David W. Hunt

Cenozoic tectono-sedimentary evolution of the western Iberian margin

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.

The depositional evolution of diapir- and fault-bounded rift basins: examples from the Lusitanian Basin of West Iberia

New data on the evolution of rift basins is presented after analysing the Late Jurassic stratigraphy of the Central Lusitanian Basin (west Iberia). Well, outcrop and regional 2D seismic reflection profiles are used to investigate the differences in stratigraphic signature between diapir- and fault-bounded sub-basins. During the Late Jurassic syn-rift phase, surface rupturing in fault-bounded sub-basins resulted in the formation of tectonic scarps from which footwall-derived gravity flows were sourced. In contrast, the diapir-bounded Bombarral-Alcobaca sub-basin evolved as a distal bowl-shaped depocentre with an axis located up to 10 km away from its basin margins. Low-gradient marginal slopes developed in the Bombarral-Alcobaca sub-basin during the Late Jurassic rifting, while growing salt pillows limited the vertical propagation of basement normal faults. Differences in tectonic evolution, basin physiography and sediment input are the main factors responsible for the distinct sedimentary evolutions recorded in the study area: (1) transverse footwall-derived sediment fans, predominant in fault-bounded regions, give place to axial southwards-prograding fluvial to shallow-marine units in the diapir-bounded sub-basins; (2) growing salt pillows, absent in the fault-bounded sub-basins, formed barriers to and limited the development of transverse drainage systems.