Spatial variability of soils developing on basalt flows in the Potrillo volcanic field, southern New Mexico: prelude to a chronosequence study

Abstract

If the systematic spatial variability of soils in a chronosequence is identified and accounted for, the accuracy of quantitative data derived from soil chronosequence studies will be increased. A sample design using landscape positions with minimal variability could result in more accurate chronofunctions from these studies. Four basalt flows in the Potrillo volcanic field, southern New Mexico, with ages ranging between 20 ka and 260 ka (40Ar/39Ar and/or cosmogenic 3He methods) provide a sound basis for a soil chronosequence study. Basalt flow surface relief in the Potrillos reduces with time as depressions fill with basalt rubble and aeolian dust. Soil variability is primarily a function of landscape position with respect to ridges and swales in the original basalt flow topography. Soils developing over original topographic lows (swale soils) form primarily in aeolian dust, have larger amounts of total carbonate and soluble salts, and display greater variability than soils developing over original topographic highs (ridge soils). It is thus concluded that ridge soils, which have minimal variability, should be employed for a soil chronosequence study of basalt surfaces in the Potrillo volcanic field. The spatial variability of swale soils results in part from the significant hydrologic variability of low-lying landscape positions. Depth profiles of chloride concentrations suggest that hydrologic variability systematically correlates with the size and shape of depressions in which soils are forming. The infilling of depressions with aeolian material results in increasingly arid hydrologic conditions both by increasing the volume of aeolian material that is being drained and by reducing the catchment area for runoff into the depression. Depressions fill at different rates, however, depending on their size, shape and catchment area. Small, narrow depressions fill quickly, and their associated soils form under more arid conditions and have stronger development than soils in large depressions. Therefore, a number of geomorphic surfaces of varying age may develop on a single isochronous basalt flow. Each of these surfaces will have unique hydrologic characteristics and consequently different degrees of soil development. The pre-burial high water flux evident in depressions suggests that basalt flows may play an important role in aquifer recharge in this area of New Mexico. Copyright © 1999 John Wiley & Sons, Ltd.