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Turbidite System Architecture and Sedimentary Processes Along Topographically Complex Slopes: the Makran Convergent Margin

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Turbidite System Architecture and Sedimentary Processes Along Topographically Complex Slopes: the Makran Convergent Margin Sedimentology (2010) doi: 10.1111/j.1365-3091.2010.01168.x Turbidite system architecture and sedimentary processes along topographically complex slopes: the Makran convergent margin JULIEN BOURGET*,1, SEBASTIEN ZARAGOSI*, NADINE ELLOUZ-ZIMMERMANN , NICOLAS MOUCHOTà, THIERRY GARLAN§, JEAN-LUC SCHNEIDER*, VALENTINE LANFUMEY– and SIGFRIED LALLEMANTà *Universite´ de Bordeaux, UMR CNRS 5805 EPOC, Avenue des Faculte´s, F-33405 Talence, France (E-mail: [email protected]) IFP, Geology-Geochemistry-Geophysics, 1 & 4 avenue de Bois Pre´au, F-92852 Rueil-Malmaison, France àUniversite´ de Cergy-Pontoise, GEC, 5, mail Gay-Lussac, F-95031 Cergy-Pontoise, France §SHOM, Centre Hydrographie, BP 426, F-29275 Brest, France –IFREMER, GM/LES, BP70, F-29280 Plouzane´ Cedex, France Associate Editor – Subhasish Dey ABSTRACT This study investigates the morphology and Late Quaternary sediment distribution of the Makran turbidite system (Makran subduction zone, north- west Indian Ocean) from a nearly complete subsurface mapping of the Oman basin, two-dimensional seismic and a large set of coring data in order to characterize turbidite system architecture across an active (fold and thrust belt) margin. The Makran turbidite system is composed of a dense network of canyons, which cut into high relief accreted ridges and intra-slope piggyback basins, forming at some locations connected and variably tortuous paths down complex slopes. Turbidite activity and trench filling rates are high even during the Holocene sea-level highstand conditions. In particular, basin-wide, sheet- like thick mud turbidites, probably related to major mass wasting events of low recurrence time, drape the flat and unchannellized Oman abyssal plain. Longitudinal depth profiles show that the Makran canyons are highly disrupted by numerous thrust-related large-scale knickpoints (with gradients up to 20° and walls up to 500 m high). At the deformation front, the strong break of slope can lead to the formation of canyon-mouth ‘plunge pools’ of variable shapes and sizes. The plunge pools observed in the western Makran are considerably larger than those previously described in sub-surface successions; the first insights into their internal architecture and sedimentary processes are presented here. Large plunge pools in the western Makran are associated with large scoured areas at the slope break and enhanced sediment deposition downstream: high-amplitude reflectors are observed inside the plunge pools, while their flanks are composed of thin- bedded, fine-grained turbidites deposited by the uppermost part of the turbidity flows. Thus, these architectural elements are associated with strong sediment segregation leading to specific trench-fill mechanisms, as only the finer-grained component of the flows is transferred to the abyssal plain. However, the Makran accretionary prism is characterized by strong along- strike variability in tectonics and fluvial input distribution that might directly influence the turbidite system architecture (i.e. canyon entrenchment, plunge pool formation or channel development at canyon mouths), the sedimentary 1Present address: Australian School of Petroleum, Reservoir Analogue Research Group, University of Adelaide, SA 5005, Australia. Ó 2010 The Authors. Journal compilation Ó 2010 International Association of Sedimentologists 1 2 J. Bourget et al. dynamics and the resulting sediment distribution. Channel formation in the abyssal plain and trench-fill characteristics depend on the theoretical ‘equilibrium’ conditions of the feeder system, which is related closely to the balance between erosion rates and tectonic regime. Thus, the Makran turbidite system constitutes an excellent modern analogue for deep-water sedimentary systems with structurally complex depocentres, in convergent margin settings. Keywords Active margin, gravity flow, Gulf of Oman, hydraulic jump, plunge pool, slope break deposits, submarine slope, turbidite system. INTRODUCTION canyons (e.g. Underwood & Karig, 1980; Huyghe et al., 2004). In addition, due to the lack of data in Early conceptual models of turbidity current modern analogues of tectonically controlled sys- deposition and submarine turbidite system devel- tems, it remains unclear whether ‘classical’ archi- opment essentially envisaged the development of tectural elements of turbidite systems, as defined channel-leve´e systems associated with uncon- by Payton (1977) or Mutti & Normark (1991), fined, fan-shaped distal sediment bodies. How- among others, are observed or not, and how their ever, a wealth of case studies from several recent geometries can differ from passive-margin