ABSTRACT

Holocene groundwater lakes (salinas) are common in the coastal zone of South Australia. They usually contain an evaporitic carbonate sequence that surrounds a central gypsum unit to form a "bull's-eye" pattern of sediment distribution in the salina. The evaporitic carbonates can be subdivided into three units: a boxwork boundstone unit, a veneer boundstone unit, and an algal boundstone unit. These units contain many interesting structures including tepees, stromatolites, and mound springs, all of which show a megapolygonal outcrop pattern. The salina carbonates have formed under a hydrological regime that has changed little since its inception some 6000 years ago. Consequently it has been possible to interpret the hydrological significance of the salina carbonates. The salinas are fil ed by schizohaline surface waters whose water levels and salinities fluctuate seasonally. All of the salina carbonates form where marine-derived groundwater resurges from a surrounding dune aquifer into the margin of the salina. Tepee structures form in the veneer boundstone as a response to groundwater-induced seasonal changes in the pore pressures of the underlying boxwork sediments. The recognition of such groundwater tepees or other groundwater-controlled structures in ancient carbonate sequences could aid paleohydrological interpretation of the ancient basin.