1, 86-87148R-2,104-105148R-3,0-3148R-3,60-61148R-3, 81-82

971.96982.34991.65

1010.531019.961020.521030.491031.401031.921032.091032.171040.161040.481040.671049.381058.941068.471087.831097.191099.161109.271117.391126.181165.941177.801203.821204.651213.041213.341223.451242.051358.031358.081358.141368.271368.541377.471378.121386.551386.991387.011396.561398.161398.271398.871399.08

Black laminated limestoneAlgal mat with black laminaeDark green claystoneBlack argillaceous laminated limestoneBlack argillaceous laminated limestoneBlack laminated limestoneBlack laminated limestoneBlack laminated limestoneBlack argillaceous laminated limestoneGray limestoneLimestone with black laminaeLimestone with black laminaeLimestone with black laminaeLimestone with black laminaeLimestone with black laminaeLimestone with black laminaeLimestone with black laminaeLimestone with black laminaeBlack argillaceous laminated limestoneLimestone with black laminaeLimestone with black laminaeGray limestoneLimestone with black laminaeGray limestone with clay layersGray limestoneLimestone with black laminaeLimestone with black laminaeLimestone with black laminaeDark gray mudstoneLimestone with black laminaeLimestone with black laminaeBlack argillaceous laminated limestoneBlack argillaceous laminated limestoneGray limestoneLimestone with black laminaeDark gray claystoneLimestone with black clayey laminaeBlack argillaceous laminated limestoneBlack greenish claystoneLimestone with black clayey laminaeBlack sandy claystoneBlack sandy claystoneBlack sandy claystoneBlack argillaceous laminated limestoneGreen claystoneLimestone with black laminae

9157287450486850797052nd826783533567347

nd890

71747151603368

504590270

6071

0442226684142

1.631.200.633.901.541.045.011.131.110.952.793.721.131.922.054.994.050.940.480.418.740.150.270.630.490.588.87

12.660.569.350.622.700.870.123.250.171.151.120.160.240.560.290.570.510.450.86

430428418426429418422

431431429427429425420422423433370*460429

445419424421408410372*402417429412

421

428426

386364*373*412435427

0.310.040.030.490.050.030.580.040.140.060.370.330.100.140.630.920.600.05

0.000.62

0.060.00

0.022.704.94

3.570.020.110.03

0.14

0.050.05

0.00

0.000.030.03

11.81.450.64

19.851.620.58

19.981.134.801.94

10.0413.264.264.26

13.9424.7411.282.800.060.34

23.03

0.580.390.310.37

49.8774.490.04

48.741.924.720.29

5.62

2.072.17

0.160.010.040.220.071.83

72112010151010556

399100432205360357375222682496279298

13*83

263

213626362

562588

7*52131217534

173

180194

285*6*

4416

212

601161814598

10040

1061271274745777243273182

145*7730

1562182241352835

228*32955895

41

8788

94207*155*14713472

Notes: TOC = total organic carbon (wt%), Tmax = maximum temperature, S, = free hydrocarbons, and S2 = pyrolyzable hydrocarbons (in mg/g of rock), HI = hydrogen index (in mgHC/g TOC), and OI = oxygen index (in mg CO2/g TOC), nd = not determined and * = doubtful data resulting from a small S2 peak.

shoulder on the 002 reflection of kaolinite at 3.57 Å (e.g., Sample143-865 A-89R-3,65-70 cm, Fig. 9) in the absence of chlorite (no peakat about 14 or 4.7 Å). This reflection is attributed to the iron-rich 1:1layer silicate called berthierine.

A strong mineralogical change occurs within Subunit IVC be-tween Samples 143-865A-89R-2, 110-115 cm (827.27 mbsf), and-89R-2, 2-4 cm (826.19 mbsf). The lowermost part of the subunit(from 827.27 mbsf to bottom) shows dominant I/S, minor kaolinite,and berthierine and the common occurrence of goethite and gibbsite.By contrast, the clay fraction from the upper part of this subunit ischaracterized by dominant kaolinite and berthierine, minor I/S, andby the occurrence of illite (Fig. 7). Goethite and gibbsite are not iden-tified in this part of the subunit.

The clay fraction of the wackestones and packstones from SubunitIVB consists of dominant random I/S associated with illite and kao-linite. According to the saddle index, which is between 0.73 and 0.95,the percentages of smectite layers in the I/S mixed-layers range be-tween 40% and 65%.

Hole 866A

The carbonate content of Units IV and V is high (up to 64%),except for a green claystone level at 658.52 mbsf, which is devoid ofcarbonate (Table 1). Units VI and VII present contrasting carbonatecontents according to their lithologies (Fig. 10). As expected, clay-stones, sandy claystones, and limestones having dark clayey laminaeare carbonate-poor (0%-50%), whereas the carbonate content reaches70% to 90% in gray wackestones and packstones.

In contrast to Hole 865 A, clay mineralogical changes do not coin-cide with unit or subunit boundaries (Fig. 11). The clay assemblagesconsist of a mixture of dominant illite and I/S mixed-layer that occurstogether with minor chlorite (0%-25%) and generally less than 10%of kaolinite, except in Samples 143-866A-55R-CC, 1-2 cm, and-120R-1, 8-9 cm, which are enriched in kaolinite. Illite and I/S showgreat variations in quantity, whereas goethite and celestite occur infew samples.

The most striking feature is the occurrence, at the base of Unit IVand in the two studied samples of Unit V, of monomineral clayfractions composed of illitic material including illite and I/S (Fig. 12).Illite is identified in Sample 143-866A-71R-2, 74-75 cm, accordingto the position of the 002 and 003 peaks at 17.7° and 26.7°, respec-tively, and to the Ir index, which is very close to 1. Small amounts ofISII mixed-layers are probably associated with illite (Srodon andEberl, 1984). Regular I/S, with a proportion of illite layers between40% and 60%, was identified in Sample 143-866A-73R-1,42-43 cm.More illitic random I/S (60%-70% of illite layers) occur in Sample143-866A-74R-2, 74-75 cm. Chemical analyses of the clay fractionof these three samples are reported in Table 2. They indicate mainlyan iron-rich composition and an increase of potassium from randomI/S through ordered I/S then to illite. TEM observations indicate thatthe illitic particles are of very small sizes (

F. BAUDIN ET AL.

Table 2. Chemical analyses of the clay fraction of selected samples from Hole 866A.

