Andrew D. Miall

A review of the braided-river depositional environment

Abstract Generalized sedimentation models have been developed from a review of more than sixty recent papers on modern and ancient braided-stream deposits. Braided rivers consist of a series of broad, shallow channels and bars, with elevated areas active only during floods, and dry islands. There are three main bar types; longitudinal, comprising crudely bedded gravel sheets; transverse to linguoid, consisting of sand or gravel and formed by downstream avalanche-face progradation; and point or side bars, formed by bedform coalescence and chute and swale development in areas of low energy. Important sediment-forming processes include bar formation, channel-floor dune migration, low-water accretion and overbank sedimentation. Braided-stream deposits consist of up to three gravel facies, five sand facies and two fine-grained facies. Vertical sequences recorded in modern and ancient deposits are of several types: flood-, channel fill-, valley fill-, channel re-occupation- and point bar-cycles. Some of these fine upward and could be confused with meandering-river sequences. Facies assemblages and vertical sequences fall into four main classes, which are proposed as sedimentation models for the interpretation of ancient braided-river deposits in the surface and subsurface: (1) Scott type: consists mainly of longitudinal bar gravels with sand lenses formed by infill of channels and scour hollows during low water. (2) Donjek type: may be dominated by sand or gravel; distinguished by fining-upward cycles caused by lateral point-bar accretion or vertical channel aggradation. Cycles commonly are less than 3 m thick, but cycles up to 60 m may be present, representing valley-fill sequences. Longitudinal and linguoid-bar deposits, channel-floor dune deposits, bar-top and overbank deposits all may be important. (3) Platte type: characterized by an abundance of linguoid bar and dune deposits (planar and trough crossbedding). No well-developed cyclicity, probably owing to a lack of topographic differentiation in the river (no evidence of deep, primary channels, abandoned areas or overbank areas). (4) Bijou Creek type: consists of horizontally laminated sand plus subordinate amounts of sand showing planar crossbedding and ripple marks. Formed during flash floods and may be most typical of ephemeral streams.

Architectural-Element Analysis: A New Method of Facies Analysis Applied to Fluvial Deposits

Abstract Existing methods of facies analysis for fluvial deposits rely extensively on vertical profile analysis and comparisons with a limited array of fixed “end member” facies models. However, vertical profiles are not sufficiently diagnostic for this purpose because they cannot adequately represent three-dimensional variations in composition and geometry. A new method of analysis is proposed which subdivides fluvial deposits into local suites consisting of one or more of a set of eight basic three-dimensional architectural elements. These are channels, gravel bars and bedforms, sandy bedforms, foreset macroforms, lateral accretion deposits, sediment gravity flow deposits, laminated sand sheets and overbank fines. Twelve fluvial styles are selected to illustrate possible combinations of these elements. It is suggested that the same methodology could be used for other clastic facies. The better documentation of three-dimensional facies variability that can be obtained should be of considerable use in interpreting sedimentary controls and in carrying out petroleum field development, reservoir engineering or ore grade studies.

The Geology of Fluvial Deposits

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Sequence stratigraphy: methodology and nomenclature

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Models of glaciomarine sedimentation and their application to the interpretation of ancient glacial sequences

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 This paper argues that glaciomarine environments can be regarded as special, glacially-influenced types of continental margin environments (e.g. continental shelf, slope, rise and basin plain). Knowledge of the stratigraphic architecture and typical sedimentary sequences of non-glacial margins is becoming well-known but remains limited for those that have been glacially modified. The principal influences on sedimentation in these environments relate to the glacial sediment input (controlled by relief of basin margin, glacier thermal regime and ice flow dynamics) and depositional environments (influence of traction currents, substrate relief and proximity to nearby ice margins). Typical ranges of sedimentation rates can be established for glacially-influenced continental margin environments and these may provide a framework for ancient sequences. Starvation of sediment supply to glacially-influenced continental margins is common. The nature of sub ice shelf sedimentation, a model that has been applied to many ancient glacial sequences is critically reviewed; the significance of such sedimentation in the rock record has probably been exaggerated because of oversimplistic interpretations of diamictite sequences. Existing process models of glaciomarine sedimentation derived from study of modern environments are sometimes difficult to employ in investigation of ancient sedimentary sequences because simple lithofacies criteria and typical vertical profiles are not available to aid in interpretation. In addition compositional data emphasized by many workers for distinguishing glaciomarine from continental glacial diamict(ite)s frequently fingerprint sediment source and not mode of deposition. The importance of facies analysis methods for isolating depositional environments is illustrated by three examples of ancient glaciomarine sequences. These are the Early Proterozoic Gowganda Formation (∼2.3 Ga) of northern Ontario, Canada; the Late Proterozoic Port Askaig Formation (∼670 Ma) of Scotland and Ireland, and the Late Cenozoic Yakataga Formation (∼20 Ma to recent) of the Gulf of Alaska. These examples illustrate the significance of detailed genetic studies of ancient glacial rocks in the interpretation of palaeogeographic and tectonic settings. Diamictite units in ancient glaciomarine sequences cannot be easily interpreted in terms of climatic or ice advance/retreat cycles, because of the varied controls on diamict accumulation and diamictite preservation in marine basins.

