Abstract:
The regional infrazonal scale for the Upper Bathonian - Lower Callovian is elaborated on the basis of successive ammonite assemblages in some Middle Jurassic sections of Central Russia. Four zones and 13 biohorizons, most of which are well correlative to the standard West European units are distinguished in the studied stratigraphic interval. The Infimum Zone including the C. infimum and C. cf./aff. infimum biohorizons is the lowermost unit of the scale. According to its stratigraphic position below the Elatmae Zone, the former unit approximately corresponds to the Discus Zone and probably the upper part of the Orbis Zone of the standard Upper Bathonian scale. The succeeding Elatmae Zone is subdivided into the M. jacquoti, C. elatmae, C. tschernyschewi, C. surensis, C. subpatruus and Ch. saratovensis biohorizons. Due to some correlative stratigraphic levels, this Zone is considered to be analogous to the basal Callovian Herveyi Zone of the standard scale. The Koenigi Zone including the K. densicostatus, K. curtilobus and K. trichophorus biohorizons is an equivalent of the synonymous Zone of the standard scale. The Lower Callovian succession is completed by the Calloviense Zone comprising the S. calloviense and S. enodatum biohorizons. It is also analogous to the synonymous standard Zone.
