Sedimentation and basin development of the Middle Proterozoic Doornpoort and Klein Aub Formations, Central Namibia

Abstract

The 1300-950 Ma old Sinclair Sequence at Klein Aub, central Namibia, is preserved between the Congo and Kalahari Cratons. It commenced with bimodal pyroclastic rocks and lava flows which partly filled a variety of graben-like basins. Siliciclastic and carbonate sedimentation dominated basin infill, developing an overall fining-upward sequence of sedimentary rocks. Deposition of conglomerates and compositionally immature arenites was initially controlled by fault activity but ultimately overstepped graben shoulders. Younger basin-wide sedimentation comprised quartz arenites and carbonaceous mudstones with subordinatelimestones deposited in association with waning fault activity. The coarse sandstones and conglomerates represent red-beds deposited on debris flow-dominated fans next to the graben fault scarps. Fan lenses are arranged in an en Cchelon fashion which suggest a strike-slip component to the marginal fault system. These fans passed axially into sandy braided stream systems and localised aeolian dunes and playa lakes. Evaporite pseudomorphs confirm arid climatic conditions. The finer clastic rocks and carbonates are organized in up to seven fining-upward cycles passing from subtidal quartz-arenites to finely interlaminated and flaser laminated pyritic intertidal limestones. This mixed siliciclastic/carbonate sequence developed in a series of prograding cycles averaging 4 m - 5 m thick, which could imply a similar maximum tidal range of macrotidal proportions. The basin developed in an extensional crustal regime with thermal uplift, followed by mechanical rifting, associated with volcanism and the deposition of coarse clastics. During the thermal subsidence phase a marine transgression gave rise to the tidal deposits of the later fine grained clastic sediments. Both tectonically and sedimentologically the Sinclair Sequence heralds the subsequent development of early Damara rifting.