Weathering and soil genesis from the Nasinu Basalt, South-East Viti Levu, Fiji

Abstract

Detailed studies of basalt weathering in the South Pacific Islands are relatively rare. In order to address this gap, this study investigated weathering changes and soil formation for the predominantly fine-grained feldspar-phyric basalt flows (6.4 Ma) in the Nasinu area, about 10 km north-north-east of Suva, Fiji. In the hot humid environment locally, these rocks have been subjected to rapid and deep weathering. Three weathering profiles (Nasinu S1, S2 and S3 at approximately 60 m elevation) show features of strong kaolinitisation. Aluminium and iron enrichment in the horizons of the Nasinu weathering profiles is marked, with the presence of kaolinite, gibbsite, goethite, haematite and magnetite (grains) confirming substantial mineralogical change. Suggested mechanisms for these changes are presented. Using Soil Taxonomy, the Nasinu soil pedon (JBS-1) developed on the weathered basalt is a Typic Palehumult, very fine, mixed, isohyperthermic. A comparison is made with the weathering pattern of the Nakobalevu basalt located only about 25 km from this location.