SOIL PROPERTIES AND GENESIS OF PANS IN TILL-DERIVED ANDISOLS, OLYMPICPENINSULA, WASHINGTON

Subsoil pans regarded as cemented are present in certain Andisols of t he Olympic Peninsula region of western Washington. Soils formed in bas alt-derived alluvium or colluvium over Pleistocene-aged compact alpine till. Theories of pan formation include pedogenic cementation by sili ca, allophane, or humic-metal complexes; or geogenic till compaction w ith no subsequent cementation. We examined the properties of three ped ons with compacted and cemented subsoil horizons to better understand the genesis of these soils and pan horizons. Pan horizons, designated as Cm, are very brittle, nearly impossible to dig with hand tools, and massive in structure. Soils are acidic, with pH(H2O) for all horizons ranging from 3.7 to 6.0. Crystalline phyllosilicates identified are g ibbsite, kaolinite, chlorite, vermiculite, and hydroxy-interlayered ve rmiculite. Allophane was detected in most mineral horizons, including Cm horizons, and allophane Al/Si molar ratios vary from 1.2 to 3.4. Lo w NaOH-extractable Si, low Si/Al molar ratios from NaOH extracts, high rainfall, and the presence of gibbsite decrease the likelihood of ped ogenic opal as a cementing agent in pans. Limited organo-metallic depo sition, low organic C, and low pyrophosphate-extractable Fe and Al in pan horizons suggest that podzolization processes related to ortstein formation are not responsible for Cm horizon cementation. Micromorphol ogic examination of Cm horizons detected a light brown to nearly color less substance that appears to be allophane around mineral grains and in pore channels. Soil chemistry data support allophane as the primary cementing agent in these pan horizons, with kaolinite, gibbsite, and Fe oxides acting as possible accessory agents. Glacial compaction, par ent material mineralogy, and cycles of wetting and drying are importan t factors affecting cementation of these pans.