Multi-stage Dolomitization of the Upper Permian Zechstein Limestone (Ca1) Carbonates (SW Poland)

Peryt, Tadeusz Marek¹, Marek Jasionowski¹ (1) Panstwowy Instytut Geologiczny, Warszawa, Poland

The earlier study on evaporite-related, Upper Permian Zechstein dolomites showed that they are products of both the early- and late diagenesis, and the pattern of occurrence of dolomite is complex. In the basinal facies of the Zechstein Limestone in SW Poland, carbonate buildups (termed reefs) were encountered which are surrounded laterally and covered by evaporites. The carbon and oxygen isotopic investigation on 213 samples showed quite varied Δ18O and Δ13C values: the range of Δ13C values is 0‰ to +6‰ PDB in calcite (average +2.7‰) and 0‰ to +8‰ PDB in dolomite (average +4.8‰), and the Δ18O values are from +3‰ to -13‰ PDB in calcite (average -5.1‰) and +3‰ to -9‰ PDB in dolomite (average -4‰). Two groups of dolomites can be distinguished. The first group is represented by laminated or structureless and cavernous dolomicrites with anhydrite and pisolitic deposits that were syndepositionally dolomitized in the sabkha environment, and their &delta18O values range from 0‰ to +3‰ PDB. Assuming the crystallization temperature of 40°C, sabkha deposits originated from water characterized by &delta18O values close to 0‰ PDB, which corresponds to slightly evaporated Permian seawater. A part of the isotopic heavier deposits is also represented by dolomitized grainstones showing perfectly preserved primary fabric (fabric-preserving dolomitization). These dolomites might have originated through dolomitization in the sabkha environment due to reflux. The isotopic composition of other dolomites is characterized by low &delta18O values (from -2‰ do -10‰ PDB) due to burial conditions at relatively high temperatures, which is compatible with the recorded presence of saddle dolomite. However, the most common petrographic texture of dolomite is planar, which is characteristic for low-temperature diagenetic environments, and thus, it is concluded that these textures are inherited after earlier, low-temperature phases of dolomite formation.