Sedimentology

The complexity of flow and wide variety of depositional processes operating in subaqueous density flows, combined with post‐depositional consolidation and soft‐sediment deformation, often make it difficult to interpret the characteristics of the original flow from the sedimentary record. This has led to considerable confusion of nomenclature in the literature. This paper attempts to clarify this situation by presenting a simple classification of sedimentary density flows, based on physical flow properties and grain‐support mechanisms, and briefly discusses the likely characteristics of the deposited sediments. Cohesive flows are commonly referred to as debris flows and mud flows and defined on the basis of sediment characteristics. The boundary between cohesive and non‐cohesive density flows (frictional flows) is poorly constrained, but dimensionless numbers may be of use to define flow thresholds. Frictional flows include a continuous series from sediment slides to turbidity currents. Subdivision of these flows is made on the basis of the dominant particle‐support mechanisms, which include matrix strength (in cohesive flows), buoyancy, pore pressure, grain‐to‐grain interaction (causing dispersive pressure), Reynolds stresses (turbulence) and bed support (particles moved on the stationary bed). The dominant particle‐support mechanism depends upon flow conditions, particle concentration, grain‐size distribution and particle type. In hyperconcentrated density flows, very high sediment concentrations (>25 volume%) make particle interactions of major importance. The difference between hyperconcentrated density flows and cohesive flows is that the former are friction dominated. With decreasing sediment concentration, vertical particle sorting can result from differential settling, and flows in which this can occur are termed concentrated density flows. The boundary between hyperconcentrated and concentrated density flows is defined by a change in particle behaviour, such that denser or larger grains are no longer fully supported by grain interaction, thus allowing coarse‐grain tail (or dense‐grain tail) normal grading. The concentration at which this change occurs depends on particle size, sorting, composition and relative density, so that a single threshold concentration cannot be defined. Concentrated density flows may be highly erosive and subsequently deposit complete or incomplete Lowe and Bouma sequences. Conversely, hydroplaning at the base of debris flows, and possibly also in some hyperconcentrated flows, may reduce the fluid drag, thus allowing high flow velocities while preventing large‐scale erosion. Flows with concentrations <9% by volume are true turbidity flows (sensuBagnold, 1962), in which fluid turbulence is the main particle‐support mechanism. Turbidity flows and concentrated density flows can be subdivided on the basis of flow duration into instantaneous surges, longer duration surge‐like flows and quasi‐steady currents. Flow duration is shown to control the nature of the resulting deposits. Surge‐like turbidity currents tend to produce classical Bouma sequences, whose nature at any one site depends on factors such as flow size, sediment type and proximity to source. In contrast, quasi‐steady turbidity currents, generated by hyperpycnal river effluent, can deposit coarsening‐up units capped by fining‐up units (because of waxing and waning conditions respectively) and may also include thick units of uniform character (resulting from prolonged periods of near‐steady conditions). Any flow type may progressively change character along the transport path, with transformation primarily resulting from reductions in sediment concentration through progressive entrainment of surrounding fluid and/or sediment deposition. The rate of fluid entrainment, and consequently flow transformation, is dependent on factors including slope gradient, lateral confinement, bed roughness, flow thickness and water depth. Flows with high and low sediment concentrations may co‐exist in one transport event because of downflow transformations, flow stratification or shear layer development of the mixing interface with the overlying water (mixing cloud formation). Deposits of an individual flow event at one site may therefore form from a succession of different flow types, and this introduces considerable complexity into classifying the flow event or component flow types from the deposits.

Fluvial landforms and deposits provide one of the most readily studied Quaternary continental records, and alluvial strata represent an important component in most ancient continental interior and continental margin successions. Moreover, studies of the long‐term dynamics of fluvial systems and their responses to external or ‘allogenic' controls, can play important roles in research concerning both global change and sequence‐stratigraphy, as well as in studies of the dynamic interactions between tectonic activity and surface processes. These themes were energized in the final decades of the twentieth century, and may become increasingly important in the first decades of this millennium.

