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Sequence Stratigraphy as a “ Concrete ” Stratigraphic Discipline Report of the ISSC Task Group on Sequence Stratigraphy

Sequence stratigraphy is a stratigraphic discipline in which stratigraphic surfaces that represent changes in depositional trend are used for correlation and for defining specific types of sequence stratigraphic units. On the basis of both empirical observations and theoretical models, numerous different types of surfaces have been proposed as unit boundaries in sequence stratigraphy. Our systematic evaluation of these proposals has revealed that only four surfaces are appropriate for bounding units in sequence stratigraphy. These are a subaerial unconformity, an unconformable shoreline ravinement, a maximum regressive surface and a maximum flooding surface. Proposed surfaces that are not suitable to act as a sequence stratigraphic unit boundary include hypothetical time surfaces at the start and end base level fall (the basal surface of forced regression and the correlative conformity), highly diachronous sequence stratigraphic surfaces (normal shoreline ravinement, regressive surface of marine erosion) and highly diachronous within-trend facies boundaries (base of turbidites or shallow water strata, marine flooding surfaces). A sequence is the main unit of sequence stratigraphy and it is defined in a generic fashion as a unit bounded by a specific type of unconformity and its correlative surfaces. Two distinct types of sequences have been recognized. A depositional sequence has a subaerial unconformity as its primary unconformity and an unconformable shoreline ravinement and a maximum regressive surface as the correlative surfaces. An R-T sequence has maximum flooding surfaces as its boundaries. Both of these sequence types can be divided into two systems tracts, a transgressive systems tract and a regressive systems tract. Proposed systems tracts, which require unrecognizable time surfaces or highly diachronous surfaces as part of their boundaries, are not scientifically valid and should be avoided. These include entities such as lowstand, highstand and forced regressive systems tracts. Highly interpretive terms such as lowstand, highstand and forced regressive can be used as descriptive adjectives for specific facies interpreted to be