A RIVER TERRACE SOIL CHRONOSEQUENCE OF THE PAKUA TABLELAND IN CENTRAL TAIWAN.

Soils science and fluvial geomorphology meet on the subject about flights of river terraces whose treads rise like stairs above active channels. The Pakua tableland in central Taiwan consists of widely unpaired terraces, including six levels in altitude formed by successive river incision. The terraces serve as geomorphic markers to gauge the differential or absolute deformation of the land surface but such application is hampered by the absence of well-documented ages. Six representative soil pedons were selected from the six levels of terraces in the tableland (from the highest pedon PK-1 to lowest pedon PK-6) to characterize the soil properties in a chronosequence and to relate the pedogenic processes in the major terraces to the formation and evolution of the landscape in the study area. The soil morphological, physical, and chemical properties as well as the clay mineral variation showed that pedogenic intensity is strongly dependent on the terrace levels with varying formation age. The soils can be divided into three domains as Hapludox for pedon PK-1, Paleudult (or Hapludult) for pedons PK-2, PK-3, PK-4, and PK-5, and Dystrudept for pedon PK-6, based on Soil Taxonomy. The degree of soil development increases with altitude in a sequence from PK-1 to PK-6 forming a postincisive type of soil chronosequence in accord with the evolution of the geomorphic surface by successive river incision in the study area. Based on the crystallinity ratios of free iron, the soils give an estimated age of 40-400 ka for the river terraces of the tableland.