The Permo-Carboniferous glacial to marine Karoo record in Southern Namibia

Abstract

English abstract provided by author:

In this interglacial flourishing life is indicated by fossil remains of paleoniscoid fishes, the bivalve Eurydesma mytiloides and algal-serpulid build-ups up to 4 m high. The content of organic carbon and the type and maturity of the organic matter makes these mudstones slightly gas-prone, with a fair to good source rock quality

Postglacial deposition of the Ecca Group was initiated by the fluvially dominated delta complex of the Nossob Member. The formation of this delta complex, which comprises two major pro-gradational phases in a south to south-westerly direction, is directly connected with the enhanced sediment supply after the final melting of the Permo-Carboniferous ice sheet. After a period of mudstone deposition, the Mukorob Member, a second period of sandy sedimentation is recorded by the Auob Member. It represents the southward progradation of a wave-dominated shoreline into the basin. In this case three progradational phases are recorded. The top of the Permo-Carboniferous succession is then formed by thick marine mudstones of the Rietmond Member and the Whitehill Formation

Apart from the main outcrop area xenoliths of Karoo origin were found in kimberlite and carbonatite pipes of the Gibeon Kimberlite Field and at the Gross Brukkaros structure. All these have formed in the Cretaceous and are located in pre-Karoo sediments west of the study area at present. At Gross Brukkaros and in the surrounding carbonatite pipes xenoliths consist of Dwyka strata only, while in some kimberlite pipes fragments of the Auob Sandstone Member are preserved in addition. As all these fragments must have fallen into the open pipe from above during the eruption, Karoo strata must have extended much further westwards in the Cretaceous than today

Associated mostly with sediments of the lower Dwyka Group acidic tuff beds with a wide lateral distribution were discovered. Also a small percentage of the glacial clasts consists of undeformed basalts, trachytes and rhyolithes. All pre-Karoo magmatic rocks in the source area are deformed and tuffs have been described only from significantly younger strata in southern Africa. Thus new evidence for the early onset of volcanic activity in southern Namibia is presented here. Contemporaneous tectonism is recorded by type-1 unconformities and systematic thickness changes across E-W trending normal faults. This period of volcanic and tectonic activity in the late Carboniferous to early Permian may mark initial extensional events along the Namibian continental margin

Interpretation of the studied succession using sequence stratigraphic techniques revealed four third-order sequences. Several of these cycles can be successfully correlated with published global changes of relative sea-level in this time interval

The stratigraphic position of the tuff beds also shows a distinct association with transgressive developments. Most of them concentrate between the upper part of the transgressive systems tracts and the lower part of the subsequent highstand systems tracts. This correlation may not only result from the enhanced preservation potential during transgressive development, but rather from a genetical coupling between magmatism, extensional tectonics and basin subsidence. This interaction is significant, not only for the recent discussion of potential controls on relative sea-level, but also for the understanding of the early geodynamic evolution of the south Atlantic continental margin