Recent Sedimentary Processes Along the Makran Trench (Makran Active Margin, Off Pakistan)

Marine Geology 271 (2010) 17–31

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Marine Geology

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Recent sedimentary processes along the Makran trench (Makran active margin, off Pakistan)

Nicolas Mouchot a,⁎, Lies Loncke b, Geoffroy Mahieux c, Julien Bourget d, Siegfried Lallemant a, Nadine Ellouz-Zimmermann e, Pascale Leturmy a a Université de Cergy-Pontoise, GEC Geosciences Environnement Cergy, 5 mail Gay Lussac, 95031 Cergy Cedex, France b Université de Perpignan, Laboratoire IMAGES, 52 av Paul Alduy, 66860 Perpignan, France c Université de Picardie Jules Verne, FRE 3298 Geosystemes, 80000 Amiens, France d Université de Bordeaux, UMR 5805 EPOC, 33000 Bordeaux, France e Institut Français du Pétrole, 1 & 2 av Bois Préau, 92500 Rueil-Malmaison, France article info abstract

Article history: A geophysical and geological survey (CHAMAK) has been carried out on the Makran accretionary wedge off Received 14 November 2008 Pakistan in order to understand the structure of the margin and the recent sedimentary processes in this self- Received in revised form 18 January 2010 maintaining prism disconnected from the modern Indus inputs (Qayyum et al., 1997; Gaedicke et al., 2002a; Accepted 21 January 2010 Schluter et al., 2002). Available online 29 January 2010 Morphostructural analysis, based on the interpretation of bathymetric data and backscatter imagery, as well Communicated by D.J.W. Piper as a 3.5 kHz echo-character mapping, allow us to distinguish three structural domains, from north to south, where sedimentary processes differ: (1) the accretionary wedge to the north, (2) the trench and (3) the Keywords: northern Murray Ridge at the seaward edge of the trench. The accretionary wedge is cut by canyons Makran margin responsible for an important erosion of the prism especially in the eastern part of the wedge. Within the morphostructure trench, sediments transported by the canyons generate sediment waves and are transported westward, echo-character mapping parallel to the E–W axis of the trench. The eastern part of the abyssal plain is eroded by strong turbidity sediment dispersal pattern currents whereas important sediment deposition occurs in the western part of the abyssal plain, as a sediment waves consequence of a decrease in the current energy. Nearly no mass transport deposits are recognized in the erosional pools study area except near the ridges forming the accretionary wedge. Small-scale slope failure scars are scours described. The prevalence of turbiditic processes and the existence of a morphological barrier formed by the Murray Ridge allow the conﬁnement of turbidites within the trench. Migrating sediment waves seem to be common sedimentary structures in this setting. These features might be produced by important velocity decrease of turbidity currents when reaching the trench. © 2010 Elsevier B.V. All rights reserved.

1. Introduction drained by small seasonal coastal rivers in arid and semi-arid environments. Detrital sediments related to sub-marine and conti- The study of sedimentary processes along active margins has two nental erosion of the wedge ﬂow through large structurally controlled main goals: i) to understand the effect of active tectonics on sediment canyons and reach the trench. The eastern part of the prism has been mobilization and in particular in the triggering of slope instabilities; recently surveyed by different groups as reported in Kukowski et al. this is very important in assessing coastal risks associated with such (2001) and Ellouz-Zimmermann et al. (2007a,b). The main deforma- settings, ii) to know the sediment content and architecture of these tional style and the morphology of the prism have been described. The systems. Accretionary prisms are indeed recognized as important trench of the Makran margin is entirely ﬁlled by sediments (Schluter petroleum provinces where various combinations of active tectonic et al., 2002; Ellouz-Zimmermann et al., 2007b) resulting in gentle and sedimentary processes (turbiditic, hemipelagic and mass wast- slopes. One peculiarity of the recent Makran accretionary prism is its ing) create a wide variety of hydrocarbon-trapping structures. disconnection since Early Miocene from the Himalayan inputs The Makran convergent margin is a wide accretionary wedge (Qayyum et al., 1997; Schluter et al., 2002). As a consequence, the located in southeastern Iran and southwestern Pakistan (Fig. 1) Makran prism has been “self-maintained” since that time, essentially built by off-scraping sediments eroded from the outcropping older parts of the accretionary wedge and arid surrounding lands (Prins et al., 2000). ⁎ Corresponding author. Tel.: +33 1 34 25 73 64; fax: +33 1 34 25 73 50. In this paper, we present an analysis of CHAMAK surface data along E-mail address: [email protected] (N. Mouchot). the very eastern Pakistani Makran margin (Fig. 2a and b). The aim of

