Geomorphologic relationships and ages of soils on alluvial fans in the Rio General Valley, Costa Rica

Abstract

Soil-geomorphic relationships on alluvial fans in the Rio General Valley, Costa Rica, show that the sequence of alluvial fan surfaces and soil development on them is related chiefly to uplift of the bordering mountain ranges and not significantly to climatic change. The oldest and most strongly developed soils are found only on the highest surfaces; successively younger and less-developed soils have lower positions. Oxisols characterize the oldest surfaces, Alfisols the intermediate ones, and Entisols the youngest surfaces. Minimum ages of these surfaces are estimated, from the extrapolation of sedimentation rates, solum development rates, and the rate of channel incision based on two radiocarbon dates, to range from about 45,000 to 65,000 years for the oldest, to approximately 100 years for the most recent surface. Pedons representative of the soils on the principal alluvial surfaces are described in terms of the physical, chemical, and mineralogic characteristic. We propose that the soils studied are a chrono-catena that represents soils development through time for this region, and can be used to establish a minimum time scale by which to guage stage and rate of soil development in a humid tropical environment.