Morphology and origin of patterned ground in a humid tropical lowland area, New Guinea

Abstract

Two types of patterned ground occur on rolling to undulating plains consisting of weathered Pleistocene deposits under a warm climate with an annual rainfall between 1650 and 2000 mm and low seasonality. Pitted soils occur on gentle slopes and are characterized by sharply defined holes or unconnected trenches developed in coarser textured upper-soil horizons, above slowly permeable, finer textured subsoils. Sorted stripes are found locally on moderate slopes and consist of a network of gravel strips aligned at about 25° to the maximum slope and separating low earthen rises. There is no evidence that deformation by swelling and shrinking, loading, or liquefaction has caused the pitted soils. On the other hand, there are strong indications that this micro-relief is produced by large earthworms. Worms are also considered to build up the earthen rises of the sorted stripes, but, in this case, are likely to act upon a pre-existing pattern of gravel distribution and differentiation in gleying caused by denudation, and minor deformation due to differential swelling and shrinking, loading, and rheo-tropic disturbance. It is suggested that the microrelief caused by worms is not older than 400-800 years.