Parasequences: Allostratigraphic misfits in sequence stratigraphy

Parasequences were introduced as the building blocks of seismic-scale systems tracts in the context of low-resolution seismic stratigraphy. Pitfalls of this concept relate to the definition of parasequence boundaries as lithological discontinuities that mark episodes of abrupt water deepening. With this general meaning, ‘flooding surfaces’ may be facies contacts within transgressive deposits, or may coincide with different types of sequence stratigraphic surfaces (maximum regressive, transgressive ravinement, or maximum flooding). In all cases, flooding surfaces are allostratigraphic contacts restricted to coastal and shallow-water settings, where evidence of abrupt water deepening can be demonstrated. Flooding surfaces may also be absent from the shallow-water systems, where conformable successions accumulate during gradual water deepening. It follows that (1) parasequences have smaller extent than systems tracts, and (2) systems tracts do not always consist of stacked parasequences. These limitations prevent the dependable use of the parasequence concept in sequence stratigraphy. Advances in high-resolution sequence stratigraphy show that the scales of sequences and parasequences are not mutually exclusive; the two types of units define different approaches to the delineation of stratigraphic cycles at high-resolution scales. Sequences that develop at parasequence scales provide a more reliable alternative for correlation, both within and outside of the coastal and shallow-water settings, rendering parasequences obsolete. Every transgression that affords the formation of a flooding surface starts from a maximum regressive surface and ends with a maximum flooding surface observed at the scale of that transgression. These systems tract boundaries are invariably more extensive than any facies contacts that may form during the transgression. Flooding surfaces remain relevant to the description of facies relationships, but their stratigraphic meaning needs to be assessed on a case-by-case basis. The use of sequences and systems tracts in high-resolution studies provides consistency in methodology and nomenclature at all stratigraphic scales, irrespective of geological setting and the types and resolution of the data available.