SOIL GENESIS ON PERALKALINE FELSICS IN BIG BEND NATIONAL PARK, TEXAS

Abstract

Soil genesis involving felsic rocks frequently involves understanding the roles of climate and time on the transformations of the primary minerals into secondary products. This study focuses on the weathering of peralkaline felsics in a semiarid climate. Soils and fresh parent materials were collected from different microclimate regimes in the Chisos Mountains of Big Bend National Park, Texas. Soils in the Chisos mountains generally have either a mollic-argillic-rock or a mollic-rock horizon sequence, whereas other sites at lower and drier locations have an ochric-rock horizon sequence. Atmospheric deposition of Ca and Sr and wind deflation of clay are important attributes of soil genesis. Elemental loss rates of Si, Al, K, Na, and other elements are appreciable, suggesting that leaching has been important to the soil formation. Iron oxide formation, which is related to the moisture regime, is greater in soils having wetter microclimates. Solution chemistry suggests that Mg concentrations are appropriate for smectite synthesis and maintenance, potassium concentrations reflect the presence of feldspar, sodium concentrations reflect their loss rates because of leaching, and calcium reflects the presence of calcite. The rare earth elements similarly demonstrate loss rates consistent with chemical weathering. In general, chemical weathering of feldspars, atmospheric deposition of Ca and Sr, and wind deflation are important pedogenic processes.