The analysis, within the framework of a chronostratigraphic scale, of depositional units that are related to one another genetically (i.e. they were formed by similar processes). There are two schools: the Exxon Production Research (EPR) depositional sequence model and the genetic stratigraphic sequence model (proposed by W. E. Galloway in 1989), differing mainly in the type of boundaries by which they define the strata.