Temporal significance of sequence boundaries

Abstract

This paper analyses the temporal significance of stratigraphic surfaces bounding the marine portions of the depositional sequence, genetic stratigraphic sequence and transgressive–regressive sequence. These bounding surfaces, known as the `correlative conformity' (c.c.), `maximum flooding surface' (MFS) and `conformable transgressive surface' (CTS), respectively, may either be defined on the basis of stratal stacking patterns (which we call `type A surfaces'), or on the basis of water-depth changes and relative sea-level changes (which we call `type B surfaces'). The type A MFS and CTS are time lines in a depositional-dip section, corresponding to the turnaround points from shoreline transgression to regression and vice versa. They separate prograding (coarsening-upward) from retrograding (fining-upward) geometries, with a timing determined by the interplay between the rates of sedimentation and relative sea-level rise in the shoreline area. The timing of type A MFS and CTS is not affected by the offshore variations in sedimentation and subsidence rates, but it is only controlled by the shoreline movements and the associated facies shifts. The type A c.c. separates rapidly prograding and offlapping forced regressive strata from the overlying lower rate prograding and aggrading normal regressive strata. This surface is diachronous, younger basinward, with the rate of offshore sediment transport. The timing of the type A c.c. in the shoreline area corresponds to the end of relative sea-level fall, but it develops under relative sea-level rise conditions offshore. The timing of the type B MFS and CTS depends on the offshore variations in the sedimentation and subsidence rates. These surfaces, defined on the basis of bathymetric changes, become younger and older seaward, respectively, tending to merge together offshore. The type B c.c. marks the end of relative sea-level fall in any point along a depositional-dip section. It is diachronous, older basinward, as its timing depends on the offshore variations in subsidence rates. The diachroneity of type B surfaces reaches a quarter of the period of the highest frequency variable, whichever that is among the eustasy, tectonics or sedimentation controls. Types A and B surfaces merge together in the shoreline area, but they become temporally divergent offshore. Deepening-upward and shallowing-upward facies should not be confused with transgressive and regressive systems tracts. The latter are strictly controlled by the shoreline movements, which determine the direction of facies shifts and the stratal stacking patterns.