Fig. 1. Regional geological sketch map of the Makran convergent margin off Pakistan. The borders between Eurasian, Arabian and Indian plates correspond to the Murray Ridge fracture, the Makran subduction and the Ornach–Nal fault (ONF). The CHAMAK survey is outlined by a white rectangle. The Murray Ridge, located south the surveyed area, delimits the Oman basin and the Indus basin respectively. this study is to complement the knowledge on sediment transfers and the Indus basin to the south filled by the modern Indus deep-sea fan processes occurring in this active margin, knowing that similar and the Oman basin to the north essentially filled by material eroded sedimentary systems may have participated to the building of the from the Makran accretionary wedge and arid surrounding lands frontal wedge since the Miocene. (Prins et al., 2000). Bathymetry and backscatter imagery are data commonly used for During the Plio-Pleistocene, the turbidite sedimentation in the the analysis of sediment processes along active margins. Echo- Makran and Indus Fan systems appears to be controlled by sea-level characters studies have been widely used in order to determine and climate (Prins and Postma, 2000) and it was more active during sediment processes in various deep-sea environments, frequently in the last glacial sea-level lowstand (Prins et al., 2000). Turbidite passive margin context (Jacobi, 1976; Embley and Langseth, 1977; activity and trench filling rates are high even during the Holocene sea- Damuth, 1980a; Damuth and Flood, 1985; Pratson and Laine, 1989; level highstand conditions (von Rad and Tahir, 1997) in both systems Damuth, 1994; Gaullier and Bellaiche, 1998; Loncke et al., 2002), but in spite of different tectonic settings. Most of Himalaya-derived more rarely in convergent margin contexts (Henry et al., 1990; sediments are trapped in the Indus fan (Prins and Postma, 2000) and Whitmore et al., 1999; Chow et al., 2001; Chiu and Liu, 2008). The only the sediments derived from rivers draining the Makran margin more rugged seafloor and the higher slope values generally observed were directly connected to the Makran canyons are involved in the in such environment may disturb acoustic acquisition and explain the turbidite system growth (Kukowski et al., 2001). Overall higher lack of interest in using echo-character mapping in convergent margin turbidite frequencies are observed in the proximity of the deforma- context. In this study, echo-character mapping has been carried out tion front of the accretionary prism (Prins et al., 2000). mainly in the trench and along the widest intra-slope basins The morphology of the Makran accretionary prism has been completing surface data analysis. previously studied (Kukowski et al., 2001; Ellouz-Zimmermann et al., 2007b) and can be divided into three domains (Fig. 3): the 2. Location and geological setting accretionary wedge, the trench and the Murray Ridge system. Ellouz-Zimmermann et al. (2007b) reported a significant change in The Makran accretionary wedge extends over 1000 km in the wedge morphology east of Pasni (63.5°E). It results in a dramatic southern Iran and Pakistan. South of the Makran margin, the decrease in size, length of thrust sheets and in distance between each northeast-southwest trending Murray Ridge system is the transten- thrust. Moreover ridges seem to be more sinuous and prominent sional boundary between the Indian and Arabian plates (Quittmeyer compared to the area investigated further west by the MAMUT survey and Kafka, 1984; Gordon and Demets, 1989; Edwards et al., 2000; (Kukowski et al., 2001). Erosion has been depicted as a major process Gaedicke et al., 2002a,b)(Fig. 1). occurring on the wedge, expressed by numerous circular or linear The build up of the accretionary wedge during the Paleocene was slump scars cutting the ridges and by large canyons cutting the wedge enhanced by a direct input of Himalayan detrital sediments to the (Ellouz-Zimmermann et al., 2007b). Makran margin through the paleo-Indus deep-sea fan system (Garzanti et al., 1996; Qayyum et al., 1997). A major uplift of the 3. Data set and methods Murray Ridge system during the Early Miocene was followed by an additional uplift in the Pliocene (Gaedicke et al., 2002a) shifting the The southeastern Makran accretionary wedge was investigated influx of Indus River to the south (Qayyum et al., 1997). The during the CHAMAK survey. This survey was carried out aboard the development of this bathymetric high has probably acted like a dam French R/V Marion Dufresne during fall 2004, and allowed us to prohibiting direct Indus sediment supply to the active margin (Clift et investigate the physiography of the northeastern Arabian Sea al., 2001, 2002; Schluter et al., 2002). In any case, the uplift of the (including the eastern Makran accretionary wedge, the trench and a Murray Ridge has divided the Arabian Sea in two sedimentary basins: part of the Murray Ridge system) using a multibeam Thomson “sea

Fig. 2. Data set acquired during CHAMAK survey. a) Shaded and contoured bathymetry. Labeled cores are indicated by diamonds. The black rectangles indicate bathymetric details presented in Figs. 4 and 5 and the white lines locate 3.5 kHz proﬁles or multichannel seismic section presented respectively in Figs. 8 and 10. The sinuous white lines on the accretionary wedge correspond to the longitudinal canyon path proﬁles plotted in the Fig. 4 and used to calculate the sinuosity (Table 3). In the bottom right corner, the box indicates tracklines (light grey lines) and 3.5 kHz data available (black lines) for this study. b) Raw backscatter imagery covering the studied area. N. Mouchot et al. / Marine Geology 271 (2010) 17–31 19 20 N. Mouchot et al. / Marine Geology 271 (2010) 17–31

Fig. 3. Morphostructural map based on the analysis of the bathymetry of CHAMAK survey. The legend, related to the morphostructural features presented on the three sedimentary domains, is put down in the grey box.

