Very thick (> 1 km) successions of matrix-supported conglomerates (diamictites) are a very distinctive component of many Neoproterozoic basins. Classically interpreted as glacially deposited sedimentary rocks, their thickness has been seen as requiring exceptional depositional conditions such as world-wide “panglacial” climates. The Neoproterozoic Grand Conglomérat Formation (GC) of Katanga Province, southeastern Democratic Republic of Congo, is a 1.8-km-thick diamictite succession hosting one of the world's largest stratiform copper deposits. Examination of more than 300 km of recently acquired large diameter (up to 4 inches, 10.2 cm) core identifies the diamictites of the GC as debrites that accumulated as part of a deep-water “mass-transport complex” in a tectonically active and volcanically influenced anoxic rift basin. Detailed sedimentological descriptions of debrite facies and their lateral and vertical variability permits new insights into processes of debris-flow formation and transport, and their wider paleoenvironmental and paleotectonic significance. A genetic model is herein presented that highlights the importance of slumping and subaqueous downslope mixing of basin-margin fan-delta gravels, fault breccias, volcanic debris from contemporaneous basaltic fissure eruptions with basinal muds, and the downslope ponding of flows in narrow fault-bounded depocenters. Preservation of such an exceptionally thick subaqueously deposited debrite-dominated mass transport complex lacking any evidence of shallow-water deposition, requires rapid subsidence. Any direct sedimentary evidence of a glacial influence on sedimentation in the wider basin hinterland (if present) has been destroyed and homogenized by mass flow. Indirect evidence of a cold-climate setting is possibly expressed as lonestones in laminated turbidite facies dropped by either glacial or seasonal ice, and by exceptionally rare scratched clasts that may have been striated subglacially. Descriptions and interpretations presented here provide clues to the origin of other unusually thick debrite and turbidite successions elsewhere in other Neoproterozoic basins; their primary paleoenvironmental significance is that they appear to record ponding and focusing of mass flows in narrow, rapidly subsiding fault-bounded depocenters, rather than any unique glacial paleoclimate.