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

Centimetre thick, laterally extensive tuff horizons occur within dark, mari ne 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 Afri ca. Four deglaciation sequences (DS I-IV) recorded in the Dwyka Group of Namibi a and South Africa are capped by mudstone units such as the 45 m thick mari ne fossil-bearing Ganigobis Shale Member in Namibia in which 24 thin ash-fa ll horizons are preserved. ion microprobe analyses (SHRIMP) of juvenile, ma gmatic zircons from the tuff horizons were used to determine their age. The y permit a new radiometric age calibration of the top of deglaciation seque nce II and of the Dwyka/Ecca Group boundary in southern Africa. Juvenile zi rcons of two tuff horizons near Ganigobis (southern Namibia) give Pb-206 ./ U-238 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 t he Western Cape (South Africa), were dated at 288.0+/-3.0 and 289.6+/-3.8 M a (earliest Asselian). According to these age determinations, the depositio n of Dwyka Group sediments in southern Africa started by the latest at abou t 302 Ma and ended at about the Carboniferous/Permian boundary, 290 Ma befo re present. (C) 1999 Elsevier Science Limted. All rights reserved.