Falcon” TSM 5265B multibeam sounder (Fig. 2a). The cruise, which (2) Classification and interpretation of 3.5 kHz echo-character on was planned for a 100% coverage of the study area, recorded also the basis of (i) acoustic penetration and continuity of bottom backscatter images of the seafloor, where variations of the acoustic and sub-bottom reflectors, (ii) microtopography of the sea- reflectivity relate to several parameters such as slope angles and floor, and (iii) internal structures. roughness of the seafloor. The latter being related to lithological and physical characters of the sediment (Fig. 2b). Multibeam data are presented as grids with a 100 m spacing, but were locally reprocessed allowing a 50 m spacing for selected bathymetric details. Simulta- Table 1 Sinuosity index (SI) calculated for main canyon systems. The SI marked by an asterisk is fi neously, near-surface sediments were imaged using a 3.5 kHz pro ler issued from Kukowski et al. (2001). of 50 to 100 m sub-bottom penetration. Deeper structures were also imaged by 6-channel seismic data (Fig. 2a). Finally, 10 piston cores Canyon system Main river SI (up to 30 m recovery) and 17 gravity cores (5 to 10 m recovery) were Save Save 1.4⁎ ⁎ collected, allowing sedimentary calibration of 3.5 kHz data (Fig. 2a). Save 1.8 Shadi Shadi 1.35⁎ In this study, we carried out the analysis of the sub-surface data Basol 1.27 following three steps: Basol 1.22 1 Basol? 1.14 (1) Morphostructural analysis of bathymetry and acoustic imagery 2 Rach 1.15 in order to point out the main bathymetric directions, 3 Hingol 1.19 4 Hingol 1.26 escarpments and sedimentary features;

Fig. 4. Morphological analysis of the canyons (see Fig. 2 for location). Local bathymetric details targeted on canyon outlets are presented with longitudinal and transversal canyon path profiles. P1 and P2 correspond of the transversal canyon path profiles, for each canyon, and are located on the corresponding bathymetric detail. Below the longitudinal profiles, the black arrows indicate the position of distinguishable thrust faults crossing the canyon path in its downstream section. DF: deformation front; EP: erosion pool; Sw/S: sediment waves/scours; Sw field: sediment waves field and Kp: knickpoint. a) Detail on the deep gorges with steep flanks characterizing the mouth of canyon 2. b) Detail on the mouth of canyons 3 and 4. See the E–W sediment waves at the canyon outlet turning N–S displaying the turbidity current direction flowing from these canyons. c) Detail on canyon 1. Notice the numerous circular failures scarps affecting the abandoned/new ? canyon pathway. d) Detail on the mouth of Save and Shadi canyons. Note the presence of large erosion pools and sediment wave field. N. Mouchot et al. / Marine Geology 271 (2010) 17–31 21 22 N. Mouchot et al. / Marine Geology 271 (2010) 17–31

Fig. 5. Local bathymetric detail on normal faults (white lines) cutting abandoned meandering bathymetric structure (see location in Fig. 2a).

(3) Mapping of the defined 3.5 kHz echo-character and sedimen- Save and Shadi canyons. They have already been partly described by tary interpretation on the basis of literature and core data. Kukowski et al. (2001). The other canyons have been termed canyons 1 to 4 from west to east. The canyons may be either connected to a 4. Results single river onshore or to several rivers onshore due to the dendritic pattern of the upstream canyon systems. However, we can suggest a 4.1. Bathymetry list of potential connections between offshore and onshore hydro- graphic systems (Table 1). 4.1.1. Accretionary wedge domain The numerous upslope small canyons and gullies merge after a An important drainage system, characterized by 6 major canyon short distance into larger canyons going downslope. They have systems, cuts the wedge (Fig. 3). From west to east, we have first the meandering morphologies because they flow along-strike until they

Fig. 6. Filtered reflectivity map enhancing the distinction of high (dark grey), average and low (light grey) reflective areas on the three bathymetric domains. Detailed features observed on raw backscatter imagery (Fig. 2b) are plotted in the figure. The black lines correspond to bathymetric ridges (Fig. 3) and the dashed line delimits a lobe-shaped body distinguished at the outlet of canyon 1. N. Mouchot et al. / Marine Geology 271 (2010) 17–31 23

