China: materials for a loess landscape. (Special Issue: Loess and dust dynamics, environments, landforms, and pedogenesis: a tribute to Edward Derbyshire.)

摘要

For various reasons it was thought that material for the great Chinese loess deposits originated in the deserts of the north - and the idea of a 'desert' origin was widely accepted. But Butler, in Australia in 1956, cast doubt on the actual existence of desert loess and this led to considerable discussion. One facet of the argument (advocated by Smalley and Vita-Finzi, 1968) proposed that there were no desert specific mechanisms which could produce the large amounts of loess material observed. Applying this idea to the Chinese loess was particularly appropriate because of the huge extent and thickness of the deposits. How was this vast amount of loess material produced? Smalley and Krinsley (1978) proposed a sequence of events that could lead to the formation of the Chinese loess. This required that the loess be a mountain loess (eventually defined by Smalley and Derbyshire, 1990) - the material was made in the mountains to the west; and the Yellow River had a role to play in bringing it to the loess deposit region; and that loess material in desert regions was in a state of transit. The 1978 model has been proved to be true by a whole sequence of ingenious and intricate experiments. In particular the analysis of zircons has produced data allowing particle sources to be identified. The zircon particle proves to be a key component of the loess landscape; but the mode particle remains the silt-sized quartz particle. The problem running in parallel with the 'how did the material form?' question is the puzzle of why loess material has such a restricted size range. What controls operate on the formation of loess material? In the quartz particles it appears that a combination and interaction of two events produce a size control. The high-low quartz transformation introduces tensile stresses; the eutectic sizing of the quartz particles in the original granite also constrains the level of stress development. The stress levels produced cause a particle size product of around coarse silt size.