Transport-of-terrestrial-organic-matter-in-the-Ogoou-deep-sea-turbidite-system-Gabon-_2011

Transport of terrestrial organic matter in the Ogoouédeep sea turbidite system (Gabon)

Laurie Biscara a ,*,Thierry Mulder a ,Philippe Martinez a ,Fran?ois Baudin b ,Henri Etcheber a ,Jean-Marie Jouanneau a ,Thierry Garlan c

a

Universitéde Bordeaux,CNRS-UMR 5805EPOC,avenue des facultés,33405Talence Cedex,France

b

UniversitéParis 6,UPMC,CNRS-UMR 7193ISTeP,Equipe Evolution et Modélisation des Bassins Sédimentaires,case 117,4place Jussieu 75252Paris Cedex 05,France c

SHOM,Océanographie/Recherche,CS 92803,29228Brest Cedex 2,France

a r t i c l e i n f o

Article history:

Received 14April 2010Received in revised form 14September 2010

Accepted 7December 2010

Available online 15December 2010Keywords:OgoouéGabon

Organic matter Turbidites

Carbon and nitrogen isotopes Rock-Eval pyrolysis

a b s t r a c t

In order to de ?ne the nature and distribution of the organic matter (OM)preserved in the modern Ogoouédeep sea turbidite system (Gabon),bulk geochemical techniques (Rock-Eval pyrolysis,elemental and isotopic analyses)and palynofacies were applied to three piston cores collected in the Cape Lopez Canyon and lobe and on the continental slope,north of the canyon.

The hemipelagic sedimentation in the study area is characterized by high accumulations of well-preserved OM (w 2e 3wt.TOC %).Bulk geochemical and palynofacies analysis indicate both a marine and terrestrial origin of the OM.Contribution of the marine source is higher on the slope than in the canyon and lobe.

OM accumulation in turbidites is strongly controlled by the combined in ?uence of the Cape Lopez Canyon and littoral drift.In the canyon and lobe,turbidites show generally low TOC content (0.5wt.%)and OM is oxidized.The origin of the OM is interpreted as both marine and terrestrial,with a higher contribution of continental source versus marine source.The low TOC contents are due to the large siliciclastic fraction transported by the littoral drift and diverted in the Cape Lopez Canyon during high energy processes (e.g.storms)which tend to dilute the OM in the turbidites.Transport by long-shore currents and/or turbiditic ?ows leads to oxidation of the OM.

On the continental slope located north of the Cape Lopez Canyon,large amounts of OM are deposited in turbidites (up to 14wt.%).The OM is predominantly derived from terrestrial land plants and has not been subjected to intense oxidation.These deposits are characterized by high hydrocarbon potential (up to 27kg HC/t rock),indicating a good potential as gas-prone source rock.Because Cape Lopez Canyon captures a signi ?cant part of the sediment transported by the littoral drift,the siliciclastic sedimentary ?ux is reduced north of the canyon;OM is thus concentrated in the turbidites.Variation in TOC content within turbidite laminae can be explained by the burst and sweep deposition process affecting the boundary layer of the turbulent ?ow.

This study con ?rms that gravity ?ows play a preponderant role in the accumulation and preservation of OM in deep water and that deep sea turbidite systems could be regarded as an environment where organic sedimentation occurs.

ó2010Elsevier Ltd.All rights reserved.

1.Introduction

Continental margins are important areas for the production and preservation of organic matter (OM)(Bauer and Druffel,1998).Terrestrial OM in marine sediments constitutes an important reservoir in the global carbon cycle,but remains poorly constrained

(Degens et al.,1991;Schlünz and Schneider,2000).Previous studies demonstrated that terrestrial organic deposition may be predomi-nant in some areas of the ocean,and especially in deep sea turbidite systems where gravity processes are active (Heezen et al.,1964;Damuth,1977;Cornford,1979;Dean and Gardner,1982;Littke and Sachsenhofer,1994;Rullk?tter et al.,1984;Cowie et al.,1995;Watanabe and Akiyama,1998;Stow et al.,2001;Huc et al.,2001;Khripounoff et al.,2003;Saller et al.,2006;Baudin et al.,2010).Sil-iciclastic turbidites represent huge volumes of sediments which are of particular interest for the oil industry (Stow and Johansson,2000).

*Corresponding author.

E-mail address:l.biscara@epoc.u-bordeaux1.fr (L.

Biscara).

Contents lists available at ScienceDirect

Marine and Petroleum Geology

journal h omepage:ww w.elsevi https://www.doczj.com/doc/b614052882.html,/locate/marp

etgeo

0264-8172/$e see front matter ó2010Elsevier Ltd.All rights reserved.doi:10.1016/j.marpetgeo.2010.12.002

Marine and Petroleum Geology 28(2011)1061e 1072

To date,these sandy facies have generally been studied for their potential as reservoirs but not as source rocks.

According to the literature,deep water settings are not suitable environments for source rock deposition because OM is intensely degraded as it settles through the water column,where it is exposed to oxygenation and biogenic degradation over a long time (Suess,1980;Pedersen and Calvert,1990).However,downslope gravity processes may lead to the deposition and rapid burial of organic-rich-sediments in the deep sea(Cornford,1979;Rullk?tter et al.,1982;Meyers et al.,1996;Peters et al.,2000;Doust and Noble, 2008)which is considered to be a favorable area for its preservation and the formation of source rocks for hydrocarbons(Stow et al., 2001;Huc et al.,2001).Turbidity currents are one of the proc-esses involved in the accumulation of both terrestrial and marine OM and the formation of organic-rich-sediments in deep water environments(Dean and Gardner,1982),as for example in the Angola Basin(up to16wt.%Total Organic Carbon(TOC);Dean et al., 1981),the Makassar Strait(>5wt.%TOC;Huc et al.,2001)or the Kutei Basin(>20wt.%TOC;Saller et al.,2006).

The main objective of this study is to provide information about the nature and distribution of the OM in the present-day Ogoouédeep sea turbidite system,offshore West Africa(Gabon),which has not previously been investigated.Three piston cores were collected both on the continental slope and in the deep sea turbidite system, in order to better understand the transfer conditions of OM by gravity https://www.doczj.com/doc/b614052882.html,ing organic carbon measurements,carbon and nitrogen isotope signatures,Rock-Eval pyrolysis and palynofacies analyses,we not only attempt to quantify the OM preserved in different parts of this deep sea sedimentary system,but also to determine its origin(terrestrial versus marine)and its quality. Finally,we explore the different mechanisms that control the transfer and the burial of organic material from shallow to deep oceanic settings in a context of active downslope gravity processes.

