Sequence Stratigraphy of Clastic Systems -

Sequence Stratigraphy of Clastic Systems

Anthony P. Hamblin
Geological Survey of Canada, Calgary, 3303 - 33 St. N.W. Calgary, Alberta, T2L 2A7

by Octavian Catuneanu Geological Association of Canada, Short Course Notes, Volume 16 2003, 248 pages, $ 85

1 Research, exploration and understanding of the stratigraphic controls on sedimentary successions and their enclosed resources, has progressed rapidly in the last quarter-century, although the real roots of the discipline date to Barrell (1917). The more recent realization that the internal architecture of sedimentary successions and the links between sedimentation, unconformities, time relationships and base-level changes can lead to novel resource exploration models and interpretations, fuelled an enormous international effort to understand the processes involved and the variety of resultant geometries. As the author of this book states, Sequence Stratigraphy of Clastic Systems is an attempt to "provide an in-depth coverage and critical assessment of all current ideas and models in the field of sequence stratigraphy", in order to "build a bridge between the various sequence models currently in use, facilitating communication among its practitioners and demonstrating that sufficient common ground exists to promote a unifying theory". These are grand aspirations for a set of Short Course Notes! Does the publication measure up?

2 This summary volume is divided into two parts, the first dealing with basic concepts and the ubiquitous jargon (including 9 chapters, 175 pages), and the second presenting practical applications of sequence stratigraphy to several clastic depositional systems (including 4 chapters, 55 pages), with a short 2-page Conclusion. The extravagant difference in page content between theory and application sections may in itself be telling us something important about one of the prime problems with sequence stratigraphy at present: too much emphasis on competing theoretical models and jargon systems (although these certainly have their place as initial hypotheses or final syntheses) and too little emphasis on the real practicalities of commonplace usage at a modestly exposed outcrop or on a small-scale well log (which geologists are actually faced with each day). Part I provides a review of the history, concepts, evolution, misconceptions, benefits and pitfalls of sequence stratigraphy, and should be required reading for all practitioners because it cautions against blind trust in any of the various methods. There is a thorough effort here to fully detail all the rival methods and jargon systems proposed so far, but with lesser critical evaluation of these different approaches, or effort to sort through all the competing terms to streamline the nomenclature. In some ways it is really an exposition on the astounding plethora of nomenclatural jargon with which sequence stratigraphy has grown up (rather than matured) and reads more like a Glossary of Geological Terms than a true summary. The point of all stratigraphic studies should be to reduce apparent chaos to a more manageable, simplistic and easily interpretable framework, rather than make it more mysterious. Having gathered everything together in one place, Catuneanu provides an excellent opportunity to sift, sort, toss out and recast this material, to create that simplified and eminently practical synthesis. But, I believe that that process requires further critical assessment than is presented here. Perhaps the discipline requires more distillation before that process can be properly undertaken. However, the final discussion of Chapter 9 of Part I (5 pages) acknowledges the central (dominant?) role of tectonics in ultimately controlling stratigraphic cyclicity, and the key concept of "shifts in depositional trend at the shoreline"as the central element in practical sequence stratigraphic analysis. Here the author provides much firmer ground for laying the foundations of synthesis, and perhaps, points the way to the future. As Embry and Catuneanu (2002) said, "The current consensus is that sequence stratigraphy has a lot to offer but that the discipline is burdened by an unwieldy and overblown jargon, and that it is long on theory but short on practical application".

3 Part II provides a review of well-known facies concepts, as often summarized elsewhere, and treatments of the application to three separate depositional systems. The first, on non-marine clastic systems, discusses the difficult application of sequence stratigraphy to fluvial systems, but gives very little information on lacustrine and aeolian systems, although there are relevant studies available. The sections on coastal/shallow marine, and deep marine systems are actually simply reviews of facies material (all availablein more useful forms elsewhere) and lots of photos of sedimentary structures, with very little discussion of sequence stratigraphy. Although Part II was meant to help geologists apply all that theory from Part I in every-day situations, this unfortunately ends up as the weakest portion of the book, with little practical demonstration of methods at all. Disappointingly, none of these sections use real outcrop photos, measured sections or well log cross sections to demonstrate principles or techniques. I'm not sure a student or professional would come away from this volume, as it stands, with a better idea of how to actually identify any of the truly important surfaces or systems tracts at an isolated outcrop, or how to correlate an ambiguous subsurface cross section. However, it is possible that exercise material presented in-class, but not included here, was more successful.

