The Oligocene–Neogene deep-sea Columbia Channel system in the South Brazilian Basin: Seismic stratigraphy and environmental changes

Marine Geology 266 (2009) 18–41

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Marine Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a r g e o

The Oligocene–Neogene deep-sea Columbia Channel system in the South Brazilian Basin: Seismic stratigraphy and environmental changes Andréa Franca Lima a, Jean-Claude Faugeres b,⁎, Michel Mahiques a a b

Instituto Oceanográfico da Universidade de São Paulo, 05508-120 Praça do Oceanográfico, 191, São Paulo, Brazil Université Bordeaux1, UMR CNRS 5805 EPOC, 2 Allée Ronsard, 33600 Pessac, France

a r t i c l e

i n f o

Article history: Received 9 July 2008 Received in revised form 11 July 2009 Accepted 13 July 2009 Available online 21 July 2009 Communicated by D.J.W. Piper Keywords: Brazilian basin Deep-sea channel mixed turbidite–contourite system seismic stratigraphy tectonic control paleocirculation

a b s t r a c t The Columbia Channel (CCS) system is a depositional system located in the South Brazilian Basin, south of the Vitoria–Trindade volcanic chain. It lies in a WNW–ESE direction on the continental rise and abyssal plain, at a depth of between 4200 and 5200 m. It is formed by two depocenters elongated respectively south and north of the channel that show different sediment patterns. The area is swept by a deep western boundary current formed by AABW. The system has been previously interpreted has a mixed turbidite–contourite system. More detailed study of seismic data permits a more precise definition of the modern channel morphology, the system stratigraphy as well as the sedimentary processes and control. The modern CCS presents active erosion and/or transport along the channel. The ancient Oligo-Neogene system overlies a “upper Cretaceous–Paleogene” sedimentary substratum (Unit U1) bounded at the top by a major erosive “late Eocene–early Oligocene” discordance (D2). This ancient system is subdivided into 2 seismic units (U2 and U3). The thick basal U2 unit constitutes the larger part of the system. It consists of three subunits bounded by unconformities: D3 (“Oligocene–Miocene boundary”), D4 (“late Miocene”) and D5 (“late Pliocene”). The subunits have a fairly tabular geometry in the shallow NW depocenter associated with predominant turbidite deposits. They present a mounded shape in the deep NE depocenter, and are interpreted as forming a contourite drift. South of the channel, the deposits are interpreted as a contourite sheet drift. The surficial U3 unit forms a thin carpet of deposits. The beginning of the channel occurs at the end of U1 and during the formation of D2. Its location seems to have been determined by active faults. The channel has been active throughout the late Oligocene and Neogene and its depth increased continuously as a consequence of erosion of the channel floor and deposit aggradation along its margins. Such a mixed turbidite–contourite system (or fan drift) is characterized by frequent, rapid lateral facies variations and by unconformities that cross the whole system and are associated with increased AABW circulation. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The sedimentary cover and record of paleoenvironmental changes in the continental rise and abyssal plain of the South Brazilian Basin (South Atlantic Ocean) is little known. Only one DSDP leg took place in this region, with a single drill in the deep ocean (site 515, Barker et al., 1983a and b). However, it is a key area for the understanding of the Cenozoic development of the South West Atlantic Ocean, where both the Antarctic Bottom Water (AABW) and the North Atlantic Deep Water (NADW) have been active and have played an important role in the abyssal sediment deposition (Gamboa et al.,1983; Mézerais et al., 1993; Viana and Faugères, 1998; Alves, 1999; Viana et al., 2002, among others), since their beginning during the Paleogene( AABW) and Oligocene (NADW) times. ⁎ Corresponding author. E-mail address: [email protected] (J.-C. Faugeres). 0025-3227/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2009.07.009

Numerous canyons and channels are present along the continental margin where most of the terrigenous supplies have been deposited on the continental slope due to a major break between the upper and middle slope and to topographic traps on the middle–lower slope. However, a part of these supplies have been transported onto the abyssal plain where they have been deposited as deep-sea turbiditic fans or sheets, or have been redistributed by the contour currents (Asmus and Guazelli, 1981; Brehme, 1984; Castro, 1992; Gonthier et al., 2003; Viana et al., 2003). In the north-eastern part of the basin (Fig. 1), a deep-sea sedimentary system is linked to one such major deep channel, the Columbia channel. This sedimentary system is called the Columbia Channel System (CCS in this paper). This system that was deposited during the Cenozoïc and still works today, results from the interplay of turbidity and contour currents as shown by Massé et al. (1998) and Faugères et al. (2002).

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Fig. 1. The South Brazilian Basin. Localization of the study area, major morphological domains, channels and structural and volcanic features (bathymetric contours extrapolated from from Cherkis (1983) and the maps GEBCO (General Bathymetric Chart of the Oceans); s.m.: sea mount; Is.: island.

The aim of this paper is to establish the morphology of the modern Columbia channel, and to present a more detailed and reliable interpretation of the stratigraphy and geometry of the CCS Cenozoïc deposits in order to characterize the processes and history of the channel formation, the growth of such system and the major controlling factors. Detailed correlations between the system and other systems previously studied in the deep or shallower parts of the basin have been realized and could drive to a better knowledge of the sedimentary cover at the scale of the whole South Brazilian Basin. 2. Morphological, hydrological and geological background The South Brazilian Basin is located on the Brazilian margin of the South Atlantic between the Rio Grande Rise in the south and the Vitoria–Trindade seamount chain in the north (Fig. 1). It comprises: 1) a shelf of variable width (from 20 to 250 km), 2) an upper slope, between 200 and 2000 m depth, dissected by numerous gullies and canyons, 3) a lower slope (2000 to 3400 m) consisting of a large plateau, the São Paulo Plateau (SPP), affected by numerous active halokinetic structures, and crossed by a very complex channel network and 4) a continental rise that gently slopes down to the abyssal plain and presents a more regular surface. The SPP channels converge towards the rise thus forming a small number of major channels like the Columbia channel. The deep part of the margin is swept by two major water masses, the NADW and the AABW. The study area is under the direct influence

of the AABW (Reid et al., 1977). The modern NADW flows slowly southward (at a velocity of less than 5 cm s− 1) at a depth ranging from around 1200 m to 4000 m and the AABW flows northward, below the NADW, and with a velocity never exceeding 10 cm s− 1 in the open basin. However more recent data have shown a far more complex modern circulation with rapid and temporary velocity and trend changes (Reid,1989,1996; DeMadron and Weatherly,1994; Siedler et al., 1996; Hogg et al., 1996; Hogg and Zenk, 1997; Hogg and Owens, 1999, among others). The current velocity and paths were also highly variable during the Quaternary (Johnson et al., 1977; Ledbetter, 1986; Massé et al., 1994; Siedler et al., 1996). Indeed, during Glacial periods, NADW was drastically reduced and replaced by AABW that penetrated further north (Oppo and Lehman, 1993; Broecker, 1997; Hagen and Keigwin, 2002, Cremer et al., 2007, among others), its upper boundary rising up to about 3500 m water depth in the Brazil Basin (Bickert and Wefer, 1996; Volbers and Henrich, 2004). During Cenozoic time, several major hydrological events have been recorded in the deep-sea deposits of the Brazilian Basin, in the form of major unconformities. They have been associated with episods of increased AABW activity (Kennett, 1982; Barker et al., 1983b; among others). Uncertainties still remain in the paleocirculation patterns in the South Atlantic Ocean, but it seems sensible to consider that the study area has been subjected to the major influence of the northward flowing deep western boundary current since the beginning of the AABW circulation at the end of the Paleogene.

