sediment ology 11

8/13/2019 Sediment Ology 11

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Definitions

Sedimentary Rocks

Defined as a geologic unit or rock unitproduced from physico-chemical

weathering of lithosphere .Sedimentary deposit

A body of solid materials accumulate at or

near surface of the earth under low T, Pwhich normally characterise thisenvironment


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Definitions

Sedimentary petrology

Is the microscopic and megascopicinvestigation of sedimentary rocks

Sediments

Sediments in its ordinary means solid

materials that had settled down from astate of suspension in a liquid but ingeological mean it is not limited.


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Sedimentology

DefinitionThe branch of geology that deals of

studying of sedimentary rocks;description, classification, genesis,diagenesis and their economicimportance.


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The classification of sedimentaryrocks

* The terrigenous sediments are those particles were originally derived from the earth ( mudrocks, siliciclastic sands and

conglomerates )* The pyroclastic sediments are the products of volcanic activity, include ashes, tuffs, volcaniclastic sands and agglomerates.* The residual sediments are those left in place after weathering,

include the laterites and bauxites.* The organic sediments are those which are composed of organic

matter of both animal and vegetal origin, include skeletal limestone and coal.

* The chemical sediments are those which form by direct precipitation in a subaqueous environment, include evaporite gypsum and rock salt , as well as tufa and perhaps some lime

mud.


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The classification of sedimentaryrocks

The five genetic classification still not solve all rock classification problem e.g phosphatized bone beds

are chemical, organic or residual deposits, and so on ?The five main genetic classes of sedimentary rocks can be divided

into two separate categories: the allochthonous and the autochthonous deposits.

** The allochthonoussediments are those which aretransported into the environment in which they are deposited. Theyinclude the terrigenous and pyroclastic, with rare reworkedcarbonates.

** Theautochthonoussediments are those which form withinthe environment in which they are deposited. They include thechemical, organic and residual classes.


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Internal Organization andStructures of sedimentary

Introduction:

To understand whatthe meaning of

structures ofsedimentary rocks

we well made a rapidcomparison betweentexture and structure

StructuresTextures

Largerorganizationalunits,

are seen orstudied best in

the outcrop

the grain tograinrelation,

seen or studiedbest in thin

section.


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Geological importance ofsedimentary structures:

a. They can be used to ascertain

sedimentary stratigraphic sequence.

b. They can be used to determine paleo-

current, paleo-slope.

c. They aid to determine agent of

transportation.

d. They aid to determine paleogeoraphy

and paleo-climate.


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Classification of sedimentarystructures

primaryorMechanicalorPhysicalstructuresare those formed at the time ofsediment accumulations or deposition, they maybe either; Hydrodynamic(produced by actionof current) or Hydro plastic( syn-

sedimentary deformation )

Chemical or diagenetic or secondary st.

are those formed after rock depositionOrganic or biogenic:are formed during the

depositional process by action of organisms.


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Characterization and restrictionof sedimentary structures:

sedimentary structures are independent ofrock composition or lithology .

examples : Graded bedding or cross

bedding characterize s.s and l.s, whileMud cracks occur only in cohesive mudsand stromatolites occur only in carbonatesediments.

Ripple do not form neither in a clay nor incoarse sand or gravel.


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The most common classification ofsedimentary structures based on theagent or the environment of deposition,other is based on the morphology.

(see pettijohn page 101)


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Inorganic StructurePrimary Structures(Mechanical)

A. Beddingstratification

B. Bedding internal structures

C. Beddingplane structures (on sole)

d. Beddingplane structures (on surface)


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Inorganic Structure Secondary Structures (chemically)

A. solution Structures 1.stylotites

2. corrosion zone 3. vugs

B. Accretionary st. Crystal aggregatesC. Composite structure

Geodes, Sepatria

Cone in cone


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Primary structures

is the common and universal;Bedding or stratification primary structures.

It is the arrangement of bed into layers stratified rock it meansedimentary rocks.

