Sequence stratigraphy of clastic systems concepts, merits, a...

Geological Society of Africa Presidential Review No. 1 Sequence stratigraphy of clastic systems: concepts, merits, and pitfalls Octavian Catuneanu * Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Sciences Building, Edmonton, Alta., Canada T6G 2E3 Abstract Sequence stratigraphy is widely embraced as a new method of stratigraphic analysis by both academic and industry practitioners. This new method has considerably improved our insight into how sedimentary basins accumulate and preserve sediments, and has become a highly successful exploration technique in the search for natural resources. The different sequence stratigraphic models that are currently in use, i.e. three varieties of depositional sequences, a genetic stratigraphic sequence, and a transgressive–regressive sequence, all have merits and limitations. Each model works best in particular tectonic settings, and no one model is applicable to the entire range of case studies. Flexibility is thus recommended for choosing the model that is the best match for a specific project. Having said that, the existing sequence models also have a lot in common, with the main difference being in the style of conceptual packaging of the same succession of strata (i.e., where to pick the sequence boundaries). Sequence stratigraphic models are centered around one curve of base level fluctuations that describes the changes in accom- modation at the shoreline. The interplay between sedimentation and this curve of base level changes controls the transgressive and regressive shifts of the shoreline, as well as the timing of all systems tract and sequence boundaries. Surfaces that can serve, at least in part, as systems tract boundaries, are sequence stratigraphic surfaces. Systems tract boundaries have low diachroneity rates along dip, which match the rates of sediment transport. These surfaces may be much more diachronous along strike, in relation to variations in subsidence and sedimentation rates. This paper presents the fundamental concepts of sequence stratigraphy, and discusses the merits and pitfalls of its theoretical framework. The deviations in the rock record from the predicted architecture of systems tracts and stratigraphic surfaces are also discussed. � 2002 Elsevier Science Ltd. All rights reserved. Keywords: Sequence stratigraphy; Eustasy and base level; Tectonic setting; Accommodation space; Architecture of systems tracts and stratigraphic surfaces Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 02 1.1. Sequence stratigraphy: a new paradigm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 02 1.2. Historical developments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 03 1.3. Definitions and key concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 04 2. Base level changes, transgressions, and regressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 06 2.1. Base level. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 06 2.2. Base level changes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 08 2.3. Transgressions and regressions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 09 3. Stratigraphic surfaces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 3.1. Types of stratal terminations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 3.2. Sequence stratigraphic surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.2.1. Subaerial unconformity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 3.2.2. Correlative conformity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3.2.3. Basal surface of forced regression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3.2.4. Regressive surface of marine erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Journal of African Earth Sciences 35 (2002) 1–43 www.elsevier.com/locate/jafrearsci * Tel.: +1-780-492-6569; fax: +1-780-492-7598. E-mail address: octavian@ualberta.ca (O. Catuneanu). 0899-5362/02/$ - see front matter � 2002 Elsevier Science Ltd. All rights reserved. PII: S0899-5362 (02 )00004-0 3.2.5. Maximum regressive surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 3.2.6. Maximum flooding surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 3.2.7. Ravinement surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 3.3. Within-trend facies contacts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 3.3.1. Within-trend normal regressive surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 3.3.2. Flooding surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 4. Systems tracts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 4.1. Methods of definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 4.2. Lowstand systems tract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 4.3. Transgressive systems tract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 4.4. Highstand systems tract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 4.5. Falling stage systems tract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 4.6. Regressive systems tract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 5. Sequence models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 5.1. Methods of sequence delineation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 5.2. Depositional sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 5.3. Genetic stratigraphic sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 5.4. Transgressive–regressive sequence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 5.5. Parasequences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 6. Time attributes of stratigraphic surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 6.1. Subaerial unconformity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 6.2. Correlative conformity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 6.3. Basal surface of forced regression. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 6.4. Regressive surface of marine erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 6.5. Maximum regressive surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 6.6. Maximum flooding surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 6.7. Ravinement surface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 6.8. Within-trend normal regressive surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 6.9. Within-trend flooding surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 7. Hierarchy of sequences and bounding surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 8. Discussion and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 8.1. Sequence stratigraphy: theory versus reality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 8.2. Sequence models: the importance of the tectonic setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 8.3. Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 1. Introduction 1.1. Sequence stratigraphy: a new paradigm Sequence stratigraphy is the most recent and revolu- tionary paradigm in the field of sedimentary geology, and completely revamps geological thinking and the methods of stratigraphic analysis. As opposed to the other, more conventional types of stratigraphy, such as biostratig- raphy, lithostratigraphy, chemostratigraphy or magnet- ostratigraphy, which are mostly concerned with data collection, sequence stratigraphy has an important built- in interpretation component which addresses issues such as (i) the reconstruction of the allogenic controls at the time of sedimentation, and (ii) predictions of facies ar- chitecture in yet unexplored areas. The former issue sparked an intense debate, still ongoing, between the supporters of eustatic versus tectonic controls on sedi- mentation, which is highly important to the under- standing of Earth history and the fundamental Earth processes. The latter issue provides the petroleum indus- try community with a new and powerful analytical and correlation tool for exploration and basin analysis. This is not to say, however, that sequence stratigra- phy is the triumph of interpretation over data, or that sequence stratigraphy developed in isolation from other geological disciplines. In fact sequence stratigraphy builds on many existing data sources, requires a good 2 O. Catuneanu / Journal of African Earth Sciences 35 (2002) 1–43 knowledge of sedimentology and facies analysis, and fills the gap between sedimentology, basin analysis, and the various types of conventional stratigraphy (Figs. 1 and 2). 1.2. Historical developments Sequence stratigraphy is generally regarded as stem- ming from the seismic stratigraphy of the 1970s. In fact, major studies investigating the relationship between sedimentation, unconformities, and changes in base level, which are directly relevant to sequence stratigra- phy, were published prior to the birth of seismic stra- tigraphy (e.g., Grabau, 1913; Barrell, 1917; Sloss et al., 1949; Sloss, 1962, 1963; Wheeler and Murray, 1957; Wheeler, 1958, 1959, 1964; Curray, 1964; Frazier, 1974). The term ‘‘sequence’’ was introduced by Sloss et al. (1949) to designate a stratigraphic unit bounded by subaerial unconformities. Sloss emphasized the impor- tance of such sequence-bounding unconformities, and subsequently subdivided the entire Phanerozoic succes- sion of the interior craton of North America into six major sequences (Sloss, 1963). Sloss also emphasized the importance of tectonism in the generation of sequences and bounding unconformities, an idea which is widely accepted today but was largely ignored by the propo- nents of seismic stratigraphy. Seismic stratigraphy emerged in the 1970s with the work of Vail (1975) and Vail et al. (1977). This new Fig. 1. Sequence stratigraphy in the context of interdisciplinary research. Fig. 2. Types of stratigraphy, defined on the basis of the property they analyze. O. Catuneanu / Journal of African Earth Sciences 35 (2002) 1–43 3 method for analyzing seismic-reflection data stimulated a revolution in stratigraphy, with an impact on the geological community as important as the introduction of the flow regime concept in the late 1950s–early 1960s and the plate tectonics theory in the 1960s (Miall, 1995). The concepts of seismic stratigraphy were published together with the global cycle chart (Vail et al., 1977), based on the underlying assumption that eustasy is the main driving force behind sequence formation at all levels of stratigraphic cyclicity. Seismic stratigraphy and the global cycle chart were thus introduced to the