A reappraisal of the Namibian Damara stratigraphy in part of the Southern Swakop Terrane and its implications to basin evolution

Abstract

The basin evolution of the Panafrican belts in Namibia was controlled by major rift systems comprising the Kaoko-West Congolian and Damara Branches with their associated parallel rift zones. Rifting along the Kaoko Branch caused continental spreading, while the Damara Branch formed the terrestrial Otjiwarongo and Windhoek Rift Zones, separated by the Usakos Horst, in which the rift related coarse clastics (Etusis Formation) and associated basinal sediments (Khan Formation) were deposited. Thermal subsidence along the Otjiwarongo Rift Zone produced an oceanic basin with thinned continental crust. In the Windhoek Rift Zone, terrestrial conditions prevailed and the newly deposited rift sequences were subjected to erosion. Succeeding the primary rifting was the Chuos Formation glacial/marine event, which produced thick deposits along the northern and western margins of the Windhoek Rift Zone. After the glaciation, a shelf carbonate unit was deposited in the slow subsiding Otjiwarongo Rift Zone. In the Windhoek Rift Zone, however, northeast trending faulting along the Okahandja Lineament produced the submerged Okawakuejawa rift unit in which the Adler dolomite and Quelle calcareous turbidites were deposited. The Okawakuejawa rift unit also divided the Windhoek Rift Zone into a northern Usakos Horst and southern Uitdraai Block. Renewed tectonism and uplift produced rifts splaying from the Okahandja Lineament northward to trend parallel to the Abbabis-Welwitchia Lineament. On the Usakos Horst, basaltic volcanism was associated with the rifting, while those on the Uitdraai block was more fissure like, non fault-related extrusions. The graben floors were submerged while the scarp shoulders were still areal as water laid and arid, non-reworked successions were deposited on them respectively. Further south in the Windhoek Rift Zone, continental separation caused the cessation of the Omusema volcanism and thermal subsidence of the entire Windhoek Rift Zone with dolomite platforms being formed on the Usakos Horst and Uitdraai block terranes. Beneath the platform edges, continuing subsidence created peri-platforms and below the Usakos Horst influx of clastic material produced the Tinkas Formation comprising the alternating dolomite and pelite sequence of the Kudulecke Member and dolomite/pelite turbiditic flows and slump deposits, in incised canyons, of the Jacobs Member. On the Uitdraai block, the Flachen quartzites were deposited onto the platform dolomite, while the Lievental Member dolomitic turbidites were deposited on the slopes. Continuing subsidence caused the north and eastward transgression of the pelitic turbidites of the Fahlwater Formation, which eventually covered the entire Windhoek Rift Zone area. Rifting of a Kibaran-aged continent produced a spreading Kaoko Branch with a subsidiary splayed Damara Branch in which the Rift Zones developed at different rates. The northern Otjiwarongo Rift Zone produced a submarine basin underlain by continental crust, while the southerly Windhoek Rift Zone stayed terrestrial. In the Windhoek Rift Zone, rejuvenation along the Okahandja Lineament lead to renewed rifting ending in continental spreading and thermal subsidence forming a passive continental margin.