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The previous chapters have described in general terms how depositional sequences form and the general facies patterns that characterize them. The important observation was made that stratigraphic successions generally can be subdivided into repetitive or cyclic stratigraphic units at varying scale. The objective of this chapter is to illustrate the well-log and facies signatures of the key surfaces that bound and punctuate these stratigraphic units to form sequences and systems tracts.

As discussed previously, cyclic variations of accommodation in either marine or continental settings result in regressive-transgressive cycles that comprise generally predictable facies associations and stratal patterns. As discussed in Chapter 2, whenever these cycles are bounded by unconformities, at least locally, they are defined as sequences. The facies associations that characterize the different phases of cyclic sedimentation within a sequence can be grouped together as systems tracts. Systems tracts comprise linked depositional systems bounded by key stratigraphic surfaces (Posamentier et al., 1988). The most important of these surfaces are the sequence boundary, the transgressive surface, the downlap suiface, and the maximum flooding surface.

Sequences can be defined and subdivided using core, outcrop, well log, and seismic data by identifying these key surfaces (Posamentier et al., 1988; VanWagoner et d., 1990; Bhattacharya, 1993). Subsequently, analysis of the stratigraphic architecture between these surfaces leads to the identification of component systems tracts. As always, it is important to note that interpretations should not be based solely on a single dat set, but must be based on the integration of all available

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