Mineral and Chemical Composition of Rudist Valves from Upper Cretaceous Limestones of Southern Istria, Croatia

Darko Tibljaš,Alan Moro,Željka Ostrež

doi:10.4154/gc.2004.05

Abstract

Turonian to late Santonian limestones of southern Istria are rich inrudist remains. The main aim of this study was to determine themineral and chemical composition of 22 samples of rudist valvesbelonging to the genera Durania, Praeradiolites, Radiolites, Gorjanoviciaand Vaccinites, and to explain any observed differences.X-ray diffraction and chemical analyses showed that in all the analysedsamples, the primary constituents of rudist shells, aragonite andlow-Mg calcite, were transformed into diagenetic low-Mg calcite.The strontium concentrations of all the analysed shells correspond topelagic bulk rock samples altered by diagenesis and are significantlylower than those for biological calcite. Observed differences in thechemical composition of diagenetically altered rudist shells belongingto different rudist genera, namely a slightly higher content of Srin specimens of Gorjanovicia and Durania and lower concentrationsof Mg in shells of Vaccinites are most probably the result of differentshell structures and original mineral constituents.

Highlights

Turonian to Upper Santonian limestones rich in rudist remains crop out in the southernmost part of Istria, at the Mrlera and Premantura peninsulas (Fig. 1)
Observed differences in the chemical composition of diagenetically altered rudist shells belonging to different rudist genera, namely a slightly higher content of Sr in specimens of Gorjanovicia and Durania and lower concentrations of Mg in shells of Vaccinites are most probably the result of different shell structures and original mineral constituents
The rudist genera Durania, Radiolites and Praeradiolites lived during the Middle to Upper Turonian, Gorjanovicia lived in the time span from Upper Turonian to Santonian while Vaccinites specimens are of Santonian age (MORO, 1997; MORO & ĆOSOVIĆ, 2000)

Summary

Introduction

Turonian to Upper Santonian limestones rich in rudist remains crop out in the southernmost part of Istria, at the Mrlera and Premantura peninsulas (Fig. 1). According to the latest stratigraphic and palaeoenvironmental investigations of the rudists from these rocks (MORO, 1997; MORO & ĆOSOVIĆ, 2000) specimens of the genera Durania, Praeradiolites and Radiolites thrived in the shallower parts of the subtidal zone in an inner shelf environment, while specimens of the genera Gorjanovicia and Vaccinites lived in the deeper subtidal environment with pelagic influxes (outer shelf). The main aim here was to determine the mineral (i.e. phase) and chemical composition of the valves of several rudist genera from pure carbonates without the

Objectives

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Results

Discussion

Conclusion