Litho- and chronostratigraphy of the Upper Quaternary sediments from the Piedra Buena Terrace, Patagonian Continental Margin (SW Atlantic)

Abstract

This paper presents a detailed description of the lithostratigraphy of mostly contouritic deposits from the Piedra Buena Terrace (PBT), collected at water depth of 2–2.5 km. It is based upon a multi-proxy investigation of five gravity cores, including visual description and scanning, grain size distributions, microfossils and sand fraction study, supported by the analysis of high-resolution sub-bottom profiles. Two older seismic units were identified in addition to those previously described in the area. The defined lithostratigraphic units were correlated to the upper seismic units, while the age of the younger units was confirmed using AMS-14C dating. In terms of the sedimentary facies, the dark quartz-glauconitic sands, which seem to be a fingerprint of the Piedra Buena Terrace, were investigated by applying the X-ray diffraction and performing grain SEM - SE images. The sandy facies correspond to sea-level low-stands during cold glacials (MIS 2 and MIS 4), when glauconite and terrigenous material were transported from the emerged shelf down and along the Patagonian continental slope by vigorous bottom currents. The latter is confirmed by an occurrence of parts of bi-gradational sequences reported from the grain-size distributions in lithostratigraphic units 2 and 4. Units 1 and 3, represented by mostly hemipelagic calcareous ooze, were formed during warm high sea-level stages (MIS 1 and 3). Unlike Unit 1, Unit 3 also demonstrates clear contourite features, similar to Units 2 and 4. They include common bioturbation, occurrence of mottles, lenses, irregular thin layers of sand, erosional boundaries and hiatuses between and within the units, good sorting of fine sands, and microfossil reworking. Together with our previous data from canyons on the Perito Moreno Terrace and the Ameghino segment, supported by available publications from the PBT and Mar del Plata areas, this study demonstrates that the Patagonian Continental Margin could be used as a natural laboratory to investigate and better understand contourite systems.