Transgressive Systems Tract

Transgressive Systems Tract

The Transgressive Systems Tract follows the Lowstand Systems Tract and comprises the deposits accumulated from the onset of coastal transgression until the time of maximum transgression of the coast, just prior to renewed regression. Parasequences onlap the sequence boundary in a landward direction and downlap onto the transgressive surface in a basinward direction. It is the middle system track of both type 1 and type 2 sequences. The sediments of the Transgressive Systems Tract onlap and retrograde across the transgressive surface. The lower boundary of this systems tract is marked by the development of the transgressive surface that steps up onto the shelf margin (see animated gif). This surface may be marked by erosion and cementation, and often Glossifungites are burrowed into this during or just after the inital transgressive phase that immediately follow sea level lowstands. The top of this systems tract is formed by the maximum flooding surface (mfs) over which the Highstand Systems Tract sediments prograde and agrade.

Stacking patterns of parasequences exhibit backstepping onlapping retrogradational aggrading clinoforms that thicken landward. Seaward the rates sediment accumulation are commonly low and condensed sections often form, particularly in association with the maximum flooding that forms the maximum flooding surface. Glauconite rich sediments are often associated with these widespread condensed sections that may merge landward with transgressive surfaces. On chronostratigraphic charts it can be seen that though the mfs is often shown to be absent offshore, undoubtedly deposition of sediment continues even if it is in much reduced quantities. Thus the chronostratigraphic significance of the mfs is that landward it represents shorter period of time, while seaward a longer period of time. Thus the upper surface of a mfs transgresses time or is diachronous.