Geochemical perspective on origins and consequences of heuweltjie formation in the southwestern Cape, South Africa

Heuweltjies are large (20 m diameter) raised mounds, spread throughout much of the South-West Cape region of South Africa. Many heuweltjies have a thick basal calcrete layer. Our analyses of this calcrete layer indicate at the landscape level it contains several tonnes per hectare of Ca, Mg, Na and K. These amounts are surprisingly large considering that the underlying Table Mountain Group sedimentary rocks contain low concentrations of these elements. The low present-day input rates of these elements from rainfall suggest that the calcrete must have formed under a different climate and depositional regime. We propose that some of these elements are derived from marine aerosol deposits. This is supported by the mean (super 87) Sr/ (super 86) Sr ratios obtained from the calcrete layer of 0.71303, which is much lower than that of the immediate underlying rocks (mean 0.74936). The delta (super 13) C values for calcretes (-4.5 to -8 per mil PDB ) suggest that Cape heuweltjies are sinks of atmospheric CO (sub 2) as the calcrete (super 13) C is different from local limestones and dolomites, which have delta (super 13) C values close to zero.