Paper No. 190-7
Presentation Time: 3:20 PM
COARSE, FABRIC DESTRUCTIVE POST-DEPOSITIONAL DOLOMITES, LATE JURASSIC-EARLY CRETACEOUS ADRIATIC PLATFORM, CROATIA: ORIGIN BY MESOHALINE REFLUX
Exposed, post-depositional Tithonian, Barremian and Albian dolomites in southern Croatia form stratiform bodies up to tens of meters thick, are medium and coarse grained (20 to over 100 um), fabric destructive, nonluminescent that replaced calcite precursors. High Ca-dolomites form cores (turbid with calcite inclusions), and initial clear rims, whereas the later rims generally are low Ca-dolomite which also variably replaced the cores. The bulk dolomites have δ18O values of +1 to +3 and δ13C values of +2 to -1, suggesting deposition from refluxing marine to mesohaline platform waters while low Mn values (10 to 30 ppm) indicate oxidizing conditions. Sr contents of bulk dolomites are low ( 30 to 250 ppm) and show little relation to Ca stoichiometry. Dolomitization was initiated in peritidal settings forming fine grained dolomite in tidal flat laminites, and dolomite nuclei in subtidal sediments. 87/86Sr values suggest that dolomitization was initiated by coeval waters but continued as younger, more radiogenic seawaters refluxed down into the section to 1 km and temperatures below 50oC.This caused the Ca-dolomites to continue to nucleate and coarsen at shallow burial depths to form medium- and coarse-grained dolomites. As limestone was depleted within dolomite units, decreasing carbonate ions in refluxing waters slowed dolomitization, forming more stoichiometric, low Sr, low d18O, radiogenic 87/86Sr dolomite that formed rim cements and partly replaced cores. The dolomites were little affected by subsequent deeper burial due to low porosity and relatively short burial time prior to rapid Cenozoic uplift and so retain their near-surface geochemical signatures.