SOURCES OF DISSOLVED SALTS IN THE CENTRAL MURRAY BASIN, AUSTRALIA

Large areas of the Australian continent contain scattered saline lakes underlain by shallow saline groundwaters of regional extent and debat ed origin. The normative salt composition of subsurface pore fluids ex tracted by squeezing cores collected during deep drilling at Piangil W est 2 in the central Murray Basin in southeastern Australia, and of su rface and shallow subsurface brines produced by subaerial evaporation in the nearby Lake Tyrrell systems, helps constrain interpretation of the origin of dissolved solutes in the groundwaters of this part of th e continent. Although regional sedimentation in the Murray Basin has b een dominantly continental except for a marine transgression in Oligoc ene-Pliocene time, most of the solutes in saline surface and subsurfac e waters in the central Murray Basin have a distinctly marine characte r. Some of the Tyrrell waters, to the southwest of Piangil West 2, sho w the increase in NaCl and decrease in sulfate salts expected with eva porative concentration and gypsum precipitation in an ephemeral saline lake or playa environment. The salt norms for most of the subsurface saline waters at Piangil West 2 are compatible with the dilution of va riably fractionated marine bitterns slightly depleted in sodium salts, similar to the more evolved brines at Lake Tyrrell, which have rechar ged downward after evaporation at the surface and then dissolved a var iable amount of gypsum at depth. Apparently over the last 0.5 Ma signi ficant quantities of marine salt have been blown into the Murray Basin as aerosols which have subsequently been leached into shallow regiona l groundwater systems basin-wide, and have been transported laterally into areas of large evaporative loss in the central part of the basin. This origin for the solutes helps explain why the isotopic compositio ns of most of the subsurface saline waters at Piangil West 2 have a st rong meteoric signature, whereas the dissolved salts in these waters a ppear similar to a marine assemblage.