The Jurassic-Cretaceous boundary: problems of definition and procedure

Abstract

Problems with the Jurassic-Cretaceous (J-K) boundary are largely the result of stratigraphic difficulties caused by a lack of a significant faunal turnover at either boundary of the Berriasian stage, and to extreme faunal provincialism as a consequence of the Purbeckian regression. There is also a lack of agreement about basic principles of procedure, which could be resolved if we accept that system boundaries are conventional boundaries. The choice of boundary level depends primarily on its correlation potential. This would put an end to fruitless discussions about whether Berriasian faunas are Jurassic or Cretaceous. Continuity of usage is another important factor. For this reason the efforts of the International Working Group on the Jurassic-Cretaceous Boundary have been concentrated on the Tethyan-Boreal correlation of the Tithonian-Berriasian boundary, though so far with limited success. Other levels should be tested in future. Another problem in inter-regional correlation is the lack of clarity in the underlying zonal concept. Fossil zones are traditionally biochronologic units based on evolutionary events. Therefore a clear distinction between biostratigraphic raw data and their biochronologic interpretation is absolutely necessary. System boundaries have to be defined by global boundary stratotype section and point (GSSP) according to the guidelines of the International Commission on Stratigraphy (ICS). This definition would normally start from a biochronologic marker level. It should be emphasized that only range zones, delimited by phyletic events, have unequivocal boundaries. Traditional ammonite zones (Oppel zones) are more loosely defined by their contents and have no clear cut boundaries. This should be taken into consideration when deriving the boundary definition from an ammonite zonation.