find a local bathymetric minimum in accretionary ridges that they are ridges, as already noticed by Kukowski et al. (2001) and Ellouz- able to erode. Generally the canyon paths easily bypass tectonic ridges Zimmermann et al. (2007b). These scars are localized and generally except in the Save and Shadi canyon systems (Kukowski et al., 2001). do not exceed 2 km in length (Fig. 4c). In Fig. 3, a long section of the Shadi canyon flows along-strike for at least 40 km. The sinuosity (the quotient of channel length and channel 4.1.2. Trench domain reach length) calculated for canyons 1 to 4 is between 1.14 and 1.26, As it is entirely filled with sediments originating from the erosion smaller than sinuosity calculated by Kukowski et al. (2001) for the of the Makran fold-and-thrust belt, both onshore and offshore, the Save and Shadi canyons (Table 1). In the Shadi canyon system, the seafloor of the trench is a flat area, morphologically undistinguishable bathymetric swath is not complete between the left and the right from the Oman abyssal plain (Ellouz-Zimmermann et al., 2007b). This branches connected onshore with Basol rivers, so we used published domain is divided in two parts by the Little Murray Ridge (LMR): a data (Kukowski et al., 2001) to evaluate the whole canyon path and western sector with the Save and Shadi canyons and an eastern sector calculate a sinuosity between 1.22 and 1.27 (Table 1). with the other canyons (Fig. 3). Longitudinal profiles have been computed in the most embanked In the eastern sector, canyon system 1 does not reach the trench but branches of each canyon system (Figs. 2a and 4a, b and c). For most of instead flows into a large piggy-back basin developed backward to the the canyons, the profiles are characterized by knickpoints that shift frontal accretionary ridge. The entrenchment in the frontal accretion- their pathways vertically. They result from the interaction between ary structures by the canyon systems 2, 3 and 4 extends for a short relief produced along the active thrust fault and retrogressive erosion distance in the trench seafloor (Fig. 4a and b). Seaward of the mouth of processes occurring in the canyon path in order to reach equilibrium canyons 3 and 4, the entrenchment is expressed as an erosive NW–SE (Huyghe et al., 2004; Mitchell, 2006). Although canyons may have corridor which turns abruptly to W–SW some 8 km from the canyon small local disturbance in their long profiles, the major knickpoints outlets (Fig. 4b). Numerous NE–SW scours or ridges, perpendicular to always occur in the downstream section in the frontal part of the this pathway have been observed there. Similar scours are also wedge. A series of transversal profiles, downstream from these major observed off canyon 2 prolonging this system and defining a more than knickpoints, reveals “V” shape gorges suggesting that active incision 80 km long sedimentary entrenchment. Erosion pools are also present occurs in this section of the pathways. at the mouth of canyons, as small circular basins less than 2 km wide Most canyon mouths are furthermore characterized by erosion except in canyon 3 where it reaches 3 km wide. At the canyons 2 and 4 pools (Fig. 4a, b, c and d). Numerous circular failure scars affect thrust outlets, two successive erosion pools are well identified (Fig. 4b).

Table 2 Echo-character analysis.

Class Echo type Occurrence Interpretation

I. Distinct Ia. Distinct sharp, continuous Mainly on the northern Hemipelagic deposits (Gaullier and Bellaiche, 1998), bottom echo, with sharp Murray Ridge, on the detrital sediments deposited by turbidity currents, parallel sub-bottom reﬂectors western part of the with alternating sandy and silty beds (Damuth, trench and locally in 1980a; Pratson and Laine, 1989) some piggy-back basins

Ib. Distinct sharp continuous Eastern part of the Coarse-grained deposits or erosional sedimentary bottom echo with indistinct trench processes (Damuth, 1975; Damuth and Hayes, 1977) sub-bottom reﬂectors

II. Indistinct IIa. Indistinct wavy bottom In the trench in front of Sediment waves generated by deep currents echo with discontinuous the mouth of canyon 3 (Bouma and Treadwell, 1975; Jacobi et al., prograding sub-bottom 1975; Damuth, 1979, 1980b) or to creeping reﬂectors deposits

IIb. Indistinct sharp undulated Restricted to the vicinity Fluid expulsion (Loncke et al., 2002) bottom echo with intermittent of mud volcanoes indistinct continuous reﬂectors

III. Hyperbolae IIIa. Hyperbolic echoes with Southwestern part of Basement highs or outcrops discontinuous parallel the northern Murray (Damuth, 1980a; Laine et al., 1986) sub-bottom reﬂectors Ridge

IIIb. Hyperbolic echoes with Toe of the thrusts Mass wasting processes (Damuth, 1980a,b, 1994) indistinct sub-bottom reﬂectors related ridges 24 N. Mouchot et al. / Marine Geology 271 (2010) 17–31

Fig. 7. Echo-character mapping on the study area. The diamonds localize the sediment cores used to calibrate the 3.5 kHz data and colors correspond to the predominant facies (see Fig. 9). Legend for the structural features in Fig. 3.

