TOWARDS A COMPOSITE D¹³C CURVE FOR THE NEOPROTEROZOIC

Marinoan glacial deposits in Australia, Namibia, and Candada are bound below by large (>10 per mil) negative d¹³C anomalies and above by isotopically and sedimentologically homologous cap carbonates. The occurrence of these distinctive pre- and post-glacial features in the Neoproterozoic succession of northeastern Svalbard suggests that both diamictites in the Polarisbreen Group and their >200 m of intervening strata belong to the Marinoan glaciation and may represent an unusually complete record of a snowball earth. It follows that the Sturtian glaciation, or proxy thereof, should occur in the thick stack of underlying sediments. We argue that the equivalent of the Sturtian cap carbonate occurs in the lower Russoya Member, at the base of the Polarisbreen Group. Insofar as this hypothesis is correct, then the underlying carbonate-rich Akademikerbreen Group provides a lengthy pre-Sturtian d¹³C record.

In northern Norway, a pre-glacial d¹³C anomaly and typical Marinoan cap carbonate bracket the Smalfjord Formation, the older of two glacigenic intervals in the Vestertana Group. Therefore, it is concluded that the younger Mortensnes Formation represents a third Neoproterozoic glaciation ("Gaskiers glaciation"). The absense of a cap carbonate to this glaciation suggests that it is not a snowball and could be diachronous. However, this glaciation appears to be related to the most extreme negative d¹³C anomaly ("Wonoka anomaly") in the Neoproterozoic. In many successions, the stratigraphic record between the Marinoan cap carbonate and the Wonoka anomaly is heavily truncated beneath deep erosional disconformities, likely related to the Gaskiers glaciation. An important exception is the Tsumeb Subgroup in northern Namibia, which may bridge the record between the Marinoan glaciation and Wonoka anomaly. Thus, a join between the Svalbard and northern Namibia stratigraphies provides a virtually continuous d¹³C record through much of the Neoproterozoic. The well documented terminal Proterozoic d¹³C record extends the composite record to the Precambrian-Cambrian boundary.