Sedimentological investigations of well-exposed slope strata in the Upper Devonian reef complexes of the northern Canning Basin lead to a sequence-stratigraphic interpretation that differs from others based on predictive models in which coarse carbonate debris deposits are regarded as lowstand deposits. In the upper Frasnian–Famennian Napier Range slope succession, we recognize an important phase of platform-margin collapse during a sea-level highstand in the middle Famennian. At this time the reef-rimmed margin was rapidly prograding, and the platform was producing carbonate grains (notably ooids and peloids) that were being transported by turbidity currents to the slope and basin floor. Periodic collapse of the platform margin led to deposition on the slope of allochthonous blocks of reefal limestone and channelized debris-flow units, intercalated with the ooid-peloidal turbidites and quartzo-feldspathic sandstones. Collapse of the early-cemented margin was probably triggered by gravitational instability caused by oversteepening during rapid progradation and/or tectonic activity, with falling blocks initiating some of the debris flows.