Soil Genesis and Landscape Evolution in Central Oahu, Hawaii

Gavenda, Robert Thomas
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The highly weathered soils of central Oahu are generally characterized as residual soils developed in basalt. These soils do not have any of the properties that are commonly associated with soils developed in volcanic ash. Most of these soils are classified as either Oxisols or Ultisols. Soils that are on the eastern footslopes of the Waianae Range are recognized as having formed in volcanic ash and are classified as Inceptisols. Soils along a transect from the Inceptisols of the Waianae Range to the Oxisols and Ultisols of the Koolau Range were examined in this study. A discontinuity was observed in all but one profile along the transect. The stratigraphic relationships between the discontinuity and landforms strongly suggest an eolian origin for the B horizons above the discontinuity. Mineralogical analysis of the soils revealed that both the highly weathered B horizons above the discontinuity in the Oxisols and Ultisols and the B horizons in the Inceptisols which developed in volcanic ash contain very small amounts of the resistant primary minerals magnetite and ilmenite relative to soil developed in basalt. These two independent lines of evidence indicate that the highly weathered B horizons above the discontinuity in the Oxisols and Ultisols developed in volcanic ash. The soils in central Oahu provide a record of alternating periods of landscape stability and instability. This record is the basis for constructing the following sequence of events for the landscape evolution of central Oahu: (1) erosion of Koolau and Waianae Volcanos; (2) formation of soils in residual basalt; (3) formation of alluvial fans at the base of the Waianae Range; (4) development of soils in the Waianae alluvial fans; (5) basin-wide truncation of soil profiles and concurrent downcutting of stream channels; (6) deposition of one or more volcanic ash layers possibly separated by erosional periods; (7) formation of soils in the volcanic ash and the concurrent accumulation of tropospheric dust from mainland Asia; and (8) further downcutting of stream channels but stabilization of interfluve surfaces as evidenced by the accumulation of tropospheric dust. The soils of the study area were also classified according to the latest version of Soil Taxonomy.
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