ABSTRACT

Hutt and Leeman Lagoons, in the Perth Basin, Western Australia, are shallow (< 11 m) evaporitic basins developed during the past 6,000 years as a result of ponding in a Pleistocene dune terrain. A comparative study of the Holocene-Recent marine, alluvial, and evaporitic sediments in the lagoons has defined the following stages of evaporite basin evolution:

1) Open marine embayment stage; represented by a sheet of lithoskeletal grainstone incorporating skeletal remains and seagrass leaf-sheaths.

2) Marine lagoonal stage; the accretion of coastal barrier beach and dune complexes progressively restricted open marine circulation resulting in changes in the faunal and floral assemblages and an upward fining of the marine sediment sequences. At Hurt, concurrent with barrier formation, the embayment was encroached by prograding alluvial suites due to Hurt River discharge; no similar alluvial suite is present at Leeman Lagoon.

3) Evaporitic pond stage; following the isolation of the lagoons from the sea, gypsum and mud layers developed as a result of alternating brine concentrations in the ponds. Rapid shoaling produced playas where lagoonal brines were confined below the sediment surface; locally subaerial exposure resulted in the reworking of earlier evaporites towards the lagoon centers. Subsequent winnowing of surface eolian sediments facilitated the development of a veneer of pellet intraclast grainstone which has since prograded basinward forming extensive barren zones and salt-marsh flats.

4) Contemporary pond-playa stage; sedimentation is related to distribution and interaction of hydrologic units (groundwater, seawater, and lagoonal brines). Precipitation is restricted to ephemeral halite in ponded waters and the vadose diagenetic emplacement of CaSO₄ minerals in playa sediments.