Sequence Stratigraphy: Applications and Perspectives

Opening Pandora 's Box: The Issue of Sequence Stratigraphic Terminology By Bill Devlin, Exxon Ventures, CIS, Houston

In October 1992, the North American Com- mission on Stratigraphic Nomenclature (NACSN) solicited ". . . opinions from the profession on whether or not to amend the 1983 North American Stratigraphic Code so as to provide formalization of sequence stratigraphic units." The NACSN also so- licited opinions on whether or not sequence stratigraphic units should be considered a special type of allostratigraphic unit (as defined in the North American Stratigraphic Code, 1983), and how they potentially re- lated to the unconformity-bounded units (synthems) of the International Subcom- mission on Stratigraphic Classification (ISSC, 1987). 1 responded to the NACSN on behalf of myself and some colleagues, and excerpts of the letter (with several ad- ditions) are presented below.

Time to Formalize Sequence Strati- graphic Units?

The purpose of this letter is to offer an en- dorsement to the proposal that the North American Stratigraphic Code be amended to formalize sequence stratigraphic units. The major reasons for this endorsement are as follows:

1. Over the past 20 years, sequence strati- graphic terminology has become en- trenched in the stratigraphic and sedimen- tologic literature. An ever-increasing num- ber of geoscientists are applying sequence stratigraphic methodology, with a concomi- tant increase of its use in technical presen- tations and publications. Although concepts and terminology are continually evolving, a core group of terms appears in most stud- ies.

2. Let's face it, some confusion exists in the geoscience community regarding the definition and use of sequence stratigraphic units, and such confusion only fosters mis- use, miscommunication, and misunder- standing among geoscientists. Formaliza- tion of sequence stratigraphic terminology

does not totally ensure against future mis- use, but NACSN acceptance and standard- ization of core terminology could be a po- tentially positive step toward rectifying the situation.

3. Since the introduction of sequence strati- graphic units and terminology, a large num- ber of additional terms have been proposed. Some of this is a result of legitimate, new perspectives and insights into sequence stratigraphy. However, in some instances (in my opinion), it seems to be nothing more than an author introducing terminol- ogy to pad their publication list. The former needs to be accommodated as a natural outcome of continuing research and devel- opment, but the latter should be discour- aged in that it can only lead to further con- fusion and frustration, especially to those who may be entering the field or trying to initiate sequence stratigraphic research. The formalization of key sequence stratigraphic units could provide a foundation for devel- oping concepts and terminology, as well as a constraint on the further introduction of redundant and/or supefluous terminology.

4. It is acknowledged that acceptance of terminology into a formal stratigraphic ode does not guarantee that geoscientists will use it. But along those same lines, sequence stratigraphic terminology is certainly more widely used than synthem terminology, and although allostratigraphic terminology is used to some degree, it is not used nearly as much as sequence strat terminology in industry or the mainstream geologic litera- ture.

5. With reference to unconformity-bounded units, there is precedence in the geologic literature for use of the term 'sequence' (e.g., Wheeler, 1958, Sloss, 1963) over both synthem (ISSC, 1987) and allostratigraphic unit (NACSN, 1983).

6. The acceptance of sequence stratigraphic units by the NACSN does not preclude the

use of either allostratigraphic units or synthems by geoscientists. As requested in the NACSN solicitation of opinions and recommendations, comments are provided below concerning sequence stratigraphic units, allostratigraphic units, and synthems.

Allostratigraphic Units

Definition: A mappable stratiform body of sedimentary rock that is defined and iden- tified on the basis of its bounding discontinuities (NACSN, 1983). Sequence stratigraphic units should not be considered a special type of allostratigraphic unit for the following reasons:

1.Allostratigraphic boundaries are defined as "bounding discontinuities that are later- ally traceable." According to the definition, the allostratigraphic unit is nolonger present once the bounding discontinuities are no longer present. Sequences, on the other hand, are bounded by sequence boundaries that are defined as unconformities and their correlative conformities. Allostratigraphic boundaries provide no allowance for the fact that most (all?) discontinuities in sedi mentary strata (i.e., unconformities) have correlative conformities. Unconformities can be commonly traced into their correla- tive unconformities on subsurface data, and in laterally continuous outcrop exposures. The sequence, as a stratal unit, does not end where its bounding unconformities end, and hence can be mapped over a broader area that is more representative of true deposi- tional relationships.

