Collinson, John_ Mountney, Nigel_ Thompson, David-Sedimentary Structures ( 3rd Edition)-Dunedin Academic Press ().pdf - Ebook download as PDF File . Sedimentary Structures: • Features in sedimentary rocks that reflect depositional or diagenetic processes. – Diagenesis: • physical and/or chemical changes to. Sedimentary structures exposed in bar tops in the Brahmaputra River, Bangladesh. Article (PDF Available) in Journal of the Geological Society Special .
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Sedimentary structures are large-scale features of sedimentary rocks which Potter, and Seiver (); Reineck and Singh (); and Collinson and. 2nd edition. Unwin Hyman, p. Sedimentary structures introduces undergraduate students to depositional and also post-depositional. SEDIMENTARY. STRUCTURES. J.D. COLLINSON & D.B. THOMPSON. University of Bergen. Senior Lecturer in the Department of. Education at the University of.
In the case of high-concentration flows. Debris flows move as a result of viscous nearlaminar shear. For most debris flows. Debris flows occur in both sub-aerial and sub-aqueous settings. Such non-cohesive flow requires an applied shear that can overcome the initial intergranular friction. The flow will then stop abruptly. On this basis. Grain-to-grain support Where a muddy matrix is mostly absent. The particles are kept apart and hence are free to move relative to one another by vigorous interparticle collisions.
Yield strength or plastic limit k is a coefficient that describes the stress—strain relationship during deformation. A general equation for describing the rate of shear strain in such deforming layers can be written as: In sub-aqueous settings. As concentration increases.
For low-concentration aqueous flows. Pouring granulated sugar or tipping dry sand from a bucket or the back of a truck are everyday examples of this behaviour. Once the applied shear falls below some critical value. The high density of the deforming matrix means that buoyant uplift is very important in debris flows. To preserve a record of internal deformation. It is possible to investigate some of these features. In general. As we shall see in later chapters.
Careful slicing of the solidified mass will allow visualization of the internal deformation that acted in the latest stages of the flow. In extreme cases. Mix clay and sand with water to give different viscosities and see how these move on slopes of differing gradient and roughness. Debris flows that maintain cohesive behaviour throughout their lives will eventually decelerate either because of a downslope reduction in gradient or.
The volumes. It is a common observation that a slope of loose dry sand is stable only below a certain gradient. See how the size and shape characteristics influence the angle. During deposition from such flows. When the shear falls below a critical value. Within such flows. Some simple experiments can illuminate this type of sediment behaviour. An everyday experience of this is the fact that Turbidity currents Turbidity currents are the most important agents for transporting sand and silt-grade sediment into deeperwater settings.
Pure grainflows occur only on steep gradients. Their mobility depends upon the sedimentary particles being supported by the upwards components of fluid turbulence. With lower sediment concentrations. First of all.
Collinson J.D., Thompson D.B. Sedimentary Structures
How does this angle vary with sediment type and conditions? When the angle of rest is exceeded. Once in motion. At the highest concentrations. Inverse grading may develop within the moving layer where a suitable mix of grain sizes is available and this may itself be frozen where the layer freezes abruptly. A second experiment is to steepen the slope of a pile of sand to see what angular difference exists between the angle of rest and the angle of slip.
This type of avalanche movement and the existence of a particular angle of rest repose are important in depositing inclined laminae on the lee faces of ripples and larger bedforms in both air and water see Ch.
In gravelly or sand-rich flows. These two processes. Alteration to any one property will trigger changes in other properties. Deposition from such a layer can occur gradually as sediment is added from suspension. These so-called modified grainflows or traction carpets depend on the shear stress applied by an overriding powerful current. In many cases however. Attempts to steepen the gradient trigger a flowing movement of the sand after the angle has been increased by a few degrees Fig.
Methods of doing this are outlined in some of the references given at the end of the chapter. Turbidity currents are driven by gravity acting on the excess density of the suspension over that of the surrounding clear water. Hindered settling. Turbidity currents. In concentrated and hyperconcentrated flows on lower gradients. This movement reduces the gradient to one at which the slope is again stable. Try the experiments in air with dry sediment and.
Hyperconcentrated and concentrated flows including grainflows Natural concentrated and hyperconcentrated flows. Each bedform will produce its own distinctive lamination as reviewed in Ch. The more rapid the deposition. In the process. The currents also exert shear stresses on the bed. Because turbidity currents are fluidal flows. Large turbidity currents commonly originate from the slumping of poorly consolidated material near the top of a slope and are important in the transport of sand to the abyssal regions of the ocean floor.
The initial slumps tend to mix with increasing volumes of sea water as they accelerate down slope and become more dilute until the sediment load is fully suspended. As the flow drops its suspended load. If a range of grain sizes is present in the turbidity current. At their highest concentrations. Similar considerations apply to accelerating currents.
Deceleration in space occurs where a constant flow decelerates as it encounters lower gradients or expands from a constriction. Certain density currents. Density currents in general. At a larger scale. In such cases homopycnal interflows develop at density interfaces within the column.
Deceleration in time occurs when the turbidity current is in the form of a surge that passes through a point and deposits its load as flow decelerates at that point.
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Fill the whole tank with water and add dye to the water in the small compartment. Pull up the gate and observe how the coloured and the clear water interact. In such a case. Now add a quantity of salt to the small chamber. How does the salinity of the introduced water influence the pattern and rate of the resultant flows?
By using a small immersion heater or by allowing ice cubes to melt in the small compartment. Modified after Simpson The higher-density cold water plunges below the lower-density warm water.
How a b c transport Heights h1 total ambient fluid column h2 ambient fluid column above flow h3 mixing zone h4 main body of flow h5 foremost point of flow Figure 3. Clear-water density currents. The popular lock-gate model of a turbidity current is only one of the possible 13 types. They originate as a consequence of holding compartment sliding lock gate main compartment tap for draining water Figure 3. Many natural currents are hybrids and they migrate from one field to another.
Recommended dimensions are: Mixing is achieved by lifting the lock gate between the two compartments. If a tank is not available. The nature of their flow is determined by their density relative to the surrounding fluid often air but sometimes water and by gravity. These experiments can be varied by using different concentrations of suspended material.
In a second series of experiments. A removable ramp can be used to study the effects of current reflections. Hydraulic jump and the transition from rapid to tranquil flow Place a sheet of glass on a flat. Shine a bright lamp on the measuring cylinder to Study techniques Field experience It is important to generate a feel for the interaction of the many variables involved in sedimentary processes: Pyroclastic falls involve the settling of sub-aerially erupted particles through air and through water.
At higher discharge. Reynolds number and Froude number. Spatial changes in pyroclastic density-current velocity are caused by factors such as downstream changes in slope. A pyroclastic density current is accumulative where it accelerates because of flow convergence or a downstream increase in gradient. In observing both products and processes. At low discharge. Gutters and beaches Flow in very shallow currents. Density and turbidity currents Flows of mud or silt at the edge of ponds.
Many pyroclastic density currents are considered to be single-surge events generated by an individual shortlived pulse that waxes rapidly and then wanes rapidly.
Repeat the experiment with further grains of varying diameters. Three simple experiments are outlined here. The behaviour of pyroclastic density currents is partly determined by the nature of the eruptive event. The abrupt transition from rapid to tranquil flow is represented by a hydraulic jump. Use the tap to control the water discharge. Pyroclastic flows and surges are end-members of a spectrum of gas-rich gravity flows that range from concentrated laminar and plug flows to dilute turbulent currents.
Waves in ponds. It is also important to develop an appreciation of certain dimensionless relation- Field programmes should include investigations. Laboratory experience Many physical experiments can be devised so as to afford direct observation of the properties of fluids and flows.
Flow of wind over dunes or obstacles Organized or lessorganized turbulence see Ch. After wet weather. Waxing of a pyroclastic density current typically results from dilation of a volcanic conduit or vent.
Documentation of slow processes e. Measure the rate of sinking of the grain through the glycerine column. Note how.
Relate these observations to Equation 3. A clear account of the basic physics of sediment transport and deposition.
Experiments in physical sedimentology. An encyclopaedic account of sedimentary structures and the physics of their development. Recommended references Allen. The physical character of subaqueous sedimentary density flows and their deposits. Physical processes of sedimentation. Sedimentary structures: A thorough account of river sediments with a strong emphasis on hydrodynamics and computer modelling.
A good general discussion of fluid dynamics in relation to sediment entrainment. Mechanics of sediment movement. Rivers and floodplains: Stand the tube upright with the attached hose at the base and half-fill it with low-density polystyrene spheres 2— 5 mm diameter. A clear and logical account of this often confusing topic. Fluid turbulence and the motion of non-cohesive particles Connect a rubber hose to one end of a transparent plastic tube 0.
