Seismic reflection profiles of the upper and lower continental slopes and intervening Exmouth and Scott plateaus of northwest Australia reveal three prominent reflectors that are traceable over a large area. From measurements made near boreholes on the shelf, these reflectors are identified as sedimentary rocks of Early Cretaceous (Aptian), early Oligocene, and early middle Miocene age. The Aptian reflector, usually the deepest reflector recorded, overlies a regional unconformity that marks the end of a phase of continental rupture and the beginning of a new phase of subsidence of the Exmouth and Scott plateaus. The overlying reflectors are sedimentary or diagenetic surfaces formed during changes in oceanic circulation, and represent second-order events of a single phase f deposition that has accompanied the subsidence. The differentiation of the northwest Australia continental margin into shallow-water and deep-water depositional environments by the Late Cretaceous is indicated by sonobuoy reflection-refraction measurements of a seaward decrease in interval velocity of laterally equivalent sedimentary rocks above the unconformity.