Sequence stratigraphy theory: remarks and recommendations

Abstract

In this paper, depositional cycles formed by changes in relative sea-level and sediment supply are discussed from a theoretical point of view. The interplay between sediment supply and relative sea-level produces shoreline migration patterns that can be described in terms of the direction of their shoreline trajectories. Depositional cycles can be bracketed by one of three key surfaces. These are the surfaces of maximum transgression and maximum regression and the subaerial unconformity (and its correlative conformity) formed during relative sea-level fall. The two latter surfaces may be replaced by a transgressive surface of erosion along parts of their extent. It is proposed that depositional cycles formed in response to alternating falls and rises of relative sea-level can be divided into four systems tracts or segments and not three as has been previously suggested. The four systems tracts can be bounded between the levels of highest, lowest, maximum regressive and maximum transgressive shoreline positions within a relative sea-level cycle. Furthermore, it is claimed that the method of using superpositioned progradational and retrogradational parasequence stacking patterns as indicative of intervening sequence boundaries is dubious. The application of the Type-2 unconformity and Type-2 sequence boundary is problematic. It is recommended that these terms are redefined or taken out of use.