Core, section,interval (cm)

143-866A-71R-2, 74-7573R-1, 42-4374R-2, 74-75106R-1,75-77117R-2, 56-57146R-2,22-24

Depth(mbsf)

658.52676.22687.52991.65

1099.161368.54

SiO2(%)

55.0054.1658.0358.1861.1056.64

A12O3(%)

20.7127.8722.1026.6122.8223.14

Fe2O3(%)

10.406.179.345.175.677.63

MgO(%)

5.172.674.624.445.844.48

TiO2(%)

1.333.291.522.750.923.00

K2O(%)

7.195.433.882.503.164.83

Na2O(%)

0.210.180.080.090.340.09

CaO(%)

0.000.230.410.280.150.17

Hole 865A Allison Guyot

Hydrogen index Oxygen Indexmg HC/g TOC mg CCyg TOC

0 400 800 0 200 400

Figure 6. Calcium carbonate percentages, organic carboncontent, and variations in hydrogen and oxygen indices ofsediments from Unit IV, Hole 865A.

from 30% to 60%. Chemical analyses of the clay fraction of these sam-ples are reported in Table 2. They indicate a dioctahedral character ofthe smectite layers belonging to the montmorillonite-beidellite group.

Organic Carbon

The distribution of total organic carbon contents observed in theholes is classified as follows: (1) very rich, more than 5%; (2) rich,1% to 5%; (3) mean, 0.5% to 1%; (4) low, 0.25% to 0.5%; and (5)very low, less than 0.25%.

Hole 865A

Low to very low organic carbon content (less than 0.5%) charac-terizes the gray green pyritic claystones and most of the bioturbatedclayey wackestones and packstones (Table 1). Some exceptions occurin Samples 143-865A-73R-1,23-24 cm;-74R-1,110-112 cm;-85R-2, 3-4 cm, and -90R-2, 33-34 cm, which present a slightly higherorganic-carbon content at about 1% (Fig. 6).

Most samples from Sections 143-865A-89R-3 to -92R-2 are richto very rich in organic carbon. The richest samples (6%-50%) corre-spond everywhere to black and brown claystones, whereas somecoaly claystones and packstones show only mean organic content(about 1% or less).

Hole 866A

TOC percentages present high-amplitude variations that rangefrom 0.1% to 34.47% (Table 1). As a general rule, the higher TOCcontents (>3%) are recorded in laminated limestones of Units IV andVII (Fig. 10), whereas algal or cyanobacterial mats and claystoneswith plant debris likewise present high TOC values in Unit VI. UnitV, investigated by only two samples, shows very low TOC values(

ORGANIC GEOCHEMISTRY AND CLAY MINERALOGY

% ILLITE % ILLITE/SMECTITE % KAOL+ BERTHIERINE650

90020 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100

Figure 7. Clay mineralogy of sediments from Unit IV of Hole 865 A.

They result from a mixture of marine and terrestrial organic matter orfrom selective biodegradation of organic matter. A fourth type oforganic matter, sometimes called type IV, corresponds to residualorganic matter that may be either recycled from older sediments byerosion or deeply altered by weathering (Tissot, 1984).

Hole 865A

The origin of organic matter is mainly terrestrial (type III) accord-ing to the relatively low HI values (around 150) and medium OIvalues (between 50 and 150) shown in Figure 15. Black or coalyclaystones demonstrate mainly a type III distribution, and only twosamples contain types I or II organic matter. These samples from thebottom of Subunit IVD reach 730 mg HC/g TOC.

It is also clear from the HI-OI diagram (Fig. 15) that low HI relatedto high OI typifies the relatively organic-poor wackestones and pack-stones. These samples are regarded here as containing an alteredorganic matter (type IV).

Hole 866A

The organic matter is widely distributed between types I and IV,according to the broad distribution of HI values, which range from 5to 784 mg HC/g TOC (Fig. 15). Laminated limestones and algal matsclearly show the highest His and are related to types I or II organicmatter. Nevertheless, such facies may likewise coincide with indicesof less than 50. These low values, correlated with low organic-carboncontents and relatively high OIs, support a hypothesis for importantamounts of alteration.

Black claystones and clays with plant debris show low to mediumHis (50 to 200), suggesting a type III organic matter. Other lithologiesgenerally have low HI and high OI values, pointing to a type IVorganic matter.

Type II organic matter is mainly distributed in Units IV and VI,whereas type IV typifies Unit V and is abundant in Unit VII (Fig. 16).Type III organic matter is mostly limited to Subunit VIA within theterrestrial-plant-rich interval of Core 143-866A-89R.

Maturation

Maturation level of the organic matter is estimated by both the Tmaxparameter provided by Rock-Eval and the vitrinite reflectance mea-surements on kerogen concentrates. The first requirement for using

865A-92R-21 75-77 cm/

I Exothermic reaction

865A-94R-3, 72-73 cm

I Endothermic reaction

100 300 500 700 900 1100T*C

Figure 8. Differential thermal analyses of the illite-smectite mixed-layers fromsediments of Subunit IVD.

Tmax data is an organic-carbon content higher than 0.25% and an S2peak greater than 0.1 mg HC/g rock (Espitalié et al., 1985a, 1985b,1986). Thus, Tmax for the shallowly buried material that has a loworganic carbon content is here disregarded. Tmax can be plotted vs. His(Fig. 17) to estimate the maturation of samples. Evolution paths forthe three reference types of organic matter (types I, II, and III) andisoreflectance curves (0.5%, 1%, and 1.5%) of vitrinite are outlinedfollowing Espitalié et al. (1985a, 1985b, 1986).

Hole 865A

The range of Tmax values observed in Hole 865 A is 409° to 433°Cfor the interpretable Tmax values (Table 1). There is no clear evolutionof Tmax with depth, and this parameter presents a large variation inamplitude for the organic carbon-rich interval that extends between800 and 850 mbsf.

181

F. BAUDIN ET AL.

co

3CO

IVD

20CuKα25 20 15 10

Figure 9. Typical X-ray diffraction patterns (glycolated) of the clay fractionfrom samples of Unit IV (Hole 865A). I/S = illite-smectite mixed-layers, I =illite, K = kaolinite, Be = berthierine.

Two organic carbon-rich samples corresponding to coaly claystonefacies reveal abnormally low Tmax values (around 390°C), which maycorrespond either to altered organic matter, or to determination ofunreliable Tmax values. We know that this Rock-Eval parameter may beerratic in coal and coaly organic matter (Peters, 1986).

Basaltic intrusions that occur in Subunit IVD do not disturb theTmax parameter, as the lowermost studied sample (143-865A-94R-3,72-73 cm), which is sandwiched between two sills, has only a rm α xvalue of 424°C. A comparable situation of basaltic intrusion and lowTmax value in the vicinity was noted in DSDP Site 368 (Deroo et al.,1977) as well as in outcrop (Correia and Maury, 1975; Baudin andTéhérani, 1991).