In the western sector, the Save and Shadi canyon outlets to the Each of the three structural provinces described previously has a trench seafloor do not display a similar configuration but instead an heterogeneous acoustic distribution pattern except along the Murray over-incision of the seafloor resulting in a closed depression Ridge flank. elongated in the flow direction at the canyon outlets (Fig. 4d), The accretionary wedge is mainly characterized by low reflectivity described in more detail as plunge pools by Bourget et al. (submitted). while canyon paths are highlighted by average to high reflectivities. The bathymetry is characterized at the mouth of these canyons by The average and high reflectivities of the Shadi canyon are present numerous sedimentary ridges, roughly perpendicular to the channel along the sections respectively parallel and orthogonal to the axis (Fig. 4d). structural trend, respectively. The Save canyon and canyon 3 exhibit discontinuous high relectivity along their paths while canyon 2 has 4.1.3. Northern Murray Ridge continuous high reflectivity. The upstream section of the canyon 4 This southern domain presents an arched surface and is the place exhibits average reflectivity while its downstream section has high of numerous meandering bathymetric structures 0.5 to 1.5 km wide reflectivity. The outlet section of the canyon 1, disconnected from the (Fig. 5). Numerous faults, some of which bound 2 km wide grabens, trench, exhibits a low reflectivity lobe-shaped body deposited in a are oriented WSW–ENE in the southwest part of this province and large piggy-back basin (Figs. 3 and 6). WNW–ESE in the northeast part (Fig. 3). The northern border of this In the trench domain, the whole seafloor is represented by an arched province is relatively sharp, even affected by gullies or slumps. average reflectivity. The high linear reflectivity patterns are artifacts By contrast, its southern border has much steeper slope, probably linked to the ship track and cannot be used to describe the sea- related to extensional processes associated with emplacement of the bottom roughness or sediment grain-size. All canyon outlets to the Murray Ridge and suffering widespread gravity gliding (collapse of trench have a rather high reflectivity. The outlet of Shadi canyon has the sediments towards the Murray depression). ahighreflective elliptic-shape surrounding average reflectivity while canyons 2, 3 and 4 are defined by high reflectivity patches. 4.2. Reflectivity The large relief of the Little Murray Ridge is entirely characterized by low reflectivity values contrasting with surrounding average Three main ranges of backscatter intensity have been filtered from reflectivity of the trench. This difference may be due to more active the raw reflectivity data: low, average and high reflectivities sedimentary transits in the trench. Two areas of the trench seafloor respectively, represented by clear, average and dark grey (Fig. 6). exhibit contrasting reflectivities defining lineaments. The first area, N. Mouchot et al. / Marine Geology 271 (2010) 17–31 25 mainly developed in front of the Save and Shadi canyons, IIb have also been observed (southeast of canyon 2). They are located corresponds to arcuate-shaped lineaments concave towards canyon in the vicinity of normal faults and seem related to dome-like outlet to the trench. East of Save canyon, lineaments are less structures, probably corresponding to mud volcanoes emplaced near developed and straighter with E–WtoNW–SE trend. These normal faults. lineaments correspond to bathymetric ridges. In the second area off canyon 2, we can distinguish 3 distinct sets of lineaments also 4.4. Sedimentology visible on bathymetry (scours described in previous sections). The first set corresponds to very small and arcuate E–W lineaments 27 sediment cores allow calibration of 3.5 kHz data. The detailed turning around the west border of the high reflective area at the description of facies association, clay mineralogy, grain-size and major outlet of canyon 2 to the trench. The second set is a well developed element geochemistry is fully presented in parent papers (Bourget et field of NW–SE trend lineaments, located southwest of canyon 2. al. submitted; Mouchot et al., submitted). In the Makran accretionary The last set exhibits few lineaments with NW–SE and NE–SW complex sediment cores, we identify two major types of sedimentary preferential orientation southeastward canyon outlet. environments (Fig. 9a and b). Seven typical cores are presented in The northern Murray Ridge is homogeneously characterized by Fig. 9a and located in Table 3. A summary of the composition of all low reflective intensity suggesting that no erosional processes available cores is given in Fig. 7. occurred recently. The fact that the meandering bathymetric The first sedimentary environment (Facies A) consists of fine- structure and the faults observed on bathymetry are not visible on grained, normally graded thin turbidite beds (FA-1a, Bourget et al., backscatter imagery suggests a widely distributed fine-grained submitted). The association Facies A forms typical cm-thick fine- sedimentation. grained turbidites corresponding to the Td to the Te terms of Bouma (1962). Thicker (dm-thick) fine sand turbidites beds (FA-2, Bourget et 4.3. 