AUTHORS

A. Viana � Petrobras, E&P, UN-EXP, 500, GeneralCanabarro St., Rio de Janeiro, 20271-200, Brazil;aviana@petrobras.com.br

Adriano Viana is an advisor for sedimentology atPetrobras E&P, which he joined in 1986. He receivedhis bachelor’s degree in geology from the Uni-versidade Federal do Rio Grande do Sul in Brazilin 1982, and his Ph.D. in marine geology from theUniversite Bordeaux 1 in France in 1998. From 1988to 2001, he studied deep-water sedimentation in theBrazilian Atlantic margin, including the character-ization of modern deep-water depositional systems,turbidite and contour-current-controlled depositionalsystems, and geohazards assessment. In 2001, hejoined the Petrobras Santos Basin Exploration Group.He also coordinates several industry-academy jointprojects.

A. Figueiredo � Laboratorio de Geologia Marinha,Instituto de Geociencias, Universidade Federal Flu-minense, Avenida Litoranea, 24210-340, Niteroi, RJ,Brazil

Alberto G. Figueiredo, Jr. was born in Mococa, Brazil,in 1947. He is a full professor at Fluminense FederalUniversity, Niteroi, Brazil; he is also researcher ofthe Brazilian National Research Council. Albertoobtained a Ph.D. at the University of Miami in 1984and undertook postdoctoral work at the State Uni-versity of New York at Stony Brook in 1991. His majorresearch interests focus on marine sedimentation,processes and products, and shelf and deep-seaenvironments.

J.-C. Faugeres � Departement de Geologie etOceanographie, UMR 5805 ‘‘EPOC,’’ UniversiteBordeaux I, Talence 33405, France;faugeres@geocean.u-bordeaux.fr

Jean-Claude Faugeres graduated from Paris Univer-sity (France) where he received a third-cycle degreein sedimentology. After a seven-year stay as lecturerat Rabat University (Maroco), where he subsequentlyundertook sedimentological research on the SouthRifan series, he moved back to France at BordeauxUniversity where he obtained a Doctorat d’EtatEs Sciences in 1978. Since that time, his researchinvolved the modern deep-sea sedimentation inpassive and active continental margins with a majorinterest on the contour-current-controlled deposits.He was professor of geology since 1991 and is nowjust retired.

A. Lima � Instituto Oceanografico, USP, Praca doOceanografico, 191, 05508-900 Sao Paulo, SP, Brazil

Andrea Franca Lima is a Ph.D. student in theInstitute of Oceanography of the University of SaoPaulo, Brazil. She graduated in geology and has an

The Sao Tome deep-sea turbiditesystem (southern Brazil Basin):Cenozoic seismic stratigraphyand sedimentary processesA. Viana, A. Figueiredo, J.-C. Faugeres, A. Lima,E. Gonthier, I. Brehme, and S. Zaragosi

ABSTRACT

The Sao Tome deep-sea turbidite system, elongated parallel to the

rise of the south Brazilian continental margin, was first interpreted

as a channel-levee system resulting from contour-current activity.

Study of new seismic data permits the proposal of a stratigraphy for

the system and a new interpretation of depositional processes. Three

major depositional units have been recognized that are separated

by major erosive discontinuities. The basal unit seems to be Paleo-

cene to lower or middle Eocene, and the second one, subdivided

into two subunits, is probably upper Oligocene to middle Miocene.

Both units show superimposed north-to-south–channelized turbi-

dite systems, with supply provided directly from a channel network

that crosses the upper margin in the north. The third unit is upper

Miocene(?) to Pliocene or Quaternary and is still under predomi-

nantly gravity processes: turbidite processes in the lower and upper

subunits, and major mass-flow processes in the median subunit. The

sediment sources are located either in the north or in the south, with

sediment provided by major deep-sea channels. The base of the upper

subunit is well marked by an erosive discontinuity (late Pliocene or

Pliocene–Quaternary boundary). Impact of the contour currents is

mainly recorded as widespread erosive surfaces (seismic discontinui-

ties) correlated to global hydrological events and transparent or wavy

deposits. Because this system contains a significant amount of upper

Quaternary sands, it suggests the occurrence of petroleum reservoirs

along the rise and the Sao Paulo Plateau in the lower continental slope.

INTRODUCTION

Giant sedimentary levees are commonly observed on the world’s

deep ocean sea floor. Various processes may be involved in their

Copyright #2003. The American Association of Petroleum Geologists. All rights reserved.

Manuscript received November 15, 2001; provisional acceptance May 20, 2002; revised manuscriptreceived August 22, 2002; final acceptance December 10, 2002.

AAPG Bulletin, v. 87, no. 5 (May 2003), pp. 873–894 873

874 The Sao Tome Deep-Sea Turbidite System (southern Brazil Basin)

formation such as gravity flows (turbidity currents, mass flows) and

bottom geostrophic currents, with processes acting either individ-

ually or simultaneously. Giant levees occur generally in turbidite

systems or contourite drifts (bibliography in Faugeres et al., 1998,

1999; Migeon et al., 2000; Zaragosi et al., 2000) and are commonly

associated with major channels along which they are located, forming

very large channel-levee systems.

Numerous turbidite channel-levee systems have been described

since the 1970s (e.g., Normark, 1978; Normark et al., 1980; Cremer

et al., 1985; Kolla and Coumes, 1987; Hesse, 1989; Pirmez and Flood,

1995; Piper and Normark, 2001). These systems are deposited down-

slope from a canyon mouth. They commonly show a trend directed

perpendicular to the continental margin because of gravity flows

but may be more or less parallel because of the Coriolis force.

Channel-overflow processes are responsible for the lateral growth

of levees. The levee deposits may sometimes contain a significant

amount of sand interfingered in muddy deposits and may form a

good-quality reservoir for hydrocarbons (Migeon et al., 2000).

Contour-current-derived channel-levee systems are less common,

yet more examples have become available in the literature during

the last decade. Various types of contourite levees have been identi-

fied in a variety of morphological and hydrological contexts (McCave

and Tucholke, 1986; Faugeres and Stow, 1993; Faugeres et al.,

1993, 1999). In many cases, the system is elongated parallel to the

margin, as the drift levees tend to be built along the current path-

way. In such systems, the levee consists of major muddy material

and minor sands. These systems are characterized as poor oil-bearing

reservoirs, but can develop large accumulations of gas hydrates (Tu-

cholke and Moutain, 1986; Dillon et al., 1996; Drury et al., 1996).