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Most of the morphological patterns of the basin have been inherited from previous margin evolution and the opening of the South Atlantic Ocean. After a rifting phase initiated during the late Aptian, the first occurrence of an oceanic crust, north of the Walvis-São Paulo Rise, is dated near the Aptian/Albian boundary (113 Ma, Chang et al., 1992). A regime of oceanic sedimentation with the construction of a prograding margin has taken place since that time and throughout the Tertiary. Oceanic crust accretion is responsible for the laying of an oceanic floor (continental rise and abyssal plain) that has today attained an average width of more than 1000 km. Features like active transform faults and volcanic seamounts are associated with expansion of the oceanic floor. That is the case of the Rio de Janeiro fault zone and of the Vitoria–Trindade volcanic chain (hot spot) where radiometric ages of around 50–40 Ma have been obtained from the rocks of Abrolhos Bank (Cordani and Blazekovich, 1970), between 10 and 50 Ma from the Abrolhos and Columbia seamounts, and younger than 3 Ma at the Trindade and Martim Vaz islands (Fodor and Hanan, 2000). The deep-sea CCS (Figs. 1 and 2) is located in the north-eastern part of the basin, on the continental rise and abyssal plain. It trends WNW–ESE, close to the south of the Vitoria–Trindade volcanic chain. The system spreads towards the SE and the Rio de Janeiro lineament that probably controlled the subsidence of the area and the location of the mouth of the modern channel in the very deep abyssal plain (Gorini and Carvalho, 1984).

3. Methodology: seismic data, correlations and stratigraphy Two previous publications have been dedicated to the study area. Massé et al. (1998) investigated the Quaternary deposits and sedimentary processes on the basis of Kullenberg core lithology and two 3.5 kHz echosounding profiles (DB and GH, Fig. 2 in this paper). Faugères et al. (2002) presented a preliminary interpretation of the deposit stratigraphy and geometry supported by only two seismic lines AB and GH (Fig. 2), in addition of a synthesis of the Quaternary CCS based on results from Massé et al. (1998). The stratigraphical interpretation in Faugères et al. (2002) was fairly hypothetical as no reliable seismic correlation was established at that time between lines AB and GH as well as between these lines and the DDSP site 515 used as a chronological reference. This paper is based on the the analyses of several new seismic lines and the re-interpretation of the AB and GH published lines. These lines come from two sources (Fig. 2): – one profile from the LEPLAC program, the N–S air gun line 500–514 that begins at the DSDP site 515 and crosses the lower continental rise northward (Alves, 1999); – several lines from the BYBLOS program (Faugères, 1988), that cross the upper and lower part of the CCS: 1650 km of water-gun seismic lines (100 Hz, single-channel unprocessed seismic) associated with 3.5 kHz echo-sounder lines.

Fig. 2. Detail of the deep part of the South Brasilian Basin with the Columbia channel system and location of the AABW currents, DSDP site 515 and LEPLAC seismic line 500–514; BYBLOS lines A–B, D1–E, F–F1, G–H, I–J, J–K, K–L, L–M. (bathymetric contours extrapolated from from Cherkis (1983) and GEBCO (General Bathymetric Chart of the Oceans) maps.

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Fig. 3. The Columbia channel morphology and echofacies. Bathymetric sections crossing the Columbia channel and showing the downslope evolution of the valley morphology and 3.5 kHz echofacies distribution. The echofacies types and interpretation are extrapolated from Massé et al. (1998); Faugères et al., 2002, Lima, 2003: -IIIB: very uniform echo characterized by regular overlapping hyperbolae with conformable subbottom reflectors; IIIB/IIID: uniform echo with regular overlapping hyperbolae and conformable subbottom reflectors sometimes associated with tangential hyperbolae; -IIID: indistinct more or less prolonged echofacies without subbottom reflectors but with regular subbottom hyperbolae with vertex tangential to the seafloor; -IIIC: discrete hyperbolae of fairly regular vertex elevation; IIB/IIID: echofacies composed of mixed IIB and IIID echo; IIIB/IIA: echofacies composed of mixed IIIB and IIA echo;-IIA: indistinct echofacies with discontinuous subbottom reflectors; IB: distinct echo;-IIB: indistinct, more or less prolonged echofacies without subbottom reflectors; IIIA: hyperbolic echofacies without subbottom reflectors, large hyperbolae with varying vertex elevation.

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Fig. 4. Stratigraphic correlations between the Byblos seismic lines crossing the Columbia Channel system.

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Fig. 5. Stratigraphic correlations between the Sao Tomé and Columbia systems.

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Fig. 6. Stratigraphic correlations between the Leplac and Byblos seismic lines south of the Columbia Channel.

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Fig. 7. LEPLAC profile (500–514) and interpretation based on Alves (1999). Location of DSDP site 515 and crossing with BYBLOS line VW (see Figs. 2 and 6).

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No detailed bathymetric map of the study area is so far available. The morphological background of the system presented here (Figs. 1 and 2) is extrapolated from Cherkis (1983) and GEBCO (General Bathymetric Chart of the Oceans) maps. Improvements in the knowledge of the morphology and physiography of the system presented below (Section 4) are based exclusively on the analyses of the BYBLOS seismic lines. We have analyzed all the available 3.5 kHz lines (AB, D1E, FF in addition of DB and GH, Fig. 3). We give here only a summary of the echofacies description and interpretation as they have been previously illustrated and published in details (Massé et al., 1998; Faugères et al., 2002; Lima, 2003). The detailed interpretation of the stratigraphy and geometry of the Cenozoïc CCS deposits (Section 5) is supported by the seismic line analyses and correlations (Figs. 2 and 4 to 7). It consists of: 1) a more precise interpretation of the already published BYBLOS lines (AB and GH), 2) reliable correlations between AB and GH by the way of lines IJ, JK, KL and LM; only a small gap exists between points B and L (Fig. 4); 3) the definition of several seismic units at the scale of the whole system, using seismic facies types and the occurrence of major unconformities at the unit boundaries; 4) correlations between the CCS and the Sao Tomé system (Viana et al., 2003) on the upper continental rise (line VW, Fig. 5); 5) correlations between the system and the LEPLAC line through BYBLOS lines VW and WA (Figs. 6 and 7). The ages proposed in Section 6 for the units and unconformities (Table 1) have been deduced from the interpretation of the LEPLAC line (Fig. 7) corroborated by DSDP site 515 (Barker et al., 1983a and b; Gamboa et al., 1983; Alves, 1993). Correlations throughout the South Brazilian Basin (Table 2) have been made using the most important studies already published (Alves, 1999; Gamboa et al., 1983; Mézerais et al., 1993, Souza Cruz, 1995; Faugères et al., 2002; Viana et al., 2003; Lima, 2003). 4. The modern Columbia Channel system: morphology, physiography and sedimentary processes

