The Syn- to Post-Rift Transition on the West Iberian Margin

Alves, Tiago M.¹; Soares, Duarte ¹
(1) School of Earth and Ocean Sciences, Cardiff University, Cardiff, United Kingdom.

A comprehensive set of 2D seismic reflection lines, borehole data from more than 50 Industry wells, stratigraphic information from key outcrops, and published DSDP/ODP data is used in the analysis of syn-to post-rift transitions offshore West Iberia, Northeast Atlantic. In this work are presented seismic reflection data, wireline, stratigraphic and seismic data to document the variations in sedimentary facies and stacking patterns immediately below and above the ‘break-up unconformity’.

In West Iberia, strata immediately below the break-up unconformity were accumulated in a relative sea-level (lowstand) and denote the progressive abandonment of the former Late Jurassic rift axes. As a consequence, stratigraphic data from key outcrop and well locations show shallow-marine to continental deposits denoting progradation towards the main axis of rifting, which was located in the present-day continental slope area. Resulting shallowing-upwards sequences are intercalated with carbonate units deposited in slowly subsiding basins with no relevant topographic features.

All over the West Iberian margin, continental break-up resulted in the sudden influx of siliciclastic material in onshore and shallow-offshore basins. This event occurred in several pulses (or events) as continental break-up migrated northwards on the margin. Strata immediately above the break-up unconformity of interpreted to represent a forced regressive event accompanied by: a) westward tilting of the proximal margin, which sourced westward-prograding units; b) complete abandonment of syn-rift depocentres, which were blanketed by post-rift successions. In Northwest Iberia, a similar episode of progradation occurred in the Porto Basin during (Aptian) continental breakup. However, the inferred forced regression across the break-up unconformity was marked by prograding reflections of Albian age denoting shallow-marine environments, not continental deposits. In contrast, a period of widespread erosion is also recorded on the modern continental shelf and slope of southwest Iberia.

We suggest the differences between the two sectors as resulting from differences in the geological processes leading to continental break-up. Regions where continental break-up occurred relatively close to the rift-shoulder areas show widespread regional hiatuses and an abrupt shallowing of sedimentary facies across the break-up unconformity.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.