Contourite drifts on early passive margins as an indicator of established lithospheric breakup

- Cretaceous lithospheric breakup triggered widespread bottom currents in NW Iberia.
- Contourite drifts can possibly show lithospheric breakup between conjugate margins.
- The NW Iberia Albian-Cenomanian Breakup Sequence (BS) is characterized.
- Genetic facies within the BS include MTDs, contourites and black-shales/turbidites.

The Albian-Cenomanian breakup sequence (BS) offshore Northwest Iberia is mapped, described and characterised for the first time in terms of its seismic and depositional facies. The interpreted dataset comprises a large grid of regional (2D) seismic-reflection profiles, complemented by Industry and ODP/DSDP borehole data. Within the BS are observed distinct seismic facies that reflect the presence of: (a) black shales and fine-grained turbidites, (b) mass-transport deposits (MTDs) and coarse-grained turbidites, and (c) contourite drifts. Borehole data show that these depositional systems developed as mixed carbonate-siliciclastic sediments proximally, and as organic-carbon-rich mudstones (black shales) distally on the Northwest Iberia margin. MTDs and turbidites tend to occur on the continental slope, frequently in association with large-scale olistostromes. Distally, these change into interbedded fine-grained turbidites and black shales showing widespread evidence of deep-water current activity towards the top of the BS. Current activity is expressed by intra-BS erosional surfaces and sediment drifts. The results in this paper are important as they demonstrate that contourite drifts are ubiquitous features in the study area after Aptian-Albian lithospheric breakup. Therefore, we interpret the recognition of contourite drifts in Northwest Iberia as having significant palaeogeographic implications. Contourite drifts materialise the onset of important deep-water circulation marking the establishment of oceanic gateways between two fully separated continental margins. As a corollary, we postulate the generation of deep-water geostrophic currents to have had significant impact on North Atlantic climate and ocean circulation during the Albian-Cenomanian, with the record of such impacts being preserved in the contourite drifts analysed in this work.

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