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Sedimentary Architecture in Meanders of a Submarine Channel: Detailed Study of the Present Congo Turbidite Channel (Zaiango Project)

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Sedimentary Architecture in Meanders of a Submarine Channel: Detailed Study of the Present Congo Turbidite Channel (Zaiango Project) Journal of Sedimentary Research, 2010, v. 80, 852–866 Research Article DOI: 10.2110/jsr.2010.078 SEDIMENTARY ARCHITECTURE IN MEANDERS OF A SUBMARINE CHANNEL: DETAILED STUDY OF THE PRESENT CONGO TURBIDITE CHANNEL (ZAIANGO PROJECT) 1 2 3 4 N. BABONNEAU, B. SAVOYE,* M. CREMER, AND M. BEZ 1Universite´ de Brest, CNRS UMR 6538 Domaines Oce´aniques, IUEM, Place Nicolas Copernic, 29280 Plouzane´, France 2IFREMER, GM-LES, BP70, 29280 Plouzane´, France 3Universite´ Bordeaux I, CNRS UMR 5805 EPOC, Avenue des Faculte´s, 33405 Talence, France 4TOTAL, CSTJF, Avenue Larribau, 64000 Pau, France e-mail: [email protected] ABSTRACT: Sinuous deep-water channels are recognized in most large deep-sea fans in the world. They present a particular interest to oil companies, since they are significant hydrocarbon reservoirs in deep offshore environments. The understanding of their geometries and their internal sedimentary architecture is necessary to better characterize reservoir heterogeneity of sinuous submarine channels. Therefore, numerous studies have been undertaken recently to better understand the behavior and sedimentary architecture of deep-water channels. The aim of this paper is to present our results concerning the development of the meandering channel of the present Congo turbidite system (or Zaire turbidite system). The study is based on high-resolution data including multibeam bathymetry, seismic lines, echosounder profiles, high-resolution side-scan sonar images, and gravity cores, collected by IFREMER along the submarine Congo channel between 1994 and 2000, during Guiness and ZaiAngo surveys. The present Congo turbidite channel is a long incised turbidite channel. It is presently active. It has been built gradually by progradation of the distal depositional area. The most distal part of the channel is the youngest part and shows an immature morphology: the channel presents a low incision and a low sinuosity. In contrast, the upper part of the channel has undergone a long evolutionary history. Its pathway is mature and complex, with numerous abandoned meanders visible in the morphology. This paper presents evidence of progressive channel migration and meander development of the Congo channel. It describes and explains the presence of terraces inside the channel. The detailed characterization of channel morphology and migration geometry shows that the evolution of the channel path is very similar to fluvial meandering systems with (1) lateral meander extension or growing, (2) downstream translation of the thalweg, and (3) meander cutoff. Seismic and 3.5 kHz echosounder profiles show that the terraces, which are visible in the seafloor morphology, are not the imprints of incisional processes. Terraces are true depositional units infilling the channel. They are built during and after the lateral migration of the channel. They are composed of (1) point-bar deposits and (2) inner-levee deposits aggrading above the point bar deposits. Point-bar deposits are characterized by low-angle oblique reflectors forming deposits with a sigmoidal shape. They seem very similar to those observed in fluvial systems. The similarity between fluvial and turbidite point bars suggests that the basal part of the turbidity currents flowing in this channel can be considered as very similar to river flow. With the high-resolution dataset collected in a present Congo turbidite channel, we provide a new description of the channel morphology and evolution, at a ‘‘reservoir’’ scale, intermediate between outcrop observations and 2D and 3D seismic data. The detailed interpretation of intrachannel sedimentation, associated with lateral channel migration, also provides new data for interpretation of flow dynamics in submarine meandering channels. INTRODUCTION Rhoˆne Fan (Droz and Bellaiche 1985; Torres et al. 1997). Submarine meandering channels present a particular interest to oil companies, since Meandering deep-water channels have been observed at the surfaces of they are significant hydrocarbon reservoirs in deep offshore environ- most of large deep-sea fans in the world: the Amazon Fan (Damuth et al. ments. The understanding of their geometries and their internal 1983; Flood and Damuth 1987; Pirmez and Flood 1995), the Mississippi sedimentary architecture is necessary to better characterize reservoir Fan (Kastens and Shor 1986; Pickering et al. 1986), the Bengal Fan heterogeneity. Therefore, numerous works have recently been carried out (Hu¨bscher et al. 1997; Schwenk et al. 2003), the Indus Fan (Kenyon et al. by both industrial and academic researchers to understand the behavior 1995; Kolla and Coumes 1987), the Nile Fan (Loncke et al. 2002), and the and sedimentary architecture of sinuous deep-water channels, including different approaches: study of modern deep-water channels, outcrop * Bruno Savoye, the second author, died in august 2008. This article is analysis (Arnott 2007; Labourdette et al. 2008), 3D seismic studies (Abreu dedicated to his memory. et al. 2003; Deptuck et al. 2007; Kolla 2007), and numerical and Copyright E 2010, SEPM (Society for Sedimentary Geology) 1527-1404/10/080-852/$03.00 JSR DETAILED SEDIMENTARY ARCHITECTURE OF THE MEANDERS OF THE CONGO TURBIDITE CHANNEL 853 FIG. 1.