The oldest Nama Group exposed: Insights from the Tsaus Mountains(Tsau Khaeb National Park)

The oldest Nama Group exposed: Insights from the Tsaus Mountains

(Tsau Khaeb National Park)

C.-I. Uahengo*¹, F.T.Bowyer², K.Kaputuaza¹, J.Ndeunyema¹, M.Yilales², R.Alexander², A.Curtis², R.Wood²

¹Department of Geosciences, University of Namibia, Namibia.

iuahengo@yahoo.com

²School of GeoSciences, University of Edinburgh, UK.

 

     The late Ediacaran Nama Group of southern Namibia and northwest South Africa preserves a richly fossiliferous mixed carbonate-siliclcastic shallow marine succession, subdivided into the Kuibis and overlying Schwarzrand subgroups. Whilst the termination of Schwarzrand Subgroup deposition (ca. 538 million years ago, Ma) is temporally well-constrained by numerous dated volcanic ash interbeds, the age of the base of the Kuibis Subgroup (>547.36 Ma) remains uncertain. Carbonates of the lower Kuibis Subgroup record recovery from a negative carbonate carbon isotope (δ¹³C_carb) excursion (Basal Nama Excursion, BANE) that has long been thought to represent the regional expression of the Shuram δ¹³C_carb excursion, which itself has recently been re-dated on multiple cratons to ~575–565 Ma. However, siliciclastic rocks of the Kuibis Subgroup preserve soft-bodied fossils of the Nama assemblage, but lack fossils diagnostic of the preceding White Sea assemblage which, along with inferred depositional rates, is commonly used to inform an age for the base of the Nama Group of ~551–550 Ma. The apparent decline in global soft-bodied fossil diversity between the White Sea and Nama assemblages has been suggested by some to reflect an extinction event that may therefore be recorded within the lowermost Kuibis Subgroup, approximately coincident with the BANE. Carbonate rocks of the Kuibis Subgroup also host the first appearances of the biomineralising Cloudina and Namacalathus, but the precise stratigraphic position of their first appearances relative to the soft-bodied fossil record and regional δ¹³C_carb profile remain poorly constrained.

     Here we present new stratigraphic, palaeontological, and geochemical (δ¹³C_carb and δ¹⁸O_carb) information from the oldest strata of the Kuibis Subgroup. These new insights were gathered during a recent expedition to the westernmost exposures of the Nama Group that outcrop in the Tsaus Mountains, within the Tsau Khaeb National Park (formerly Sperrgebiet). We present the first detailed tectonic and lithostratigraphic assessment of the Tsaus Mountains, including a revised regional geological map, in addition to a high resolution δ¹³C_carb chemostratigraphic profile. We use these data to provide a holistic litho- and chemostratigraphic correlation framework for the Kuibis Subgroup of the southern (Witputs) Sub-basin. All published palaeontological information, in addition to important new soft-bodied and skeletal fossil occurrences from the Tsaus Mountains succession, are correlated within this framework and discussed.