Temporal significance of sequence boundaries

This paper analyses the temporal significance of stratigraphic surfaces bou nding the marine portions of the depositional sequence, genetic stratigraph ic 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 eithe r 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-leve l changes (which we call 'type B surfaces'). The type A MFS and CTS are tim e lines in a depositional-dip section, corresponding to the turnaround poin ts from shoreline transgression to regression and vice versa. They separate prograding (coarsening-upward) from retrograding (fining-upward) geometrie s, with a timing determined by the interplay between the rates of sedimenta tion 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 a nd subsidence rates, but it is only controlled by the shoreline movements a nd the associated facies shifts. The type A c.c. separates rapidly progradi ng and of flapping forced regressive strata from the overlying lower rate p rograding and aggrading normal regressive strata. This surface is diachrono us, younger basinward, with the rate of offshore sediment transport. The ti ming of the type A c.c. in the shoreline area corresponds to the end of rel ative sea-level fall, but it develops under relative sea-lever rise conditi ons offshore. The timing of the type B MFS and CTS depends on the offshore variations in the sedimentation and subsidence rates. These surfaces, defin ed on the basis of bathymetric changes, become younger and older seaward, r espectively, 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 sectio n. It is diachronous, older basinward, as its timing depends on the offshor e variations in subsidence rates. The diachroneity of type B surfaces reach es a quarter of the period of the highest frequency variable, whichever tha t is among the eustasy, tectonics or sedimentation controls. Types A and B surfaces merge together in the shoreline area, but they become temporally d ivergent offshore. Deepening-upward and shallowing-upward facies should not be confused with transgressive and regressive systems tracts. The latter a re strictly controlled by the shoreline movements, which determine the dire ction of facies shifts and the stratal stacking patterns. (C) 1998 Elsevier Science B.V. All rights reserved.