4 So, Sequence Stratigraphy of Clastic Systems does provide encyclopaedic, in-depth and unbiased coverage of all current ideas and models in the field, and I commend the author for this effort. It certainly gives us the opportunity to try to sort out and improve the sequence stratigraphymess. However, it is less expansive on critical assessment, analysis or synthesis of those theoretical concepts and jargon-filled models. Overall, this book presents more detail on the theoretical side and copious terminology of the rival model systems than we can absorb, but less useful demonstration of the practical applications. Here, in microcosm, we have the classic duel between the seductiveness of jargon-filled, model-driven science (which might lead the enthusiastic initiate into boundless over-interpretation beyond the real data) vs. the drab actualistic data-driven science (which, conversely, might lead the cautious practitioner to miss extending standard interpretations to the next, perhaps more valuable and imaginative, level). This may be, unfortunately, an encapsulated commentary on the state of sequence stratigraphy today. The next step of providing a clear, concise theory and methodology perhaps can be a major goal of a later edition: as yet, the hoped-for "Unifying Theory" is not well articulated here.

5 I can recommend this book as a stand-alone guide, but with some reservations. Catuneanu has covered the ground in excruciating detail, more than most people could comfortably absorb, but provided less guideline information on the real threads of logical truth that matter, or on the realistic application in the less-than-ideal real world most geologists toil in. How do we unequivocally identify each of the various surface types on a well log or outcrop, and in fact, which surfaces are truly crucial to most workers' practical needs? Which of the numerous systems tracts touted by various authors will really help us map out an important facies? This voluminous outpouring of contending theoretical postulates, giving each researcher's system its fair due, is commendable but, ultimately, confusing. There is some critical evaluation buried here and there, but finding it requires wading through a sea of subtly different concepts. It certainly provides a baseline record of where we are and how we got here (muddled as that may be), but is less successful in pointing the way to the future. In the end, we are left with the advice to use our common sense in each different situation - a state of mind most of us had already adopted, waiting for the dust to clear. For those interested in an encyclopaedic catalogue of every theoretical idea and term of jargon which has yet been proposed, this is your dream come true; for those interested in one workable practical application of a "quick-and-dirty" methods approach, utilizing real examples from the real world, I would recommend Embry and Catuneanu (2002). However, I repeat my previous caveat, that taking the classroom short course itself likely provided much more "hands-on" experience.

6 Not surprising in a set of Short Course Notes, there are a number of minor spelling/grammatical/editing errors (eg. "Walter's Law" appears several times instead of Walther's Law), which will likely be rectified in later editions. Although the volume is well-illustrated and the photographs are of high quality, there is a preponderance of sedimentological photos that don't effectively demonstrate the principles of sequence stratigraphy, and of model-driven line drawings. The spiral-bound, soft-cover format is convenient, low-cost and appropriate for a publication that could receive daily use for a few years (particularly by students), later to be replaced by updated versions (already planned) as the content evolves.

7 The author states in his Preface "This volume should therefore be seen as work in progress, and comments from the readership will be most welcome". I encourage professionals and students alike to read carefully, sift, absorb, and analyse the contents, then take the author up on his offer for comments. Not only this book, but also sequence stratigraphy itself, is still a work in progress.

Barrell, J. 1917. Rhythms and the measurements of geological time. Geological Society of America Bulletin, v. 28, p. 745-904.

Embry, A.F. and Catuneanu, O. 2002. Practical Sequence Stratigraphy: Concepts and Applications. Canadian Society of Petroleum Geologists, Short Course Notes, 147p.