engleski

Introduction to the bulk rock chemistry of the Eocene hemipelagic/pelagic deposits in the Dinaric foreland basin

The Eocene sediments along Adriatic coast deposited in the Dinaric foreland basin were processed sedi- mentologically, micropaleontologically, mineralogically and geochemically to assess the continuity of sed- imentation, determine the age, composition of the foraminiferal assemblages and environmental conditions. This work presents the bulk rock chemistry of hemipelagic/pelagic deposits (H/P sediments) collected in four geographically separated regions: Pićan-Pazin area on the Istrian Peninsula (Jakomići, Kožljak and Racani sections, 3 samples), Selce-Bribir (Kostelj section, 4 samples), Hvar Island (Zaraće Bay, 4 samples and Podstine Bay, 3 samples) and Krk Island (Baška beach section, 5 samples). These results were used to define the sediment maturity, the type of sources of deposited sediments and the geotectonic environment of marls deposition. The analyzed samples show a wide range of SiO2 contents. The values between 15 and 30 wt% were recorded in Istrian sediments, sediments from Podstine Bay and from the lower part of the Zaraće section, while significantly higher values (41–50 wt%) were measured for the Kostelj section, sediments from is- land of Krk and the upper part of the Zaraće Bay section. The Al2O3 wt% content is the lowest in the Pods- tine section (4–7 wt%). Sediments from Istria and the lower part of the Zaraće Bay section show the same alumina content (7–9 wt %). The samples from island of Krk have values between 8 and 13 wt%, while the samples from Kostelj and the upper part of the Zaraće section have the highest Al2O3 values (13–16 wt%). The lowest ICV values (Index of Compositional Variability (ICV = [Fe2O3 + K2O + Na2O + CaO + MgO + MnO + TiO2)/Al2O2]) have the samples from Kostelj section (1.40–2.09) and the upper part of the Zaraće section (1.75 and 1.90). Slightly higher values have the sediments from Baška beach (2.75–3.22). The samples from the Istrian peninsula and the samples from the lower parts of two sections on the island Hvar have similar ICV index values (from 4.08 to 6.48). The highest calculated ICV values (8.51 and 10.85) of the studied samples have the upper part of the Podstine Bay section on the island of Hvar, which can be ascribed to a high carbonate content. The studied samples have the chemical index of weathering (CIA = [Al2O3/(Al2O3 + CaO* + Na2O + K2O)] x 100) between 62 and 76, which describes an intermediate weathering (Nesbitt and Young, 1982) and could be interpreted as conversion of feldspars, volcanic glass and other labile components to clay minerals. Clay mineral assemblage contains illite, illite-smectite, chlorite, dioctahedral vermiculite and sparse kaolinite and generally corresponds to ICV and CIA values. Low CN abundance of marls from Istria (< 11%), suggests a drop in marine carbonate productivity linked to climatic changes and/or a sea level rise. Acknowledgements. This research is supported by Croatian Science Foundation (IP 2019-04-5775).

Chemical index of weathering, ICV values, Eocene, Marls, External Dinarides

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