SEDIMENT TRANSPORT PROCESSES AT THE ARGENTINE CONTINENTAL MARGIN SINCE THE OLIGOCENE

Sedimentary processes on the Argentine margin are largely controlled by the interaction of the seafloor with northward flowing Antarctic water masses (Antarctic Intermediate Water, AAIW; Circumpolar Deep Water, CDW; Antarctic Bottom Water, AABW) and southward flowing North Atlantic Deep Water (NADW).

A complex contourite drift system on the southern Argentine slope and rise consists of two major sectors: A submarine canyons and channels sector in the north mainly shaped by down-slope processes and an escarpment and terraces sector in the south dominated by along-slope sediment deposition and erosion. Based on regional correlation of seismic reflection profiles a seismostratigraphic model for the sedimentary development of the area is developed. Seven seismic units from regional reflector AR4 (~34 Ma) upward are identified and mapped. The shape and location of depocentres provide indications for major transport and depositional processes.

From ~34 to 17 Ma sediments in both sectors were mainly deposited on the continental slope and formed the Piedra Buena Terrace (2000-3000m water depth today). The depocenter stretching along the continental slope and narrowing to the North indicates that sediment was likely delivered by a contour following northward directed current that was flowing in the depth range of todays Lower Circumpolar Deepwater (LCDW). Seaward of the depocenter sedimentation was moderate (canyons sector) to very low (terraces sector) indicating non-deposition or erosion over quite long time intervals.

A major change in deposition after 17 Ma is indicated by generally decreased deposition in the canyons sector and eastward shifted depocenters in the terraces sector which can be explained by a change in AABW circulation that was more confined to the western flank of the Valentine Feilberg Terrace. Changes in depositional style during growth of this terrace reflect a sequence of flow variations: 1. Low bottom current activity during the Mid-Miocene Climatic Optimum (~17-14 Ma). 2. A current reorganization through the emergence of LCDW ocuured at ~14-12 Ma. 3. Moderate flow speed of LCDW and AABW from ~12-6 Ma. 4. Strengthening of bottom flow after 6 Ma due to a general change in deep water mass organization following the closure of the Panamanian gateway.