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

Abstract

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 was isochronous.