Southard Pass an air supply down the hose and into the base of the tube a regulated air bottle is ideal. Alexander A classic in its time and still one of the clearest accounts of the basic hydrodynamic principles. Note the effects that particle size. Earth surface processes. Outlines a series of experiments concerning flows and sediment transport that can easily be conducted in a standard laboratory.
They can. As with most depositional structures Chs 5—7. Observed relief therefore reflects only the minimum thickness of sediment removed. Three broad categories are 4. Widespread erosion of a large thickness of sediment may result in preservation of only small-scale features. For erosional structures to be preserved.
The coarser-grain sediments are commonly sandstones. Where relief is observed. Erosion is also recognized in vertical sections by truncation of bedding or lamination in the sediment below the erosion surface.
Resumption of deposition of fine-grain sediment. Background deposition of mud is punctuated by sudden Classification of erosional structures has to be arbitrary. This chapter deals with features that indicate that erosion has taken place. Even in areas of net long-term accumulation. Erosion of mud and deposition of coarser material can often be phases of the same current.
The scheme adopted here is based on both descriptive and genetic criteria Fig. Many erosional structures are valuable indicators of both way-up and palaeocurrent direction. The cohesive strength of sediment allows details of the erosional relief to be maintained until they are buried by coarser-grain material Fig.
Small-scale erosional structures are almost always recognized as relief on the base of the bed immediately overlying the erosion surface. Subsequent lithification usually renders the coarsegrain sediment more resistant than the finer material to eventual weathering.
The sole marks result from the erosion of cohesive fine-grain sediment. Sole marks are typically the products of environments characterized by episodic sedimentation. It is very important to understand this mode of preservation and to recognize that the structures observed are negative impressions of the erosional relief.
A variety of shapes occur. They may occur as isolated casts or in groups covering a bedding surface in distinctive patterns. Interpretation of sole marks should initially be restricted to the processes involved.
Sole marks are divided here into two broad classes that differ principally in the way the structures are generated: It was once thought that sole marks were diagnostic of turbidites. Four main groups cover the range of forms: Current from top left to bottom right. Obstacle scours are not very common as sole marks. Place a pebble or some other obstruction on the bed and see what happens when the streamflow or the backwash of waves passes over the bed.
Sub-aerial erosion Figure 4. Obstacle scours Large clasts such as pebbles. Hecho Group. A crescentic scour trough will commonly develop around the obstruction. These ridges are casts of troughs developed around the large clast. The ridges are commonly crescentic or of horseshoe shape. The eddies are directed vigorously downwards onto the bed on the upstream side of the obstacle.
They occur both as isolated features and as collective groups that share a common origin and form distinctive patterns. In some cases. The scour trough is caused by the accelerated flow around the obstacle.
Individually they vary in shape and size. Sub-aerial erosion c Way-up Tectonic overturning stream. The development of these structures can be readily observed on a sandy beach or on sandy stream beds over which there is quite a strong flow of water.
Several smaller pebbles also show their own scours. Some flutes have highly twisted shapes particularly close to the nose Fig. The deepest part i. These steps can be generally related to the lamination or thin bedding in the underlying sediments. Try to judge the palaeocurrent direction in each case.
Flutes typically range in length from 5 cm to 50 cm. The sides of some flutes are unusual in showing a pattern of small-scale steps. Flutes are characterized by a rounded. Photos a and b courtesy of Gilbert Kelling. Example c is unusual in showing both flutes and tool marks. In shape they range from highly elongate forms. Figure 4. As well as being a valuable indicator of way-up in deformed sequences. Linear patterns occur where the flutes are arranged longitudinally. The shape of the flute is intimately related to the structure of eddies in the nose region.
The associated higher shear stresses lead to erosion. Eventually evidence of the initial irregularity will be destroyed. Where erosion was sustained. Small bumps and depressions on the bed cause acceleration of the flow. Erosion is most concentrated near the nose of the flute. Some may develop from the lateral merging of longitudinal scours.
Not all flutes develop from initial irregularities of the bed. Flutes can be produced experimentally when water flows over a surface of cohesive sediment or over a slightly soluble substrate.
The shape and pattern of flutes bear quite a close relationship to the shape and distribution of the initial irregularities if the scour and growth of the flutes did not last very long. The scale of separation and the erosional relief increase together. Descriptions of flutes should include measurements of their dimensions. After Allen The lineation along which descending vortex limbs impinge on the bed will be sites of high stress and rapid erosion.
