The elusive snowball earth: U-Pb zircon ages from the upper diamictite of the Gifberg Group, South Africa

Abstract

U-Pb dating of detrital zircons was undertaken in the matrix component of the Swartleikrans Bed (Bloupoort Formation, Gifberg Group), located in the Vredendal Outlier of western South Africa. Results from this study indicate detrital zircons were derived from both local and regional (or distal) sources. The deposit is reportedly Neoproterozoic in age, but convincing age constraints are absent. These meta-diamictitic rocks were interpreted as glacial and are correlated with other diamictites in SW Gondwana, in the wake of the snowball Earth hypothesis. While sedimentological data is interpreted as glacial, geochemical proxies (Mo, Cr, U/Th, Ce*, V) point to an oxygenated depositional environment. Thus, if the deposit was related to a snowball Earth glacial event, then deposition must have been distal to the ice sheets.Nearly 50% of the dated detrital zircons (n = 68) have crystallization ages that cannot be correlated with known sources in the locally exposed basement. These predominantly Mesoproterozoic grains were either derived from distal sources or are reworked from unknown, now eroded, sedimentary rocks. Only 14% of all concordant zircon have an Early Mesoproterozoic to Late Palaeoproterozoic age, and a single grain relates to an older Paleoproterozoic event.Results are similar to other glacial deposits in the region (further north, in Namibia). In total, sedimentological observations and detrital zircon age populations point to a large catchment area, characteristic of glacial events of regional significance. However, direct regional correlation is still not possible using maximum depositional ages indicated by detrital zircons.