Abstract

Dish-shaped lenses of dolomitic limestone commonly occur in the arenaceous Lower Tertiary and Quaternary eolianites of the Central Namib Desert region. They are interpreted as the deposits of vegetated playa lakes and, as such, have been described as "fossil oases." Regional stratigraphic studies have shown that there were two separate wet episodes during which these playa lakes formed. These are the Zebra Pan Carbonates of the Late Eocene-Early Miocene Tsondab Sandstone Formation and the Khommabes Carbonates in the partially consolidated Late Pliocene-Recent Sossus Sand Formation. In plan view, the dolomitic limestone beds have a roughly concentric distribution of lithofacies and trace fossils that reflect the water-table depth and the interaction of the vegetated lake margins with the surrounding dunes. The facies change laterally from outer arenitic dolocrete with branching unlined cylindrical Planolites burrows, some with indistinct meniscae (possibly Rutichnus) and ovoid Termitichnus nest structures to massive carbonate with desiccation, syneresis, and tepee structures containing abundant Taenidium burrows and reed-stem casts. The central facies comprises microlaminated carbonate with stromatolite mounds and algal matted surfaces with no burrowing infauna. The controlling factor over the stratigraphic distribution of paleo-playas is climate. Both intervals are linked to "wet desert" phases when the climate changed from an arid to relatively wet fluvial phase with perennial rivers and seasonal floods, through to semi-arid, with a more flashy fluvial discharge, and eventually returning to arid conditions. The spatial distribution of these assemblages is controlled by interdune topography and river-flood discharge in the case of the Zebra Pan-type end-point playas, and the elevation of groundwater table by bedrock highs in the case of Khommabes-type oases.

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