Viana et al., 1998

Sedimentary Geology

ELSEVIER Sedimentary Geology 1 15 (1998) 133-157

Hydrology, morphology and sedimentology of the Campos continental margin, offshore Brazil

A.R. Viana a,b,*, J.C. Faugères b, R.O. Kowsmann ", J.A.M. Lima ", L.F.G. Caddah ", J.G. Rizzo a

Petróleo Brasileiro S.A. - Petrobras, DepedDesud, Av Elias Agostinho 665, Macaé, RJ 27913-350, Brazil Universiré de Bordenux I, Département a'e Géologie et Océanographie, U.R.A. 197, CNRS, av. des F a ~ t t l t é ~ , 33405, Talence, France

Received 15 Febmary 1995; accepted 5 June 1997

Abstract

Slope sand deposits have accumulated from at least the Neogene to the Present on the southeastern Brazilian continental margin (Campos Basin area). This region shows sand accumulations concentrated on the upper portion and on the base of the continental slope with a middle to lower slope bypass zone. A synthesis of preliminary result:;, supported by recent cores, high-resolution geophysical surveys, geotechnical investigations and environmental research, is presented and permits a prelirninary analysis of the sedimentological mechanisms operational in this area. These point toward a temporal and spatial multiscale set of phenomena responsible for sand deposits. At any sea-leve1 stand these deposits are dependent on: (1) a suitable sediment source; (2) offshelf transport mechanisms; (3) a morphostructural antl hydrodynamic context responsible for the deposition of these sands in the upper portion of continental slopes. The proposed scenario of depositional processes concerns: (1) a set of hydrological processes such as surface currents and counter-isurrents, waves, tides and eddies with sufficient energy to form submarine sand dune fields at the outer shelf; (2) the offshelf export of this sediment under a combined action of spillover, interna1 waves, eddies 'seafloor polishing effect' and gravity processes (turbidity currents); and (3) the slope sand deposits and their distribution controlled by the action of contour currents, mass movemeiits and the morphological coritext, such as canyons, gullies or scarps. O 1998 Elsevier Science B.V. A11 nghts reserved.

Keywords: sedimentology; submarine physiography; sedimentary facies; water masses; oceanic processes; Quaternary; Campos Basin

1. Introduction

The presence of modern and aincient deep-water sand bodies has always interested geoscientists and petroleum geologists. These sands are analogues for potential oil-bearing deposits and consequently have a large economic importante.

*Corresponding author. Tel.: +33 5684 :3853; Fax: +33 5684 0848; E-mail: viana0geocean.u-bordeaux.fr

The dynamics of water masses play a decisive role in transfemng to, storing, and reworking sediments in deep-water. This role is controlled by the geo- morphology of the basin and ultimately by relative sea-leve1 oscillations. From the interplay between hydrologic factors, sediment availability and basin morphology we may distinguish: (a) processes that occur onshelf from those that occur offshelf; (b) processes that occur along the isobaths from those that occur crossing the isobaths; and (c) processes

0037-0738/98/$19.00 O I998 Elsevier Science B.V. AI1 rights reserved. PII SOO37-0738(97)00090-O

134 A.R. Kana er a[. /Sedirnentav Geology 115 (1998) 133-157

that occur in different time sca1e:s: seconds (waves), hours (tides), days to months (mesoscale synoptic eddies) and even hundreds or thousands of years (large oceanic currents and oceari stratification).

The case study of the Campos margin off southeastem Brazil will be examined in order to analyse the imprint over the modem sediments of the local oceanographic phenomena, 21 complex interplay between different geostrophic currents and hydrological processes, occurring fronl the outer shelf to offshore. A more detailed approach on the Cam- pos Basin continental slope facies is presented by Caddah et al. (1998).

The South Atlantic large-scale hydrodynamics have not significantly changed sirice the latest Oligo- ceneEarly Miocene (Wright, 1Y91), thus the processes observed in the Quatemary may, in general terms, be extrapolated back to that time. This scenario provides a good basis for the preparation of an analogue model, possibly valiid for the whole of the late Cenozoic, when the mosi. extensive Campos Basin deep-water sand deposits accumulated (Carmi- natti and Scarton, 199 1).

The principal aim of this paper is to present new data on the hydrological context of the Campos margin and to provide a preliminary interpretation of the interactions between physical oceanographic phenomena that play an important role on shelf- edgelslope sedimentation. The sei. of these processes will be divided into four approaches in order to make their understanding easier. We will address succes- sively: (a) shelf processes (tides, storm waves, synoptic eddies and shelf currents); (b) deep-water processes (geostrophic/thermohaline currents, synoptic eddies and tide waves); (c) comparison between along-isobath and cross-isobath p hysical phenomena and the topographic control on those processes; and (d) the influente of intermediate- and bottom-water circulation changes on facies and geometry through the uppermost Quatemary.

2. Data and methods

Prospecting in the Campos Elasin and adjacent areas has already provided an excellent set of information on deep-water processes. The data supporting this paper comprise 850 m of short (up to 9 m long) and long (up to 150 m) piston-cores. A11 oxygen-iso-

tope analyses and absolute 14C dating were carried out by Beta Analytics Inc., Coral Gables, USA. More than 450 km of air-gun high-resolution multi- channel seismic lines, 4000 km of sparker analogue seismic lines, 1800 km of SMS960, 800 m swath deep-water side-scan sonar, 120 km of Klein 150 m swath side-scan sonar records were collected and studied. Bottom-currents were registered with the current meter Aandera RCM-7 during eight months in three different sites, two at the shelf-edge, at 110 m water depth, and one set at 400 m water depth. One last set deployed at 800 m water depth was in operation for only 45 days (Xavier r:t al., 1993). The instruments were moored 3 m from the sea-floor. Complementary data carne from engineenng and environmental surveys, including 4Cl in-situ geotechnical log-profiles from long boreholes (up to 250 m long) in depths up to 1000 m, wave measurements over a two-year period, and infrared AVHRR (Ad- vanced Very High Resolution Radiometer) satellite images, from TIROSLWOAA- 1 1 satellites, processed by INPE (Instituto Nacional de Pesquisas Espaciais, the Brazilian agency for spatial research).

3. Regional setting

3.1. Geological setting

The Campos Basin is located on the southwestem margin of the South Atlantic Ocean and occupies a portion of the Brazilian continental margin between 20.5"s (Vitoria High) and 24"s (Cabo Frio High), in an area of more than 100,000 kmz (Fig. 1). More than 70% of this basin is at water depths greater than 200 m (Carminatti and Scaríon, 1991). The continental shelf has an average width of 100 km and the shelf break varies from 80 m water depth in the northern area to 130 m water depth in the south, with an average depth of 110 m. The slope extends over 40 km with an average gradient of 2.5". Its base is shallower at the north (1500 m water depth) than in the south (2000 m) due to the development of a submarine cone connected to the Almirante Camara submarine canyorl (Paraiba do Sul submarine fan, Brehme, 1984). The passage from the continental slope to the continental rise is marked by an intermediate province, the São Paulo Plateau, a low-gradient area (1 : 100) influenced by salt diapirs,

A.R. Iliana et al. /Sedimentary Geology 115 (1998) 133-157 135

Fig. 1. General physiographic and bathymetric chart of the southeastem Rrazilian continental margin. Dots iridicate long-term bottom-current measurements (a and b at - 100 m, c at -400 m, and d at -800 m). The area inside the polygon is the most extensively surveyed for high-resolution geological, geciphysical and environmental data. A-B is the location of seismic section presented in Fig. 12. Submarine canyons are distinguished by iiumbers: I = Almirante Camara; 2 = Itapernirim; 3) =São Tomé; 4 == Grupo Sudeste. Continental slopeISão Paulo Plateau passagi: near 2000 m isobath. Isobath contour at each 100 m.

extending from 2000 m to 3500 m water depth. 3.2. Oceanographic context It is the place where the most important slope base deposits are found and presents a complex submarine Oceanic physical phenomena have different time drainage system (Castro, 1992). and space scales and may be observetl in a11 oceans

(Fig. 2). These various processes (many of which are

A..R. Viana et al. /Sedimenta? Geology 115 (1998) 133-157

-

Fig. 2. Timexspace relationship of the different physical processes in the ocean that have an impact on sedimentation: l = surface waves; 2 = intemal waves; 3 = tidal oscillations (a = semi-diurnal, b = diurnal); 4 = oceanic eddies and Rossby waves; 5 = seasonal variations; 6 = major oceanic currents; 7 = stratification of the ocean. (From Kamenkovich et al., 1986.)

atmosphenc-driven) lead to inhomogeneities in the distribution of properties and chiiracteristics of sea- water (Kamenkovich et al., 198611. Among them, the water column stratification in different water masses with particular thermodynamic properties (temperature, salinity. etc.), and their motion characteristics (velocity components and sea-surface leve]), play an important role in sedimentation.

