
Kimmeridgian

In the geologic timescale, the Kimmeridgian is an age or stage in the Late or Upper Jurassic epoch or series. It spans the time between 157.3 ± 1.0 Ma and 152.1 ± 0.9 Ma (million years ago). The Kimmeridgian follows the Oxfordian and precedes the Tithonian.
Stratigraphic definition.
The Kimmeridgian stage takes its name from the village of Kimmeridge on the Dorset coast, England. The name was introduced into the literature by French geologist Alcide d'Orbigny in 1842. The Kimmeridge Clay Formation takes its name from the same type location. It is the source for about 95% of the petroleum in the North Sea.
Historically, the term Kimmeridgian has been used in two different ways. The base of the interval is the same but the top was defined by British stratigraphers as the base of the Portlandian ("sensu anglico") whereas in France the top was defined as the base of the Tithonian ("sensu gallico"). The differences have not yet been fully resolved; Tithonian is the uppermost stage of the Jurassic in the timescale of the ICS.
The base of the Kimmeridgian is at the first appearance of ammonite species "Pictonia baylei" in the stratigraphic column. A global reference profile for the base (the GSSP of the Kimmeridgian stage) had in 2009 not yet been assigned. The top of the Kimmeridgian (the base of the Tithonian) is at the first appearance of ammonite species "Hybonoticeras hybonotum". It also coincides with the top of magnetic anomaly M22An.
Subdivision.
The Kimmeridgian is sometimes subdivided into Upper and Lower substages. In the Tethys domain, the Kimmeridgian contains seven ammonite biozones: