pet_08 texture of sedimentary rocks

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TEXTURE & STRUCTURE IN SEDIMENTARY ROCKS

Handout : Petrology - 08

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MAJOR TYPES OF SEDIMENTARY ROCKS

Siliclastic : accumulation of mostly silicate mineral fragments Sandstones, mud rocks, conglomerate and breccias

Biochemical : accumulation of particles produces by precipitation from living organism

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Limestones and cherts

Chemical : formed by direct chemical precipitation from water Halite, gypsum

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TEXTURE OF SEDIMENTARY ROCKS

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Nature of source rocks Determines the original shape of grains and

mineralogical composition

Textures depend on process :

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Strength of wind or water current Determines whether or not grains are transported or

deposited Deposition process controls structures preserved in the

sediment and give clues to the environment of deposition

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• Distance transported or time in transportation process- Longer transported distance will change shape


• Textures depend on process :

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g p g pand become sorted of size and mineralogy

- Control the break down to stable mineral during transportation process

• Biological activity with the sediment prior to diagenesis- Burrowing organisms can redistribute sediment after

deposited, and erasing some clues to original environment


• Textures depend on process :

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deposited, and erasing some clues to original environment of deposition

• Chemical environment under which diagenesis occurs- during diagenesis grains are compacted, new minerals

precipitate in pore spaces, some minerals continue to react to produce new minerals, and some minerals re-crystallize

Richard Harwood, 2009


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GRAIN SIZE

Clastic sediments/sedimentary rocks are classifiedbased on predominant grain size of clasts in therockWentworth Scale


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Wentworth Scale, 1922


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Wentworth Scale, 1922


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SORTING Refers to the uniformity of grain size in a sediment or

sedimentary rock Particles become sorted on the basis of density of the

energy of the transporting medium High energy (high velocity) currents can carry larger

fragments, as the energy or velocity decreases, heavier particles are deposited and lighter fragments continue particles are deposited and lighter fragments continue to be transported. Sorting by density

If the particles have the same density, and the heavier particles will also be larger, so the sorting will take place on the basis of size.

Sorting can be classify from : well sorted to poorly sorted

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SORTING

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SORTING

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SORTING

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SORTING

Beach sands and dune sands tend to be well-sorted because the energy of the waves or wind is usually rather

constant. The coarse grained sediment is not carried inbecause the wave or wind velocity is too low to carry suchlarge fragments, and the fine grained sediment is kept insuspension by the waves or wind.

Mountain streams usually show poorly-sorted

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because they have many turbulent eddies where thevelocity of the stream change suddenly on the bottom ofthe stream channel.

Glacial till tends to show poor sorting because it is deposited in places as glacial ice melts, and is

not transported by water

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SHAPE - ROUNDING

During the transportation process, grains may bereduced in size due to abrasion. Random abrasionresults in the eventual rounding off of the sharpcorners and edges of grains

The degree of rounding of grains give us clues tothe amount of time a sediment has been in the

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transportation cycle Rounding is classified on relative terms The rounding is not the same a sphericity Sphericity is controlled by the original shape of the

grain

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SHAPE - ROUNDING

Angularity or roundness depends on how far the rocktransported

Roundness : degree of edge and corner removal Sphericity : degree to which fragment approaches

spheroid shape

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Rospheroid shape

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SHAPE - ROUNDING

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High Sphericity

Low Sphericity

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POROSITY

Porosity is the percentage of the volume ofthe rock that is open space (pore space).This determines the amount of water orother fluids, like petroleum, that a rock cancontain

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Porosity of sedimentary rocks depend on : Grain size, Shape of the grains Degree of sorting Degree of cementation

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Well-rounded coarse-grained sediments usually have higher porosity than fine-grained sediments, because the grains do not fit together well

Angular grains of fine grained sediment can be compacted to fit together better, and thus the porosity is reduced

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POROSITY

p y Fine grain size of mudrock usually have very low

porosities

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Poorly sorted sediments usually have lowerporosity because the fine-grained fragments tend tofill in the open space

Cementation that takes places during diagenesistends to fill in the pore space.

Highly cemented sedimentary rocks have lower

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POROSITY

Highly cemented sedimentary rocks have lowerporosity than do poorly cemented sedimentaryrocks

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POROSITY

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Poor sort

PACKING OF GRAINS

Packing refers to the arrangement of clasticgrains entirely apart from any authigeniccement that may have later crystallizedbetween them.

If the clastic grains touch each other

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throughout, the rock is said to be GrainSupported

If the rock is poorly sorted and the grainsare separated by a mud or silt matrix, therock is said to be Matrix Supported

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INDURATION

Induration refers to the hardness of therock or how easly it breaks apart.

Well indurated rocks are difficult to breakwith a hammer

Moderately indurated rocks can be easlybroken with a rock easly in your hand

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Poorly indurated or friable rock break aparteasily in your hand

The term non-indurated would describe asediment that has not undergone anycementation

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TEXTURE MATURITY

The longer sediment is involved in thetransportation cycle, the more time it has tobecome well-sorted

The longer the sediment is transported, themore time is available for grains to losetheir rough edges and corners by abrasion

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Texturally mature sediment to be sedimentthat is well-sorted and well-rounded

Sediment tends to become both texturallyand mineralogically mature the longer it isin the transportation cycle

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DESCRIPTIONS OF TEXTUREP

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1. Size of the grains2. Sorting3. Degree of roundness and sphericity of the

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Complete description of the texture of a sedimentary rock should include :

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grains4. An estimate of the porositity of the rock5. Packing of the grains6. A description of the matrix7. Induration of the rock8. A statement about the textural maturity of the

rock

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Texture & Structure

Breccias

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Limestone

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GRAIN SIZE

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GRAIN SIZE

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SORTING & FABRIC

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ROUNDNESS

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Diameter shape of pebbles : S = Short; I = Intermediate; L = Long

ORIENTATION OF GRAINS & PEBBLES

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Crystalline rocks

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SEDIMENTARY STRUCTURES

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TYPES

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Erosional Structures Flute casts Groove casts Tool marks Scour marks

Depositional Structures Bedding & lamination Ripples, dunes & sand-

waves Cross-stratification osan

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Scour marks Soured surfaces Channels etc

C oss s a ca o Flaser, Lenticular, wavy

bedding Tidal cross-bedding etc

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Flute marksGroove marks

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Fluvial channel

Current ripples

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Lenticuler bedding

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Plannar cross- bedding

Columnar Stromatolites

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Bed Thickness

Types of bedding osana 2012

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Types of bedding

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Bedding planes & Bed Contatcs

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TO BE CONTINUE

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pet_08 texture of sedimentary rocks

Documents

sedimentary rock particles

accumulation of particles

sorted of size

heavier particles

mud rocks

sedimentary rockshandout

larger fragments

lighter fragments