Calcic soils of the southwestern United States

Calcic soils are commonly developed in Quaternary sediments throughout the arid and semiarid parts of the southwestern United States. In alluvial chronosequences, these soils have regional variations in their content of secondary calcium carbonate (CaCO 3) because of (1) the combined effects of the age of the soil, (2) the amount, seasonal distribution, and concentration of Ca ++in rainfall, and (3) the CaCO 3content and net influx of airborne dust, silt, and sand. This study shows that the morphology and amount of secondary CaCO 3(cS) are valuable correlation tools that can also be used to date calcic soils. The structures in calcic soils are clues to their age and dissolution-precipitation history. Two additional stages of carbonate morphology, which are more advanced than the four stages previously described, are commonly formed in middle Pleistocene and older soils. Stage V morphology includes thick laminae and incipient pisolites, whereas Stage VI morphology includes the products of multiple cycles of brecciation, pisolite formation, and wholesale relamination of breccia fragments. Calcic soils that have Stage VI morphology are associated with the late(?) Miocene constructional surface of the Ogallala Formation of eastern New Mexico and western Texas and the early(?) Pliocene Mormon Mesa surface of the Muddy Creek Formation east of Las Vegas, Nevada. Thus, calcic soils can represent millions of years of formation and, in many cases, provide evidence of climatic, sedimentologic, and geologic events not otherwise recorded. The whole-profile secondary CaCO 3content (cS) is a powerful developmental index for calcic soils: cS is defined as the weight of CaCO 3in a 1-cm 2vertical column through the soil (g/cm 2). This value is calculated from the thickness, CaCO 3concentration, and bulk density of calcic horizons in the soil. (See Soil Survey Staff, 1975, p. 45–46, for a complete definition of calcic horizon.) CaCO 3precipitates in the soil through leaching of external Ca ++that is deposited on the surface and in the upper part of the soil, generally in the A and B horizons. The cS content, maximum stage of CaCO 3morphology, and accumulation rate of CaCO 3in calcic soils of equivalent age can vary over large regions of the southwestern United States in response to regional climatic patterns and the influx of Ca ++dissolved in rainwater and solid CaCO 3 Preliminary uranium-trend ages and cS contents for relict soils of the Las Cruces, New Mexico, chronosequence show that 100,000- to 500,000-year-old soils have similar average rates of CaCO 3accumulation. Conversely, soils formed during the past 50,000 years have accumulated CaCO 3about twice as fast, probably because the amount of vegetative cover decreased in the Holocene and, hence, the potential supply of airborne Ca ++and CaCO 3to the soil surface increased. The quantitative soil-development index cS can be used to estimate the age of calcic soils. This index can also be used to correlate soils formed in unconsolidated Quaternary sediments both locally and regionally, to compare rates of secondary CaCO 3accumulation, and to study landscape evolution as it applies to problems such as earthquake hazards and siting of critical facilities.