Mineralogical-petrographical study of evaporites from Mali Kukor, Vranjkovići and Slane Stine quarry (Upper Permian evaporites from Dalmatia, Croatia)

Željko Dedić,Nikolina Ilijanić,Slobodan Miko

doi:10.4154/gc.2018.02

Abstract

The evaporite deposits examined in this study are located in the central part of middle Dalmatia, Croatia. In this region, Upper Permian evaporite sediments were deposited under favourable conditions onto the Variscan basement around the northern margins of Gondwana. These sediments can be subdivided into three members, a lower evaporite unit (an anhydrite member), a middle evaporite unit (a gypsum member), and an upper unit (a clastic member), and are mainly comprised of secondary gypsum that formed via the hydration of precursor anhydrite rocks. The middle evaporite unit comprises beds of gypsum as well as early diagenetic dolomites that contain gypsum sequences, extending up to 60 m maximum thickness, and overlying clastic sequences that themselves are up to 20 m thick. These Upper Permian evaporite sediments contain horizontal, irregular, gypsum lithofacies that exhibit pronounced enterolithic and boudinage structures. The characteristics of these sediments are indicative of deposition in supratidal and sabkha settings (i.e., early diagenetic dolomites and evaporites) within a shallow epicontinental marine environment with highly varied coastlines, bays, and lagoons. The secondary gypsum seen within this Upper Permian middle evaporite unit displays alabastrine and porphyroblastic secondary textures and includes corroded anhydrite relics; associated minerals include muscovite, chlorite, potassium (K)-feldspar, quartz, and amphibole. The Upper Permian evaporite sediments discussed in this study are composed of irregular, locally brecciated secondary gypsum that probably formed as a result of multiple synsedimentary collapse of pre-existing soluble mineralsand/or synsedimentary and post-sedimentary tectonics.

Highlights

IntroductionGypsum is readily prone to dehydration, producing water and anhydrite, when heated to a temperature that is high enough as a function of the salinity of coexisting fluids (HARDIE, 1967; AZIMI et al, 2011; MA et al, 2015)
Gypsum is by far the most abundant secondary mineral in sedimentary settings, sometimes forming the calcium sulfate mineral anhydrite in arid and hot supratidal environments in the presence of concentrated brines, but rarely at the surface (KINSMAN, 1966; BUTLER, 1969; SHEARMAN, 1985; STROHMENGER et al, 2008; ZAKI et al, 2011; ABRANTES et al, 2016).Gypsum is readily prone to dehydration, producing water and anhydrite, when heated to a temperature that is high enough as a function of the salinity of coexisting fluids (HARDIE, 1967; AZIMI et al, 2011; MA et al, 2015)
The evaporitic sediments investigated in this study are mainly secondary gypsums that contain dolomite, anhydrite, muscovite, calcite, quartz, amphibole, and clay minerals

Summary

Introduction

Gypsum is readily prone to dehydration, producing water and anhydrite, when heated to a temperature that is high enough as a function of the salinity of coexisting fluids (HARDIE, 1967; AZIMI et al, 2011; MA et al, 2015). The most common process that causes the dehydration of gypsum, and the formation of anhydrite, is burial to depths below the reaction isotherm, in settings where fluids can still escape the system. If anhydrite is uplifted back above the reaction isotherm, it can return to gypsum, provided sufficient water is available. The aim of this study is to highlight the depositional environments and phases of diagenetic evolution that led to the formation of these Upper Permian evaporites based on the results of petrographic and sedimentological investigations of beds within the sequences at these sites

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Conclusion