Sedimentary structures

Home > Earth and the Environment > Geology and Oceanography > Geology and Oceanography

sedimentary structures Fluids transporting sediment particles, their subsequent movement within sediments, the action of gravity, and the effects of biological and chemical changes in the deposited sediment: all these factors control the architecture of the resulting deposit. The operation of any of these processes leads to the formation of sedimentary structures, which are the visible signs of the various processes as preserved in the rock record.

Sedimentary structures range from centimetres to kilometres in scale. They provide invaluable information about the mode and direction of transport and deposition, as well as subsequent changes. Because particular processes are characteristic of different environments of deposition, the study of sedimentary structures makes it possible to reconstruct environments of deposition and hence palaeogeographies.

Episodes of sedimentation produce layers or strata which vary in thickness from laminae (less than 1 cm thick) to beds (more than 1 cm thick). Varying current conditions produce mixed deposits: inter-laminated sediments of unlike type, for example mud and sand (i.e. heterolithic stratification) (Fig. 1, a, b, c). When sand is being transported by ripples and current velocity falls, the sands often become cloaked by a thin film of mud. Remobilization of the ripples by a new current leads to erosion of the mud except in the ripple-troughs, where it remains. The resulting structure is termed flaser bedding. More persistent mud layers are deposited under lower-energy conditions during longer periods of low-velocity currents. Wavy bedding then results. Where mud is the dominant sediment and sand is supplied spasmodically, the muddy sediment encloses linked or isolated sand ripples and the resulting structure is called lenticular bedding.

Sediments that slump under water can develop into a mixture of sediment and water that flows downslope as a density current along lake or sea floors. Various types of flow develop: turbidity currents where the sediment is supported mainly by the upward component of flow; fluidized-sediment flows where the sediments are supported by water escaping from between grains as they settle; grain flows where sediment is supported by grain-to-grain collision; and debris flows where grains are supported by a mixture fine sediment and water. Turbidity currents produce upward-fining beds, expressed by graded bedding. The beds show a sequence of structures produced by the waning flows. Such graded beds commonly display a Bouma sequence, named after the Dutch geologist who first recognized the phenomenon (Fig. 1, d(i)). Other mechanisms produce distinctive sedimentary sequences containing characteristic structures (Fig. 1, d (ii), (iii), (iv).

A unidirectional current of water moving over sand moulds the sediment into a series of bedforms which migrate downcurrent. The size pattern and sequence depend upon the magnitude of stress exerted by the fluid and the grain size of the sediment. As the velocity increases over a bed of initially smooth, fine-medium sand, the sediment is fashioned into small ripples (from one to a few centimetres high with wavelengths of decimetres). At first these are regular crested ripples which become linguoid as the current velocity increases. Larger ripples (sometimes called dunes or megaripples), which again are initially straight-crested and then develop irregular forms, subsequently give rise to a flat bed (plane-bed) before finally becoming an undulating bed of antidunes. (Antidunes, unlike ripples, migrate upstream as sediment is eroded on their downstream (lee) slopes and is deposited on the upstream (stoss) slope of the next antidune downstream. The latter sometimes develop into beds or irregular chutes and pools. In very fine sands, the sequence of bedforms is small ripples, plane beds, antidunes, whereas in coarse sands and gravels the sequence is plane beds with irregular scours parallel to the current and then large ripples and plane beds.

As ripples migrate, sediment moving up the gentle stoss slopes is deposited on the steeper lee slopes to produce inclined cross-stratification dipping in the direction of flow. In long crested ripple marks of all sizes this produces a series of cross-stratified sediments with individual sets representing the passage of a single bedform. The sets may have non-erosional or slightly erosional contacts with underlying sets (tabular cross-stratification) (Fig. 1, e (iv)). Short crested ripples usually have scour-pits developed on their downcurrent side into which the cross-stratified sediment advances in a series of spoon-shaped depressions infilled with cross-stratified sands which conform to the scour surfaces. Individual sets of cross-strata fill the scour surface (trough cross- stratification or festoon cross-bedding: Fig. 1, f). The axis of an individual set forms parallel to the current. Small-scale ripples, under decelerating flow, migrate to form ripple-drift cross-stratification, the angle of climb depending upon the rate of sediment deposition. Some large-scale ripples have counter flows in the shelter of the ripple which produce small-scale ripples that move upstream at the base of the lee slope.

Flat beds produce flat laminated deposits (plane bed lamination: (Fig. 1, g), with linear streaks or ridges only a few grains thick aligned in the direction of the current flow, giving rise to a structure called primary current lineation. Problems arise in the interpretation of plane beds in very find sands, since a very similar structure can be produced when sediment is falling out of suspension against a background of slowly moving currents. However, the presence of primary current lineation is usually sufficient to distinguish one from the other. Changes in velocity, sediment supply, or even exposure lead to the modification of the ripple surfaces and truncation of cross-stratified units to produce so-called reactivation surfaces. Reversing tidal currents are usually invoked to explain reversals of dip in succeeding cross-stratified units (herring-bone cross-stratification: Fig. 1, h).

