engleski

Cycles in Middle Eocene marls of Istria (Croatia)

Small-scale cyclicity is observed in Middle Eocene marls of Istria, Croatia. On steep, well exposed outcrops alternations of two types of marls can be visually identified based solely on their relative differences in resistance to weathering. The more resistant marls gently standout from the rock surface compared to less resistant marls between them. Such alternations have been recognized at many localities ; however, due to gradual transitions, identical coloration, similar physical properties, and in places subsequent tectonic disturbance, the observable difference between the two marl types is often lost. A multi-proxy approach was used to investigate the nature of the cyclic alternations, and ultimately, to shed light on the possible mechanisms that might have contributed to their formation. The studied marls belong to the Globigerina Marl unit, which is widespread in the northern part of the Istrian peninsula, and constitutes a part of the Pazin Basin fill (Dinaric foreland zone). The unit represents hemipelagic sediments deposited after the subsidence of the carbonate platform and preceding onset of flysch deposition which is related to the advance of the Dinaric orogen. The interval chosen for the study is in the middle part of the Globigerina Marl, and it belongs to the lower part of the planktonic foraminiferal Zone 12 of Berggren (1995). Investigations of planktonic and benthic foraminifera suggest deposition at depths of about 1000m (Gohrbandt, 1962, Zivkovic & Babic, 2003). Ten samples (5 of each marl type), taken from an outcrop with well developed alternations, were analyzed for their carbonate content and stable isotopes. Major and trace element concentrations were determined by ICP-MS in 6 of the samples. TOC content was also determined on 6 samples. Carbonate content, element concentrations and stable isotopes show clear systematic differences between the two marl types, thus confirming the visually observed alternations. More resistant marls have higher carbonate content and lower Al, Si, Ti, Fe, Na and K than neighboring less resistant marls. Both δ 18O and δ 13C show more positive values in the more resistant marls. TOC is slightly lower in more resistant marls. The higher δ 13C values in the more resistant marls could be a consequence of higher primary productivity in surface waters. Values of the productivity proxy Ba/Al (Latimer & Filippelli, 2002) are also higher in the more resistant marls, which is in agreement with such an assumption. Likewise, preliminary results of nannoplankton abundances seem to support this interpretation. The positive correlation between δ 18O and δ 13C indicates that a process which simultaneously affects both δ 18O and δ 13C values, such as variations in surface runoff or diagenesis, could have contributed as well. TOC content does not correlate positively with productivity, and is probably related to detrital input and/or oxygenation of deep water.

cycles; marls; Eocene; Istria; geochemistry

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