Soil Development of a Fluvial Terrace Chronosequence in the Oregon Coast Range

Pedogenic evolution was evaluated for a fluvial terrace chronosequence in the Oregon Coast Range. Soil chemical and physical analyses were coupled with an elemental mass-balance approach to determine time-dependent changes in soil properties. The seven soil profiles formed in poorly sorted, mixed Eocene alluvium and colluvium of sequential fluvial terraces and range in age from several thousand to ~990ka. Dominant processes in soil evolution include (1) depletion of Si, Na, Ca, K, and Mg; (2) conservation and redistribution of Al and Fe; and (3) transformation of Al and Fe from the parent material to secondary pedogenic oxides. Si represents the largest and most continuous net elemental loss with increasing age and changes from -28.7g/cm² at 40ka to -219g/cm² by 990ka. Na and Ca reach maximum elemental depletion by 140ka with a net absolute loss of 5.62g/cm² and 3.41g/cm², respectively, and remain equally depleted with increasing terrace age. Maximum elemental depletion of K and Mg is reached by 200ka, and depletions subsequently lessen with increasing age. Mineral analyses corroborate mass-balance results of complete removal of Na and Ca with increasing age and a shift of Si, Mg, and K from sand to clay-sized particles. Al and Fe are redistributed within the profiles but differ with rate and duration of systematic change. Pedogenic forms of both Al and Fe increase with soil age, and while pedogenic Al consistently remains a small fraction of total Al, pedogenic Fe comprises an increasing amount of total Fe in the soils. Redness index increases with soil age as does the concentration of clay cutans, percentage of clay content, and abundance of concretions. Evaluation of the soil chronosequence suggests a shift from desilication and acidification as the dominant processes for the first 200ka to the subsequent production and redistribution of secondary iron oxides and clay minerals.