Models for evaporite, selenite and gypsum microbialite deposition in ancient saline basins

Abstract

A group of integrated hydrological and sedimentary qualitative models is introduced for evaporite and selenite (coarsecrystalline gypsum) deposition in ancient drawdown saline basins (salinas). The general model of a salina basin as a framework for intrabasinal models of selenite and gypsum microbialite (a variety of fine-grained gypsum) sedimentation is given. Selected aspects of evaporite, selenite and gypsum microbialite deposition are reviewed and discussed. A salina basin is a depression supplied with marine water by seepage and occasional surface inflows. The intrabasinal environments comprise: (i) ephemeral saline pans, evaporite shoals, and the peculiar majanna environment (recognised in the Recent MacLeod salina, Australia); and (ii) perennial saline pans. The sedimentary dynamic of these environments is controlled largely by seasonal brine level and groundwater table level fluctuations. The perennial saline pans are characterised by three basic hydrological states: (i) meromixis – with a permanent pycnocline, (ii) monomixis to polymixis – with a seasonal or periodic pycnocline, and (iii) polymixis - without a constant pycnocline. Monomictic saline pans showing stratification in the wet period (during seasonal highstand) and mixis in the dry period of the year (during seasonal lowstand) are the most significant for subaqueous evaporite and selenite deposition. Evaporite deposition takes place mainly during a mixis period coinciding with a dry season lowstand and increased evaporation. Within intrabasinal environments selenite crusts can be occasionally deposited from permanent brine sheets on evaporite shoals or majanna flats, but are mainly the product of bottom crystallisation in the hypolimnion of the monomictic (and/or polymictic) saline pans. Shallow-brine and deep-brine selenite pans are distinguished from each other on the basis of the relationship of the seasonally fluctuating pycnocline to the bottom of the pan. Selenite deposition in the mixolimnion of a deep meromictic basin is also possible. The qualitative models can be used for sedimentological analyses of ancient selenite-evaporite basins.