SOIL DEVELOPMENT ON A PLEISTOCENE TERRACE SEQUENCE, BOISE VALLEY, IDAHO

Study of a sequence of terraces in the western Snake River Plain of Id aho reveals a record of at least seven terraces, the ages of which spa n the Pleistocene. In the Boise Valley, the youngest terraces are less than similar to 14,500 yr and the oldest terraces are similar to 1.7 Ma. Within this sequence, several relationships exist between soil mor phology and terrace chronology. On terraces older than similar to 14,5 00 yr, argillic horizon development generally increases with terrace a ge with maximum development occurring in soils of the oldest terraces. CaCO3- and SiO2-cemented duripans are found in soils on terraces that are late middle Pleistocene and older. By virtue of their physical an d chemical properties, duripans are very resistant to erosion, and the refore provide stable records of CaCO3 and SiO2 accumulation throughou t multiple cycles of loess deposition onto the terrace treads, pedogen esis, and partial erosion. Mean duripan thickness increases with age t o a maximum of 0.66 m on the oldest terraces. Our results suggest that a geomorphic surface age of approximately 130,000 yr is required to f orm the initial plugged horizon that is characteristic of a duripan. C aCO3 and SiO2 accumulation is most rapid in duripans occupying geomorp hic surfaces with ages ranging from 130,000 to 300,000 yr. After this, apparent accumulation rates decrease and little additional accumulati on of these cementing agents occurs with time.