Margin Shape Response to Changes in Submarine Turbiditic Systems Along the NE Iberian Margin, NW Mediterranean Basin

Submarine canyons can efficiently drain continental margins just as river systems drain subaerial catchments. Highly detailed seafloor topography acquired by state-of-the-art multibeam echosounders provides the opportunity to compare submarine and subaerial morphologies at a similar resolution. New bathymetric data show geomorphic similarities between fluvial and turbiditic systems, including: a) long-profile channel concavity, b) tributary branching, c) tributary confluences at converging elevations, and d) knickpoints. Recent models explaining these features are formulated using morphodynamic and hydrologic relations developed to study subaerial systems. Here we apply these models using multibeam, seismic reflection, sidescan sonar and core data from the NE Iberian margin (Catalano-Balearic Sea, NW Mediterranean). The margin is drained by the Valencia deep-sea channel, which collects sediment from canyon-channel systems and unconfined mass-wasting events along most of its length before debouching onto the Valencia Fan. The Valencia Channel has been active since the late Miocene, evolving in response to Plio-Quaternary episodes of erosion and deposition. Seismic records from beneath the modern Valencia Channel show that mass-failure deposits have periodically filled the channel through this period. Observations from the modern seafloor suggest that the most recent major failure, dated at 11,500 yr B.P., perturbed the Valencia Channel long profile and may have caused readjustment of the entire submarine drainage network. The modern Valencia Channel shows no major discontinuities at junctions with canyon-channel tributaries, indicating that all the channels have been active with similar time-averaged erosion rates at these locations (analogous to Playfair's Law for fluvial systems). However, several tributaries contain knickpoints and evidence for retrogradation of canyon heads. These tributaries join the Valencia Channel downstream from a major change in its long-profile concavity that coincides with the terminus of the 11,500 yr B.P. event deposit. Fragmentation of the canyon-channel network by the failure with a consequent reduction in drainage area may have triggered re-excavation of the Valencia Channel, incision of canyon heads, and upstream migration of knickpoints. Alternatively, the features may be relict, reflecting adjustment to earlier failures observed in seismic data. Detailed morphometric data allows us to assess the efficiency of long-profile adjustment from erosion by turbidity currents and the degree to which the modern submarine drainage network has been modified by the recent failure. The analysis is a first step towards quantifying long-term erosion rates in a submarine channel system that acts as the primary sediment pathway on the `sink' side of the NE Iberian margin.