A COMPLEX ORIGIN FOR AN UNUSUAL PALEOSOL INTERVAL WITHIN THE KOLOA VOLCANIC SERIES, KAUAI, HAWAII

On a rocky headland of Koloa Volcanics (1.5-0.5 Ma) near Poipu Beach, southeastern Kauai, an interval of brown, erosionally-resistant, goethite-rich saponitic clay bears abundant cm- to dm-scale, subrounded to subangular masses of vesicular olivine basalt, as well as calcified root traces and thin pedogenic carbonate crusts. Although preliminary evidence suggested an in situ differential weathering origin for the brown matrix (Griffing et al., 2011), detailed petrographic and chemical analysis of the interval reveals a more complex pedogenic history than in situ differential weathering of Koloa basalts might produce.

It is likely that both the basalt and the brown clay matrix components were subject to sedimentary modification/transport prior to soil formation, based on: 1) very thin (mm-scale) alteration rinds on unaltered basalt clasts, 2) the absence of relict vesicles in the brown matrix surrounding the vesicular basalt clasts, 3) rare rounded silt- and sand-sized ultramafic phenocryst fragments within the brown matrix, and 4) a distinctly bimodal grain-size distribution among paleosol components (mostly cobble-sized basalt clasts and clay- to silt-sized grains in the brown matrix). Also, cm-scale pedogenic carbonate crusts preserve marine skeletal sand, including abraded coralline algal fragments and pristine micro-gastropod shells. Marine sand inclusion suggests paleosol development at least within reach of storm surge. Given the textural characteristics of the paleosol interval, the young age of Koloa Volcanics, and the low topography of this area today, it is more likely that the interval represents floodwater muds deposited within nearshore basalt cobbles than the distal portion of a debris flow.

Bulk chemical analysis of the brown matrix indicates that mobile and immobile elements are depleted and enriched as one would expect from a soil developing on a basaltic source in a humid climate. However, the unusual similarity of trace element concentrations within the basalt clasts and the brown matrix is much harder to interpret. This similarity could be explained by either a more arid climate during the formation of the brown matrix or by fluid exchange with a later basalt flow that could have partially indurated the clays and allowed the matrix to reacquire the basaltic trace element signature.