Monomineralic carbonate ooid types in the Triassic sediments from Northwestern Bulgaria
Authors: Chatalov Athanas 
Published: January 2023

Oolitic rocks occur in almost all lithostratigraphic units from the Iskur Carbonate Group (Lower – Upper Triassic) in NW Bulgaria. Primarily monomineralic ooids with homogenous cortices belong to several fabric types: micritic/microsparitic, brickwork, sparry relic, blocky, radial fibrous, radial sparry, shrunken and mottled. In addition, ooids with compound cortical fabrics were distinguished: blockymicritic, micritic-radial and radial-brickwork. Cracked, spalled, broken, abraded, distorted and pitted types were differentiated according to outer deformed shape. Apart from the single individuals polyooids were found in some rocks. The primary ooid fabrics were modified by various isochemical diagenetic processes including microbial micritization, compaction, aragonite transformation, recrystallization, dissolution and cementation. These proceeded in the marine-phreatic, meteoric-phreatic and deep-burial connate diagenetic environments. Only one specific ooid association shows relation with a particular depositional setting. At the very base of the Lower Triassic peritidal sequence (Mogila Fm) mottled and radialbrickwork ooids coexist with bimineralic individuals and broken shapes. The formation of such association is attributed to hypersaline conditions, as this interpretation is based on other sedimentological data and analogy with similar occurrences from the geological record. The original mineralogy of some ooid types was inferred mainly on the basis of petrographic and geochemical data, but actualistic evidence was considered as well. The precipitation of aragonite and high-Mg calcite ooids was proved. Their stratigraphic occurrence conforms well to the global distribution of carbonate ooids through the Triassic period. Thus aragonite ooid precipitation was predominant during Late Olenekian – Middle Anisian times. The main control was high carbonate saturation state of seawater promoting higher precipitation rates for aragonite due to kinetic reasons. In this particular case, the high degree of seawater carbonate saturation was probably further promoted by an increased salinity reflecting intense evaporation in an arid peritidal setting. The contrastingly large predominance of calcitic ooids in the Upper Anisian – Lower Carnian sediments corresponds to the secular increase of calcite abiotic precipititation during the Middle Triassic epoch. This change was related to the appearance of global highstand, more intense seafloor spreading, warmer climate, higher CO2 levels, and consequently, to low carbonate saturation state of seawater favouring calcite precipitation. In this particular case, the seawater reached normal salinity from the Middle Anisian onwards, thus probably further promoting calcite ooid formation. Locally controlled short-term shifts in the carbonate precipitation also took place in the epicontinental Triassic sea. This is inferred from the coexistence or interbedding of primary aragonitic and calcitic ooids, as well as presence of bimineralic ooid cortices. It is suggested that subtle temperature and/or salinity changes ultimately controlled the carbonate saturation state of seawater. Unlike the established trend of primary ooid mineralogy, marine phreatic cements in the studied Triassic rocks were predominantly composed of high-Mg calcite. This discrepancy is generally explained by the lower supply of carbonate ions into interparticle pores, which favoured calcite cementation, although poor preservation of marine aragonite cements is another possibility. Further evidence on the abiotic carbonate precipitation might be obtained from investigation of the limestone micritic matrix.