Measured vitrinite reflectance data range from 0.33% to 0.47% Rr(Table 3). Accordingly, all samples are immature, as already sug-gested by the low Tmax values, the abundance of smectitic layers in I/S,and the strong positive alteration of the fluorescence intensity of algi-nite (see below). Nevertheless, vitrinite reflectance seems relativelyhigh, considering the shallow depth of burial. Perhaps this and thelarge scatter of the reflectance values are an effect of the basaltic sillsor the result of oxidation of organic matter during sedimentation.

Hole 866A

The range of interpretable Tmax values in Hole 866A has a largeamplitude, from 386° to 434°C (Table 1). Three samples present highTmax values (452° to 551°C), which are abnormal considering theshallow depth of burial and the probable absence of a thermal anom-aly. It is obvious from the Rock-Eval pyrograms that these sampleshave a complicated (bimodal or trimodal) S2 peak. This may representa mixture of immature and very mature (charred?) organic matter,which disturbs the Tmax determination. Cracking of unusual carbon-ates (siderite, dawsonite, nacolite, or others) also may explain abnor-mal Tmax measurements. Except for this anomaly, all the other studiedsamples are immature. Our conclusion of immaturity is also sup-ported by vitrinite reflectance data that range from 0.28% Rr at a depthof 512 m to 0.42% Rr at a depth of 1110 m (Table 3).

In the HI vs. Tmax diagram (Fig. 17), most data plot in the areabelow the Tmax threshold of 435°C and the 0.5% isoreflectance curve,further evidence of an immature stage of all samples from Holes 865 Aand 866A.

Petroleum Potential

Total hydrocarbons (S{ + S2) expelled during Rock-Eval pyrolysisrepresent the petroleum potential, expressed in kilograms of HC perton of rock.

Hole 865A

Very low to low petroleum potentials, less than 1 kg/t, characterizethe carbonate-rich wackestones and packstones, whereas medium togood potentials (1-20 kg/t) can be observed in most carbonaceousfacies. The sample containing up to 50% TOC would be able togenerate only 23 kg/t, suggesting a low hydrogen content of thisorganic matter.

Two samples from the bottom of Subunit IVD that correspond toblack claystones and gray wackestones, respectively, however, dohave good potential for petroleum. Sample 143-865A-91R-5,130-132cm, may generate 50 kg/t, and Sample 143-865A-92R-2, 58-60 cm,may generate up to 180 kg/t, implying a high hydrogen content oforganic matter.

Hole 866A

Very low to low petroleum potentials (

ORGANIC GEOCHEMISTRY AND CLAY MINERALOGY

Table 3. Maceral percentages and mean vitrinite reflectance values of samples from Holes 865A and 866A.

Core, section,

interval (cm)

143-865A-74R-1, 110-11289R-6, l l-15 a

90R-2, 33-34a

90R-3, 37-3890R-3, 95-9690R-3, 112-11391R-5, 130-13292R-2, 44-4792R-2, 58-60

143-866A-56R-1,42-4363R-2, 109-11185R-3, 50-5286R-1, 143-14586R-2, 111-11389R-1,98-99110R-1,89-90111R-1, 86-87118R-2, 97-98144R-1,53-54

Depth

(mbsf)

680.80831.48833.27834.61835.19835.36848.06849.07849.21

512.02581.89795.25802.93804.11831.58

1030.491040.161109.271358.03

Vitrinite

(%)

n.d.97n.d.979780

56126

25

Traces21

35100

26n.d.

Inertinite

(%)

n.d.1

n.d.Traces

02

Traces4

Traces

Traces100121

Traces2

n.d.

Higher-plant

liptinite (%)

n.d.1

n.d.31111

Traces

000004

Traces06

n.d.

Telaginite

(%)

n.d.0

n.d.001

862455

0150000001

n.d.

Lamalginite

(%)

n.d.0

n.d.001332

633867235124341713

n.d.

Bituminite

(%)

n.d.0

n.d.0213

Traces78

344131754735538050n.d.

Liptodetrinite

(%)

n.d.0

n.d.0

Traces25

Traces9

1Traces

2TracesTracesTraces

232

n.d.

Vitrinite

Rr(%)

0.400.400.330.420.410.470.350.410.41

0.28?0.370.38?n.d.n.d.

0.400.38n.d.0.42n.d.

Reflectance

n

404930505045215050

530

7

3739

50

s

0.050.050.070.030.050.050.030.050.05

0.020.060.07

0.040.03

0.05

Note: n.d. = not determined.a Whole-round sample.

Hole 866A Resolution Guyot

0 20CaCO3

60 100 0I L_

% TOC5

Hydrogen Indexmg HC/g TOC

Oxygen Indexmg CO2/g TOC

10 400 800 0 200 400

400 π

600-

800-

f l 1000-α>Q

1200-

1400"

nit

z>

IV

V

VI

VII

05

A

C

ABC"D

Figure 10. Calcium carbonate percentages, organic carbon content, and variations in hydrogen and oxygen indices of sediments from Units IV to VII, Hole 866A.

stones and claystones with plant debris display medium to goodpotentials (1-18 kg/t).

Kerogen Study

Hole 865A

Eight samples were selected for the preparation of kerogen con-centrates. Most samples are from the lowermost part of Unit IV; onlyone is from the top of Unit IV. The data derived from kerogenmicroscopy are summarized in Table 3.

Vitrinite is the prevailing maceral group in most samples from Sub-unit IVD. Sample 143-865A-90R-3,37-38 cm, contains nearly exclu-sively completely gelified telinite. Suberinite is the main maceral of the

liptinite group in this sample. Resinite occurs rarely. Other vitrinite-rich samples are characterized by higher ratios of vitrodetrinite overtelinite. In these samples, alginite, bituminite, and liptodetrinite occuronly in traces. Microcracks in a few vitrinite particles from Sample143-865A-90R-3, 112-113 cm, may indicate oxidation. Because py-rite is not weathered, oxidation must have occurred before burial.

The organic material from samples between 848.00 and 849.25mbsf is characterized by significant amounts of telaginite, with well-preserved cell structures derived from colonial algae. The telalginiteexhibits a yellow to orange fluorescence. Its intensity of fluorescenceincreases significantly during 30 min of irradiation (= positive altera-tion). The greatest amount of telaginite (86%) occurs in Sample143-865A-91R-5, 130-132 cm, which contains only minor vitrinite

183

F. BAUDIN ET AL.

% CHLORITE % ILLITE% ILLITE/SMECTITE

MIXED-LAYERS % KAOLINITE

400

600

if)CQ2

£1000LUQ

1200

1400

UNIT IV & V

UNIT VI

UNIT VII

• i . i . i

0 40 80 120 0 40 80 120 0 40 80 120 0

Figure 11. Clay mineralogy of sediments from Units IV to VII of Hole 866A.

