Stranded parasequences and the forced regressive wedge systems tract: deposition during base-level'fall

Abstract

To overcome inconsistencies in the Exxon sequence stratigraphic model as applied to silicielastic and carbonate shelf margins, it is proposed that an ideal sequence should consist of four systems tracts. In addition to the transgressive and highstand systems tracts, developed during rising base-level, it is suggested that there should be two systems tracts associated with falling and lowstands of relative sea-level. These are: the forced regressive wedge systems tract formed during falling base-level, bounded below by the ‘basal surface of forced regression’ and above by the sequence boundary, representing the lowest point of sea-level fall, and the lowstand prograding wedge systems tract, developed as relative sea-level begins to rise after sequence boundary formation. This systems tract downlaps the basin-floor forced regression deposits in a basinwards direction and onlaps forced regressive wedge sediments on the slope. The forced regressive wedge systems tract consists of shallow-water stranded parasequences deposited on the upper slope to the shelf, and basin-floor fan or apron sediments, deposited at the toe-of-slope and derived from erosion of the stranded parasequences and/or erosion of the previous highstand shelf and shelf-margin sediments.