well-defined terrace occurs on both flanks and is slightly deeper on the northern flank (around 4600 m). It seems to result from current erosion and/or from slide scarps as suggested by the echofacies (Massé et al., 1998). On the following deeper line (D1–E), the section is still asymmetrical but the steepest flank is that to the south. An erosive terrace is still present on both flanks. The deepest lines F–F1 and G–H show some contrasting patterns. The northern flank is significantly higher than the southern one suggesting more active deposition in the northern area, and a terrace only exists on the northern flank on line F–F1 that could be the continuation of the feature observed on D1E. The southern depocenter (Figs. 2, 8 and 10) belongs to the lower continental rise and is characterized by a fairly regular surface that slopes down gently towards the east (the deep abyssal plain) and the north (into the channel); the sea floor shows predominant undulating echofacies and muddy sediments (red clays) deposited under sluggish contour currents (Massé et al., 1998). The northern depocenter (Figs. 2, 8 and 10) is about 500 km long and 100 to 200 km wide. In the shallow NW depocenter, the sediment surface is very flat and presents an indistinct echofacies with discontinuous subbottom reflectors that corresponds to turbiditic deposits (Massé et al., 1998). In the deep NE depocenter, the bottom presents a slightly mounded morphology and a very thick undulating echo-type with stationary surficial sediment waves and conformable subbottom reflectors, some of them associated with tangential hyperbolae, that is typical of the contouritic deposit in this region (Faugères et al., 2002). In the transitional area between the NW and NE depocenters, echofacies and core data suggest turbidity and contour current interaction (Massé et al., 1998). All these data show that the modern Columbia channel is an active channel. South and north of the channel, the deposit pattern fits with a fan drift (Lewis, 1994; McCave and Carter, 1997) or a mixed turbidite– contourite system (Mulder et al., 2008). 5. The Oligocene–Neogene Columbia Channel system

The CCS is about 500 km long with a width that increases downstream from about 200 km to 350 km. Three major environments have been defined (Fig. 2): 1—the channel itself; 2—south of the channel, a part of the continental rise we have called the southern depocenter; 3—north of the channel, what we have called the northern depocenter, bounded to the N by the Vitoria–Trindade chain. The shallower part of the northern depocenter (shallow NW depocenter) shows a fairly flat morphology, gently sloping down towards the south (from the Vitoria–Trindade chain towards the channel) and towards the east (the deep abyssal plain). The deeper part of the northern depocenter (deep NE depocenter) presents a slightly mounded morphology suggesting a levee-like sediment body (McCave and Tucholke, 1986; Faugères et al., 1999). The deep Columbia channel is connected with the upper slope. However, there are no available data to trace the exact course of the channel between the upper slope and the lower rise. On the lower rise (Figs 2 and 3), the channel runs from a depth of 4200 m to 5200 m. It trends initially in a WNW–ESE direction and then changes slightly to a NW–SE direction. The depth of the valley decreases down slope from about 400 m to 250 m, and its width is of about 20 km at the top of the valley flanks and 4–5 km on the very flat valley floor. The irregular flanks of the valley show evidence of active erosion: reflection truncations, flat terraces and erosive scars. The lower flanks and the channel bottom show indistinct echofacies suggesting erosive processes and active sediment transport mainly by turbidity currents, as confirmed by recent sandy turbidite deposits on the valley seafloor (Massé et al., 1998). The cross-section of the valley (Fig. 3) shows variations from W to E. On the shallower line (AB), the altitude of the south and north flanks is fairly similar and the section of the valley is dissymmetric with a southern flank dipping more gently than the northern one. A

5.1. Definition of seismic units and chronostratigraphy The chronostratigraphic framework of the Cenozoïc unconformities and seismic units defined in the CCS is presented in Table 1. The system began to develop at the end of the Paleogene above a sedimentary substratum. This substratum overlies a volcanic basement (Faugères et al., 2002) that could correspond to the oceanic crust but no reliable evidence are today available to support this hypothesis. If it is the case, the age of the sedimentary substratum is assumed to be “upper Cretaceous–Paleogene” because the continental break-up responsible for the South Atlantic Ocean opening occurred during Aptian–Albian times (Asmus and Guazelli, 1981) and the study area is very close to the outermost limit of the continental margin. The sedimentary substratum forms the seismic unit I (U1). This unit is subdivided into two subunits (U1a and U1b) separated by unconformity D1. D1 is correlated with reflector I present at the DSDP site 515 but there is no direct evidence for its age as drilling stopped before D1 (Gamboa et al., 1983). As the oldest drilled sediments were dated from the Early Eocene, D1 should have been formed before that time, probably somewhere during the Paleocene (D1: “Paleocene” unconformity in this paper, Tables 1 and 2). The CCS system itself was mainly deposited during the Oligocene– Neogene times. It developed above a major erosive unconformity D2 and consists of two seismic units U2 and U3. D2, between U1 and U2, is a prominent unconformity correlated with reflector II of Gamboa et al. (1983) that presents a hiatus spanning about 22 Ma. Its age is not well-constrained due to the deposit drastic erosion: from the end of the Eocene to the base of the Upper Oligocene (Rupelian–Chattian unconformity, Duarte and Viana, 2007). D2 is named “late Eocene– early Oligocene” unconformity in this paper.

Table 1 Correlations between seismic and sedimentologic data from the Columbia Channel system (Lima, 2003) and DSDP site 515 (Gamboa et al., 1983).

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28 Table 2 Stratigraphical correlations between DSDP site 515 (Gamboa et al., 1983), the deep basin (LEPLAC line 500-0514, Alves, 1999), the continental slope (Souza Cruz, 1995), the deep Vema Fan, (Mézerais et al., 1993), the upper rise (Sao Tomé system, Viana et al., 2003; Lima, 2003) and the deep Columbia Channel system (Faugères et al., 2002; and new interpretation of Lima, 2003 and Lima et al., this study).