Sandy drift levees or sheets are deposited only in a certain hydrologi-

cal and morphological setting (Viana et al., 1998), such as along the

upper slope of the Brazilian margin (Viana and Faugeres, 1998) or

in the Gulf of Cadiz (Faugeres et al., 1985; Nelson et al., 1993;

Habgoad et al., 2000).

As a consequence, the levee patterns (trends, bedforms, deposit

geometry, and lithology) vary according to the predominant process

involved. However, overlapping features commonly make the rec-

ognition of the levee-building processes difficult (Masse et al., 1998;

Faugeres et al., 1998, 1999). Such recognition is still more difficult

when both processes are interactive in an area. That is why recog-

nition of deep-sea levee-building processes remains a challenge, es-

pecially for petroleum research.

This paper is a study of an example of such deep-sea levees, the

Sao Tome channel-levee system located on the rise in the southern

Brazil Basin (Viana, 1998).

REGIONAL SETTING OF THE STUDY AREA

The Sao Tome channel-levee system is located in the southern

Brazil Basin, which lies between the Vitoria Trindade volcanic

M.S. degree on shallow seismic in coastal region.Presently, she works on the recognition of contouriteand turbidite-current processes in the Braziliancontinental margin.

E. Gonthier � Departement de Geologie et Ocea-nographie, UMR 5805 ‘‘EPOC,’’ Universite BordeauxI, Talence 33405, France

Eliane Gonthier is researcher in a Centre Nationalde Recherche Scientifique laboratory of Bordeaux IUniversity (France). Eliane obtained a Ph.D. in 1972.Since that time, she has worked on modern deep-sea sediments with a major interest on sedimentaryfacies characteristics and deposit origin and dis-tribution in turbidite and contourite systems.

I. Brehme � Laboratorio de Geologia Marinha,Instituto de Geociencias, Universidade FederalFluminense, Avenida Litoranea, 24210-340, Niteroi,RJ, Brazil

Isa Brehme is a geology professor at FluminenseFederal University, Niteroi, Brazil. She received herPh.D. at the University of Bremen in 1991. Since thattime, her major research interest focus on glacio-marine and marine deep-sea sedimentation, espe-cially in the Antarctic and the south Brazilian margins.

S. Zaragosi � Departement de Geologie et Oceano-graphie, UMR 5805 ‘‘EPOC,’’ Universite Bordeaux I,Talence 33405, France

Sebastien Zaragosi received a Ph.D. in 2001 fromBordeaux I University (France). He investigates thephysiography and Quaternary sediment distribution andprocesses of deposition of the deep-sea fans in the Bayof Biscay in response to sea level and global climaticvariations. He is now a lecturer in Bordeaux andfocuses his interest on deep-sea sand accumulationand paleoenvironments.

ACKNOWLEDGEMENTS

The research presented in this paper was conductedunder a program of cooperation between the Uni-versite Bordeaux I (France) and the UniversidadeFederal Fluminense (Laboratorio de Geologia Ma-rinha), Niteroı, Brasil. Lowrie and Kumar are grate-fully acknowledged for their reviews that helped tosignificantly improve the paper. The job done by theeditor to improve the final copy and the Englishwriting is highly appreciated. This is an UMR/EPOCCNRS 5805 contribution no. 1475.

seamounts to the north and the Rio Grande Rise to the

south (Figure 1).

The continental margin of this basin (Leyden et al.,

1976; Asmus and Guazelli, 1981; Gorini and Carvalho,

1984; Cande et al., 1988; Alves et al., 1997; Aguiar, 1997)

is a large plateau, the Sao Paulo Plateau, located on the

slope at a depth ranging from 2000 to 3400 m (Figures

2, 3). This plateau is affected by numerous halokinetic

structures related to the Aptian salt deposits and sepa-

rates the steep uppermost part of the slope from the

rise. The downslope boundary of the plateau is marked

by the Sao Paulo Plateau escarpment, about 200 m

high, that shows a north-south trend more or less par-

allel to the trend of the margin in the study area. The

escarpment corresponds to the external limit of the

thick salt sequence (Castro, 1992) and is located where

the continental and oceanic crust transition occurs. Be-

yond the escarpment, the continental rise is a more reg-

ular surface that gently dips toward the abyssal plain

and is locally disturbed by volcanic seamounts such

as Almirante Saldanha. A complex network of channels

crosses the plateau where the channel courses are con-

trolled by active diapiric structures. Beyond the escarp-

ment, these channels converge into a small number of

major channels that run across the rise (Brehme, 1984;

Mello, 1988; Castro, 1992; Miller et al., 1996; Alves,

1999). They are active channels transporting sands dur-

ing the late Quaternary (Machado et al., 1998; Viana,

1998). Locally, the escarpment is strongly eroded by

secondary channels.

Between about 22j and 24jS, a gentle accumula-

tion zone (Figures 3, 4), about 100 km long and 60 km

wide, is located at the foot of the northern part of the

escarpment. It is separated from the escarpment by a

channel-like depression. This system was originally iden-

tified in seismic profiles collected as part of the Bra-

zilian REMAC Project (Reconhecimento global da

Margem Continental Brasileira) (Asmus and Guazelli,

1981) and more recently in maps and data published

by Cherkis (1983), Mello (1988), and Castro (1992).

Viana et al. 875

Figure 1. Localizationof the study area in thesouthern Brazil Basin.

Sediment distribution on the rise was interpreted as

mainly controlled by the Antarctic bottom-water cur-

rents (Damuth and Hayes, 1977; Kumar and Gamboa,

1979; Barker et al., 1983a,b; Gamboa and Rabinovitz,

1981, 1983; Johnson and Rasmussen, 1984; Brehme,

1984; Mello et al., 1992; Mezerais et al., 1993; Masse

et al, 1994, 1996; Petchick et al., 1996; Faugeres et al.,

2002). Likewise, the system was interpreted as de-

rived from contour-current activity and was named the

Guanabara Channel and Drift (Mello et al., 1992).

