Ammonites in the NW Adda-1XA well. A contribution to The Jurassic Petroleum System in the Danish Central Graben - The PETSYS Project

Finds of ammonites in a cored interval (depths: 2887–2851m / 9479’04” – 9354’01”) in the NW Adda-1Xa well (Fig. 1) are studied with the purpose of adding biostratigraphic data to the succession previously dated mainly by palynostratigraphy.

Ammonites are relatively abundant in the c. 38 metre thick interval. 17 levels were sampled (Figs. 3–11, Table 1) and fragments were observed in additional two levels.

The interval covers the Jurassic–Cretaceous boundary. This interval is characterized by extreme provincialism, and it is commonly difficult, if not impossible, to correlate between areas. Accordingly separate zonations have been established in different areas and even separate stage names are used (Fig. 2). The ammonites recovered in the NW Adda-1Xa core belong to forms which are here related to the zonation used in eastern England (Casey 1973; Rawson 2006). Casey (1973) identified forms related to Greenland and Russia and therefore adopted the Boreal stage names Volgian and Ryazanian for the boundary interval, i.e. different from the usage in southern England where the stage names Portlandian and ‘Purbeckian’ has been locally developed and also different from the Tethyan stages names Tithonian and Berriasian.