2. Allostratigraphic units can contain strata that are "lithologically heterogeneous" ac- cording to the NACSN, and some of these strata can be designated as allomembers. Sequence stratigraphic terminology ac- counts for the observation that, within a sequence, "lithologically heterogeneous" strata commonly display an ordered, repeat- able, and therefore predictable succession

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of stratal stacking patterns. Furthermore, within the sequence, these stratal stacking patterns are separated by readily identifi- able surfaces. Allostratigraphic nomencla- ture does not adequately allow for a) the description of stratigraphic architecture between unwnformities (sequence bound- aries), b) the chronostratigraphic signifi- cance of surfaces within that architecture, and c) the lateral relationship of sedimen- tary facies and lithology within the unit.

Synthems

Definition: Unconformity-bounded strati. graphic unit (ISSC, 1987).

1. The term synthem is rarely used in main. stream geologic literature.

2. As defined, synthems have the same in- adequacies as allostratigraphic units in that they do not account for correlative confor- mities and internal depositional relation- ships.

3. Synthems occur in all rock types with- out distinction. In addition, the definition indicates that synthems are not lithostratigraphic, chronostratigraphic, or biostratigraphic units. Sequences, on the other hand, are primarily sedimentary units that emphasize the chronostratigraphic sig- nificance of the sequence itself and espe- cially its internal components. Along the same lines as the points presented for allostratigraphic units, the distinction is fundamental and significant enough to con- sider sequence stratigraphic units as a sepa- rate typeof unit from synthems. The ISSC itself (1987, p. 234) noted that ". . . unconformity-bounded units have acertain amount of chronostratigraphic significance

because all the rocks below anunconformity or discontinuity are older than all of those above, and t ime l ines d o not cross unconformity surfaces."

Recommendations

Recognizing that 1) there is an ongoing evolution of concepts in this rapidly grow- ing field, and 2) that several distinct philo- sophical schools exist with regard to their approach to sequence stratigraphy, the fol- lowing terms are generally recognized and used by most stratigraphers and sedimen- tologists, and are proposed for consider- ation by NACSN: Sequence, Sequence Boundary, Sequence Set, Parasequence, Parasequence Set, Parasequence Boundary, Systems Tract, Downlap Surface, Con- densed Section.

The above sequence stratigraphic terms should be defined in terms of observable physical criteria, independent of interpre- tations regarding their origin of deposition. Furthermore, the above terms should be defined independent of thickness and du- ration over which each of the above units is deposited.

As a point of initiation, the definitions of the above terms should be taken after those presented in Mitchum et al. (1977). Van Wagoner et al. (1988, 1990). and Mitchum and Van Wagoner (1991). It is acknowl- edged that some clarification may be re- quired for some of the definitions in the above publications before acceptance by the NACSN.

Your Opinion

The NACSN is still considering action on formalizing sequence stratigraphic units. I have recently contacted them and offered

Houston Geological Society Bulletin

to solicit opinions from the HGS member- ship, which I will summarize and forward to the Commission (and back to y'all). If there is a great enough positive response, it could serve as a stimulus for a position paper to be submitted to the Commission to serve as a "strawman" for technical de- bate on this issue.

What do you think? Please send your com- ments and opinions to me via the Net (bdevlin-ix.netcom.com), or c/o the Edi- tor of the Bulletin.

References

International Committee on Stratigraphic Classification, 1987, GSABuII., v. 98, p. 232- 237.

Mitchum, Jr., R.M., Vail, P.R., and Thompson, S. 111, 1977, The sequence as a basic unit for stratigraphic analysis: in AAPG Memoir 26, p. 53-62.

Mitchum, Jr., R.M. and Van Wagoner, J.C., 1991, High frequency sequences and theirstacking patterns: sequence stratigraphic evidence of high frequency eustatic cycles: Sedimentary Geology, v.70, p.131- 160.

North American Committee on Stratigraphic Nomenclature, 1983, AAPG Bulletin, v. 67, p. 841-875.

Sloss, L.L., 1963, Sequences in the cratonic interior of North America: GSA Bull., v.74, p. 93-114.

Van Wagoner, J.C., Posamentier, H.W., Mitchum, R.M.,Vail, P.R., Sag, J.E. Loutit, T.S., and Hardenbol, J., 1988, An overview of the fundamentals of sequence stratigraphy and key definitions: in SEPM Special Publication 42, p.3946.

Van Wagoner, J.C., Mitchum, R.M., Campion,K.M., and Rahmanian,V.D., 1990, Siliciclastic sequence stratigraphy in well logs, cores, and outcrops:AAPG Methods in Exploration Series No. 7.

Wheeler, H.E., 1958,Time-stratigraphy: AAPG Bulletin, v. 42, p. 1047-1063..

November 1995