This system, 3500–4000 m deep, corresponds to

the transitional zone between the North Atlantic deep

water and the Antarctic bottom water (Reid et al.,

1977; Reid, 1996; Hogg et al., 1982). The North At-

lantic deep water flows southward with a very low ve-

locity (less than 5 cm/s) at a depth of 1200–4000 m,

and the Antarctic bottom water flows northward at a

depth greater than 4000 m at a velocity not exceeding

10 cm/s in the open basin. However, recent data (Reid,

1989, 1996; DeMadron and Weatherly, 1994; Siedler

et al., 1996; Hogg et al., 1996) have shown a more

complex circulation with rapid and temporary velocity

and trend changes. Uncertainty in the paleocirculation

patterns since their initiation during the late Eocene

to early Oligocene (Kennett, 1982) makes the inter-

pretation of sediment patterns difficult. Several Ceno-

zoic hydrological events are responsible for major dis-

continuities in the deep-sea deposits, as observed at a

876 The Sao Tome Deep-Sea Turbidite System (southern Brazil Basin)

Figure 2. Physiography of the South Brazilian continental margin: (a) the channel network (after Castro, 1992) and the location ofthe Sao Tome channel-levee sytem; (b) 3-D view of the sea-floor morphology along the Brazilian margin off Cape Sao Tome. Note in theupper slope the Almirante Camara canyon (1) and the Sao Tome canyon (2) that provide sediments to the modern Sao Tome deep-seaturbidite system (ST-DSTS), the complexity of the submarine morphology and drainage complex over the Sao Paulo Plateau as a resultof the Aptian salt halokynesis, and, in the first plan, the Sao Paulo Plateau outer escarpment, the Guanabara Channel, and the SaoTome levee (ST-DSTS).

global scale. The Quaternary has also seen a variety of

current variations (Johnson et al., 1977; Ledbetter, 1986;

Masse et al., 1994, 1996).

DATA AND METHODS

During the BYBLOS cruise (Faugeres et al., 1998), 470

km of air-gun seismic lines and echo-sounder lines and

seven Kullenberg cores were collected in the study area

(Figure 3). These new data were used to prepare a de-

tailed bathymetric map and to determine the distribu-

tion of depositional environments in the area (Figure

4). However, the available seismic data are not suffi-

cient to map any of the facies over large areas. Corre-

lation with data from Deep-Sea Drilling Program site

515, located south of the study area, and more recent

studies in the southern Brazil Basin (Mezerais et al.,

1993; Castro,1992; Alves, 1998; see Table 1) suggest

that the sediment deposition in the study area took

place mostly during the Oligocene and Neogene, and

the basal sediments were deposited during the Paleogene

or possibly later (see age discussion in Stratigraphy).

Sao Tome Channel-Levee System Morphology

The Guanabara Channel is about 70 km long. Its width

ranges from 2 km in the north to 10 km in its central

part and 4 km in the south. It is approximately 200 m

deep with respect to the top of the escarpment and

as much as 75 m deep with respect to the levee in its

middle part. The relative depth decreases northward

and southward. The sediment accumulation area shows

a sector of maximum deposition along the channel, here

called the levee (s.s.), and a transitional area between

the levee and the rise (Figure 4).

The north-south–elongated levee (s.s.) is 70 km

long to 30 km wide. It is separated into northern and

southern parts by an east-west depression. This de-

pression is about 40 m deep and opens downslope. In

the north, the levee deepens southward, and has a short

western flank toward the channel and a wide eastern

flank gently sloping basinward. The sea floor displays

large flat areas with a few small sediment mounds scat-

tered throughout. In the south, the levee has a more

symmetric shape and regular sea floor molded by sed-

iment waves and uncommon small mounds. Here, the

Viana et al. 877

Figure 2. Continued.

top of the accumulation reaches the shallowest depth

(3500 m) and gently deepens northward.

The transitional area presents a far more chaotic

sea floor, with gullies, sediment wavelike and hum-

mocklike bedforms in the north and the middle part of

the system, and more regular wavy bedforms and west-

east–directed gullies in the south (Figure 4).

Two major deep-sea channels (about 15–20 km

wide and 40–60 m deep) supply sediments to the area.

The Macae Channel to the north (Figures 2, 4) is part

of a continuous feature (Figure 2) that connects the

continental shelf and upper slope (Almirante Camara

canyon, Machado et al., 1998; Gorini et al., 1998) to

the deep rise and the Columbia Channel (Brehme,

1984). It plays a major role in feeding the northern part

of the system. Farther south, secondary channels may

feed this area directly across the escarpment (Figure

2). The Carioca Channel (Brehme, 1984; Mello, 1988;

Castro, 1992; Alves, 1999) in the southernmost part

of the study area cross the rise close to the southern end

of the system, where our profiles cross this channel only

once (Figures 2–4). However, we suspect that such a

major channel plays a significant role on the deposit

geometry in this area. The linkage between the Carioca

Channel and the upper margin channel network re-

mains unclear. It may be linked to the Sao Tome canyon

system and/or to southern Campos basin canyons.

Neogene Deposit Geometry

Three depositional units, units I, II, and III, respec-

tively, from the base to the top of the sedimentary se-

quence (Figures 5–10) have been defined. However,

neither an acoustic basement nor a clear basal boundary

for unit I is recorded by most of the BYBLOS seis-

mic lines except for the southernmost one (Figure 7),

where a volcanic basement might be present. The total

878 The Sao Tome Deep-Sea Turbidite System (southern Brazil Basin)

Figure 3. Detailed bathymetric map of the Sao Tome channel-levee system and location of the seismic lines and cores (11,12,. . . 17).

Figure 4. The Sao Tome channel-levee system and the differ-ent modern depositional areas and associated channels; arrowssuggest turbidity-current pathways.

thickness of the deposits ranges from about 1 to 1.7 s

(two-way traveltime) and represents about 900–1500 m

of sediments, assuming a mean velocity of wave prop-

agation of 1800 m/s. Each of these units are bounded

by major discontinuities that are spread all over the en-

tire accumulation area.

Unit I

The basal unit I is observed in all the profiles, and it

has a mean thickness of 0.5–0.6 s. Nearly transparent

reflections form the deepest layers. In the upper part,

unit I shows high-amplitude, continuous to semicontin-

uous, parallel reflections with locally wavy geometry.

It is bounded upward by a gently erosive discontinuity

(R1) that deepens slightly southward.

In the north, there are some shallow channels and

mounded geometry (Figures 9, 10b) that seems to con-

verge southward into a major mounded depositional

system (Figure 5). This system could be elongated north

to south, as it seems to extend up to the southern end of

the study area where unit I shows a slightly mounded,

lobelike geometry (Figures 7, 11). The system axis (Fig-

ure 5) corresponds to a fairly high relief with high-

amplitude reflectors and some channel features. East-

ward and westward from the sediment high, downlapping

and offlaping reflections suggest the occurrence of lat-

eral levees that gently deepen toward lower relatively

flat areas. This geometry suggests a turbidite-perched

channel-levee system (in Bouma et al., 1985; Pirmez

and Flood, 1995). At the foot of the escarpment, evi-

dence of shallow channels is observed (Figures 5, 6c, 9).

