Octavian Catuneanu

Sequence stratigraphy of clastic systems; concepts, merits and pitfalls

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 accommodation 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.

Sequence stratigraphy: methodology and nomenclature

The recurrence of the same types of sequence stratigraphic surface through geologic time defines cycles of change in accommodation or sediment supply, which correspond to sequences in the rock record. These cycles may be symmetrical or asymmetrical, and may or may not include all types of systems tracts that may be expected within a fully developed sequence. Depending on the scale of observation, sequences and their bounding surfaces may be ascribed to different hierarchical orders. Stratal stacking patterns combine to define trends in geometric character that include upstepping, forestepping, backstepping and downstepping, expressing three types of shoreline shift: forced regression (forestepping and downstepping at the shoreline), normal regression (forestepping and upstepping at the shoreline) and transgression (backstepping at the shoreline). Stacking patterns that are independent of shoreline trajectories may also be defined on the basis of changes in depositional style that can be correlated regionally. All stratal stacking patterns reflect the interplay of the same two fundamental variables, namely accommodation (the space available for potential sediment accumulation) and sediment supply. Deposits defined by specific stratal stacking patterns form the basic constituents of any sequence stratigraphic unit, from sequence to systems tract and parasequence. Changes in stratal stacking patterns define the position and timing of key sequence stratigraphic surfaces. Precisely which surfaces are selected as sequence boundaries varies as a function of which surfaces are best expressed within the context of the depositional setting and the preservation of facies relationships and stratal stacking patterns in that succession. The high degree of variability in the expression of sequence stratigraphic units and bounding surfaces in the rock record means ideally that the methodology used to analyze their depositional setting should be flexible from one sequence stratigraphic approach to another. Construction of this framework ensures the success of the method in terms of its objectives to provide a process-based understanding of the stratigraphic architecture. The purpose of this paper is to emphasize a standard but flexible methodology that remains objective.

Precambrian clastic sedimentation systems

Abstract The unique and evolving nature of the Precambrian geological environment in many ways was responsible for significant differences between Precambrian clastic sedimentary deposits and their Phanerozoic-modern equivalents. Some form of plate tectonics, with rapid microplate collisions and concomitant volcanic activity, is inferred to have led to the formation of greenstone belts. Explosive volcanism promoted common gravity-flow deposits within terrestrial greenstone settings, with braided alluvial, wave/storm-related and tidal coastline sediments also being preserved. Late Archaean accretion of greenstone terranes led to emergence of proto-cratons, where cratonic and rift sedimentary assemblages developed, and these became widespread in the Proterozoic as cratonic plates stabilised. Carbonate deposition was restricted by the paucity of stable Archaean terranes. An Early Precambrian atmosphere characterised by greenhouse gases, including CO2, in conjunction with a faster rotation of the Earth and reduced albedo, provide a solution to the faint young Sun paradox. As emergent continental crust developed, volcanic additions of CO2 became balanced by withdrawal due to weathering and a developing Palaeoproterozoic microbial biomass. The reduction in CO2, and the photosynthetic production of O2, led to aerobic conditions probably being achieved by about 2 Ga. Oceanic growth was allied to atmospheric development, with approximately 90% of current ocean volume being reached by about 4 Ga. Warm Archaean and warm, moist Palaeoproterozoic palaeoclimates appear to have become more arid after about 2.3 Ga. The 2.4–2.3 Ga Huronian glaciation event was probably related to continental growth, supercontinent assembly and weathering-related CO2 reduction. Despite many analogous features among both Precambrian and younger sedimentary deposits, there appear to be major differences as well. Two pertinent examples are rare unequivocal aeolian deposits prior to about 1.8 Ga and an apparent scarcity of Precambrian foreshore deposits, particularly those related to barrier island systems. The significance of these differences is hard to evaluate, particularly with the reduced palaeoenvironmental resolution because of the absence of invertebrate and plant fossils within Precambrian successions. The latter factor also poses difficulties for the discrimination of Precambrian lacustrine and shallow marine deposits. The temporal distribution of aeolian deposits probably reflects a number of possible factors, including few exposed late Archaean–Palaeoproterozoic cratonic areas, extensive pre-vegetative fluvial systems, Precambrian supercontinents and a different atmosphere. Alternatively, the scarcity of aeolian deposits prior to 1.8 Ga may merely reflect non-recognition or non-preservation. Precambrian shallow marine environments may have been subjected to more uniform circulation systems than those interpreted from the Phanerozoic-modern rock record, and Precambrian shelves probably were broad with gentle seaward slopes, in contrast to the narrow, steep shelves mostly observed in present settings. Poorly confined Precambrian tidal channels formed sheet sandstones, easily confused with fluvial or offshore sand sheets. Epeiric seas were possibly more prevalent in the Precambrian, but active tectonism as proto-continents emerged and amalgamated to form early supercontinents, in conjunction with a lack of sufficient chronological data in the rock record, make it difficult to resolve the relative importance of eustatic and tectonic influences in forming epeiric embayments and seaways. Other differences in Precambrian palaeoenvironments are more easily reconstructed. Ancient delta plain channels were probably braided, and much thicker preserved delta successions in the Precambrian are compatible with the inferred more active tectonic conditions. Pre-vegetational alluvial channel systems were almost certainly braided as well. Common fluvial quartz arenites are ascribed to differences in weathering processes, which probably changed significantly through the Precambrian, or to sediment recycling. Although Precambrian glacigenic environments were probably the least different from younger equivalents, their genesis appears to reflect a complex interplay of factors unique to the Precambrian Earth. These include emergence and amalgamation of proto-continents, the early CO2-rich atmosphere, the development of stromatolitic carbonate platforms, early weathering, faster rotation of the Earth and the possible role of changes in the inclination of the Earths axis.