2.General setting

2.1.Environmental setting

The OgoouéRiver drains nearly the whole of Gabon(Fig.1A),and is characterized by a moderate catchment area(203,000km2),com-pared to the Congo(3,550,000km2)and the Niger(1,100,000km2) rivers(Mahéand Olivry,1995).The OgoouéRiver drainage basin is essentially set over the Congo craton and Proterozoic orogenic belts (Séranne et al.,2008).The vegetation on the watershed consists mostly of rainforest with some savanna(Mahéet al.,1990).

The river?ows west e northwest and discharges into the Gulf of Guinea,south of Port Gentil,on both sides of the Mandji Peninsula where it forms a large delta(Fig.1B).The OgoouéRiver is charac-terized by a mean annual discharge of4700m3/s(Mahéet al.,1990), with a low water discharge variability(979e11,300m3/s;http:// https://www.doczj.com/doc/b614052882.html,/).The equatorial climate generates two periods of?ooding,in May and November,relating to the Atlantic monsoon.The sediment load of the OgoouéRiver is not precisely determined although Syvitski et al.(2005)estimated the prehuman sediment?ux between1and10?106t/year.The suspended sediment(37.8mg/L),dissolved organic carbon(8.33mg/L)and particulate organic carbon(2.41mg/L)concentrations were punc-tually measured at an upstream site in November1979(Cadée, 1984).Even if the samples collected were discrete,these values are representative of the most important period of river discharge. The particulate organic carbon(POC)content expressed as the particulate organic carbon/suspended material ratio delivered by the OgoouéRiver is around6.4%,which is similar to the Congo River (Coynel et al.,2005)and high compared to other major rivers (e.g.w1%for the Amazon River;Seyler et al.,2005).These data suggest that the Ogoouéwatershed would be a signi?cant source of organic terrigenous sedimentation on the equatorial West-African continental margin.

On entering the delta,the freshwaters of the OgoouéRiver diffuse into a mixed layer.The thermohaline structure of the upper ocean off the Ogoouédelta can be schematized by a20-m thick mixed layer with low salinity(w29psu)overlapping highly saline waters (w36psu)(Jourdin et al.,2006).The Ogoouéplume is composed by a“mineral plume”extending20km seaward of the river mouth (Jourdin et al.,2006)and characterized by low turbidity(<0.2 Nephelometric Turbidity Unit)and silicate concentrations (4e6m mol/kg).Beyond the mineral plume,a river-induced primary production is observed.This“algal plume”is characterized by an increase of chlorophyll a(w0.1m g/L)at the surface and shows similar extent as the mineral plume(Jourdin et al.,2006).Enhanced productivity is also observed along the South Gabonese margin where seasonal upwelling occurs from July to September(Picaut, 1983;Fig.1A).This equatorial upwelling is explained by the effect of a rapid intensi?cation of easterly winds in the western basin, generating Kelvin waves that propagate eastward along the equator. When they reach the west coast of Africa,the waves bring cooler water near the surface(Hardman-Mountford et al.,2003).

The African coast is affected by a strong,northward-?owing alongshore current(littoral drift).The sedimentary transit associ-ated to this littoral drift in the area of Port-Gentil(Gabon;Fig.1)is estimated to be between300,000and400,000m3/year(Bourgoin et al.,1963).It is responsible for the formation of sandy bars and most particularly of the Mandji Peninsula that forms the north end of the OgoouéDelta(Fig.1).Le Fournier(1972)indicated that the formation of the Cape Lopez Canyon is related to the development of the Mandji Peninsula.The deep incision into the continental shelf by the Cape Lopez Canyon(Fig.2)combined with littoral drift favors the capture of sandy sediment by the canyon(Reyre,1984). Capture of coastal sediments by submarine canyons has already been described in the literature(Shepard and Dill,1966;Burke, 1972)and more particularly in West Africa,with the head of the Congo Canyon(Heezen et al.,1964;Rigaut,1997).At the OgoouéRiver mouth,littoral drift associated with energetic processes on the continental shelf(?oods,storm waves,swells)probably generates gravity?ows in the canyon.

2.2.Physiographic setting and studied sites

2.2.1.Physiographic setting

The sediment load of the OgoouéRiver contributes to the formation of the present-day Ogoouéturbidite system.Although the Cenozoic turbidite system has been studied in details by Mougamba(1999),the present deep sea turbidite system has not previously been investigated.

The Ogoouédeep sea turbidite system is located in the northern Gabonese basin between0and1 S(Fig.1B).The continental shelf is relatively narrow,decreasing in width from60to5km in a northward direction(Mandji Peninsula).The mean slope along the continental shelf is0.5 .The continental shelf break is found at a depth around150m,and is characterized by high slope value(6 ). The slope then decreases slightly to reach2e3 from300to500m water depth and1e2 from500to900water depth.

South of Cape Lopez,a ramp of more or less sinuous canyons extends over50km(Fig.1B).These tributary canyons merge around 1500e1700m water depth and form four channel-levee systems which extend down to the distal part of the basin.The northern-most canyons of this ramp are deviated by the Mount Loiret,an inactive submarine volcano(Reyre,1984;Fig.1B).Several pock-marks are also present in the vicinity of Mount Loiret.Most of them are aligned or grouped into?elds.

L.Biscara et al./Marine and Petroleum Geology28(2011)1061e1072 1062

2.2.2.Studied areas

Three piston cores located in the northern part of the Ogoouéturbidite system were selected for this study (Fig.2).Two cores were collected in the Cape Lopez Canyon area,and one in a pock-mark alignment,further north.

The Cape Lopez Canyon is situated north of Mount Loiret.The head of the canyon is located close to the coast,in about 5m water depth (Fig.2).The canyon incision is small:w 60m and w 90m at distances of 1and 5km from the coast,respectively.The upper part of the canyon (2km)is highly sinuous and straightens with depth.Seismic pro ?les collected in this area (CGG,1972)indicate that the canyon ?oor is formed by indurated sediment and is draped by a recent soft sediment layer with a thickness decreasing towards the coast,from 50m to about 10m at 10e 20m water depth.

