STRATIGRAPHY OF THE PORT NOLLOTH GROUP OF NAMIBIA AND SOUTH AFRICA AND IMPLICATIONS FOR THE AGE OF NEOPROTEROZOIC IRON FORMATIONS

ABSTRACT. Uncertainties in the number and age of glacial deposits within the Port Nolloth Group have hindered both structural and stratigraphic studies in the Neopro-terozoic Gariep Belt of Namibia and South Africa. These uncertainties are com-pounded by major lateral facies changes that complicate correlations locally. Herein, we report the results of integrated geological mapping, chemo- and litho-stratigraphic, and sedimentological studies that shed light on the age and stratigraphic architecture of the Port Nolloth Group. Particularly, we have distinguished an additional glacial deposit, herein referred to as the Namaskluft diamictite, which is succeeded by a ca. 635Ma basal Ediacaran cap carbonate. This interpretation indicates that the stratigraph-ically lower, iron-bearing Numees diamictite is not Marinoan or Gaskiers in age, as previously suggested, but is instead a ca. 716.5 Ma Sturtian glacial deposit. A Sturtian age for the Numees Formation is further supported by the discovery of microbial roll-up structures in the dark limestone of the Bloeddrif Member that caps the diamictite. A re-evaluation of the age constraints indicates that all Neoproterozoic iron formations may be of Sturtian age, and thus indicative of secular evolution of th