Le plateau Sud-Tasman (Australie): collage de deux fragments du Gondwana oriental? (The South Tasman Rise (south of Tasmania): a collage of two fragments of eastern Gondwana?) : Rollet N., Royer J.-Y., Exon N.F. & Hill P.J., Comptes Rendus-Academie des Sciences, Serie II: Sciences de la Terre et des Planetes, 1996, 323/10 (865–872)

Abstract

The Nama foreland basin developed in Neoproterozoic times as a peripheral foreland basin between the Kalahari Craton in the east and the partly contemporaneous Damara and Gariep orogenic belts in the north and west. The foreland basin is subdivided into three basins, from north to south the Zaris, Witputs, and Vanrhynsdorp basins. Deposits of the Nama Group proper fill the first two basins, whereas the third contains the Vanrhynsdorp Group which differs in many aspects from the Nama Group. Only during its latest phase of development did the foreland basin form one continuous depository.The stratigraphic sequences in all three basins are suggested to be genetically related to orogenesis, flexural bulging and sea-level changes that affected the basin throughout its history. Palaeocurrent studies show that clastic sediments were initially derived from the craton and transported west and southwestwards in fluvial environments along the entire length of the basin. As mountain belts emerged along the interactive northern and western orogenic margins, transport directions switched to southerly and southeasterly. Foredeep flysch and ramp carbonates grade upwards into reddish conglomeratic molasse derived entirely from these orogens.Three major unconformity bounded sequences (composite sequences), probably of second-order cyclicity, can be recognised. The uppermost composite sequence (CS3) comprises the Fish River Subgroup, whereas the lower composite sequence (CS1) is composed of the Flaminkberg Formation and the Kwanous, Kuibis and lower and middle Schwarzrand subgroups. CS2 comprises the Knersvlakte, Brandkop and upper Schwarzrand subgroups. CS1 is composed of transgressive and highstand sequence sets onlapping the craton and truncated by a “Type 1” unconformity at the base of the Nomtsas Formation. CS2 contains a conformable lowstand flysch delta partly derived from CS1 deposits eroded on flexural bulges as well as an orogenic source in the northwest. CS3 is entirely composed of orogen-derived molasse onlapping a basal “Type 2” unconformity and overstepping older sequences in a cratonward direction. A number of intrabasinal unconformities is ascribed to flexural bulging in response to tectonic and sedimentary loading.The age of the Nama and Vanrhynsdorp groups is estimated at between 600 Ma and 530 Ma as determined by the age of the basement, the age of detrital micas and the age of subsequent thrust and fold tectonic events that vary in vergence from southeast to northeast, which affected Nama and Vanrhynsdorp rocks along the active basin margins. Marginal tectonogenesis of the Nama foreland basin resulted from northwesterly subduction of the Kalahari plate underneath the Congo and South American plates through a process of sinistral transpression along the western Kalahari margin leading to sequential closure of the proto-Atlantic (Adamastor) Ocean from north to south.