Core KS06was collected in the thalweg of the Cape Lopez Canyon (288m water depth)(Fig.2),where the slope is the steepest (6 ).The transition between the canyon and the channel-levee complex is at about 550m water depth where the slope-angle signi ?cantly decreases (w 2 ).Channelization disappears close to the Mount Loiret (w 700m water depth).Core KS02was collected at the end of the channel-levee complex (733m water depth).Imagery data show high re ?ectivity in this area over 90km 2,from 400to 780m water depth.SBP120pro ?les show a high energy chaotic echo-facies in the area that differs from the surrounding areas characterized by low-energy continuous re ?ectors (Fig.2).These data suggest a non-channelized sandy body corresponding to a lobe.The presence of Mount Loiret modi ?es the slope and produces a relatively ?at surface (w 1 ),where sediment trans-ported by gravity ?ows are deposited.

Core KS09(837m water depth)was collected from the slope,north of Cape Lopez in a large pockmark (diameter:400m;depth:40m;mean slope:3.5 ;Fig.2).The location of the core in a pock-mark allows performing a detailed study of non-channelized gravity ?ows along the continental slope.

The area is characterized by several pockmarks aligned between 400and 850m water depth.They tend to connect to form a large submarine valley (w 1km width,up to 150m deep).Pockmark

diameters vary from 150to 400m and they are about 30m deep.The results of Pilcher and Argent (2007)showed that a series of buried slope channels underlie the pockmark alignment.According to these authors,coarse-grained deposits ?lling the channels facilitate the upward migration of ?uid to the subsurface.The origin of the ?uid has not been determined and the channel is presently buried and inactive.The absence of carbonate concretions and ?uid perturbation (de-watering structure)in the core indicates that this pockmark is currently inactive.3.Material and methods

The three Kullenberg-type piston cores analyzed in this study were collected during OPTIC CONGO cruise (2005)(Fig.2and Table 1).103samples were collected from these cores for sedi-mentological and OM analysis with a resolution varying from 0.5cm in beds interpreted as turbiditic deposits,to 20cm in hemipelagic deposits.Only the uppermost meter of core KS06was analyzed because the lower part showed sediment perturbation due to coring.

Core sections were analyzed using the SCOPIX X-ray image processing tool (Migeon et al.,1999).Grain size analyses were performed using a Malvern ?Mastersizer ‘S ’(particle size range from 0.05to 878.67m m).The particulate activity of the excess 210Pb (210Pb exc,Table 2)was measured in order to estimate sedimentation rates during the last hundred years (210Pb half life ?22.3years;Jouanneau et al.,2008).8AMS 14C dating were done on the three cores (Table 2),from vegetal remains sampled in turbidites.A conversion of these radiocarbon ages to calendar ages was made with the Calib 6.0program using the SHcal04calibration (McCormac et al.,2004)and UWSY98calibration for samples younger than 35014C yr BP (Stuiver et al.,2005).3.1.Geochemical analysis

For each sample in the three cores,an aliquot was dried in an oven in order to de ?ne Total Organic Carbon (TOC),C/N and isotopic

ratios.

Fig.1.A)General map of central Africa and the Gulf of Guinea in eastern equatorial Atlantic Ocean,showing the OgoouéRiver,its main distributaries and its watershed,the surface and subsurface currents and highly productive areas (modi ?ed from Schneider et al.,1994);B)General bathymetric map showing the Ogoouédelta and the morphology of the proximal part of the Ogoouéturbidite system.

L.Biscara et al./Marine and Petroleum Geology 28(2011)1061e 10721063

Inorganic carbon was removed prior to TOC and d 13Corg measurements using 1N HCl and dried at 60 C for 24h.The TOC analysis was carried out using a high temperature combustion analyzer on a LECO CS 125analyzer.TOC contents are expressed as a percentage of dry weight of sediment (wt.%).Analytical accuracy,determined from replicate measurements of the same standard sample was generally better than ?5%.Because TOC contents are in ?uenced by sediment grain size (Hunt,1979),a study of the organic contribution in coarse (>150m m),medium (63e 150m m)and ?ne (<63m m)sediment was also performed on core KS09.The different fractions were separated by wet sieving using a 150and 63-m m sieve.The stable carbon and nitrogen isotope composition of the sedimentary OM was determined by on-line combustion in a Carlo-Erba elemental analyzer interfaced with a Micromass-Iso-prime mass-spectrometer.13C/12C and 15N/14N are expressed by the conventional d notation in &relative to PDB and air,respectively.Based on replicates of international standards and samples,analytical precision is ?0.1&for d 13Corg and ?0.3&for d https://www.doczj.com/doc/b614052882.html,rmation on the type and thermal maturity of the bulk OM was obtained by Rock-Eval pyrolysis using Rock-Eval 6Turbo devices (Vinci Technologies)under standard conditions (Espitalièet al.,1985;Behar et al.,2001).Hydrocarbons (HC)are detected by a ?ame ionization detector,and CO 2by a thermal conductivity detector or infra-red detector.58samples were analyzed,repre-senting all the sedimentary facies observed in the cores.Hydrogen and Oxygen Index (HI,mg HC/g TOC;OI,mg CO 2/g TOC)and Petroleum Potential (PP,mg HC/g or kg HC/ton of Rock)were determined.The precision for the parameters is ?10mg HC/g TOC for HI,?5mg CO 2/g TOC for OI and ?1kg/t for PP.20samples,representative of the different sedimentary facies observed in the three cores,were also prepared for palynofacies analysis.Neither oxidation nor ultrasonic probes were carried out during prepara-tion,to avoid accidental destruction of particles.Analysis of the samples mounted on glass microscope slides was performed on

Table 1

Location of the studied cores (Latitude,Longitude and water depth),penetration into the sediment,length of studied interval and number of studied samples.Core Latitude ( S)Longitude ( E)Water depth (mbsl)Location

Penetration (mbsf)Studied

interval (m)Number of samples KS0600035.57300839.821288Cape Lopez Canyon 1.86118KS02000 31.406008 32.803733Cape Lopez lobe 1124KS09

000 28.590

008 32.169

837

Pockmark

5.15

5.15

61

Fig.2.A e Bathymetry (EM120)and B e Acoustic imagery (EM120)of the study area,C e SBP120chirp pro ?le on the Cape Lopez lobe.