In the southernmost part of the system (Figure 7), the

distal lobelike deposit overlays layered deposits filling

up the relief irregularities of what could be the oceanic

crust. The unit total thickness here is about 1 s, in-

cluding about 0.5 s for the lobelike upper part. In the

north, near Almirante Saldanha Seamount (Figures 4,

10a), shallow channels, probably directed southwest-

northeast, develop during the last stage of unit I.

All these patterns point out a predominantly

turbidite-depositional process, with a northern source

and a major north-south–directed system (Figure 11).

Unit II

Unit II is the thickest unit (0.5–0.95 s, 400–800 m),

and the maximum thickness is developed in a western

Viana et al. 879

*Dce = discontinuity; see text for further explanations.

Table 1. Tentative Correlations of the Seismic Major Sequences and Unconformities in the Southern Brazil Basin from PreviousPublished Works and Correlations with the Proposed Stratigraphical Interpretation for the Sao Tome Deep-Sea Turbidite System*

880 The Sao Tome Deep-Sea Turbidite System (southern Brazil Basin)

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depression inherited from R1 erosive surface (Figures

5, 9). This unit’s reflections are predominantly trans-

parent to chaotic. The unit is divided into two subunits

by a semicontinuous, irregular, high-amplitude reflec-

tor (discontinuity R1a) that shows erosive features in

the west and becomes more regular and undulated east-

ward (Figure 5). However, in the south, this disconti-

nuity has disappeared (Figure 7), and no subunits can be

distinguished.

Subunit IIa

The basal subunit IIa is the thickest (0.4 s, 360–400

m) in the west, close to the Sao Paulo Plateau (Figure

5). There, it shows semicontinuous reflections that,

eastward, onlap onto the channel-levee relief inherited

from unit I. That supports the occurrence of a new

north-south–feeding channel system (Guanabara Pa-

leochannels), located between the Sao Paulo Plateau

escarpment and the unit I channel system (Figure 11).

Farther east, the deposit thickness decreases (0.25 s,

230 m) and the facies becomes more or less transparent

with some tiny wavy reflections of either turbidity or

contour-current origin.

Subunit IIb

The overlying subunit IIb is thicker than subunit IIa

(as much as 0.6 s, 550 m) with a more predominant

transparent facies (Figures 5–10b). This facies is asso-

ciated with discontinuous high-amplitude reflections

scattered throughout, most of them stacked in a column-

like shape (Figure 5). Some more continuous reflections

show a divergent geometry (Figure 5) that suggests a

possible mounded deposit center similar to the unit I

system. However, such a feature is not visible farther

north (Figure 9) and south (Figure 7).

At the foot of the escarpment (Figures 5, 6c), well-

stratified, more or less discontinuous, high-amplitude

reflections form a thick lens of deposits that are inter-

digitated eastward into the transparent deposits. West-

ward, they show small-scale moats and minor channels

and levees that present no drift geometry. Such depos-

it patterns suggest the presence of active Guanabara

Paleochannels at that time, with dominant turbidity cur-

rents and little or no activity of contour currents. Far-

ther north (Figure 9), these deposits show more irregular

discontinuous reflections and lenses of transparent fa-

cies. They could be caused by either depositional-gravity

Viana et al. 881

Figure 6. Details of the BC profile (location in Figure 5): (a) active diapirlike structures associated with subunit IIIb, and subunit IIIcdeposits trapped and deformed between these structures, (b) slumped deposits. AB profile crossing Guanabara Channel (location inFigure 3); (c) seismic line and interpretation. Twt = two-way traveltime.

882 The Sao Tome Deep-Sea Turbidite System (southern Brazil Basin)

Figu

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processes (e.g., debris flows or turbidity currents) or post-

depositional sediment deformation (e.g., slide or tec-

tonic). Such deposits would correspond to a north-south

system trapped at the foot of a probably already existing

escarpment (Figure 11).

The upper part of the subunit is characterized by

transparent facies that are very thick toward the west,

where they fill the depression between the lenticular

system in the west and the mounded system in the east

(Figure 5). At the top, wavy reflections are widespread

(Figures 5–7, 9), which suggests sediment waves con-

trolled by contour currents.

In the southern end of the system (Figure 7), no

subunits can be defined. Most of unit II consists of trans-

parent facies associated with high-amplitude discontin-

uous reflections showing chaotic dips (hummocklike

reflections) that suggest deposit deformations induced

by argilokinetic processes or by overpressured water ex-

pulsion (Cartwright, 1994). At the base of the unit, a

lens of more continuous well-stratified reflections with

an erosive top surface is visible to the west. It could be

intrepreted as lobelike deposits linked to the unit IIa tur-

bidite system (Figure 11). Laterally to the east, the trans-

parent facies merges into high-amplitude reflections

suggesting a channel-levee geometry (Carioca Paleochan-

nel, Figure 8). That supports the activity of a southern

sediment source, during the base of unit II deposition.

In the northern end of the system, there is no clear

evidence of the occurrence of an active Macae Paleo-

channel at the foot of Almirante Saldanha Seamount.

The unit II tentative reconstruction (Figure 11) sug-

gests depositional conditions in some way similar to

those of unit I as we infer stacked north-south–elongated

systems. These systems could be fed by a major turbi-

dite supply coming from the north and by minor slides

and debris flows coming probably from the escarpment

directly. A southern minor source was active at least

at the time of lower unit II. The transparent chaotic

deposits interbedded in the turbidite systems could

have been deposited by contour currents similar to the

Columbia channel-levee system farther north (Fau-

geres et al., 2002). However, postdepositional distur-

bance by fluid overpressure might also be responsible

for this seismic facies.

A major discontinuity (R2) strongly marks the units

II and III boundary (Figures 5, 7, 9). It is underlined by

a very irregular surface showing erosive depressions (as

much as 3–4 km large, 50 m deep) and a seismofacies

change. The modern erosive Guanabara Channel is al-

ready well developed, and was probably initiated slightly

earlier (Figures 5, 6c).

Unit III

Unit III shows far more complex deposit geometry

and variations in thickness that do not exceed 0.4 s

(350 m). According to the geometry, three subunits

can be recognized.

