Pedogenic clay formation from allochthonous parent materials in a periglacial alpine critical zone

Abstract

This project investigated the origin of clay minerals in regolith present above quartzite bedrock in the alpine zone of the Uinta Mountains, Utah, USA. Geochemical and mineralogical analysis indicates that these clays formed through chemical weathering of two allochthonous parent materials. Smectite-rich clays, with Mg abundance distinctly higher than modern dust or local bedrock, formed through weathering of andesitic ash layers that were deposited during the Oligocene from eruptive centers in the Basin and Range. In contrast, various stages of interstratified kaolinite-illite–smectite clays have formed through weathering of mica and feldspar delivered by eolian processes that continue today. Local bedrock appears to have played only a minor role as a parent material for clay neoformation. These results demonstrate that pedogenic clay can be common even in cold climate soils and regolith overlying weathering resistant bedrock. They also emphasize that the clay inventory in soil and regolith can reflect processes that operated before modern climatic conditions were established, and reinforce the key role played by exogenous mineral inputs in the functioning of the alpine critical zone.