DENUDATION AND DEPOSITION IN A CLOSED SEDIMENT ROUTING SYSTEM: A CASE STUDY FROM THE NEOGENE AMUR RIVER AND ITS DELTA, RUSSIAN FAR EAST

The Amur river built a large delta on the northern part of the island of Sakhalin from the Early Miocene (22 Ma) to Pliocene. The deltaic deposits are mostly enclosed within basement highs, which are parts of a Cretaceous-Paleogene forearc. The sediments are sand-dominated, with subordinate muddy sand and very sandy mud and rare claystones; mudstone units are rare. The present-day catchment of the Amur is also almost completely surrounded by basement ridges which are Oligocene or older. None of these ridges show any sign of having been breached by the Amur. This situation means that the maximum possible extent of the catchment is fixed for the period represented by deltaic sedimentation on Sakhalin. Since the total amount of deltaic sediment can be assessed, a first order mass balance between source and sink is possible. This provides insight into the rates of hinterland denudation. Work in progress suggests that the total volume of deltaic sediment on Sakhalin is of the order of 240,000 km³, which translates to a mass of 4.5x10¹⁴ t of material. This requires a fluvial sediment yield about half that of the present-day Amur. Since the Neogene sediment yield was higher than the present-day rate, this suggests that a large quantity of the Amur fluvial product is not represented by deltaic sediments. There are three possible explanations: 1. there is an unidentified bypass route through/around the delta; 2. significant amounts of sediment are trapped in internal sedimentary basins within the catchment; and 3. material is being removed from the catchment by non-fluvial processes. Since no bypass route has yet been identified, we conclude that the apparently low denudation rate is a function of internal trapping and aeolian removal of fines, leading to a very sand-rich deltaic system.