The Lower Jurassic Sub-system comprises four stages, in chronological order, the Hettangian, Sinemurian, Pliensbachian and Toarcian. Each stage is subdivided into a sequence of ‘standard zones’ (= chronozones) and subzones – each correlated primarily on the basis of its ammonite fauna. A further increase in stratigraphical resolution is available by the use of intra-subzonal units known collectively as ‘horizons’. The close link between ammonites and chronostratigraphy means that faunal provincialism may determine which zonal framework, and therefore which subdivision of the Lower Jurassic, applies in different regions of Europe. Such provincialism is of minor importance in the early Jurassic (Hettangian – Lower Pliensbachian) but increases significantly in the Upper Pliensbachian and into the Toarcian where at least three ammonoid faunal provinces are distinguishable. The standard zonal schemes for each relevant faunal area are discussed here, with greatest emphasis being placed on the Northwest European Province, which is characteristic of much of northern Europe throughout most of the Early Jurassic. Intra-subzonal units have only been described in certain regions for parts of the Lower Jurassic but where recognisable these are introduced.

Lithostratigraphic and magnetic-susceptibility logs for four sections in the Blue Lias Formation are combined with a re-assessment of the ammonite biostratigraphy. A Shaw plot correlating the West Somerset coast with the Devon/Dorset coast at Lyme Regis, based on 63 common biohorizon picks, together with field evidence, demonstrate that intra-formational hiatuses are common. Compared to laminated shale deposition, the climate associated with light marl is interpreted as both drier and stormier. Storm-related non-deposition favoured initiation of limestone formation near the sediment–water interface. Areas and time intervals with reduced water depths had lower net accumulation rates and developed a greater proportion of limestone. Many homogeneous limestone beds have no ammonites preserved, whereas others contain abundant fossils. Non-deposition encouraged shallow sub-sea-floor cementation which, if occurring after aragonite dissolution, generated limestones lacking ammonites. Abundant ammonite preservation in limestones required both rapid burial by light marl during storms as well as later storm-related non-deposition and near-surface carbonate cementation that occurred prior to aragonite dissolution. The limestones are dominated by a mixture of early framework-supporting cement that minimized compaction of fossils, plus a later micrograde cement infill. At Lyme Regis, the relatively low net accumulation rate ensured that final cementation of the limestones took place at relatively shallow burial depths. On the West Somerset coast, however, much higher accumulation rates led to deeper burial before final limestone cementation. Consequently, the oxygen-isotope ratios of the limestones on the West Somerset coast, recording precipitation of the later diagenetic calcite at higher temperatures, are lower than those at Lyme Regis.

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The meaning and usage of Horizons is discussed and then applied to the Sinemurian (Lower Jurassic) sequence within the Lower Lias Group of the British Isles. These rocks are of fundamental importance in Lower Jurassic stratigraphy and have yielded the type specimens of 15 of the 20 zonal and subzonal indices of the NorthWest European Standard for the Sinemurian Stage. By the recognition of Horizons a threefold increase in the resolution of the ammonite-correlated "time-scale" or Standard Zonation for the Sinemurian is possible (with 61 potentially correlatable hori zons identifiable within 17 established subzones). Comparisons and correlations with early and recent detailed subdivisions of the Sin em urian Stage are discussed. Resume. La signification et I'utilisation des horizons sont discutees et ensuite appliquees a la sequence Sinemurienne Ourassique inferieur) dans Ie groupe Lias inferieur des lies Britanniques. Ces roches sont d'une importance fondamentale dans la stratigraphie du Jurassique inferieur et en ont donne les specimens types de 15 a 20 indices zonaux et sous-zonaux du Standard Nord-Ouest Europeen pour l'etage Sinemurienne. Par la reconnaissance des horizons une echelle plus detaillee est possible et correlee avec les zones et sous-zones du Sinemurien. Les 61 horizons sont identifiables en 17 zones etablies. Les comparaisons et les correlations avec des anciennes et des recentes subdivisions detaillees de I'etage Sin emu rienne sont examinees.

The stable isotope data presented here significantly extend and expand upon previous isotopic investigations of the Middle to Late Jurassic interval. The belemnite samples collected from the Staffin Bay and Staffin Shale formations from the Isle of Skye, Scotland, yielded oxygen isotope values consistent with Callovian-Kimmeridgian palaeotemperatures of 6.7-20.6 8C. The carbon isotope data comprise one of the first moderately high-resolution investigations of the relationship between terrestrial ä 13 C org (predominantly fossil wood debris) and marine ä 13 C carb (belemnites) as derived from a geologically coeval record. The Staffin Bay data reveal a broad Early to Mid-Oxfordian positive carbon isotope excursion. The excursion maximum occurs in the cordatum Zone (British Boreal ammonite zonation), although high values persist into the tenuiserratum Zone. The correspondence between the marine and terrestrial records indicates a strong coupling of the ocean-atmosphere system and suggests that the total exchangeable carbon reservoir would have been affected at this time. The Mid-Oxfordian negative carbon isotope excursions identified in published Tethyan records and commonly attributed to methane release are not recorded in the Staffin Bay data, which may suggest that the Tethyan excursions do not represent fluctuations in the global carbon reservoir and that the fidelity of the methane hypothesis should be re-evaluated.