Oscillatory flow under gravity-driven waves produces ripples which show laminae dipping away from each side of the crest to give a chevron pattern (oscillatory ripples) (Fig. 1, i (i)). As waves commonly show variable degrees of asymmetry in shallow water the resulting ripples are asymmetric and show an asymmetric internal structure (Fig. 1, i (ii)), similar to those formed during unidirectional flow. Air flowing over sands produces ripples which are superficially like aqueous ripples but they have lower height/wavelength ratios and long crests, and are slightly asymmetric. In strong winds they are destroyed and a plane bed develops. Aeolian ripples are formed by saltating grains and the wavelength is approximately that of the saltation path of the grains. These ripples commonly do not show an internal structure of cross-laminae. Ripples of coarser sand and gravel produced by surface creep of grains show a coarse cross-stratification. Aeolian sand transport fashions sand into large-scale bedforms which often have complicated internal structures (dunes and draas).

A fluid together with its entrained load moving over a cohesive bed erodes longitudinal furrows or grooves when the stress exceeds the critical erosion velocity. At higher velocities spoon-shaped scours develop, with their blunt end on the upstream side (flutes). Transported fragments scour the bottom to form similar grooves and pits. These, when infilled with sediment, can be seen on the base of the succeeding bed, usually sand, as casts (groove casts and flute casts) (Fig. 1 j). The former record the sense of direction of current flow, and flutes also demonstrate its polarity from their asym-metry.

A curious structure found in coarse silt and fine-grained marine sands that has attracted a great deal of interest in the last few decades is hummocky cross-stratification. It consists of very gently inclined medium to large-scale cross-stratification with the laminae in each set parallel or nearly parallel to a lower, gently dipping erosion surface (Fig. 1, k). Hummocky cross-strata are fan-shaped and dip in all directions. When exhumed during later erosion they form an irregular surface of unorientated hummocks and depression 10–50 cm high, spaced a metre to a few metres apart. Units with such structures commonly show an erosional base cut in fine-grained deposits and are succeeded by oscillation-rippled sand and bioturbated sediments. They are thought to be produced by short-lived events such as storms.

Sand deposited on soft muds with a high water content sinks into them because of their greater density; structures of all sizes at the sand–mud interface show downward protuberances (load casts) (Fig. 1, l). These appear to develop in some instances as the sand is being deposited, and protrusions of mud occasionally project into the sand and bend in the direction of transport. These are flame structures (Fig. 1, m). Sometimes, the sand becomes detached from the main sand unit and sinks into the mud to form sand balls or pseudonodules (Fig. 1, n), which resemble true chemical nodules. They probably form by normal loading but they can be triggered by shock waves (e.g. earthquakes) which liquify the mud. In large deltas during times of rapid deposition of nearshore sediments over pro-delta sediment, muds flow upwards from hundreds of metres depth to regions of lesser stress, to form mud-diapers or mud lumps on the surface around delta river mouths. Slow creep of pro-deltaic muds under the load of the delta-top sands can produce a radial pattern of grabens andhorsts and an accompanying tangential pattern of growth faults.

Water-saturated sands are sometimes disturbed by fluid flow or an object being dragged over the surface. In this way overturned cross-stratification can be formed. Convolutions can form the direction of flow in interlaminated sand or mud beds (convolute bedding). When sands have been deposited very quickly, the particles do not adopt their most stable packing, and the load is carried by interstitial water as well as by the sediment framework; shock waves or pressure differences due to passing waves can dewater the sand; water escaping upwards produces vertical pipes which are vertically stratified (pipe structures), and where they emerge on the sediment surface sand volcanoes (Fig. 1,o) can be formed. Between the pipes, the sediment is deformed and forms downward protrusions of sand with the original stratification retained but distorted (ball and pillow structure) (Fig. 1, n). Fine-grained sediment is sometimes concentrated to produce upwards concave structures (dish structures) (Fig. 1, o).

