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Abstract:

Lake Hayward is a small coastal hypersaline lake in the Clifton-Preston lakeland system of southwestern Australia. The lake is a Na-Cl-SO4 brine, compositionally similar to seawater, with a salinity exceeding 200 g/l in summer. Although only 2.75 m deep, it is thermally and chemically stratified between April and January.

Groundwater tepees, desiccation features, crusts, gas escape structures, and degrading benthic microbial mats (BMC) occur around the lake margins. Pleistocene carbonate pinnacles acted as nucleation sites for carbonate-precipitating microbialite communities. In the lake center, the surface is covered with non-carbonate precipitating BMC communities, underlain by a thin gypsum layer.

The stratigraphy established from 20 cores shows a disconformable transition from Pleistocene eolian coastal-dune sedimentation at the base of the cores to a Holocene limnic swamp. The swamp phase is dated at about 8,000 years BP. Holocene deposition began when rising sea level caused the freshwater swamp to develop over the Pleistocene dune sand and calcretized calcarenite. Wetland conditions became predominant with progressively higher sea level and concomitant rise in water table. Dune barriers were breached and for a short interval Hayward became part of a complex marine inlet with a marine fauna.

Gradual lowering of sea level isolated Hayward from the rest of the system. Since 7,000 yr BP, a saline lacustrine environment has prevailed. The lacustrine sediments are laminated and dominated by chemically and biologically precipitated carbonates. Significant amounts of diagenetic dolomite are present. The lake-center cores contain -25 percent TOC.

At Lake Clifton, the latest part of a pre-lacustrine marine inlet phase has been dated at between 4,670 and 3,890 yr BP, ending later than at Lake Hayward. Lake Hayward apparently became isolated from the ocean earlier than Lake Clifton and consequently has a longer lacustrine sedimentary record.

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