MARINE PLATFORM DOLOMITES, LATE JURASSIC EARLY CRETACEOUS BAHAMA TYPE PLATFORM, CROATIA

Marine platform dolomites form thick, widespread units interlayered with limestones in the Croatian Jurassic-Cretaceous platform interior. The carbonates commonly are highly cyclic, the cycles consisting of meter to several meter-thick shallowing upward parasequences. Fine grained dolomites (less than 20 microns) are fabric preservative, and associated with peritidal laminites. However the most common dolomites are medium (20 to 100 microns) and coarse (greater than 100 microns), fabric destructive dolomites. The medium and coarse dolomite crystals have inclusion rich cores (common calcite and organic inclusions) and clear rims and are euhedral, subhedral and anhedral. No cathodoluminescent zonation is present in the dolomites (nonluminenscent); element maps were made on the microprobe to see if any element zoning was present. The dolomites range from stoichiometric to Ca-rich, Sr is 50 to 150 ppm, and Mn is very low (5 to 30 ppm). Stable isotope values of the Late Jurassic lime muds are d¹⁸O -.5 to -2.0‰ and d¹³C +1 to +3‰; the dolomites are d¹⁸O +1 to +3‰ and d¹³C of 0 to +2‰. The general absence of evaporites from the section, the Sr and Mn values and the stable isotope compositions all suggest precipitation from oxidizing Jurassic seawater (normal marine or slightly evaporated) near the platform surface, rather than precipitation from highly evolved evaporite-related brines or burial fluids. The dolomites are strikingly similar to many young dolomites described from beneath Late Cenozoic platforms, and Paleozoic relict non-luminescent early dolomites (which become subsequently overprinted by burial dolomite).