RELATIONSHIPS BETWEEN ISOTOPIC COMPOSITION AND PETROGRAPHIC INDICATORS FOR MARINE, BRACKISH AND FRESHWATER CARBONATE DEPOSITION: PALEOGENE OF ISTRIA, CROATIA

Isotopic compositions of fossiliferous Liburnia (or Kozina) strata (Paleogene) in Istria, Croatia were examined to develop relationships with paleoenvironmental indicators that may be useful for interpreting deposits of uncertain origin elsewhere. The lower Liburnia unconformably overlies Cretaceous limestone and consists of alternating layers of coal and a bituminous gastropod mudstone/wackestone and microbially laminated mudstone deposited under freshwater-to-brackish conditions in karst depressions on an isolated carbonate platform. Previous interpretations of the coal as marginal marine and sapropelic in origin support a coastal setting with periodic marine influences. The appearance of miliolid mudstone/wackestone with brackish biota, followed by foraminifera-rich wackestone/packstone with diverse marine fossils indicates a gradually increasing marine influence upsection.

Compared to estimated Paleogene marine calcite compositions (d¹⁸O=-2 to -1‰; d¹³C=0 to +2.5‰; VPBD) all samples analyzed are depleted in ¹⁸O and many are depleted in ¹³C, suggesting formation and/or diagenesis in the presence of meteoric fluids. The compositional fields of micrite from gastropod-rich and laminated mudstones (d¹⁸O=-8.4 to -2.6‰; d¹³C=-3.7 to +2.7‰) overlap entirely and show a positive covariance between d¹⁸O and d¹³C. These two lithofacies formed under similar conditions and their most negative isotopic values reflect low salinity and maximum ¹²C input from decaying organic matter. The d¹⁸O and d¹³C covariance may indicate formation from waters of variable residence time and/or mixing between fresh and marine waters. Mixing with marine fluids is supported by the overlap with the compositional field of micrite from miliolid mudstone/wackestone (d¹⁸O=-6.5 to -3.7‰; d¹³C=-1.7 to +1.2‰). The more narrow range of isotopic compositions of miliolid limestone, however, reflects a slightly less restricted, brackish lagoonal environment. Most micrites from marine foraminiferal wackestone/packstone have d¹³C values (-0.3 to +1.2‰) within the estimated marine range, but their d¹⁸O values (-7.9 to -5.3‰) indicate diagenetic alteration, which may have been promoted by elevated (intragranular) porosity.