Only few rare sediments such as Tillite are without internal

stratification, also some Igneous, metamorphic and lava flows arebedded.Stratum: is a layer greater then > 1 cm in thick visually

separable from other layer this is due to change inlithology or to sharp physical break.

Lamination: is restricted to similar units 1 cm in thick.

Fissile, shaly:is applied to shale strata and formed due to arrangement of

micaceous Laminas


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Diagram showing different shapes that can beacquired by beds and laminae and the

corresponding descriptive terms


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Nomenclature of bed accordingto their thickness

According to Meckee and wear

CharacterNomenclatureThickness

FlaggyVery thin bedded1-5 cmSlabbyThin bedded5-60 cm

BlockyThick bedded60-120 cm

MassiveVery thick bedded> 120 cm

The thinner strata are Laminated if 2mm., it named thinly


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Terminology for thickness of bedsand laminae


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Morphological description ofbedding.


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Flow Mechanisms

It is very important to know howparticles will behave when the fluid ismoving.

The physics of this situation isexpressed by the Reynolds equation:R = vdp / Where R is the reynolds numberV: velocity of the particleD: id the diameter of the particleP: is the density of the particle: is the viscosity of the fluid.


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Flow Mechanisms

Reynold number

For low R. number ( 2000) the fluid is

turbulent, generating eddies and vortices flow.--------------------------


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Ripple marks also


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Ripple marksalsoindicate currents, andare preserved onbedding planes


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5) Mud cracksindicate alternate drying and wetting

of the surface

The important idea

the characteristics ofsediments and sedimentary strata tell usabout the past environmental conditions ina particular area & how they changed over

time


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Sole marks


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Flute marks preserved as flute moulds on the lower surfaceof a bed. Flute are arranged parallel to the current. The

current is from L to R.


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Vertical plastic deformational structures

pseudonodule


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Biogenic Sedimentary Structures

Four main types of structure

Biogenic tracks ( vertebrate activities)Biogenic trails (vertebrate homes)

Biogenic burrows ( soft sediments)

Biogenic borings ( hard rocks)All the mentioned features known as trace fossil

or Ichnofossils


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Organic Structures

A. petrification

B. BeddingC.Miscellaneous

Borings

Casts


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Allochthonous Sediments Classification

Allochthonous sediments consists of the

terrigenous and pyroclastic classes

The allochthonous sediments may conveniently

be classified using the end-member triangle

Studies of modern sediments are predominant

emphasizes grain size and texture rather than

mineral composition

Here we prefer the classification which is based

on the grain size and composition..


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Allochthonous Sediments Classification

Fourmain types of allochthonous sedimentswell now be described. These may be groupedinto the:

Mudrock,Pyroclastic,

Sandstone

Rudaceous.


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Gravel and Conglomerates( Rudaceous Sediments )

Most gravels consist of a framework and void.

The framework is composed ofgravelsizematerial varying from pebble to cobble and

Boulder)

The voids are the openings between theseframework elements, voids are rarely empty,they are generally filled with detritus sand orsmaller size or with a cement.

Gravel with unfilled voids have been termedopenwork gravel or Beach gravel


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Gravel and Conglomerates

Texture of Gravel and Conglomerate

Size of grains

Sorting or modalitySize of gravel

Shape of gravel

Roundnesssurface textures


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Classification of Gravels and conglomerate

Conglomerates and gravels have been classified in various ways.

e.g: boulder conglomerate)Texture (size* Based on cobble conglomerate

e.g: Limestone conglomerate, Chertcomposition* Based on conglomerate

e.g: Ferrugineous conglomerate, Calcareouscement* Based on conglomerate

of depositionagent or to the environment** According to the e.g : beach conglomerate, Fluviatile conglomerate

e.g: Epiclasticfragmentation** Based on the process responsible for conglomerate, Cataclastic conglomerate and Pyroclastic

conglomerate** Based on the origin of the Conglomerates and breccias, there are

those five major categories will shown in table.,Five types


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C l i B i


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Cataclastic Breccias( fault, fold breccia)

* Movement of large masses of rocks past one

onother

* Movement that occurs along a fault surface

give rise to fault breccia

* In case of overthrust, the breccia form thin

blanket along the sole of such overthrust.