3.5 kHz echo-sounder data al., submitted) are only observed in the MD04-2849 core between 6 and 14.1 mbsf (Fig. 9a and b). These deposits (Facies A) correspond On the basis of reflection characters (e.g. clarity, continuity, to deep-sea plain like turbidites in low density turbidity currents amplitude and geometry of bottom and sub-bottom echoes; Roksan- (unsteady turbulent flows), lower fan to proximal basin plain (Mattern, dic, 1978), six echo types grouped into three main classes have been 2005). distinguished on the 3.5 kHz profiles data set (Table 2). The second sedimentary environment (Facies B) is typically The extent of these six echo types has been mapped within the encountered at the top of the Makran accretionary complex sediment whole study area, except in the accretionary wedge where most echo cores. On the bathymetric highs (e.g., the Murray Ridge) and the types are hyperbolic due to artifacts related to high slope gradients trench, it mostly consists of grey olive to brown, laminated clays more (Fig. 7). or less carbonaceous, with abundant scattered fossils and sometimes wood (Fig. 9a and b). In core MD04-2858, located in an upper-slope 4.3.1. Accretionary wedge domain piggy-back basin (Figs. 2 and 9a), we observed alternation of a few The province is essentially characterized by hyperbolic echo-types mm-thick olive-grey laminated clays with abundant organic matter corresponding to slope artifacts. Given the slope gradients character- and biogenic calcareous fossils. The Facies B corresponds to hemi- izing this province, echo-character mapping is not an appropriate pelagic and/or pelagic (background) sedimentation. The laminated method to define recent sedimentary processes. However, hyperbolae facies in core MD04-2858 has been commonly described in the echo type IIIb is mainly confined to the toe of thrust faults in the east Makran continental slope and is interpreted as “varved-like” of the accretionary wedge. Distinct echo type Ia characterizes the sediments related to river-derived deposits related to flood events widest piggy-back basins that have probably caught an important part (Lückge et al., 2001; von Rad et al., 2002). of sediments (Figs. 7 and 8a). Three bent core barrels (Table 3) collected in the canyons 2 and 4 (Fig. 7) reveal that, locally, the seafloor is characterized by highly 4.3.2. Trench domain indurated clayish sediments, older than Quaternary. Four echo types are represented in the trench: echo type Ib The cores in the accretionary wedge are characterized by Facies A covering most of the plain, echo type IIa off canyon 2, and hyperbolae and B. The cores with a dominant turbiditic facies (Facies A) are echo type IIIb between canyons 2 and 3 (Fig. 7). A progressive located close to canyon paths while the cores with a hemipelagic evolution of echo-types and sedimentary structures is observed off dominant facies (Facies B) are generally located at the top of ridges or canyon 2 (Figs. 7 and 8b and c). Echo type IIa (bedded wavy in piggy-back basins far away from canyon paths (Fig. 7). Two cores sediments) evolves downslope to echo type Ib (rough sediments) and with no dominance of turbiditic or hemipelagic facies are located finally to echo type Ia (bedded sediments). Then, echo types become close to the path of the Shadi canyon. rough and then bedded. The eastern border of canyon 2 is Thecoresinthetrenchdomainaretakenclosetothe characterized by hyperbolic echo type IIIb associated with gullies deformation front except two cores taken atop the high LMR already identified on bathymetry. Finally, distinct echo-type Ia are (Fig. 2a). The cores are characterized by dominant hemipelagic mainly restricted (Fig. 7) to the western part of the central province, facies (Facies B) and dominant turbiditic facies (Facies A) except the either at the toe of the frontal thrust, when not cut by sedimentary cores atop of the high LMR which are only characterized by Facies B pathways, or southwest of the LMR. (Fig. 7). On northern Murray Ridge domain, whether cores are from a 4.3.3. Northern Murray Ridge meandering bathymetric structure or not, they are only characterized This province is mainly characterized by echo type Ia (Figs. 7 by hemipelagic facies (Facies B). and 8d), except along its northern border where echo type Ib is observed, and along its southern border where intense faulting and 4.5. Multichannel seismic data slope gradients generate hyperbolic echo type IIIa comparable to artifacts (Figs. 7 and 8e). The bedded echo type Ia recorded on the The multichannel seismic section line CHAMAK 11 (Fig. 2a) crosses province probably correspond to hemipelagic sedimentation slowly the trench west of Save canyon outlet in the Makran trench fill covering the flank of the northern Murray Ridge. The northern border sequence M3 (Fruehn et al., 1997; Gaedicke et al., 2002a). Seismic of this province seems to correspond to a transition zone toward the pattern is characterized by 0.7 s (two-way travel time; TWT) thick abyssal plain. Along the northern border, some transparent echo type of mostly continuous and wavy high amplitude reflectors packets 26 N. Mouchot et al. / Marine Geology 271 (2010) 17–31 N. Mouchot et al. / Marine Geology 271 (2010) 17–31 27

