Sedimentological and Hydrodynamical Processes Along the Var Turbiditic System (SE France)

Virginie Mas¹, Bernard Dennielou¹, Thierry Mulder², Bruno Savoye¹, Sabine Schmidt², Alexis Khripounoff³, and Annick Vangriesheim^3
1IFREMER GM/LES Brest, France
²University Bordeaux 1, France
³IFREMER EEP/LEP Brest, France

Introduction
The Var turbiditic system is located in the Western Mediterranean offshore Nice. It is connected to the Var River through a steep canyon. As a result, the system is subject to seasonal river discharges. The sediment transport to the basin is mainly via turbidity currents and mass-wasting. The three kinds of turbidite processes have been described: (i) ignitive high-density turbidity currents, generated by large slope failures, (ii) small-sized turbidity currents related to small failures and (iii) hyperpycnal flows generated during high magnitude floods.

Aims & dataset
The aim of the project is to determine the spatio-temporal relative proportions of these processes in the sedimentary record in the turbidite system.

Our investigation is based on EM 300 bathymetry and SAR side-scan sonar and a comprehensive set of sediment cores retrieved every 6 months from September 2005 to September 2007.Core analysis includes visual descriptions, X-Ray imagery, grain size analysis, X-ray fluorescence and resin embedding. The stratigraphic framework for the last century is constrained by ²¹⁰Pb _exc and ¹³⁷Cs measurements. Five mooring lines and a benthic observatory were deployed along the turbidite system from September 2005 to September 2007. Acquired data are particle flux, temperature, pressure and current direction and velocity. These data are compared with the historical Var River discharge (from 1973 to 2007).

System description & sediment distribution
Previous studies showed that the canyon floor is a conduit for sediment transport and displays a wide range of deposits. Sediment waves made of gravels and cobble-sized clasts, cover the canyon bed. In the upper part of the valley, muddy sediments partially infill the intervals between dunes. Downstream, in the middle part of the valley, gravel can appear through large scours, but are caped by wide areas of coarse to fine deposits. Most recent deposits are composed by fine sand to muddy sediments. Their thickness is variable spatially and floor sedimentation is episodic and localised.

In the canyon and the upper valley, terraces reach 30 to 70 m above the channel floor. At the left-hand side, they display many bedforms and erosional structures. The sediment generally consists of coarse silt to gravel. Turbidite sequences are truncated and erosional contacts are very common. Sedimentation rate (including hemipelagic and terrigeneous sedimentation) for the last century is about 2 mm ^y-1. On the right-hand side terraces there are fewer bedforms. Turbidite sequences are well defined, and composed of coarse sand to silty mud. The sedimentation rate is greater than 10 mm y^-1. These observations suggest that the right-hand side terraces likely undergo less energetic flows.

Southward to the middle valley floor, the hypertrophied right-hand levee consist of a 350 m high sedimentary ridge. Sediments on top are composed of coarse silt to mud. Sequences are thiner, finer and better preserved than on the terraces. Sedimentation rate is about 0.8 mm ^y-1.

Right hand terraces in the upper valley contain the best deposition records of the gravity processes in the Var system for the last century. Theses terraces have been cored 14 times since September 2005, in four principal areas. Spatial correlation between cores allows the reconstruction of lateral variability of gravity currents through the interpretation of their deposits. Detailed analyses allow to identify the processes responsible of each sedimentary sequence. The four main sequence types identified are (i) turbidites (generated by slope failures), (ii) hyperpycnite, (iii) hemipelagite and (iv) deposition by decantation of turbid clouds generated by hypopycnal flows. The high sedimentation rates measured on the right hand terraces in the upper valley are mostly due to hyperpycnal deposition.

Record of present sedimentary activity
Mooring datasets show that several hyperpycnal flows have occurred since September 2005.
One event, recorded on the 3^rd December 2005, in the Var canyon and in the upper Valley channel is characterized by a brief but strong peak in current velocity and high particule flux increase. This event correlates with a river flood and has been interpreted as hyperpycnal flow that transported ~150 tons of material through the Var canyon.

In the latest cores recovered on the canyon terraces (i.e. taken after the events recorded by the moorings), new sediment sequences can be observed, but the record of the hyperpycnal events is intermittent. More detailed analysis is under way to identify the major factors controlling sedimentation.

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