80 120

(PI. 2, Fig. 1). Sample 143-865A-92R-2, 4 4 ^ 7 cm, contains 24%telaginite and 61% vitrinite, and Sample 143-865A-92R-2, 58-60cm, contains 55% telaginite and 26% vitrinite (PI. 2, Fig. 2). Othermacerals (inertinite, bituminite, lamalginite, higher plant liptinites,liptodetrinite) occur only in small amounts (

ORGANIC GEOCHEMISTRY AND CLAY MINERALOGY

*20 Cu Kα 25 20 15 10

Figure 12. X-ray diffraction patterns (glycolated) of illite/smectite mixed-lay-

ers identified in Hole 866A.

-

0.5 µm

0.49

1.04

92

<

<

4

2.38

68

14.38

/30

4.70

318

Figure 13. Transmission electron micrograph of the clay fraction of Interval143-866A-71R-2, 67-71 cm.

87η

cm

89-

91

93-

95

97

99

101

103

105

Figure 14. Photograph of Interval 143-866A-89R-1, 87-105 cm, showing dark

green olive to black finely laminated shales of Aptian age. Higher-plant

remains are abundant at the base of the dark facies. Total organic carbon content

(%TOC) and hydrogen index (HI, in mg hydrocarbons per g TOC) of studied

samples are shown on the right.

fluorescing bituminite (PI. 2, Fig. 6). The precursor of this bituminitevariety, which displays a reflectance up to 0.34%, is not known.Perhaps it is severely degraded algal material, or humic substances(Teichmüller and Ottenjann, 1977). Although these samples are richin liptinites, they are characterized by relatively low HI values, indi-cating that the HI of weakly fluorescing bituminite is significantlylower than the HI of other liptinite macerals. Therefore, the correla-tion between HI and liptinite content is not as good as that for Hole865A (Fig. 18).

Rock-Eval characterization of kerogen concentrates indicates aTOC content that varies from 21% to 65%. The most deeply buriedkerogen (at 1358.03 mbsf) has the lowest TOC content (Table 4).

TOC

HI

185

F. BAUDINETAL.

Table 4. Pyrolysis data, elemental composition, and ash content of kerogen concentrates from Holes 865A and 866A.

Core, section,interval (cm)

143-865A-74R-1, 110-11290R-3, 37-3890R-3, 95-9690R-3, 110-11190R-3, 112-11391R-5, 130-13292R-2, 44-4792R-2, 58-60

143-866 A-56R-1, 42-4358R-1, 98-9963R-2, 109-11185R-3, 50-5286R-1, 143-14586R-2, 111-11389R-1, 97-9889R-1,98-99110R-1,89-90111R-1, 86-87113R-CC,44^5118R-2,97-98144R-1,53-54

Depth(mbsf)

680.80834.61835.19835.34835.36848.06849.07849.21

512.02531.98581.89795.25802.93804.11831.57831.58

1030.491040.161058.941109.271358.03

CaCO3(%)

68n.d.5600

n.d.00

7197n.d.n.d.79n.d.258768n.d.53n.d.50

TOC(%)

23.7836.4961.6769.9833.3351.8133.4152.90

58.1153.3954.0559.7664.9159.2359.4254.4548.9056.2540.4941.9121.25

(C)

413404405404409427422422

410403416401416417410407406410410415415

HI

37265

14571

123839399579

669608417697749821183383523500584504413

OI

9173343766285137

35566231192650544639304270

C(%)

25.3652.3555 8760.4643.2457.7841.5761.03

66.3955.1357.9363.1465.8663.8956.9253.7351.2759.8941.4147.5523.38

H(%)

2.693.844 164.243.086.933.635.99

7.156.686.087.618.078.234.675.295.256.164.934.842.42

O(%)

13.3524.7425.7328.7720.5114.3614.7718.70

13.0314.4320.2411.6512.0910.2620.7213.0113.4714.619.56

13.026.67

N(%)

0.940.430.550.550.560.590.610.73

1.911.531.671.711.131.211.071.321.452.300.931.290.78

Stot.(%)

30.748.247.123.91

16.9810.7721.12

8.13

8.277.928.129.348.18

10.758.42

11.1516.0510.7822.43

18.537.6

Fe(%)

24.955.974.071.01

12.437.57

16.824.70

2.621.821.960.261.173.363.835.768.993.78

15.3412.1528.69

Ash(%)

1.974.432.501.063.202.001.480.72

0.6312.495.606.293.502.304.379.743.522.485.402.650.46

Pyrite(%)

53.4612.798.722.16

26.6316.2236.0410.07

5.613.904.200.562.517.208.20

12.3419.268.10

32.8726.0361.47

Sorg.(%)

2.221.422.462.762.772.111.902.76

5.275.845.889.046.846.914.044.575.776.464.904.614.81

H/C

1.270.880.890.840.851.441.051.18

1.291.451.261.451.471.550.981.181.231.231.431.221.24

O/C

0.390.350 340.330.360.190.270.23

0.150.190.260.140.180.160.270.240.190.250.170.200.21

Note: n.d. = not determined.

ü

800

700

600

üX 500en

x 400(DT3

§ 30°2

^ 200

100

0.25-0.5 O 0.5-1.0 o 1.0-5.0 O >5.0 %TOC

\'91R5, 130 132cm

7 // type I or II

92R2, 58-60cm

865A

° altered OM

i0 0 •n i ~

V type I 866A

altered OM

100 200 300 400 0 100 200

Oxygen Index (mg Cθ2/g TOC)

300 400

Figure 15. Hydrogen index (HI) vs. oxygen index (OI) diagrams for Holes 865A and 866A. Circles are proportional to total organic content (TOC, in wt%). Note

that black circles correspond to prepared kerogens.

186

ORGANIC GEOCHEMISTRY AND CLAY MINERALOGY

type I

12

O altered• D type

D Limestone with black laminae

• Algal mat

0 Black daystonβΦ Claystone or limestone with plant debris

O Other lithologies

800

600

400

200

0800

600

400

20 40 60

— % CaCO,

80 100 0 100 200 300 400

Oxygen Index *~

Figure 16. CaCO3-TOC and HI-OI diagrams by unit and lithological charac-teristics of studied samples from Hole 866A.

