Temporal evolution, petrography and composition of dolostones in the Upper Devonian Plavinas Regional Stage, southern Estonia and northern Latvia

The Upper Devonian Plavinas Regional Stage in southern Estonia and northern Latvia is represented by dolostones containing interlayers of dolomitic marlstones and limestones. Petrographic, cathodoluminescence, electron microprobe and isotope techniques were used to investigate diagenetic evolution of dolostones. The rock succession has been affected by multiple diagenetic events. Based on petrographic and geochemical data, six dolomite textures were identified. The crystal size (5ñ1200 µm) and morphology of dolomites are variable. Commonly, dolomites are close to the stoichiometric composition, with low iron and manganese content. Their stable isotope composition (δ13C, δ18O) differs greatly from that of dolomite precipitated from Frasnian seawater. Dolomitization is more pronounced in the lower part of the studied sequence, in the Snetnaya Gora Formation and Lower Pskov unit where carbonates are completely dolomitized, whereas upwards in the section the dolomite content decreases. Voids and pores of the Lower Pskov unit are mainly open, but partly or completely occluded in the Upper Pskov unit. Void-filling dolomite has slightly and void-filling calcite notably depleted stable isotope signatures. Major dedolomitization and calcite-filling processes took place during the final uplift and emergence of the northern part of the Baltic basin, connected with the migration of karst-related meteoric waters into previously dolomitized horizons within carbonate rocks.