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Detailed petrographic studies of recent or sub-recent evaporites where many critical environmental parameters are defined can help in the analysis of ancient deposits and in the establishment of chemical and environmental models for evaporite formation.

The Prungle Lakes are part of the now dry Willandra Lakes system in semi-arid southwestern New South Wales. The Willandra Lakes have been a site of cyclic Quaternary sedimentation in response to the well documented global climatic fluctuations associated with the glacial-interglacial cycle. At times of hydrologic stress, low lake levels are associated with high salt volumes, inherited from evaporative concentration in large shallow lakes upstream and the inflow of saline groundwaters. Thus, at Prungle, a complex array of concentric shorelines, lake floor terraces and lunettes has developed as basin-within-basin landforms which formed in response to filling and drying cycles. Sediments associated with these geomorphic units include:

(1) Freshwater littoral, nearshore and deep-water deposited laminated sands and muds, with biogenic carbonates chiefly of ostracodal and algal origin;

(2) Evaporites comprising interbedded clay and laminated gypsum-clay couplets, which consist of discrete thin laminae of sugary white euhedral prismatic gypsum separated by detrital clay layers with a high degree of optical orientation. Primary accessory celestite (SrSO4) occurs as discrete patches within the gypsum laminae;

(3) Groundwater pyramidal, discoidal or lenticular gypsum crystals, grown from supersaturation produced by evaporation at the capillary fringe of the water table. These have formed displacively within the sediments and caused considerable disruption of the sediment matrix;

(4) Aeolian deposition in irregular transverse ridges of sand-sized reworked groundwater gypsum and disrupted lacustrine clays. The gypsum crystals are predominantly discoidal pyramidal forms which are conspicuously oriented parallel to bedding, well sorted and extensively corroded. The clay pellets are well sorted and moderately well rounded. Higher in the profile, bedding is indistinct due to pedogenic processes, which include modification of detrital grains and secondary crystallization of gypsum and carbonate. A thin gypcrete horizon occurs at the surface.

The last cycle of sedimentation in the Prungle Lakes is believed, from correlation with lakes higher in the Willandra System, to have occurred during the period 50,000-16,000 yr B.P. Since then groundwaters have dropped below the zone of interaction with the lake floor and surface water no longer reaches the lakes. Sedimentation has ceased and soil formation with secondary carbonate and gypsum has continued to the present.

A range of sedimentary environments is identified which depend on strong seasonal oscillations. These environments are virtually absent from the Australian continent today, and their occurrence during the Pleistocene supports the contention of enhanced climatic seasonality at that time. The array of sediment types and mineralogies is almost identical to that characteristic of coastal sabkha sequences highlighting the difficulty of relying on unusual evaporative mineralogy to differentiate marine and non-marine environments.

Detailed petrographic analyses of the Prungle sequence have been made in association with a review of theoretical, experimental and observational studies of the environmental controls of calcium sulfate mineralogy and crystal habit. This has enabled the correlation at Prungle of depositional environment with gypsum petrofacies and associated compositional, fabric or structural features. These observations are more generally applicable to other evaporite deposits. Many gypsum evaporites reported in the literature have been compared with this scheme developed at Prungle.