Gutter casts Gutter casts generally occur as isolated elongate ridges on the bases of sandstone or coarse-grain limestone beds. Some patterns are parallel and continuous. If there is any uncertainty. Superficial examination could cause transverse scours to be confused with straight or sinuously crested ripples. Adjacent eddies have opposite senses of rotation. Some die out by coalescing with a neighbour.
Wider ridges with distinct noses are gradational to flutes. Longitudinal scours result from patterns of smallscale eddying close to the bed. Where sandstone casts have rounded noses. They protrude into the underlying finer-grain sediment from an otherwise rather flat bedding surface. These are generally symmetrical. In transverse cross section. Where the coarse-grain sediment does They are commonly up to 10 cm wide and almost as deep.
Location unknown photo courtesy of Gilbert Kelling. Mam Tor Formation. Upper Carboniferous. Although the overall pattern is one of parallelism. In most cases it is possible to tell only the trend of flow and not its sense of direction. Once localized on scour features.
Longitudinal scours are useful indicators of way-up and of palaeocurrent trend. The spacing of the ridges is typically 0. Pairs of vortices are probably responsible.
Smaller features commonly tool marks may be superimposed on the walls and floors of the gutter casts. They appear to reflect a pattern of helical vortices with their horizontal axes parallel to the flow. Campanuladal Formation. Gutter casts are the product of fluid scour. Tabernas Basin. Some ridges are gently curved in plan. Sometimes their ends are seen and these may be quite steep. In plan.
Sedimentary structures third edition pdf
Note the sinuous shape of the gutter and the slightly anastomosing pattern. The smaller. The identity of the tool will usually be unknown. In transverse vertical section they show an irregular sharply defined relief.
Ends of groove casts are seldom seen. Chevrons Rarer than grooves. More rarely.
On surfaces where more than one trend is apparent. They occur in isolation or in parallel groups. A simple morphological classification is: Sharp and irregular profile: The twisted appearance of some grooves reflects rotation of the tools as they were dragged along the bed.
They also have rather more sharply defined shapes. It is important to take care in measuring and recording the orientation of groove casts. Dragging a stick through soft mud or any very viscous liquid Groove casts result from the infilling of erosional relief gouged by an object. Most bedding planes with groove casts show only one trend of groove. The individual linear zones are seldom more than 3 cm wide and the relief is generally less than 5 mm.
Ainsa Basin. The V-shape ridges. Larger examples may show the superimposition of delicate ribbed relief comparable to that seen on grooves. Prod marks and bounce marks Sharply defined discontinuous marks. Note particularly that the asymmetry of prods is opposite to that of flutes. Both types vary in size. Depths are roughly proportional to width. With prod marks. Prod and bounce marks record the impact of larger objects on the bed. Section view along chevron axis Like other sole marks.
In describing and recording these marks in the field. Krosno Beds. Where the marks are almost continuous and approach the appearance of grooves. In many sequences.
In most cases it is impossible to identify the tool. No asymmetry is thereby produced and so only the trend of the current can be deduced. This suggests that tool marks represent superficial erosion by relatively weak or shortlived currents.
Differences in the shape of bounce marks can sometimes be related to rotation of the tool as it bounced along. Bounce marks reflect a lower approach angle of the tool to the bed. In some cases skip marks may be very closely spaced and almost gradational with a groove. Krosno Formation. Skip marks represent the repeated bouncing of the same tool above the bed Fig. The individual marks need not be identical. On rippled sand. They occur on both sandy and muddy surfaces.
Rill marks Rill marks are small-scale. Note the deposition of sand ridges in the lee of the boulder. They result from the emergence of pore water from within the sediment. They are often very subtle features. Both water and wind can act erosively on sediment surfaces.
They are found on modern sand and silt surfaces. They have no palaeocurrent significance. On flat muddy areas. In this case the obstacle was an ice block that has subsequently largely melted away. They are almost invariably destroyed by a rise of water level and thus they have a very low preservation potential.
They occur most commonly on beaches and on flanks of larger tidal bedforms at low tide. They are parallel to dominant currents and are probably related to spiral patterns of secondary circulation in the water cf.
If the mineral composition of the sand is varied. Water erosion gives rise to obstacle and longitudinal scours and rill marks. The trough is usually deepest along the upstream side or around the flanks of the obstacle. The sides of the scour are quite steep. When found. The lower parts of the slopes and the floors of the structures are commonly decorated with current ripples. Note the current ripples on both the surrounding bedding surface and the floor of the megaflute.
