The Units of Sequence Stratigraphy,

Part 2: Time-based Depositional Sequences

by Ashton Embry

As described in the last article on material- based sequences (Embry, 2009b), a sequence is best defined generically as “a stratigraphic unit bound by a specific type of unconformity and its correlative surfaces.” Two specific types of sequences have been defined in the literature – the genetic stratigraphic sequence of Galloway (1989) (part of a maximum flooding surface for defining unconformity) and the depositional sequence of Mitchum et al. (1977) and Van Wagoner et al. (1988) (subaerial unconformity for defining unconformity).

The boundaries of a genetic stratigraphic sequence are always material-based for all sequence models and consist of maximum flooding surfaces. However, proposed boundaries for a depositional sequence are much more diverse. The proposed, material- based boundaries were described and evaluated in the last article (Embry, 2009b). The proposed boundaries for a depositional sequence which include timebased surfaces as correlative surfaces are described and evaluated herein. These time-based, depositional sequences are somewhat controversial as to their validity and utility.

Time-based, Depositional Sequence Time-based, Depositional Sequence Time-based, Depositional Sequence Time-based, Depositional Sequence Time-based, Depositional Sequence Boundaries

Boundaries Boundaries Boundaries Boundaries

In the time-based approach to sequence stratigraphy, two time-based surfaces are recognized as valid surfaces of sequence stratigraphy. These surfaces were introduced by Hunt and Tucker (1992) and are: 1) the basal surface of forced regression (BSFR), which equates to the time surface (depositional surface) at the start of base- level fall and 2) the correlative conformity (CC), which equates to the time surface (depositional surface) at the start of base- level rise. These time-based surfaces were discussed in detail in a previous article in this series (Embry, 2009a).

Employing the Correlative Conformity Employing the Correlative Conformity Employing the Correlative Conformity Employing the Correlative Conformity Employing the Correlative Conformity

One proposed, time-based, depositional sequence boundary uses the correlative conformity (CC) as a key correlative surface of a subaerial unconformity so as to extend the sequence boundary well into the basin (e.g., Hunt and Tucker, 1992; Helland-Hansen and Gjelberg, 1994). In a sequence model

with a ramp physiography and a fast, initial rise, the correlative conformity joins the basinward end of the shoreline ravinement (SR-U) which in turn truncates the subaerial unconformity (SU) as previously discussed (Figure 10.1, see also Figure 2 in Helland- Hansen and Gjelberg, 1994). Thus, the CC is an acceptable correlative surface of an SU in this model and such a depositional sequence boundary (SU/SR-U/CC) is theoretically valid. Figure 10.2 (page 48) illustrates a time-based, depositional sequence boundary using a CC as a correlative surface for a sequence model with ramp physiography and a slow initial rise (see also Figure 1 in Helland-Hansen and Gjelberg, 1994). As was previously discussed in Embry (2009b), there are no material-based, correlative surfaces for the SU in such a model. However, in the time- based approach, the CC provides such a correlative surface because it adjoins the basinward termination of the SU as shown in Figure 10.2 (page 48). Once again, such a depositional sequence boundary (SU/CC) is theoretically valid.

In a shelf/slope basin model, the CC closely coincides with the MRS as was discussed in Embry (2009a). As illustrated in Figure 10.3 (page 48), a continuous boundary consisting of an SU, an SR-U, an SOS, and a CC can be delineated on such a sequence model. Thus, such a time-based, depositional sequence boundary would also be theoretically valid. Although depositional sequence boundaries which employ a CC as a correlative surface are theoretically valid, the practical utility of such boundaries is debatable. The reason for this is that no published studies have demonstrated how a CC can be delineated and correlated in well exposed strata or on closely spaced well logs with abundant core (see Embry, 2009a for a detailed discussion). Unconstrained interpretations of a CC on seismic data have been offered (e.g., Catuneanu et al., in press) but these have not been corroborated by rock-based data and remain questionable. As discussed by Embry (2009a), such seismic reflectors, which are interpreted to encompass timebased CCs, may actually be harbouring material-

Figure 10.1. The boundaries of one proposed, time-based, depositional sequence are shown in red on this sequence model characterized by a ramp setting with a slow initial base-level rise rate. The correlative conformity (CC) is the only theoretically possible correlative surface of the subaerial unconformity for such a model. Unfortunately a CC has no physical characteristics which allow its delineation.

based MRSs. Overall, much more research is necessary before a depositional sequence boundary which employs a CC can be accepted as having practical utility.

Employing the Basal Surface of Forced Employing the Basal Surface of Forced Employing the Basal Surface of Forced Employing the Basal Surface of Forced Employing the Basal Surface of Forced Regression

Regression Regression Regression Regression

Another time-based depositional sequence boundary which has been proposed involves the use of the basal surface of forced regression as a correlative surface of an SU (Posamentier and Allen, 1999; Coe, 2003). This sequence boundary comprises the same combination of surfaces (SU/BSFR) for all sequence models. Such a depositional sequence boundary is shown for a ramp setting with a slow initial base-level rise (Figure 10.4).

