Hybrid turbidite-contourite sediments transport system in the Eastern Mediterranean upper continental slope

Nile derived siliciclastic sediments are the main source for sedimentation along the Levant continental margins. The sediments are transported along the southeastern Mediterranean coast via jet and longshore currents, mainly operating along the shelf. However, the cross shelf component of sediments transport, responsible for conveying sediments towards the upper slope, is less known. To better understand the cross-shelf vs. the longshore components of sediment transport, we studied two ~5.5 m piston cores: DOR280 and DOR350, sampled on the upper continental slope at 280 m and 350 m water depth, respectively.

We analyzed the particle size distribution (PSD) as well as the benthic-foraminiferal assemblages and their shells taphonomy, for documenting both the source and the transport mechanism of the upper continental-slope sediments. The radiocarbon sediment age at the DOR280 core-base is ~660 ±70 Cal Yrs. B.P., indicating an exceptionally high average sedimentation rate of ~800 cm/kyr. DOR280 consists of alternating two sedimentary facies: (1) Laminated (L) intervals with bimodal PSD and high ratio of allochthonous vs. autochthonous (allo/auto) foraminiferal species, characterized by a high percentage of benthic-foraminiferal broken and poorly preserved shells, indicating contribution of transported sediments originating from mid-shelf habitats. (2) Non-laminated (NL) intervals with unimodal PSD, low allo/auto ratio (<1) and low percentage of broken shells, indicating mostly in-situ deposition. The L intervals are interpreted as sediment laden gravity currents, possibly turbidites. Numerous centimeters-thick turbiditic events were identified, based on grain-size grading and discontinuous eroded lower stratigraphic-contacts. Sedimentation rate calculated only for the NL intervals is still exceptionally high, excluding hemipelagic sedimentation as the sole deposition. Thus, a contour bottom-current transported component is suggested for the NL sediments of DOR280 (i.e. contourites). DOR350 reveals higher sedimentation rates (age of ~350 ±80 Cal Yrs. B.P. at the core-base) and consists mostly of the L facies. Hence, the sediments of DOR350 are mostly consist of transported (by turbidities) sediments with only minor contribution of hemipelagic sedimentation or contourites.

We conclude that a hybrid contourite-turbidite system actively prevails along the Levant upper continental slope offshore Israel, apparently at water depth of less than 350 m.