Interplay of static loads and subduction dynamics in foreland basins: Reciprocal stratigraphies and the 'missing' peripheral bulge

Foreland basins are created by superimposed mechanisms that flex the lithosphere. In addition to static loads, dynamic loading below the basin by viscous mantle corner flow above a subducting plate may cause long-wavelength subsidence. It is proposed that the interaction of the static and dynamic forces is responsible for the formation and preservation of recently recognized “reciprocal stratigraphies” in retro-foreland regions above subducting slabs. Reciprocal stratigraphies refer to a correlative succession of strata characterized by contrasting stacking patterns that reflect opposite base-level changes between proximal and distal settings. The same interactions may also modify the stratigraphy of the flexural peripheral bulge and mask its presence. An example from the Late Cretaceous of the Western Interior basin, Canada, illustrates the concepts.

Stochastic surface-based modeling of turbidite lobes

Flow event deposits in turbidite lobes are modeled with stochastic surface-based simulation. This method honors the geometries and compensational stacking of flow event deposits. Flow event deposit geometries are based on a flexible lobe parameterization. Compensational stacking is the tendency of flow event deposits to fill topographic lows and to smoothing of topographic relief. The surface-based model may be conditioned to well data.Models of reservoir properties such as porosity and permeability are constrained by the resulting geometric models. This approach is applied in a geostatistical workflow to better integrate available geologic information. The resulting models may improve the accuracy of model reservoir response and account for the uncertainty in the heterogeneity of turbidite lobes.

Maastrichtian-Paleocene foreland-basin stratigraphies, western Canada: a reciprocal sequence architecture

