Bottom currents and their influence on the sedimentation pattern in the El Arraiche mud volcano province, southern Gulf of Cadiz

Abstract

The completion of IODP expedition 339 within the Cadiz contourite depositional system, along the southern Iberian margin (which was created and maintained by the Mediterranean Outflow Water) has brought increased amounts of attention to this natural contourite laboratory. In contrast, a lot less attention is given to the southern Gulf of Cadiz despite the ubiquitous presence of tectonic ridges, small contourite deposits, mud volcanoes and cold-water coral mounds. The El Arraiche mud volcano province (EAMVP) is located in the southern Gulf of Cadiz and is characterized by an extensional regime, creating two tectonic ridges named Renard and Vernadsky. Also, nine mud volcanoes and numerous cold-water coral mounds are present in this area.The northward flowing bottom currents are deflected by the topographic obstacles and flow in a westward direction at the foot of the NW–SE-oriented Renard Ridge. Calculations indicate that this bottom current is capable of turning around the tip of the Renard Ridge and continues its path along its northern edge. The locations of the contourite deposits at the foot of the Renard Ridge are controlled by the steepness of the ridge: slopes of more than 12° are associated to contourite deposits, while less steep ones merely show hemipelagic deposits. The moats around the mud volcanoes originate due to a combination of subsidence and the action of bottom currents, as the seismic data show separated mounded drift deposits perpendicular to the moats as well as subsidence rims. Some mud volcanoes have a less incised northern and a deeper southern moat, which indicates eastward flowing bottom currents. This orientation is consistent with the shoreward component of the internal tides, which flow vigorously in this area with peaks of speed over 30cm/s. The integration of geophysical and hydrographic datasets in the EAMVP sheds new light on the dynamic nature of the interaction of bottom currents and topographic features.