Soil genesis on peralkaline felsics in Big Bend National Park, Texas

Soil genesis involving felsic rocks frequently involves understanding the r oles of climate and time on the transformations of the primary minerals int o secondary products. This study focuses on the weathering of peralkaline f elsics in a semiarid climate. Soils and fresh parent materials were collect ed 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 site s at lower and drier locations have an ochric-rock horizon sequence. Atmosp heric deposition of Ca and Sr and wind deflation of clay are important attr ibutes of soil genesis. Elemental loss rates of Si, Al, K, Na, and other el ements are appreciable, suggesting that leaching has been important to the soil formation. Iron oxide formation, which is related to the moisture regi me, is greater in soils having wetter microclimates. Solution chemistry sug gests that Mg concentrations are appropriate for smectite synthesis and mai ntenance, potassium concentrations reflect the presence of feldspar, sodium concentrations reflect their loss rates because of leaching, and calcium r eflects the presence of calcite. The rare earth elements similarly demonstr ate loss rates consistent with chemical weathering. In general, chemical we athering of feldspars, atmospheric deposition of Ca and Sr, and wind deflat ion are important pedogenic processes.