CoachA systems tract is the set of depositional systems (river, coast, shelf, slope) that were active together during one part of a relative sea-level cycle, recognised by its position in the sequence, its bounding surfaces and its stacking pattern. Four are distinguished:
- the falling-stage systems tract, built by forced regression while relative sea level falls: downstepping foresets detached from the old shelf;
- the lowstand systems tract, built by normal regression once relative sea level starts to rise again, at the lowest shoreline position: a wedge that aggrades and progrades;
- the transgressive systems tract, built while the shoreline retreats landward: backstepping units onlapping the old shelf;
- the highstand systems tract, built by normal regression late in the rise, when supply again outpaces accommodation: foresets that prograde and downlap onto the deposits below.