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Three subunits (U2a, U2b and U2c) are defined in U2. They are bounded by three unconformities: D3, D4 and D5 (Table 1). D3 is correlated with reflector III of Gamboa et al. (1983), dated from the end of the Oligocene (D3: “Oligocene–Miocene boundary” unconformity in this paper). D4 is correlated with reflector IV (Gamboa et al., 1983). According to these authors, this unconformity probably represents a hiatus important (upper Miocene) and was probably formed during the late Miocene but the available stratigraphic data did not allow to give a precise age as they cannot firmly associate R IV with one of the two major sea-level drops that occur during this period, either at 11.5 Ma or at 6.6 Ma (Haq et al.,1987). We give a “late Miocene” age to D4 in this paper. D5 is correlated with unconformities R3 of Viana et al. (2003) and R4’ of Mézerais et al. (1993) and assumed to occur during the Pliocene by the authors. It could be correlated with a global hydrological event associated with the closure of the Panama seaway, at about 3 Ma (Kennett, 1982) and therefore interpreted as “late Pliocene” in age (Table 2). U1 at the top would represent the upper Pliocene–Quaternary sedimentary cover of the system. The origin of the unconformities that bound the seismic units is discussed in more detail in Section 6.3.2. Correlations between the shallow and deep parts of the CCS and between the CCS and others systems in the South Brazilian Basin are synthesized in Table 2. These correlations point out a few mistakes in previous seismic interpretations of the Columbia and the Sao Tomé systems. In the CCS, Faugères et al. (2002) recognized two major unconformities R1 and R2. Based on correlations with LEPLAC line 500–514 and the DSDP site 515, they were assumed to be respectively “Paleocene” and upper “Eocene–lower Oligocene” in age. Their location in the seismic line AB crossing the north of the system was misinterpreted by these authors: R1 was confused with the unconformity we have called D2 in this paper (“upper Eocene–lower Oligocene” in age) and R2 with D3 (“Oligocene– Miocene boundary”; compare Figs 2 and 3 in Faugères et al., 2002 with our Figs. 8 and 10, and see the new interpretation of Lima (2003), in Table 2). In the same way, correlation with the Sao Tomé system points out the misinterpretation of the age of unconformities R1 and R1a and associated seismic units (Units I and IIa) described by Viana et al. (2003). R1 that was interpreted as upper Eocene–lower Oligocene in age is correlated with D1: it is therefore “Paleocene” in age; R1a was interpreted as Oligocene–Miocene boundary in age, but is correlated with D2 and is thus “upper Eocene–lower Oligocene” in age (Table 2; Fig. 7). 5.2. Seismic stratigraphy and deposit geometry The study area presents a thick accumulation of sediments (1 to 1.8 s Twt on average) above the volcanic basement. The modern

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Columbia channel “cuts” deep into the upper part of the sedimentary series. In this section we define the deposit geometry and stratigraphy in the “upper Cretaceous–Paleogene” sedimentary substratum and the overlaying CCS, and then the relationship between the channel development and processes, and the evolution of the system during the Oligocene–Neogene times. 5.2.1. The “upper Cretaceous–Paleogene” sedimentary substratum Seismic unit U1 (Fig. 11) forms the “upper Cretaceous–Paleogene” sedimentary substratum (0.4 to 0.6 s Twt) and is composed of two subunits, U1a and U1b, bounded at the top by unconformities D1 (“Paleocene”) and D2 (“late Eocene–Early Oligocene”) respectively. This is observed throughout most of the area, with the exception of the deep NE depocenter and beneath the channel where seismic artefacts may occur. Subunit U1a, at its base, overlies an irregular volcanic basement. In the shallow NW part of the system, this basement gently dips from south to north before rising again in the vicinity of the Vitoria– Trindade chain; several faults may be responsible for the basement surface irregularities and unconformities (Figs. 8 and 9). In the deep NE part, it remains very irregular and difficult to observe (Fig. 10). U1a is characterized by fairly transparent facies and more or less irregular or chaotic reflections. It is bounded by unconformity D1 (“Paleocene”) marked by a fairly continuous and high amplitude reflector and local erosion. In the shallow NW part of the system, the thickness of the deposits diminishes both towards the south and the north. A rapid increase below the channel is suggested by the unit thickness, significantly greater south of the channel (“m.t.1a”, Fig. 8) than in the north. Then the thickness decreases again further towards the south. In the deep NE part, a similar increase in the deposit thickness seems to occur below the channel area, where the basement forms a shallow depression (Fig. 10). Subunit U1b, at the top, shows predominantly high amplitude, irregular, more or less continuous reflections (Figs. 8, 10 and 12). It presents local variations, like transparent and chaotic reflections closed to the Vitoria–Trindade Chain in the shallow NW part of the system (Figs. 13–15). The deposits seem to increase in thickness, as does U1a, close to the channel (Fig. 8). The D2 unconformity (“late Eocene–early Oligocene”) corresponds to a major erosive surface that cuts into unit U1 (see interpretation, Figs. 8, and 10, 16 and 17). It is clearly recorded throughout the system, and is well-correlated (Fig. 4) between the deep NE part (line GH) and the shallow NW part of the system (line AB). In this NW part, it is still more erosive than in the southern depocenter and cuts drastically into

Fig. 8. Seismic line A–B crossing the shallow western part of the Columbia channel system: profile (with Fig. 16 location) and interpretation (m.t.1a and m.t. 1b: maximum thickness of units U1a and U1b respectively, close to the channel).

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Fig. 9. Scheme of the tectonic evolution during the déposition of unit U1 and lower and middle part of unit U2 (subunits U2a and U2b?) in the shallow part of the Columbia Channel system (interpreted from AB seismic line); -a: deposition of U1a with active F1 vertical faults (crustal distension with mini-grabens formation) to explain the rapid increase of U1a thickness just below the Columbia channel (thicker south than north of the channel); -b: deposition of U1b with probably a similar slight tectonic activity: -c and -d (m.ch.: minichannels in the graben area): at the end of U1b deposition, a first tectonic event, responsible for the uplift of the northern side of the graben (-c), may explain the strong erosion of U1b north of the channel (-d); it is synchronous with D2 erosive event;-e and-f: after D2 erosion, a second tectonic event opposite to the first tectonic event is responsible for an important down lift of the northern part of the system; it is synchronous of the lower (-e) and middle (-f) part of U2 deposition to explain the high thickness of the sediment series in the northern depocenter compared to the southern depocenter;-g: interpretation of the seismic line AB.

the U1b subunit, (Fig. 8). Such massive erosion might be explained as due to an uplifting of the NW area that did not affect the southern area (see Section 6.3.1 and Fig. 9). 5.2.2. The Oligocene–Neogene depocenters A fairly thick Oligocene–Neogene sedimentary series (up to 1 s Twt) is deposited above the D2 unconformity. In contrast with unit U1, which shows fairly uniform deposit facies on the scale of the study area, the series shows significant deposit variations: a homogeneous, more or less transparent facies in the southern depocenter and various deposit facies in the northern depocenter (Figs. 4, 6, 8 and 10). Two

units are defined: the thick basal unit U2 subdivided into three subunits (U2a, U2b and U2c) and, at the top, a very thin unit, U3. Three unconformities D3, D4 and D5 are defined at the top of respectively U2a, U2b and U2c (Fig. 11). 5.2.3. Unit U2 (“upper Oligocene–lower Pliocene”) Unit U2 is characterized by major transparent facies thoughout the system and a mounded geometry in the deep NE depocenter. The thickness is significantly greater in the northern depocenter (up to 1 s Twt both in the NW and the NE) than in the southern depocenter (0.5 to 0.8 s Twt, in the SW and the SE respectively).

