Upper Karoo sediments (Middle to Upper Triassic) in NE Swaziland consist of an upward-fining, fluvially dominated sequence (Molteno and Elliot Formations) followed by the mainly aeolian Clarens Formation. Diamonds and associated kimberlitic garnets are strata-bound in two thin upward-fining sequences (4–16 m) at the base of the Elliot Formation. These sequences comprise erosionally based, clast-supported conglomerate channel-fill, overlain by coarse, feldspathic, cross-stratified sandstone and rippled siltstone with gritty layers. Diamonds and garnets occur in the matrix of the conglomerate, but are five times more abundant in the coarse feldspathic sandstone. They also occur in the sandy mode in the overlying siltstone, but in greatly diluted concentrations. The diamonds were locally derived and released to the sedimentary system following a climatic change when conditions favoured more intense erosion of nearby kimberlite pipes. Diamonds and garnets released to the passing sediment were concentrated locally in thin (<2m), extensive channel sands deposited by overloaded ephemeral flood flows. Source area denudation and the increasing aridity of the climate during Upper Karoo times led to a gradual reduction in slope and sediment supply, with the result that river energy and bedload diminished, producing a gross upward-fining trend in the succession.