The texture of sediments
The texture of sediments
1- Definition of texture: It’s the shape, size and arrangement of component minerals of a nock.
Texture attributes :it’s a more description of the nock if its coarse grained, regular on imbricate note: some texture properties are not simple attributes the panasity which dependent an packing, an grain shape and an sorting.
What’s the difference between texture & structure texture :- Shasta do with the grain to grain relations & texture is studied in a thin section or by analysis of small sample.
Structure:
1) Structure deals with the larger feature of the rack as “bedding and ripple marking”
2) Structure is usually studied in the out crap and less commonly in the hand specimen.
The difference between The frame work of grains of igneous, metamorphic rocks and sedimentary rocks.
· The grains of igneous and metamorphic rocks are in continuous contact with their neighbors “very little on no porosity.
· The sediment grains are generally in tangential contact only and form on open, three dimensional network “high pores volume” inter angular space
Texture of sediments
Sediments exhibit two fabrics “hydrodynamic and diagenetic”.
Hydrodynamic texture: “The grains of most sediment preformed are emplaced as solid particles in the fabric of the rock by the movement of fluids under the in flounce of gravity.
Diagenetic texture, with passage of time, the fluid filled pones are the loci deposition from satiation of new mineral “the cementing minerals”. These gradually reduce ponasity to zero
Particle size of detrital rocks
The grain size of detrital sediment is important because of:
1) The size of sediment can subdivision it into conglomerates, sandstones and shales.
2) The size and santing are measures the competence and efficiency of transporting agent.
3) In normal terriginaus water- deposited material, size is a measure guide to the proximity source area (coarseness usually have not moved for”.
Concept of size:
If the particles composing a sediment were all spheres, no special difficulty would arise in determining their size and its diameter would suffice. But if it’s particles composed of gravel “are highly irregular”, So what is the diameter?
Concept and measurement of pebble diameter.
Left the b-axis in position, center the caxis in position right the pebble in perspective
Concept of size by analytical method:
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100 |
10 |
1.0 |
0.1 |
.01 |
.001 |
.0001 |
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mm
Gravel à Sand à Silt à clay à colloids à
Size terms:
Boulder was defined as :a detached rockmoss, some what naunded on modified by abrasion in transport, and larger than cobble with a minimum size 256.
A pebble :- rock fragment larger than a coarse sand grain on granule and smaller than a cobble which has been rounded on abraded by the action of water, wind or glacial ice gravel: the unconsolidated accumulation of pebbles, cobbles and boulder and the consolidated equivalent is conglomerate.
Rubble: is an unconsolidated accumulation of angular rock fragments.
Sand: the term sand used to denote an aggregate of mineral an rock grains greater than and less than 2 mm.
Size limits of common grade and rock terms of sedimentary
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Size |
Rounded & sub rounded, sub angular |
Angular | ||
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Fragment |
Aggregate |
Fragment |
Aggregate |
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256 |
Boulder |
Boulder Gravel & conglomerate |
Block |
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65mm |
Cobble |
Cobble gravel &conglomerate |
- |
Rubble |
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4mm |
Pebble |
Pebble gravel &conglomerate |
- |
Breccia |
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2mm |
Granule |
Granule gravel |
- |
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1/10mm-1mm |
Sand |
Sand stone |
- |
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1/2mm-1/256mm |
Silt |
Silt stone |
- |
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1/256mm |
Clay |
Clay shale |
- |
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Representation of size frequency distributions:
The histogram and cumulative curve are used for representation the size of sediment “sand, pebble and the like J which would found to differ in size.
Note: Both histogram 2 cumulative curve devices for presenting frequency distribution a long king.
Histogram: is the relation between thesize “ 2 mm, 1m , 1/2,1/4,1/8,1/16,1/52mm) converted it to δby “the negative log of the diameter” lour, and weight percent.
Mathematical nature of size frequency distractions:
· This is Just another way of saying that the size distribution is more or less symmetrical only basis.
· The size distribution of many classic sediment, was loy normal.
Grain size distribution and casual factors:
· In general the interpretation of grain size analysis has followed three paths:
1) One path relates the characteristics of the grading carve to hydrodynamics, the coarser mode being a product of traction transport, the lesser and fine mode being the result of salutation transport.
