Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021 Sequence stratigraphy: a revolution without a cause? R. C. L. WILSON Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, UK Abstract: The paper presents personal reflections on the origins and utility of sequence stratigraphic models. These focus on two questions: (1) does sequence stratigraphy represent a revolution in our understanding of the stratigraphic record, and (2) does it provide a new means of global correlation? The f-n-st question is answered in the affirmative, at least insofar as sequence stratigraphy enables us to integrate a wide range of data and interpretations across a huge range of spatial and temporal scales. The recognition of the importance of stratal surfaces has led to a greater understanding of the response by sedimentary to climatic, tectonic and eustatic changes. But it has yet to be shown that eustatic signals can be detected unequivocally in the stratigraphic record. Therefore this 'new global stratigraphy', based on the premise that sequence boundaries are primarily controlled by eustatic changes, is not yet a reality. Testing this hypothesis is beyond the resolution of current biostratigraphic and chronostratigraphic techniques. When I was invited to contribute to the Lyell to the subject are given by Wilson (1992) and bicentenary meeting, I was asked to talk about Christie-Blick and Driscoll (1995). Textbooks by 'Sequence stratigraphy and sea-level change'. It Miall (1996) and Emery and Myers (1996) provide was hardly surprising that 'sea-level change' was in-depth coverage, with Miall's text offering much included in the title, because over 150 years after constructive criticism. The best introduction from Lyell addressed the sea-level controversy, we are the Exxon school is the colourful volume on still trying to unravel tectonic and eustatic signals siliciclastic stratigraphy (Van Wagoner et al. from the sedimentary record. But sequence strati- 1990). Reflective articles by Posamentier and graphy is not just about sea-level change. It Weimer (1993) and Walker (1990) are well worth identifies genetic packages of strata bounded by reading. time-related physical surfaces: unconformities and their correlative conformities, and surfaces caused by flooding events. Peter Vail and the 'Exxon Method, problems and doctrine school' claimed that eustatic sea-level changes are The beginnings the dominant control on stratal geometries and facies distributions within them. For readers not The roots of sequence stratigraphy lie in the recog- familiar with it, the sequence stratigraphic nition, some 40 years ago, of packages of strata approach is summarised in Fig. 1. It develops a bounded by continent-wide unconformities. Tech- simplified version of a frequently used diagram nological advances that enabled the petroleum which is often referred to as the 'Exxon slug'. exploration industry to move offshore played a key To some geologists, the sequence stratigraphic role in stimulating stratigraphers to take this 'big approach heralded the possibility of the 'new global view'. High-quality marine seismic data enabled stratigraphy' whereby stratigraphic surfaces caused stratigraphic architecture to be determined at the by globally synchronous eustatic sea-level changes basin and continental margin scales. Out of this could be used as a means of global correlation. grew the conceptual framework that enables the However, others regarded it as dressing up integration of a range of scales of stratigraphic regressions, transgressions etc. in unnecessary new information from a single laminae to a first order terminology. stratigraphic sequence. This paper presents personal reflections on the I first learnt about seismic and sequence sequence stratigraphic approach gained both as a stratigraphy through a workshop run by Esso UK teacher and researcher. It focuses on the questions soon after the 'old testament' was published explicitly and implicitly stated in the title, but does (Payton 1977). They ran another in the 1980s, not attempt to provide a thorough description or around the time of the appearance of the 'new comprehensive critical review. Brief introductions testament' (Wilgus et al. 1988). The first meeting WILSON, R. C. L. 1998. Sequence stratigraphy: a revolution without a cause?. 303 In: BLUNDELL,D. J. & SCOTT, A. C. (eds) Lyell: the Past is the Key to the Present. Geological Society, London, Special Publications, 143, 303-314. Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021 304 R.C.L. WILSON was, of course, seismically oriented, and the second which I find more use with students is given in Fig. one focused more on the outcrop and well log 2 -but this is not proof of the proposition! scales. At the first meeting there was some robust How can eustatic signals be determined from discussion about a number of problems and coastal onlap charts ? contentious issues, which was hardly surprising Exxon charts have a characteristic saw-tooth shape with people such as Drummond Matthews and with abrupt initial falls followed by gradual rises. Tony Hallam present. A few of these problems are The early derivative sea-level curves reflected such discussed below. asymmetry. In the early 1980s revised smoother curves were published, as by then it had been recognised that the landward encroachment of Are seismic reflections really time lines? sediments across older strata (onlap) along The examples given by the Exxon group at the Esso continental margins, as seen on seismic sections, workshop (e.g. Vail, Todd & Sangree 1977, figs involved marine and fluvial sediments. The latter 3-6) were not entirely convincing. A diagram for were deposited during periods when coastlines Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021 SEQUENCE STRATIGRAPHY 305 Fig. 1. Sketch cross-sections illustrating the basic concepts and terminology of sequence stratigraphy. Sketches (a) to (g) illustrate the stages in the development of one depositional sequence during one cycle of relative sea-level change. Slightly modified from Skelton et al. (1997). moved seaward, yet on seismic sections landward new testaments, and especially the famous - or onlap continued. infamous - Haq curve (Haq et al. 1988), heralded the start of a new era of stratigraphic studies. How can sea-level curves for individual locations Perhaps a means of global chronostratigraphic yield a means of global correlation ? correlation was within reach? This euphoria This was, and still is, a minefield. Different probably resulted from the fact that, to many approaches, and different authors, produce different people, traditional stratigraphy had slipped into the curves for the same period of time (Fig. 3). Twenty doldrums. Many with an historical geology bent years after the first global curves were published, had taken refuge in sedimentology and found that all the evidence on which a series of Exxon school the sequence stratigraphic approach widened their curves are based have still not been published horizons - in space and time - as they attempted to although some is contained in the appendix of Haq analyse basin-wide data sets and interpretations. et al. (1988). Despite such shaky foundations (and The historical development of sequence strati- others which are discussed by Miall 1996) many graphy was significantly different from the way geologists felt that the contents of the old and Lyell's ideas were introduced to, and eventually Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021 306 R.C.L. WILSON Fig. 2. A cartoon to explain why seismic reflections are time lines, and do not follow lithostratigraphic boundaries. Changes in tone density depict facies changes; these are relatively abrupt across time lines, but gradual parallel to them. accepted by, the scientific community, or for that that it provides a new means of global correlation matter the exposure to peer review of later based on the supposed eustatic signals. So method conceptual advances. Seismic stratigraphy came became tainted by doctrine. Lyell avoided this trap, out of the corporate closet in the late 1970s, about as discussed by Jim Secord during his Linnean ten years after it had been routinely applied during Society lecture at the Bicentenary Meeting (see also petroleum exploration activities. The birth and Secord 1997). early childhood of sequence stratigraphy were not Therefore it is important to distinguish the subjected to open debate as were Lyell's analytical framework of sequence stratigraphy from observations and interpretations. Perhaps there debates about eustatic signals and the 'new global were robust debates within Exxon, but they failed stratigraphy'. With this in mind, this paper addres- to ensure that the method was distanced from ses two questions contained within its title: (1) does eustatic explanations when the subject entered the sequence stratigraphy represent a revolution in our public domain in 1977. Unfortunately, this led to understanding of the stratigraphic record, and (2) judgements about the value of the sequence does it enhance our ability to determine the causes stratigraphic approach being clouded by the claim (i.e. the relative contributions made by eustasy, sea level rise lOOm 50 0 50m ,i it 4 -= Berriasian ,0) LUL) o Volgian ~ Kimm. .J Oxfordian ~,.2 Callovian Fig. 3. Late Jurassic sea-level curves compared. (a) Haq et al. (1988) based on a compilation of coastal onlap and sequence stratigraphic