Opoka – a mysterious carbonate-siliceous rock: an overview of general concepts

The opoka is a carbonate-siliceous marine sedimentary rock, forming a thick succession of Upper Cretaceous age in Poland and in another regions of Europe. This rock has been studied for over 150 years, but only the use of modern analytical techniques enables for the formulation of its mineralogical definition, which identifies the distinct features of opoka and allows it to be distinguished from other rocks (e.g. chalk, gaize). Parallel to the petrographic research on opoka, its palaeobathymetric interpretations, which were based on the palaeotectonic models of Danish-Polish Trough inversion has been revised. Depending on the model of palaeotectonic history, opoka has been interpreted as a deep-water or shallow facies, without detailed petrographic studies of its mineralogical composition. The paper presents various aspects of opoka, including: history of the term, nomenclature, mineralogical composition, microtexture and palaeoecological significance of Cretaceous opoka. New data which permit precise definition of this rock term, and its mineralogical composition are discussed in the light of palaeoecological reconstructions, bathymetry and existing models of opoka distribution.