Palynological and magnetostratigraphic chronostratigraphic correlations of lower Maastrichtian to Paleocene strata along an east–west Western Canada Basin transect allow for the recognition of a reciprocal sequence architecture in nonmarine strata. Reference sections include three Canadian Continental Drilling Program Cretaceous–Tertiary Boundary Project core holes and outcrops in Alberta, southern Saskatchewan, and north-central Montana. The spatial and temporal position of the third-order sequences provides evidence for the correlation of proximal sector regional disconformities and sedimentary wedges with distal sector sedimentary wedges and regional disconformities, respectively. The boundary between the two sectors is represented by a hingeline, which separates the foreland-basin “syncline” from the “peripheral bulge.” The stratigraphies defined by reciprocal third-order sequences are complicated by fourth-order boundaries, developed within proximal sedimentary wedges and with no correlative distal strata. These results support tectonic control on foreland-basin sedimentation. A model for interpreting the various types of sequences in terms of foreland-basin evolution, vertical tectonics, and orogenic cycles is provided. It is argued that nonmarine sequence boundaries (times of maximum uplift in the foreland region) may be expressed as disconformities, incised valleys, top of mature paleosol levels, or base of fluvial channels, whereas nonmarine equivalents of marine maximum flooding surfaces (times of maximum basinal subsidence) may be indicated by extensive coal seams and (or) lacustrine sediments. Résumé: Les corrélations palynologiques, magnétostratigraphiques et chronostratigraphiques des strates échelonnées du Maastrichtien inférieur au Paléocène, le long du transect est-ouest du bassin de la Cordillère occidentale au Canada, permettent la reconnaissance d’une ordonnance architecturale de séquences réciproques dans les strates continentales. Les coupes de référence incluent trois trous de forage carottés du «Canadian Continental Drilling Program Cretaceous– Tertiary Boundary Project», et des affleurements localisés en Alberta, dans le sud de la Saskatchewan et dans le centrenord du Montana. La position spatiale et temporelle des séquences de troisième ordre plaide pour la mise en corrélation des discordances régionales et des dépôts sédimentaires en coin dans le secteur proximal avec les dépôts sédimentaires en coin et les discordances régionales dans le secteur distal, respectivement. Une ligne de jonction représente la limite entre les deux secteurs, qui sépare le bassin d’avant pays «en forme de cuvette» du «bombement périphérique». Les stratigraphies définies par les séquences de troisième ordre réciproques sont embrouillées par les limites de quatrième ordre, développées au sein des dépôts sédimentaires en coin proximaux sans la présence de strates distales corrélatives. Ces résultats appuient la thèse d’un contrôle tectonique sur le bassin sédimentaire d’avant-pays. On présente ici un modèle interprétant les divers types de séquences en termes d’évolution d’un bassin d’avant-pays, de tectonique verticale et de cycles orogéniques. Nous tentons de démontrer que les discordances, les vallées entaillées, les horizons sommitaux de paléosols matures ou la base des chenaux fluviaux peuvent aider à définir les limites des séquences continentales (périodes de soulèvement maximum dans la région de l’avant-pays), tandis que les couches de charbon étendues et (ou) les sédiments lacustres représentent plutôt les équivalents continentaux des aires d’inondation maximale (périodes de subsidence maximum du bassin). [Traduit par la Rédaction] Catuneanu and Sweet 703

Late Archaean superplume events: a Kaapvaal–Pilbara perspective

Abstract The 2714–2709 Ma Ventersdorp Supergroup overlies Mesoarchaean basement rocks and sedimentary strata of the Neoarchaean Witwatersrand Supergroup. The latter basin was inverted by tectonic shortening and suffered the loss of up to 1.5 km of stratigraphy prior to deposition of the Ventersdorp volcanics. Thermal uplift and fluvial incision prior to the basal Klipriviersberg Group flood basalts appear to have been limited, but this could also reflect a hot dry palaeoclimate acting on a peneplained plateau. Rapid ascent of ponded magma beneath thinned sub-Witwatersrand lithosphere, transported laterally from a mantle plume starting head possibly situated marginally to the Kaapvaal craton is inferred for this unit of up to 2 km of predominantly tholeiitic basalts with subordinate, basal komatiites. Crustal extension related to ascent of the ponded magma followed, leading to the formation of a set of graben and half-graben basins, in which immature clastic sedimentary, and felsic to mafic lavas and pyroclastics of the Platberg Group were laid down. The Platberg basins show no evidence for reactivation of pre-existing crustal structures. The Fortescue Group of the Pilbara craton has an analogous lower flood basaltic succession, followed by graben-fills similar to those of the Platberg Group. Differences in the Fortescue include evidence for significant thermal uplift prior to the onset of volcanism, subaqueous basalts in the south of the Pilbara craton, evidence for two episodes of flood basaltic volcanism, possibly related to two plumes at c. 2765 and 2715 Ma, and graben basins aligned along existing cratonic structures. Both Kaapvaal and Pilbara flood basalts and graben-related sedimentary-volcanic deposits are thought to have been part of a c. 2.7 Ga global superplume event. The plume inferred for the Fortescue Group flood basalts was probably related to rifting and the breakup of a plate larger than the preserved Pilbara craton. Uppermost Ventersdorp units (Bothaville Formation terrestrial clastic and Allanridge Formation tholeiitic rocks) suggest a combination of thermal subsidence, allied to continued plume (minor komatiites) and graben basin influences. In the Kaapvaal craton, the Transvaal Supergroup lies unconformably above the Ventersdorp. Basal “protobasinal” successions reflect discrete fault-bounded basin-fills, analogous to those of the Platberg Group; however, it is inferred that the former depositories were related to craton marginal plate tectonic influences, specifically the c. 2.6 Ga Limpopo orogeny. Thin fluvial sheet sandstones of the Black Reef Formation unconformably succeed the protobasinal rocks and reflect the transition to an epeiric drowning of much of the Kaapvaal craton. A shallow shelf carbonate-banded iron formation platform succession (Chuniespoort-Ghaap Groups) developed in two sub-basins on the Kaapvaal craton. They are mirrored by the approximately coeval Hamersley chemical epeiric sediments on the Pilbara craton, and both Kaapvaal and Pilbara transgressive successions are related here to a possible second, c. 2.5 Ga superplume event, which raised sea levels globally. Evidence for the younger superplume event is less clear than for the c. 2.7 Ga event.

