DURIPANS OF SOUTHWESTERN IDAHO - POLYGENESIS DURING THE QUATERNARY DEDUCED THROUGH MICROMORPHOLOGY

There is uncertainty regarding the pedogenesis of duripans on the Owyh ee Plateau of southwestern Idaho. Micromorphology was used to deduce t he role of polygenesis in their formation. Data suggest that during th e Pleistocene, duripans were cyclically subaerially exposed, then reco vered by eolian dust. Two lines of evidence indicate past duripan expo sure. First, sinuous laminar fabrics with lenticular voids, which occu r within most duripan plates, are similar to the fabrics produced in a rid regions by the action of lichens and cyanobacteria at the soil sur face. Second, a laterally extensive air-fall tephra layer in the middl e of a duripan plate was deposited at one sampling location, indicatin g the duripan or protoduripan was exposed at the ground surface. A sug gested model to explain the data is as follows: (i) during cold stages of the Quaternary, periglacial processes, possibly assisted by eolian deflation, caused the truncation of loose soil material overlying the duripan; (ii) cryptogamic: organisms then colonized the exposed durip an surface and produced distinctive fabrics; (iii) in response to the increased production and delivery of eolian dust during interglacials, the duripan was recovered. Pedogenesis during interglacial times resu lted in precipitation of CaCO3, thereby thickening the upward-developi ng duripan and preserving micromorphological signatures of subaerial e xposure. At higher elevations in this region, large-scale removal of u nconsolidated soil may have occurred during full glacial stages of the middle and earlier Pleistocene. Maximal development of duripans may r equire polygenesis.