Article: Biohorizons and zonules: intra-subzonal units in Jurassic ammonite stratigraphy
Intra-subzonal units, known collectively as 'horizons', are employed in Jurassic ammonite stratigraphy. Two basic types of 'horizon' are used by different authors. In France the unit is commonly employed as a sub-subzonal division which can be termed a 'zonule'. In Britain and Germany, however, the intra-subzonal unit typically used is more closely comparable to a faunal event. These faunal or 'biohorizons' are analogous to the hemerae of S. S. Buckman. Zonules, therefore, have a significant time duration, whereas biohorizons have a negligible duration but typically a significant inter-biohorizon time interval. As intra-subzonal units 'horizons' offer the potential for much finer scale correlation of marine Jurassic successions than is possible by any other means. The average zonule duration or biohorizon-plus-gap duration currently possible for the British Jurassic is around 200 000 years. Nevertheless, calculations of average duration on a stage by stage basis in the Jurassic suggest that at some levels this figure may be as little as 80 000 years.