The geochronology and significance of ash-fall tuffs in the glaciogenic Carboniferous-Permian Dwyka Group of Namibia and South Africa

Abstract

Centimetre thick, laterally extensive tuff horizons occur within dark, marine mudstones of the Carboniferous-Permian Dwyka Group (Karoo Supergroup) in southern Namibia and South Africa. These pyroclastic deposits preserve the earliest evidence of volcanism in Karoo-equivalent strata of southern Africa. Four deglaciation sequences (DS I–IV) recorded in the Dwyka Group of Namibia and South Africa are capped by mudstone units such as the 45 m thick marine fossil-bearing Ganigobis Shale Member in Namibia in which 24 thin ash-fall horizons are preserved. Ion microprobe analyses (SHRIMP) of juvenile, magmatic zircons from the tuff horizons were used to determine their age. They permit a new radiometric age calibration of the top of deglaciation sequence II and of the Dwyka/Ecca Group boundary in southern Africa. Juvenile zircons of two tuff horizons near Ganigobis (southern Namibia) give 206∗Pb 238U ages of 302.0 ± 3.0 Ma and 299.2 ± 3.2 Ma (latest Kasimovian) for the top of DS II. Juvenile zircons from two tuff horizons of the basal Prince Albert Formation, sampled north of Klaarstroom and south of Laingsburg in the Western Cape (South Africa), were dated at 288.0 ± 3.0 and 289.6 ± 3.8 Ma (earliest Asselian). According to these age determinations, the deposition of Dwyka Group sediments in southern Africa started by the latest at about 302 Ma and ended at about the Carboniferous/Permian boundary, 290 Ma before present.