Controls on sedimentation in submarine canyons: Nazare, Setubal and Cascais canyons, West Iberian Margin

Abstract

This thesis presents one of the most comprehensive studies on submarine canyons yet. It
integrates data on the geology, geochemistry, sedimentology and oceanography of the
Nazaré, Setúbal and Cascais canyons, west Iberian margin, in order to constrain the
processes and controls of past and present sedimentation in this area. The results
indicate that, during the glacial stages, turbidity currents are the dominant process of
sediment transport, erosion and deposition in these canyons. Turbidity currents are
mostly in the form of small-volume, high-frequency events that are generated by fluvial
and hydrodynamic processes, and the flows remain mainly within the upper canyon. A
smaller proportion of turbidity currents are large-volume, low-frequency, seismictriggered
events that flush through the entire canyons. Catastrophic mass wasting is
most prominent in Setúbal Canyon due to its closer proximity to the region’s active
fault zone along the southern Iberian margin.
During the Holocene, sedimentary activity in Setúbal and Cascais canyons continued in
the form of frequent, canyon-flushing turbidity currents that ceased abruptly ca 6.4 ka.
This interval corresponds to both a regional aridification event that affected the
Mediterranean and northern African regions, and to a decline in rising sea level. A
combination of the two events is interpreted as being the direct cause of the cessation in
sedimentary activity on the continental margin. In Nazaré Canyon, sedimentary activity
during the last ~1000 years has been dominated by a mid-canyon resuspension
depocentre that is controlled by small-volume, low-energy, hydrodynamic-generated
turbidity currents. This depocentre provides a unique high-resolution record of recent
sedimentation in a deep-sea setting, preserving a potential forest fire debris signal that is
linked to a change in climate during the Little Ice Age.
The conclusions from this work are that sedimentation in the west Iberian canyons is
controlled by the complex interplay between several variables, the most important ones
being the source and supply of sediment, the hydrodynamic conditions on the shelf and
slope, and the canyon morphology. The sedimentary activity in the river-fed Setúbal
and Cascais canyons is found to be affected more by regional climatic changes than by
eustatic sea-level changes, and in Nazaré Canyon by the supply of sediment along the
shelf.

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