A Late Cretaceous mixed (turbidite-contourite) system along the Uruguayan Margin: Sedimentary and palaeoceanographic implications

The Polonio Drift System (PDS), occupies the middle to lower-slope of the Uruguayan Margin. The PDS is characterised by four channel-drifts, which propagate obliquely (120-170°) from the palaeoslope and exhibit over 800 m of local relief relative to the surrounding seafloor. Seismic interpretation identified four seismic units (SU.1-SU.4), separated by five major discontinuities (R.1-R.5). These record the onset (Albian-upper Turonian, 113-89.8 Ma), growth (Coniacian-upper Maastrichtian, 89.8-66 Ma), and burial (Paleocene, 66-56 Ma) phases for the PDS. During the onset stage, down-slope processes dominate but, three major shifts in the sedimentary stacking pattern occur. These coincide with palaeoceanographic changes in the South Atlantic during the Cenomanian-Turonian (100.5-89.9 Ma), Campanian-Maastrichtian (83.6-66 Ma) and Paleocene (66-56 Ma), following the opening of the Falkland/Malvinas-Agulhas Seaway (FAS), the subsidence of the Walvis-Ridge and the opening of the Vema channel. Bottom current influence first occurs during the growth stage (Coniacian-Campanian) when mounded deposits shift from aggradational and slightly progradational towards the N (SU.2), to clear lateral migration towards the S (SU.3). This sedimentary pattern indicates a regional palaeoceanographic shift in bottom current flow from northerly, to a subsequent southerly direction. Bottom currents do not appear to have actively eroded sediment, although they apparently had the potential to pirate and entrain fine-grained material from turbidity plumes, which build on underlying bathymetric features such as basement horsts and channel-levees. These produce large mounded morphologies adjacent to channels. Drift growth ceases following the termination of shelf progradation (R.4), enabling sluggish bottom currents contributed to a homogeneous sedimentary drape. Bottom currents intensify in the beginning of the Paleocene, forming contourite depositional systems with well-developed contourite terrace along the middle slope. Further research can elucidate the nature and mechanics of turbidite-contourites interactions along continental margins. This work offers new insights for the implications and consequences of the interaction of turbidites and contourites, which can improve the characterisation of deep-water deposits and understanding palaeoceanographic interpretations.