Glacigenic contouritic sediments at the Argentine Continental Margin in deepwater channels and pockmarks

MARIA ELENA CERREDO; TILMANN SCHWENK; ALEJANDRO TASSONE; MARCELA REMESAL; , DANIELA SPOLTORE; ROBERTO VIOLANTE; GRAZIELLA BOZZANO; TILL HANEBUTH; JOSE ISOLA

Roma

Encuentro; 34th IAS Meeting of Sedimentology; 2019

Asociación internacional de sedimentologia

The Argentine Continental Margin (ACM) is swept by along-slope bottom currents that originate in the Antarctic region and flow from south to north forming a complex Contourite Deposition System (CDS), originally described by Hernandez-Molina et al. (2009). The margin is also incised by five Submarine Canyon Systems (SCS) that interact with the CDS at different temporal and spatial scales. Two sectors of the ACM are the focus of the present contribution: the Mar del Plata (MdP) Canyon area (~38° S) and the Patagonia SCS (~47° S). During the SO260 cruise, performed in January 2018 on board the German R/V Sonne, three secondary channels of the MdP Canyon have been mapped in detail and sampled with box corers and dredges. They rout the middle slope at water depths that range from 600 to 1100 m; two of them have an along-slope S-N orientation, whereas the third one has a SE-NW direction, oblique to the slope. Coarse sediments and rock fragments, including pebbles and cobbles, floor these channels and part of the canyon head area. Petrographic and morphologic analyses performed on these rocks allow us to interpret them as dropstones, as they show large diversity in lithology with a composition and morphology that recalls glacial erosion. In the MdP Canyon area, these dropstones seem to represent contouritic lag deposits accumulated over the Quaternary within channels that are swept by bottom currents strong enough to carry the finest fraction away. The other sector of the ACM, the Patagonia SCS, has been surveyed during the Geo2 cruise, performed in November 2017 on board the Argentinean R/V Austral; here, 20 gravity cores have been retrieved from the upper to the lower slope (700-3400 m water depth). This study area is characterized by a complex seafloor morphology due to the presence of seven blind canyons, with their multiple branches and gullies, contouritic terraces, scours and fluid escape features such as pockmarks (Lastras et al., 2011). In the sediment cores open so far from the continental slope of the Patagonian SCS, dropstones have been regularly found in the sediment archives, in locations that are not specifically related to deepwater channels. However, in 2 sediment cores retrieved close or within the pockmarks at 2500-2600 m water depth, erratic pebbles were found to be abundant and associated with medium sand as well as gravel. The working hypothesis is that the interaction between bottom currents and the irregular sea floor morphology of the pockmarks play a key role in the acceleration of the flow, able to erode and transport the very coarse fraction of the sediment. Future efforts will be addressed to the understanding of the glacigenic contouritic facies formation and evolution in the ACM, their genetic link with other gravity-driven processes and their potential use for the estimation of bottom current velocity.