This review paper provides a historical perspective on the development of ideas in the fields of geomorphology/Quaternary geology vs. sedimentary geology, and then summarizes key processes that operate to produce alluvial stratigraphic records over time‐scales of 10³−10⁶ years. Of particular interest are changes in discharge regimes, sediment supply and sediment storage en route from source terrains to sedimentary basins, as well as changes in sea‐level and the concept of accommodation. Late Quaternary stratigraphic records from the Loire (France), Mississippi (USA), Colorado (Texas, USA) and Rhine–Meuse (The Netherlands) Rivers are used to illustrate the influences of climate change on continental interior rivers, as well as the influence of interacting climate and sea‐level change on continental margin systems.

The paper concludes with a look forward to a bright future for studies of fluvial response to climate and sea‐level change. At present, empirical field‐based research on fluvial response to climate and sea‐level change lags behind: (a) the global change community's understanding of the magnitude and frequency of climate and sea‐level change; (b) the sequence‐stratigraphic community's desire to interpret climate and, especially, sea‐level change as forcing mechanisms; and (c) the modelling community's ability to generate numerical and physical models of surface processes and their stratigraphic results. A major challenge for the future is to catch up, which will require the development of more detailed and sophisticated Quaternary stratigraphic, sedimentological and geochronological frameworks in a variety of continental interior and continental margin settings. There is a particular need for studies that seek to document fluvial responses to allogenic forcing over both shorter (10²−10³ years) and longer (10⁴−10⁶ years) time‐scales than has commonly been the case to date, as well as in larger river systems, from source to sink. Studies of Quaternary systems in depositional basin settings are especially critical because they can provide realistic analogues for interpretation of the pre‐Quaternary rock record.

Recent alluvial sediments are reviewed with respect to their geometrical, textural, structural and biological characteristics. These properties are related to the physiographic occurrence and hydraulic geometry of streams and to the dynamics of flowing water as controlling sediment transport‐deposition and stream morphological activities. Based on this data, three‐dimensional facies models are presented as an aid to the identification of ancient alluvial sediments, which are briefly reviewed also.

Three processes of water escape characterize the consolidation of silt‐, sand‐and gravel‐sized sediments. Seepage involves the slow upward movement of pore fluids within existing voids or rapid flow within compact and confined sediments. Liquefaction is marked by the sudden breakdown of a metastable, loosely packed grain framework, the grains becoming temporarily suspended in the pore fluid and settling rapidly through the fluid until a grain‐supported structure is re‐established. Fluidization occurs when the drag exerted by moving pore fluids exceeds the effective weight of the grains; the particles are lifted, the grain framework destroyed, and the sediment strength reduced to nearly zero. Diagenetic sedimentary structures formed in direct response to processes of fluid escape are here termed water escape structures.

Four main types of water escape structures form during the fluidization and liquefaction of sands: (1) soft‐sediment mixing bodies, (2) soft‐sedimsnt intrusions, (3) consolidation laminations, and (4) soft‐sediment folds. These structures represent both the direct rearrangement of sediment grains by escaping fluids and the deformation of hydroplastic, liquefied, or fluidized sediment in response to external stresses.

Fundamental controls on sediment consolidation are exerted by the bulk sediment properties of grain size, packing, permeability, and strength, which together determine whether consolidation will occur and, if so the course it follows, and by external disturbances which act to trigger liquefaction and fluidization. The liquefaction and fluidization of natural sands usually accompanies the collapse of loosely packed cross‐bedded deposits. This collapse is commonly initiated by water forced into the units as underlying beds, especially muds and clays, consolidate. The consolidation of subjacent units is often triggered by the rapid deposition of the sand itself, although earthquakes or other disturbances are probably influential in some instances.

Water escape structures most commonly form in fine‐ to medium‐grained sands deposited at high instantaneous and mean sedimentation rates; they are particularly abundant in cross‐laminated deposits but rare in units deposited under upper flow regime plane bed conditions. Their development is favoured by upward decreasing permeability within sedimentation units such as normally graded turbidites. They are especially common in sequences made up of alternating fine‐(clay and mud) and coarse‐grained (sand) units such as deep‐sea flysch prodelta, and, to a lesser extent, fluvial point bar, levee, and proximal overbank deposits.

