Gradual Avulsion, River Metamorphosis and Reworking by Underfit Streams: a Modern Example from the Brahmaputra River in Bangladesh and a Possible Ancient Example in the Spanish Pyrenees

Abstract

Avulsion of rivers from one course to another may result in metamorphosis of channel pattern. The Brahmaputra River, a large sand-bed braided river, avulsed into its present course along the Jamuna channel over 100yr ago. Cartographic evidence indicates that the avulsion was gradual rather than instantaneous and that the new course of the river has changed from sinuous to braided. The former course is occupied by the Old Brahmaputra, a meandering river that is reworking the top of the deposits of the abandoned braided channel belt. This situation, where an underfit stream reworks a partially abandoned channel belt, is probably quite common, but not typically recognized in the rock record. River abandonment is unlikely to be instantaneous and reworking of channel-belt sediments by an underfit stream following avulsion should be expected. A multistorey sandstone capped by an exhumed meander bend in the Oligocene Campodarbe Group, in the Spanish Pyrenees, may be an example. The superposition of a fine-grained meander belt on top of a coarse-grained multistorey sandstone, which probably was deposited by a braided river, can be explained by avulsion. The implications are that braided-river sand bodies may be capped by meandering-river deposits, as a consequence of avulsion, and that river metamorphosis can be autocylic, rather than allocyclic, in avulsive river systems.