Large-scale disturbances resembling those produced by tectonic deformation can occur in sediments. Many of these phenomena are slumps. They differ from tectonic structures in that they are usually confined between undisturbed sediments. The distinction is not, however, always easy, since slumps and tectonic folds can have similar orientations. Sediments deposited on slopes of even 1° slump when the shear strength exerted by gravity exceeds the shear strength of the sediment. The shear stress can be increased by the dumping of new sediment on top of a mud, and the shear strength decreased by increased pore pressure or fluidization caused by sudden sediment loading, storm waves, tsunamis, or earthquakes. Such disturbances vary from gentle folds with small thrusts, when the beds retain coherence, to those in which the sediment completely loses its coherence and forms a sediment gravity flow. A sheet of mud and sand behaving chaotically can develop from such movements. Sheets of this kind are known as olistrostromes. They contain isolated clasts, called olistoliths. Olistrostomes can be very extensive, covering hundreds of square kilometres. Almost all the thick and extensive examples have been triggered by earthquakes. Slumps (Fig. 1, p), many of them covering hundreds of square kilometres, are common on basin margins, on delta slopes, and along modern continental slopes, particularly those of active margins.

Loss of water by desiccation, and shrinkage on exposure of sediments to the atmosphere, produce one of the most widely used and least ambiguous pieces of evidence for former exposure in ancient deposits: mudcracks and desiccation cracks (Fig. 1, q). The polygonal pattern of cracks often becomes filled with sediment and they are thus preserved as downward V-shaped projections filled with sediments from the succeeding bed. Muds sometimes lose water even when permanently submerged, to produce a rather similar pattern of cracks called syneresis cracks, which are less regular but can be difficult to distinguish from subaerial mudcracks. Crystallization of cements by salt and evaporites in subaerial or very shallow water, and by calcium carbonate in waters of variable depth, produces horizontal expansion in surface sediments to produce a polygonal pattern of slabs (several metres or tens of metres across) with upturned edges (tepee structures, pseudo-anticlines, gilgai: Fig. 1, r).

Bottom-living organisms leave traces—trace fossils—and usually have a profound effect on the sedimentary structures of a deposit. The intensity of this type of disturbance, or bioturbation, (Fig. 1, s) and the relative proportions of physically and biologically produced structures provides useful information of the environment and rate of deposition. Whereas the main effect of burrowing organisms is to destroy stratification; other organisms produce delicate stratification by trapping fine-grained sediment to produce stromatolitic bedding.

Many sediments contain cemented patches—nodules—formed by chemical precipitation of a mineral cement developed around local centres of chemical activity, such as an organic fragment, or a zone of increased permeability such as a burrow (Fig. 1, t).

G. Evans

Bibliography

Allen, J. R. L. (1985) Principles of physical sedimentology. Allen and Unwin, London.
Collinson, J. D. and and Thompson, D. B. (1982) Sedimentary structures. Allen and Unwin, London.
Leeder, M. R. (1982) Sedimentology process and product. Allen and Unwin, London.

sedimentary structure The external shape, the internal structure, or the forms preserved on bedding surfaces, generated in sedimentary rocks by sedimentary processes or contemporaneous biogenic activity. Internal sedimentary structures include: those formed by physical depositional processes (cross-stratification, flat bedding (see PLANE BED), lamination, and heterolithic structures); those due to post-depositional deformation (convolute bedding, slump structures, dish and pillar structures, flame structures, ball and pillow structures, etc.); those caused by organic disturbance (bioturbation, trace fossils); or by post-depositional chemical disturbance (enterolithic structures, collapse and solution structures, concretions, etc.) Structures preserved on the tops of beds include: those formed by depositional processes (ripple marks, primary current lineations); erosional structures (flutes and scour marks, see SCOUR AND LAG); structures caused by the transportation of an object over the bed (tool marks); and other features such as desiccation and syneresis cracks, sand volcanoes, adhesion ripples and warts, rain prints, and biogenic traces and trails. Structures preserved on the bases of beds (sole marks) include load casts, the casts of flutes, trails and tool marks, and the fill of erosional scours. The external form of sedimentary units (sheet-like, channel-fill, reef or mound (see MUD MOUND), lenticular, etc.) is a function of the depositional environment and sometimes of post-depositional compaction.

cross-stratification A family of primary sedimentary structures formed by the migration of the slip-faces of rippled bedforms or of bars. It is characterized by inclined laminations (foresets) bounded by planar surfaces (planar or tabular cross-stratification), or by scoop-shaped surfaces (trough cross-stratification). The foresets dip at the angle of repose of the sediment on the ripple slip-face and are oriented in the direction of migration of the ripple (see PALAEOCURRENT ANALYSIS). Tabular cross-stratification is produced by the migration of straight-crested, asymmetrical ripples or sand waves. Trough cross-stratification is generated by the migration of linguoid ripples or dunes. The term ‘cross-lamination’ is applied to cross-stratification formed by the migration of ripples; ‘cross-bedding’ is used for cross strata formed by the migration of large-scale forms such as dunes, sand waves, or bars. The term ‘cross set’ is used to define the cross-stratification preserved between any upper and lower bounding surface. Where the original bedform which produced the cross set is preserved and forms the upper bounding surface to the set, the term ‘form set’ is used. A number of cross sets preserved within a single bed are called a ‘coset’.