The water masses and circulation pattern in the South Atlantic Ocean are among the best defined at the present day (Garfield, 1990). Nevertheless, some divergence occurs in literature when defining the circulation regime above the main thermocline situated near the 500 m isobath in the study area (Evans et al., 1983; Piola and Gordon, 1989; Reid, 1989; Peterson and Strama, 1991). For the purposes of this study the water masses will be vertically divided into two major groups: the superficial waters (the Thermocline Waters of Schmid et al., 1995) and the deeper waters (Fig. 3).

The superficial waters are infliienced principally by the wind-driven anticyclonic regime and comprise two distinct water masses: the Superficial Tropical Water (STW) and the South Atlantic Central Water (SACW).

40" W 35" W 30" W

Fig. 3. Water-mass stratification off the southeastem Brazilian margin at 22%. STW = Superficial Tropical Waters (mainly composed of Brazil Current and shelf waters); SACW = South Atlantic Central Water; AAIW = Antarctic Intermediate Water; NADW = North Atlantic Deep Water; AABW = Antarctic Bottotn Water. Symbols inside the circles indicate the general direction of flux. BC = Brazil Current; cBC = counter Brazil Current; bold lines indicate the actual region of current action in study area; dashed lines indicate region of activity of those currents as inferred by some authors (see text for more details).

(a) The first and more superfici~il layer, the Su- perficial Tropical Water (STW), is the result of the mixing of three water types: the Tropical Water (temperature > 18"C, salinity >36%0), the Littoral Water and periodic upwelling of the South Atlantic Cen- tral Water (6" < T t 18°C 34.5%0 < S <36.0%0). This Superficial Water is camed southward by the Brazil Current (BC) and is well characterized at up to 2501300 m water depth (Signorini, 1978; Miranda, 1982; Evans et al., 1983).

(b) Flowing under the Superficial Tropical Water (STW), between 300 m and 550 m water depth, is the South Atlantic Central Water (SACW), here considered as the northward flowing BC: countercun-ent. Its origin is controversial: some authors characterise it as the freshened southerly flow ol' Tropical Water (Garfield, 1990), while some others (Deacon, 1933; and Sverdrup et al., 1942. cited by Miranda, 1982; Garfield, 1990) recognise it as the upper and warmer branch of the Antarctic Intermediate Water (AAIW).

Undemeath the SACW both temperature and salinity show an abrupt decrease with depth (Pierre et al., 1991).

3.2.1. The deeper water rnasses present below the themzocline are

(a) Antarctic Intermediate Water (AAIW) formed in the Sub-Antarctic Front at 45"s. l'his is the shal- lowest distinct layer below the superficial waters (Reid, 1989) and irrigates the middle slope between the isobaths of 550 m and 1200 m. AAIW is well characterized by temperatures varyirig between 6°C and 2°C a high dissolved-oxygen content and a salinity minimum of 34.2%0 (Miran'da, 1982; Piola and Gordon, 1989; Reid, 1989). According to Reid (1989) AAIW flows northward, to the north of Cabo Frio, after contouring the Rio Graride Rise (Reid, 1989; Boebel et al., 1994).

(b) Circumpolar Deep Water (CPDW) is a thin, nutrient-rich and oxygen-poor water, that originates near Antarctica and flows northward. At 55"s it bifurcates into two layers, the Upper and Lower CPDW, that sandwich the North Atlalntic Deep Water (NADW; Reid, 1989). Our data corifirm the observations of Pierre et al. (1991) and Tsuchyia et al. (1994), and do not distinguish special dynamic characteristics for the Circumpolar Waters in the region. For this reason, in this paper we will not consider UCPDW as an individual water body. On the con- trary, it will be assumed as the lciwer portion of AAIW.

(c) North Atlantic Deep Water (NADW) is a large water mass more than 2 km thick and 800 km wide, flowing southward, highly saline, niitrient-poor and oxygen-rich, that is encountered at ciepths from the lower slope to the extemal border of the São Paulo Plateau, from 1200 m to more thari 3500 m water depth (Reid, 1989; Schmid et al., 1995). It must have a slow flux, possibly slower i.han 5 c d s , as shown by the associated pelagic sediinentation found under its domain (Viana et al., 1994; Faugères et al., 1994). DeMadron and Weatherly (1994) observed a significant decrease in the NAIIW flow speed (20 c d s to less than 5 crnls) southward of the Columbia-Trindade Seamount Chaii~ at 21°S on the northem edge of the study area.

(d) The dense and cold Antarctic Bottom Water (AABW) occurs beneath NADW, in waters deeper

than 4000 m, far away from the outer limit of the study area.

Among a11 these water masses and currents, the upper margin of the Campos is strongly influenced by the western-boundary Brazil Currerit (BC) generated by the subtropical anticyclone that dominates the motion characteristics of the South Atlantic superficial circulation. The vertical and horizontal circulation of the BC are not well known in detail (Garfield, 1990). Some authors consider that the southward flow of the wind-driven westem-boundary Brazil Current involves also waters at the depth of SACW, AAIW and even NADW (Reid, 1989, Garfield, 1990, Peterson and Strama, 1991).

4. Hydrological data

New hydrological data have shown that the study area has a complex hydrodynamic context, with the shelf edge and the continental slope presenting a multilayer water mass stratification with different circulation patterns. In addition, tide-related bottom- currents. high-amplitude storm waves, synoptic eddies and a meandering of the Brazil C u ~ ~ e n t are usu- ally observed. The different hydrologic: factors will be discussed in order to address their role in: offshelf sand exportation, downslope and along-slope sediment transport, erosion and deposition, and the related facies and geometry.

4.1. Shelf-edge phenornena

Shelf-edge bottom-current data of the Campos Basin indicate a bi-directionality of shoreward (west) and slopeward (east) flows (Fig. 4a). Mathematical analyses of bottom-current measurements indicate that the principal oceanographic phenoinena that af- fect this circulation, are strongly influirnced by the wind-regime (NE trade winds) and by the passage of SW storm fronts, with a frequency rariging from 4 to 11 days (in accordance with the values observed by Stech and Lorenzzetti, 1992). Also iimportant are the higher-frequency processes (waves and tides), as well as those of larger periods (meso-scale eddies and meandering of the BC, period > 15 days), both influencing the resultant shelf bottom-cnirrents.

Bottom-currents near the shelf break have mean velocities of 25 c d s (Fig. 1, moorings a and b).

A.R. Viana et al./Sedimentary Geology 1 I5 (1998) 133-157

DIRECTION ( O )

I -...- Longitudinal Component

Frequency (cph)

Fig. 4. (A) Histogram of the measurement of a 100-m water depth near-bottom current, relating current speed and direction of flow. The bi-directionality of the record clearly shows offshelf flow (50-100"; NE to E ) and a secondary shoreward flow (250-300"; W to NW). (B) Spectral analysis of bottom-current records from shelf-edge mooring a. Both transversal and longitudinal components of the flux are represented.