These rather uncommon features clearly record erosion of a sand surface by sustained fluid scour by a current that did not deposit any sediment as it eventually waned. The pattern of small dendritic channels is cut by water emerging from within the sand during falling and low-water level. Note the massive nature of the eroded sandstone and the predominantly muddy nature of the fill of the megaflute.
The only examples of such structures in present-day settings are on the surfaces of deepwater submarine fans. Down stream the features die away gradually. In plan view. They are commonly up to a few centimetres wide and up to a few tens of centimetres long.
Tana Delta. Those of finer grain fill may both onlap and drape the margins of the scour. In vertical section. A blunt nose points up wind with a tail streaking out down wind Fig. Ross Formation. Ventifacts have a high preservation potential and. Ebro Delta. The sharp ridges usually face in an upwind direction and can therefore be used to assess predominant wind direction.
They are rather uncommon in the rock record. Another feature is that of wind-faceted pebbles or boulders or both. Such ventifacts are characterized by one or several sharp ridges that bound smooth faces on individual clasts Fig.
Askja Sandsheet. Larger clasts shells act as obstacles. Huab Basin. Slides integrity of stratification largely retained and slumps strata dis- 9 rupted and jumbled are common, particularly on the relatively steep, sediment-fed continental slope and rise D.
Lewis, ; Coleman et al. The other types of mass flow are grain flows,composed predominantly of sand, and mud-rich debris flows similar in character to terrestrial examples Shepard, ; Middleton and Hampton, , ; Embley, ; Lowe, a, b; Stanley and Taylor, Slumps and debris flows, through prolonged dilution with seawater, apparently can change into turbidity currents Van der Knaap and Eijpe, ; Allen, b; Hampton, , which are vigorous river or surge-like flows sustained by an excess of density imparted by dispersed sediment.
These currents are regarded as primarily responsible for submarine canyons and channeled deep-sea fans and cones. These deeper currents e. Stander et al. Zimmerman, ; Connary and Ewing, ; Eittreim et al. Locally, as in the Mediterranean Sea Lacombe, ; Heezen and Johnson, , an increase of density due to evaporation leads to undercurrents.
Glaciers are gravity-driven streams of ice of importance as transporting agents Fig. Land-based glaciers are customarily divided between temperate, when some slip is possible between the ice and its meltwater-lubricated bed, and cold, when the ice is so chilled that it firmly adheres to the substrate.
Stagnating glaciers generally include systems of dendritic internal drainage tunnels that collect and transmit meltwater to their downslope margins. This zone of retardation is a boundary layer, the flow in it being either laminar, when fluid particles follow smooth parallel streamlines, or turbulent, when there is an irregular eddying motion of relatively high velocity.
The flow is two-dimensional when the same profile of velocity is observed over the extent of the layer, but three-dimensional when the velocity must be resolved between three orthogonal components. Boundary layers can, furthermore, be steady, when the flow remains constant in time at a station, or unsteady, when conditions change with time. Some boundary layers are uniform, maintaining a constant character over their extent, whereas others are non-uniform, varying spatially.
The boundary layers of interest in environmental fluid dynamics all occur in the context of a rotating spherical Earth, and rotational effects are negligible only for those of a sufficiently small scale.
Rotating flows Two fictitious forces on fluid elements must be accounted for when using the rotating Earth as a frame of reference for boundary-layer study Craig, The second is the Coriolis force, which is zero for particles that are stationary with respect to the Earth, but non-zero when the particles are moving. The magnitude of its horizontal component is 2muU sin 8, where U is the particle velocity and 8 the latitude of the motion.
This component acts perpendicularly to the right of the motion in the Northern Hemisphere and to the left in the Southern Hemisphere; its value is zero at the Equator and a maximum at the Poles. The boundary layer formed on a flat disc rotating uniformly about an axis perpendicular to its plane in an infinitely extensive Newtonian fluid otherwise at rest illustrates well the influence of centrifugal force and the nature of three-dimensional boundary layers Schlichting, Fluid adjacent to the disc is transported with the disc because of viscous drag, but on account of the centrifugal force is thrown outward, a compensating axial flow toward the disc being induced Fig.
Viewed from a stationary frame of reference in the undisturbed fluid, and compared to the radial velocity of the disc, the radial velocity component U reaches a maximum at about the outer edge of the boundary layer, while the circumferential component W is a maximum at the surface 11 IY Fig.
Schematic radial, axial, and tangential profiles of velocity generated within the boundary layer on a smooth, circular disc rotating in an otherwise still fluid. Velocity measured relative to stationary coordinates.
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