A PALEOSOL ORIGIN FOR AN UNUSUAL BROWN BASALTIC INTERVAL WITHIN KOLOA VOLCANICS, POIPU BEACH, KAUAI, HAWAII

An unusual decimeter-scale interval sandwiched between basaltic lava of the Koloa Volcanics (1.5-0.5 Ma) contains centimeter- to decimeter-scale, angular to round clasts of vesicular olivine basalt surrounded by a vesicular brown groundmass. Exposures along a rocky promontory at Poipu Beach (southeastern Kauai) show that the brown groundmass is indurated and only slightly less resistant to wave erosion than the basalt clasts. X-Ray diffraction reveals that the brown groundmass is largely composed of chlorite-vermiculite and saponite group clays mixed with calcite and relict plagioclase, olivine and clinopyroxene grains. Comparisons of REE plots show clearly that the brown groundmass material inherited its trace element geochemistry directly from the underlying basalt. Trace element concentrations of the brown groundmass and the underlying vesicular basalt are nearly identical with the exception of U, Th, and As, which are markedly enriched in the brown groundmass. Enrichments in these elements may reflect interaction with biologic materials and/or seawater. Petrographic analysis reveals saponitic clay rims on most grains along the basalt clast margins and on a few phenocrysts of plagioclase and olivine that persist within the brown groundmass. Sinuous tube-shaped, branching millimeter- and submillimeter-scale calcite masses and vesicles partially filled with calcite resemble root traces. A few larger, elongate open vesicles within the brown groundmass contain a mixture of marine skeletal and angular basalt sand grains. The textural and compositional evidence suggests that this interval represents a regolithic paleosol horizon that was baked and indurated by the later overlying flow.