|
Working depositional models |
bel
Institute of Geology, Warsaw University, Al. 
wirki i Wigury 93, PL-02-089 Warszawa, Poland (e-mail: m.babel{at}uw.edu.pl)
An integrated group of conceptual models for evaporite deposition is presented for a shallow salina-type basin, supplied both with marine and non-marine water, and with a water level separated from and drawn down below world sea-level. Knowledge of, and terminology for, modern limnology is used in these models in order to build a new and more comprehensive link between the hydrography and hydrochemistry of brines and the depositional and stratigraphical record anticipated in such basins. In these modelled basins it is assumed that (as in saline lakes): (1) the evaporite deposition reflects the stratification-mixing cycles in the brine column; (2) the evaporative crystallization of salts culminates during the mixing periods; and (3) the evaporite facies are linked to the position of water zones separated by a horizontal pycnocline. The models are prepared especially for interpretation of subaqueous coarsely crystalline gypsum (selenite) deposition in perennial-to-ephemeral saline pans, and also for fine-grained gypsum deposition (clastic, microbialite and pedogenic) on specific, flat, semi-emerged shoals (majanna-type shoals), commonly flooded by wind-driven brine sheets, in between them. Coarse, crystalline selenites appear to form below the pycnocline and the crystallization of particular well-developed grass-like selenite beds is thought to be connected with the pycnocline highstands. Such beds are both marker beds and ideal datum surfaces. The models presented are used for sedimentological interpretation of the various gypsum facies of the Badenian (Middle Miocene) evaporite basin in the northern Carpathian Foredeep. The observed architecture of these facies suggests that the margin of the basin was occupied by a system of variable saline pans (dominated by selenite deposition) and evaporite shoals (dominated by gypsum microbialite deposition); some of these shoals were majanna-type. Many features of the studied facies can be clearly explained by the models presented, particularly by the rapid and variable fluctuations in water and pycnocline levels incomparable in time scale and size with any global sea-level changes. All these features together consistently suggest that the Badenian gypsum basin was a salina-type basin with a water level below global sea-level, although salts in the basin are/were basically marine in origin.
