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Delineation between brackish and freshwater carbonate depositional environments: Upper Cretaceous and Paleogene of Istria, Croatia

The objective of this study of fossiliferous strata of known origin is to establish criteria for distinguishing between marine, brackish and freshwater carbonate settings that can be subsequently applied to interpreting depositional environments of poorly fossiliferous strata of uncertain origin. Eastern Istria, Croatia provided an ideal research site as it is a relatively small study area with well-exposed, fossiliferous carbonate strata representing various depositional settings. During fieldwork in Croatia, a stratigraphic section through the Upper Cretaceous strata, an unconformity marking the regional K/T (Cretaceous/Tertiary) boundary, and the Lower Paleogene (Paleocene) Liburnia strata was measured and described near the village Paradiz. Samples were collected at this site and also at three additional outcrops of the Upper Paleocene to Eocene strata. Alternating layers of coal and a dark bituminous limestone with freshwater to brackish biota of the lower Liburnia (or Kozina) strata unconformably overlie the Cretaceous marine limestone at Paradiz. The lower Liburnia strata were deposited in freshwater lakes that formed in karst depressions created on an isolated carbonate platform during prolonged subaerial exposure. Brackish conditions imply a coastal setting that experienced periodic marine influences. This is supported by previous interpretations of the coal as being paralic (deposited along the margin of the sea). The occurrence of marine fossils in the upper Liburnia strata at Paradiz indicates a gradually increasing marine influence. Overlying the Liburnia strata are Upper Paleocene to Eocene marine limestones rich in foraminifera, representing a return to predominantly marine deposition. Future research will focus on a detailed geochemical (stable isotope) analysis of these strata to establish a relationship between petrographic and geochemical indicators of marine, brackish and freshwater carbonate depositional environments. The results will provide useful information for interpretation of carbonate strata that lack key lithological or paleontological characteristics.

carbonates; Cretaceous; Paleogene; Croatia; Istria

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