Because the BSFR is developed long before the start of base-level rise, it intersects the SU landward of the basinward termination of the SU (Figure 10.4). It is also slightly offset by the regressive surface of marine erosion (RSME) if such a surface is developed. As was discussed by Embry (2009a), the BSFR has no physical characteristics making it of dubious value for comprising part of a sequence boundary. However, more importantly, the BSFR cannot be considered to be a valid correlative surface of an SU because it does not join with the end of the SU as shown in Figure 10.4. The use of such a sequence boundary results in much of the subaerial unconformity being inside the proposed sequence rather than on its boundaries (Figure 10.4), an inappropriate relationship for a depositional sequence. This would suggest that such a proposed depositional sequence boundary (SU/BSFR) be rejected as a possible option. Summary Summary Summary Summary Summary

By defining a sequence as a generic unit which is bound by a specific type of unconformity and its correlative surfaces, two specific types of sequences are recognized – a depositional sequence (SU, defining unconformity) and genetic stratigraphic sequence (part MFS, defining unconformity). Numerous combinations of material-based and timebased surfaces have been proposed for a depositional sequence boundary. In a time-based approach, a correlative conformity (CC) which represents a time surface (depositional surface) at the start of base-level rise is advocated for use as a correlative surface for extending the boundary well into the basin. Although the CC is a theoretically valid correlative surface, its use as part of a sequence boundary is compromised by a lack of physical characteristics that would allow a CC to be delineated and correlated with reasonable objectivity.

Figure 10.3. The boundaries of a time-based, depositional sequence are shown in red on this sequence model characterized by a shelf/slope/basin setting with a fast initial base-level rise rate. The correlative surfaces of the SU are the shoreline ravinement (SR-U), most of the slope onlap surface (SOS) and the correlative conformity (CC) (time surface at start base-level rise).

Another proposed, time-based, depositional sequence uses a basal surface of forced regression (time surface at start base-level fall) as a major part of the sequence boundary. The largest objection to such a proposal is that the BSFR is not a correlative surface of an SU because it is truncated by the SU far landward of the basinward termination of the SU. Such a proposed boundary is not compatible with the established definition of a depositional sequence.

The main proposed material-based and time- based sequence boundaries are summarized

in Figure 10.5 and most are for a depositional sequence boundary. The only boundaries which have widespread utility are material- based ones and include the MFS of the genetic stratigraphic sequence and the combined SU/ SR-U/MRS, with or without an SOS, for the depositional sequence. All other proposed boundaries use an inappropriate correlative surface (e.g., BSFR, facies change) or include a correlative surface that cannot be recognized in most situations (e.g., CC). The next article will examine systems tracts which are component stratigraphic units of

Figure 10.2. The boundaries of a time-based, depositional sequence are shown in red on this sequence model characterized by a ramp setting with a fast initial base-level rise rate. The correlative surfaces of the SU are the shoreline ravinement (SR-U) and the correlative conformity (time surface at start base-level rise).

Figure 10.4. The boundaries of another proposal for a time-based, depositional sequence are shown in red on this sequence model characterized by a ramp setting with a slow initial base-level rise rate. In this case, the basal surface of forced regression (BSFR) (time surface at start base-level fall) is used as the primary correlative surface. Such a proposal is not reasonable because, as illustrated, the BSFR does not join the end of the subaerial unconformity.

Figure 10.5. A summary of the various combina- tions of surfaces for the different types of sequence boundaries which have been proposed.

a sequence. Once again, both material-based and time-based systems tracts have been defined. The main ones in each approach will be discussed and appraised as to their validity and utility for mapping and communication.

References References References References References

Catuneanu, O., et al., in press. Towards the Standardization of Sequence Stratigraphy. Earth Science Reviews.

Coe, A. (ed.). 2003. The sedimentary record of sea-level change. Cambridge University Press, New York, 287 p.

Embry, A.F. 1993. Transgressive-regressive (T-R) sequence analysis of the Jurassic succession of the Sverdrup Basin, Canadian Arctic Archipelago. Canadian Journal of Earth Sciences, v. 30, p. 301- 320.

Embr y, A.F. 2009a. Practical Sequence Stratigraphy VIII: The Time-based Surfaces of Sequence Stratigraphy. Canadian Society of Petroleum Geologists, The Reservoir, v. 36, issue 1, p. 27-33.

Embr y, A.F. 2009b. Practical Sequence Stratigraphy IX: Part 1 Material-based Sequences. Canadian Society of Petroleum Geologists, The Reservoir, v. 36, issue 2, p. 23-29. Galloway, W. 1989. Genetic stratigraphic sequences in basin analysis I: architecture and genesis of flooding surface bounded depositional units. American Association of Petroleum Geologists Bulletin, v. 73, p. 125-142.

Helland-Hansen, W. and Gjelberg, J., 1994, Conceptual basis and variability in sequence stratigraphy: a different perspective. Sedimentary Geology, v. 92, p. 1-52.

Hunt, D. and Tuc ker, M. 1992. Stranded parasequences and the forced regressive wedge systems tract: deposition during base-level fall. Sedimentary Geology, v. 81, p. 1-9.

Mitchum, R, Vail, P., and Thompson, S. 1977. Seismic stratigraphy and global changes in sea level, part 2: the depositional sequence as the basic unit for stratigraphic analysis, In: Seismic stratigraphy: application to hydrocarbon exploration. C. Payton, (ed.). American Association of Petroleum Geologists Memoir 26, p. 53-62.

Posamentier, H. and Allen, G. 1999. Siliciclastic sequence stratigraphy – concepts and applications. Society of Economic Paleontologists and Mineralogists, Concepts in Sedimentology and Paleontology, no. 7, 210 p.

Van Wagoner, J.C., Posamentier, H.W., Mitchum, R.M., Vail, P.R., Sarg, J.F., Loutit, T.S., and Hardenbol, J. 1988. An overview of the fundamentals of

sequence stratigraphy and key definitions, In: Sea level changes: an integrated approach. C. Wilgus, B.S. Hastings, C.G. Kendall, H.W. Posamentier, C.A. Ross, and J.C. Van Wagoner, (eds.). Society of Economic Paleontologists and Mineralogists, Special Publication 42, p. 39-46.