Evolution of the Gulf of Cadiz and west Portugal contourite depositional system: tectonic, sedimentary and paleoceanographic implications from IODP Expedition 339
Monday, 14 December 2015: 09:40
2005 (Moscone West)
The contourite depositional systems (CDS) along the southwestern Iberian Margin (SIM) bear the unmistakable signal of Mediterranean Outflow Water (MOW) exiting the Strait of Gibraltar. This locality records key information concerning the effects of tectonic activity on margin sedimentation, the effects of MOW dynamics on Atlantic circulation, and how these factors may have influenced global climate. The Integrated Ocean Drilling Program (IODP) Expedition 339 recently drilled five sites in the Gulf of Cadiz and two sites on the western Iberian margin. The integration of core and borehole data with other geophysical databases leads us to propose a new stratigraphic framework. Interpretation of IODP Exp. 339 data along with that from industry sources and onshore outcrop analysis helps refine our understanding of the SIM’s sedimentary evolution. We identify significant changes in sedimentary style and dominant sedimentary processes, coupled with widespread depositional hiatuses along the SIM. Following the 4.5 Ma cessation of a previous phase of tectonic activity related to the Miocene-Pliocene boundary, tectonics continued to influence margin development, downslope sediment transport and CDS evolution. Sedimentary features indicate tectonic pulses of about 0.8 Ma duration with a pronounced overprint of ~2 -2.5 Ma cycles. Two major compressional events affecting to the Neogene basins at 3.2-3 Ma and 2-2.3 Ma help constrain the three main stages of CDS evolution. The stages include: 1) the initial-drift stage (5.33-3.2 Ma) with a weak MOW, 2) a transitional-drift stage (3.2-2 Ma) and 3) a growth-drift stage (2 Ma-present time) with enhanced MOW circulation and attendant contourite development due to greater bottom-current velocity. Two minor Pleistocene discontinuities at 0.7-0.9 Ma and 0.3-0.6 Ma record the effects of renewed tectonic activity on basin evolution. This research identifies time scales of tectonic controls on deep-marine sedimentation, specifically over periods of 2.5->0.4 Ma Shorter term climatic (orbital) mechanisms control sedimentation at time scales of ≤0.4 Ma. The role of bottom water circulation in shaping the seafloor and controlling the sedimentary stacking pattern on continental margins has to be seriously reconsidered in future studies.