Geochemistry of the Pronksberg Bentonite of the Upper Elliot Formation (Early Jurassic), Eastern Cape, South Africa

Abstract

Located within the continental red beds of the upper Elliot Formation (Eastern Cape, South Africa), the Lower Jurassic Pronksberg bentonite is in close proximity to the boundary between the Triassic and Jurassic, which coincides with one of Earth’s largest mass extinction events. Sedimentological, mineralogical, and major and trace element geochemical data indicates that: (1) the bentonite formed by devitrification of felsic volcanic ash; (2) the deposit was reworked by gentle currents in an ephemeral lake where intense chemical weathering of the volcanic ash occurred; and (3) the source of the original felsic volcanic material was probably in southwest Gondwana (now South America) where explosive silicic igneous activity has been documented not only in the Permian, but also in the Late Triassic and Early Jurassic.