L.Biscara et al./Marine and Petroleum Geology 28(2011)1061e 1072

1064

a Zeiss Axoplan microscope.Relative percentages of surface covered by each type of particle were determined,which limits interpretation errors compared to particle counting.The following classi?cation of particles was applied(Tyson,1995):

e Phytoclasts:comprise all opaque and translucent land plant

debris.Two subgroups are most represented in the studied samples:Ligneous Organic Matter(LOM),including all opaque and translucent vascular tissues of land plant debris and Cellular Organic Matter(COM),such as cuticles and epidermis.

e Fungal debris:comprise all the?lamentous segmented parti-

cles and sclerotes.

e Palynomorphs:include all land-derived pollen grains and

spores(sporomorphs)and organic walled algae still identi?able.

e Amorphous Organic Matter(AOM)includes all particular

organic components without microscopic structure;

4.Results

4.1.Cores sedimentology

Core KS06was collected in the thalweg of Cape Lopez Canyon (Fig.2).

The bottom of the core is composed of massive?ne to medium sand with sediment perturbation due to coring at its base.These sandy beds are interpreted as turbidites(T a division of Bouma sequence;Bouma,1962).Between46and58cm,sediment is characterized by massive ochre?ne sand(D50w160e190m m) showing millimetric laminae of organic remains(Fig.3).These sandy beds are also interpreted as turbidites(T a division of Bouma sequence).Finally,the top40cm of the core are made of normally graded millimetric?ne sandy to silty deposits(Fig.3)showing a basal erosive contact and layered with centimetric to decimetric homogenous silty-clay deposits(D50w10e15m m,Fig.3).Sandy to silty beds are interpreted as deposits from turbidity currents(T a division of Bouma sequence)and silty-clay deposits are interpreted as hemipelagic sedimentation.

Activity of210Pb exc activity in hemipelagites indicates that the top 70cm of the core are recent(<100years,Table2).These values suggest that sedimentation in Cape Lopez Canyon is currently active.

Core KS02was collected offshore Cape Lopez,at the mouth of the channel-levee complex(Fig.2).

The bottom of the core consists of massive?ne sand(D50 w150e200m m)showing no sedimentary structure with scattered, compact,silty-clay clasts(D50w10e15m m)of variable size(max. 10cm)(Fig.3).Clasts have irregular shapes and edges.The clay matrix in the sandy facies is almost absent(<3%).The identi?cation of benthic microfauna in these clasts and the study by X-ray imagery (unpublished data)suggest that the deposits were originally composed of layered,sandy and silty-clay beds;the latter was deformed during coring.Massive sand deposits are interpreted as turbidites(T a;Bouma,1962)and silty-clay clasts as hemipelagites.

The top10cm of the core are characterized by a dark-brown sandy-silty bed,rich in organic remains(millimetric to centimetric in size)(Fig.3).This deposit can be related to gravity?ows,possibly related to?ood events of the OgoouéRiver.

Core KS09was collected in a pockmark located on the conti-nental slope(Fig.2).The core shows decimetric gray e brown homogenous silty-clay beds interbedded with dark-brown?ning-upward beds(?ne sand to silty-clay),which are millimetric to decimetric in size,showing an erosive basal contact(Fig.3).Most of these beds are characterized by an alternation of siliciclastic and organic laminae(millimetric to centimetric)with sharp contacts, where planar and cross laminations can be https://www.doczj.com/doc/b614052882.html,anic laminae are abundant in organic remains and pluri-millimetric mica?akes.The?ning-upward beds are interpreted as classic turbidites(Bouma,1962)and silty-clay deposits as hemipelagites.

Activity of210Pb exc activity in hemipelagites indicates that the top170cm of the core were deposited during the last hundred years(Table2).Using these values,a hemipelagic sedimentation rate of w1m/100years can be proposed,calculated by subtracting the turbidite thickness to the whole sediment thickness.

https://www.doczj.com/doc/b614052882.html,anic matter distribution

Among the three cores studied,the mean TOC content observed in core KS06is the lowest(mean:1.1wt.%;Fig.3).Tmax values range between419 C and430 C,indicating that the samples studied are immature in respect to hydrocarbon generation.

Silty-clay beds show high TOC content(mean:2.85wt.%;Fig.3) and a relatively high hydrocarbon generation potential(4.67kg HC/t rock).Mean values of d13C(à27.4&)and C/N ratio(17.4)indicate a mixture of marine and terrestrial OM.Measured HI(145e150mg HC/g TOC)correspond to a Type III kerogen(100

Massive?ne sands show a low TOC content(mean:0.5wt.%; Fig.3)and hydrocarbon generation potential(mean:0.57kg HC/t rock).These low TOC values prevent the characterization of the OM by stable nitrogen isotopes.d13C values are slightly higher(mean:à27&)compared to silty-clay bed.HI values(59e107mg HC/g

TOC)of these deposits are low,therefore OM can be interpreted as Type IV kerogen(HI<100mg HC/g TOC).Palynofacies analysis shows that organic particles are composed of?ne AOM(50%),LOM (20%),partially degraded LOM(20%)and COM(10%).

Sandy beds with laminations of organic remains display and a very low TOC content and hydrocarbon generation potential (mean:0.5wt.%and0.43kg HC/t rock respectively).This facies is characterized by high d13C values(mean:à27.9&).According to the HI values(66e140mg HC/g TOC,mean:84.7mg HC/g TOC),the OM can be interpreted as Type IV kerogen.Terrestrial particles are recognizable in palynofacies,particularly COM(50e60%),LOM (30%)and partially degraded LOM(10e15%).

Table2

Radiocarbon ages and measured sedimentation rates(210Pb exc activity)of cores KS06,KS02and KS09.

Core number Depth

(cm)

Uncorrected

14C age BP

Calendar

age BP

Origin Measured

sedimentation rate

(210Pb exc Activity)

KS0656e>1950LSCE https://www.doczj.com/doc/b614052882.html,b.0.7m/100years

KS0210e>1950LSCE https://www.doczj.com/doc/b614052882.html,b.e

KS0955425?30472LSCE https://www.doczj.com/doc/b614052882.html,b. 1.7m/100years

KS09100140?3095LSCE https://www.doczj.com/doc/b614052882.html,b.

