A CRYO-CALCIC THRESHOLD MODEL OF PEDOGENESIS: THE UPPER GREEN RIVER BASIN IN WESTERN WYOMING

This study documents the effects of periglacial and eolian processes on a calcic soil chronosequence formed on a flight of ten fluvial terraces in the upper Green River Basin in western Wyoming.

Calcic horizons on the upper eight terraces all have pedogenic ooids, nodules, and fragments of petrocalcic material embedded in a friable, carbonate-rich matrix, but none of the soils reach Stage IV carbonate morphology. Dust thickness increases with increasing terrace height, and calcic horizons increase in thickness, carbonate and sepiolite content. However, the overlying A and Bt horizons and stone pavements show minimal changes across the landscape.

Many relict periglacial features occur on the T-2 through T-10 surfaces, including fossil sand wedge casts, oriented clasts, injections, and cryoturbated calcic horizons. The sand wedge fills are better-sorted and coarser than host materials and overlying sediments. Microscopic features include common frost-shattered sand, nodules, and pendants, vertically-oriented carbonate fragments, orbiculic fabric, suscitic fabric, sub-cuboidal microstructure, injections and flows.

I propose a model of pedogenesis, the Cryo-Calcic Threshold Model, which explains these trends in three stages. In Stage 1 (interglacial climate), the overall trend is slow accumulation of desert dust, translocation of silicate clay and soluble salts, and the development of incipient petrocalcic horizons. In Stage 2 (full glacial climate), the soils are frozen, surface horizons are deflated, and the calcic horizons are disrupted by thermal cracking, cryoturbation, and dissolution by cold meltwater. In Stage 3 (return to interglacial climate) accumulation of desert dust resumes and the permafrost features are slowly buried. This model is almost identical to the sequence proposed by Hall (1999), but extended from terminal moraines to the basins.

The concept of cryoturbation and enhanced dissolution during full-glacial climate explains why calcic soils on this landscape do not reach the advanced stages of morphology seen in warm-dry regions, even though carbonate content increases with time. Understanding that these soils experience processes that differ from other widely-cited models may lead to better ways of interpreting chronological and paleoenvironmental data derived from them.