Cenozoic tectono-sedimentary evolution of the western Iberian margin

Abstract

Bathymetric, 9.5-kHz long-range sidescan sonar (OKEAN) and seismic reflection data are used to characterise the Cenozoic tectono-sedimentary evolution of the Porto, Lisbon and Alentejo continental margins, offshore Portugal, where the presence of submarine fans, slope gullies, submarine canyons and seamounts was confirmed. Sediment drifts, some of probable contouritic origin, were recognised over the upper continental slope (500–1000 m) and surrounding the Vigo seamount. Seven echo types offshore Porto (types IA-1, IB, IB-2, IC-2, IIA, IIB, IIIA) and five echo types offshore Lisbon (types IB, IIA, IIB, IB-2, IIIA) were identified on 3.5-kHz profiler data. In addition, 11 Meso–Cenozoic seismic units off Lisbon and Alentejo, plus three post-Turonian seismic packages offshore Porto were interpreted and dated by well, dredge and DSDP/ODP stratigraphic data. During the Cenozoic, the tectono-sedimentary evolution of the studied areas depended on their position in relation to the locus of compression in Iberia. Accommodation space and sediment pathways varied in relation to distinct pulses of uplift or subsidence occurring at different times in the three studied regions. The Porto and Lisbon margins record extensional collapse respectively after the early Eocene and early Chattian, interrupted by short episodes of uplift related to distinct tectonic phases. As a result, gravity flows dictated deposition on these margins in most part of the Cenozoic. Seamounts and halokinetic structures controlled deposition on the Porto margin by inducing topographic barriers to the westward progradation of slope-derived sediment. The relative proximity of the Alentejo margin to the Azores–Gibraltar Fracture Zone resulted in folding and exposure during the middle Oligocene, but subsidence after the early Chattian generated a palaeoslope buried under Neogene units. Oligocene and Burdigalian canyon incision offshore Alentejo preceded the emplacement of modern channels during the Pliocene. These presently transport sediment derived from the shelf and major onshore drainage catchments into abyssal areas.