KS091432170?302075LSCE https://www.doczj.com/doc/b614052882.html,b.

KS09303.5970?30844LSCE https://www.doczj.com/doc/b614052882.html,b.

KS09344.51195?301033LSCE https://www.doczj.com/doc/b614052882.html,b.

KS094591270?301123LSCE https://www.doczj.com/doc/b614052882.html,b.

L.Biscara et al./Marine and Petroleum Geology28(2011)1061e10721065

KS02has a higher mean TOC content than core KS06(2.6wt.%and 1.1wt.%,resp.;Fig.3)but is also characterized by a low thermal maturity (420 C

The TOC content is relatively high in the mud clasts (mean:2wt.%;Fig.3).Hydrocarbon generation potential in these deposits is about 2.29kg HC/t rock.d 13C values are stable (mean:à27.5&),and the C/N ratio shows values greater than 20.According to HI values (96e 124mg HC/g TOC),the OM can be classi ?ed as Type III kerogen.However,the organic particles observed in the samples are relatively similar to those observed in KS06silty-clay beds,with dominance of AOM (70e 80%).Partially degraded LOM (10%),LOM (5e 10%)and COM (5e 10%)are also present in lesser proportions.

The sandy-silty bed rich in organic remains,displays the highest mean TOC content of the core (mean:5wt.%;Fig.3).Hydrocarbon generation potential in this type of deposit is rela-tively high (mean:6.50kg HC/t rock).d 13C values are stable in these deposits (mean:à28.3&)and the C/N ratio is high (>20).HI values (96e 143mg mg HC/g TOC)can be interpreted as Type III kerogen.Palynofacies analysis indicates that cellular and ligneous particles are predominant (COM:40%;LOM:40%;partly degraded LOM:5e 10%;AOM:<5%).

Core KS09show the highest TOC content with a mean value of 4.2wt.%(Fig.3).All the samples studied display a low thermal maturity,with Tmax ranging between 419and 429 C.

The TOC content in the silty-clay facies is very homogenous (mean:3.4wt.%).Similarly to core KS06,hydrocarbon generation potential in the silty-clay beds is signi ?cant with a value of 5.4kg

HC/t rock.Mean d 13C values (à25.2&)and C/N ratio (14.3)are the lowest of the study area.HI values varying from 150to 170mg HC/g TOC indicate Type III kerogen.Palynofacies analysis shows an important fraction of AOM (80%),followed by LOM (5%),COM (5%)and partially degraded LOM (5e 10%)(Fig.4).

Turbidite beds show a very high TOC content,up to 14wt.%(mean:5.4wt.%;Fig.3).Hydrocarbon generation potential in these deposits is the richest of the study area (up to 27.6kg HC/t rock,mean:9.2kg HC/t rock).Carbon isotopes (à27.2&)and C/N ratio (20.3)mean values suggest a terrestrial origin of the OM.HI values ?uctuate from 77to 300mg HC/g TOC,with a mean value of 176mg HC/g TOC.Thus,the origin of the OM can be interpreted either as (1)a Type III kerogen showing an important proportion of hydrogen-rich components such as plant cuticles or (2)as a Type II/III kerogen i.e.a mix between terrestrial and marine organic material.Analysis of the palynofacies indicates that these two types are present in the turbidites:OM in a turbidite bed between 45and 100cm consists predominantly of higher land plants:40%COM,30%LOM and 10%partially degraded LOM.HI values in this turbidite bed range from 131to 185mg HC/g TOC.However,palynofacies in another turbidite bed between 301and 306cm suggest a different origin with a high proportion of AOM (50e 80%),which is con ?rmed by higher HI values (up to 300mg HC/g TOC).

The TOC content variability exists also within the turbidite (Fig.5).The detailed study of a sandy turbidite bed in core KS09(Fig.5)shows alternating siliciclastic and organic laminations with sharp contacts.TOC content is relatively low in the siliciclastic laminae:0.45wt.%at the base,and 2.90wt.%at the top of the https://www.doczj.com/doc/b614052882.html,anic laminae show higher concentrations,between 1.70wt.%and 4.90wt.%at the base and the top of the bed respectively.d 13C values increase from à25.9&to à27.6&for terrigenous facies and from à27.3&to à28&for organic facies.Similar trends are observed with nitrogen isotopes whose values range from 4.4&to 6.4&and from 3.8&to 4.6&for siliciclastic and organic laminae

respectively.

Fig.3.Vertical distribution of median grain-sizes (D50,m m),Total Organic Carbon (TOC,weight %),d 13C (&),d 15N (&),C/N and Hydrogen Index (mg HC/g TOC)of the three cores studied.See Fig.2for core location.

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4.3.Estimation of total organic carbon as a function of grain size In order to estimate the OM contribution in different grain size fractions,TOC measurements were performed on core KS09for three grain size intervals:<63m m,63e 150m m and >150m m.The TOC content for each grain size interval is de ?ned as follow:

TOC fn ewt :%T?h TOC f ewt :%T ?efraction weight T

i.

?etotal sediment weight T

Where TOC f is the TOC content measured of the grain size interval and TOC fn the TOC content of the grain size interval normalized to the total sediment weight.Results are summarized in Table 3.

First,mean TOC contents obtained (i)from total sediment and (ii)by the summing the three fractions,are very similar:3.40wt.%and 3.35wt.%for hemipelagic sedimentation,5.50wt.%and 5.15wt.%for turbiditic deposits respectively (Table 3).This suggests that even if TOC values derived from fractions are always lower compared to TOC values derived from total sediment analysis (due to a loss of organic particles during the sieving),such differences are low enough to make these ?rst estimates signi ?cant.TOC contribution is extremely dominant in the <63m m fraction (97%)for hemipelagic sedimenta-tion.Due to their lower density than minerals,OM particles are mainly associated with the ?nest grains constituting the sediment (Hunt,1979).However,in turbidites,the TOC contribution is similar for <63m m and >150m m fractions (w 46and 49%resp.,Table 3).In all cases,TOC present in the 63e 150m m fraction is negligible (1.5%and 6%for hemipelagic and turbiditic deposits

respectively).