Fig. 9. Sediment cores. a) Lithological log description of seven typical abyssal plain and piggy-back basins sediment cores (see Fig. 2a for location). b) Pictures of facies A and B.

Fig. 8. Typical echo types in the three domains (see Fig. 2a for location) a) Distinct echo type Ia through a large piggy-back basin. b) SW–NE profile displaying transition between wavy echo type IIa to rough echo type Ib in the trench. c) WNW–ESE profile displaying transition from rough echo type Ib to bedded echo type Ia. d) SW–NE profile in the northern Murray Ridge area displaying a channel axis with levees covered by bedded echo type Ia. e) Channel axis covered by bedded echo type Ia and disturbed by normal fault (see Fig. 5). The southern end of the profile is defined by echo type IIIa related to slope artifacts on the northern Murray Ridge. 28 N. Mouchot et al. / Marine Geology 271 (2010) 17–31

(Fig. 10). The crests of upwards successive ripples migrate upslope. Table 3 The structure of internal reflectors appears similar and continuous Location of sediment cores. from one wave to the next, suggesting sediment waves rather than Core Location (lat N, long E) Setting Water Core shortened sediment packages (Bourget et al. submitted). The number depth length dimensions of the ripples are comprised between 1 and 3 km long (m) (m) with an amplitude that reaches 10 to 20 m high, which is comparable MD04-2849 24.4280000 64.5503333 Trench 2980 33.91 to the ridges described on bathymetry in Section 4.1. MD04-2858 24.7488333 64.3251667 Trench–slope 1456 24.59 basin MD04-2864 24.2603333 63.9135000 Trench 3095 33.62 5. Discussion MD04-2867 24.2666667 63.9083333 Trench 3130 7.32 MD04-2868 24.6040000 63.8388333 Trench–slope 1718 4.50 5.1. Insights from surface data basin MD04-2871 23.5600000 63.8808333 Northern Murray 1850 5.34 Ridge (1) The accretionary prism is incised by numerous canyons. The MD04-2872 23.6655000 63.8273333 Northern Murray 2204 10.98 important entrenchment of V-shaped canyons (e.g., Kukowski Ridge et al., 2001), numerous knickpoints and high reflectivity values along their pathways indicate that erosion and incision are very active in these systems. High reflectivity values are frequently related either to the presence of coarse-grained sediments or to bedded echo type (Ia) is commonly attributed to terrigenous fine- indurated sea-bottom. This implies that sediment bypass and grained sediments deposited by turbidity current processes (Gaullier erosion are the most predominant processes in the canyons, at and Bellaiche, 1998). The core MD04-2867, located in the western least locally. The activity of these canyons is probably related to part of the trench (Figs. 2a and 7; Table 3), consists of Facies A in the respective sedimentary loads of the corresponding onshore agreement with the turbidity current processes described by Gaullier rivers which have currently a seasonal activity (Lückge et al., and Bellaiche (1998). However, the cores MD04-2864, located in the 2001). The coarse deposits that reach the eastern abyssal plain same sector (Figs. 2a and 7), consists of thin-bedded medium to fine- probably come from both continental erosion of the Makran grained sands overlain by massive muds to silts association, which prism and sub-marine erosion processes (many failures have characterize low density turbidity current sedimentation followed by been observed along canyon paths). Due to tectonic activity a pelagic to hemipelagic sedimentation. So, the echo type Ia can and related uplift of the wedge structures, canyons can be represent different successions of sedimentary processes at local trapped in large piggy-back basins (for example, the canyon 1 scale: turbidite sediments (Loncke et al., 2002)orpelagicto doesn't reach the trench). Considering sinuosity and reflectivity hemipelagic sediments (Le Cann, 1987). values, the canyons of the eastern part of the prism (canyons 1 Echo type Ib is located on the eastern part of the trench south of to 4) seem more erosive than the Shadi and Save canyons. the toe of canyons 2, 3 and 4 and in the trench south of the large (2) Along the trench, bathymetric data show sediment related to piggy-back basins (Fig. 7). These echo types can be attributed to high energy flows from the main canyons (erosional pools) sediments that contain rather high concentrations of coarse-grained (Fig. 4a, b and c). In the trench itself, numerous scours show detrital sediments. It can also be attributed to environments where westward sediment transport with predominant erosional erosional processes are dominant, such as channel axes (Damuth, processes in the eastern part of the trench. An 80 km long E–W 1975; Damuth and Hayes, 1977). Indistinct echo types IIa and IIb are erosive channel has been observed in the prolongation of confined in the eastern part of the trench (Fig. 7). Echo type IIa is canyons 3 and 4 (Figs. 3 and 4b). Further to the west, we encountered in the trench at the toe of the canyon 2. These echo types observed a series of smoother elongated structures on the generally emphasize sediment waves deposits generated by deep seafloor, roughly perpendicular to the local trench axis. In front of currents (Bouma and Treadwell, 1975; Jacobi et al., 1975; Damuth, Shadi and Save canyon mouths, similar structures correspond to 1979). Echo type IIb is restricted to the vicinity of an inferred mud sediment waves generated by turbiditic fluxes reaching the volcano discovered in the study area (Fig. 7). These transparent- trench. We do not observe any channel–levees systems in the bedded echo types (IIb) commonly correspond to deposits partly abyssal plain. Sediment flows in the western and central abyssal disorganized by mass-flow processes or fluid-rich bodies (Loncke et plain are not channelized but rather diffuse on the seafloor. al., 2002). Hyperbolae echo types IIIa and IIIb are confined to the toes (3) Finally, although the northern Murray Ridge is characterized by of thrust faults in the eastern part of the accretionary wedge and SW wide meandering channel-like systems and appears highly of northern Murray Ridge, respectively (Fig. 7). Hyperbolic echo type faulted, the seabed is very homogenous from a reflectivity IIIa is recorded on the northern flank of the trough located on the point of view. This suggests that hemipelagic deposition southwestern part of the northern Murray Ridge. It is generally dominates the area and that this raised domain is now associated with irregular topographies such as fault scarps and rugged disconnected from active turbiditic pathways. slopes in which energy diffusion highly perturbs the 3.5 kHz data acquisition. So, it cannot be used to reflect depositional processes as 5.2. Insights from echo-character mapping and core calibration explained by Damuth (1975, 1980a). Hyperbolic echo type IIIb is restricted to the toe of the thrust related ridges. They are both Based on core analysis and numerous studies of seafloor sampling associated with diffracting blocks contained within mass transport providing a basis for allocating specific sedimentary types and, finally, deposits (Damuth, 1975; Jacobi, 1976; Le Cann, 1987) or with surface depositional processes for most of the observed echo types (Fig. 7), we ridges generated by contour currents (Damuth, 1980a,b, 1994). Given speculate the following links between echo-characters, type of the location of echo type IIIb down the thrust related ridge, the mass sediments and associated depositional processes (Table 2). Distinct wasting process hypothesis has been preferred. echo types Ia and Ib are predominantly observed in the study area Echo-character mapping allows completing our vision of recent (Fig. 7). Echo type Ia is well identified on the northern flank of the sedimentary processes from north to south: Murray Ridge and on the western part of the trench, mainly close to the deformation front. The depositional pattern in the scarce thrust- (1) In the accretionary wedge, echo-character mapping is not top piggy-back basins, revealed on the echo-character data, is also successful in characterizing recent sedimentary processes emphasized by the echo type Ia (Fig. 7). The distinct sharp continuous because of important slope gradient generating slope artifacts. N. Mouchot et al. / Marine Geology 271 (2010) 17–31 29

Fig. 10. Multichannel seismic proﬁle across sediment wave ﬁeld in the trench west of the mouth of canyon 1 (see Fig. 2a for location).

