Andrzej Wierzbowski

SUBOREAL/BOREAL AMMONITE SUCCESSION AT THE OXFORDIAN/KIMMERIDGIAN BOUNDARY IN THE FLODIGARRY SECTION (STAFFIN BAY, ISLE OF SKYE, UK)

The Flodigarry section at Staffin Bay, Isle of Skye, has yielded numerous ammonites both of Subboreal and Boreal affinity. This has enabled close correlation of the Subboreal and Boreal ammonite zonations, and provided new palaeontological data on levels which may be considered as a potential GSSP for the Oxfordian/Kimmeridgian boundary. The traditional Oxfordian/Kimmeridgian boundary placed at the Pseudocordata/Baylei zones boundary of the Subboreal scheme corresponds precisely to the Rosenkrantzi/Bauhini zones boundary of the Boreal scheme: this level is characterized by appearance of Pictonia together with Prorasenia replacing an older assemblage of Ringsteadia-Microbiplices (Subboreal), as well as by first occurrence of small-sized Amoeboceras ( Plasmatites ) – mostly A. praebauhini (Boreal). A further level which may be considered as the Oxfordian/Kimmeridgian boundary is the boundary between the Bauhini Zone and Kitchini Zone of the Boreal scheme characterized by first occurrence of Amoeboceras ( Amoebites ) of the A. bayi group. This level corresponds to the Planula/Galar subzones boundary of the Submediterranean scheme, i.e. it lies close to the currently accepted Oxfordian/Kimmeridgian boundary in the Submediterranean Succession.

Towards a consistent Oxfordian/Kimmeridgian global boundary : current state of knowledge

New data are presented in relation to the worldwide definition of the Oxfordian/Kimmeridgian boundary, i.e. the base of the Kimmeridgian Stage. This data, mostly acquired in the past decade, supports the 2006 proposal to make the uniform boundary of the stages in the Flodigarry section at Staffin Bay on the Isle of Skye, northern Scotland. This boundary is based on the Subboreal-Boreal ammonite successions, and it is distinguished by the Pictonia flodigarriensis horizon at the base of the Subboreal Baylei Zone, and which corresponds precisely to the base of the Boreal Bauhini Zone. The boundary lies in the 0.16 m interval (1.24–1.08 m) below bed 36 in sections F6 at Flodigarry and it is thus proposed as the GSSP for the Oxfordian/Kimmeridgian boundary. This boundary is recognized also by other stratigraphical data – palaeontological, geochemical and palaeomagnetic (including its well documented position close to the boundary between magnetozones F3n, and F3r which is placed in the 0.20 m interval – 1.28 m to 1.48 m below bed 36 – the latter corresponding to marine magnetic anomaly M26r).The boundary is clearly recognizable also in other sections of the Subboreal and Boreal areas discussed in the study, including southern England, Pomerania and the Peri-Baltic Syneclise, Russian Platform, Northern Central Siberia, Franz-Josef Land, Barents Sea and Norwegian Sea. It can be recognized also in the Submediterranean-Mediterranean areas of Europe and Asia where it correlates with the boundary between the Hypselum and the Bimmamatum ammonite zones. The changes in ammonite faunas at the boundary of these ammonite zones – mostly of ammonites of the families Aspidoceratidae and Oppeliidae – also enables the recognition of the boundary in the Tethyan and Indo-Pacific areas – such as the central part of the Americas (Cuba, Mexico), southern America, and southern parts of Asia. The climatic and environmental changes near to the Oxfordian/Kimmeridgian boundary discussed in the study relate mostly to the European areas. They show that very unstable environments at the end of the Oxfordian were subsequently replaced by more stable conditions representing a generally warming trend during the earliest Kimmeridgian. The definition of the boundary between the Oxfordian and Kimmeridgian as given in this study results in its wide correlation potential and means that it can be recognized in the different marine successions of the World.

On the Proposed Oxfordian–Kimmeridgian (Upper Jurassic) Boundary Stratotype and Its Potential for Correlation

The primary standard for the Oxfordian–Kimmeridgian boundary is the base of the Sub-Boreal Baylei ammonite Zone, which corresponds to the base of the Boreal Bauhini Zone as recognized in the proposed GSSP section at Staffin Bay, Scotland. New data from the Sub-Mediterranean succession indicate that the boundary in question may be recognized fairly low within the so-called “Sub-Mediterranean upper Oxfordian”, not far from the boundary of the Hypselum (Semiarmatum) Subzone/Zone and the Bimammatum Subzone/Zone. This improves the detailed correlation between a large part of the “Sub-Mediterranean upper Oxfordian” and a lower part of the Boreal/Sub-Boreal lower Kimmeridgian, which would result in a change in the position of the Oxfordian–Kimmeridgian boundary in Sub-Mediterranean–Mediterranean successions.