Exploring the genesis of glaucony and verdine facies for paleoenvironmental interpretation: A review

Green marine clays, known as glaucony and verdine facies, are unique iron-rich minerals typically found in shallow marine sedimentary environments covering large areas of continental shelves today. While on some occasions they have been used effectively as paleoenvironmental indicators associated with transgressive and highstand system tracts, their occurrence can vary in different sedimentary environments. This paper aims to provide a review of the nomenclature and mineralogical composition of these facies, their habits and morphologies, as well as the physico-chemical characteristics of their genesis. This includes water depth, temperature, and geochemical factors that influence the local environments and characteristics of their formation. The article also presents examples from the NW Galician Continental Shelf and discusses the variety of mechanisms behind their genesis and evolution, as well as the challenges surrounding their chronology. Comparisons are drawn with the old facies (ironstone), which can occur in a wider range of marine environments and are sometimes found alongside glaucony and verdine facies in ancient deposits. This highlights the importance of accurately reconstructing the environment of each deposit. The literature reveals many exceptions in relation to the zonal and bathymetric distribution, sedimentation rates, age, etc. of these facies, emphasizing the need for further investigation into their genesis in current and ancient environments. Such research would provide valuable insights for paleogeographic, paleoenvironmental, and stratigraphic interpretations.