Single-zircon U-Pb dating of Carboniferous-Permian tuffs, Namibia, and the intercontinental deglaciation cycle framework

Abstract

The Carboniferous-Permian Dwyka Group in southern Namibia is subdivided into four upward-fi ning deglaciation sequences, each of which is capped by fi grained glaciolacustrine or glaciomarine deposits. Both the second and the third deglaciation sequences are associated with mudstone units that are particularly widespread—the Ganigobis Shale Member and the Hardap Shale Member. An abundance of marine macrofossils and ichnofossils and extrabasinally derived fallout tuff horizons characterize these mudstones and provide the basis for an integrated high-resolution biostratigraphic and tephrostratigraphic framework. Juvenile magmatic zircons separated from tuff horizons at the base of the Ganigobis Shale Member (top deglaciation sequence II) yield 206 Pb/ 238 U sensitive high-resolution ion microprobe (SHRIMP) ages of 302.3 ± 2.1, 302.0 ± 3.0 Ma, and 299.5 ± 3.1 Ma, whereas a tuff associated with the upper part of the Hardap Shale Member (top deglaciation sequence III) reveals a SHRIMP age of 297.1 ± 1.8 Ma. Since macrofaunal occurrences reveal the Hardap Shale Member to be a correlative of the Gondwana-wide Eurydesma transgression, this datum is an important age calibration for a well-established global event. Tuff beds sampled next to the boundary between the glacial Dwyka Group and the postglacial Ecca Group yield SHRIMP ages of 290.9 ± 1.7 Ma and 288.5 ± 1.6 Ma, providing an age proxy for the end of the late Paleozoic ice age in southern Africa.