Fine-scale structure and micromorphology of the Cricket Flat paleosol, Elgin, Oregon, USA

The Cricket Flat paleosol is located about 9 km east of Elgin, Oregon on Oregon Route 82. The paleosol underlies an olivine basalt of the Powder River Volcanic Field, a sequence of Middle Miocene to Pliocene lavas that overlie the Columbia River Basalt Group in northeastern Oregon. The parent material of the paleosol is a felsic to intermediate lahar that contains leaf and twig fossils as well as tree casts. While some researchers have measured the bulk chemistry and clay mineralogy of this paleosol, no study of its micromorphology has been attempted. In this study we viewed polished thin sections with a petrographic microscope to examine both the parent material and the paleosol. Scanning electron microscopy was used to understand the composition of minerals. Soil texture was analyzed using point counts. Skeleton grains inherited from the parent are mainly plagioclase, orthoclase feldspar, quartz, and volcanic glass. Accessory minerals include titanite, epidote, apatite, and zircon. The paleosol has an argillic horizon with vertic features that are not apparent at the field scale. Diatoms, palynomorphs, and root traces are relatively common in the A horizon of the paleosol. Strong sepic plasmic fabrics and redoximorphic features suggest an environment that was at least seasonally waterlogged and subjected to shrink-swell processes.