2) A second approach considers the grain size description largely a product of the sediment generative process, in this cars the distribution is attributed to the source materials and the size distribution, generated by their disintegration.
3) A third approach is to make an empirical study of grading characteristics of sediments from various natural geometric environments to see what relation
Grain size and provenance:
× The bed material ranges from 5mm in diameter to boulders 100mm or larger.
× It seems more probable that the processes of rock disintegration would produce more of certain size and less of others and thus create an initial deficient of certain size, three separate classes of particle might be expected from breav down or grains larger than 1mm are rare the blocks generated because rebbles whose abrasion produces fine silt or lay sized material not sand. The decomposition products are of clay size.
× Of clastic sedimentary round, brean down will sihplylead to the liberation of particles fronanearlier of primer production.
× Grain size and transportation :to what extent and in what way are size and size distributions modified?
× In general the gravels carried by streams appear to decrease in size downstream. And because the corner and surfaces of the larger materials are rounded and smoothed. It has been presume that abrasion is an active process during transport and that the down current decline in size, therefore caused by such wear.
× Abrasion: is a general term meaning wearing away oratihtion as such it is applied to almost any mechanical process of size reduction. Mavsh all defined three process.
1) Abrasion. 2)Impair. 3)grinding.
× Abrasion: Is the effect – of rubbing one pebble against another. It is by far the slowest process of wear.
× Impact: is the effect of definite blows of relatively larger fragments on others of smaller size.
× Grinding: is the crushing of small grains by continued contact and pressure of the pebble, of some what larger size.
× Wadell: recognized four size reduction process solution attrition, chipping and splitting, the differences are primary a matter of the ratios of the size of the pervade removed to the size of the original fragment.
× If the particles removed are visible but less than that of the fragment, being worn, the process termed artirior. If the material removed is still larger, such as results from flaking of the corner the term chipping is applied if the destruction process produces two substance equal fragment the term spliting may be employed.
× Normal attrition of grauev product silt and clay – sized the percentage of pebble, and couple, one rounded and now broken greatly exceed, that in the bar, of the present day Columbia river.
× Sorting action which takes place in the natural environment and which move it difficult to separate the effect of size reduction than others if the finer products are removed by sorting the result, an increasing in mean size of the residue and an improvement in sorting.
× The rate of size, reduction is greatest in the early stage of the process and tend, to decline exponentially with tie or distance.
× The size distribution is a product of hydraulic, rather than abrasive action and in general actual size is inherited from the parent or is a product of rock disintegration and not a result of transported agent or process.
× Garnishee and depositional process: a transporting medium tends to carry and to deposit more of two certain sizes of material than of other sizes the principle deposit it make will have an excess of an her considerably coarser ingredient in which it can roll.
× A bric and frame work geometry: fabrics been used to elucidate the transport process itself. Fabrics have an important bearing on the physical properties of rock, such as thermal electrical fluid and sonic conductivity.
× Fabrics: the spatial arrangement and orientation of the fabric element, Gefuge include such properties as grain size sorting porosity.
× A fabric element may be a single crystal, a pebble orsahd graw.
× Packing is the spacing ordenisty of the fabric element although fabric and paching are closely related they aren’ the same thing.
× Fabric type : it may be is oiropic opines tropic
× Fabric of sediments: Gravel fabric
The overlapping shingling effect of flat pebbles in certain gravely and conglomerates has been described as an intricate structure the mean upstream inclination in flu vial deposits was is to 30 degrees, marine deposits showed indications of only 2 to 15 degree, in general flat pebble, had a lower inclination than less flat ones the larger sizes were better oriented than the smaller.
× Till fabric: the orientation of till stones has been used as criterion of the direction of ice flow? Till tend to be paralled to the direction of ice movement. Till fabrics have in general proved a useful tool in determining the direction of ice flow.
× Sand fabric: is less well known because of the difficult of the fabrics that relate to current flow are those seen in sections cut paralled to the bedding, parricullay of those sandstone, A similar agreement between grain fabric and dielectric an sotropy was noted by Mclver.
