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.

CONCLUSION

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.

References:

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