The Oxfordian/Kimmeridgian stage boundary in Late Jurassic sedimentary rocks of the Swiss Jura range

Abstract

This paper presents a review of the criteria for and limitations to the definition of a usable long distance correlation scheme for the Oxfordian/Kimmeridgian stage boundary, based on the example of the Late Jurassic of the Swiss Jura range. Two main issues and their stratigraphic consequences are more specifically addressed, namely the interpretation of calcareous oolites in terms of depth of deposition and the dependence of ammonite distribution on the paleoenvironment. Calcareous oolites are not diagnostic for depth of deposition, as demonstrated by the conflict with other detailed sedimentary observations. When the sedimentary record is correctly interpreted in terms of water depth, the distribution of ammonites proves to be dependent on the paleoenvironment. Therefore ammonites cannot be directly used as a time correlation tool in a laterally heterogeneous regional context such as the Swiss Jura range. As a consequence, additional proxies are needed to circumvent the limitations of ammonite biostratigraphy in shallow carbonate platform environments. These proxies can ultimately be calibrated with high-resolution ammonite biostratigraphy in basin setting, which remains the most accurate method for long distance time correlation (provided it is not based on endemic or low diversity assemblages). The analysis of the biostratigraphic and lithostratigraphic schemes for the Swiss Jura range with respect to the position of the Oxfordian/Kimmeridgian boundary leads to the conclusion that the former suggestion of a suitable type section on the slope of Summerhalde (3 km NNW of the city of Schaffhausen) has to be replaced by another section outside Switzerland. Indeed, the Oxfordian/Kimmeridgian boundary in northern Switzerland lies within the Knollen Bed (uppermost part of the Gredingensis Zone) rather than at the base of the Reuchenette, Burghorn and Schwarzbach Formations (boundary between Galar and Platynota Zones with low long distance correlation potential).