Tephra Influence on Spokane‐Flood Terraces, Bonner County, Idaho

Abstract

In a previous study, our team described five Pleistocene Spokane‐flood terraces with a ridge‐and‐trough surface topography in very gravelly outwash deposits we thought differed in age by <2000 years, but others later found to differ by about 6000 years (19000 and 13000 14C yr BP). Given the possibility of a longer time difference for soil development before deposition of any tephra, we obtained analytical data and reexamined the study thinking that an age difference of 6000 yr would have implications for soil classification and possibly suggest new series. Holocene tephra in the very fine sand (vfs) fraction (0.05–0.10 mm) ranges from 1% in the ridges to 93% in the troughs. The mean of the vfs fraction tephra content of soils on ridges of the higher, and presumably older, terraces (11%) is significantly less than that of soils on ridges of the lower and younger terraces (45%). The mean tephra content in the troughs (77%) does not vary by terrace level. The maximum fine‐earth (<2 mm) allophane content of the youngest ridge is 96.3 g kg−1 and that of the youngest trough is 95.4 g kg−1 These amounts are greater than five times those of ridge soils on the older terraces. Only about 20 to 50% of the estimated Mount Mazama tephra fall remains on the older Bonner and Kootenai soils, whereas the older Rathdrum soil retains about 400% of the estimated Mount Mazama tephra fall. Minor amounts occur in the Histosol on the flood plain. Isotic 50‐ to 100‐μm thick coatings on some A and B horizon sands indicate that weathering has been minor. Low spodic‐horizon indicators (Alo and Feo in subsurface horizons), and high ammonium oxalate extract optical densities of surface horizons, also suggest no fulvic acid leaching. Below the tephra influence, however, the outwash grains have anisotropic clay minerals showing that some weathering may have occurred before deposition of the tephra. The higher allophane content, higher NaF pH, and higher P‐retention of lower terrace ridge soils, as compared with higher terrace ridge soils, results mostly from variation in the amount of their tephra and now justifies classifying ridge soils on higher and lower terraces into different taxa and opens the possibility of differentiating soil series by terrace level.