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Event stratigraphy of the Badenian selenite evaporites (Middle Miocene) of the northern Carpathian Foredeep

Bąbel M.

Acta Geologica Polonica

2005

Vol. 55, no. 1

9-29

The Middle Miocene (Badenian) evaporites of the northern Carpathian Foredeep were deposited in a salina-type basin. Calcium sulphate sediments were deposited mainly on the broad northern margins of the basin, on vast evaporite shoals (mainly as fine-grained microbialite gypsum) and in large shallow saline pans (as coarse-crystalline selenites). 125 sections of these primary deposits, exposed from Moldova, Ukraine, Poland to the Czech Republic, were subjected to stratigraphic analysis based principally on the methodology of event and high-resolution stratigraphy. Due to an extremely gentle relief and a predominantly aggradational type of deposition, typical of a salina basin, the environmental changes or events were recorded nearly instantaneously in the whole area. Some events, such as water-level or brine-level fluctuations (emersions and floods, which can be very rapid in a salina basin),fluctuations in the average pycnocline level, aeolian dust or ash falls, accretion of specific gypsum microbialites, produced sets of marker beds which are perfectly correlated over distances of tens to hundreds of kilometres. Some thin grass-like selenite beds, representing deposits of shallow flat-bottomed saline pans, were correlated precisely over such great distances and are interpreted as isochronous or near-isochronous. Each bed was presumably deposited during the average pycnocline level highstands in the saline pan. The thick-bedded selenite units do not show long-distance bed-by-bed correlation, presumably because they were deposited in deeper pans in which the pycnocline fluctuations were recorded by bedding planes (i.e. by intercalations of fine-grained gypsum or dissolution surfaces) only on the shallow slopes or swells. However some apparent growth zones in the gypsum crystals from such selenite units were correlated throughout the basin, proving that the selenite growth However some apparent growth zones in the gypsum crystals from such selenite units were correlated throughout the basin, proving that the selenite growth was isochronous.

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.