Equating the basal unconformity with the palaeoplain: a model for passive margins

The palaeoplain is the terrestrial continental surface, little changed from the landsurface that existed before continental breakup. The basal unconformity occurs offshore, separating older continental rocks from younger post-rift sediments. This paper considers the palaeoplain and the basal unconformity to be the same surface. It presents a hypothesis for continental passive margins that integrates the geomorphology near the continental boundary with the offshore geomorphology. By recognising the identity of the palaeoplain with the basal unconformity the hypothesis links the terrestrial story to the offshore story. Many details that were separated in earlier models come together in a single model. Continental rifting starts with a rift valley stage. At this stage the pre-breakup landsurface, the palaeoplain at the edge of the continent, is downwarped. On the seaward side post-rift sediments accumulate on the submerged palaeoplain, which thus becomes the basal unconformity. odel explains the parallel structures of continental divide, present coast line, and continental edge. It also explains the creation of the basal unconformity in a short time, and the comparative volumes of onshore erosion and offshore sedimentation. Onshore, the model explains the modifications of drainage and the great escarpments that are characteristic of passive margins. It also explains oldlands observed near the coast.