fan and ancient turbidite systems worldwide makes it architectural elements. As an example, the impor- clear that sediment distribution and geometries of tance of tectonically induced break of slopes has depositional bodies vary significantly within sys- been evidenced by the occurrence of abrupt tems located on topographically complex slopes canyon terminations in abyssal plains (for exam- (e.g. Kneller & McCaffrey, 1999; Prather, 2000; ple, submarine ‘plunge pools’ of Farre & Ryan, Sinclair & Tomasso, 2002; Prather, 2003; Lomas & 1985; Lee et al., 2002). Such atypical architec- Joseph, 2004; Smith, 2004). Reservoir models tural elements are poorly documented, and the derived from Neogene Gulf of Mexico salt- sedimentary processes related to them are still withdrawal basins (e.g. Prather et al., 1998; unknown. In accretionary prisms, strong breaks of Beaubouef & Friedman, 2000) are usually cited slope are observed classically at the deformation in studies of structurally complex slope depocen- front, corresponding to the continental slope/ tres. However, there is significant variation in the abyssal plain transition. However, how does characteristics of tectonic deformation and sub- uplift of accretionary ridges at the deformation marine slope topography between Gulf of Mexico front impact the sedimentary processes and the intra-slope basins and thrust-belt controlled geometry of sedimentary bodies? Does it control slopes as developed in accretionary prism set- the mechanisms of sediment transfer and trans- tings (e.g. Covault et al., 2009), that may lead to port in the trenches? both different sediment distribution and turbidite These questions have been investigated in the system architecture. Smith (2004) proposed that Oman basin, for which the present data cover sediment distribution systems on topographically 166 700 km2. This basin offers an excellent complex slopes can consist of cascades of silled opportunity to investigate the sediment dispersal basin (or disconnected, intraslope basins, such as patterns and their architectural elements in the the Gulf of Mexico slope with salt-withdrawal- context of convergent margins. The Makran created mini basins) and a connected tortuous accretionary prism exhibits: (i) high convergence ) corridor [defined as a laterally confined depres- rates of about 2Æ5 to 4 cm year 1 (Ellouz-Zimmer- sion (or a canyon) evolving on a topographically mann et al., 2007a) resulting in highly deformed complex slope, that can connect several intra- sea floor, composed of intra-slope (piggyback) slope basins]. Fold and thrust-belt controlled basins and accreted ridges; and (ii) significant submarine slopes of accretionary prisms, which continental erosion by numerous ephemeral usually show several partially silled basins with streams located in an arid environment controlled lateral escape paths, lie within the latter one. by the Asian monsoon climate, that supports high However, continental slope morphology and rates of sediment transfer to the deep sea (von Rad downslope sediment transport along accretionary et al., 1999a). The large data set available on this prisms are often complex, and primarily depend system (multibeam echo sounder data, sedimen- on the balance between deformation rates (uplift tary cores and seismic lines acquired during the of slope ridges) and erosion rates by submarine MARABIE and CHAMAK surveys) has been Ó 2010 The Authors. Journal compilation Ó 2010 International Association of Sedimentologists, Sedimentology The Makran turbidite system 3 analysed in order to address: (i) the overall (Kukowski et al., 2001; Ellouz-Zimmermann et morphology of the present-day turbidite system al., 2007a; Grando & McClay, 2007). The western and its architecture; (ii) the sediment dispersal part of the Arabian Sea lithosphere belongs to the pattern and the role of pre-existing sea floor Arabian plate, whereas the eastern part belongs to topography on the gravity current deposition at the Indian plate; their present-day plate boundary the deformation front (i.e. continental slope to runs along the dextral transform Owen Fracture abyssal plain transition) and downstream; and Zone (Gordon & Demets, 1989; Fournier et al., (iii) the along-strike relationships between the 2001). The plate boundary shows a nearly east– margin-scale tectonic regime, the fluvial input west bend along the Murray Ridge before inter- and the architecture of the Makran turbidite secting the subduction boundary west of Karachi system at both the system and architectural City (Ellouz-Zimmermann et al., 2007b; Fig. 1). element scales. The actual plate convergence rate has been esti- ) mated at 4 cm year 1 using classical plate mod- elling (DeMets et al., 1994), whereas recent space REGIONAL SETTING geodesy modelling and a GPS-based model esti- mated the present-day velocities between ) 2Æ5 cm year 1 in the western Makran area and Geodynamic setting ) 3 cm year 1 in
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