Antecedent topography controls preservation of latest Pleistocene-Holocene transgression record and clinoform development: the case of the Sao Francisco delta (eastern Brazil)

摘要

The present study used shallow high-resolution seismic surveys to characterize the architecture and stratigraphic evolution of the last depositional sequence of the Sao Francisco Delta (SFDS). The sequence was accumulated within a bathymetric low (BL) on the shelf located in front of the Sao Francisco river. This antecedent topography provided additional accommodation of tens of meters in comparison with the rest of the shelf. The BL and its sediment-trapping effect allowed the accumulation of a complete stratigraphic record of the eustatic sea-level rise since the Last Glacial Maximum (LGM). The SFDS comprises six stratigraphic units. Because cores and radiocarbon dates are not available for this study area, the depth of occurrence of the top and base of each unit and their lateral relationships with wave-cut terraces carved into the BL walls were used in an effort to tentatively tie the mapped units to known eustatic sea-level curves available in the literature. Results suggest that major episodes of accumulation occurred during intervals of reduced rates in sea-level rise. The lower five units mapped accumulated in a large bay-estuarine protected environment, which resulted from the inundation of the BL. Only the topmost unit (the present clinoform) accumulated in an open ocean environment when sea level reached approximately its present position after 8 ka and the whole shelf was flooded. The most important delta backstepping event was probably associated with Meltwater Pulse 1B, which followed the Younger Dryas and produced one of the most significant stratal surfaces of the SFDS. Among the existing wave-dominated deltas in the Brazilian coast only, the Sao Francisco was able to construct a well-developed sigmoidal muddy clinoform in an otherwise very shallow shelf subjected to high wave-energy levels. This was only possible because of the existence of a bathymetric low in front of the river mouth, which illustrates the role that antecedent topography might pla