A.F. Lima et al. / Marine Geology 266 (2009) 18–41 Fig. 10. Seismic line G–H crossing the deep eastern part of the Columbia Channel system: profile (with Fig. 17 location) and interpretation: m.t.2a: maximum thickness of U2a; m.t.2b: maximum thickness of U2b; I: crossing with seismic line IJ (see Figs. 2 and 12).

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Fig. 11. Synthetic scheme showing the spatio-temporal variations of the seismic units in the Columbia Channel system.

Fig. 12. Seismic line I–J and interpretation.

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Fig. 13. Seismic line J–K and interpretation (M: crossing with seismic line LM, see Figs. 2 and 14).

5.2.4. Subunit U2a (“upper Oligocene”) In the major part of the system, subunit U2a (0.1 to 0.4 s Twt, maximum sedimentation rate of about 70 m/Ma, using a seismic velocity of 1.800 m/s Twt and a sequence duration of about 5 Ma) is represented by more or less transparent facies, with wavy reflections at the top (Figs. 8 and 10). In the shallow NW depocenter (Fig. 9a), the deposit is thickest and the deposit geometry is tabular and similar to that in the southern depocenter. However, in the deep NE depocenter, the unit is significantly thinner on average but presents a slightly mounded shape (maximum thickness of 0.4 s Twt in the vicinity of “M.t.2a”, Fig. 10). The unconformity D3 between units U2a and U2b, is characterized by a slightly irregular surface marked by a drastic seismofacies change (from wavy below D3 to transparent facies above D3) and, locally, a few erosional features (Fig. 15). It is sometimes not clearly identifiable as in the northernmost part of the study area (Fig. 4 point J). 5.2.5. Subunit U2b (“lower–middle Miocene”) Subunit U2b (from 0.1 to 0.6 s Twt) shows seismic facies similar to U2a (predominant transparent facies with some wavy reflections at the top) south of the channel and in the deep NE depocenter (Figs. 8, 10 and 12). In this depocenter, it shows a significant mounded shape

and reaches its maximum thickness (0.6 s Twt, sedimentation rate of about 45 m/Ma in the vicinity of “M.t.2b” Fig. 10, using a seismic velocity of 1.800 m/s Twt and a sequence duration of about 12 Ma). In the shallow NW depocenter, the location of D4 is more speculative due to a facies lateral change (point J in Fig. 4): the fairly transparent facies of U2b merges progressively into high amplitude fairly wellstratified reflections that form the whole unit close to the Vitoria– Trindade chain (Figs. 13, 14 and 15). These reflections dip gently and thicken slightly towards the south (Figs. 8 and 11). The unconformity D4 between subunits U2b and U2c is marked by local erosion (Figs. 12 and 15) and by a seismic facies change (from wavy to transparent) similar to that associated with D3. As for D3, it is sometimes not clearly visible as the overlying subunit U2c seems to have been eroded locally like in the SE depocenter (point G in Fig. 6 and Fig. 11) or difficult to localize because of lateral facies changes like in the NW depocenter (between points A and B, Fig. 11; points J and M in Fig. 4; Fig. 12). 5.2.6. Subunit U2c (“upper Miocene–lower Pliocene”) The fairly thin subunit U2c (≤0.1 s Twt) occurs in the N depocenter but seems to have been almost entirely eroded in the S depocenter. In

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Fig. 14. Seismic line K–L and interpretation.

the NW depocenter, high amplitude well-stratified reflections, similar to those of U2b, merge towards the channel into wavy and chaotic reflections (Figs. 8 and 16). In the NE depocenter, it still presents a

predominant transparent facies (Figs. 10, 13 and 17). There, the sedimentation rate is significantly lower than those observed in U2a and U2b but the subunit still shows a slightly mounded geometry.

Fig. 15. Seismic line L–M and interpretation.

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Fig. 16. The A–B BYBLOS profile crossing the shallow western part of the system, detail of the seismic line in the vicinity of the channel (location in Fig. 8), and interpretation (1—minor channels).

Subunit U2c is bounded at the top by D5, a well-marked erosive unconformity that is characterized by an undulating surface in the southern depocenter and the deep NE depocenter (point I in Fig. 4; Figs. 10, 12 and 17) and a very flat erosive surface in the shallow NW depocenter (Fig. 8).

in Fig.4; Figs. 10 and 17) and more or less continuous and wavy reflections in the southern depocenter (point W in Figs. 5 and 8). On the other hand, in the NW depocenter and close to the Vitoria– Trindade Chain, more or less discontinuous flat horizontal reflections are sometimes present (Figs. 8, 13 and 16).

5.2.7. Unit U3 (“upper Pliocene–Quaternary”) Unit U3 is a very thin unit (b0.1 s Twt) that forms a blanket of more or less draping deposits over the whole system. It consists of wellstratified undulating reflections in the deep NE depocenter (points I

5.2.8. Geometry of the deposits in the vicinity of the channel The Columbia channel cuts into the units U3 and U2a. Below the channel valley, minor shallow channels are present in the upper subunit U1b (Figs. 9, 16 and 17), and in the lower subunit U2a where

Fig. 17. The G–H BYBLOS profile crossing the deep eastern part of the system: detail of the seismic line in the vicinity of the channel (location in Fig.10), and interpretation (1: minor channels; 2: eastward channel migration).