* Crush conglomerate are produced by deformation of brittle, closely jointed rocks.


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Landslides and slump Breccia

* Most slump breccia are formed due to

simple gravitational srtess

* They are localy and quantitavely

unimportant.

* Slump and landslides occur either

subarial or saubaqueous

* landslides are related to slope,

structure and lithology


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Impact or fallback Breccias

* Shown in a few terrestrial impact Craters.

* Seen through the effects of shock

metamorphism ( effect of shock on qz)

* Shatter cones in basement quartzites

and features attributable to shock metamorphism in the breccia


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Internal fabric of gravel and conglomerate:

Gravels tend to have a preferred orientation.

It has been observed that a flatter stone

have up current dip.Imbricationsalso have been observed in

both fluvial and marine gravels.

The long axesof the pebble are said to be

oriented in the direction of the currentflow.

P d l t d


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Pseudoconglomerates andpseudobreccias

* Formed due to the weathering of diabases and related rocks (rounded

boulders of exfoliation surrounded by

clays)

* Sandstone packed with many rounded

concretionary bodies.

* Limestone consists entirely of algal balls-

pebbely sized concretionally structure

oncollites.

* diagenetic processes produces a pseudo-

breccias in some l.s


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Diagenesis of conglomerates

Conglomerates, like all sedimentary rocks undergomodifications after deposition as lithification, solution ext..

Lithification,the addition of a cement lead to

binds the constituents clasts together.Solution;when intrastratal solution begin to

play, it dissolve the contact between pebbles,and finally give stylolitic boundaries

Lead to deformationMechanical pressures:of pebbles some quartz or quartzite displayfractures or micro-fault


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Type of Sphericity

F t t l th t it f th


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Factors control the maturity of thesediments


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Rock components of detrital sediments


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Uses and Economic importance of


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Uses and Economic importance ofSandstone

Sandstone deposits can be a beautiful part of the naturalenvironment and apart of the breath-taking views at many naturalparks and preserves.

Mined sandstone can be cut, polished and carved for many uses. Itis used as ornamental rocks for buildings, monuments, gravestones, bookends, beverage coasters, tiles and many other possible

uses.Sandstone can also provide silica for glass production.

Some mineral ores are found in sandstone. concentrate in uraniumminerals in sandstone deposits and many uranium ore bodies arethe result.

Heavy minerals such as rutile, gold, diamonds and others can befound in sandstones from prehistoric placer deposits that are nowworked for their economic value.


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Economic Importance

1.Many sands and sandstones are economic resources

as abrasive. Raw materials in chemical, glass and

metallurgical industries, as constructive material, both

building stones and as ingredient of plaster and concrete.

2. Some sandstone constitute important reservoirs for

natural gas, oil and artesian water

3. Some placer sands are a source of ore minerals and

gem.

4. Sand erosion and deposition are of engineering

importance on beaches, in river and in dune area.


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Academic Importance:

* They help for understanding the geologic

history;

* Their composition consider as clue to

provenance .

* Their directional structure are guide to paleo

-current.* Their geometry and internal structure shed

light on depositional environment.


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Mud rocks

Definition:

The term mud is illdefined and looselyused. In recent deposits sedimentsreferred to as a mixture of wet clay withcertain amount of silt and sand.

If lithified equivalent is termed

mudstone.


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MUDSTONE & CLAYSTONE

Claysare sediments with particles < .0039 mm.If lithified it give claystones.

Siltshave a grain size between .0039-.00625

mm. If lithified it give siltstones. Shaleis another term applied to fine grained

sediments, it does not differentiate silt fromclay-grade sediments. Shaley or fissile

phenomena is due to traces of mica aligned onlaminae.


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Type of Clay Minerals


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Type of Clay Minerals


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sediment ology 11

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