Sedimentology and structural geology of the Gariep belt in Southern Namibia.

Abstract

The Gariep Belt is an arcuate north-south trending tectonic unit extending ;along the western coast of southern Namibia and northwestern South Africa. It forms part of the Pan-African Damara Oragert, which consists of a north-south trending coastal branch (Kaoko and Gariep Belts) and an east-west trending inland branch. The Gariep Belt is subdivided into an eastern parautochthonous passive continental margin on the western edge of the Kalahari Craton, the Port Nolloth Zone, and a western allochthonous ophiolitic terrane, the Marmora Superterrane, thrusted on the Port Nolloth Zone. The sedimentary evolution of the Gariep Belt was initiated with the deposition of sediments of the Rosh Pinah Formation into fluvial, alluvial and lacustrine depositional systems, which are associated with bimodal volcanic activity and related Zn-Pb-Cu-Ag- (+Ba) sulphide mineralization. The overlying mixed continental/shallow marine deposits of the Gumchavib Formation are overlain by the carbonate dominated Pickelhaube Formation, which was deposited in shallow marine to pelagic depositional settings. The Rosh Pinah, Gumchavib and lower Pickelhaube Formations are intruded by basic volcanic sills of the "Gannakouriep dyke swarm", The conformably overlying Obib Peak Formation was deposited into a fluvial/alluvial palaeoenvlronrnent, The Numees Formation unconformably overlies the underlying stratigraphic units and consists predominantly of glaciomarine massive diamictites and occasionally interbedded iron formations, which are occasionally interbedded with interglacial sediments and volcanics. The regional structural pattern is characterized by three phases of deformation. The earliest deformational event, D1, comprises intrafolial small scale recumbent and isoclinal F1 folds, a penetratlve beddlnq-subparatel 81 cleavage and a preferred elongation of boulders, pebbles, grains and minerals (11). Associated with D1 are bedding-subparallel thrust faults. The D2 deformational phase is characterized by small to large scale, north to northwest trending F2 folds with a generally easterly vergence, which can change into a westerly direction due to backfolding. The F2 folds are associated with a penetrative axial planar S2 cleavage. Thrusting continued during the D2 deformational phase. The latest deformational event, D3, is characterized by small to large scale open F3 folds with southerly to south-westerly trending fold axes. The metamorphic evolution of the Gariep Belt during the Damara Orogeny was associated with barrovtan-type metamorphism with a geothermal gradient of about 20oC/km, reaching greenschist to lower amphibolite facies metamorphism. The sedimentological and structural features are explained in a tectono-sedimentery model, which started with rift initiation during the deposition of Rosh Plnah Formation lithologies along old tectonic weakness zones of Middle Proterozoic age at about 780Ma. Supercontinenta! breakup is documented by the initial transgression of Gumchavib Formation sediments and the continuous trensgression of the Pickelhaube Formation, which is related to thermal subsidence after the initiation of oceanic spreading of the Adamastor Ocean. Renewed rifting and/or glacial outwash heralding' the advance of the Numees glaciation and associated seale': :'changes may be responsible for the sudden break in marine sedimentation during the deposition of the Ohio Peak Formation. The Numees glaciation is probably contemporaneous with the 670Ma old Varangian glacial and the Rapitan glacial episodes and provides a minimum age for the deposition of Gariep Group lithologies. Rifting and spreading was followed by the closure of the Adamastor Ocean, associated with. a southeastward directed subduction. of oceanic crust under the Kalahari Craton and was accompanied by intense southeastward and subsequently eastward directed tectonic transport during 01 and D2. During D3, a late Sinistral movement developed along tre defined fault zones in the late stages of the Adamastor Orogeny.Metamorphic ages from the Nama and Vanrhynsdorp Groups indicate that that the tectonic evolution of the Gariep Belt only ceased at about 500Ma.