Fig.4.Microphotographs of palynofacies observed in KS06,KS02and KS09samples of the Ogoouéturbidite system.A:phytoclast (wax);B:cellular organic matter (cuticle);C:light ligneous organic matter with pyritic crystals;D:Black ligneous organic matter;E:Amorphous organic matter;F:Amorphous higher land plant debris.

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5.Discussion

5.1.Controls on the distribution and accumulation of organic matter in the Ogoouédeep sea turbidite system

Although it is dif ?cult to de ?ne a model for OM distribution based on three cores,our results suggest that two different types of deposition can be observed in the Ogoouédeep sea turbidite system:(1)in the Cape Lopez Canyon and lobe and (2)on the continental slope,north of the Cape Lopez Canyon.

5.1.1.In the canyon and lobe (KS06and KS02)

Hemipelagic sedimentation in the Cape Lopez canyon and lobe is characterized by high TOC content (2e 3wt.%).Although the HI values indicate a type III kerogen (Fig.7),the analysis of d 13C and palynofacies indicate both a marine and terrestrial origin of the OM.Moreover,d 13C measurements suggest a predominant terrestrial origin of the OM in these deposits (Fig.6).Therefore,the relative low HI probably re ?ects the dilution of the marine OM by less reactive terrestrial OM.

OM present in sandy turbidites in the canyon and lobe shows generally low TOC content (0.5wt.%).From the palynofacies

analysis,the origin of the OM in these deposits is interpreted as both marine and terrestrial.OM is probably characterized by a higher contribution of continental source versus marine source (Fig.6).Moreover,the low HI values observed indicate that the OM is oxidized (Fig.7).Terrestrial OM distributed out by the mouths of the Ogoouédelta located south of the Cape Lopez is transported by the littoral drift to the end of the Mandji Peninsula.Cores collected in this area indicate that organic remains are commonly observed in the canyon and on its northern ?ank (CGG,1972).During high energy processes (e.g.storm waves),gravity ?ows are generated in the canyon and the large siliciclastic fraction transported by the littoral drift and diverted in the Cape Lopez Canyon tend to dilute the OM in the turbidites.Ages obtained by 14C dating of organic remains in the turbiditic deposits are in agreement with the sedimentation rate measured by 210Pb exc activity (Table 2),suggesting the terrestrial OM results from a direct transport during high energy events.The oxidation of the OM observed in Rock-Eval pyrolysis (Fig.7)may result either from degradation during its transport by littoral drift and its deposition around the canyon,or/and by the frequent remobilization of sediments by turbiditic ?ows in the canyon.

However,the deposition of a high amount of terrestrial OM in this area is possible.The ?rst 10cm of core KS02are characterized by an organic-rich sandy-silty turbidite bed (5wt.%).d 13C measurements (Fig.6)and palynofacies analysis indicate clearly a terrestrial origin of the OM.The HI values show that the OM in this deposit is less oxidized compared to the sandy turbidites observed in the canyon and the lobe (Fig.3).Consequently,turbi-ditic ?ows carrying large amount of well-preserved OM exist in the canyon.Such events could be directly linked to large ?ood events of the OgoouéRiver,explaining the good preservation of the OM by the reduced transport time,the absence of intermediate deposition on the continental shelf and ?nally,its rapid burial.This interpre-tation is con ?rmed by the present ages obtained on the organic remains by 14C dating (Table 2).

5.1.2.On the continental slope (KS 09)

Pockmarks are morphological depressions that can act as sedi-ment traps for gravity ?ow deposits.The location of a core

in

Fig.5.X-ray Imagery,D50,TOC,d 15N,d 13C and interpretation of a turbidite in core KS09.

Table 3

Estimates of TOC content in different grain size fractions (<63m m;63e 150m m and >150m m)for hemipelagic and turbiditic deposits in KS09core,and the contribution of each TOC fraction to the total TOC content of the sediment.Type of sedimentation/Grain size fraction TOC fn (wt.%)TOC fn

contribution (%)Hemipelagic deposits:<63m m (n ?38) 3.2597.0Hemipelagic deposits:63e 150m m (n ?13)0.05 1.5Hemipelagic deposits:>150m m (n ?4)0.05 1.5Hemipelagic deposits:Sum of grain size fractions

3.35100.0Hemipelagic deposits:Total sediment 3.40e Turbiditic deposits:<63m m (n ?23) 2.3545.5Turbiditic deposits:63e 150m m (n ?23)0.30 6.0Turbiditic deposits:>150m m (n ?23) 2.5048.5Turbiditic deposits:Sum of grain size fractions

5.15100.0Turbiditic deposits:Total sediment

5.50

e

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a pockmark allow us to perform a detailed study of OM transport by non-channelized gravity ?ows along the continental slope.

The TOC content is constant in hemipelagic deposits (w 3wt.%).Although the HI values indicate a type III kerogen (Fig.7),the analysis of d 13C and palynofacies indicate both a marine and

terrestrial origin of the OM.Moreover,the contribution of the marine source in these deposits is higher compared to the one estimated in the canyon and lobe (Fig.6).The terrestrial fraction originates from the settling through the water column of small organic particles transported in the turbid surface plume of the OgoouéRiver.Phytoplanctonic material is probably derived from the river-induced primary productivity,observed offshore of the Ogoouédelta (Jourdin et al.,2006).

The highest TOC contents (up to 14wt.%)were measured in turbidites.The OM is predominantly derived from terrestrial land plants (Fig.6)and has not been subjected to intense oxidation (Fig.7).Because Cape Lopez Canyon captures a signi ?cant part of the sediment transported by littoral drift,the siliciclastic ?ux north of the canyon is reduced.OM is thus concentrated in gravity ?ows,leading to the high amounts of TOC observed in the turbidites.

In this area,turbidity currents may have two possible origins:(1)instabilities at the continental shelf/slope transition after a resting time on the continental shelf or (2)direct transport by river-fed hyperpycnal ?ows during ?oods.The excellent preserva-tion of the organic remains and their low oxidation state is consistent with the second hypothesis.However,the 14C ages obtained on organic remains in the turbidites are much older than the presumed ages from an hemipelagic sedimentation rate esti-mated w 1m/100years (Table 2).This suggests that these organic deposits have been stored temporarily on the shelf before being remobilized by turbiditic ?ows.The good preservation of the organic remains could be explained by a high sedimentation rate on the continental shelf,leading to enhanced preservation of the OM.

