Stratigraphy of the Port Nolloth Group of Namibia and South Africa and implications for the age of Neoproterozoic iron formations

F. A. Macdonald, J. V. Strauss, C. V. Rose, F. O. Dudas, D. P. Schrag
2010 American Journal of Science  
Uncertainties in the number and age of glacial deposits within the Port Nolloth Group have hindered both structural and stratigraphic studies in the Neoproterozoic Gariep Belt of Namibia and South Africa. These uncertainties are compounded 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
more » ... rchitecture 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. 635 Ma basal Ediacaran cap carbonate. This interpretation indicates that the stratigraphically 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 the redox state of the ocean. introduction The Port Nolloth Group (PNG) of South Africa and Namibia hosts glacial deposits, iron formations, mixed carbonate-siliciclastic rocks, enigmatic microbialites, economically significant sedimentary exhalative Pb-Zn deposits, and datable volcanic rocks (Rogers, 1915; Kröner, 1974; Frimmel, 2008) . However, due to structural complexities and large lateral facies changes, stratigraphic correlations within the PNG have remained unclear. Particularly, much debate has centered around the number and age of the glacial deposits (Jasper and others, 2000; Frimmel, 2008) . The most recent review of the stratigraphy of the PNG concludes that the Kaigas Formation (table 1) is a pre-Sturtian ca. 750 Ma glacial deposit, the Numees Formation and associated iron formation are ca. 580 Ma Gaskiers-age glacial deposits, and that both the ca. 716.5 Ma Sturtian and ca. 635 Ma Marinoan glaciations are missing (Frimmel, 2008) . This interpretation has significant implications not only for the tectonic evolution of the Kalahari Craton, but also for the evolution of ocean chemistry. The presence of extensive iron formation is an important indicator of the redox state of the ocean, and thus constraining the temporal range of major iron deposits in Earth history is critical to a better understanding of the co-evolution of oxygen, climate, and life. Previous studies have relied on regional lithostratigraphic correlations of the diamictites (for example Von Veh, 1993). However, such correlations can be compromised by complexities in the stratigraphy of glacial deposits: lateral facies changes are abundant, and glaciers access different sedimentary sources over time. In contrast, carbonate rocks that bound Neoproterozoic diamictites are likely to reflect conditions in a well-mixed ocean reservoir and have distinct geochemical and sedimentological
doi:10.2475/09.2010.05 fatcat:nokqcdghcvaudnnrimmb3l44im