Subunit IIIa (0–205 m)

This subunit corresponds to a major change in the sed-

iment distribution compared to unit II. It is well de-

veloped in the northeastern part of the system (Figures

9, 10b) but has not been deposited (or preserved) in the

south (Figures 5, 7). In the northernmost part of the

system (Figure 10a), these deposits are clearly related to

the Macae Channel, now an erosional-depositional chan-

nel (Clark and Pickering, 1996). They consist of high-

amplitude continuous reflections that are parallel or

divergent and downlap southward, suggesting turbidite-

overflowing processes for most subunit IIIa deposits. No

deposits in the south suggest no active southern source

of sediments (Figure 11).

Subunit IIIb (0–200 m)

This subunit is a distorted layer well developed in the

central part of the system (Figures 5, 9, 11). In this area,

Viana et al. 883

Figure 8. Details of the LA profile (see location Figure 7).Detailed deposit geometry is associated with the Carioca Chan-nel and Paleochannel. Twt = two-way traveltime.

884 The Sao Tome Deep-Sea Turbidite System (southern Brazil Basin)

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e.

Viana et al. 885

Figure 10. Profiles crossing the north-ern Sao Tome system. (a) IJ1 strike pro-file crossing the northernmost part ofthe Sao Tome system and the MacaeChannel and profile interpretation. (b)J1J2 dip profile crossing the midpart ofthe Sao Tome system and profile inter-pretation. Twt = two-way traveltime.

its top surface is very irregular with reliefs as much as

40 m. Transparent facies with some chaotic reflections

are predominant. Diapirlike structures, rotational blocks,

minor faults, and compressional ridges or wavy bed-

forms may occur (Figures 5, 6, 9). Locally, the struc-

tures suggest slumped deposits (Figures 5, 6a). In some

other places, the deposits may have nearly completely

disappeared (Figure 9). Most of these features may be

caused by active argilokinetic or halokinetic deforma-

tions. In case of halokinetic processes, the subunit could

correspond to a giant mass flow of salt coming from the

Sao Paulo Plateau. The diapiric processes, whatever the

886 The Sao Tome Deep-Sea Turbidite System (southern Brazil Basin)

Figure 11. Diagrams showing the major stages of the Sao Tome system evolution. SPPl = Sao Paulo Plateau; SPPlE = Sao PauloPlateau Escarpment; aAS = active volcanic Almirante Saldanha Seamount; AS = nonactive seamount; a = Antarctic bottom water; b =North Atlantic deep water; numbers 5–13 refer to the figure that illustrates the seismic facies and deposit geometry associated withthe environments mapped for each unit.

origin, have been active recently, as the overlying deposits

(subunit IIIc) are deformed between active diapirs (Figure

6b). Subunit IIIb could then correspond to a major

slope failure with a mass flow directed eastward (Figure

11). That could explain the central depression observed

today in the system.

Westward, at the foot of the Sao Paulo Plateau es-

carpment, the Guanabara Channel is well developed,

with deposits showing both high-amplitude, continuous

reflections (probably turbidites) and chaotic to transpar-

ent reflections (slide, debris flows[?]) (Figures 5, 6c, 9).

In the southern end of the system (Figure 7), subunit

IIIb presents no evidence of postdepositional deforma-

tion or sliding, and the thickness drastically decreases.

The deposits show discontinuous to chaotic reflections

in the west and merge eastward into more continuous

and undulated reflections. Farther down, the deposits

are bounded by the Carioca Channel that erodes the

Sao Tome levee deposits down to unit II (Figure 8).

Most of the channel erosion is synchronous of subunit

IIIc basal erosive event (R3). However, the channel was

probably active during subunit IIIb deposition and could

have provided sediment to the system.

In the northern end of the system, the deposits pinch

out westward (Figure 10b) close to Guanabara Channel

and extend eastward (0.2–0.3 s in thickness). Stratified

reflections similar to that of subunit IIIa are associated

with minor lenses of transparent facies similar to that

of the mass flow in the central depression (Figure 10).

They probably correspond to mass-flow sediments inter-

digitated into turbidites overflowing the Macae Chan-

nel (Figure 10a).

Subunit IIIb sediments seem then to have been sup-

plied, for one part, by the very active Macae Channel

in the north and, for another part, probably by the

Carioca Channel in the south (Figure 11). The origin

of the sediments that form the slide deposits, however,

remains speculative.

Subunit IIIc

The major discontinuity (R3) at the base of subunit IIIc

shows reliefs both inherited from the subunit IIIb top

surface and from erosion at that time. A drastic erosion

affects the southern part of the Guanabara Channel (Fig-

ures 5, 6c), which cuts down throughout subunits IIIb

and IIb. Farther north, the channel erosion decreases pro-

gressively (Figures 9, 10b), suggesting processes caused

by northward-directed turbidity currents. That fits with

the eastward minichannel migration observed in the chan-

nel infill (Figure 5). In the south, active erosion by the

Carioca Channel also occurs (Figures 7, 8) as mentioned

previously. Only gentle erosion takes place in the Macae

Channel at that time (Figure 10a).

The southern Guanabara Channel infill shows a

maximum thickness (0.25 s; as much as 250 m), and the

adjacent levee, only thin deposits (0.05 s); the chan-

nel plays as a sediment trap for the turbidite supply.

Northward, the deposit thickness decreases along the

channel (0.15 s), but increases relatively on the levee

(0.09 s). That suggests deposition by turbidites over-

flowing the levee (Figures 5, 9). In the north (Figure

10b), the channel deposits increase again slightly (0.12

s). Such sediment distribution is consistent with a ma-

jor source in the south (Carioca Channel) and a minor

source in the north (Macae Channel) (Figure 11).

The deposits consist mainly of well-stratified re-

flections interpreted as turbidites. They form locally a

continuous cover on the levee (Figures 5, 9) or pounded

turbidites that fill in small troughs between mounded

(diapiric?) structures (Figure 6b). Wavy bedforms north

and south of the Carioca Channel (Figure 7) could be

turbidity or contour-current sediment waves.

During the deposition of subunit IIIc, west- to east-

directed turbidite systems have controlled the sedi-

mentation, and the overflowing processes are far more

active in the south than in the north.

The predominance of turbidite processes is well

demonstrated for the Quaternary sediment deposition

by the Kullenberg core data (Viana, 1998; Gonthier et al.,

personal communication). The Guanabara Channel de-

posits consist of thick (as much as 30 cm) sandy turbi-

dites (median diameter as much as 140 mm) and debris

flows interbedded in hemipelagites (Figure 3, core 11).