Terrestrial OM in the turbidites is associated predominantly with <63m m and >150m m fractions (w 46and 49wt.%resp.,Table 3)while it is negligible for the 63e 150m m fraction (5.62wt.%,Table 3).This means that in the Ogoouéturbidite system,OM transported by turbiditic ?ows exists both in the form of very ?ne OM linked to ?ne particles (silt and clay)and of large terrestrial organic remains.

The TOC content variability in the turbidites can be explained by the existence of two types of particles with different densities:organic and siliciclastic.As the density of organic remains is smaller than the density of the siliciclastic fraction,the latter has a higher settling velocity than the former.Deposit of laminae characterized by contrasting densities translates rapid ?uctuations of velocity and shear stress near the bottom.These velocity ?uctuations

are

Fig.6.OM type in the sediments studied from the Ogoouéturbidite system as de ?ned by the cross-plot of d 13Corg and TOC.Two trends may be highlighted:hemipelagic deposits display constant TOC contents but variable d 13Corg signature.Turbiditic deposits show a relatively clear d 13Corg signature but highly variable TOC

contents.

Fig.7.Kerogen type in the sediments studied from the Ogoouéturbidite system as de ?ned by the cross-plot of TOC and pyrolysis S2parameter (after Langford and Blanc-Valleron,1990).Type III Kerogen is dominant but several samples from the KS06and KS02cores contain type IV Kerogen.

L.Biscara et al./Marine and Petroleum Geology 28(2011)1061e 10721069

probably the result of burst and sweep processes,which affects the boundary layer of the ?ow and generates the alternation of silt and clay laminations in turbidites located on levee (Hesse and Chough,1980;Migeon et al.,2001;Gervais et al.,2001).Periods of silici-clastic deposition (i.e.with low TOC content)occurs during successive burst and sweep https://www.doczj.com/doc/b614052882.html,anic laminae (i.e.with high TOC content)would be deposited when the bursting cycles cease (Hesse and Chough,1980).

5.1.3.Estimation of the terrestrial organic carbon contribution

In order to estimate the terrestrial organic carbon contribution to the Ogoouéturbidite system,we used binary mixing models with different sets of proxies (d 13C,d 15N,N/C ratio)de ?ned as follows:

Fter e%T??eXsample àXmar T=eXter àXmar T ?100

Where Fter is the terrestrial organic carbon contribution and Xsample depends on the proxy analyzed.Xmar and Xter corre-spond respectively to the marine and terrestrial end-member values of these proxies and are determined from the available literature.Because terrestrial OM is relatively depleted in nitrogen,the fraction of terrestrially derived organic carbon is under-estimated by the C/N ratio,and results that were interpreted as the fraction of terrestrial derived carbon should be re-interpreted as the fraction of terrestrially derived nitrogen (Perdue and Koprivnjak,2007).Therefore,the N/C ratio was preferentially used for this estimation.End-member values used for terrestrial OM are:d 13C ?à29&(Bird et al.,1994,1998;Hedges et al.,1986);d 15N ?1(Meyers,1997);N/C ?1/30(Hedges et al.,1986).These values have been reported for equatorial forests and are considered represen-tative of the Ogoouéwatershed.End-member values used for marine OM are:d 13C ?à21&(Tyson,1995;Hedges et al.,1997);d 15N ?9(Meyers,1997);N/C ?1/7(Hedges et al.,1986).

The Fter values calculated using these proxies display generally the same trends:for hemipelagic deposits,the terrestrial contri-bution is larger in the canyon and the lobe than on the slope (65e 90%and 50e 65%respectively;Table 4).Moreover,in the canyon and lobe area,the terrestrial contribution seems relatively similar for hemipelagic and turbiditic deposits while north of the Cape Lopez Canyon,the terrestrial contribution becomes predom-inant in turbiditic deposits compared to hemipelagic ones (60e 80%and 50e 65%resp.,Table 4).

5.2.Hydrocarbon generation potential of deep water organic facies 5.2.1.In the Ogoouéturbidite system

S1values are less than 0.1kg HC/t rock,indicating that sediments examined in this work are immature with respect to petroleum generation and that they cannot contain more than traces of migrated petroleum.The low Tmax values (<431 C)of these samples con ?rm this observation.Mean HC generation potential of studied samples range between 0.1and 27.63kg HC/t rock with a mean about 5.76kg HC/t rock.With an HC generation potential greater than 5kg HC/t rock,the studied samples could be considered as a good future gas-prone source rock,as de ?ned by Espitalièet al.(1985)and Peters (1986).Samples largely exceeding this threshold correspond essentially to turbidites deposits on the slope.

Three parameters contribute to the high amounts of OM in this turbidite system:

-Connexion between the sedimentary source and the canyon :The very limited extent of the continental shelf in the northern part of the Gabonese margin (5e 10km width)favors the transfer of terrestrial OM from the delta to the deep sea turbidite system.Terrestrial OM transfer to deep water was probably even enhanced during sea level lowstand when submarine canyons were directly linked to the river mouths (Degens and Mopper,1976).

-Dilution degree of the OM :The combined action of the Cape Lopez Canyon and littoral drift causes an OM ?ux in turbiditic ?ows to be signi ?cantly higher north of the canyon than in south.Moreover,high energy processes (e.g.storms,instabil-ities,?ood events)will play a signi ?cant role in the temporal dilution/concentration of the OM in gravity https://www.doczj.com/doc/b614052882.html,rge OM accumulation observed in the lobe could be related to a direct transport of the OM from the river mouth to the canyon by turbiditic ?ows generated during large ?ood events.