Slopeward currents (eastward trend) may reach peak velocities greater than 50 c d s . Preliminary spectral analysis reveals a dominante of supra-inertial frequency phenomena (greater than 3 1 h), mainly related to storm fronts and the: passage of eddies (Fig. 4b). Also semi-diurna1 tide:s have a strong signature. In addition, other observed bottom-currents have a southwest trend, probably related to the meandering of the Brazil Current.

4.1.1. Waves and tides Local data reveal that south/southwest storm

fronts have a frequency between 4 and 11 days. Very strong storms produce waves with amplitudes higher than 6 m and pet-iods longer than 10 S. Such storms have a 3- to 4-month freqiiency (Viana and Rizzo, 1994) and may induce bottom orbital velocities between 8 and 12 c d s , potetitially capable of overcoming the threshold velocities for movement of

A.R. \'iana et al. /Sedirnentay Geology 115 (1998) 133-157

WAVE HEIGTH (rn)

WAVE HEIGTH (rn)

Fig. 5. (a) Graph showing an empincal relation between wave amplitudes and the maximum water depth where the orbital velocity is greater than the threshold velocity for different grain sizes. The wave period corresponds to a normal storm wave in the Campos Basin (13 s). The substrate is a mixture of siliciclastics and bioclastics with a density of 2.68 &m3. Numbers over the lines represent textura1 classes: 1 = coarse silt: 2 = very fine sand; 3 = fine sand; 4 = medium sand; 5 = coarse sand; 6 = very coarse sand; :7 = granules; 8 = pebbles (6 mm); 9 = pebbles (12 mm); 10 = pebbles (24 mm). Adapted from Viana and Rizzo (1994). (b) Maximum depth at which a 9.60-m amplitude Campos Basin storm wave may induce orbital velocities greater than the threshold velocities for different grain sizes. Several periods are considered and it may !be noticed that the greater the period the deeper the wave action. The numbers in the curves correspond to the same grain size classes as in Fig. 4a. Sediment density is 2.68 g/cm3. The white area on the left represents the region where the ratio wave heightlwave length is less than 117; in this situation no wave can exist in nature. breaking and assuming other parameters (from Viana and Rizzo, 1994, at'ter Komar and Miller, 1975)

silt and very fine sand at the shelf iedge (Fig. 5). NE shore, these winds induce an Eckmari effect-related trade winds are responsible for the coastal circula- offshore shift of the bottom currents. tion, with an oblique approach of waves producing Even being a continental margin with a low- moderate to strong along-shore currents towards the amplitude tide regime (amplitude 0.5 to 1.5), the southlsouthwest, restricted to the Littoral area. Off- analysis of shelf-edge bottom-current data of the

140 A.iP. Viam et aí. /Sedimentay Geology 115 (1 998) 133-157

study area revealed a bi-directional inshore-offshore predominant flux direction. Preliminary statistical analyses suggested that the bi-tlirectionality could be associated with tidal processe:s (Fig. 4a) with an offshore principal component, with peak velocities attaining more than 50 crnls. In fact, such bi-directionality is mainly related to the frequency of storm front passages, with tides being more important for the along-shore (northeastward) component of the bottom currents (Fig. 4b). The frontal passages induce a seaward rotation (countt:rclockwise) in the wind field and bring about a complete reversal in the flow (Stech and Lorenzzetti, 1992:).

4.1.2. Eddies The presence of synoptic mesoscale eddies in

the Campos Basin area has been known since the late 1970's from classical oceai~ographic observations (Signorini, 1978; Miranda, 1982; Evans et al.,

1983). Recent infrared satellite images of sea-surface temperature (SST) obtained from AVHRR sensors (Fig. 6), conventional CTD data and bottom-current measurements have shown that the Campos Basin cold-core, cyclonic, synoptic eddies may attain di- ameters of 50 to more than 100 E;m (Silva et al., 1994). They are enabled by a strong instability of the Brazil Current, possibly associated with the following factors: (1) shearing and exfoliation of the BC after being funnelled and accelerated at the offshore projection of Cape São Tomé (Fig. 7); downstream, the topographic re-inversion expands the BC flux, lowering its velocity and enabling its exfoliation and the production of a rotatory motion that contributes to the vortices generation; and (2) thermal differ- ences between upwelled cold waters and the warm and saline BC (Fig. 8).

The vortices move from their birth place over the central outer shelf at the west/iiorthwest of the

Fig. 6. Sea-surface temperature (SST) olstained from the AVHRRINOAA-I 1 satellite, October 11, 1993. The dark part represents the continental emerged area: the E-W littoral represents the southern part of Rio de Janeiro State; R J = Rio de Janeiro city; CF = Cabo Frio (23"S, 42"W); CST = Cape São Tomé (22"S,42"W). White parts represent southward-going warm waters of the Brazil Current (BC). A well-defined mesoscale synoptic eddy is observed offshore Cape São Tomé, from the mid shelf to the upper slope. Immediately north of the vortices the funnelling area of the BC is found. South of Cabo Frio, another gyre is developed. Image processed by INPEIDCT (Earth Sciences Dept. of the Brazilian Spatial Agency).

A.R. Mlzna et al./Sedimentary Geology 115 (1998) 133-157

a ) Plan view A HIGH SPEED WARM CURRENT

\,(BRAZIL CURRENT)

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SHELF WATER EXFOLIATION . . . . . . . . . . . . . . . . . . . . . . .....) . . . . . . . . . . . . . . . . . . . . . . . . . . . . POINT jy . . . . . . . . . . . . . . . . . . . . . I a I I BASE OF SLOPE , ( - 2 0 0 0 m l

.ING / WATER)

b ) Section view A

Fig. 7. Scheme of fumelling and exfoliatioii of the Brazil Current at the offshore physiographic expression of Cape São Tomé. The current is laterally confined and accelerated north of the cape, reaching superficial velocities up to 2 mls. After the cape it expands, decelerating, and assumes a rotatory movement reinforced by contact with the cold upwelling SACW waters.

study area toward the east1southea:;t in an isobath transversal trend, with displacement velocities varying between 4 and 35 c d s (Silva et al., 1994). Over the shelf, this cross-isobath track is topograph- ically driven, according to the observations made by Schmid et al. (1995) over an eddy in the Vitona- Trindade region, a few hundreds of I~ilometres north of the study area. In the Campos Ilasin the eddies reach the continental slope near the head of the São Tomé canyon, where they are advected southward by the Brazil Current, following the bathymetric contours, being dissipated at the northern border of Cabo Frio (Tanaka and Lorenzzetti, 1991). Their frequency is not yet well established, varying between more than 15 and less than 45 days, and the resi- dente period fluctuates between a few days to less than one month. Spectral analysis of a 7-month series of bottom-current data indicates, a strong energy of the system in the band of 15 to 21, days (Fig. 4b).

A preliminary rotatory spectral analysis and other statistical investigations, conducted over near-bottom-current records obtained from the outer shelf to 800 m water depth (Fig. 1, mooring d) revealed that these oceanic features may reach velocities of up to 35 c d s at 400 m water depth. These velocities are similar to the superficial swirl velocities of the Vitoria eddy (Schmid et al., 1995) antl corroborate the study by Schmitz (1980) that has demonstrated a weak depth-dependence of eddy energy: only a slight decay in current velocities from surface to depth was observed down the core of an eddy.

4.2. Slope-water dynamics in the Campos Basin area

Severa1 discontinuous oceanographic surveys have been carried out through the years in parallel with hydrocarbon exploitation. 'f ie available interna1 reports from Petrobras and the most recent

A R. Viunu er al. /Sedimentary Geology 115 (1998) 133-157

1 eddie I I

Time (hours)

Fig. 8. Downslope (+) and upslope (-) components (SEíNW) of near-bottom current velocities (bold line) measured at 400 m water depth (moonng c), from early July (houi- 500) to late September 1992 (hour 2500), and corresponding water temperature (light line). The suggested interpretation of observed phenomena indicate: ( I ) an upwelling betrveen hours 940 and I 160, probably inducing (2) the cold- core eddy generation between hours 1270 and 1850. The core of the eddy is interpreted as being located between hours 1550 and 1720.

unpublished data have demonstrated that the water-mass stratification in the Campos Basin shows a classical structure, with an upward shift of interna1 boundaries close to the continental slope and an expressive dynamic behaviour strongly controlled by the wind-regime and by the subniarine topography.

