Biohorizons as infrazonal biostratigraphic units: An attempt to refine the Jurassic stratigraphy based on ammonites

Abstract

The biohorizons (faunal horizons) as infrazonal units are the smallest correlatable biostratigraphic units. Their main features are: (1) potential indivisibility based on taxonomic differentiation of guide fossils; (2) determinancy of both lower and upper boundaries in the geological section; (3) identification by a single index species/subspecies. First such units were defined at the end of the 19th century and since the 1980s have been widely used in biostratigraphic investigations of Jurassic and, later, Cretaceous systems. The biohorizons are characterized by phylogenetic or immigrational paleobiological nature and geologically they are connected with depositional and postdepositional transformation (and, consequently, structure) of the sedimentary succession. Based on parallel sequences of phylogenetic and separate immigrational biohorizons, they are integrated into different zonal scales and an integrated regional scale. The problems related to the lack of universal criteria for defining and using biohorizons are discussed. The basic nomenclature rules, which are aimed at regulation of the use of these units in practical stratigraphic investigations, are suggested for their recognition and description.