-Preservation after burial :OM in marine surface sediments is subject to intense biogeochemical processes under oxic and anoxic conditions (Pedersen and Calvert,1990).Fast deposition rapidly moves OM through this zone of preferential degrada-tion and favors its preservation (Hedges and Keil,1995).Thus,high sedimentation rates observed in this area promote the OM preservation in deep water.

https://www.doczj.com/doc/b614052882.html,parison with modern and ancient turbidite systems

Large terrestrial OM accumulations in modern gravity deposits have been reported in several localities:the Makassar Strait (>5wt.%TOC,Huc et al.,2001),the Congo submarine fan (w 3wt.%TOC;Jansen et al.,1984;Treignier et al.,2006;Baudin et al.,2010,)or the Amazon submarine fan (up to 1wt.%TOC,Debyser et al.,1975;Schlünz et al.,1999).However,as far as we know,TOC contents measured in our study area are among the highest ever encoun-tered in a modern turbidite system.These TOC contents are equally high compared to most of fossil analogs (e.g.Boutefeu and Labo ?na,1980;Shipboard Scienti ?c Party,1990;Meyers et al.,1996;Watanabe and Akiyama,1998;Caja and Permanyer,2008;Deniau et al.,2010).Among the different studies published in the litera-ture,the Kutei Basin (Indonesia)represents the best fossil analog of the modern Ogoouéturbidite system.Geochemical analysis real-ized on Miocene sandstones deposited in slope and basin-?oor environments by turbiditic ?ows indicate that the OM transported is extremely concentrated,up to 50wt.%TOC and derived predominantly from leaf fragments (Saller et al.,2006).This terrestrial OM is preferentially concentrated in channel and lobe-related sandstones.According to these authors,fossil leaf fragments were apparently carried from shelf-margin deltas into deep water by turbidity currents during sea level low-stands and are inter-preted as the source for oil and gas in the Kutei Basin.More than 6tcf of gas and 200million bbl of oil condensate have been found in these deposits,in water depth greater than 900m (in Saller et al.,2006).

Table 4

Estimates of Fter contribution (%)of the total TOC content observed for hemipelagic and turbiditic deposits in the studied cores.Estimations are based on the binary mixing models of different proxies.

KS06

KS02

KS09

d 13C

d 15N

N/C d 13C

d 15N

N/C d 13C

d 15N

N/C

Hemipelagic deposits 79.8(n ?6)66.5(n ?6)77.5(n ?6)80.6(n ?3)78.7(n ?3)90.1(n ?3)52.2(n ?36)51.5(n ?36)65.9(n ?36)Turbiditic deposits

79.2(n ?18)

e (n ?0)

e (n ?0)

82.2(n ?15)

89.4(n ?8)

94.7(n ?8)

77.5(n ?25)

58.8(n ?25)

82.8(n ?25)

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6.Conclusions

OM accumulation in the Ogoouédeep sea turbidite system is strongly controlled by the combined action of the Cape Lopez Canyon and littoral drift.

The hemipelagic sedimentation in the study area is character-ized by high accumulations of well-preserved OM(w2e3wt.TOC %).Bulk geochemical and palynofacies analysis indicate both a marine and terrestrial origin of the OM.Contribution of the marine source is higher on the slope than in the canyon and lobe.

By diverting a signi?cant part of the siliciclastic?ux from the littoral drift,the Cape Lopez Canyon in?uences the organic sedi-mentation in turbidites.In the canyon and lobe,turbidites show generally low TOC content(0.5wt.%)and OM is oxidized.The origin of the OM is interpreted as both marine and terrestrial,with a higher contribution of continental source versus marine source. The low TOC contents are due to the high siliciclastic fraction transported by the littoral drift and diverted in the canyon during high energy processes(e.g.storms)which tend to dilute the OM in the turbidites.Transport by long-shore currents and/or turbiditic ?ows probably lead to oxidation of the OM.However,high accu-mulations of well-preserved terrestrial OM have been found in turbidites deposited on the lobe(5wt.%).This large OM accumu-lation and its good preservation could be related to a direct trans-port of the OM from the river mouth to the canyon by turbiditic ?ows generated during large?ood events.

On the continental slope located north of Cape Lopez Canyon, large amounts of OM are observed in turbidites.(up to14wt.%)The OM is predominantly derived from terrestrial land plants and has not been subjected to intense oxidation.These deposits are char-acterized by the highest HC generation potential of the study area (up to27kg HC/t rock),indicating a good potential gas-prone source rock.Because Cape Lopez Canyon captures a signi?cant part of the sediment transported by the littoral drift,the siliciclastic sedimentary?ux is reduced north of the canyon.OM is thus concentrated in the turbidites.14C dating and210Pb exc activity suggest that the OM has been stocked temporarily on the shelf before being remobilized by turbiditic?ows.The TOC content of the turbidites is derived from very?ne OM associated with particles <63m m and from large vegetal remains(>150m m)while the 63e150m m is negligible.TOC variation related to laminae in

turbidites can be explained by burst and sweep processes affecting the boundary layer of the turbiditic?ow(Hesse and Chough,1980).

This study con?rms that gravity?ows play a preferential role in the accumulation and preservation of OM in deep water and that deep sea turbidite systems could be regarded as an environment where organic sedimentation occurs.Therefore,submarine fans and especially those located in high productive areas will be characterized by higher TOC concentrations than many other deep sea sediments.It also suggests that potential reservoir and source rocks could exists simultaneously in these speci?c areas.

Special attention should be given to the spatial variability of OM deposition in the various architectural elements of turbidite system (canyon,channel-levee,lobes)in relation with the different sedi-mentation processes of gravity?ows within these elements.Future detailed investigations into the evaluation of TOC storage ef?ciency in these architectural elements are essential to clearly evaluate hydrocarbon potential in submarine fans.

Acknowledgements

We are grateful to the SHOM for making its data available and we thank all the crewmembers of the R/V Beautemps-Beaupréfor technical assistance during the OPTIC CONGO cruise.We are indebted to J.Saint-Paul,B.Martin and G.Chabaud for their technical assistance in core sampling.H.Derriennic and K.Charlier are also gratefully thanked for TOC analysis and isotope measurements.We gratefully acknowledge A.Coynel and J.-M.Moron for their critical and constructive reading of an earlier version of this paper and S. Laugier for English polishing.Finally,we bene?t from constructive reviews by Henrik I.Petersen,anonymous reviewer and the co-editor Eric Hiatt.We acknowledge the ARTEMIS AMS14C French INSU https://www.doczj.com/doc/b614052882.html,urie Biscara’s PhD thesis is funded by a DGA(French Ministry of Defence)-CNRS doctoral fellowship.This is a UMR CNRS 5805EPOC(University Bordeaux1)contribution n.1768. References

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