Hydrogen indices of kerogen are always higher than the whole-rock equivalent. Some samples (e.g., 143-866A-118R-2, 97-98 cm)present HI values that are twice as high for kerogen, implying a strongmatrix effect (Fig. 19).

Measured Tmax values on kerogen vary from 403° to 417°C. Theseare slightly lower than for whole rocks, but confirm the immaturestage of the studied samples.

800

700

üor -üX

E,×ΦT3

_C

Φ

600

500

400

300

200

100

• 865A

α 866A

vitrinite

isoreflectance

300 350 400 450 500 550 600

Tmax (°C)

Figure 17. Yi\-Tmax diagram for Holes 865A and 866A.

Elemental analysis of kerogens reveals 0.5% to 6% of pyrite andrelatively low ash amounts (0.5%-6%), except for Samples 143-866A-58R-1, 98-99 cm, and -89R-1, 98-99 cm, which contain 12%and 10%, respectively, of other minerals (Table 4). Carbon percent-ages determined by elemental analysis are slightly higher than thoseestimated by Rock-Eval pyrolysis, but a good correlation exists.Organic sulfur is relatively abundant (4%-9%), and nitrogen also ismore important here than in Hole 865A.

The H/C-O/C diagram implies an immature stage of all the kero-gen concentrates from Hole 866A (Fig. 20). The two samples fromCore 143-866A-89R show H/C values of about 1. Six kerogen con-centrates are located along the type II evolution path, with H/C ratiosranging from 1.22 to 1.29. The others are related to the type I evolu-tion path, with the highest H/C ratios (1.43 to 1.55). No clear correla-tion exists between lithology and type of kerogen. For the samelithology, different samples present high (1.25) as well as very high(1.55) H/C ratios. O/C ratios are lower here than in Hole 865A andfluctuate between 0.14 and 0.27.

Dichloromethane Extracts Study

Hole 865A

The bitumens collected from seven kerogen concentrates yielded1.56 to 21.96 wt% of extract relative to TOC (Table 5). As is usuallyfound in immature samples, N,S,O-compounds with high molecularweight predominate in the bitumens. Their amount ranges from 63%to 86% of the total extract, whereas saturates+unsaturates represent3.9% to 28.3% and aromatic hydrocarbons, 5.9% to 9.7%.

The molecular distribution in gas-chromatograms (Fig. 21 A) mustbe considered carefully because of the relatively low saturates+unsaturates fraction and the immaturity of the organic material.

Four samples present a small mode in the range of C π to C1 9 and apredominance of odd-numbered ra-alkanes in the C2 5 to C3 5 fraction.

187

F. BAUDIN ET AL.

800 -

ü2 600 -σ>üX| 400 -

X

200 -

o -

• - • - • Site 865— B — Site 866

•

• o, - * ' α

* * &\k , ' *» ify>^

r T ^ " ^ D r

20 40 60Liptinite (Vol %)

80 100

Figure 18. Relation between hydrogen index (HI) of kerogen concentrates andcontent of organic matter in macerals of the liptinite group. There is a strongcorrelation in the case of Hole 865A (correlation coefficient r2 = 0.96), whereasthe correlation is considerably weaker in the case of Hole 866A (r2 = 0.54).

Important amounts of C2 9 hydrocarbon, biomarkers, and relativelyhigh carbon preference index (CPI between 2.64 and 3.42) were notedin these samples. The predominance of molecules having an odd num-ber of carbon atoms can be measured by the CPI (i.e., the weight ratioof odd to even molecules; Tissot and Welte, 1984). All these charac-teristics correspond to an immature terrestrial organic material (Tissotand Welte, 1984), in agreement with the type III kerogen assignment.

Other samples present a bimodal distribution of hydrocarbons.The larger mode is in the range of C1 7 to C 1 9 π-alkanes, which sug-gests a phytoplanktonic origin, whereas the second mode correspondsto odd-number π-alkanes between C25 and C35. These chromato-grams, associated with high H/C ratios and CPI values below 1.9,indicate a mixture of type I and III organic matter.

Three samples from the coaly interval in Section 143-865A-90R-3present a large unresolved hump in the region of C3 5 + hydrocarbons.Clayton and Swetland (1978) showed that weathering of organic ma-terial results in a selective loss of π-alkanes and the lower carbonnumber molecules, producing a more pronounced envelope in the highmolecular weights. We interpret the unresolved hump seen in some gaschromatograms as probably having been caused by weathering.

Isoprenoid amounts, mainly phytane and pristane, are very low tomedium, ranging from 0.4% to 17%.

Hole 866A

Only four kerogens from Hole 866A yielded enough organicmatter concentrate to be extracted (Table 5). The bitumen quantitiesrange from 1.31% to 11.09% ext/TOC. High molecular compoundsdominate the bitumens, with 75.5% to 94.5% of the total extract. Thesaturates+unsaturates fraction varies from 2.8% to 21%, whereasaromatic hydrocarbons always represent less than 5%.

Gas chromatograms (Fig. 2IB) of Samples 143-866A-56R-1,42-43 cm, and -118R-2, 97-98 cm, show a bimodal distribution. Thefirst mode occurs at C1 6 to C I 8, with a slight predominance of even-numbered n-alkanes. The second mode occurs at C27 to C29, with apronounced odd-number predominance that is typical of terrestrialorganic matter. Although the n-alkane distribution of these samples,in many cases, is bimodal with a maximum at C2 7 to C29, the terrestrialorganic matter contribution is thought to be overestimated owing tothe immaturity of samples (Tissot and Welte, 1984).

Sample 143-866A-86R-2, 111-113 cm, presents an increasingpercentage of n-alkanes from C1 6 to C25, with a slight predominanceof odd numbers. Biomarkers, generally characteristic of autochtho-nous marine organic matter, are important in all three samples. They

Φ

X

ORGANIC GEOCHEMISTRY AND CLAY MINERALOGY

Table 5. Composition of extracts from Holes 865A and 866A kerogen concentrates.