The Brazil Current (BC) is 300 km wide and topo- graphically controlled, with its iriner margin roughly coincident with the shelf break. In the study area, the surface expression of this current is well recorded in sea-surface temperature (SST) satellite images (Fig. 6). The available data demonstrate that the thickness of the BC increases offshore the shelf- edge, varying from 250-300 m at its western boundary, to more than 500 m thickneiss nearly 50 km offshore (Lima, 1992). Recent meatiurements (Xavier et al., 1993) recorded its southward near-bottom flux mostly confined to slope waters between the shelf- edge (around 150 m water deptlh) and the 300-350 m isobath. Occasional onshelf penetration and meandering of the BC may be linked to weak to moderate southwestward records on the bottom-current data.

The submarine physiographic context may accel- erate or reduce BC flux speecl as seen above in the eddy-generation processes. R~xent oceanographic data were obtained in the area c~f current-funnelling

and eddy generation, in a mooring-deployed system at 200 m water depth, in the wall of the shelf-break escarpment, at the offshore projeciion of Cape São Tomé, north of the São Tomé canyon. Preliminary analysis of these data revealed frequent peak velocities of 6 knots (circa 3 d s ) for supt:rficial flux of the BC and 0.50 rnls at 4 m from the sea-bottom. Sedi- ments and bottom features of this area are typical of a high-energy environment (see follo~ving sections).

The flux of South Atlantic Central Water (SACW) in the study area probably represeiits the BC countercurrent, formed after the sinking of the BC in the subtropical convergence. SACW flows northward beneath the superficial waters (ST'W) down to the 550 m isobath. It sweeps the outer upper slope with peak velocities of 50 c d s recorded in a near-bottom mooring at 400 m depth (Fig. lc) south of the São Tomé canyon. In addition, strong dciwnslope currents (55 cmls) were also recorded at this mooring (Xavier et a]., 1993). Preliminary interpretation suggests that the northward SACW flux intensity is reduced after crossing the São Tomé canyon, in a similar way, but with an opposite trend, to the EIC flux reduction south of the canyon (Fig. 7).

Antarctic Intermediate Water (AAIW) is a well constrained, south-originated watcr layer (Fig. 8)

A.R. I.Tana e! al./Sedimenraty Geology 115 (1998) 133-157 143

that sweeps the middle slope, from 550 m to 1200 m, in a northeast along-isobath direction. Its near-bottom peak velocities, at 800 m water depth, attained 30 cm/s during 30 days of bottom-current measurements (Xavier et al., 1993).

No data from the near-bottoni expression of deeper water masses are available in the study area.

5. Geomorphological context

The study area comprises the cirntral portion of the Campos Basin, from its outer shelf to the lower slope. The physiography is characterized by a relatively flat continental shelf that passes abmptly (shelf-edge escarpment) to a broad and smooth slope, limited to the north and to the south by submarine canyons that dissect the slope in a west to east trend, transverse to the isobaths (Figs. 1 and 9). Down- slope, the continental slope merges into the São Paulo Plateau after developing a con spicuous escarpment along the whole area (lower slope escarpment).

The outermost portion of the continental shelf lies between 70 and 120 m water depth. It is marked by a flat relief, where different bedforms (ripples to sand dunes) are developed, intemipted by ancient beach rocks, hundreds of metres loiig and up to 5 m high, and by wide carbonate banks (coral and alga1 build-ups), more than 5 km in diamlzter, that develop positive reliefs of up to 10 m. One of these banks is responsible for the offshore projection of São Tomé Cape, in the north of the study area. High-resolution seisrnic profiles indicate that these topographic highs may act as sediment traps, damming sand wave migration. The shelf-edge escarpmeni occurs between 120 and 250 m, and may acquire steep slopes up to 15", where outcrops of (uppennost Pleistocene mud) have been exposed, probably as a result of mass wasting during sea-leve1 loweiing (Viana et al., 1994) and of erosion by oceanographic processes (geostrophic currents plus coastal processes during periods of maximum relative lowstand of sea-level).

The northern limit of the stildy area is the Itapemirim canyon (Fig. 9), characierised by an am- phitheatre-like head developed from the upper slope down to the lower slope. It does not develop any depositional feature in its passage to the São Paulo Plateau where it feeds directly intci the broader São Paulo Plateau submarine drainage system, controlled

by salt diapirs (Castro, 1992). To the south, a set of parallel canyons (the Southeast Group) head in the upperlmiddle slope region (around 600 m water depth). The São Tomé canyon, with a distinc- tive bifurcated head, splits the slope of the study area into northem and southern halves (Viana et al., 1994). Analyses of seismic profiles indicate severa1 episodes of shelf indentation by this canyon, generally linked to relative sea-leve1 lowstands.

The slope compartment south of the São Tomé submarine canyon is relatively smootl~, with gradients averaging 2" (Fig. 9). The shelf edge is marked by carbonate banks with steep seaward scarps incised by gullies. These gullies represent patkiways through which relict siliciclastic sands from the outer shelf are being transported to the upper slope (see Sec- tion 6). The upper slope is flat to concave-shaped grading to a convex externa1 geometry in its middle section, between 600 and 1200 NI. The convex geometry of the middle slope is atti-ibuted to the stacked accumulation of mass-moveinent deposits that developed an irregular bottom topography. They were formed by creep-slump processes (Costa et al., 1994) and occupy an area of 1600 km2, with six events being identified in high-resolution multichan- nel seismic profiles, a11 of them younger than Late Miocene with the youngest placed between 85 and 53 ka (Kowsmann et al., 1994). Elongated patches of deep-water coral mounds (see Section 6) are observed between 570 m and 800 m water depth, over the topographic highs that represent the surface expression of the buried compressional slump ridges. The lower slope is marked by a conspicuous scarp, common to both southern and northem compart- ments, where sediment remova1 by recurrent mass flows has occurred (Kowsmann and Viana, 1992).

The slope compartment north of São Tomé submarine canyon (Fig. 9) differs in many ways from its southern counterpart. The upper slope is wider presenting a 8-km-wide erosional terrace between the 200 m and 400 m isobaths, with an average gradient of only 0.6". Its terrace is intensely sculpted resulting in a system of low and narrow erosional ridges and valleys, between 250 and 350 m water depth, NNW- SSE elongated, 50 m high, 0.5-1.0 krn wide and up to 10 km long. These features are alniost connected to the head of the São Tomé canyon. Older cut and fill features are also obsemed in the Quatemary sec-

NW SOUTHOFSAO T O M t CANYON SE

1 40m (nedimmt)

2km C1 b;-.:! .-,.... gmps

Fig. 9. Plan view of the study area and superficial distribution of main circulaiion pattems, with the on-shelf penetration of the Brazil Current and eddy generation. Notiwurthj: is thi iiurthward bend o:' the muuth :ube üf the São TüiiiL caiiyiin. Tiitei-vã1 cuiiiuui 100 111. ProPilçs (a) and (b) are schemaric venicai sections representing the main distribution of facies, seismic reflectors and trends of propagation of water masses.

A.R. Viana et al. /Sedimentary Geology 115 (1998) 133-157 145

tion underneath the terrace. The largest of these (3 km wide and 80 m deep) represents a buried headward extension of the Itapemirim canyon. 'The middle slope, seaward from the terrace, comprises a clinoform set with 4.5" average inclination. It is abruptly intenupted by a large escarpment, 200 m high and 30 km long, which connects the Itapeminm canyon in the north to the São Tomé canyon in the south (Fig. 1) with a trend oblique to the isobaths. Recent unpublished biostratigraphic analyses confirmetl that this escarpment is the present expression of episodic reworking processes which occurred from the latest Pliocene to Late Pleistocene, resulting in a remova1 of more than 160 m of sediment column.

