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Monsoon-Induced Hyperpycnal Flows Recorded in the Gulf of Oman (NW Indian Ocean)

J. Bourget1, S. Zaragosi1, T. Mulder1, T. Garlan2, N. Ellouz-Zimmermann3, A. VanToer4, and J-L. Schneider1
1Université Bordeaux I, Département de Géologie et Océanographie, UMR 5805 EPOC, 33405 Talence cedex, France
2SHOM, Centre Hydrographie, BP 426, 29275 Brest cedex, France
3IFP, 4 rue Bois Préau, Rueil-Malmaison 92141 cedex, France
4LSCE/IPSL, Laboratoire CEA-CNRS-UVSQ, Avenue de la Terrasse 91198 Gif sur Yvette cedex France

The Gulf of Oman (Arabian Sea) is a 3,400 m deep triangular shaped sedimentary basin (Figure 1), located between the passive Oman margin and the Makran accretionnary prism (Iran and Pakistani margins), which developed in response to the subduction of the Arabian Sea since the Late Cretaceous (Ellouz et al., 2007). The Gulf of Oman is also affected by the Asian monsoon-climate (Figure 1), characterized by strong seasonality with annual flash-floods of the numerous small sporadic streams (wadis) that are located all along the Oman and Makran coasts during the summer monsoon.

The MARABIE (2000, 2001) and CHAMAK (2004) surveys allowed us to investigate the Quaternary deep sea gravity sedimentation along both the Oman and Makran margins. In this study we examine several piston cores recovered in a widespread range of environments (e.g. slope ridges, levees, channels floor, abyssal plain). Several facies could have been recognized (i.e. slumps, lobes sands, fine grained turbidites, thick ponded muds, hemipelagites and pelagites).

Very high resolution grain size analysis, X-ray images, geochemical data, petrographical analysis on thin sections of indurated sediment allowed to describe the turbidites beds morphology and nature (Figure 2).

The Oman margin

The south-western Oman margin is characterized by the development of numerous small channel-levees systems on the slope, connected to seasonal wadis landward. The largest system, the Al Batha turbidite system, extends in the abyssal plain with a 1500 km2 sandy lobe. Several cores recovered in these systems show that turbidite activity remains high during the present sea-level highstand.

We suggest that this is due to enhanced sediment transfer during floods and/or storms associated with summer monsoon.

The Makran margin

The northern margin is characterized by the Makran accretionnary wedge which correspond to a very fine-grained slope-apron (Stow et al., 2002), multi-sourced system. It is also sediment-fed by the wadis that form a relatively dense network along the Iran and Pakistani coasts. The frontal part of the submerged prism is marked by thrust faulting, responsible of the formation of accreted ridges with steep flanks (Ellouz et al., 2007), and dissected by several canyons that reach the abyssal plain at 3300 m. Slump or slide scars and deposits are observed throughout the slope: they cause dissection of the ridges crest, permanently uplifted. They also generate retrogressive erosion of headless canyons in the upper and middle slope. Although distinct channels are incised into the ridges, most of the turbidity currents generated from the shelf edge or upperslope must follow complex pathways, dividing between the slope ridges, and fill or bypass small intraslope (piggyback) basins. Some of theses flows can reach the abyssal plain and build large muddy sediment-wave field on the lower slope, downward the canyons mouth.

The Abyssal plain

Sedimentation in the deep, central abyssal plain is characterized by more rare, very fine-grained thick ponded turbidites, probably originated from the Makran margin. Here the flows are not channelized, and form sheet-like deposits. Most of the turbidites show a complex structure with silt-beds recurrence and wispy laminae in the Te subdivision, suggesting that the flows at the origin of such deposits did not occur as discrete flow, but probably as successive surges occurring in a single, thick muddy turbidity current (Tripsanas et al., 2004b). Such deposits are believed to be generated by successive retrogressive mass-failures in the prism.

Controversly, inversely-graded basal layer in some of the turbidites (Figure 2) suggest that, at least some of them are flood-generated (Mulder et al., 2003). This implies that sustained turbidity currents generated at the shelf edge by very intense flash-floods can cross the Makran slope and create very fine grained hyperpycnites as far as 220 km from the shelf break.


The Gulf of Oman is an interesting modern analogue for deep water systems located in a complex tectonic and climatic setting.

