Soil-forming processes and rates on uplifted marine terraces in southwestern Oregon, USA

Soil chronosequences were examined on flights of tectonically uplifted marine terraces near Cape Blanco and Cape Arago, southwestern Oregon. All of the soils contain abundant organic C and are strongly acidic (pH in H2O < 4.7) with < 10% base saturation and > 30% of the exchange sites saturated with Al, The soils in the two areas show a progression in development from Spodosols to Spodosols with clay-enriched horizons and eventually to Ultisols. Although the spodic and argillic horizons appear to be forming concurrently in the soils, evidence is presented to support the concept that the spodic horizon is eventually masked by clay accumulation because of weathering in situ, neoformation of clays, and clay translocation. The primary age-related trends include increases in (1) the relative abundance of silt and clay, (2) the ratio of quartz to feldspars in the very fine sand (100-50 mu m) fraction, and (3) the amounts of dithionite-extractable Fe and Al and crystalline forms of Fe in the profile.