To the north of the study area is the Almirante Camara canyon, the last to be connected with the most expressive fluvial drainage of the area (Paraiba do Sul river). The Almirante Camara and São Tomé canyons develop fan-shaped lobes at the base of slope, in their passage to the São Paulo Plateau. To the south of the study area, there is a series of large, irregular and very shallow valleys (up to 50 m in depth), mass-flow derived, cross-cutting the continental slope (Brehme, 1984; Castro, 1992).

The submarine drainage network on the southeastern Brazilian continental margin is similar to that observed by Schlee and Robb (19'91) on the westem margin of North Atlantic Ocean. The canyons and other sea-valleys converge down-dip over the upper São Paulo Plateau. From this ai-ea, a simpler and more expressive drainage system is developed flowing away eastward amid the irregular salt-diapirized topography converging again at the outer border of the São Paulo Plateau (Castro, 1992; Viana et al., 1994).

The slope base scarp is the superficial expression of the steepest portion of the clirioforms that con- stitute a 1-km-thick Lower to Midtile Miocene sediment package. This represents a third-order lowstand prograding wedge, composed of siltstones, shales and marls. It is truncated at the top by an early-Late Miocene type I unconformity which separates this third-order lowstand systems tract from the overly- ing Upper Miocene to Quaternary highstand systems tract (Viana et al., 1990, 1994). The highstand sys- tenis tract results from the intense basinward shift of the sedimentation. It thns downslope partially due to episodic remova1 by mass movements at the steep slope base scarp ('toboggan effect' of Kowsmann

and Viana, 1992; Kowsmann et al., 1994) serving also as a by-pass zone. Only a thin veneer of tens of centimetres of sediments younger than 53 ka (oxygen isotopic stage 3, M.A. Vicalvi, pers. commun., 1994) was found covenng that 8" steep scarp.

6. Sedimentology

The pattern of modern sedimeritation of the Campos Basin was reconstructed from the available Brazilian National Oceanographic Data Bank (BNDODHN), from recently collected cores and side-scan sonar data. These data are here summarised for the deep-water regions (Fig. 10).

6.1. Shelf sediments

Siliciclastic to bioclastic sands occupy the inner to middle shelf. Shelf muds derived froni the discharge of the Paraiba do Sul river develop small mud ponds and large accumulations in an area of near-zero current energy, adjacent to the Buzios and Cabo Frio region, 150 km south of the nver mouth, a11 along the inner shelf.

The modern outer shelf is dominated by sand- sized sediments, mainly siliciclastic (quartz-feldspathic), and secondarily carbonatic (derived from red and green algae), up to 20 m thick. Sands are moderately to well-rounded, fine to coarse (finer to the south and to the inner shelf and coarser to the centrallnorthern study area), quartzose, some- times glauconitic or bioclastic (rhoclolites and reworked alga1 fragments, some molluscs, bryozoa and miliolid foraminifera), with a low to moderate content of labile and heavy mineral:;. These sands cyclically overlie carbonate accumulations that develop coarsening/shallowing upward sequences from wackestone to grainstone/bioherms, 10 to 20 m thick. Subaereal exposure of the top of carbonate cycles is suggested by grain dissolution and calcareous ce- mentation immediately overlain by yellow quartz- feldspathic terrigenous sands. The carbonate banks sustain the topography at the shelf edge and are traversed by bed-load transported sands developing sand ribbons and dune fields.

In the central-north portion of the study area, from the middle to the outer shelf and in water depths of 70-110 m, the sands are better sorted,

146 A.17. Viana et al./Sedimentary Geology 1 I5 (1998) 133-157

n Fig. 10. Superficial distribution of modem sediments.

rnedium to coarse, essentially quartzose, tanned by Fe-oxide and form large subaqueous dunes. These bedforms have an average heiglrit of 0.5 to 1.0 rn and a length of tens of metres, with a NE trend of propagation (Fig. 11). They occupy an area of 100 km2 and their thickness varies from metres to tens of rnetres. These sands accumulate against the topographic highs (beach rocks and carbonate banks) that also divert their offshore migration.

Another subaqueous dune field, being smaller and confined in area and composed of medium sand sediments, is observed at the passa&: from the shelf to the continental slope, south of São Tomé canyon, on a

narrow erosional plateau. Side-scan sonar records and robot video images revealed that they reach a maximum amplitude of 0.5 m, with their crests alignedpar- allel to the isobaths, indicating an offshore rnigration direction. Following the observations of Dalrymple et al. (1992) on the Westem Grand Banks of New- foundland, the 'fresh' appearance of' these bedforms indicate that they are modem features.

6.2. Upper slope

The base of the shelf-edge scarp represents an irnportant depositional site for shelf-spill over sands.

A.R. Viana er al. /Sedimenrary Geology 115 (1998) 133-157 147

Fig. 11. Side-scan sonar irnage of outer-shelF rnegaripple field observed in the central portion of the study area. The bedfoms are 1 m in height and 15-20 rn in length. At the right corner of the image we may observe the presence of two NNE parallel beach-rocks, indicating the - 110 rn latest Quatemary sea-leve1 posii.ion. Large arrow indicates the propagation direction of the sand dunes (to NE). Srnall mow indicates a W-E high-frequency interference pattem, probably related to tides andlor eddies.

These sands are fine- to coarse-grained, finer to the south, cleaner and coarser to the north of the São Tomé canyon. They are siliciclastic and bioclastic in composition, with some glauconite and micas. To the south of the São Tomé canyon, mud-free, thick-bed- ded fine sands (tens of cm thick), generally structureless and with sharp basal contacts, occur close to the shelf-edge base, developing conic features attached to gully mouths. Normal grading and mud clasts are observed in some beds. Muddy sands occur over a greater area (more than 100 km2), developing flat accumulations up to 10 m thick, with superimposed layers being tens of centimetres to a few metres thick. They are very fine-grained, extensively bur- rowed, with some bioclasts (planktonic foraminifera, Caddah et al., 1998). The few primary structures preserved indicate the action of bottom currents in developing these deposits: upper and lower sharp contacts, inverselnormal grading, t.hin intemal bed- ding. They are spread over the entire upper slope of the study area (200-550 m water depth; Viana et al., 1994; Caddah et al., 1998).

In the northern compartment of the slope, between the Itapemirim and São Tomé canyons where the Brazil Current attains its highest velocities, piston-cores were collected from the shelf edge to the

upper slope, between water depths of 150 and 300 m. The cores revealed the presence of extremely clean, well rounded siliciclastic structureless coarse sands to pebbly sands, with red alga1 nodules. On the adjacent downslope terrace, between water depths of 300 and 450 m, high-resolution seismic sections show truncation of the reflections close to the sea- bottom and a thinning downslope to a few metres thick cover of shelf-spill over sands reworked by the BC and SACW. The erosional depressions in this terrace are partly infilled by coarse to fine clean siliciclastic sand. Rhodolite nodules consisting mainly of red algae are obsemed at the remnant topographic highs. Their age (ca. 14,000 years) is coincident with the settlement of carbonate banks on the present outer shelf adjacent to the terrace, supporting an al- lochthonous origin for these nodules. Fine-grained turbidite sands are observed infilling ancient erosional features below the terrace.

6.3. Middle slope

The middle slope (550-1200 m:) is characterized by iron-rich laminated indurated fine sands and deep-water coral mounds that overlie a thick, metres to tens of metres, package of silty--mud to sandy

148 A.R. Hana et al. /Sedimentary Geology I15 (1998) 133-157

laminated mud (Caddah et al., 1998), in some places disrupted by mass-flow processe!;.

The iron-rich surface encrusted layer is generally 10 cm thick, comprising siliciclastic and biogenic fine silty sands (Caddah et al., 1994). They are yellowish brown, more rarely recldish ('rusty'). They show planar lamination marked by intercalations of fine sand and silt layers, sornetimes masked by Planolites-type bioturbation (Cacldah et a]., 1998).