Foredeep submarine fans and forebulge deltas: orogenic off-loading in the underfilled Karoo Basin

Abstract Third-order sequence stratigraphic analysis of the Early Permian marine to continental facies of the Karoo Basin provides a case study for the sedimentation patterns which may develop in an underfilled foreland system that is controlled by a combination of supra- and sublithospheric loads. The tectonic regime during the accumulation of the studied section was dominated by the flexural rebound of the foreland system in response to orogenic quiescence in the Cape Fold Belt, which resulted in foredeep uplift and forebulge subsidence. Coupled with flexural tectonics, additional accommodation was created by dynamic loading related to the process of subduction underneath the basin. The long-wavelength dynamic loading led to the subsidence of the peripheral bulge below base level, which allowed for sediment accumulation across the entire foreland system. A succession of five basinwide regressive systems tracts accumulated during the Artinskian (∼5 My), consisting of foredeep submarine fans and correlative forebulge deltas. The progradation of submarine fans and deltaic systems was controlled by coeval forced and normal regressions of the proximal and distal shorelines of the Ecca interior seaway respectively. The deposition of each regressive systems tract was terminated by basinwide transgressive episodes, that may be related to periodic increases in the rates of long-wavelength dynamic subsidence.

Sequence analysis of the Ecca—Beaufort contact in the southern Karoo of South Africa

Sequence analysis of the Ecca—Beaufort boundary in the southern Karoo Basin has revealed three separate facies associations spanning the stratigraphic interval between the top of the Fort Brown Formation and the lowermost maroon mudrocks of the Beaufort Group. This sequence was deposited in prodelta, delta front, and delta plain environments respectively. The lithological contact between the rocks deposited in the delta front and delta plain represents the palaeoshoreline, and occurs only once in the stratigraphic sequence, suggesting a continuous normal regression from Ecca to Beaufort times. This diachronous shoreline is associated with deltaic progradation within ahighstand systems tract. Sediment deposition was mainly a result of ephemeral flash floods, but perennial rivers also flowed from melting ice-capped highlands to the palaeosouth. Two separate fossil associations have been recognized and correlate with the lithological subdivisions. The lower fossil assemblage occurs in the Fort Brown andWaterford formationsand is characterized by silicified wood, comminuted plant material, and fish scales. The upper association occurs only in the lower Beaufort Group and includes in situ equisitalean, and well-preserved Glossopteris plant fossils and tetrapods of the Eodicynodon Assemblage Zone. The Beaufort—Ecca boundary coincides with the position of the palaeoshoreline. This lithological contact also marks achange in depositional style and palaeontological character between the two groups. The new placementof the boundary is some 300— 650m below the presently mapped contact in the south of the basin.

Basement control on flexural profiles and the distribution of foreland facies: The Dwyka Group of the Karoo Basin, South Africa

Flexural partitioning of a foreland system into foredeep, forebulge, and backbulge is the result of the interplay of lithospheric deflection under orogenic loading (flexural tectonics) and the structure of the underlying basement. Flexural tectonics is commonly invoked as the sole control on flexural profiles, which are modeled as dampened sine curves with wavelengths and magnitudes controlled by a number of physical parameters of the lithosphere and the applied loads. Basement tectonics is shown here to be an equally important control on the scale and shape of flexural provinces, by controlling the location of flexural hinge lines and the tilt directions within the foreland system. The case study for this paper shows that basement heterogeneity coupled with the selective reactivation of major crustal faults may lead to distortion of the ideal flexural profile, by modifying the wavelength and shape of individual flexural provinces.