—Shaded bathymetry on the Congo deep-sea fan (EM12 multibeam data), showing the network of meandering submarine channels diverging from the canyon. The present turbidite system is underlined: the present channel in black, levees and lobe in gray. experimental modeling (Corney et al. 2006; Das et al. 2004; Imran et al. Submarine meandering channels are often compared to fluvial channels 2007; Kassem and Imran 2004; Keevil et al. 2006; Peakall et al. 2000; for their similar planform shape. However, deep-sea channels are known Straub et al. 2008). to exhibit several differences with rivers in terms of channel behavior as The aim of this paper is to present our results concerning the migration, aggradation, incision, or levee development (Abreu et al. 2003; development of the meandering channel of the present Congo turbidite Kolla et al. 2007). Based on high-resolution data, the characterization of channel. Based on high-resolution data acquired on the presently active the Congo turbidite channel provides new information for comparison Congo channel, we describe the detailed sedimentary architecture linked with fluvial channels, existence of lateral-accretion deposits (point-bar to meander development and intrachannel deposition, and analyze the deposits), and interpretation of flow processes near the channel bottom of possible channel behavior and flow processes. Finally, we propose a turbidite channels. model explaining the evolution of channel morphology through time, focused on meander development. GEOLOGICAL SETTING AND PREVIOUS WORK Our work results from the study of the Congo (or Zaire) modern deep- The Congo–Angolan Margin and the Congo Deep-Sea Fan sea fan, conducted within the ZAIANGO project, in collaboration between IFREMER and TOTAL. The study of meandering morphology The Congo Deep-Sea Fan is located on the Congo–Angolan margin, of the present Congo turbidite channel was performed using a complete which is a mature, passive continental margin resulting from the Early set of high-resolution data: bathymetry, backscatter sonar image, Cretaceous opening of the South Atlantic Ocean (130 Ma) (Jansen et al. echosounder profiler, and gravity piston cores (Babonneau et al. 2002). 1985; Marton et al. 2000). The depositional limits of the fan extend from The Congo channel is presently active. The meandering and incised the base of the continental slope, at the outlet of the Congo Canyon, to channel morphology is dynamic and presently continues to evolve. the abyssal plain. The submarine Congo Canyon marks the limit between Babonneau et al. (2002) described the channel morphology and the the Congo and the Angola margins. It is strongly incised across the shelf general seismic structure of the turbidite system from the head of the and the continental slope (Ferry et al. 2004). The Congo Canyon is canyon down to the channel-mouth lobe. The work presented in this directly connected to the Congo River mouth (Fig. 1). The gravity paper focuses on the detailed architecture of several meanders with high- currents generated in the canyon head transfer most of river sediment resolution tools to address the sedimentary structures linked to meander discharge towards the deep-sea fan at more than 3000 m water depth migration and intrachannel deposits. The high-resolution dataset includes (Babonneau et al. 2002; Savoye et al. 2009). high-resolution bathymetric data (EM300), side-scan sonar, seismic and The deep-sea fan displays a complex paleochannel network extending echosounder data, and piston cores. It provides a new description of the over 300,000 km2 (Savoye et al. 2000). Most of the paleochannels still visible evolution of channel morphology, at a scale intermediate between at the present basin seafloor are Quaternary in age (Droz et al. 2003). The outcrop observations and seismic data. present turbidity currents arriving from the Congo Canyon flow along a 854 N. BABONNEAU ET AL. JSR FIG. 2.—A) Longitudinal depth profile of the Congo canyon and channel. B) Sinuosity of the present Congo thalweg. C) Simplified map of the present Congo turbidite system from the canyon to the distal lobe area. unique active channel located in the central part of the fan (called the Axial channel–levee system corresponding to the upper-fan valley, (3) the Fan by Droz et al. 2003)
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