Tropical weathering in the territory of Papua and New Guinea

Abstract

Pure physical weathering is rare in New Guinea, but three main types of physicochemical weathering are recognized. These are skeletal weathering, in which physical changes such as fragmentation, exfoliation, disaggregation, and softening outweigh the associated chemical/mineralogical changes, and immature and mature weathering in which chemical/mineralogical changes predominate. Immature and mature weathering are subdivided, according to the kind of clay minerals formed, into smectite weathering (montmorillonite, illite, vermiculite), kandite weathering (kaolinite, halloysite, allophane), and sesquox weathering (gibbsite, goethite). In New Guinea mature weathering is uncommon despite the generally favourable climatic and lithological conditions. This is due to youthful and unstable land forms, and in the highest areas also to low temperatures. Equilibrium between rapid weathering and erosion maintains skeletal to immature weathering profiles on most slopes, and outcrops of fresh rock are rare. Kandite weathering is by far the most common type, and is often associated with gibbsite sesquox weathering in very wet areas and mainly in immature stages. Only two instances of goethite sesquox weathering have been observed, both on ultrabasic rock. Smectite weathering is rare and associated with low rainfall, basic or calcareous rock, and low topographic position. Strong weathering can mostly be related to old land surfaces, and is characterized by parallel zonation from mature weathering down to bedrock. More complex patterns can arise from renewal of weathering after dissection, differential weathering of rocks, burial of weathered profiles, or remain unexplained. The maximum depth of weathering recorded is only 32 m, but on the other hand weathering to a depth of less than 3 m is uncommon on consolidated rocks except limestone. Very limited data on the rate of weathering suggest that immature weathering requires at least 5000 years to develop, while mature weathering requires at least 20,000 years. Whereas weathering at depth appears to occur under conditions of constant volume, there is evidence near the surface of strong expansion in kandite weathering, and of moderate to very great volume decreases in sesquox weathering. Examples of the chemical changes occurring during weathering are presented in a triangular diagram relating percentages of SiO,, (Al2O3, Fe2O3), and (K2O, Na2O, CaO, MgO), and in a diagram relating the weathering potential index to the weathering direction or product index. The position of materials in these diagrams is strongly related to the clay minerals present. Deviations in the trends are caused by rejuvenation by volcanic ash, pedogenetic clay migration, sedimentary discontinuity, and differences in quartz content.