The Keis Supergroup

Abstract The Keis Supergroup comprises a geological succession dating from the Palaeoproterozoic to Mesoproterozoic eras, with an estimated thickness of approximately 28 km, inferred from interpretations of structurally complex geology. It consists of predominantly metasedimentary and subordinate metavolcanic rocks exposed between Olifantshoek, Upington, Marydale and the Korannaberg Mountains. It is subdivided into four major lithostratigraphic groups: the Elim-, Olifantshoek-, Groblershoop- and Wilgenhoutsdrif groups. The Elim Group comprises upward-fining fluvial to shallow marine sediments of the Mapedi Formation, overlain by stromatolitic carbonates and arenites of the >1 916 Ma Lucknow Formation. The Olifantshoek Group lies unconformably on the Elim Group and consists of the conglomeratic Neylan Formation and the volcanic Hartley Formation (~1 916 Ma), succeeded by fluvial arenites of the Volop- and Top Dog formations. The Groblershoop Group represents a ~14 km-thick package of upward-coarsening marine shelf deposits consisting of arenites and schists, interpreted as progradational passive margin successions that were deposited prior to 1 290 Ma. The uppermost Wilgenhoutsdrif Group includes quartz arenites, mafic and felsic volcanic rocks, serpentinites, carbonates and phyllites, deposited in a forearc or back-arc basin, with felsic lavas dated at ~1 290 Ma. The Keis Supergroup defines the Kheis Terrane and records sedimentation along the western margin of the Kaapvaal Craton, from post-rift to forearc basin development, and preserves the effects of crustal shortening during the Kheis Orogeny (~1 290 to 1 114 Ma). Structural, geophysical and provenance data indicate that the Kheis Terrane, formed as a distinct crustal domain as a result of the collision between the Rehoboth- and Kaapvaal cratons, prior to overprinting during the Namaqua Orogeny. The Keis Supergroup provides a critical window into early Mesoproterozoic basin evolution and tectonic reworking at the interface between the Kaapvaal Craton and the Namaqua-Natal Belt.