Organic-walled microfossils and biostratigraphy of the upper Port Nolloth Group (Namibia): implications for latest Neoproterozoic glaciations

Abstract

The occurrence of organic-walled microfossils is reported for the first time from the Neoproterozoic Port Nolloth Group, Gariep Belt (southern Namibia). Acritarchs assigned to Bavlinella faveolata occur in the Hilda Subgroup below the younger of two glaciogenic diamictite units (Numees Formation) within the Port Nolloth Group. The microfossil assemblage in the overlying upper Holgat Formation, above the Numees Formation, is characterized by low diversity (six species), dominance of Soldadophycus bossii, and absence of acanthomorphic or large sphaeromorphic acritarchs. The agglutinated foraminifer Titanotheca also occurs in the Holgat Formation. Combined with available chemostratigraphic data and Pb–Pb ages, this microfossil assemblage indicates an upper Ediacaran age of around 555 Ma for the Holgat Formation. Virtually identical microfossil assemblages, negative-to-positive δ¹³ C trends,⁸⁷Sr/⁸⁶Sr values between 0.7080 and 0.7085, as well as Pb–Pb carbonate ages, make it possible to correlate the Holgat Formation with the Buschmannsklippe Formation (Witvlei Group, central Namibia), the Kombuis Member (Cango Caves Group, southern South Africa) and the uppermost Polanco to lowermost Cerro Espuelitas Formation (Arroyo del Soldado Group, Uruguay). Based on these data, the underlying Numees Formation, the age of which has been only loosely constrained so far and subject to considerable debate, can now be assigned to the c. 580 Ma Gaskiers or the possibly younger (< 570 Ma) Moelv glacial event. The Numees glacial event may be represented in the uppermost Nooitgedagt Member (Cango Caves Group, South Africa) and the lower Barriga Negra formations (Arroyo del Soldado Group, Uruguay), characterized by a negative δ¹³C excursion and a strong sea-level drop. If this correlation is confirmed, lack of glacial deposits there might have implications for the palaeogeographic extent of upper Ediacaran glaciations. Our preliminary studies show that acritarch biostratigraphy can make a significant contribution to unravelling the stratigraphy of Neoproterozoic glacial deposits, especially when combined with C and Sr isotopic data.