Abstract
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.
Highlights
The Lower Jurassic Sub-system comprises four stages, in chronological order, the Hettangian, Sinemurian, Pliensbachian and Toarcian
Many other stage names have been proposed for parts of the Lower Jurassic in Europe, some have continued to be used for divisions at the level of substage, but most are redundant
The Marmorea Chronozone has been the subject of some discussion as to its meaning and whether it ranges into the Sinemurian (Taylor 1986), but in a European sense at least, a wholly Hettangian age is most likely (Bloos 1983, 1984)
Summary
Mountains of eastern France and Switzerland, first recognised as having rocks of a distinctive geological division by Alexander von Humbolt in 1795. It was not until 1829, that the term ‘Jurassique’ was introduced by Brongniart (1829), in a restricted sense essentially for what is known as the Middle. Many other stage names have been proposed for parts of the Lower Jurassic in Europe, some have continued to be used for divisions at the level of substage, but most are redundant. A full list of such terms was provided by Arkell (1933), together with extensive discussion of the origins of the various schemes for subdividing the Jurassic System. Substage names are occasionally used and these are introduced in the relevant sections below
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