Evolution of the gulf of Cadiz margin and southwest Portugal contourite depositional system: Tectonic, sedimentary and paleoceanographic implications from IODP expedition 339

Abstract

International audienceThe contourite depositional system (CDS) along the southwestern Iberian Margin (SIM), within the Gulf of Cadiz and offshore areas of western Portugal 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. Over the last four decades, numerous studies have been conducted on the late Miocene, Pliocene and Quaternary sedimentary stacking pattern of Neogene basins along the SIM for both academic and resources exploration purposes. However, understanding of the region rests primarily on basic seismic stratigraphy calibrated with limited data from only a few exploration wells. 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 sedimentation style and dominant sedimentary processes, coupled with widespread depositional hiatuses along the SIM within the Cadiz, Sanlucar, Doñana, Algarve and Alentejo basins. 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 0.8–0.9 Ma duration with a pronounced overprint of ~ 2–2.5 Ma cycles. These more protracted cycles relate to the westward rollback of subducted lithosphere at the convergent Africa-Eurasia plate boundary as its previous NW–SE compressional regime shifted to a WNW–ESE direction. 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 into the Atlantic and associated 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, appearing most prominently in the Doñana basin. Several discontinuities bounding major and minor units appear on seismic profiles. Quaternary records offer the clearest example of this, with major units of about 0.8–0.9 Ma and sub-units of 0.4–0.5 Ma. Sedimentation is controlled by a combination of tectonics, sediment supply, sea-level and climate. This research identifies time scales of tectonic controls on deep-mari...

Impact of sea-level rise on earthquake and landslide triggering offshore the Alentejo margin (SW Iberia)

“…For instance, in the case of the Alentejo margin, the present interpretation indicates that the nature and physical properties of sediments are an important preconditioning factor. The Pliocene-Quaternary record of the Alentejo margin is dominated mostly by the deposition of contourite sediments corresponding to calcareous mud interbedded with minor sandy layers, turbidites, and debrites (Hernández-Molina et al 2016). According to Laberg and Camerlenghi (2008), the occurrence of mass movements in contourite drifts is partly favored by the composition, distribution and physical properties of the sediments themselves.…”

“…The three events recognized by Masson et al (2011) in the TAP area correlate with E8, E9, and E11. Sea-level curve Preferred ice-volume equivalent sea-level solution from Lambeck et al (2014) the Alentejo margin were provided by IODP Expedition 339 results (Hernández-Molina et al 2016).…”

“…4) illustrates how mass movement processes affect the Quaternary sediments. The main sedimentary facies consist of calcareous muds with interbedded minor sandy layers (Hernández-Molina et al 2016). The mass movement deposits correspond to rotational slides with stratified parallel acoustic facies and slumps with chaotic facies.…”

“…Several studies in the area have considered landslides and turbidite records as proxies of paleoseismic activity (Gràcia et al 2010;Masson et al 2011). Recently, deep drilling during IODP Expedition 339 at Site U1391 revealed the existence of several debris flow events interbedded within the sedimentary succession of the Alentejo margin (Hernández-Molina et al 2016). Thus, mass movement deposits along the Alentejo margin seem to be even more conspicuous than previously thought, making this an ideal setting to test the causal relationship between sea level-induced earthquakes and submarine landslides.…”

“…The present study reports further evidence of an extensive area of slope failure adjacent to the PSF that has been recently discovered (Roque et al 2015). This is the area of the Sines contourite drift deposited by the Mediterranean Outflow Water circulation around the continental slope of the Alentejo margin (Hernández-Molina et al 2016).…”