AAIW oxygen-rich flow prc~bably oxidises the ocean bottom creating iron-rich sandy crusts, and carries ahermatipic coral larvae ('Viana, 1994). These larvae grasp the gentle topographic mounds inher- ited from Late Pleistocene mass movements (Kows- mann et al., 1994; Viana et a]., 1994) and develop coral banks elongated down-current. These deep- water coral banks may attain hundreds of metres in length, tens of metres in width, 10 to 15 m in height and develop a 40-km-long coral field. These large accumulations are found only urider the upper flow of the AAIW, from 570 to 850 m of water depth (Viana, 1994). Fewer and minor deep-water coral accumulations and the persistence of an iron-rich laminated sand cover at the very top of the sea-floor are seen as far down-slope as the passage between the still vigorous flow of the AAIW and the calmer and nutrient-poor NADW.

The sandy-silty laminated mud occurs below the sandy crust and comprises very thin sand beds ( c 3 cm) irregularly interbedded in a muddy sequence. This facies resembles the bottonl-current reworked sands described by Schanmugarn et al. (1993) in Plio-Pleistocene sections of the Gulf of Mexico. The Campos Basin bottom-reworked sands are pre- dominantly fine-grained to silty, show sharp upper contacts and sharp to gradational basal contacts, rare laminations and cross-lamination., and a high degree of bioturbation (Caddah et a]., 1998). This facies is found from the upper-middle slope to the upper São Paulo Plateau. It is Late Pleistocene in age, being younger on the middle slope (17 ka B.P.) than on the Plateau (26 ka B.P.).

Unlike the southern area where slump deposits only reach as far as the middle slope, the northem mass-movement deposits were displaced further down-slope. The higher sedimeritation rate of this area (120 cmka), in contrast to th,e southem area (70 cmka), and the abundante of t~irrigenous organic

matter observed in piston-cores indicate a greater proximity to a continental feeder source. The observed widening of the lower slope results from the interplay between slumped sediments and the sediments from the Paraíba do Sul subniarine fan.

6.4. Lower slope

Below 1200 m water depth, where the southward NADW flows, a thin (< I0 cm thick) nannoforam ooze overlies the iron-rich crust with a sharp non-erosive contact. First radiometric dating and isotopic analyses indicate that the ooze is Holocene in age, whereas the iron-rich crust is diachronous: the mid-slope crust is Holocene ancl the lower-slope crust is latest Pleistocene (Fig. 12, and Viana, 1994; Viana et al., 1994). The iron-rich crust, from the middle to the lower slope, is everywhere underlain by sandy-silty mud of a hemipelagic aspect (Caddah et al., 1994, 1998).

7. Modern sedimentation on the Campos margin - a discussion

7.1. Modern highstand processes

7.1.1. Outer shelfupper slope Bed-load transport and bedform tlevelopment un-

der the action of combined shelf and oceanic processes are well known from the literature (Hunt et al., 1977; Flemming, 1978; Pattiararchi and Collins, 1984; Dalrymple et a]., 1992; Rarnsay, 1994). In the Campos Basin, outer shelf bed-load transport is related to the interaction of many hydrological factors resulting in a dominant cross-slope flow. Low-frequency currents (longer than 1 day) tend to be constrained by geostrophy to flow along isobaths. Hence cross-isobath water fluxes are a consequence of ageostrophic processes (Huthnace, 1992, 1995). Local, non-linear and time-dependent flows result from coastal trapped waves (wind- and pressure- forced, topography-related and generated by oceanic eddies), tides, upwelling and downwelling. Along- shore gradients in either wind or pressure result in a cross-slope flow (Huthnace, 1995). Sediment export to the upper slope occurs pseferentially via low-density gravity flows and possible suspension cascading, that may be induced by the temperature

A.R. 'Viana et al. /Sedimentary Geology 115 (1998) 133-157

- OOZE

- HARDGROUND

- DEEP-WAlElrl CORM MOUND

- HEMIPEUIGIC MUD

- SLUMP DEKi8ITS

Fig. 12. Scheme showing the interplay between sedimentation and deep-ocean circulation fluctuation in the Campos Basin continental slope. Coral mounds and hardgrounds (irun-nch cmst) are found beneath the AAIW influence, with nannoforamiiiiferal ooze under the NADW. Figure not to scale: hardground and ooze are 10-20 cm thick; coral mounds are 10-m-thick, hemipelagic muds and mass-rnovement deposits are metres to tens of metres thick. The presence of hardground overlain by ooze at the base of the slope and the I4c data suggest a downslope shift frorn the present position of the AAIW during the latest Pleistocene (26 ka BP to li 1 ka BP). Contact between modern NADW (ooze) and AAI-' (hardground) occurs near the 1200-m isobath. Adapted from Viana et al. (1994).

gradient between cooler, higher-density shelf waters and warmer, lower-density slope waters. The process resembles a downwelling wiirh the dense shelf waters flowing down the slope urider gravity. Fric- tion and topographic channelling compensate the geostrophic tendency for along-slope flow (Huth- nace, 1995).

7.1.1.1 Tides and waves. Storm fronts and related storm waves in the Campos Basiri propagate to the northlnortheast and induce near-bottom orbital velocities capable of overcoming the threshold velocity for fine sediments (Fig. 5). Distinct storm events occur at least three times per year but the outer- shelf submanne sand dunes may be reworked even under calmer conditions (Viana ;md Rizzo, 1994). The bi-directionality of outer-shelf currents (Fig. 4)

is strongly controlled by the passage of storm fronts with a dorninant frequency of 4- 12 driys.

Semi-diurna1 tides are important in along-isobath transport, mainly in the south with a secondary cross- slope component important in the central-northern portion of the study area, in the vicinity of the São Tomé canyon head. Additionally, the area of the Brazil Current flow reversals (northward), induced by its clockwise vorticity (eddy) with consequent on-shelf penetration, is placed near the large sand dune field.

The bottom currents caused by this combined influence of storm waves, tides and geostrophic currents (eddies and meanders) result in the generation, migration and reworking of the shelf-edge dunes. These bedforms require near-bottom-current speeds of 0.5 to 1.0 rn/s (Southard and Boguchwal, 1990, cited by Dalrymple et al., 1992), values observed

150 A.R. Viana et al. ISedimentary Geology 115 (1998) 133-157

in Campos Basin measurements. This mechanism is possibly also responsible for reworking the large sand stock on the outer shelf arid for destabilising the sands by: (I) providing high biological activity; (2) winnowing out fine material. and therefore, (3) diminishing the cohesiveness of the sediment. This then leads to export of shelf sediments on to the upper slope. Presently the sand wilve migration trend is coincident with the head of submarine incised valleys and the São Tomé canyon head.

The trend of rnedium-sized sand dunes obsemed on a narrow plateau below the shelf-edge scarp south of the São Tomé canyon, at a viater depth of 250 m, indicates a cross-isobath downslope flow, which is in accordance with the bottom-currents measured in the adjacent areas, that have a strong component offshore. These features may be also related to the above-cited mechanism.

7.1.1.2 Brazil Current. The near-bottom action of the western-boundary Brazil Cunent is observed, in particular, on the upper slope against the shelf edge, and secondarily as the penetration of eddies and meanders on the outer shelf. The northern part of the area is most affected by this superficial geostrophic current, as the BC is subjected to the funnelling increasing its velocity and hence its capacity to erode and transport sediments. l'reliminary analyses of oceanographic data collected in a mooring anchored at 200 m water depth indicate superficial peak velocities greater than 3 m/(; and, at 5 m from the bottom, velocities of up to 1 d s . In this area very coarse sands and gravelly sediments were cored and seismic sections indicate continuous and intense episodes of sea-floor erosion. Downslope, this current and its countercurrent SACW are responsible for the development of a thin (ft:w to tens of centimetres thick) Holocene sand veneer. In the southern area the BC is less intense, and is confined by the shelf-edge scarp developing erosional scars in which Pleistocene muddy sediments outcrop. The thermal gradient between cooler shelf watt:rs and the warmer Brazil Current slope waters (Fig. 6) leads to the transfer of shelf sediment to the upper slope via low- density turbidity currents, in pla~ies where several gullies flow down intersecting the shelf edge. This results in gravity-derived deposits in an environment of strong geostrophic along-slope circulation.

