Sediment starved or accommodation-nourished basins during North Atlantic continental rifting?

Late Jurassic-Cretaceous rifting and continental break-up of the North Atlantic Ocean was markedly complex, and present-day paleogeographic reconstructions do not address the position of micro-plates such as Iberia's. As an alternative method to predict source and reservoir potential on North Atlantic rift basins, the focus has changed in recent years towards understanding the topography of paleo-rift axes, continental slopes, marginal highs and shelves, through the analysis of seismic and stratigraphic data from multiple sources.

This presentation will focus on the Late Jurassic-Early Cretaceous depositional record of West Iberia, extrapolating the results to conjugate margins off Newfoundland, Ireland and · the UK and Norwegian North Sea. Well-dated unconformities in West Iberia accurately mark the main rift-related tectonic events that precede continental breakup. In such a setting, West Iberia differs from the North Sea, proximal Newfoundland basins and Ireland by presenting excess accommodation space for sediment sourced from hinterland areas very early in its evolution. In effect, the Lusitanian Basin was a very marginal depocentre through which a substantial volume of sediment was transported on its way to deep continental slope basins. This character marked important by-pass of sediment towards the North Atlantic rift axis, with excess accommodation space resulting in enhanced conditions for the generation of 'black shales' in these regions.

This talk will focus on: a) the recognition of Late Jurassic-Early Cretaceous migration of lope facies towards the west and southwest in West Iberia, b) the predominance of a slope- ed depositional system through the latter stages of rifting and continental break-up, c) the near-permanence of topographically sheltered, 'sediment starved' conditions in the future region of continental break-up during the Late Jurassic and Early Cretaceous. This setting was maintained until continental break-up was achieved throughout the West Iberian margin, from which point slope progradation ceased, subsidence increased dramatically in offshore basins and bypass of sediment predominated on the continental slope through major submarine canyons. The locus of sediment starvation shifted at this stage from the future

Abyssal Plain region to the newly-formed continental slope. Hydrocarbon potential in distal rift axes, or troughs, will vary depending on multiple factors. Of importance to the (relatively) accessible continental-slope basins is the correct assessment of paleodepths during the phases of slope progradation, and post-breakup erosion on the proximal margin. Exhumation and recycling of sediment in 'continental breakup' conditions is particularly noted in ODP and DSDP data, together with the establishment of strong oceanic currents during the later Early Cretaceous. Point-sources for sediment derived from the hinterland are crucial in terms of assessing source and reservoir potential in deep offshore basins. Also important is the correct assessment of magmatic events on what is a 'non-volcanic margin' with abundant evidence for Late Jurassic to Late Cretaceous volcanism.