This paper reports on the identification of peperite, a breccia comprising angular to lobate variably vesiculated basalt clasts in a mudstone matrix, at the upper margin of a basalt sheet of the Sabie River Basalt Formation, northern Lebombo. The peperite formed as a result of emplacement of the basalt sheet as a shallow sill that burrowed into and migrated laterally within an unconsolidated water-saturated sediment pile. Although breccias at the upper margin of basalt units are widely recognized from the Karoo of southern Africa, they have generally been interpreted as blocky "autobreccia" or "flow-top breccia" on lavas. Consequently, many mixed basalt-sediment breccias may have been viewed as sedimentary infill of these "flow-top breccias". To the best of our knowledge, this paper represents the first documentation of an intrusive or invasive mode of formation. The new shallow sill or invasive flow interpretation has significant implications on the perceived sedimentary architecture and basalt emplacement model for the Karoo, and perhaps other flood basalt provinces. It is suggested that the widespread distribution and consistent thickness of individual "flows" in the Sabie River Basalt Formation can sometimes be attributed to emplacement as shallow sills that have propagated through a regionally extensive sediment pile.