7.1.1.3 Eddies. Oceanographic mesoscale clockwise eddies seem to act as a 'submariní: floor-polishing machine', i.e., the vertical downwater transference of the momentum generated by eddy rotation is strong enough to induce high bottom-current velocities that, associated with the upwelling Eckman pumping wich is characteristic for cyclonic eddies (Kamenkovich et al., 1986), apparently produce a tornado-like effect along its core.

Such eddy-induced bottom currents in the abyssal domain have been documented arid modelled by several authors (Weatherly and Kelley, 1985; Mul- hearn et a]., 1986; Peggion and Weatherly, 1991). They lead to remobilisation and resuspension of fine material, as confirmed by recent studies in the Cal- ifornia Borderland that showed tht: remobilisation of sediments under the influence of eddies in water depths of 100 to 600 m (Washburn et a]., 1993). The resuspended sediments form a nepheloid cloud and may be kept in suspension for :years and transported for thousands of kilometres (McCave, 1986; McCave and Tucholke, 1986; Hollister and Nowell, 1991). Hollister and McCave (1984) relate extremely high current velocities (up to 73 c d s ) , resuspension of sediments and the generatiun of bedforms to the path of the Gulf Stream warm-core rings in waters deeper than 4000 m. They also comment that possibly a11 sites of large-scale oceanic eddies are subjected to abyssal storms. Some authors suggest that resuspension of sedirnents occuirs after sea-bottom 'preparation' by biological activity and a change in current direction (Gross and Williams, 1991).

The birth place of the Campos Basin eddies is exactly over the large outer shelf area of mud-free sand dunes. A large amount of sand is available for reworking and transport along the eddy track. This process may result in a near-bottom tlense cloud and in a series of mixed traction-susperision events, as suggested by unpublished 12-kHz echo-bathymetric records. Such events propagate from the outer shelf to the mid-slope with a cross-isobath trend indepen- dent of the local topography. Side-sczm sonar records and piston-cores show that the shelf-edge gullies and the canyon heads in the area beneath the track of the eddies are filled with recent sands. Data collected during an eddy passage at mooring c (upper shelf, 400 m, Fig. 1) show that the directiori of bottom-current flow changed by 90" to a northwç:sterly direction

A.R. Viana et al. /Sedimentary i

by the normal conditions of northvvesterly flow. Pis- ton-cores collected in this area are mud-depleted and highly bioturbated.

The eddy diameter is considerably reduced along its vertical axis, producing a 'tea-cup effect'. Taking a conservative approach, the eddy velocity decreases to negligible at a depth of approximately 0.01 of the eddy diameter. Thus, an eddy of 80 krn in diameter, as normally observed in the Campos Basin, could disturb a water column up to 800 m deep. On the other hand, in an experimental model de- velopped by Nishimura and colleagues (Nishimura et al., 1984), a 1/10 relation was observed, yield- ing sediment reworking at abyss.sl depths (depths greater than 5000 m). These results are in accordance with the observations of Hollister and McCave (1984) and Hollister and Nowell ( 1991) that suggest a surface-related phenomenon (Gulf Stream eddies) as the origin for the abyssal stonns on the eastern margin of United States. Unforturiately, the intense activity of burrowing organisms, the relative high frequency of abyssal storms (15-45 days) and the circular motion of eddies mask tlie core record of such phenomena.

After the eddy passage, the normal basin circulation is re-established and the remobilised fine sediments are moved out of the area. They may settle rapidly if they are moved into an adjacent low- energy environment (for instante, into the NADW influenced zone), or be transportecl great distances if the permanent current regime is (sufficiently strong (AAIW or AABW zones, in the Campos Basin example, or any other strong bottoni current, as cited by Hollister and Nowell, 1991).

The sum of a11 of these processes (storm fronts, waves, tides and eddies) presents an offshore resultant vector that leads to offshelf sand export. The principal passage for these sands is through valleys, gullies and canyons incised into tlie shelf edge, par- ticularly in the centre and northeast of the study area. Modern supply of shelf sands to he upper slope is suggested by side-scan sonar records and Remote Operated Vehicle (R.O.V.) video inspections. Sub- manne robots recorded an overburden on the chain of a rig anchor by a 50 cm thick sand package, at 400 m water depth, one year after its installation in the thalweg of a valley in the shelf etlge. Sonar records revealed the development of sand lobes connected to

shelf-edge valleys. After arriving on the slope, sand deposition or further removal is coritrolled principally by morphology, with deposition in topographic lows caused by previous mass movements for example, and by exposure to an active hydrodynamic context.

7.1.2. Deep-water processes In deeper waters, the coarse sedirnents that spill

over to depths below the BC influence (i.e. >250- 350 m) are then submitted to the strong northeastward flux of the SACW. Piston-core analyses suggest that this current is capable of winnowing and reworking of fine-grained sand on the upper slope. The SACW is another factor resporisible for sand influx into of São Tomé canyon head, at 400 m water depth. The surface sediment distribution chart (Fig. 10) demonstrates the enlargement of the upper slope sand deposits to the north, towards the São Tomé canyon. In an inverse trend to that proposed for BC exfoliation and eddy generation, the SACW must also suffer a decrease in intensity after crossing the canyon. Nevertheless, since the velocities are less than for the BC, probable SACW exfoliation would be less strong, resulting in partia1 loss of competency and load deposition rather than eddy generation. Sand-size sediments in this area are found down to 750 m water depth, suggesting downslope prograda- tion of the upper-slope sand sheet.

Holocene sedimentation rates in the upper- middle slope of the northern area are: twice as great as those in the south (14 crníka versus 7 cm/ka). The upper course of the São Tomé canyon is presently under the influence of the SACW and cores collected from within the canyon indicate infilling with modern Holocene sand.

Cores from various water depths (550 to more than 2000 m) highlight some facies distinctions between the middle and lower slopes. On the middle slope, below the AAIW, hemipelagites are overlain by a sandy crust, whereas below the NADW the vertical succession is composed of hemipelagites, sandy crust and, at the top, a pelagic ooze drape (Fig. 12). The flow of the AAIW between 550 and 1200 m water depth is responsible for the principal characteristics of the middle-slope facies: (1) the development of an iron-rich cmst that partly indurates a laminated fine-grained sand, and (2) the generation

Fig. 13. High-resolution mul~ichannel seismic dip section off Cape São Tomé, from the outer shelf to the middle slope. This section displays in general the upper slope stacked erosional features, occumng from the upper Miocene to Recent. This region occurs at the Brazil Current funnelling and accelerating area, to the north of the eddy dominatcd area. Erosional features are always parallel to sub-parallel to the isobaths and connccted to the adjacent subinarine canyons. Small arrow indicates the geotechnical borehole of Fig. 14.

A.R. \'iana et al./Sedimentary Geology I I5 (1998) 133-157 153

of large deep-water coral mounds in the centre of the study area.

Below 1200 m, under the influence of the NADW, Holocene calcareous ooze overlies an latest Pleis- tocene iron-rich sandy crust (Viana, 1994). However, preliminary oxygen isotope analyst:~, together with radiocarbon dating and detailed exrimination of this crust, suggest that it was formed under the influence of a water mass with characteristics similar to the modern AAIW rather than the NADW. We therefore suggest that this facies is diachronous and its deposition would have occurred at the slope base during the last glacial maximum when the AAIW flow was deeper, and migrated progressive1.y upslope to its present position above the middle slope. This crust differs from that described by McGi:ary and Darnuth (1973) which is synchronous, and niarks the passage from Pleistocene to Holocene independently of the depth.

