TIMING OF THE ONSET OF ARIDITY IN SOUTHEASTERN AUSTRALIA, SUPPORTING EVIDENCE FROM THE SALINITY STRUCTURE OF A REGIONAL AQUITARD

Much of Australia is characterized by severe aridity, and the most critical environmental problem on that continent is ground water salinization. However, information on the timing of the onset of fully arid conditions is fairly sparse. Previous magnetostratigraphic studies of a number of evaporitic lacustrine sequences in Australia suggest the shift from wet to arid conditions may have occurred in central Australia at about 900 ka and at about 500 ka in southwestern and southeastern Australia. Additional support for a 500 ka or older date in southeastern Australia comes from the analysis of the salinity structure of a portion of the Gerra Clay, a regional marine aquitard situated in the center of the Murray basin. The Piangil study area is located 40 km northeast of Lake Terrell, a large saline lake situated in a structurally and topographically closed basin known as the Terrell trough. Shallow ground waters which discharge into the lake are evaporated to halite saturation, and Lake Terrell brines recharge into the underlying Pliocene aquifer, creating ground waters having highly elevated salinities. At Piangil, groundwater salinities increase downward in these Pliocene sands to 90 g/L at the top of the Geera. There is then a pronounced linear decrease in salinity downward over a depth of 120 m within the Geera to 15 g/L at its base. Based on both water level data and Rayleigh analysis, it is most probable that the salinity profile within the Geera was generated by diffusion-controlled solute transport. Numerical simulations indicate that it would take on the order of 500 ka for such a profile to develop, which is consistent with magnetostratigraphic dating of evaporitic sequences from the Lake Terrell area. Lake Terrell is known to have oscillated between saline and fresh water conditions in the geologic past, but it is unlikely that shallow fresh ground waters recharged from the lake could have displaced dense subsurface brines previously ponded within Pliocene sands in the Terrell structural trough.