On the levee (cores 12, 15, and 13), the deposits are com-

posed of frequent (5 sequences/m), thin (centimeter-

scale), sandy-silty (mean diameter as much as 74 mm) to

silty-clayey turbidites (as much as 25 mm). The turbi-

dites are interbedded in hemipelagic and pelagic de-

posits. The ratio of sandy silt to mud ranges from 0.10

to 0.22. These deposits result from turbidity currents

overflowing the Guanabara Channel and come from

both southern and northern sources. Similar deposits are

present in the transitional zone (cores 13, 14, 16, and 17).

However, in this area, the turbidite frequency may be

slightly higher (as much as 6 sequences/m), and the ratio

of sandy silt to mud is as much as 0.29. These features

suggest that the turbidites in the transitional zone did

not overflow directly from the Guanabara Channel but

from other channels (Carioca and Macae channels). No

striking evidences of contour-current–controlled deposits

are observed in these cores. The deposits that indicate

the impact of contour current are yellowish, brownish,

Viana et al. 887

silty-clayey beds associated with erosional surfaces and

black laminations with manganese oxides.

DISCUSSION

Sedimentary Processes

The predominant sedimentary processes involved in

the building of the Sao Tome system are downslope

processes, including major turbidity currents. They form

(Figure 11) (1) north-south channel-levee systems and

lobes (units I and II), (2) overbank deposits associated

with major west-east channels (subunits IIIa and IIIc),

(3) major mass flow (giant sliding) (subunit IIIb), and

(4) small-scale slides or debris flows (units II and III)

originating probably from the Sao Paulo Plateau escarp-

ment. The seismic data show little reliable evidence of

contour-current control on the deposit distribution; only

some wavy reflections observed at the top of subunits

IIa, IIb, and IIIb, as well as IIIc southeastward, could

have been formed by such currents.

However, we know that the study area has been

swept by the active Antarctic bottom-water and North

Atlantic deep-water circulation during the Paleogene to

Neogene. Then, the question is What is the real impact

of contour-current processes in such an area that has

suffered such active turbidite processes, and how was

it recorded?

The widespread discontinuities appear to be ero-

sive surfaces that represent time lines. In a turbidite

system, major discontinuities are commonly discon-

tinuous and diachronous surfaces, as they are mainly

formed through channel migration. That is why the dis-

continuities observed here seem to have been formed

by contour currents (Faugeres et al., 1999) instead of

turbidity currents. Consequently, we have tentatively

correlated them with the major hydrological events

known at the global scale.

The transparent facies of unit II has been inter-

preted elsewhere in the southern Brazil Basin as con-

tourite deposits (Castro, 1992; Alves, 1999; Faugeres

et al., 2002). Why not a similar interpretation for the

Sao Tome levee? In our study area, as these deposits are

clearly associated with turbidite systems, the role of the

contour current could have mainly concerned the re-

distribution of the fine-grained sediment pirated from

turbidite plumes. However, no seismic evidences sim-

ilar to those in the Antarctic margin (Rebesco et al.,

1997) are present here.

Stratigraphy

We know that during the Paleogene–Neogene, events

of very intense Antarctic bottom-water and North At-

lantic deep-water circulation occurred several times:

Paleocene–Eocene boundary (50–55 Ma), early to mid-

dle Oligocene (40–34 Ma), end of the Oligocene (25

Ma), during the late Miocene (12–8 Ma) and late

Pliocene (3–2.5 Ma) (e.g., Kennett, 1982; Johnson,

1983; Mountain and Tucholke, 1985; Tucholke and

Mountain, 1986; McMaster et al., 1989; Locker and

Laine, 1992; Berger and Wefer, 1996). These episodes

have been recorded all over the world’s oceans in the

form of major erosive surfaces and a large hiatus in the

sediment record. We have collected the available data

concerning the records of these events in the southern

Brazil Basin (DSDP site 515, Figure 1; Barker et al.,

1983a,b; Gamboa et al., 1983; Viana et al., 1990;

Castro, 1992; Alves, 1999; Mezerais et al., 1993). All

of these data have been summarized and correlated

with the Sao Tome system data (Table 1).

The most reliable correlation that clearly arises

is between the R2 discontinuity and unconformity

IV of site 515, DVI of Castro, R4 of Mezerais, and D4

of Alves, corresponding to the middle to late Miocene.

This event is marked by a high-amplitude reflector

all along the east Brazilian margin dated at 10.8 Ma

(reflector Cinza of Viana et al.,1990). The age of the

underlying and overlying discontinuities is more spec-

ulative. However, as shown in Table 1, we propose the

following assumed stratigraphic frame for the Sao Tome

system: unit I = Cretaceous(?)–Paleogene; unit IIa =

upper Oligocene; unit IIb = lower to middle Miocene;

subunits IIIa and b = upper Miocene(?)–lower Plio-

cene; subunit IIIc = upper Pliocene(?)–Quaternary.

Sao Tome System Evolution

A six-stage scenario may be proposed for the system

growth since the Paleogene.

Stage 1 (unit I, 450-m-thick deposits, Paleocene[?]–

Eocene) deposits seem to correspond to a major turbi-

dite depocenter directed north to south (about 40 km)

along the middle part of the study area. It merges south-

ward into a lobelike accumulation. Sediments were sup-

plied from the northern plateau. Such a deep turbidite

system fits with the abundant terrigenous supply, tur-

bidite complexes (Barracuda/Caratinga of Assis et al.,

1998), and large-scale debris flows (pebbly sandstone

of Guardado et al., 1990) present at that time in the

888 The Sao Tome Deep-Sea Turbidite System (southern Brazil Basin)

upper slope. This stage stopped with the R1 erosive

event (late Eocene to Oligocene).

Stage 2 (subunit IIa, 400-m-thick deposits, late

Oligocene) deposition was still controlled by a north-to-

south–directed turbidite system, but the system shifted

westward. A minor source was probably active in the

south. This stage corresponds to a global sea level fall

during the late Oligocene (Chatian–Rupelian, 35 Ma)

and the huge amount of sediment delivered into the

deep sea (Vail et al., 1977; Haq, 1991). Eastward, trans-

parent reflections could correspond to sediment redis-

tributed by contour currents. A second major event (R1a)

(Oligocene–Miocene boundary) is responsible for local

erosion and wavy bedforms.

Stage 3 (subunit IIb, 550-m-thick deposits, early to

middle Miocene) deposits seem to have been controlled

by contour currents and have transparent reflections.

They are associated with a well-stratified turbidite sys-

tem, restricted at the foot of the escarpment, and still

directed north to south. A Sao Tome paleosystem would

have been initiated at that time. This stage is synchro-

nous with an episode of low sea level and abundant

sediment supply on the upper slope (Castro, 1992; Car-

minatti and Scarton, 1991; Souza Cruz, 1995). A third

erosive event (R2) marks the end of the stage (middle/

upper Miocene boundary).