Deep sea sedimentation in the Gulf of Oman include from point source “channel-levee-lobe” systems (Oman margin), complex multisource mud-rich slope apron (Makran margin), and basin-wide sheet depositional system in the abyssal plain.

Turbidites with inversely graded basal unit (hyperpycnites) have been recognized in different depositional environments (Oman levees and lobes, Makran slope, abyssal plain), and allowed to further describe theses particular turbidites sequences in terms of structure and nature.

The evolution of the gravity sediment supply since the Last Glacial Maximum has been studied in order to evaluate the impact of eustatism, tectonics and climate on the turbidite activity in the Gulf of Oman, along two different margin morphologies (i.e. Makran and Oman margins), both submitted to the same Asian-monsoon forcing.

Investigating the gravity sedimentation in such a complex area, where strong tectonics and climatic forcings interplay, should provide significant insights on the impact of external forcings on deep sea clastic sedimentation.


Ellouz-Zimmermann, N., Deville, E., Müller, C., Lallemant, S., Subhani, A., Tabreez, A., 2007. The control of convergent margin tectonics by sedimentation along the Makran accretionary prism (Pakistan), in: Lacombe, O., Lavé, J., Vergès, J., Roure, F. (Eds.), Thrust Belts and Foreland Basins: from Fold Kinematics to Hydrocarbon Systems. Springer-Verlag, in press.

Fleitmann, D., Burns, S.J., Mangini, A., Mudelsee, M., Kramers, J., Villa, I., Neff, U., Al-Subbary, A.A., Buettner, A., Hippler, D., Matter, A., (in press). Holocene ITCZ and Indian monsoon dynamics recorded in stalagmites from Oman and Yemen (Socotra). Quaternary Science Reviews.

Mulder, T., Syvitski, J.P.M., Migeon, S., Faugeres, J.-C. and Savoye, B., 2003. Marine hyperpycnal flows: initiation, behavior and related deposits. A review. Marine and Petroleum Geology, 20(6-8): 861-882.

Prins, M.A., Postma, G., Weltje, G.J., 2000. Controls on terrigenous sediment supply to the Arabian Sea during the late Quaternary: the Makran continental slope. Marine Geology 169, 351-371.

Stow D.A. V., Tabrez A. R., and Prins. M. A., 2002. Quaternary sedimentation on the Makran margin: turbidity current-hemipelagic interaction in an active slope-apron system

Geological Society, London, Special Publications, volume 195: 219-236.

Tripsanas E.K., W.R. Bryant, and B.A. Phaneuf, 2004. Uniform Mud Deposits (Unifites) in a Complex Deep-Water Environment, Hedberg Basin, Northwest Gulf of Mexico, in W.W. Sager/ E.Doyle/ W.Bryant, eds., High-Resolution Geophysical Studies of Continental Margin Geohazards, American Association of Petroleum Geologists Special Publication American Association of Petroleum Geologists Special Publication (Special Volume) of the AAPG Bulletin. Volume 88, Number 6, June 2004, ISSN 0149-1423, pp 825-840.

Uchupi, E., Swift, S. A. and Ross, D. A., 2002. Morphology and Late Quaternary sedimentation in the Gulf of Oman Basin. Marine Geophysical Researches, 23(2): 185 - 208.

Figure 1. Localisation of the study area, 100 m-spaced bathymetric contours (source: ETOPO2 data), and limit position of the Intertropical Convergence Zone (ICTZ, white dashed line). Black arrows indicate the main wind directions during summer monsoon (after Fleitmann et al., in press).

Figure 2. X-ray picture from the core KS16 (3357 m water depth in the abyssal plain – 200 km from the shelf break) and close up on the hyperpycnite sequence (mean grain size, scanned thin slab, and interpretative log). The inversely graded basal layer (Ha) consists of silt laminae, and begins with a gradual transition. The normally graded silty layer (Hb) begins with an erosional contact, and consists of silt layers with ripple cross laminations, multiple internal erosional surfaces, and then sub-planar laminae. The whole sequence is overlain by hemipelagic deposits (bioturbated, foraminifer-rich clay, which is lighter on X-ray picture).


AAPG Search and Discovery Article #90079©2008 AAPG Hedberg Conference, Ushuaia-Patagonia, Argentina