Only wide piggy-back basins return echo type Ia suggesting progressive decrease of turbidity current energy depositing their turbiditic and/or hemipelagic sedimentation. Piston cores coarse-grained sediment fraction in the east. Fine-grained echo indicate a dominance of turbiditic deposits in this province. type Ib could result from the mixed deposition of turbidity Far from the main canyons, hemipelagic sediments have been plumes and hemipelagic sediments. The concordance between recovered. echo-character mapping and morphostructural analysis suggests (2) In the trench, the echo-character mapping confirms the that the trench is characterized by an axial sediment transport tendencies depicted by morphostructural analysis. Indeed, from east to west (Fig. 11). In a first step, the density currents echo types indicate a progressive evolution from predominant flowing through the canyons reach the trench and erode the erosional processes (echo type Ib) in the eastern part of the seafloor as attested by scours and erosion pools. In a second step, trench to predominant depositional processes to the west an axial transport of sedimentary loads settles from east to west (echo type Ia). This transition is probably the result of the in the trench. The current energy decreases, probably due to the

Fig. 11. Synthetic and interpretative map of active sedimentary processes and sediment dispersal pattern in the study area. The black arrows indicate main sedimentary paths. 30 N. Mouchot et al. / Marine Geology 271 (2010) 17–31

drastic decrease of seafloor slope at the toe of the accretionary mass transport deposits. In this active setting, slope instabilities seem wedge, and favours the deposition of sediment load westward in frequent but limited in size. A large scale study of seismic data would the trench as testified by sediment wave fields and distinct echo be useful to better constrain the typology and extent of eventual mass type Ia. Although 3.5 kHz data is lacking for the very western area transport deposits in the trench and the eventual associated risks. off Save and Shadi canyons, the seismic record (Fig. 10)confirms the existence of important sediment wave fields in the trench off Acknowledgments Save and Shadi canyons. The CHAMAK survey was carried out by the Institut Français du In the northern Murray Ridge, distinct bedded echo type Ia Pétrole (IFP), the University of Cergy-Pontoise (UCP) and the National dominates. The cores MD04-2871 and MD04-2872 (Table 3), sampled Institute of Oceanography of Pakistan (NIO). We are very grateful to in meandering bathymetric structure (Fig. 2a), have been used to Ronan Hebert and Sébastien Vasseur for their English checking. We calibrate 3.5 kHz profiles on the northern Murray Ridge. They are thoroughly thank David J.W Piper, co-editor in chief, as well as Nina composed of Facies B. Kukowski and anonym reviewers for their very helpful, detailed and The meandering bathymetric structure have been reported by constructive comments. Ellouz-Zimmermann et al. (2007b) and Gaedicke et al. (2002a) to correspond to inactive meandering channel marking the migration of the Indus deep-sea fan. Thus, differential compaction between fine- References grained levees and coarser sediments of the channel or low sedimentation rates are the only processes able to maintain the Bouma, A.H., 1962. Sedimentology of Some Flysch Deposits: A Graphic Approach to well-preserved bathymetric signature of the meandering channels on Facies Interpretation. Amsterdam, Elsevier. 168 pp. the northern Murray Ridge. Bouma, A.H., Treadwell, T.K., 1975. Deep-sea dune-like features. Marine Geology 19 (5), M53–M59. Chiu, J.K., Liu, C.S., 2008. Comparison of sedimentary processes on adjacent passive and 6. Synthesis and conclusions active continental margins offshore of SW Taiwan based on echo character studies. Basin Research 20 (4), 503–518. This study confirms that the eastern Makran accretionary wedge is Chow, J., Lee, J.S., Liu, C.S., Lee, B.D., Watkins, J.S., 2001. A submarine canyon as the cause — – – mainly eroded by very active canyon systems as already proposed by of a mud volcano Liuchieuyu Island in Taiwan. Marine Geology 176 (1 4), 55 63. Clift, P.D., Shimizu, N., Layne, G.D., Blusztajn, J.S., Gaedicke, C., Schluter, H.U., Clark, M.K., Ellouz-Zimmermann et al. (2007b). Only small and localized failure Amjad, S., 2001. Development of the Indus Fan and its significance for the erosional scars characterize this prism. They are either associated with canyon history of the Western Himalaya and Karakoram. Geological Society of America – pathways or with active thrust ridges. Easternmost canyons seem to Bulletin 113 (8), 1039 1051. Clift, P., Gaedicke, C., Edwards, R., Lee, J.I., Hildebrand, P., Amjad, S., White, R.S., Schlüter, be more erosive than the Shadi canyon already described by Kukowski H.-U., 2002. The stratigraphic evolution of the Indus Fan and the history of et al. (2001). Turbiditic facies characterize the axial trench while sedimentation in the Arabian Sea. Marine Geophysical Researches 23 (3), 223–245. hemipelagic facies drape the outer trench slope. Several sedimentary Damuth, J.E., 1975. Echo-character of the western equatorial atlantic floor and its relationship to the dispersal and distribution of terrigenous sediments. 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