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Selenite-gypsum microbialite facies and sedimentary evolution of the Badenian evaporite basin of the northern Carpathian Foredeep

Bąbel M.

Acta Geologica Polonica

2005

Vol. 55, no. 2

187-210

Facies analysis was applied to the six main facies of the Badenian (Middle Miocene) gypsum deposits exposed along the margin of the Carpathian Foredeep basin, from Moldova to the Czech Republic. These facies, recognised within primary selenite and fine-grained gypsum deposits, are: (i) selenites with vertical crystals; (ii) selenites with horizontal crystals; (iii) selenite debris flow facies; (iv) selenite debris facies; (v) gypsum microbialite facies; and (vi) alabastrine facies. The facies represent various environments (from shallow-brine to subaerial) of a giant salina-type basin without open-water connections with the sea and showing evaporite drawdown. Integration of facies analysis and event stratigraphic studies in the gypsum basin allowed reconstruction of its sedimentary history.The architecture of the gypsum facies suggests that the margin of the basin was occupied by a system of variable perennial saline pans (dominated by selenite deposition) and evaporite shoals (dominated by gypsum microbialite deposition). The basin was infilled with evaporite deposits by aggradation. After initial evaporite drawdown, the northern margin of the basin evolved from a large perennial saline pan (or system of pans) into an evaporite shoal and then back again into a perennial pan, whereas the east area of the basin was a vast evaporite shoal dominated by gypsum microbialites. Separate selenite pans of oligotrophic-type developed both at the periphery and in the interior of this shoal. Later, predominantly clastic gypsum deposition developed throughout the basin margin, presumably due to a drastic change in the chemistry and salinity of the brine. Evaporite deposition was arrested by a flood of marine waters and rapid deepening.

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

Bąbel M.

Acta Geologica Polonica

2004

Vol. 54, no. 2

219--249

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 evaporition. 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 basic 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.