Depositional processes and growth patterns of isolated oceanic basins: the Protector and the Pirie basins ofthe Southern Scotia Sea (Antarctica)

Abstract

Sedimentary processes in small, isolated oceanic basins that form adjacent to continental margins but detached from continents remain poorly understood. This work describes two such basins located in the southern Scotia Sea, the Protector and Pirie basins. We analysed multichannel seismic profiles to interpret morphostructural features and stratigraphy of these basins. Sedimentary stacking patterns and depocentre distribution illustrate basin development patterns. Basal units infill basement depressions formed by the submerged banks of thinned continental crust that abut the basin plains. These lower and middle deposits of the sedimentary record are interpreted as pre- and syn-rift deposits. The laterally extensive upper deposits are interpreted as post-rift deposits. These include five discrete units evident in seismic profiles. A prominent regional reflection referred to as Reflector-c, separates in these upper deposits two sets of seismic units that have recorded major shifts in the dominant sedimentary processes, stacking patterns and paleo-environmental conditions. The most important processes controlling deposition of the older units (those beneath Reflector-c), include down-slope gravity processes that infill depressions created by crustal thinning and seafloor spreading. These occurred under the coeval influence of Circumpolar Deep Water circulation. The major processes influencing younger units (those above Reflector-c) include bottom water circulation of the Circumpolar Deep Water and Weddell Sea Deep Water water masses,which coursed along bathymetric contours of the seafloor. The Reflector-c discontinuity developed concurrently with middle Miocene tectonic changes, which led to the opening and deepening of deep gateways in the South Scotia Ridge. These facilitated overflow of Weddell Sea Deep Water from the Weddell Gyre into the Scotia Sea. This overflow in turn forced the Circumpolar Deep Water northwards. Analysis of the Protector and Pirie basins shows that their tectonic evolution influenced regional deepwater circulation patterns in an area that makes a significant coldwater contribution to the global conveyor belt system. As a long-termfactor controlling basin evolution and sedimentary processes, tectonics events in this region therefore influenced the present day climate system. These results further clarify our understanding of deep, isolated oceanic basins in terms their sedimentologic, climatologic and oceanographic significance.