Identification of Paleosols and the Fort Hall Geosol in Southeastern Idaho Loess Deposits

Information on the origin and history of the southeastern Idaho loess deposits was obtained through examination of 11 loess sections for evidence of paleosols. These sections comprised three transects traversing the loess deposits adjacent to the Snake River flood plain near Pocatello, Idaho. Characteristics identifying paleosols were found in 7 of the 11 sections. The paleosols were identified by zones in the loess section having one or more of the following properties: (i) a twofold increase in calcium carbonate equivalent, (ii) a sharp increase to a strong maximum in soluble salts, and (iii) an increase of 5 to 10% in the amount of clay. The paleosols show as strong or stronger development than the present modern soil. When a paleosol can be positively identified as occurring at more than one site, it becomes a marker used in the study of geologic formations such as loess deposits. A paleosol that serves this purpose is termed a geosol, and becomes a valuable tool in correlation of soil parent materials. The Fort Hall Geosol was identified in loess sections at 7 of the 11 study sites. The Fort Hall Geosol differed from the rest of the loess section and the other paleosols by having higher total clay content and finer clay, more montmorillonite, higher amounts of coarse silt, very fine sand and coarser sands (1.0 to 0.1 mm); and higher calcium carbonate and total salts. These identifying characteristics infer the nature of events which took place to form these paleosols. The characteristics suggest polypedogenesis. The initial event in the formation of the paleosols resulted in accumulations of relatively insoluble carbonates and clays. The Fort Hall Geosol represents a much stronger set of soil forming factors than those responsible for the formation of the existing modern soil or the other palesols in the section. Later, soluble salts accumulated in or above the zone of carbonate and clay accumulation. The soluble salts appear to be related to a subsequent deposition followed by a period of leaching.