Reworked calcareous nannofossils as ocean dynamic tracers: The Guadiana shelf case study (SW Iberia)

For most micropalaeontological studies the presence of reworked specimens is usually considered as a disturbing factor where palaeoenvironmental or biostratigraphic interpretations are to be made. However, reworking of microfossils such as calcareous nannofossils can be used as an additional parameter in routine micropalaeontological work, providing useful information on erosion-transport processes acting on silty-clay sized particles deriving from areas surrounding the sedimentary basin under study. Most of the reworked calcareous nannofossils found in the Guadiana estuary water column (September 2001), adjacent shelf surface sediments, and a slope section of the Gulf of Cadiz were mainly of Upper Cretaceous age (Campanian to Maastrichtian), being dominated by Biscutum spp., Cribrosphaerella ehrenbergii, Prediscosphaera cretacea and Watznaueria barnesae. Since there are no Upper Cretaceous outcrops in the Guadiana’s hydrographic basin or southern Portugal, the source of this material must be located elsewhere. Matrix breccias from mud volcanoes discovered in deeper water southern Portuguese and western Moroccan margins of the Gulf of Cadiz during the TTR-10 cruise in July/August 2000, as well as silty-clay sediments from its slopes, revealed similar calcareous nannofossil assemblages as those found on the Algarve continental shelf and inside the Guadiana’s estuary. This may indicate that most of the reworked nannofossils have their origin in deepwater areas of the Gulf of Cadiz. The good preservation of most of the reworked Cretaceous nannofossils suggest that, shortly after resuspension, swift transport occurred from the source area to the coastal region of the Guadiana’s river mouth, implicating a northward change in the deep Mediterranean water direction within the Gulf of Cadiz, which reaches the upper southwest Iberia slope. The Mediterranean Outflow is, therefore, the major mechanism accountable for transporting Upper Cretaceous material to the southwest Iberian margin and the Atlantic inflow, together with longitudinal coastal currents, is the probable responsible for carrying and spreading eastward these reworked silty-clay sized particles over the Portuguese and Spanish continental shelf. This case study is a clear example of the potential importance of calcareous nannofossils as ocean dynamic tracers for particles within the silty-clay fraction.