New Mexico Geological Society Annual Spring Meeting — Abstracts

One of the most widely referenced soil-geomorphic studies in the US is the Desert Project, near Las Cruces, New Mexico, in which Gile et aI. defined stages of calcic horizon development in soils of arid environments. Each stage of calcithorizon development incorporates an age range of several thousands of years. To place better age constraints on these stages of development in the desert southwest, soils forming on well dated basalt flow surfaces in the Potrillo Volcanic Field, Dona Ana County, New Mexico were examined.

Soils were described on four basalt flows (Aden, AD; Afton, AF; and two flows from Little black mountain, LBM & LBMX). Surfaces were dated using ⁴⁰AI/³⁹AI and surface dating methods and their are: AD, ~20 ka; AF, ~90 ka; LBM, ~188 ka;, LBMX >200 ka. Basalt flow surfaces evolve topographically through time as depressions are filled with basalt rubble and eolian dust. Young flows have as much as 4 meters of relief while the surfaces of older flows are virtually flat. Soil development on the surfaces varies according to where the soil is formaing in relation to highs and lows in the flow topography. Soils developing in low areas where more eolian material has accumulated show cummulic profiles and more apparent development than the soils of higher areas. Separate chronofunctions for soils developing on highs and lows are therefore required to adequately describe soil evolution through time.

Chronofunctions of profile carbonate accumulation suggest a break in slope around 150 ka. Carbonate percentages increase more rapidly with age in the LBM and LBMX soils and they have significantly higher pH and conductivity levels in depth profiles than AD and AF soils. LBM and LBMX soils are also sandier in texture than the AD and AF soils.

The sandier texture of LBM and LBMX soils could be influencing carbonate precipitation, pH, and conductivity. Higher sand and carbonate components of LBM surfaces can be explained in at least 2 ways or a combination of both: 1) A change in dust composition, i.e. to more sandy and/or more CaCO3 rich, around 150 ka. 2) Dust size decreases across the Potrillo Volcanic Field with increasing distance from the Rio Grande. A change in dust composition in this area has climatic change implications for the southwest, suggesting increasing aridity during the Quaternary.