Structure and Cenozoic evolution of the Kolyma river valley, eastern Siberia, from its upper reaches to the continental shelf

With a length of over 2600 km. the Kolyma is the longest river draining eastern Siberia into the Arctic Ocean, crossing part of the high Arctic that has not experienced Pleistocene lowland glaciation. The upper reaches of this river system began to develop in the Cretaceous, when the main drainage divide between the Sea of Okhotsk and the Arctic Ocean formed. Surface uplift has occurred across this region throughout the Cenozoic, superimposed in some localities onto vertical crustal motions caused by active faulting. However, climatically forced incision and aggradation have been the most important factors affecting the disposition of the fluvial sediments. Incision has occurred during dry episodes within Quaternary climatic cycles, with accumulation during humid intervals in both warm and cold stages. The evolution of the Lower Kolyma has also been affected by crustal deformation related to the Cenozoic opening of the Arctic Ocean Basin and subsidence of the associated passive continental margin. Faulting and sea-level fluctuations have caused repeated eastward adjustments and reorientations of the lower Kolyma channel system. The modern Lower Kolyma is thus much younger than its principal right-bank tributaries. It did not become connected to the Upper and Middle Kolyma until the Early/Middle Pleistocene; indeed, its present form only developed at the end of the Late Pleistocene.