Core, section,interval (cm)

143-865A-90R-3,37-3890R-3,95-9690R-3,110-11190R-3,112-11391R-5,130-13292R-2,44-4792R-2,58-60

143-866A-56R-1,42-4386R-2,111-11389R-1,97-98118R-2, 97-98

Depth(mbsf)

834.61835.19835.34835.36848.06849.07849.21

512.02804.11831.57

1109.27

TOC(%)

36.4961.6769.9833.3351.8133.4152.90

58.1159.2359.4241.91

max(%)

404405404409427422422

410417410415

Ext/roc(%)

5.789.491.56

19.8621.9618.745.54

4.4711.09

1.313.91

Sat.+unsat.(%)

27.223.978.76

28.338.23

13.7322.36

2.788.02

11.0920.80

Aro.(%)

9.669.699.505.905.966.236.41

2.815.354.943.66

NSO(%)

63.1286.3481.7465.7885.8180.0571.24

94.4186.6383.4775.54

Pr(%)

4.221.399.451.732.761.870.44

2.590.245.290.88

Ph

(%)

8.902.91

16.281.973.691.870.73

3.460.724.971.48

Pr/Ph

0.470.480.580.880.751.000.60

0.750.341.060.60

CPI24-34

2.642.761.543.421.451.872.97

1.241.391.432.43

Notes: Sat.+unsat. = saturated and unsaturated hydrocarbons, Aro. = aromatic fraction, NSO = heavy compounds, Pr = pristane, and Ph = phytane.

o

o

εo

H/C

(<

1.8

1.6

1.4

1.2

1

0.8

0.6

0.4

0.2

0

typel D

/ O-TJP*

- r

• t . i

8

i

•

type III

#

D, 1

pyritβdisturbance

865A

866A

1000

8

0.1 0.2 0.3

O/C (atomic ratio)

0.4 0.5 0 50 100 150 200

Oxygen Index (mg CO2 /g TOC)

Figure 20. Location of kerogens selected from Holes 865 A and 866A on a van Krevelen diagram (H/C vs. O/C atomic ratios), compared to the distribution of the

same kerogens in an HI-OI diagram. The three reference evolution paths for types I, II, and III are reported following Tissot et al. (1974) for the van Krevelen

diagram, and Espitalié et al. (1985a, 1985b, 1986) for the HI-OI diagram.

Two levels, however, indicate a different source of organic matter.Samples 143-865A-91R-5, 130-132 cm, and -92R-2,58-60 cm, whichcorrespond to a gray bioturbated wackestone and a black claystone,respectively, contain algal organic matter. High His (730 and 592 mgHC/g TOC), abundance of telalginite, high H/C atomic ratios of thekerogen, and a large mode in the range of C17 to CI9 n-alkanes in thechromatograms indicate a type I (bacterial) or II (marine) organicmatter. Thin-sections, which exhibit yellow organic particles, and pal-ynological investigations suggest that the two samples contain a mix-ture of colonial algae, with minor terrestrial organic debris.

Hole 866A

With the exception of Core 143-866A-89R, the organic matter ofBarremian to Aptian organic carbon-rich sediments from Hole 866Ahas a marine primary source. Hydrogen indices that reach 800 mgHC/g TOC and high H/C ratios indicate type I or II organic matter.Petrographical results show that lamalginite and bituminite are themost important macerals. Bituminite with weak fluorescence also was

...nd and is considered to have been derived from several decayedalgae or humic substances (Teichmüller and Ottenjann, 1977). Al-

though the n-alkane distribution is bimodal, with a mode around C25to C29, the terrestrial contribution is thought to be small. This is alsosupported by the conspicuously low abundance of vitrinite and in-ertinite macerals. Most of the organic matter preserved in Units IV, VI,and VII of Hole 866A derives from cyanobacterial mats and is more orless oxidized, as attested by decreasing TOC contents and His in somebioturbated samples.

Some samples, especially in Section 143-866A-89R-2, contain ter-restrial organic matter. Higher plant contribution is here evidenced bycore description (Shipboard Scientific Party, 1993), thin section obser-vations, and petrographical investigations. Relatively low His (about150 mg HC/g TOC) associated with high TOC content, as well as lowH/C atomic ratios, are characteristic of terrestrial organic matter.

Significance of Clay Minerals

Hole 865A

In Subunit IVD, the I/S are rich in smectite layers that have a mag-nesian composition. Examination of thin sections of basaltic sills indi-cates that the ferromagnesian minerals have been pseudomorphed by

189

F. BAUDIN ET AL.

865A-90R3, 96-96 cm

TOC .13.45'/.HI = 1 4 4H/C 0 . 8 9P r / P h = 0 . 4 8

865A-90R3, 110-111 cm

TOC «50.60%HI 61H/C »0.84Pr/Ph» 0.58

866A-90R3, 112-113 cm

TOC =14.80%HI = 4 1H/C = 0.85Pr/Ph= 0.89

865A-91R5, 130-132 cm

w

TOC = 7.08%HI = 730H/C = 1.44Pr/Ph= 0.75

865A-92R2, 44-47 cm

TOC = 8.62%HI = 234H/C » 1.05Pr/Ph= 1.00

i l l

866A-92R2, 68-60 cm

TOC =31.11*/.HI = 592H/C = 1.18Pr/Ph= 0 . 6 0

NSO,.Aro

Sat+i Uπsat

20 25 30 35C A R B O N A T O M N U M B E R

866A-66R1, 42-43 cm

TOC = 4.08%HI = 578H/C = 1.29Pr/Ph= 0.75

866A-86R2, 111-113 cm

TOC = 7.41%HI = 784H/C = 1.55Pr/Ph= 0.34

866A-89R1, 97-98cm

TOC =14.38%HI = 130H/C =0.98Pr/Ph= 1.06

15 20 25 30 36C A R B O N A T O M N U M B E R

Figure 21. A. Gas chromatograms of selected samples from Unit IV of Hole 865A. B. Gas chromatograms of selected samples from Hole 866A. Half pie-graphs

show the proportion of the different fractions of the bitumen. Sat+unsat = saturated and unsaturated hydrocarbons, Aro = aromatic fraction, NSO = heavy

compounds, Pr = pristane, and Ph = phytane.

190

ORGANIC GEOCHEMISTRY AND CLAY MINERALOGY

clay minerals and that glass has now been converted to clay (ShipboardScientific Party, 1993). In the ocean floor, sediments interbedded withor deposited just above basalt commonly contain abundant Mg-richsmectite coming from the weathering of the volcanic rock (Chamley,1989). A similar origin is highly probable for the sediments of SubunitIVD. In Subunit IVC, the amounts of I/S diminish because of thedecreasing volcanic influence. At the base of the subunit, I/S are stillabundant, but contain more illite layers here than in Subunit IVD, sug-gesting another origin. The occurrence of terrestrial plant debris (abun-dant type III organic matter) points to the erosion of nearby islands. Inthis context, I/S from the base of Subunit IVC may be partly detrital.The detrital influences also are expressed by increasing percentages ofkaolinite. This mineral, which occurs together with gibbsite, also indi-cates hydrolyzing (warm and humid) climatic conditions. In Section143-865 A-89R-2, illite and 1:1 layer minerals (including kaolinite andberthierine) increase, whereas I/S strongly decrease. The coeval in-crease of detrital illite and kaolinite suggests a strong erosion of nearbyland areas, but, surprisingly, TOC (mainly type III) decreases. This canresult from either oxidation of organic matter in the marine environ-ment or dilution of organic matter by clastic sediments eroded fromislands. Oxidizing conditions are evidenced by the occurrence of ber-thierine. In modern sediments, this ferric iron-rich mineral occurs inshallow (water depth of about 50 m), well-oxygenated coastal environ-ments. This mineral, which is restricted to tropical areas and commonlyassociated with kaolinite, is very common in most oolitic ironstones(Odin and Matter, 1981; Debrabant et al., 1992). According to labora-tory experiments, early diagenetic remobilization of iron initiates trans-formations of detrital kaolinite to berthierine (Bhattacharrya, 1983).

