Neogene evolution of the margin adjacent to the La Plata River Delta: Sedimentary pathways, clinoforms and the origins of the Rio Grande Cone

The Neogene evolution of the Pelotas Basin, located off the southern coast of Brazil and Uruguay, presents an intriguing case of high terrigenous sedimentation in an area without major river systems. This unusual sedimentation is exemplified by the Rio Grande Cone, one of the largest submarine fan-like feature on Earth. While most continental margins with high terrigenous input are associated with large deltas and rivers, the Pelotas Basin defies this pattern, making its sedimentary pathways enigmatic. To understand the region’s sedimentary history, we analyze 13 exploratory wells and 700 seismic lines to perform seismic-stratigraphic and clinoform analysis. Our findings reveal three distinct depositional environments: (1) on the shelf, upper Miocene to Pliocene fluvial channels delivered sand onto a mud-dominated shelf; (2) on the slope, sediment instability led to structural deformation and several phases of mass transport deposition; (3) on the slope and abyssal plain, large contourite drifts formed due to the reworking of sediments by bottom currents. Clinoform analysis shows that deltaic environments existed on the inner platform during the Neogene, with three separate shelf-slope sedimentary pathways. However, the limited extent of these incised valleys suggests that additional sedimentary pathways may have contributed to sediment transport into the basin. We propose that the desiccation of an epicontinental sea over the La Plata Basin during the Miocene played a key role in enabling the influx of large volumes of fine sediments into the region. The drying of this sea likely allowed for the transport of sediments via the La Plata plume, which carried sediment-laden water into the margin. Additionally, the intensification of ocean currents during the middle Miocene contributed to the formation of contourite drifts and submarine megaslides, such as the Rio Grande Cone. In summary, the Neogene evolution of the Pelotas Basin was driven by a combination of factors: anomalous fine sediment input, sea-level changes, slope instability, and the intensification of bottom currents. These processes led to the creation of submarine megaslides and widespread contourite drifts, providing new insights into the complex evolution of the SW Atlantic margin.