The thinnest recognizable strata in modern eolian dune sands can be grouped into six classes. They are herein named planebed laminae, rippleform laminae, ripple‐foreset crosslaminae, climbing translatent strata, grainfall laminae, and sandflow cross‐strata.

Planebed laminae are formed by tractional deposition on smooth surfaces at high wind velocities. They are very rare in the deposits studied. Grainfall laminae are also formed on smooth surfaces, largely by grainfall deposition in zones of flow separation. They are much more common than planebed laminae, which they closely resemble.

Eolian climbing‐ripple structures are composed primarily of climbing trans‐latent strata, each of which is the depositional product of a single climbing ripple. Climbing translatent strata that formed at relatively high or supercritical angles of ripple climb are typically accompanied by rippleform laminae, which are wavy layers parallel to the rippled depositional surfaces. Ripple‐foreset crosslaminae, which are incomplete rippleform laminae produced when the angle of ripple climb is relatively low or subcritical, are rarely visible in eolian sands.

Sandflow cross‐strata are formed by the avalanching of noncohesive sand on dune slipfaces. Their form varies with slipface height and with other factors.

ABSTRACT Increased knowledge of modern glacial depositional environments has resulted in rapidly evolving classifications of glacial tills. These are based to a large degree on theoretical considerations of likely depositional processes. The classifications are sophisticated and more advanced than the establishment of simple field criteria whereby individual till facies can be identified in Quaternary and Pre‐Quaternary successions. This situation is compounded in many Quaternary terrains by the continued description of ‘tills’ in terms of laboratory‐derived analytical data only, reflecting a traditional interest in stratigraphic correlation rather than reconstruction of depositional environment. Detailed sedimentological logging of lithofacies is rarely undertaken. There is thus considerable confusion as to what is being described or sampled when analytical data are presented for many Pleistocene ‘tills’. The same remarks apply to Pre‐Pleistocene ‘tillites’.

A lithofacies code is presented here for the rapid description and visual appraisal of field sequences or drill cores containing unconsolidated diamicts or lithified diamictites; the term‘till’is not used as it has a strict genetic definition referring to direct aggregation and deposition by glacier ice. Use of a four part code, in conjunction with codes already published for fluvial sediments, allows fundamental field properties to be depicted independent of genetic terminology and provides a firm basis for subsequent environmental interpretation and analytical work. The value of this approach is illustrated by comparing a representative suite of vertical profiles of diamict assemblages deposited by modern grounded glaciers with a classic late Pleistocene glacigenic sequence at Scarborough Bluffs, Ontario.

Short term variability in delta form and process can be partly explained by the relative strength of hydraulic parameters such as river discharge, discharge variability, wave energy flux and tidal range. However, the calibre or grain size is also important. The amount, mode of transport and grain size of the sediment load delivered to a delta front have a considerable effect on the facies, formative physical processes, related depositional environments and morphology of the deltaic depositional system. The available grain size influences (1) the gradient and channel pattern of the fluvial system on the delta plain; (2) the mixing behaviour of sediment as it discharges into the ambient basin waters at the river mouth; (3) the type of shoreline, whether reflective or dissipative, and its response to both wave energy and tidal regime; and (4) the deformation and resedimentation processes on the subaqueous delta front. Long term aspects of deltaic sedimentation, including a few generalized relationships between sediment supply and physiographic setting, are briefly introduced. The need for further detailed research on modern and ancient deltaic dispersal systems is emphasized, and specific suggestions are given for future research.

