CONCEPTUAL BASIS AND VARIABILITY IN SEQUENCE STRATIGRAPHY - A DIFFERENT PERSPECTIVE

Sequence stratigraphic models are: (1) discussed from a theoretical point of view, with emphasis on a systematical discussion of the breakdown of depositional cycles produced by changes in relative sea-level and sediment supply into systems tracts and, (2) questioned by discussing and schematically showing alternative scenarios to the established ones. The ''shoreline trajectory'', defined as the cross-sectional shoreline migration path along the depositional dip, is a useful building block for describing the internal architecture of the depositional cycles and their contained systems tracts. The shoreline trajectories can be grouped into discrete classes including accretionary and non-accretionary forced regression, normal regression, and accretionary and non-accretionary transgression. Depositional cycles formed as a response to successive rises and falls of relative sea-level should be divided into four, and not three systems tracts, which is the most common in the literature. Three key surfaces can encompass a complete cycle. These are the surfaces of maximum regression and transgression, and the subaerial unconformity formed during relative sea-level fall. The correlative conformity to the subaerial unconformity should correspond to the time of lowest relative sea-level. Variability of the lowstand wedge and highstand systems tracts can be treated together, since both are taking place during rising relative sea-level with sediment supply being larger than the accommodation being generated. The resultant progradation may or may not be interrupted by transgressive events. Three end-member scenarios for the transgressive systems tract can be envisaged: non-accretionary transgression; accretionary transgression; and backstepping by combined transgressions and normal regressions. Important controls on the variability of the forced regressive systems tract are the gradients of the shoreline trajectory and the fronting depositional foundation. Basin floor mass-gravity deposition may occur within all four systems tracts and will eventually take place both in a ramp and shelf-slope-basin setting if the receiving basin extends into deep waters and is oversupplied with sediments.