Architectural elements and bounding surfaces in fluvial deposits: anatomy of the Kayenta formation (lower Jurassic), southwest Colorado

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.

Exxon global cycle chart: An event for every occasion?

Abstract Three well-exposed outcrops in the Kayenta Formation (Lower Jurassic), near Dove Creek in southwestern Colorado, were studied using lateral profiles, in order to test recent regarding architectural-element analysis and the classification and interpretation of internal bounding surfaces. Examination of bounding surfaces within and between elements in the Kayenta outcrops raises problems in applying the three-fold classification of Allen (1983). Enlarging this classification to a six-fold hierarchy permits the discrimination of surfaces intermediate between Allens second- and third-order types, corresponding to the upper bounding surfaces of macroforms, and internal erosional “reactivation” surfaces within the macroforms. Examples of the first five types of surface occur in the Kayenta outcrops at Dove Creek. The new classifications is offered as a general solution to the problem of description of complex, three-dimensional fluvial sandstone bodies. The Kayenta Formation at Dove Creek consists of a multistorey sandstone body, including the deposits of lateral- and downstream-accreted macroforms. The storeys show no internal cyclicity, neither within individual elements nor through the overall vertical thickness of the formation. Low paleocurrent variance indicates low sinuosity flow, whereas macroform geometry and orientation suggest low to moderate sinuosity. The many internal minor erosion surfaces draped with mud and followed by intraclast breccias imply frequent rapid stage fluctuation, consistent with variable (seasonal? monsonal? ephemmeral?) flow. The results suggest a fluvial architecture similar to that of the South Saskatchewan River, through with a three-dimensional geometry unlike that interpreted from surface studies of that river.

Reservoir Heterogeneities in Fluvial Sandstones: Lessons from Outcrop Studies

The basic premise of the recent Exxon cycle chart, that there exists a globally correlatable suite of third-order eustatic cycles, remains unproven. Many of the tests of this premise are based on circular reasoning. The implied precision of the Exxon global cycle chart is not supportable, because it is greater than that of the best available chronostratigraphic techniques, such as those used to construct the global standard time scale. Correlations of new stratigraphic sections with the Exxon chart will almost always succeed, because there are so many Exxon sequence-boundary events from which to choose. This is demonstrated by the use of four synthetic sections constructed from tables of random numbers. A minimum of 77% successful correlations of random events with the Exxon chart was achieved. The existing cycle chart represents an amalgam of regional and local tectonic events and probably also includes unrecognized miscorrelations. It is of questionable value as an independent standard of geologic time.

Facies Architecture in Clastic Sedimentary Basins

Heterogeneities in fluvial sandstones can be classified in outcrop using a sixfold hierarchy of lithosome-bounding surfaces. Individual lithofacies units or multistory units are bounded by first- and second-order surfaces, respectively. These surfaces may be readily identified in core, but can rarely be correlated reliably between wells because of their limited areal extent (less than 10 ha. or 25 ac). Outcrop studies provide information on typical geometries and size ranges for input into reservoir model studies. Macroforms are complex, compound bars (e.g., point bars) bounded by fourth-order surfaces. Accretionary phases may be separated by internal, low-angle third-order surfaces. These depositional units are 102-103 m (328-3,280 ft) across and, because of their limited size and internal complexity, may require well spacings of 32 or 16 ha. (80 or 40 ac) for reliable mapping. Fifth-order surfaces define major channel bodies, ranging from ribbon to sheetlike in geometry. Sheet sandstones may be mappable using closely spaced well data, but ribbon sandstones are difficult to correlate except in the most well-developed field. Three-dimensional seismic is a powerful new tool for mapping fifth-order surfaces. The largest scale represents units at the member or submember level, bounded by sixth-order surfaces. These units are mappable using conventional wireline log correlation. The area of a shale bed depends on the scale of the lithosome with which it is associated. For example, shales associated with sixth-order bounding surfaces may represent basin-wide periods of low fluvial energy or lacustrine flooding, whereas those lying on second-order surfaces are the result of mud drape over bed forms in small abandoned channels, and may have areal extents of only a few square meters. Such data need to be built into computer models of reservoir flow behavior.