RE-INTERPRETATION OF NORTHERN GREAT BASIN PALEOSOLS

Fossil soils between lake deposits of the last two deep-lake cycles in the northern Great Basin have multiple carbonate-engulfed horizons, and compound calcic and argillic properties at depth. They are difficult to explain pedogenetically because they bifurcate in places, especially in basin centres where they appear separated by sediments of a minor lake expansion of probable early Wisconsinan age.

A sequence of processes may account for all observed phenomena: two cycles of colluviation and synchronous admixture of carbonate-bearing loess particles were each succeeded by the formation of an argillic over a calcic horizon. The first of these cycles started at the end of oxygen isotope stage 6, the second one around the transition between stages 4 and 3. Carbonate enrichment resulting from the later soil formation often affected the older, buried soil. This led to welding of the two soils and the formation of a compound horizon carrying the argillic properties of the first and the calcic properties of the second pedogenic phase. Steady eolian influx to the same, former soil surface, a commonly invoked mechanism, cannot explain the bifurcated palaeosols and is not needed to explain the welded ones. One important implication of these findings is that paleosols in this area may bear information on paleoclimate to a greater extent than, or even in contrast to, previous interpretations.

On surfaces, which were not covered by lakes at 13ka B.P., a relict soil of intermediate maturity has developed in another loess-mixed colluvial layer. At paleolake margins the three different deposits and soils merge into one multilayered soil profile with disconformities still representing the stratigraphic borders. These may be traced into mountains surrounding the Great Basin.