× Fabrics of limestones and dolimites: lavgely growth fabrics in pores and other openings produced by druse like implantation of crystals on the walls of such cavities.
× Fossil orientation: if moved by a current that orientation tends to become uniform oriented fossils may also be indices of current direction orientations tended to have their longest dimensions the parallel to or presendicuor to the pararippe of the same bed.
Frame work Geometry of detritus sediments:
Packing:
a- Close packing reduces both volume of pore space and size of the pores.
b- Alters both porosity and perm ability.
*The frame work elements of coorser clastic sediments are the pebbled and sand grains of which these deposits are composed. These clastic elements are non spherical and non uniform in size.
1) Porosity: It’s the percentage of pore space in the total volume of the rock.
× The original porosity of a sediment is affected by uniformity of grain size, shape of the grains, method of deposition, packing of the sediment and compaction during and after deposition.
× The actual size of grain has no influence on porosity.
× The fine grained sediments have higher porosity than the coorse. Grained sediments.
× The addition of clay increases the porosity.
2) Permeability : It’s the property of a rock which allows the passage of fluids without impairment of its structure or displacement of its parts.
× A rock is said to be permeable if it permits an appreciable quantity and impermeable if the rate of passage is negligible.
× The coefficient of permeability, k, of an unconsolidated sand is affected by grain size, grain sorting, grain shape and packing.
× The permeability increasing as size of grain increases.
× The permeability dependent on the packing arrangement because the pore dimensions are dependent on the style of packing.
× The relation between porosity, permeability and grain size:- the finer the grain, the smaller the permeability. Any change in packing increases the porosity will increase the permeability. Non porous rock is also non permeable, highly porous rock isn’t necessarily permeable.
Crystalline textures:
× All rocks are crystalline but the term crystalline is usually reserved for those rocks which show an interlocking aggregate of crystals (such as rock salt) the frame work elements are grains of feldspar and quartz each briously crystalline, but in deference to the detrital nature of these grains the roc is said to have a clastic rater than a cry stalline texture.
Crystal elements:
× The basic components of a crystalline fabric are its individual crystal elements:
1- Large: so the texture is macro crystalline.
2- Small: so the texture is micro crystalline.
3- In between: so the texture is meso crystalline.
× The shape of the crystal elements may be described in terms of the extent to which they display external crystal faces and symmetry : 1- An hedral : no crystal races, 2-En hedral bounded by well-formed crystal faces, 3-sub hedral: show few faces.
Re crystallization fabrics:
× Many sedimentary rocks recry stallinze at normal pressures and temperatures. This is particularly true of many carbonate rocks but is also true of gypsum, an hydrite, and even chert, the conversion of an hydrite to gypsum or the reverse is a good example of this kind of re crystallization.
× Crystallization or re crystallization in the solid state produces textures essentially metamorphic. It may decrease the grain size, more commonly it leads to a coarsening of the rock texture.
Texture of replacement:
× Minerals which are chemically precipitated and exhibit a crystalline texture or fabric may be formed at the time of accumulation of the sediment; or they may be formed later or perhaps in part replacing chemical.
To understand the history of a sedimentary rock it’s necessary:
1- To discriminate between detrital minerals and those chemically precipitated.
2- To determine the relative ages of the several precipitated minerals.
3- To ascertain the manner of emplacement of the precipitated materials, whether formed in openings or formed by replacement.
O lites, spherulites:
× Many sediments contain small spherical bodies of diverse mineral composition and various internal organization.
1-Oo lites:
× A rock may be said to possess an oolitic texture if it consists largely of oolites. These are small spherical or subspherical, accretionary bodies, 0.25 to 2.00 m in diameter most commonly they are 0.6 to 1.0 mm in size if they are over 2.00 mm in diameter, they are termed pisolites.
2-spheralites:
× The term spherulite has been applied to any spherical body with a rodial structure.
Bio genic fabrics:
× Many sedimentary rocks contain fossils as an integral part of the rock. They may be a minor component of the rock or as in the case of some limestone, they may be the dominant constituent.
Composition and mode of preservation:
× Fossils are any evidence of past life. They may be buried such as the more resistant organic structure – bones, teeth, shells, most shells and similar structures are originally calcium carbonate, but even these show a varied composition in terms of magnesium content and minor elements others are phosphatic, siliceous or chitinoid.
