Setting and sedimentary facies of late Proterozoic alkali lake (playa) deposits in the southern Damara belt of Namibia

The late Proterozoic Damara Belt of Namibia has evolved from an elaborate system of continental rifts in which the basal portion (Nosib Group) of the Damara Sequence was deposited. In the southern rift, situated at the southern margin of the Damara Belt, the Nosib Group is represented by coarse elastic sediments (Kamtsas Formation) and fine-grained partly dolomitic deposits (Duruchaus Formation). Both formations occur, with interfingering relationships over nearly the total length of the rift. In the Geelkop Dome area the pelitic-dolomitic Duruchaus Formation includes, in its upper part, a sequence of sediments that are characterized by cyclical deposition, high sodium contents, abundant albite pseudomorphs after primary evaporite minerals, and concordant solution and collapse breccias. This 300 m thick sequence has been interpreted as deposits of an alkali lake or playa complex. Based on the playa lake models of Eugster and Hardie (1975) and Rowlands et al. (1980) four distinct facies have been distinguished in the upper part of the evaporitic Duruchaus Formation: (1) the submergent lake facies, represented by laminated shales containing layers of laminated siltstone; (2) the submergent/emergent mud flat facies, represented by laminated siltstone and calcareous shale with layers of dolomitic mudstone, calcareous shaly siltstone with scapolite and laminated sandy albitic dolomite with abundant pseudomorphs of albite after primary evaporite minerals (e.g. shortite, thermonatrite, borax); (3) the exposed saline crust facies, characterized by solution breccias, "albitolite" and "albitolite" breccias, all originating from former salt crusts; and (4) the elastic marginal facies, represented by quartzites of fluviatile and partly aeolian origin. From identified primary and diagenetic to metamorphic minerals, and from the composition of several generations of fluid inclusions, it is concluded that the brines of the alkali lake were of a NaHCO ₃(KBClS) type. Material creating the extremely alkaline environment was derived from eroded acid-to-basic magmatic basement rocks and from coeval alkali-rich volcanism. During tectogenesis the evaporite sequence was overridden by nappes approaching from the closing Damara geosyncline. Saline crust horizons were thereby partly mobilized and the resulting mush of dolomite and crystal fragments was squeezed into the thrust planes of the nappes where they acted as lubricants which enhanced late-orogenic thrusting along the southern margin of the Damara Belt.