淡水成デルタと海成デルタの堆積作用 その２： 形成過程から見た比較

Abstract

The sediment load brought by rivers to lakes and seas is controlled by the climate, geology, and geography of the drainage basins. In most fine-grained deltas the density relationship between discharged river water and the recipient water body dictates that homopycnal-type inflows occur commonly in lacustrine deltas, and hypopycnal-type (buoyant) inflows in marine deltas. Since sea water has a higher cation content than lake water, the suspended load in marine rivermouths is rapidly flocculated and settled as delta sediments. This flocculation explains why large-scale deltas consisting of fine-grained sediments occur more commonly in marine rivermouths than in lacustrine rivermouths.Lacustrine deltas are less subjected to tidal influences than are marine deltas. Therefore, estuarine morphologies which may occur in marine deltas are unlikely to be found in lacustrine deltas. Water-level changes are mainly controlled by tectonics of the receiving delta basins in lakes, and by eustatic sea-level changes in seas. Recent marine deltas, therefore, have highly uniform bottom ages corresponding to the climax age of the Holocene Jomon transgression or Frandrian transgression. In contrast, recent lacustrine deltas have various bottom ages. Gentle basin slopes may result in shallower water depths at the rivermouths, especially in larger-scale delta bodies. With increasing basin slope, wave and tide energies are increassed, resulting in small-scale delta bodies and in the characteristic continuity of sand bodies. Gentle basin slopes occur in epeirogenetic lake basins and on marine continental shelves. The other marine processes have basically the same effect on delta formation as processes in lakes.Morphologies of delta bodies are determined not only by fluvial and marine (or lacustrine) processes, but also by the tectonics and geometry of the receiving delta basins.