× In many cases neither the original organic structure nor its replacement is present, in stead are found cavities, called molds, formed by removal in solution of the original shell.
× If a mold is filled with foreign matter the filling is properly termed a cast. The filling of the interior body cavity of abivolve is often called a cast of the interior.
× Fecal pellets: are present in modern marine deposits and in some sedimentary rocks inhere they have been lithified the pellets are the product of mud- eating forms and hence consist of silt and mud particles bound by organic matter.
× Coprolites: are larger objects with an origin similar to that of fecal pellets. They are generally light to dark brown or black, avoid to elongate structures 1 to 15 cm in length.
Bedding
- Many bodies of sediment possess more or less planar bottom and top surface and are very extensive laterally in relation to their thickness. Sedimentation untis greater than I am thick are known as beds and their upper or lower surface are known as bedding or bounding planes. Below 1 cm sedimentation units are termed lainae. Beds may incorporate units thicker than 1 cm which are referred to informally as layers or strata. Strata and laminae may help to make up beds and may deposited at an angle to the main depositional surface and hence be called cross beds. Groups of beds may form cosets or bed bed sets which may be simple or composite many beds and bedsets forming at the present day maintain their shape and thickness for considerable lateral distances, although all eventually thin our or change their nature either gradatinally or suddenly when traced for enough.
Preliminary observation and recording of bedding:
- Several level of observation and investigation of bedding are possible. At first it is useful to scan exposures from distance in order to decide the general attitude and orientation of the beds. It also helpful to work out. At an early stage. The way up of the succession and where in general, the older and youger strata are to be found.
- Bedding has to distinguished from cleavage, from joints and faults, and from colour banding due to diagenesis and weathering changes in composition or grain size are the best guide, to identifying bedding and these may often be more apparent on weathered surface.
Feature within the bed or bedset:
- Beds and bedsets may be
a) homoeneous or heteroeneous
b) rhythmic
c)gradational
Homogeneous (e. well- sorted s.s or silt stone) and apparently structure less beds some times reveal ansuspected internal structure if special techniques are applied to slabbed speciment in the laboraty some bedsare heterogeneous due to sorting into layers, often showing repeated inter lamination of material of contrasting composition or grain size e.g silt and sand silt and mud.
The nature of the bounding surfaces (bed contacts) bed contacts can be gradational orsharp. They may be marked by stuble or abrupt changes of composition and colour texture and structure. Sharp changes may be non- erosional or erosional. The latter bein marked by cross- artting relation ships, as at the base of a channel or where there are down ward projections on the sole of the overlying bed.
Sequences and patterns: the classification of bedding patterns of sedimentary units in roups of beds and bed-sets may be discerned, where thickness, grain size, composition or structure varies systematically.
(a) Grain size of units fining up wards from coarse s.s to siltstone and mudstone.
(b) Grain size of units coarsening up wards from shale to coarse sandstone (S.S).
(c) Patterns of units such as shale - sandstone.
(d) Patterns involving units carbonate - sulphate, hydras and an hydrous sulphate.
(e) Patterns of inter bedded sand stone and shale units (as in beds deposited from alternately strong and less strong and or dense or less dense flows).
The significance of bedding :
Bed contacts, true bedding planes and bounding planes represent changes in conditions. Some contacts however, record gradational changes. Laminae are produced as a result of minor fluctuations in the constant conditions over the same or a smaller area than the bed. Simple bedsets or cosets are the result of repetitions of genetically related variations in conditions.
Several processes contribute towards establishing the characteristics of abed these are physical, chemical, biological and diagenetic.
Physical processes:
Most sedimentary rocks result from deposition of material transported as individual grains in suspension or near the bed by water flows of low sediment concentration. Although some are detived from denser flows or at the extreme from mud flows. The nature of the transporting processand its intensity are controlled by physical parameters such as grain size and current strength, wave intensity and viscosity of the transporting medium and, in some cases the flowdepth A change in any of these parameters might result in a change in the nature of the deposit and in the initiation of a new bed.