7.2. Glacial lowstand processes

7.2.1. Outer shelf/upper slope During the last glacial maximum lowstand of sea-

leve1 the surficial oceanographic processes and water masses both shifted basinward. The erosional terrace on the uppermost slope of the northern area, at the offshore projection of Cape São Tomé, is subjected to the strongest flux of the BC. 1The sediment facies clearly show an area of a high-energy (current- dominated) carbonate-siliciclastic transition. Seis- mic records show the persistent erosional effects of the BC in this area, including top-truncated reflectors, cut-and-fill features, large abrasional surfaces and narrow along-isobath channels at the foot of the shelf-edge scarp (200-300 m of water depth; Figs. 9 and 13).

High-resolution seismic-stratigraphic and biostratigraphic (forams and coccoliths) analyses show that the seaward extension of erosional events is mainly associated with relative sea-leve1 lowstands, being especially well documented for those of the last glacial maximum (oxygen isotope stage 2: f 24 to f 12 ka). At that time rather more than 30 m of sediment (i.e. 107 m3) wei-e removed from the upper continental slope between the São Tomé and Itapemirim submarine canyons as revealed by geotechnical surveys (Fig. 14). More recent sedi-

mentation has so far reconstmcted only one half of the total thickness of the sediments removed.

The flattened topography, the absence of associated mass-flow deposits (slump or creep) and the residual erosional features, together with the wide range of sedimentological and oceanographic data collected in the Brazil Current domairi, suggest that this current is the only possible agent capable of causing such major erosion. The BC was also an important canyon-head coarse-sediment feeder especially during the periods of low sea-level. Sedi- mentation rates on the upper-middle slope further suggest an efficient southward transport of sediments from the north by the BC. During the latest Pleis- tocene, between 11 and 40 ka, the southern area sedimentation rate of 74 cmlka was twice as great as that observed to the north of the São Tomé canyon (38 cmka), both of which are more than five times greater than the respective Holocene rates. Suspen- sion cascading and downwelling was most likely inhibited during lowstands so that along-slope processes would have been dorninant on tkie upper slope. The behaviour of the SACW and its i-elationship to sedimentation processes in glacial times are not well understood.

7.2.1.1 Deep water. The vertical and spatial variation of the three facies observed in cores bellow the AAIW andlor NADW (i.e. ooze, crust and hemipelagites; Fig. 12) reflect major changes in flow regime and water-mass characteristics changes re:lated to pale- oceanographic oscillations. The deposition of the crust at the slope base from the last maximum relative lowstand of sea-leve1 to the beginning of its rise suggests a moderate- to high-energy, well-ven- tilated, along-slope near-bottom circulation during that period. Gravity processes probably also played an important role in deep-water sedirnentation, par- ticularly via the numerous submarine canyons. The interplay between both down-slope arid along-slope processes appears to be marked by the along-slope (northward) deflection of base-of-slope features such as the canyon-mouth lobe of the São Tomé canyon (Fig. 9).

Specific facies characteristics indiciitive of this interplay might be revealed with further research, for example turbidite/contourite couplets in an amalga- mated succession of top-reworked turbidites.

A.R. Viana et al. /Sedimentarj Geology 115 (1998) 133-157

In-situ undrained shear-stregth, Su (kPa)

thickness of the removed section

(- 40m)

Fig. 14. In-situ Undrained Shear Strength (Su) log-profile recorded in a geotechnical borehole drilled at 350 m water depth, in the northeast region of the study area. The profile break at 20 m depth records an erosional event of the latest Pleistocene which removed about 40m of sedimentary column. The estimation of thickness removal is obtained by the intersection of the overconsolidated curve trend projection with the projection of the depth (vertical) axis.

I \ trend of normal consolidation 2w \ of the eroded sedirnents

I I \

A.R. Eina et al. /Sedimentay Geology 115 (1998) 133-157 155

8. Conclusions

The modem sedimentation pattern on the southeastem Brazilian continental margin is representative of relative highstands of sea-leve]. The geological and oceanographic data suggest that a continuous and significant transfer of shelf sedi~rients to the continental slope occurs at the present day. They suggest also that different thermohaline currents have played a major role in determining sedimentation pattems on the slope at least during the late Quatemary.

During the late Holocene, tides, storm-waves and on-shelf penetration of the surficial geostrophic Brazil Current, as synoptic eddies arid meanderings, are modem amplifiers of the outer-shelf bottom current. These processes may therefore act as important factors in shelf-edge sand accumulation and off-shelf sediment export. Bedforms and bottom-current data indicate a dorninant off-shelf sedimeint movement.

The eddies develop in the middle-outer shelf area as a consequence of: (1) a sudden loss in intensity of the Brazil Current after passing through a physiographically induced current-funnelling area, and (2) the interplay of the Brazil Clurrent with upwelled waters. The eddy spinning movement reaches the sea-bottom down to depths of 800 m or even deeper. Such a process and its capacity to resus- pend and transport sediments is here compared with a sea-floor polishing machine. Both suspended and bed-load material can thus be transfeued to the upper slope where deposits occur as: (1) lobes attached to the mouths of shelf-edge gullies, (2) flat accumulations spread along the concave base of the shelf-edge scarp, and (3) the infill of bottom-current-derived erosional features.

The upper slope sediments are then affected by less strong but still active BC, SACW and AAIW flows.

A nannoforam pelagic ooze is presently developed beneath the flow of the North Atlantic Deep Water.

During the last relative lowstand of sea-leve1 and early sea-leve1 rise, the basinward h i f t of the sedimentary and oceanographic procesi,es were accom- panied by some changes in ocean-circulation-related facies. Superficial waters (STW), es.pecially the BC, were reinforced and responsible for upper-slope erosional scars, and locally, for the removal of more

than 30 m of sediment column. The AAIW probably shifted down-slope and led to development of the sandy cmst at the base of the continental slope as well as to the northward deflection of sediment bodies deposited by down-slope processes.

Further research is critica1 for an improved understanding of sedimentation and oceanographic features on the Campos Basin margin. Theije include:

(1) a complete evaluation of down-slope and along-slope near-bottom processes by additional ac- quisition of bottom-current data, core analysis and outcrop studies;

(2) a precise sediment signature for each distinct hydrological process;

(3) deeper water information to evaluate the basinward continuity of the processes and the associated geometry and facies;

(4) a longer-tem oceanographic investigation integrating data from meteorologically induced oceanographic phenomena to near-bottom processes;

(5) the relationship between the high- and low- frequency processes (here involving tectonic pulses and basin subsidence);

(6) the elaboration and very-high-resolution pale- oceanographic reconstruction of short-tem high-frequency phenomena with high impact in sedimentation.

Some of these problems may be solved by the incorporation of complementary data arid multidisci- plinary studies that will probably provide:

(1) the development of a multilayer numerical model integrating satellite images, oceanographic data, sonar images and sedimentary facies, in order to evaluate the action of the diverse phenomena and to provide a quantitative basis to calculate the arnount of deep-basin sand supply, and;

(2) the adaptation of such a model to different oceanographic contexts and sea-levels in any sedimentary basin.

Acknowledgements

We thank the people from Petrobras Marine Geology Section (DESUDISEGMAR), Geotechni- cal and Environmental Section (SEGENISEGDAD) and Research Centre Oceanography Section (CEN- PESISEPRON) for their assistance and able work in collecting, processing and managing the data. The

1.56 A. R. Vana er al. /Sedirnentary Geology 115 (1998) 133-157

sedimentologists and the techniicians of Petrobras Sedimentology Laboratory at IMacaé were instru- mental in the core descriptions. M.A. Vicalvi, R.L. Antunes and A. Grassi provided biostratigraphic age-dates. We are especially grateful to J. Heiner- ici, M. Carminatti, C.L. Silva Jr. and W. Tavares Jr. for insightful discussions, and to D.A.V. Stow and 0. Moeller Jr. for their valiiable reviews of the manuscript. This paper is published by permission of Petrobras Exploration Departmei~t (DEPEX).

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