The stratigraphy of Zechstein strata in the East European Craton of Poland: an overview

Abstract

The sedimentary and stratigraphic patterns established for Zechstein of the western part of the Peribaltic Syneclise (and in particular the eastern Łeba Elevation) were applied to other parts of the East European Craton (EEC) in Poland: the eastern Baltic Syneclise and the Podlasie region. A very large number of mostly fully-cored borehole sections in the Puck Bay region certainly predestines the eastern Łeba Elevation area to use it as a model. The most part of the EEC, except of its part adjacent to the Teisseyre-Tornquist Zone, during the Zechstein deposition represents the marginal parts of the basin. The fauna occurring in the Zechstein carbonate deposits of the EEC makes it possible to distinguish between the Zechstein Limestone and the younger carbonate strata, but certainly not between the Main Dolomite and the Platy Dolomite and hence the facies models for the Zechstein that have been previously developed in the western part of the Peribaltic Syneclise augmented by sequence stratigraphic approach seem to be the best tool to apply in other peripheral areas in the EEC area. The Zechstein sequence in the western part of the Peribaltic Syneclise consists, in general terms, of three parts: (1) carbonate platform of the Zechstein Limestone (occurring only in the north-westernmost corner of the study area and passing into basin facies dominant in the most part of the area); (2) the PZ1 evaporite platform system composed of sulphate platforms and adjacent basin system and constituting the major part of the Zechstein sequence; and (3) the Upper Anhydrite-PZ3 cover. There is a consensus, as far as the western part of the Peribaltic Syneclise is concerned, that the Platy Dolomite platform is wider than the Main Dolomite platform. In the easternmost part of the Peribaltic Syneclise, the stratigraphical interpretations are diverse. We have included the anhydrite overlying the Zechstein Limestone into the Upper Anhydrite, and concluded that the overlying interbedded mudstone and anhydrite also belong to the Upper Anhydrite. When above the Upper Anhydrite one carbonate unit occurs, it is assigned either to the Main Dolomite and Platy Dolomite, or to the Platy Dolomite. The same conclusion is proposed for the marginal parts of the Podlasie Bay. The deposition of Zechstein Limestone resulted in the origin of carbonate platforms along the basin margins which changed an inherited topographic setting. The Lower Anhydrite deposits are lowstand systems tracts (LST) deposits, lacking in more marginal parts of the western and eastern Peribaltic Syneclise and in the major part of the Podlasie Bay. The accommodation space existed and/or created during the Lower Anhydrite and the Oldest Halite deposition in the Baltic and Podlasie bays was filled and at the onset of the Upper Anhydrite deposition, a roughly planar surface existed except in the area adjacent to the main Polish basin. The Upper Anhydrite deposits are transgressive systems tracts deposits and then highstand systems tracts deposits and they encroached the Zechstein Limestone platforms. The Upper Anhydrite deposition was terminated by sea level fall, and the Upper Anhydrite deposits in the marginal areas became subject to karstification. The Main Dolomite transgression took place in several phases but its maximum limit did not reach the Upper Anhydrite limit. The deposition of the PZ2 chlorides (LST deposits) resulted in the filling of the accommodation space that was inherited after the deposition of the Main Dolomite and the Basal Anhydrite. Subsequently, the area became exposed, and marine deposits (Grey Pelite and Platy Dolomite) related to the last major transgression during the life of the Zechstein basin that resulted in a flooding of the exposed surface of older Zechstein deposits, including the area that was emergent during deposition of the PZ2 cycle. Microbial carbonates, being stromatolites and thrombolites, are a common feature of all Zechstein carbonate units but in particular this is the case of the Platy Dolomite. There are no direct premises allowing for convincing settlement doubts regarding the stratigraphical position of the upper carbonate unit in many cases, but several lines of evidence suggest that, as in the entire Zechstein basin, the Main Dolomite considerably shifted basinward, and the Platy Dolomite – landward, although it is difficult to ascertain whether the original Platy Dolomite extent was similar to or greater than the limit of the Zechstein Limestone as elsewhere in the Zechstein Basin.