Chemical processes:
Much material, particularly in sea water and in certain lakes, is carried in solution. In favorable conditions brought about by changes intem perature, pressure of carbondioxide or concentration of lons, these solutes may be precipitated as minerals, either directly on the floor of the basin or as loose particles in suspension such precipitates are susceptible to reworking by physical, processes.
Biological processes:
The reater part of the calcium carbonate, which composes limestone and present day carbonate sediments, results. From the activities of organisms which precipitale calcium carbonate as part of their metabolic processes. Other organisms secrets silica or phosphate which may also contribute to sediments. Change in the dominant organisms may produce changes in characteristics of sediment and may lead to the generation of beds. Abed may be formed from the hard parts of organisms more or less remaining insitu, but the skeletal material is most commonly redistributed by physical process to form bed of a homogeneous nature, changes in organic activity commonly redistributed by physical processes to form bed of a homogeneous nature. Changes in organicactiviy commonly reflect changes in physical and or chemical processes
Diagenetic processes:
The final appearance of abed in the stratigraphic column results not only from the conditions of its deposition but also from its subsequent history during burial. The process of post depositional change may vary with, for example changing pore-water chemistry and as a result distinct beds may be due to diabetic changes rather than changes during deposition.
Changes of process and bed generation:
There are two main process of change:
( 1 ) Changes due to lateral migration. An area that once accumulated sand, for example, might later be a site of mud deposition and, if net sediment accumulation prevails, we might except to find mud over lying sand in a trench dug at the site, in other words lateral migration of sub environments under stable conditions can create changes beds ) in theoretical sequence, this idea developed.
( 2 ) Changes due to temporal fluctuations: in contrast to the steady - state, lateral migration mechanism many beds reflect changes due to external effects. On a lake floor. For ex. Coarser silts and sands may reflect periods of high river discharge, and muds may be deposited during quieter periods. With time this results in the inter lamination of sand, silt and mud. Similarly on the shallow sea floor, rain sizes and sedimentary structure may fluctuate and thus reflect periods of stormy and calmer weather, in both these example the environment is not changing. But rather the vertical sequence result from natural variation of processes within that environment. In some settin it is reasonable to distinguish normal deposit from catastrophic deposit. Example of such stein is a deep sea continental margin where, under normal conditions, line grained sediment accumulates dorm suspension. Occasionally, however this process is punctuated by a catastrophic density current generated on the continental slope of shelf edge. This event deposits a sheet of sand in amatter of hours but may not recur for 100 or 1000 of years. In the resultant sequence, however, the sand beds may be more abundant than the normal mterbedded muds. Short. Catastrophic events can therefore make a contribution to the rock record out of an proportion to their duration’s.
The significance of sharp and gradational changes between beds: the importance of bedding planes.
A radational boundary may well suggest that alateral migration of processes was taking place under steady- state conditions. Sharp junctions between contrasting bed might lead us to suggest catastrophic events.
Beds deposited from floods, dust storms and ash clouds, these beds form part of thick uniform sequence which, when dated appear to have accumulated over considerable periods of geoloical time, often millions of years. This paradox of relatively swiftly deposited beds, making up sequences representing many millions of years, has led some geoloists to coin some what intuitive statements such as 98% of geological time must lie on the bedding planes, thus hih lighting the importance of trying to recognize significant time gaps in seemingly continous sedimentary sequence.
Unconformity
An unconformity is a break in a stratigraphic sequence resulting form a change in condition that caused deposition to cease for a considerable time. There are various type of unconformity:
( 1 ) non- conformity: where sedimentary rocks succeed igneous or metamorphic rocks.
( 2 ) Angular- unconformity: where they are laid upon previously folded strata.
( 3 ) Dis conformity: where local successions contain two sets of conformable beds seprated by mere changes.
( 4 ) Para conformable: where the sediment types are virtually.
Preservation potential of bedding:
The preservation of a group of beds is due to the products of a set of depositional events escaping reworking or partial removal and bieng captured by net subsidence. This is a chance occurrence in many environments, particularly where powerful a gents of erosion operate, for example in shallow marine environments, in flood plains where rivers are constantly shifting their channel, or where strong winds are eroding down to a water table.