Stage 4 (subunit IIIa, late Miocene[?] to early Pli-

ocene) shows a new sediment-distribution pattern with

a maximum thickness of 250 m in the north and no

sediment in the south. The sediment still came from

the northern plateau but converged into a west-east

major Macae Channel that had just been formed on

the northern boundary of the system. Only turbidity

currents overflowing from this channel fed the system.

There is no evidence of a significant southern source of

sediments.

Stage 5 (subunit IIIb, [?] late Neogene) is marked

by a major slope failure and a giant mass flow in the

central part of the system. However, the northern

source is still active, with turbidite overflowing from

the Macae Channel. The southern source becomes active

again, with turbidites overflowing the southeastern end

of the system from the erosive Carioca Channel. The

R3 event at the top (Pliocene/Pleistocene boundary)

is responsible for drastic erosion caused by turbidity

currents and possibly contour currents. It induces the

modern system morphology and fits with active ero-

sion on the upper slope (Viana et al., 1999).

During Stage 6 (subunit IIIc, late Pliocene(?) to

Quaternary) deposition, the major source of sediment

was in the south. The sediments were first transported

into the deeply eroded southern end of the Guanabara

Channel where ponded turbidites were deposited. Tur-

bidity currents flowed northward along the channel and

overflowed onto the top of the levee. Probably, another

major part of the available supply flowed downward

along the Carioca Channel from where it may have

overflowed onto the southern system. Some sediments

were also provided by the Macae Channel.

Sediment Origin and the Sea Level and Tectonic Control ofSedimentary Processes and Deposits

Most of the sediments that were deposited in the Sao

Tome system came from the upper part of the margin

after crossing the Sao Paulo Plateau. There were at least

two major drainages: one in the north, with sediments

transported by the Almirante–Itapemirin canyon net-

work (Figure 2b) and connected farther down with

Macae Channel, and one in the south, with sediments

transported through Sao Tome or Campos basin south-

ern canyons beyond the plateau to the Carioca Chan-

nel. Variations in the impact of these sources upon

sedimentation may be explained by variations of sea

level and the location of the major river mouths, the

channel network on the plateau caused by sedimentary

processes or salt-diapiric activity, and the continental

margin trend and trend of the dip caused by tectonic

and volcanic activity.

Unit I (Paleogene(?)–Eocene) corresponds to a pe-

riod of high volcanic activity in the southern Brazil

Basin (Cordani and Blazekovich, 1970; Fodor and Hanan,

2000), which could have controlled the deep-sea to-

pography and favored the triggering of sediment de-

livery. The rise of the Almirante Saldanha Seamount

and Vitoria Trindade chain (40–50 Ma) north of the

study area may have induced a southward dip of the

sea floor and generated north-south turbidite systems

during unit I and II deposition. Later during unit III

deposition, the decrease of the volcanic activity may

have caused the sea floor to die to the east, which is the

dip of the present margin. At that time, the downslope

currents that crossed the rise were directed eastward,

excavating the Macae Channel. The high-amplitude

to transparent reflectors and prolonged sea-floor echo

observed inside that channel suggest coarse-grained

sediment transport. Sand supply by the northern chan-

nel network upslope was previously demonstrated by

Machado et al. (1998).

Modifications of Sao Paulo Plateau morphology

may have occurred throughout the Cenozoic because

of salt diapirism. Diapirism would have induced shifts

Viana et al. 889

in the course of the channels as they crossed the es-

carpment. These processes may explain the variations

of the sediment volume delivered to the north and to

the south of the study area and the variation of the pre-

dominant source during deposition of unit IIIa (Macae

Channel northern source) and unit IIIc (Carioca Chan-

nel southern source).

A direct relationship between sea level fluctua-

tions and the volume of deposits is difficult to address,

because of the low resolution of the available seismic

lines and probable interaction between the relative sea

level and bottom-current activity. Based only on the

upper Quaternary data, major periods of sediment

input to the deep sea via turbidity currents seem to be

related to sea level falls (Gonthier et al., personal com-

munication). During unit IIb deposition (early to mid-

dle Miocene), the widespread transparent seismic facies

could be associated with bottom-current activity. At this

period, huge sedimentary drifts developed at the upper

margin (Souza Cruz, 1995). That also corresponds to a

major sea level fall, which provided the basin with a

large quantity of sediment (Carminatti and Scarton,

1991; Souza Cruz, 1995; Appi, 1995).

Hydrocarbon Implications

Extremely deep-water prospects (i.e., depths greater

than 3000 m) have not yet been targeted in hydrocar-

bon exploration along the southeastern Brazilian mar-

gin and the Santos, Campos, and Espırito Santo basins.

According to Mello et al. (2001), the origin of the hy-

drocarbons in those sedimentary basins is related to

Barremian anoxic lakes that ranged from freshwater to

saline, Late Cretaceous anoxic global events, and a Ter-

tiary marine delta complex, the occurrence of which is

presently seen only in the Espırito Santo Basin. Most of

the oil discoveries are related to turbidite reservoirs

formed after deposition of the Aptian salt. These res-

ervoirs were charged with hydrocarbons formed in la-

custrine source rocks that were deposited before salt

deposition.

As stated previously, the seismic-stratigraphic anal-

ysis indicated that an Albian to Quaternary sequence

was deposited on the outer southeastern Brazilian mar-

gin, including the continental rise off the Sao Paulo

Plateau. The continental rise in the study area develops

basinward from the foot of the plateau escarpment,

above an oceanic crust formed after the continental

breakup at the Neocomian. Sedimentation processes

that followed the breakup phase included the trans-

gression of the basin margins by thick marine carbonate

successions (Armentrout, 1999). That phase is followed

by the Late Cretaceous and Tertiary reactivation of

older crustal structures, resulting in strong onshore up-

lift and exhumation, an increase in sediment supply to

deep-water settings, and offshore subsidence, increasing

the sea-floor gradient and accelerating the sediment-

rich gravity flows. Thus, thick terrigenous clastic wedges

prograded into the basin from the middle to Late Cre-

taceous, forming the Namorado turbidites in Campos

Basin and the Ilhabela turbidites in Santos Basin. The

extensive Late Cretaceous progradation (Pereira, 1990;

Cobbold et al., 2001) suggests that a large quantity of

gravity-flow sediments could have reached a very dis-

tal setting.