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they show a north-eastward migration (Fig. 17). Just above D2 (“upper Eocene–lower Oligocene”), the slight dip of some reflectors, from both sides of the channel towards the modern channel axis, suggests the occurrence of a single shallow channel at that time. These early channels are associated with deposits characterized by chaotic and discontinuous reflections that appear to be truncated. These deposits merge laterally into very well-stratified reflections towards the north and more irregular reflections (transparent, discontinuous, wavy) towards the south. That suggests a turbiditic sheeted system fed by several minor channels merging upward into a single-channel. Between D2 and D5 (“late Pliocene”), both in the shallow and deep parts of the system, the deposits are more or less chaotic on the channel flanks and pass laterally to more clearly stratified deposits sometimes associated with wavy geometry, then to transparent and/ or irregularly stratified facies. The chaotic-stratified facies is more extensive at the base (subunit U2a), and then decreases upward (subunits U2b–U2c), especially in the deep part of the channel (Fig. 17). The chaotic facies could result from slope failures and highenergy channel deposits. The stratified deposits are developed on both sides of the channel and are very similar to turbidite/hemipelagite seismic facies, reflecting an overspilling process. These features suggest sediments mainly deposited by turbidity currents with major aggradational processes building sediment levees. During U2, the accumulation of sediments along the channel is interrupted by several episodes of erosion (Figs. 16 and 17). At least five erosive surfaces are recorded in the deposits. The oldest erosive surface can be correlated with D2 (“upper Eocene–lower Oligocene”), and one more recent one to D4 (“late Miocene”), as observed south of the channel in the shallow system (Fig. 16). These surfaces underline a progressive increase of the channel depth that is already significant at the end of U2b. A more recent surface fits with the erosive D5 unconformity (“late Pliocene”). Finally, a last erosive event occurs recently, as evidenced by the modern channel surface that truncates D5 and the older surfaces and reflectors. Such features indicate that the Columbia channel was active throughout the Oligocene–Neogene period, with alternating periods of deposition and erosion. Such a channel history fits in with the Guanabara, Carioca and Macaé channel history as recorded in the adjacent upper continental rise (Viana et al., 2003). 6. Discussion: deposits and sedimentary processes, evolution of the system, controlling factors 6.1. Deposits and processes The “upper Cretaceous–Paleogene” sedimentary substratum shows fairly uniform sediment features at the scale of the study area. It is characterized by transparent seismofacies associated with chaotic and discontinuous irregular reflections. The deposits probably consist of pelagic–hemipelagic and turbiditic sediments as suggested by data from DSDP site 515 (Table 1). The basin was smaller than it is nowadays. As no channel patterns are present in the sediment pile, the deposits may consist of sheeted turbidites. No major deep oceanic circulation is observed despite the global tectonic activity in the Southern Atlantic Ocean and climatic changes (Kennett, 1982; Thomas et al., 2006). The rapid lateral variations in seismofacies that characterized the Oligocene–Neogene CCS may be explained by the interaction of turbidity current and contour current processes. During the system deposition, the turbiditic deposits first start spreading largely from a shallow channel. Then they are progressively more restricted in the vicinity of the channel axis and pass laterally into a transparent deposit showing discontinuous wavy reflections (U2 unit). The high similarity of the U2 transparent facies in the southern depocenter and in the deep NE depocenter strongly suggests the same depositional

process in both areas. Because this seismic facies is similar to the facies of the Quaternary contourites, we suspect that most of the U2 unit consists of contouritic deposits. Such an interpretation fits in the interpretation of the late Oligocene–middle Miocene deposits further south at the DSDP site 515 (Gamboa et al., 1983; Table 1). It fits also with the mounded geometry of U2 in the NE depocenter (drift-like levee). A great part of the material of this levee may come from suspended particles, pirated from turbidity currents and redeposited by the contour currents. South of the channel, the deposits form a contourite sheet drift. In the shallow NW depocenter, beyond the channel, the transparent deposits are mainly restricted to subunit U2a. They are overlain by well-stratified facies interpreted as turbidites. These facies gently dip and thicken from the Vitoria–Trindade chain towards the channel. That suggests that some turbidity currents could originate from the Vitoria–Trindade chain and interact with the channel-driven turbidity currents and/or contour currents as depicted by more disturbed sediments close to the channel. Then, in the CCS, the predominant process may be different according to the depth and the locus of the domain (north or south of the channel, either close to or far from the Vitoria–Trindade Chain), over a given period. However, both turbidity and contour processes may play a major role alternatively or interact in certain other periods. That results in an interfingering of both types of deposit on the scale of the whole system: this system can then be interpreted as a fan drift system (Lewis, 1994; McCave and Carter, 1997), called also a mixed turbidite–contourite system by Mulder et al. (2008). In the Quaternary CCS (Massé et al., 1998; Faugères et al., 2002), the processes that predominated along the channel resulted from gravitational processes (turbidity currents and sliding). The downflowing turbidity currents were deflected towards the north by the action of both the Coriolis force and the deep western boundary current. In the shallow NW depocenter, mostly major turbidites were deposited reflecting frequent turbidite overspilling processes. However turbidites are absent in the deep NE depocenter, where only muddy contourites are present: that could be explained by the downslope decreasing energy of the turbidity currents and then decreasing overspilling process as well by the increased intensity of the AABW below 4500 m (Hogg et al., 1996; Harkema, and Weatherly, 1996). In the southern depocenter, where only the contour currents are active, muddy contourites are deposited. 6.2. Evolution of the system The Columbia channel system first formed during the upper Eocene–lower Oligocene and is synchronous with the creation of the D2 erosive surface. Above the D2 unconformity, the Oligo-Miolower Pliocene series starts with the deposition of transparent contouritic sediments (U2a) on the scale of the whole system. It means that the contour current processes were predominant at that period (U2a). Turbiditic deposits that were derived from the upper continental slope are restricted to the channel that is already clearly defined. Unconformity D3 (“Oligo-Miocene boundary”) is not always clearly distinguished in the deposits (change in the seismic facies with only small erosive features). However, it is associated with an important change in the sedimentation processes and distribution. During U2b, if major contour current processes still controlled the deposition south of the channel and in the deep NE depocenter, turbidity current processes became predominant close to the Vitoria– Trindade Chain and in the shallow NW depocenter. That suggests an increase in the continental sediment supply together with sediment derived from the volcanic chain that should have undergone a significant uplift and formed significant relief. That fits in with the abundant sediment supply observed on the upper slope and the rapid shelf edge progradation at this time (Castro, 1992; Carminatti and

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Scarton, 1991; Souza Cruz, 1995; Duarte and Viana, 2007) and the global sea-level variations marked by several rapid and high amplitude sea-level falls (Haq et al., 1987; Fig. 18). Unconformity D4 (“late Miocene”), at the top of U2b, shows patterns similar to those of D3. The overlying Unit U2c does not present any major changes in the processes and deposit distribution as compared with those of U2b. The last depositional phase (Unit U3, upper Pliocene to recent) deposited a thin blanket of sediment all over the system. Both contour

37

and turbidity currents still controlled most of the depositional processes. The modern channel is recently active. 6.3. Major controlling factors 6.3.1. Regional tectonics The deposition of the “upper Cretaceous–Paleogene” sedimentary substratum was influenced by the tectonic background. Deposit thickness changes in Unit 1 can be interpreted as resulting from vertical

Fig. 18. Correlations between the seismic units and unconformities defined in the Columbia Channel system and the sea-level variations, ice sheet developments and global tectonic events (G.S.R.: Greenland Scotland Ridge; I.F.R.: Iceland Faroe Ridge).

