Deep basin record evolution of chemical weathering and physical erosion as response to the tectonic uplift of the South African Plateau during the upper Cretaceous

During the upper Cretaceous the South African Plateau underwent an uneven uplift period that coincided with the onset of a long-term climate cooling. This uplift recorded two pulses, an early pulse during the Turonian that affected the eastern margin and a later pulse occurred during the Campanian affecting the western margin (van der Beek et al., 2002; Braun et al., 2014; Baby et al., 2020). We aim here to determine the response of physical erosion and chemical weathering to this tectonic event, that may have played a role in the long-term climate cooling by promoting silicate weathering and associated atmospheric CO₂ drawdown. In this study we targeted the IODP 361 site located in the Cape Basin. We applied a new proxy (De_Hf) of chemical weathering intensity based on the coupled Lu-Hf and Sm-Nd isotopic systems in clay fractions of the sediments. This approach is combined with X-ray diffraction analyses to determine the concomitant evolution of clay mineralogical assemblages, which in active tectonic settings can provide insights on the evolution of mechanical erosion.

Twenty-two samples were analyzed for Hf-Nd isotopic compositions and 99 samples were analyzed for X-ray diffraction analyses. Clay e_Nd values remain relatively stable throughout the studied interval, mainly within -8 to -9 e-units, but down to -10 e-units in the Maastrichtian, suggesting a relatively stable source of the sediments. These values are coherent with a sediment source from the Karoo basin and/or the Cape Belt (Dia, 1990; Garzanti et al., 2014). Clay mineral assemblages are dominated by smectite (about 65 to 98%) but show an increase in illite proportions during the Campanian-Maastrichtian, up to about 25%. This increase in primary clay minerals may point to an increase in mechanical erosion, that could be linked to the uplift pulse affecting the western south African margin at that time. De_Hf values represent the departure of e_Hf from the clay array (Bayon et al., 2016), and display a marked increase during the Campanian-Maastrichtian as well, suggesting concomitant enhanced weathering of the nearby margin at that time.

Altogether, our data suggest a coupled response of mechanical erosion and chemical weathering of the southwestern African margin during the upper Cretaceous tectonic uplift of the margin, which also may have played as an active CO₂ consumption mechanism.