Oman Margin

The southwestern Oman margin is characterized by the development of numerous small channel-levee systems on the slope, connected landward to seasonal wadis. 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.  

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 Iranian and Pakistani coasts. The frontal part of the submerged prism is marked by thrust faulting, responsible for 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 permanently uplifted ridge crests. 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 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 of the canyons mouth.

Abyssal Plain

Sedimentation in the deep, central abyssal plain is characterized by uncommon very fine-grained thick-ponded turbidites, which probably originated from the Makran margin. Here the flows are not channelized but form sheet-like deposits. Most of the turbidites show a complex structure with silt-bed recurrence and wispy laminae in the Te subdivision (in Bouma sequence), 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., 2004). Such deposits are believed to be generated by successive retrogressive mass-failures in the prism.

Conversly, 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 includes 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 us further to describe these particular turbidite 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 eustacy, tectonics, and climate on the turbidite activity in the Gulf of Oman, along two different margin morphologies (i.e., Makran and Oman margins), both of which are subjected to the same Asian-monsoon forcing.

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

Selected Bibliography

Ellouz-Zimmermann, N., Deville, E., Müller, C., Lallemant, S., Subhani, A., and 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., and 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, v. 20, p. 861-882.

Prins, M.A., Postma, G., and Weltje, G.J., 2000, Controls on terrigenous sediment supply to the Arabian Sea during the late Quaternary: the Makran continental slope: Marine Geology, v. 169, p. 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 of London Special Publications, v. 195, p. 219-236.

Tripsanas E.K., Bryant, W.R., and Phaneuf, B.A., 2004, Uniform mud deposits (unifites) in a complex deep-water environment, Hedberg Basin, Northwest Gulf of Mexico, in Sager, W.W., Doyle, E., and Bryant, W., eds., High-Resolution Geophysical Studies of Continental Margin Geohazards: AAPG Bulletin, v. 88, p. 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, v. 23, no. 2, p. 185-208.