Opoka: Polymorphic composition, genesis, Problems of study

Abstract

This paper is a comprehensive review devoted to topical issues in the study of opoka – marine sedimentary siliceous rocks of the Cretaceous-Paleogene age. For almost two hundred years of studying these sedimentary formations, the term “opoka” itself has not received proper recognition globally due to the lack of a detailed mineralogical definition and the general imperfection of the systematics of siliceous rocks. A distinctive feature of the opoka is the presence of opal-CT lepispheres, which form a recognizable framework of siliceous rock or form an obscure lepispheric/ cryptocrystalline mass. The article discusses various aspects of opoka – history of study, nomenclature, mineralogical composition, microtexture, the distribution of Cretaceous- Paleogene opoka, etc. The etymology of the genesis of the opoka and models of its origin at the early and late stages of diagenesis are critically discussed. In this review, attention is focused on recent studies, that provide detailed information about silica polymorphic forms distributed in the opoka, as well as its typical microtexture types. A model of the genesis of opoka and the relationship with global biogeochemical cycle of silica is presented. Differences recorded in opoka of different ages, geological and tectonic positions, indicate both the predetermination of many parameters of the mineral composition from the very genesis, and the disproportionate contribution of individual factors to the evolution of opoka rocks, namely the source of silicic acid, the role of siliceous bios in the mobilization of dissolved silica, the nature and volume of terrigenous influx from the adjacent landmass, secondary changes and mineral formation, etc.

Keywords

• opoka
• silicites
• opal-st
• opal
• cristobalite
• silicic rocks
• silica cycle
• trans-urals acknowledgements