Abstract
M.Sc.
The Lower Devonian Bokkeveld Group is the Middle unit of the tripartite Cape Supergroup, which outcrops along the western, southern and eastern coastline of South Africa. A well-established sedimentary and stratigraphic understanding of the Bokkeveld Group allowed for geochemical and geochronological investigation in order to gain insight into the provenance characteristics, as well as the paleotectonic environment of the provenance areas. In order to observe any changes within the Bokkeveld Basin, complete profiles for geochemical investigation were sampled in the western, southern and eastern parts of the basin, and compared. Major and trace element patterns suggest that the western part of the basin received detrital input from felsic, magmatically evolved, and possibly alkaline sources, and that the sediment was highly recycled before deposition. Furthermore, the geochemistry suggests that the western part of the basin experienced “passive margin” type sedimentation. The geochemistry of the southern basin, in contrast, suggests input from less evolved, non-alkaline sources, and predicts sedimentation under “active margin” conditions for the lower part of the group. The eastern basin is geochemically intermediate between the western and southern basins. Zircon populations for the three parts of the basin further suggest that sources of different ages fed the three parts of the basin. The zircon population of the western basin suggests that the Namaqua Natal Belt (Mesoproterozoic) and Neoproterozoic cover successions were the major source of detritus, with only minor input from Paleozoic sources. The eastern basin also appears to have sourced mainly Namaquan aged material
as well as Neoproterozoic material, with no Paleozoic input. The southern basin has a remarkably different zircon population, with the majority of grains being Paleozoic in age, and only a few Neoproterozoic and Mesoproterozoic grains. Furthermore, many of the grains are younger than any known source-rocks on the Kalahari Craton, and thus allude to input from an extra-Kalahari source into the southern part of the basin. The youngest grain from the southern basin overlaps with the established depositional age of the Bokkeveld Group, suggesting some syn-depositional or briefly pre-depositional magmatic activity in the source area(s) of the southern basin, as predicted by the geochemistry. The complete lack of zircon ages older than the Namaqua Natal Belt (Mesoproterozoic), would suggest that the Archean to Paleoproterozoic inner part of the Kalahari Craton, the Kaapvaal Craton, was not sourced by the Bokkeveld Group. This is most likely due to the Namaqua Natal Belt having served as a large east-west trending morphological divide during Bokkeveld deposition, preventing transport of detritus from the craton interior. Remarkably, this would suggest that the Namaqua Natal Mountain Range must have survived erosion and persisted as a morphological boundary for ca. 600 Ma to serve as the major source of detritus for the Bokkeveld Group. Even an extensive, craton-fringing sedimentary cover-succession such as the Bokkeveld Group, may thus not provide a “detrital fingerprint” of the craton interior, and paleogeographical implications must be taken into consideration during provenance studies. Paleocurrent directions for the Bokkeveld Group indicate a west to east transport direction in the southern part of the basin, and as such, a western, extra-Kalahari source, most likely the Rio de La Plata Craton and surrounds,
is expected to have been the source of both the young Paleozoic zircons, as well as undifferentiated material as revealed in the geochemistry.