Turbidites in the Damara Orogen

Abstract

Three pre-orogenic turbidite sequences are developed in the north-east-trending branch of the Damara Orogen. They are all related to rift structures of the early Damaran “graben stage”.

The Chausib turbidite sequence (Chausib Member), 1500 m thick, is situated near the southern margin of the Damara Orogen. The turbidites are quartzitic in composition and are intercalated in biotite and graphite schist. They are derived from and partly interfinger with the Hakos Quartzite of the Hakos Formation.

The Tinkas turbidite sequence (Tinkas Member), 4000–5000 m thick, is exposed in a narrow zone, 300–350 km long, in the centre of the Damara Orogen. The calcareous (calc-silicate) Tinkas turbidites are genetically related to the northerly adjoining dolomites and limestones of the Karibib Formation and are intercalated in Kuiseb schist type metapelites and siltstones. The Tinkas Member grades upwards into and is partly coeval with the Kuiseb Formation. The spatial relationship between the source area and the basin, and the southward gradation of the Tinkas Member into the Kuiseb Formation suggest the existence of an extensive basin (“Khomas Trough”) towards the south. The Chausib turbidites in the southern Damara Orogen may have been deposited at the southern margin of this basin.

The Okonguarri turbidite sequence (Okonguarri Formation) occurs in the northern Damara Orogen and may be up to 4900 m thick. Two types of turbidites are developed: Calcareous turbidites which are clearly related to stromatolithic reefs and reef detritus of mainly the Okotjize Formation; and siliceous turbidites which are probably derived from Nosib type deposits. The calcareous turbidites have been transported episodically from a northern slope into the Okonguarri basin and are concentrated in nine “dolomite zones”. The siliceous turbidites are derived from the east and have been transported along the axis of the basin.

Geodynamically, the development of the rapidly subsiding turbidite depositories mark the change from a mature graben or continental rift environment to the initial stage of a more general subsidence.