GENESIS AND RELATIONSHIP OF MACROMORPHOLOGY AND MICROMORPHOLOGY TO CONTEMPORARY HYDROLOGICAL CONDITIONS OF A WELDED ARGIXEROLL FROM THE PALOUSE IN IDAHO

Soils formed in distal loess of the Palouse region have welded profile s that make assessment of their genesis and pedostratigraphic relation ships difficult. These soils typically have a mollic-cambic horizon se quence that is underlain by a well-expressed albic E horizon and a str ongly developed argillic horizon. In this study, we utilized hydrologi cal monitoring combined with micromorphological analysis and new strat igraphic data to help establish the genesis of a representative Argixe roll of the region and the relationship of its morphology to ancient a nd contemporary hydrological conditions. Monitoring results indicate t hat the Argixeroll is seasonally episaturated for approximately 5-6 mo nths per year. Ferrimanganiferous nodules, segregations, hypocoatings, and depletion mottles are observed in thin section throughout the ped on, but dominate within, just above, and below the present-day E horiz on. Measured soil Eh is sufficiently low for Fe and Mn reduction to oc cur for 1-3 months during the period of episaturation, suggesting that at least some of these redoximorphic features are likely to be contem porary. Other compound redoximorphic features are clearly relict, howe ver, as they are coated by illuvial clay which is unaffected by Fe/Mn redistribution processes. Thick illuvial clay coatings on interpedal f issures of the argillic horizon postdate pockets of Holocene tephra wi thin deeply bioturbated aggregates, while other channel argillans in t he same part of the pedon are clearly older. This indicates that at le ast two distinct phases of clay accumulation have occurred, the most r ecent being contemporaneous with loess aggradation during the Holocene . Our results suggest that these welded profiles consist of a mollic-c ambic horizon sequence that has formed primarily in Holocene loess and an albic-argillic sequence that has formed in Wisconsinan loess. The modem-day E horizon was initiated as the clay-eluvial surface horizon of the regionally recognized Washtucna paleosol that continued forming into the late-Wisconsinan cold phase and, upon burial by Holocene loe ss, it has been transformed into an albic horizon and even enlarged as a result of redoximorphic processes associated with seasonal saturati on. (C) 1998 Elsevier Science B.V.