Meisling et al. (2001) suggested that the deep-water

plays in the southern Brazilian margin are influenced by

the presence of the widely distributed lacustrine, rift-

related source rocks. However, we think that the devel-

opment of a thick column of marine sediments above

an oceanic crust can also provide a complete petroleum

system, as it does in other world-class oil-prone basins

such as the Gulf of Mexico (Roberts and Reilly, 2001)

and the western margin of Africa (Costa et al., 2001).

This system would include (1) the Albian or Cenoma-

nian to Turonian marine source rocks related to anoxic

events, (2) thermal efficiency of the oceanic crust pro-

viding heat to mature the source rocks, as observed in

the Red Sea where geothermal gradients range from

3 to more than 12jF/100 ft (�16 to �11jC/30.5 m)

(Clifford, 1986), (3) the potential for vertical and lat-

eral migration pathways through accommodation faults,

oceanic crust faults, and porous, coarse-grained turbidite-

carrier beds, (4) the presence of turbidite reservoirs,

mainly those related to the Late Cretaceous and lower

Tertiary, similar to the Holocene examples in the study,

and (5) efficient hydrocarbon-charging mechanism in

a thick, mud-rich, deep-marine system.

Following the above statements, we can expect the

development of such a sand-rich turbidite petroleum

system in the study area. In turbidite systems, channels

and lobes are the features most rich in sand and com-

monly form oil reservoirs. Roncador field, recently dis-

covered in Campos Basin, is located at a depth ranging

from 1700 to 200 m and is about 130 km upslope of our

study area. Its reservoir rocks are Campanian to Mio-

cene (Guimaraes et al., 2001). The lower reservoirs

include coarse-grained turbidites with channel-fill con-

glomerates and coarse sandstones (30% porosity and

one- to three-dimensional permeability). Upper reser-

voirs consists of confined and unconfined lobes and

890 The Sao Tome Deep-Sea Turbidite System (southern Brazil Basin)

have porosities ranging between 29 and 33% and per-

meabilities between 400 and 700 md (Guimaraes et al.,

2001; Barroso et al., 2000). Those reservoir features

indicate the high energy of the feeding system and con-

firm the possibility of delivering sand into deeper water

in the direction of our study system, as demonstrated

previously in this study.

In the study area, we mainly expect reservoirs of

the turbidite-levee type in addition to possible lobe and

channel reservoirs. Turbidite levees commonly consist

of thin-bedded deposits and are developed over a large

area. They are known to be gas-prone reservoirs (Pea-

kall et al., 2000) and have a net sand percentage ranging

from 30 to 55% (Kendrick, 1999; Field et al., 2000).

The existence of good lateral reservoir continuity in levee-

hosted sheet sands has also been noted by Kolla et al.

(1998). The presence of such widespread, thin, silty-

sand layers is expected in the studied system. These

layers would have formed mostly in the periods that

followed huge margin sediment delivery, as suggested

by our interpretation of the available cores and seis-

mic lines.

Because of the great water depths involved, such

a geologic setting still remains unexplored along open

oceanic margins. Major geologic uncertainties remain,

including the thermal state of potential source rocks

and the preservation of original fabric-supported po-

rosity as stated by Roberts and Reilly (2001) for the

deep-water plays of the Gulf of Mexico. Besides those

uncertainties, engineering and economic challenges in-

volving exploring and producing in water depths greater

than 2500 m still remain to be overcome.

CONCLUSION

The Sao Tome channel-levee system is located on the

rise of the South Brazilian margin. It is composed of the

elongated north-south Guanabara Channel, parallel to

the margin trend, at the foot of the Sao Paulo Plateau

escarpment, and a fairly flat levee adjacent to the chan-

nel that deepens eastward. It was first interpreted as a

system built by contour currents derived from the Ant-

arctic bottom-water circulation. Study of new seismic

data allowed us to reconstruct the system stratigraphy

and to propose a new interpretation of the predomi-

nant depositional processes.

1. Three major depositional units, which were divided

in subunits, have been recognized. They are separated

by major erosive discontinuities.

2. Gravity processes are predominant in the system

sedimentation. Unit I (Cretaceous[?]–Paleocene) is

characterized by a north-to-south–channelized tur-

bidite system. The overlying unit II is composed of

two subunits: lower to middle Miocene subunit IIb

with basal discontinuity R1a and upper Oligocene

subunit IIa with basal discontinuity R1. It shows

several superimposed turbidite channel-levee sys-

tems still elongated north to south that shift west-

ward compared to unit I. Most of the supply seems

to be brought along by a channel network that

crosses the upper part of the margin in the north of

the system. A minor southern source of sediment

may have been active at least during deposition of

lower unit II. Unit III (upper Miocene(?)–Pliocene–

Quaternary) at the top of the series has a complex

geometry with three subunits stacked on a basal

erosive discontinuity (R2). Turbidite processes are

still predominant during subunits IIIa and IIIc, with

supply coming either from the north (Macae Chan-

nel, subunit IIIa) or from the south (Carioca Chan-

nel, subunit IIIc). In between, subunit IIIb corre-

sponds to a major mass flow that affects the central

part of the system. The base of subunit IIIc corre-

sponds to a major erosive discontinuity (R3).

3. Impact of the contour currents linked to the Ant-

arctic bottom water or the North Atlantic deep water

has been recorded in the form of widespread ero-

sive surfaces (discontinuities R1 to R3) correlated to

major hydrological events known at the scale of the

Brazil Basin and the world ocean, such as the middle

to upper Miocene event (R2) and the upperEocene(?)–

lower Oligocene event (R1). Transparent facies of

unit II and some wavy bedforms of unit III are thought

to partly correspond to contouritic deposits.

4. The Guanabara Channel levee morphology and pro-

cess of deposition seems to have been initiated dur-

ing the early to middle Miocene (unit IIb), where

evidences of channelized deposits have been observ-

ed at the foot of an already existing plateau escarp-

ment. However, the modern system seems to have

been definitely established above the R3 discontinui-

ty (upper Pliocene or Pliocene–Quaternary boundary).

5. An along-slope trend of the channel-levee system is

here associated with gravity processes. Such a trend

might be controlled by faults involved in the south-

ern Brazil Basin oceanic expansion and by the plateau

escarpment caused by the Aptian salt diapirism.

6. Presence of fine-grained, sand-silt turbidites in Quater-

nary sediments suggests the deposition of signifi-

cant sand bodies at the scale of the whole Sao Tome

Viana et al. 891

turbidite system and potential hydrocarbon reser-

voirs in the deep, south Brazilian margin, especially

in the Sao Paulo Plateau.

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