In Subunit IVB, the decrease of illite and kaolinite is balanced byan increase of I/S and probably suggests less erosion, which may bethe result of the progressive submersion of the volcanic islands.

Hole 866A

A detrital origin of chlorite and kaolinite is likely. Small amountsof these minerals point to weak detrital influxes, which also areevidenced by the occurrence of organic matter of marine, instead ofterrestrial, origin. In most sediments that have not undergone deepburial, illite is considered as detrital and shows fluctuations parallelto chlorite or kaolinite (Chamley, 1989; Deconinck, 1992). As indi-cated by the occurrence of immature organic matter and smectite-richI/S, the influence of thermal diagenesis is negligible in Hole 866A.Nevertheless, some observations suggest that illite is not entirelydetrital. Variations in the percentages of illite are not parallel to thoseof chlorite and kaolinite. Chemical analyses point to an iron-richcomposition, and TEM observations reveal the small sizes of theillitic material, whereas detrital illites usually exhibit aluminum com-position and commonly occur as relatively large particles. Illitic ma-terial showing similar X-ray and chemical features already has beendescribed in lacustrine sediments (Porrenga, 1968; Newman andBrown, 1987), in paleosoils (Robinson and Wright, 1987), and inthe clay fraction from green marls occurring at the top of upward-shallowing sequences of Lower Cretaceous carbonate platforms inthe Tethyan realm (e.g., Purbeckian platform: Deconinck andStrasser, 1987; Deconinck et al., 1988). The formation of illitic min-erals probably occurs at surface temperatures in potassium-rich su-pratidal environments, where smectites and smectite-rich I/S are sub-mitted to wetting and drying cycles. Laboratory experiments haveshown that smectites submitted to such cycles are progressively trans-formed into I/S (Srodon and Eberl, 1984; Eberl et al., 1986). Orderedand random I/S, which occur in some samples, correspond to incom-plete transformation of smectite. The origin of smectite that consti-tuted the father mineral is more difficult to establish.

Depositional Conditions

Hole 865AFacies analysis of Subunit IVD at Hole 865A indicates that the

deepest sediments recovered at Allison Guyot corresponded to shal-

low-marine environments. Locally, brackish-waters environments orstagnant marine-water pools certainly have existed, favoring the devel-opment of colonial algae, as shown in Cores 143-865 A-92R and -9 IR.At the base of Subunit IVC (Cores 143-865A-90R and -89R), abun-dant terrestrial organic matter, roots in growth position, and relativepaucity of fauna indicate more restricted environments, such as amarsh. A humid climate, favoring coastal vegetation and intense run-off, probably dominated at that time. Soil-derived materials, includingI/S, kaolinite, and terrestrial organic matter, were probably suppliedfrom volcanic islands. Higher-plant debris was partially oxidized be-fore it was transported into the swamp, as attested by cracks in thevitrinite particles and relatively low TOC amounts of the carbona-ceous-rich facies. This land-derived influx became less abundant up-ward and disappeared within Subunit IVB, suggesting that the volcanicedifice was progressively submerged at that time. The progressivedisappearance of terrestrial organic matter runs parallel to an increaseof OIs, which suggests that more oxygenated platform conditions,unsuitable for organic matter preservation, prevailed during the timeSubunits IVB and IVA were deposited. The water certainly was oxy-genated enough to sustain the infaunal activity, clearly demonstratedby abundant fossils and the intense bioturbation of the sediment.

The presence of pyrite associated with organic matter impliesimportant bacterial sulfate reduction during the time of deposition ofSubunits IVD and IVC. Gas chromatography results show that thephytane generally predominates over pristane in all samples fromHole 865A. It is well known that the depositional paleoenvironmentcan influence the formation of isoprenoids issuing from phytol (Tissotand Welte, 1984). The richness in phytane should indicate a restrictedenvironment, suitable for organic matter preservation, whereas pris-tane predominance indicates unfavorable conditions for preservationof organic matter (Brooks et al, 1969; Didyk et al., 1978). Pris-tane:phytane ratios become reliable indicators of the environmentonly in the early mature zone, where phytanic acid has become moreor less converted to pristane. Recent studies have shown multiplesources for precursors of pristane and phytane and have questionedthe use of pristane:phytane ratios as redox indicators (Goossens et al.,1984; ten Haven et al., 1987). The predominance of phytane in sam-ples from Hole 865A may be here regarded as a consequence of toolow maturation, rather than a clear indication for a reducing environ-ment. However, reducing environmental conditions existed at leastduring the deposition of alginite-rich sediments from Cores 143-865A-92R and -90R.

Hole 866A

Sequence-stratigraphic analysis of the sediments at Hole 866A(Arnaud et al., this volume; Strasser et al., this volume) suggests thatthe clay- and organic-rich facies were mainly deposited during lagoonalto peritidal phases of the history of Resolution Guyot.

Comparison of geochemical data with petrographic analysesclearly shows that the density and thickness of laminae increase insamples having high TOC contents. As a general rule, the moreimportant the organic content, the more continuous are the laminae.On the other hand, when ligneous fragments and dispersed brownalgae predominate, TOC contents are low.

Several centimeter-thick organic-rich layers present a sharp in-crease of organic carbon content and HI from bottom to top. Thisupward increase probably was caused by flooding of the depositionalsite and better preservation of the organic matter under anoxic condi-tions. Only interval 143-866A-89R-1, 92-100 cm, presents a reversetrend with an upward decrease of organic content. Total organiccarbon percentages pass from 14.38% at the base to 0.49% at the top(Fig. 14). This interval contains mainly terrestrial organic matter,which generally display a strong heterogeneity in organic content.But in this case, probable subaerial alteration induced an upwarddecrease in TOC content and HI.