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movements of blocks in the volcanic basement, probably linked to activity on crustal lineaments (Rio de Janeiro or Martin Vaz Fracture Zones). More particularly, the rapid increase in the thickness of U1a and U1b, just below the modern channel, could be interpreted as due to a graben-like structure that favoured the establishment of the first minor channels preceding the formation of the major channel (Fig. 9). Such a structure would have controlled the location of Columbia channel at the boundary between a southern and a northern block. In the southern depocenter (sheet drift), the slight erosion of U1b by the D2 unconformity, and the uniform sedimentation during U2 deposition suggests a more uniform subsidence of the continental rise. The deep NE depocenter (mounded contourite drift) with similar features has also not recorded any particular tectonic control of the deposit distribution. On the other hand, the shallow NW depocenter seems to have undergone a more active tectonics. There, the drastic erosion of U1b at the D2 unconformity may be seen as a response of a very active circulation to a paleotopography shaped by local tectonics. The scenario proposed in Fig. 9 seems the most sensible hypothesis to explain such erosion and the deposit distribution in the overlying units: – at the end of the U1b deposition, a first tectonic event responsible for the uplift of the northern side of the graben might explain the strong erosion of U1b north of the channel; this would have been synchronous with the D2 erosive event; – after the D2 erosion, a second tectonic event contrary to the first would have been responsible for considerable subsidence of the northern part of the system; it would be synchronous with the lower (Fig. 9e) and middle (Fig. 9f) U2 deposition and may explain the thickness of the sediment series greater in the northern depocenter than in the southern depocenter. As the Vitoria–Trindade chain has been built progressively over the last 50 Ma, the volcanic activity may be responsible for the observed changes in the tectonic regime (uplift then downlift north of the channel) and may then have influenced the distribution of the system deposits. The uplift event might be the consequence of a high activity of the hot spot that lifted the basement rocks. That fits, on a smaller scale, with the recent local uplift of the volcanic basement responsible for the sediment mound located in the northern depocenter (Fig. 12). The following subsidence event, after D2, might be associated with the end of a period of intense emission of lava in the chain and the cooling of the volcanic basement. However there is no proof in the literature to support such a volcanic scenario during the late Eocene–Oligocene times. 6.3.2. Sediment source, climate, paleocirculations and unconformities Three sources of sediments may be involved in the supply delivered to the CCS (Massé et al., 1996): – a major part of the sediments coming from the upper margin through various drainage systems; – a minor part that consists in clayey sediments and siliceous biogenic particles, coming from the deep Argentine and South Brazil basins, and transported by the AABW contour currents; – a part which is difficult to assess (but probably minor as suggested by Quaternary data), delivered from the surrounding volcanic seamounts (Vitoria–Trindade chain). The sediments from the upper margin could have been delivered to the system either directly by turbidity currents, or indirectly after the fine-grained turbiditic material was pirated and transported northward by the AABW contour currents into the CCS. As the two other sources could not play a major role, we assume that the amount of available sediments that fed the system is then largely controlled by the global climatic variations and associated sea-level changes. Several periods of active sediment supply have been recorded in the Cenozoic sedimentary cover of the margin. Most of them fit with episodes of major sea-level falls (Figueiredo and Mohriak, 1984; Guardado et al., 1990; Mohriak et al., 1990; Carminatti and Scarton,

1991; Peres, 1993; Castro, 1992; Souza Cruz, 1995; Bruhn, 1998; Cainelli and Mohriak, 1999; Souza Cruz, 1998; Viana et al., 2003; Duarte and Viana, 2007; Moraes et al., 2007). Several sea-level falls occurred during the deposition of the “upper Cretaceous–Paleogene” sedimentary substratum of the CCS. Although their amplitude is relatively low compared to some younger sea-level falls (Haq et al., 1987; Fig. 18), significant terrigenous supply formed turbidite complexes on the upper slope (Assis et al.,1998). Paleoclimates remained warm with marine water temperatures relatively high; however climatic conditions were by no means stable with a number of colder and warmer intervals and a gradually increasing of the latitudinal climatic differentiation (Kennett, 1982; Thomas et al., 2006). The deep-sea circulation was at an early stage and not supposed to have been very active (Thomas et al., 2006), even it could have been responsible for some deposit features as observed at DSDP site 515 (onlap, infill, reflectors truncated by D1, Gamboa et al., 1983). Therefore, the age and the origin of unconformity D1 (“Paleocene”) still remain speculative. At the base of the system, D2 (“upper Eocene–lower Oligocene”) is the most conspicuous unconformity (Fig. 18). It is related to the Eocene– Oligocene transition (35–34 Ma) where occurred the most profound oceanographic and climatic change of the past 50 Ma (e.g., Zachos et al., 2001). This event is marked by a major global cooling, the beginning of an active deep thermohaline circulation and a major sea-level fall during the late Oligocene (e.g., Kennett,1982; Haq et al.,1987; Miller et al.,1991; 2008). It is interpreted as resulting from active global tectonics and the opening of oceanic seaways in the south hemisphere (Drake Passage, South Tasman Rise, Scher and Martin, 2006; Thomas et al., 2006) that drove major climatic changes marked by the growth of the ice cap in the Antarctica and an enhanced production of AABW and deep circulation (Miller et al., 2008). This event is well-recorded in the Indian and Pacific ocean (e.g., Kennett, 1982; Tucholke and Embley, 1984; UenzelmannNeben et al., 2007), all along the western Atlantic margin (e.g., Mountain and Tucholke, 1985; Locker and Laine, 1992; Berger and Wefer, 1996; Stoker et al., 1998), and in the South Brazilian Basin (Barker et al., 1983a and b; Gamboa et al., 1983; Viana et al., 1990; Castro, 1992; Mézerais et al., 1993; Alves, 1999; Faugères et al., 2002; Lima, 2003; Viana et al., 2003; Table 2). During the deposition of the Oligo-Neogene system, three major unconformities are observed (D3, D4 and D5). They have also been associated with episodes of intensified thermohaline circulation resulting from global climatic changes and/or tectonic modifications and subsequent expansion of ice in the polar areas (e.g., Kennett, 1982; Gamboa et al., 1983; Moran et al., 2006; Thomas et al., 2006; Fig. 18). D3 (“Oligocene–Miocene boundary”) occurred during the progressive deepening of the Drake and Tasman seaways responsible for the increased isolation of the Antarctic continent, and a major episode of global cooling marked by the expansion of the east Antarctic ice sheet (e.g. Naish et al., 2001). D4 (“late Miocene”) occurred after a global cooling event and a ice cap and sheet expansion in the Antarctic (at about 12–14 Ma, Shevenell et al., 2004), and a subsidence episode of the Greenland–Scotland and Iceland–Faeroe ridges (at about 12 Ma). These changes allowed a drastic increase of the NADW production and global oceanic circulation (e.g., Kennett, 1982; Wright and Miller,1996). D5 (late Pliocene) is associated with the strengthening of the NADW production as a response of Arctic areas to the closure of the Panama seaway: cooling of the northern hemisphere and build-up of sea ice at about 3 Ma on the Arctic ocean and major ice sheets in adjacent continental areas (Greenland ice cap at 3.2 Ma; e.g., Moran et al., 2006; Lunt et al., 2008). Unconformities D4 and D5 coincide with major sea-level falls (Fig. 18) at the end of the middle Miocene and in the middle Pliocene respectively (Haq et al., 1987). Several less important sea-level falls took place during that time: one at the end of the Oligocene that can be approximately correlated with D3, and another one, at the end of the late Miocene, could be involved in D4 development (see Section 5.1). Whatever their amplitude, these episodes of sea-level falls may have resulted in periods of turbidity currents of higher frequency and energy

