Preservation patterns of Late Cenozoic fluvial deposits and their implications: results from IGCP 449

Abstract

IGCP 449 (2000–2004) amassed data on fluvial systems world-wide, concentrating on key sequences, especially those benefiting from multiple lines of dating evidence. These archives are preserved either in terraced or stacked sequences, the latter confined to areas of subsidence. Terrace staircases record repeated incision (in response to progressive surface uplift), which, alternately with aggradation, is thought to have been climatically triggered. This triggering can be attributed to Quaternary (Milankovitch) climatic fluctuation, but the number of terraces produced in each Middle–Late Pleistocene 100 ka climatic cycle varies significantly between different systems. An unexpected result of this data collection has been the recognition of differing patterns of fluvial sedimentation and valley evolution over Neogene and Quaternary timescales, apparently related to different types of continental crust with different uplift/subsidence histories. These fall into three groups: (1) a typical uplifting pattern, with extensive terrace staircases of the type that dominate the global fluvial archive; (2) a subsiding pattern, with stacked fluvial sediments, usually coinciding with major depocentres; and (3) a stable pattern, with preservation of deposits related to channel diversion rather than significant incision. The third type, in which Early and pre-Quaternary deposits occur within a few metres of modern river level, is generally restricted to ancient cratonic or shield areas, which have apparently experienced minimal Late Cenozoic uplift, in marked contrast with most continental areas on more recently formed crust. There is also an intermediate situation, in Early Proterozoic crust, involving alternations of uplift and subsidence, with little resultant net vertical motion.