Climatically controlled river terrace staircases : a worldwide quaternary phenomenon.

摘要

A comparison of fluvial terrace sequences from around the world, based on data collected as part of International Geoscience Programme (IGCP) Project No. 449, has revealed significant patterns. River terraces provide important records of uplift, which is essential for their formation, and of landscape evolution. Their cyclic formation, however, almost invariably seems to have been a response to climatic fluctuation. Sequences in the European core area ofudIGCP 449, which has the longest and most extensive research history, have been used as templates for worldwide comparison. There is evidence for a global acceleration of uplift at the time of, and perhaps in response to, the Mid-Pleistocene Revolution, when climatic fluctuation switched to 100 ky Milankovitch cycles. Terraces formed prior to this generally consist of wide aggradational sheets that probably each represent formation over several 41udky cycles. Subsequently, river valleys became more steeply entrenched and terraces formed in response to the stronger 100 ky climatic forcing, in many cases at approximately one per cycle. This paper uses the new data resource to explore differences between records in different climate zones, between sequences with variable numbers of Middle-Late Pleistocene terraces and between systems in which the all-important incision event has occurred in different parts of climatic cycles. Key records discussed include European examples from the Rhine, Thames, Somme, Dniester, Dnieper, Don, Volga and Aguas; from Asia the Gediz (Turkey) and Orontes (Syria); from North America, the South Platte and Colorado; from South Africa the Vaal and Sundays; from Australia the Shoalhaven; and from South America, the Amazon, Paraguay and tributaries of the Colorado and Negro.