The sedimentary evolution of the Damara sequence in the lower Khan river valley, Namibia

Abstract

Abstract provided by author:

A sedimentological facies study has been undertaken on the Late Proterozoic Damara Sequence in the lower Khan River valley area in central Namibia. The sequence was highly deformed and metamorphosed up to upper amphibolite facies conditions during the Damaran orogeny at ca. 530 Ma. Standard sedimentological techniques were modified for analyzing these rock sequences, and a surprising amount of information is preserved and was recorded. This has enabled a reconstruction of the sedimentary evolution of the Damara Sequence in the study area

The quartzo-feldspathic gneisses of the Nosib Group were deposited mainly in a South Saskatchewan-type braided fluvial environment. Distal fluvial and flood-plain environments of these rivers accumulated calcareous, fine-grained sediment which produced the calc-silicate rocks of the Khan Formation. A subsequenct marine transgression led to the deposition of carbonate on a ramp in an epicontinental platform setting. The ramp was subject to abundant siliciclastic influxes which interfered with carbonate production, giving rise to the varied lithologies of the Rossing Formation. A major erosional unconformity was then developed on top of which the Chuos Formation was deposited. The Chuos diamictites are reworked glaciomarine rocks, mainly deposited by glacial debris (mud) flows. Cessation of glaciation led to resumption of carbonate deposition (Karibib Formation) in a shelf setting in which a shelf edge can be identified. This was subsequently suppressed by a major fine-grained siliciclastic influx (Kuiseb Formation)

Recognition of unconformity-bounded sequences (UBS) has enabled a syn-sedimentary tectonic reconstruction of Damaran evolution in the Central Zone. UBS 1 was deposited during the rift stage of the both branches of Damara Orogen. UBS 2 records the transgression associated with sea-floor spreading in the Adamastor Ocean (coastal branch) and the thermal subsidence which followed. UBS 3 was deposited on the newly-formed northern passive continental margin of the Khomas Sea following break-up and initiation of sea-floor spreading in the inland branch. Integration of data from the other zones has enabled a revised asymmetric double-detachment model for the early rift structures of the inland branch of the Damara Orogen to be proposed, in contrast to the previous symmetric, pure shear models