Provenance of the cape supergroup in the Cape Fold belt of South Africa — preliminary results
|Location||International Geological Congress,oslo 2008|
|Author||Zimmermann, Udo۱; Fourie, Pieter۲; Naidoo, Thanusha۲; Simonetti, Antonio۳|
|Holding Date||05 October 2008|
During the Lower Palaeozoic a thick siliciclastic sequence was deposited on the Kalahari craton, which comprises a number of sedimentary cycles. The Cape Supergroup is divided in three groups, where the lower one, the Table Mountain Group is characterized by mainly quartz-arenites and a Hirnantian Tillite; the overlying Bokkeveld Group by alternating quartz-arenites and shales and finally the uppermost Witteberg Group by similar lithotypes but with slightly more shaley components. The rocks were deformed at least during the Uppermost Palaeozoic or during the Mesozoic, which is controversively discussed. All three groups are subject of a systematic chemostratigraphy and detrital zircon dating as well as petrographic studies.
The Cape Supergroup surrounds the southern coast of South Africa - but not every formation is exposed in the entire depositional area - and is c. 100 km long and more than 8 km thick. Lithostratigraphy and petrography of the rocks is well understood (compiled in Thamm and Johnson, 2006). Sampling was performed in three profiles, in the western and eastern subbasins, and in areas in the central part where palaeocurrents are changing and point to a sediment transport from the southwest.
First detrital zircon data for the Table Mountain Group in the eastern basin displays a dominance of Mesoproterozoic and Upper Neoproterozoic zircons (Naidoo, 2008). This is in contrast to deposits of the Table Mountain Group in the west, where a wider range of ages could have been observed including Archaean grains, absent in the east.
This study will therefore give an excellent fingerprint of a typical Kalahari craton signature using detrital zircons from its western area and its eastern region.
During the Cambrian a larger rift event is proposed and a crustal fragment was separated from the Kalahari craton (Thamm and Johnson, 2006). This fragment might have been Patagonia and the Falkland/Malvinas Plateau (Naidoo, 2008), but more geochemical and isotope geochemical data are necessary. Ideally, exposures of sandstones and shales of the Bokkeveld Group in the central part of the basin received their detrital material from the southwest and might give insight in the composition of this crustal block, which then, still influenced the supracrustal record of the Kalahari craton.