The existence of planktonic organisms, such as coccoliths andforaminifers, at Hole 866A indicates oxygenated surface waters with

F. BAUDIN ET AL.

access to the open-marine environment, at least during some timeintervals. This strongly suggests that the water column was not an-oxic, but that the redox boundary lay at, or close to, the water/sedi-ment interface and probably in the cyanobacterial mat itself. Envi-ronmental signification of Pr/Ph ratios is doubtful, owing to the lowquantity of these isoprenoids and the weak maturity of the studiedsamples. Nonetheless, a general tendency of reducing environmentsis shown by low Pr/Ph ratios in algal or cyanobacterial mats, whereasa more oxygenated environment is recorded in Sample 143-866A-89R-1, 97-98 cm, rich in terrestrial organic matter.

The high organic sulfur content, characteristic of the organic mat-ter from Hole 866A, is probably the result of intense bacterial sulfatereduction associated with low amounts of reactive iron in interstitialwaters. Sulfate-reducing bacteria depend on a continuing supply ofsimple organic acids derived from the activities of heterotrophic bac-teria (Kenig and Hue, 1990). Hydrogen sulfide in turn is poisonous,and continuing degradation of organic matter depends on the removalof hydrogen sulfide by reactive iron oxides. When reactive ironcontents are low, sulfur is incorporated into organic matter, and theresidual organic matter has a high content of organic sulfur (Berner,1984;Gautier, 1986).

No direct evidence is available regarding the temperatures andsalinities of the waters. Only indirect hypotheses may be formulatedfrom the existence of cyanobacterial mats that point to relativelywarm waters, such as exist today in Shark Bay (western Australia),Laguna Guerrero Negro (Mexico), Solar Lake (Red Sea), and AbuDhabi lagoons (United Arab Emirates). The water temperature re-ported for Shark Bay is between 18° and 28°C. Salinities associatedwith present algal or cyanobacterial mats are higher than in normalseawater (i.e., hypersaline in Solar Lake; Krumbein et al., 1977), and40 to 70‰ in Abu Dhabi (Kenig et al., 1990; Kenig, 1991). On Reso-lution Guyot, evaporite pseudomorphs in several levels point to re-currently hypersaline conditions, but they are not consistently associ-ated with microbial mats (Arnaud et al., this volume). Normal salineconditions may also support cyanobacterial mats, as mentioned byGlikson and Taylor (1986).

Comparison of DSDP Site 463 with Hole 866A

DSDP Site 463, drilled during Leg 62 (Thiede, Vallier, et al.,1981), is located 44.4 km northeast of Site 866 (Fig. 1) and representsa part of the basinal sequence correlated to platform limestones en-countered on Resolution Guyot (Shipboard Scientific Party, 1993).

Three beds of siliceous limestone containing between 2% and 7%organic carbon occur between Cores 62-463-69 and -72 within thelower Aptian sequence. The most remarkable structures in these coresare laminations in the dark gray clayey intervals intercalated in blu-ish-white to dark greenish-gray limestones. The thicknesses of thelaminated intervals range from a few centimeters to more than 60 cm.The darker intervals are especially abundant in Cores 62-463-70 and-71, but they are difficult to distinguish from volcanic-ash layers.These facies do not display evidence of emplacement by turbidity cur-rents (Thiede, Vallier, et al., 1981). They are mainly strongly silicifiedlimestones, with some chert and volcanic-ash layers. Radiolarians areabundant, but poorly preserved, compared to the radiolarians aboveand below (Schaaf, 1981). This suggests that the silica for the diage-netic silicification of the limestone was, at least partly, biogenic;hence, possibly the surface water was highly fertile. Only three levelscontain high organic carbon concentrations that reach 7% TOC (Deanet al., 1981; Mélières et al., 1981). Hydrogen indices generally arelow (

ORGANIC GEOCHEMISTRY AND CLAY MINERALOGY

consequence of a local thermal perturbation caused by basaltic sills inHole 865 A. High Tmax values suggest a probable charcoal content insome samples from Hole 866A.

4. The composition of clay-mineral assemblages results fromvarious early diagenetic, volcanic, and detrital influences during theevolution of Early Cretaceous sedimentary environments of Allisonand Resolution guyots. Early diagenetic changes are responsible forthe replacement of detrital kaolinite by berthierine in oxidizing con-ditions (Hole 865A, Subunit IVC). Sediments of Hole 866A arecharacterized by the occurrence of illitic minerals that resulted fromthe replacement of smectitic minerals submitted to wetting and dryingcycles at surface temperatures.

Volcanic influences are particularly well expressed at the base ofHole 865A, where smectite-rich I/S originated from the weatheringof basaltic rocks.

Significant detrital components are recorded at Hole 865A, wherekaolinite associated with terrestrial organic matter is abundant andreflects active erosion of the nearby land area. The decrease of illiteand kaolinite, balanced by increasing amounts of I/S, in Subunit IVBprobably results from the progressive submersion of volcanic islands.By contrast, the scarcity of kaolinite and the occurrence of marineorganic matter in Hole 866A point to a more gentle terrestrial erosion.

5. Marshy environments probably existed during Aptian/earlyAlbian time on Allison Guyot. The humid climate favored growth ofcoastal vegetation and intense terrestrial run-off. Such environmentswere suitable to deposition of terrestrial organic matter, whereas localquiet pools supported the development of abundant colonial algae.The land-derived organic matter was progressively more oxidizedand became less abundant with the progressive submergence of thevolcanic edifice and the replacement of the marshes by shallow-water platform.

During the stage of active carbonate platform growth on Resolu-tion Guyot, the platform stayed at or near sea level, allowing for thedeposition of lagoonal to peritidal organic-rich facies. These faciesare mainly represented by finely laminated limestones and cyanobac-terial mats. Terrestrial organic matter occurs here and there through-out the lagoonal and peritidal facies and is testimony to the existenceof vegetated islands.

6. Comparison of Hole 866 A with DSDP Site 463 reveals that Res-olution Guyot was the source of the detrital organic matter redepositedat Site 463 during early Aptian time. Organic-rich facies from SubunitVIA probably correlate with the Oceanic Anoxic Event of this age.

ACKNOWLEDGMENTS

The authors are grateful to ODP for inviting three of us to partici-pate in Leg 143. We are indebted to IFP and KFA for analyticalfacilities. HA, FB, and JFD acknowledge the French "GeosciencesMarines" program for financial support. RFS thanks the Alexandervon Humboldt Foundation its granting of a research fellowship. ASacknowledges the support of the Swiss National Science Foundation.The authors thank W. Riegel (Göttingen) for palynological help. Themanuscript benefitted from the critical comments of R. Moberly, P.Meyers, and E.L. Winterer.

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