Late Quaternary soils on clay dunes of the Willandra Lakes, New South Wales

Abstract

The clay dunes of the Quaternary lakes of the Willandra system of New South Wales provides a framework for a comparative study of soil development. Extensive work by earlier researchers has resulted in a detailed soil stratigraphy, and palaeo-environmental history of the study area, all based upon an absolute chronology developed by radiometric dating techniques. The main elements of the dune stratigraphy are the Golgol unit (120 000+ to 44 000 years BP), the Mungo unit (44 000 to 18 000 years BP) and the Zanci (18 000 to present). Two thin eolian units (Eo2 and Eol) post-date the Zanci in some parts of the area. Clay dunes of the Zanci have a narrow range of parent materials which have been divided into two main groups, the sandy clay and the clay dune facies. A comparison is made between soils on crests of two dunes of the same age (15 500 years) but with contrasting parent materials. Solonetzes are characteristic of the clay dune facies, earthy sands of the sandier materials. Soil processes such as illuviation of clay, carbonate, gypsum, and sodium salts, clay pellet collapse, and weathering are identified and compared between the two lithogies. The main effect of parent material on pedogenesis concerns depth of leaching and the mobility of salts. The low clay content of the sandy materials does not allow the development of solonetzic characters; the sodium salts are flushed with little effect on the profile. Uniform parent materials of the Zanci clay dune facies in some dunes allows the influence of topography to be assessed in a single valley. Here the movement of solutes and suspensions through the valley was controlled by (a) valley morphology, and (b) orientation of the valley with respect to solar radiation. Solonetzes are formed on the crests and slopes, but solonetzic - chernozems have formed on the valley floor. Similar processes operate over the whole valley; only rates and depths of pedogenesis processes vary. Time as a control in soil development is studied by comparing two profiles of earthy sands on the sandy clay dunes. Soils of two pedoderms are compared, the Upper Mungo (19 500 BP to present) and the Zanci (15 500 BP to present); the longer period of soil development in the former resulted in increased depth of solum, a stronger red colour in the B horizon and marginally higher subplasticity ratings. Polygenesis is identified even in the youngest soil stratigraphic unit of the study area, with two main phases of soil development occurring during the last 15 500 years. The main characters of the dune soils were formed during the longer Phase B (15 500 to 2500 BP) with the effects of a shorter Phase A (2500 BP to present) being restricted to the modification of the Phase B soil characters. Mobile salts respond to these separate soil development phases by being redistributed through the dune according to their relative mobilities. The result of such sequences of salt mobility is the formation of superimposed patterns (soil chromotographs) from each phase. Eolian accession of salts to the soil surface is a significant feature of the soil history. Local sodium salts derived from the lake floor have resulted in the development of solonization during Phase B. Regional salt and dust acce4ssions have added clay, silt and carbonate to the soil over the last 15 500 years, yet these materials are indistiguishable from the rest of the parent materials. Wustenquarz contents of the soils and sediments, which are interpreted as evidence of periods of mobility/stability of the linear dunefields to the west, are combined with other data to produce a geomorphic history of the clay dunes. The development of type sites in the study area for the Zanci on soil morphological evidence, without the need for radiocarbon dates.