Supplement to the Desert Project guidebook, with emphasis on soil micromorphology
Authors: Gile L. H. , Hawley J. W. , Grossman R. B. , Monger H. C. , Montoya C. E. , Mack G. H. 
Published: March 2023

Companion to Memior 39. This supplement, which was prepared for a field trip in conjunction with an international meeting of soil micromorphologists, presents additional information for many of the original Desert Project study areas and presents new information for five additional study areas. Major changes in the classification of the Aridisols were made in 1994; the new system and its relation to the old one are shown.

Illustrative soils and landscapes range in age from late Holocene to late Pliocene. Photomicrographs in color include a variety of argillans and calcitans, evidence for dissolution of primary grains, silica nodules that post-date argillans in an ancient pipe, and both clay formation and illuviation in a soil formed in bedrock.

The five additional study areas are located on detailed soil maps and include sites illustrating (1) formation of the stage IV carbonate horizon and its radiocarbon chronology, the first to be reported in the world literature on genesis of soil carbonate; (2) profound changes in carbonate morphology (stage III to IV) associated with change in particle size; (3) a rare bedrock-defended, ancient fan and soil in Ice Canyon of the Organ Mountains; (4) a scarplet terrain in high-carbonate sediments where argillic horizons have formed in late Pleistocene soils but not in Holocene soils; and (5) a middle Pleistocene soil with evidence for dissolution of silicate grains by pressure solution, for precipitation of calcite by microorganisms, and for neoformation of palygorskite.

When the Desert Project began in August 1957, one of the problems for study concerned the origin of horizons of silicate clay accumulation. At the time, the prevailing opinion was that horizons of clay accumulation in desert soils formed by weathering in place. This view was still common well into the 1960s. Thin section studies are one way of learning more about the origin of these horizons. Thin sections showed that nearly all bed surfaces and pores lacked clay skins in the arid part of the study area, pipes being the only exception. Instead, coatings of oriented clay on sand grains and pebbles were found to be characteristic of the Bt horizons. Field studies, laboratory analyses, and thin sections all indicate that the red to brown horizons of silicate clay accumulation in the Desert Project area contain illuvial clay. A summary of Bt horizons and evidence for illuviation is presented in the Desert Project Guidebook.

Photomicrographs illustrate illuvial clay and carbonate in soils that range in age from late Holocene to middle to early Pleistocene. Coatings of oriented clay on sand grains and pebbles are characteristic of the Bt horizons. Grain argillans of many Bt horizons have been partly to completely obliterated by carbonate. Calcified root hairs, calcite filaments, and framework grain coatings are the youngest forms of carbonate accumulation. They are currently forming and are the major morphological expression of carbonate in late Holocene soils. Progressively older carbonate forms increase in density and hardness with increasing carbonate content. Thin sections show evidence of dissolution of primary grains in soils of late middle Pleistocene age and older. Companion to Memoir 39.