Lithostratigraphy of the Kirkwood Formation (Uitenhage Group), including the Bethelsdorp, Colchester and Swartkops Members, South Africa

Abstract

Abstract The Jurassic – Cretaceous Kirkwood Formation forms part of the Uitenhage Group, the earliest deposits to fill Mesozoic rift basins that developed in what is now the southern Cape of South Africa during the breakup of Gondwana. The Kirkwood Formation is not only palaeontologically extremely important, having yielded diverse assemblages of vertebrate, invertebrate and plant fossil taxa, but also contains suitable source rocks for hydrocarbon systems offshore of the southern Cape of South Africa. The Kirkwood Formation comprises chronostratigraphic markers in the form of radiometrically dateable volcaniclastic deposits and age-diagnostic invertebrate fossils, which may provide robust dates for the depositional history of the Uitenhage Group. The Kirkwood, including the Bethelsdorp, Colchester and Swartkops Members, is 2210 m at its thickest and comprises sandstones and mudstones deposited in fluvial and lacustrine depositional settings from the Tithonian to Valanginian. These microfossil-based ages may only apply to the unit in its type area in the Algoa Basin and not to its lithostratigraphic correlatives in the other Mesozoic rift basins, which have highly variable ages ranging from Middle Jurassic to Early Cretaceous.