The quantitative model presented simulates the development of a two‐dimensional alluvial sedimentary succession beneath a floodplain traversed by a single major river. Several inter‐related effects which influence the distribution of channel‐belt sand and gravel bodies within overbank fines are accounted for. These are (a) laterally variable aggradation, (b) compaction of fine sediment, (c) tectonic movement at floodplain margins, and (d) channel avulsion. Selected experiments with the model show how the interconnectedness and areal density of channel‐belt deposits decrease with increasing floodplain width/channel‐belt size, mean avulsion period, and channel‐belt aggradation rate. Separation of stream patterns based on interconnectedness and channel deposit density is difficult. Tectonic movements do not have a significant influence upon the successions unless a preferred direction of tilting is maintained (half‐graben). Then channel‐belt deposits showing offlap tendencies tend to cluster adjacent to the active floodplain margin, leaving dominantly fine‐grained alluvium to accumulate on the inactive side. Individual channel‐belt deposits thicken during aggradation, although a self‐regulating limit to such thickening is likely to operate. ‘Multistorey’features resulting from aggradation may be difficult to tell apart from those arising through superposition of distinct channel‐belt deposits of avulsive origin.

The spatial and temporal distribution of diagenetic alterations in siliciclastic sequences is controlled by a complex array of interrelated parameters that prevail during eodiagenesis, mesodiagenesis and telodiagenesis. The spatial distribution of near‐surface eogenetic alteration is controlled by depositional facies, climate, detrital composition and relative changes in sea‐level. The most important eogenetic alterations in continental sediments include silicate dissolution and the formation of kaolinite, smectite, calcrete and dolocrete. In marine and transitional sediments, eogenetic alterations include the precipitation of carbonate, opal, microquartz, Fe‐silicates (glaucony, berthierine and nontronite), sulphides and zeolite. The eogenetic evolution of marine and transitional sediments can probably be developed within a predictable sequence stratigraphic context. Mesodiagenesis is strongly influenced by the induced eogenetic alterations, as well as by temperature, pressure and the composition of basinal brines. The residence time of sedimentary sequences under certain burial conditions is of key importance in determining the timing, extent and patterns of diagenetic modifications induced. The most important mesogenetic alterations include feldspar albitization, illitization and chloritization of smectite and kaolinite, dickitization of kaolinite, chemical compaction as well as quartz and carbonate cementation. Various aspects of deep‐burial mesodiagenesis are still poorly understood, such as: (i) whether reactions are accomplished by active fluid flow or by diffusion; (ii) the pattern and extent of mass transfer between mudrocks and sandstones; (iii) the role of hydrocarbon emplacement on sandstone diagenesis; and (iv) the importance and origin of fluids involved in the formation of secondary inter‐ and intragranular porosity during mesodiagenesis. Uplift and incursion of meteoric waters induce telogenetic alterations that include kaolinitization and carbonate‐cement dissolution down to depths of tens to a few hundred metres below the surface.

Rhizoliths are defined as organosedimentary structures resulting in the preservation of roots of higher plants, or remains thereof, in mineral matter. They are abundant and characteristic features of Quaternary terrestrial carbonates (calcretes and aeolianites) from coastal regions of the western Mediterranean. Field and petrographic observations indicate that five basic types of rhizoliths can be recognized: (1) root moulds, which are tubular voids that outline positions of former, now decayed roots; (2) root casts, which are sediment‐ and/or cement‐filled root moulds; (3) root tubules, which are cemented cylinders around root moulds; (4) rhizocretions s.s., which are pedodiagenetic mineral accumulations (here low magnesian calcite) around living or dead plant roots; and (5) root petrifactions, which are mineral impregnations or mineral replacements of organic matter whereby anatomical features of roots have been preserved partially or totally. Apart from rhizoliths themselves, roots of higher plants are responsible for the formation of numerous and characteristic features of pedogenetically affected terrestrial carbonates. Plant roots are responsible for, or contribute to, the formation of alveolar textures, in situ brecciation (rhizobrecciation) textures, horizontal sheet cracks, vertically elongate glaebules (concretionary soil structures) and micritization (rhizomicritization) within terrestrial carbonates. Rhizoliths, together with the above features, are products of pedodiagenesis. More significantly, rhizoliths and related features are indicators of palaeosols and hence of subaerial vadose environments in ancient (post‐Silurian) successions.