geo- logical community as an inseparable package of new stratigraphic methodology. These ideas were then pas- sed on to sequence stratigraphy in its early years, as seismic stratigraphy evolved into sequence stratigraphy with the incorporation of outcrop and well data (Posa- mentier et al., 1988; Posamentier and Vail, 1988; Van Wagoner et al., 1990). The global-eustasy model posed two challenges to the practitioners of ‘‘conventional’’ stratigraphy: (1) that sequence stratigraphy, as linked to the global cycle chart, constitutes a superior standard of geological time to that assembled from conventional chronostratigraphic evidence, and (2) that stratigraphic processes are dominated by the effects of eustasy, to the exclusion of other allogenic mechanisms, including tec- tonism (Miall and Miall, 2001). Although the global cycle chart is now under intense scrutiny and criticism (e.g., Miall, 1992), the global-eustasy model is still used for sequence stratigraphic analysis in some recent pub- lications (e.g., de Gracianski et al., 1998). In parallel to the eustasy-driven sequence stratigra- phy, which held by far the largest share of the market, other researchers went to the opposite end of the spec- trum by suggesting a methodology that favored tec- tonism as the main drive of stratigraphic cyclicity. This version of sequence stratigraphy was introduced as ‘‘tectonostratigraphy’’ (e.g., Winter, 1984). The major weakness of both schools of thought is that an a priori interpretation of the main allogenic control on accom- modation was automatically attached to any sequence delineation, which gave the impression that sequence stratigraphy is more of an interpretation artifact than an empirical, data-based method. This a priori interpreta- tion facet of sequence stratigraphy attracted consid- erable criticism and placed an unwanted shade on a method that otherwise represents a truly important ad- vance in the science of stratigraphy. Fixing the damaged image of sequence stratigraphy only requires the basic understanding that base level changes can be controlled by any combination of eustatic and tectonic forces, and that the dominance of any of these allogenic mecha- nisms should be assessed on a case by case basis. It became clear that sequence stratigraphy needs to be dissociated from the global-eustasy model, and that a more objective analysis should be based on empirical evidence that can actually be observed in outcrop or the subsurface. This realization came from inside the Exxon research group, where the global cycle chart originated in the first place: ‘‘Each stratal unit is defined and identified only by physical relationships of the strata, including lateral continuity and geometry of the surfaces bounding the units, vertical stacking patterns, and lat- eral geometry of the strata within the units. Thickness, time for formation, and interpretation of regional or global origin are not used to define stratal units. . ., [which]. . . can be identified in well logs, cores, or out- crops and used to construct a stratigraphic framework regardless of their interpreted relationship to changes in eustasy’’ (Van Wagoner et al., 1990). The switch in emphasis from sea level changes to relative sea level changes in the early 1990s marked a major and positive turnaround in sequence stratigra- phy. By doing so, no interpretation of specific eustatic or tectonic fluctuations was forced upon sequences, sys- tems tracts, or stratigraphic surfaces. Instead, the key surfaces, and implicitly the stratal units between them, are inferred to have formed in relation to a more ‘‘neutral’’ curve of relative sea level (base level) changes that can accommodate any balance between the allo- genic controls on accommodation. 1.3. Definitions and key concepts Figs. 3 and 4 provide the most popular definitions for sequence stratigraphy and the key sequence stratigraphic concepts. In contrast with all other types of stratigraphy (including allostratigraphy), and in spite of becoming such a fashionable method of stratigraphic analysis, sequence stratigraphy has not yet made it into the North American Code of Stratigraphic Nomenclature. The reason for this is the lack of agreement on some basic sequence stratigraphic concepts, including the definition of a ‘‘sequence’’, and also the proliferation of an in- credibly complex jargon that is next to impossible to standardize. The fact that several different sequence models are currently in use does not make the task of finding a common ground easy, even for what a ‘‘sequence’’ should be. Part of the problem comes from the fact that the position of the sequence boundary (both in space and time) varies from one model to another, to the extent that any of the key stratigraphic surfaces may become the (or part of the) sequence boundary. Never- theless, all versions of sequence boundaries include both unconformable and conformable portions, which means that the original definition of Mitchum(1977) (Fig. 4) still fits in most of the cases. It is important to note that no scale is associated with the definition of sequence stratigraphic concepts (Figs. 3 and 4). This means that the same terminology can and should be applied for sequences, systems tracts, and surfaces that develop