SOIL CHRONOSEQUENCE AND GEOMORPHOLOGY IN A SEMIARID VALLEY IN THE ANDES OF SOUTHERN PERU

Soil development, particularly in relation to relative age of geomorph ic surfaces, was studied in a high-altitude valley on the semi-arid we stern side of the Andes in southern Peru. Seven major fluvial surfaces in the Colca Valley were recognized, including floodplains, stream te rraces, and fans associated with the Rio Colca and its tributaries. So ils were also briefly examined on Pleistocene andesite flows within th e valley, and on high volcanic plateaus surrounding the valley. Field and laboratory work indicate that most soils on stable surfaces above the floodplain are Mollisols and that there is a consistent pattern of increasing subsurface horizon development on successively higher and older fluvial surfaces from 3300 to 3800 m elevation. A developmental sequence of argillic horizons, duripans, and carbonate accumulation is present on these fluvial surfaces. Soils on the oldest fluvial surfac es exhibit strongly developed argillic horizons underlain by calcic ho rizons and strongly cemented duripans that contain illuvial carbonate and clay. Substantial development of these horizons also occurs on col luvial surfaces with slopes up to at least 65%. Relative depths of sub surface horizons on steeper colluvial surfaces differ from those on ge ntly sloping fluvial surfaces in that argillic horizons and duripans a re amalgamated. This study indicates that significant chemical weather ing and strong subsurface pedogenesis occur in high-altitude, mountain ous environments within the tropics, in contrast to findings at high a ltitudes in other climatic zones.