In the genetic stratigraphic sequence model used in sequence stratigraphy, a sigmoid clinoform produced by onlap under conditions of rising sea level due to the rapid subsidence of the basin. It is a type of lowstand systems tract produced under special conditions. If the rate of subsidence was at no time high enough to outpace the rate of sedimentation, thus allowing transgression, the tract is known as a midstand (or forced regressive) systems tract.

Regressive systems tract