Sequence stratigraphy attempts to subdivide the rock record into genetically related – unconformity-bounded – rock units or sequences (Emery & Myers, 1996; Jones, 1996; Holland, in Briggs & Crowther, 2001; Sharland et al., 2001; Coe, 2003; Catuneanu, 2006; Jones, 2006; Simmons et al., 2007; Catuneanu et al., 2009; Miall, 2010). The methodology lends itself well to the interpretation of seismic data, on which unconformities are readily identifiable on the basis of – real or apparent – reflector terminations (erosional truncation below, and transgressive onlap and associated landward facies shift, above, on basin margins). The unconformities are generated by base-level fall, in turn driven by glaciation or glacio-eustasy, and/or by structuration or tectonism. Global coastal onlap and ?glacio-eustatic sea-level charts have been constructed for the Carboniferous–Recent. Sea-level curves have also been constructed for the Cambrian, Ordovician, Silurian and Devonian.
A Vailian sequence is defined as a ‘stratigraphic unit composed of a relatively conformable succession of genetically related strata bounded at its top and base by unconformities or their correlative conformities’ (Vail et al., 1977). As intimated above, Vailian sequence boundaries (SBs) (bounding unconformities) are readily identifiable on seismic stratigraphic criteria (truncation below, and onlap and associated landward facies shift, above, on basin margins; and development of deep-sea fans in basin centres). They are also recognisable on palaeontological, sedimentological and petrophysical criteria (truncation below, and landward facies shift above, on basin margins; development of deep-sea fans in basin centres; and associated lithological indications on wireline logs).