Geochronological Constraints from the Ghaub Formation, Namibia: Implications for the Timing of Marinoan Glaciation
Extended Abstract – Prato 2004

K.H. Hoffmann1, D.J. Condon2, S.A. Bowring2, A. R. Prave3 & A.E. Fallick4
1Geological Survey of Namibia, Windhoek, Namibia
(email: khhoffmann@mme.gov.na)
2EAPS, Massachusetts Institute of Technology, Cambridge, MA, USA
3School of Geography and Geosciences, University of St Andrews, Scotland
4SUERC, East Kilbride, Glasgow, Scotland

Number of Words: 312
Number of References: 1
Number of Figures: 0 
Number of Tables:  0

Running header text: Ghaub Formation and Marinoan glaciation

The Neoproterozoic succession of Namibia contains an exceptionally well-preserved carbonate and glacial sedimentary record.  Two glacial units, the Chuos and the Ghaub formations, have been correlated to the global Sturtian and Marinoan episodes, respectively.  These correlations are based on lithofacies similarities of post-glacial cap carbonates, overall physical stratigraphy and positive - negative carbonate C isotopic excursions associated with both intervals.  A third positive (up to +11 % VPDB) excursion and subsequent sharp decline in 13C values occurs in the post-Ghaub carbonate rocks of the upper Tsumeb Subgroup.
Dropstone-bearing glaciomarine sedimentary rocks of the Ghaub Formation within metamorphosed Neoproterozoic basinal strata (Swakop Group) in central Namibia contain interbedded mafic lava flows and thin felsic ash beds.  U-Pb zircon geochronology of an ash layer associated with mafic volcanic rocks within basinal dropstone-bearing strata of the Ghaub Formation provides a reliable age of 635.5 ± 1.2 Ma for the deposition of the glaciomarine sediments (Hoffmann et al. 2004).  We interpret this age as constraining the timing of the Ghaub glaciation, providing an age for what has been described as a “Marinoan-type” glaciation. In addition, this age provides a maximum limit for the proposed lower boundary of the Terminal Proterozoic (Ediacaran) System and Period.  Combined with reliable age constraints from other Neoproterozoic glacial units - the ca. 713 Ma Gubrah Member (Oman) and the 580 Ma Gaskiers Formation (Newfoundland) - these data provide unequivocal evidence for at least three, temporally discrete, glacial episodes during the Neoproterozoic.  Furthermore, we suggest that the upper Tsumeb Subgroup 13C decline and overlying base Mulden unconformity as a proxy of the 580 Ma Gaskiers glaciation indicates that the interglacial periods, with maximum durations of ~50–80 Ma, were characterized by prolonged positive 13C excursions. 

Reference
Hoffmann, K.H., Condon, D.J., Bowring, S.A., and Crowley, J.L. 2004. U-Pb zircon date from the Neoproterozoic Ghaub Formation, Namibia: Constraints on Marinoan glaciation. Geology, 32(9), 817-820.