Jurassic stage definitions: the value of global sequence stratigraphy

Five years ago we demonstrated the occurrence across the Arabian Plate of 11 synchronous Maximum Flooding Surfaces (MFS) during Jurassic time (Sharland et al. 2001, 2004). Ongoing work, incorporating all the stratigraphy of North Africa and examination of other regions, can now demonstrate the occurrence of 15 Jurassic 3rd order surfaces, and intervening sequence boundaries. Each of our MFS is defined in a Reference Section. This is a location with good sedimentological and/or wireline log evidence for a MFS, supported by biostratigraphy. The biostratigraphy also provides constraint on the correlation of the MFS to its occurrence in other locations. Each of our MFS can be shown to be occurring within the same biozone, or correlative, in different basins across the Middle East and North Africa region. Data from differing fossil groups have been calibrated to the standard zonation for each period. Sequence boundaries are age-calibrated and correlated in a similar manner via their correlative conformities. The ability to recognize and correlate depositional sequences across Arabia and beyond, across basins with differing subsidence and sedimentation rates, indicates that these sequences are driven by synchronous eustatic sea-level change. This observation is of value in providing guidance for the definitions of the fundamental units of chronostratigraphy - stages. Each Phanerozoic stage requires a Global Stratotype Section and Point (GSSP), that is to say, a location and bedding plane where the base of each stage is defined. This definition is tied to an event in the rock record useful for correlation. Progress in defining Jurassic GSSPs has been delayed because of difficulties in choosing the most appropriate (bio)event to relate to definition. It is recommended that stage boundaries be related to bioevents associated with correlative conformities of sequence boundaries. This links chronostratigraphy and sequence stratigraphy and honours the original concepts upon which many stages were first described in the 19th Century.