THE NAMA FORELAND BASIN - SEDIMENTATION, MAJOR UNCONFORMITY BOUNDED SEQUENCES AND MULTISIDED ACTIVE MARGIN ADVANCE

The Nama foreland basin developed in Neoproterozoic times as a periphe ral foreland basin between the Kalahari Craton in the east and the par tly contemporaneous Damara and Gariep orogenic belts in the north and west. The foreland basin is subdivided into three basins, from north t o south the Zaris, Witputs, and Vanrhynsdorp basins. Deposits of the N ama Group proper fill the first two basins, whereas the third contains the Vanrhynsdorp Group which differs in many aspects from the Nama Gr oup. Only during its latest phase of development did the foreland basi n form one continuous depository. The stratigraphic sequences in all t hree basins are suggested to be genetically related to orogenesis, fle xural bulging and sea-level changes that affected the basin throughout its history. Palaeocurrent studies show that elastic sediments were i nitially derived from the craton and transported west and southwestwar ds in fluvial environments along the entire length of the basin. As mo untain belts emerged along the interactive northern and western orogen ic margins, transport directions switched to southerly and southeaster ly. Foredeep flysch and ramp carbonates grade upwards into reddish con glomeratic molasse derived entirely from these orogens. Three major un conformity 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 s equence (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 compose d of transgressive and highstand sequence sets onlapping the craton an d truncated by a ''Type 1'' unconformity at the base of the Nomtsas Fo rmation. CS2 contains a conformable lowstand flysch delta partly deriv ed from CS1 deposits eroded on flexural bulges as well as an orogenic source in the northwest. CS3 is entirely composed of orogen-derived mo lasse onlapping a basal ''Type 2'' unconformity and overstepping older sequences in a cratonward direction. A number of intrabasinal unconfo rmities is ascribed to flexural bulging in response to tectonic and se dimentary loading. The age of the Nama and Vanrhynsdorp groups is esti mated at between 600 Ma and 530 Ma as determined by the age of the bas ement, 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 ma rgins. Marginal tectonogenesis of the Nama foreland basin resulted fro m 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 th e proto-Atlantic (Adamastor) Ocean from north to south.