2.17-2.07 GA EOLIANITES AND EVAPORITIC CONTINENTAL PLAYA SEDIMENTARY ROCKS FROM THE DEWERAS GROUP, ZIMBABWE: THE WORLD’S OLDEST DESERT?

The Deweras Group, the basal part of the Paleoproterozoic Magondi Supergroup, unconformably overlies the Neoarchean granite-greenstone terrain of the Zimbabwe Craton. It comprises a red bed sequence, up to 1.3 km thick, of meta-arenites, rudites, pelites and minor dolostones and sulfate evaporites, together with subalkaline basaltic lavas, pyroclastic rocks, and sills (Master et al., 2010, Prec. Res.). Detrital zircon U-Pb dating gives a maximum age of 2171 ± 11 Ma for the basal Deweras sedimentary rocks, which are overlain by ca. 2070 Ma Lomagundi Group rocks (Glynn et al., 2012, IMSG Meeting Abstr., Johannesburg). The Deweras Group was deposited in rift-related continental alluvial fan, braided stream, playa flat, and playa lake environments.

We have identified eolian facies in the Deweras Group arkosic arenites, in three localities: (1) underground exposures at the Shackleton/Avondale Cu mines, situated at 30°02.0’E, 17°18.5’S; (2) outcrops along the Angwa River between Alaska Siding and Alaska Mine on Belltrees Farm at 30°03.0’E, 17°23.2’S; and (3) in a vertical borehole MJZM/5 on Inyati Farm at 30°11.103’E, 19°04.360’S.

The eolianites (having respective thicknesses of ~5, ~30 and 49 m) overlie fluvial facies, and are overlain by, or interbedded with, playa flat/lacustrine facies (with desiccated shales deposited in ephemeral playa lakes in interdunal pan environments, and rippled clastic anhydrite beds reworked on the shoreline) in the first two localities, and by a mafic volcanic succession in the borehole. The three localities are found over a region 33 km long and 5 km wide. These occurrences may be part of a continuous wedge-shaped body of sandstone, containing about 3.3 km³ of eolianite, representing a small sand sea.

The eolianites are recognised based on the presence of: (1) medium-grained arkosic sandstones, with well sorted, well rounded grains; (2) large-scale planar crossbeds, locally with tangential bottomsets; (3) inversely graded cm-scale wind ripple deposits (subcritically climbing translatent strata) with local cross laminations; (4) wedge-shaped grainflows; (5) pinstripe laminations, and (6) Stokes surfaces. The Deweras Group eolianites and associated evaporitic playa facies possibly represent the oldest recognised desert environment in the sedimentary rock record.