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and then in more active erosive processes along the channel. As a consequence it seems sensible that both turbidity currents and contour currents interacted in the development of the major erosive unconformities observed in the system. However the relationship between turbiditic and contouritic processes in response to sea-level variations (active at the same times, or alternating through time) remains questionable. Our data as well as data from the literature (e.g., Faugères et al., 1999; HernándezMolina et al., 2008; Knutz, 2008) show that there is no systematic link between sea level and rates of drift destruction or accumulation at a global scale. Each episode of increased bottom current circulation linked to a global hydrological event typically corresponds to a widespread surface of erosion or non-deposition in the deep-sea systems. Such events can have either a climatic origin or a tectonic cause. In the first case they are linked to changes in the extent of polar ice caps and sea ice like during the later Quaternary, and then may be associated with the interglacials or the glacials as well as with the deglaciation according to the oceanic areas and water masses (Faugères et al., 1999): there is no link with the sea-level stand. In the second case, a tectonic cause (for example the opening and closing of sills and gateways, like during the Cenozoic) may control bottom water exchange between ocean basins and be responsible for a global climatic and hydrological event, and the development of a major unconformity in the oceanic deposits. Such events may be associated with a drastic sea-level fall like at the end of the middle Miocene as mentioned before. Significant accumulation on contourite drifts is mostly favoured by a moderate intensity of bottom currents (otherwise erosion is predominant), and needs significant but not too high rates of sediment supply via turbidity currents or other mass flows (otherwise they would mask the contourite sedimentation). We may notice that in the CCS, the highest sedimentation rates are observed during U2a (upper Oligocene) just after a major fall of the sea-level (end of the early Oligocene), and during U2b (lower and middle Miocene), associated with several sea-level falls (end of the late Oligocene and during the early-middle Miocene, Haq et al., 1987). During the last two decades, numerous works supported by very high resolution analyses of the deep marine sediment record give a more and more detailed interpretation of the thermohaline paleocirculation and paleoclimate during the Cenozoic and Quaternary. However, at a global scale, uncertainty still remains today in relating particular climatic conditions (and hence sea level) to particular intensities of the thermohaline circulation. We might therefore conclude that it is not possible to fit contourite processes and depositional systems in a global sequence-stratigraphic model. However at a local scale, it may be possible to define a “regional sequence-stratigraphic model” like in the Gulf of Cadiz, for the Meditterranean outflow and associated contouritic deposits (Llave et al., 2007; Hernández-Molina et al., 2008). To summarize, the deposit patterns of the CCS result from the combination of several determining factors. Tectonic movements mainly control the system location and sediment distribution. Climate and eustasy must have controlled the flux of terrigenous sediments transported by turbidity currents and local erosion. Active deep circulation plays a major role in the sediment distribution and deposit erosion (major unconformities).

1. The system is developed on an “upper Cretaceous–Paleogene” sedimentary substratum that shows fairly uniform seismic facies thoughout the area. It started during a major paleoenvironmental change marked by a major erosive unconformity formed during the “upper Eocene–lower Oligocene” (D2). 2. The system worked throughout the Oligocene–Neogene as shown by the striking difference between the deposit pattern north of the channel and that to the south of the channel, and by the deposit geometry on both flanks of the channel. The depth of the channel increases progressively as a consequence of deposit aggradation north and south of the channel, and erosion on the channel floor and flanks. On both sides of the channel, the deposits are interpreted as turbidites. The modern channel is still active as proved by the channel physiography and down slope evolution. 3. The Oligocene–Neogene deposits are formed of several units and subunits bounded by unconformities, respectively “Oligocene– Miocene boundary” (D3), “late Miocene” (D4) and “late Pliocene” (D5) in age. These major unconformities, as well as D2, can be traced at the scale of the South Brazilian Basin and represent time lines for the system and basin stratigraphy. They can be correlated with unconformities known all along the Western Atlantic Ocean margins and are associated with global hydrological–climatic events. 4. Interaction between contour currents and turbidity currents result in sediment features typical of a mixed turbidite–contourite (fan drift) system: major turbiditic deposits along the channel; higher sedimentation rate on the flank of the channel where the turbidity currents are deflected by the Coriolis force (N depocenter); prominent turbiditic deposits where the deflected turbidity currents are of high-energy and the contour currents less active or masked by the turbidity currents (shallow NW depocenter, proximal part of the system); major contouritic deposits in areas where the turbidity currents are absent (southern depocenter) or of decreasing energy and the contour currents more active (deep NE depocenter channel, distal part of the system); turbidite– contourite interfingering between these areas with, more particularly, rapid lateral passage from turbiditic to contouritic deposits in both sides of the channel; and finally the occurrence of widespread erosive unconformities that cross the whole system. 5. Climatic global changes and associated eustatic variations have controlled most of the sediment supply. Important sea-level falls, as at the end of the lower Oligocene and during the Miocene, might explain the high sedimentation rate observed during the upper Oligocene and the lower–middle Miocene. Three episodes of sealevel falls seem to be synchronous with major hydrological events suggesting that turbidity and contour currents may have interacted to form the major unconformities. 6. Tectonics has played a significant role in the location of the channel and sediment distribution. Major crustal lineaments and the volcanic activity of the Vitoria–Trindade chain seem to have determined variations in the tectonic behaviour of crustal blocks.

7. Conclusions

References

The Columbia channel system in the deep South Brazilian Basin is a mixed turbiditic–contouritic system composed of the channel itself and two depocenters located north and south of the channel. Detailed study of seismic lines allows reconstruction of the stratigraphy and evolution of the system and to interpret the sedimentary processes and control.

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Acknowledgments The authors would like to thank M. Cremer for valuable discussions, F.J. Hernandez-Molina, A. Viana, two anonymous reviewers and the editor comments for their very helpful critical review that greatly improved the manuscript.

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