Soil chronosequence and geomorphology in a semi-arid valley in the Andes of southern Peru

Abstract

Soil development, particularly in relation to relative age of geomorphic surfaces, was studied in a high-altitude valley on the semi-arid western side of the Andes in southern Peru. Seven major fluvial surfaces in the Colca Valley were recognized, including floodplains, stream terraces, and fans associated with the Rio Colca and its tributaries. Soils were also briefly examined on Pleistocene andesite flows within the 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 surfaces exhibit strongly developed argillic horizons underlain by calcic horizons and strongly cemented duripans that contain illuvial carbonate and clay. Substantial development of these horizons also occurs on colluvial surfaces with slopes up to at least 65%. Relative depths of subsurface horizons on steeper colluvial surfaces differ from those on gently sloping fluvial surfaces in that argillic horizons and duripans are amalgamated. This study indicates that significant chemical weathering and strong subsurface pedogenesis occur in high-altitude, mountainous environments within the tropics, in contrast to findings at high altitudes in other climatic zones.