Sedimentary stacking pattern of plastered drifts: An example from the Cenozoic on the Uruguayan continental slope

Abstract

Plastered drifts are a complex type of contouritic drift, very common along continental slopes, although their precise sedimentary stacking pattern and long-term evolution are not well understood. In this work we used 3D and 2D multichannel reflection seismic and well datasets to characterize a Paleogene plastered drift along the Uruguayan continental margin. A large buried drift running parallel to the middle and lower slope was identified, comprising five main seismic units (SU1-SU5) and a number of subunits subdivided by internal widespread erosive discontinuities. An extensive contourite terrace is developed on the landward top of the drift, while smaller-scale bottom current features (channels and bedforms) denote a hierarchy of features related to water mass circulation and interfaces, as well as associated oceanographic processes. Four long-term evolutionary stages were decoded in the plastered drift formation: I) Onset Stage (66 Ma – 56 Ma), whose basal surface represents a prominent erosional surface marking the onset of drift, after which extensive sheeted deposits develop; II) Growth Stage (Eocene ~ 56 – ~38 Ma) with a prominent backstepping sedimentary stacking pattern; III) Maintained Stage (~38 Ma – ~20 Ma) of limited growth of the drift, characterised by aggradational sheeted deposits and extensive erosion; and IV) Burial Stage (<20 Ma), which determines a major change in the margin evolution —the main depocenter shifts to deeper domains, leading to the final burial of the drift. The plastered drift formation is attributed to the influence of a deeper and weak water mass and a shallower but more vigorous water mass, as well as their interface. The aforementioned evolutionary stages and the greatest changes in the drift depositional style would be a consequence of spatial and vertical changes in these water masses over millions of years, the Growth Stage being related to the expansion and intensification of deep-water circulation that modulated the formation of the proximal terrace at its top and resulted in the backstepping stacking pattern. The smaller lateral and vertical changes in the seismic units and subunits along the drift are linked to local bottom current processes and their interaction with the slope morphology, the slope gradient playing a key role in the lateral bottom current behavior. This study shows the complex lateral and temporal sedimentary stacking pattern and evolution of a contouritic drift, and decodes the dominant oceanographic and depositional processes in its long-term formation. In doing so, we demonstrate the requirement of extensive 2D and 3D seismic datasets for accurate characterisations. Still, similar research in other continental margins is needed to better understand how and when (in geological time) large contouritic drifts are generated, in light of their implications for basin analysis, paleoceanographic reconstructions, and energy geosciences.