STRUCTURE AND VARIABILITY OF CONTOURITE SEDIMENTATION BY MEDITERRANEAN OUTFLOW WATER: PRELIMINARY RESULTS OF IODP EXPEDITION 339 IN THE GULF OF CADIZ AND WEST OF PORTUGAL

Bottom-currents play an important role in distributing sediment in ocean basins and in creating deposits which can be large with coarser and finer beds and a range of sedimentary structures. Current-created deposits are becoming better understood as a result of a number of studies, but few extensive sequences of facies with coarser beds formed by bottom currents have been sampled in their modern settings. IODP Expedition 339 drilled 7 sites in the Gulf of Cadiz and off the west Iberian margin in 2011/2012 to study the history of Mediterranean Outflow Water (MOW) through the Gibraltar Gateway for the past ~5 Ma. MOW has influenced global circulation and climate and created a number of sediment drifts along the continental slope, and its flow speed varies due to climatic, sea level and tectonic forcing. We recovered sequences of silty and sandy beds which show bi-gradational, inverse and normal bedding, often bioturbated, with gradational, sharp or erosional contacts and which have thicknesses ranging from a few cm to several m in proximal and distal settings. These beds are interpreted as contourites, and the structure and timing of these beds, and of several hiatuses, will give important information about the character and history of this important current. Several of the beds will be studied in detail with thin section, X-ray, high-resolution XRF, micro-CT and grain size techniques to better characterize the beds and the events which created them. Also, at most sites, two or three holes were drilled 10 to 100 m apart, and sediments in adjacent drill holes are correlated based on downcore proxies. While the nature and sequence of contourite layers in adjacent holes are often quite similar, correlated beds can have somewhat different thickness, grain sizes and layering patterns. This lateral variability may be due to the presence of larger bed forms or other depositional and erosional process due to the local behavior of the MOW. We need to distinguish between bed variations related to current flow history and those related to bed forms or other local sedimentary processes to fully characterize the nature and effects of these deep currents.

Coauthors: IODP Expedition 339 Scientists