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

Abstract

The augite-olivine flows (5.3 Ma)

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

“…Iron shows enrichment towards the top of the profiles, e.g., ranging from approximately 10.5% Fe 2 O 3 to about 18% at the S1 site. Al 2 O 3 values are also enriched with decreasing depth, but the degree of enrichment in all 3 Nasinu profiles is significantly less than in the laterite development in the Nakobalevu basalt about 25 km away (Bonato and Morrison, 2012). Silica (SiO 2 ) decreases markedly (~30% loss), similar to other tropical basalt weathering sequences (Herbillon and Nahon, 1988;Ollier, 1984).…”

“…Although thin section study did indicate the presence of opaques, and some olivine in the lowest horizons of the S1 and S2 sites, the amounts were minimal and they did not appear in the XRD outputs of the whole fraction samples. The Nasinu profiles were in general of a uniform mineralogical composition, and the elemental data has indicated several cycles of weathering and soil formation; the colours of the profiles are more consistently uniform than found at Nakobalevu (Bonato and Morrison, 2012), and the distinction between pallid and mottled zonation is not clearly evident. The Nasinu profiles therefore belong to the kaolinitised lateritic profile or to the weak laterisation as described by Schellman (1981) and Herbillion and Nahon (1988) in their discussion of the limits of kaolinitisation and laterisation fields for the case of basaltic weathering.…”

“…Details of the procedures used are given in Bonato (1997) and Bonato and Morrison (2012). In summary, clay, silt, and sand fractions were separated by sieving (2, 0.05, 0.02 mm) and sedimentation, and treated with hydrogen peroxide and dithionite-citrate-bicarbonate (DCB) to isolate the aluminosilicate clays.…”

“…The weathering conditions support rapid change and such situations lead to problems of soil use and management for agriculture (Liedtke, 1989). Some research has been completed on the weathering of basalts in Fiji (Bonato and Morrison, 2012;Morrison et al, 1986Naidu et al, 1987), but few studies have looked comprehensively at the parent material chemistry, mineral transformations during weathering, and the resulting soil chemistry. There has also been ongoing interest in 'laterisation' (Brandes and Nakayama, 2010;Ollier, 1994;Schellmann, 1981), as this has been observed frequently in parts of the Fiji Archipelago, particularly associated with pedological studies, and the bauxite and manganese mining operations of the 1940s and 1950s (Colley, 1976;Morrison et al, 1987;Twyford and Wright, 1965).…”