2005 lecture 4 texture and grains

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Textural Analysis of

Clastic SedimentaryRocks



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Texture of SedimentaryRocks Part 1

• Aspects of textural analysis

– Grain Size – Grain Morphology

– Grain Surface Texture

– Sedimentary Fabric (grainsput together)

Reflects the processes of

deposition



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Grain Size

• Defined by Udden-WentworthScale (handout)



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Udden-Wentworth ScaleGrain SizeAnalysis



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ΦΦ = -log2G (grain size in mm)

– Makes calculations easier: – Large Φ finer grain size

Grain-size measurement:

Ruler

Sieve (unconsolidated sands)

Graticule measurement in t.s(consolidated sedimentary rock)

Settling/ Laser techniques on

clay suspensions

Grain SizeAnalysis



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Assessing Grain Size

Distributions• Plot histogram of number of grains of a given

size range against all size ranges

Grain SizeAnalysis



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• Histogram may be:

– Unimodal

– Bimodal

– Polymodal

Grain SizeAnalysis



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Sorting:

Measure of the standard deviation

of the distribution

Grain SizeAnalysis Sorting



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Grain SizeAnalysis Sorting gauge



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• Sorting shows how effectivethe depositional medium

(gravity, water, wind) is in

separating grains of differentsize.

• Desert: good sorting.

• Glacier: very poor.

• Factors involved:• Source.

• Grain size.

• Depositional mechanism.

Grain SizeAnalysis



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Good sorting



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Poor Sorting



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Skewness• Symmetry about the mean grain

size distribution• E.g. beach deposition tends to be

negative: waves winnow the fine particles

Grain SizeAnalysis



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Grain Size Trends

• Deposition over large area

often shows a trend• Deltas: finer grained seawards

• Rivers: finer grained downstream

• Sea: continental shelf deposition

Grain SizeAnalysis



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Graded Bedding

• Coarse base fine top.

– Decelerating flow (e.g. turbidity currents).

– Common.

• Fine base

coarse top. – Reverse/ inverted grading : uncommon.

– E.g. Delta deposits.

Grain SizeAnalysis

Keep an

eye out

for this in

thin

sections



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Grain SizeAnalysis Graded bedding



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Grain Morphology• Shape

• Measured in x,y,z dimensions

• Sphericity• How closely the grain approximates a sphere

• Roundness• Degree of curvature of grain corners

• N.B. High roundness does NOT necessarily imply

high sphericity



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Grain Shape• Grain shape comprises attributes

which refer to the external

morphology of particles. Theseinclude surface texture, roundnessand form. Grain shape (Bustin,

1995) is determined by:

internal structure, (mineral cleavage);

characteristics of source rock such as

jointing and bedding; lithology

hardness

fracture

transport



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Form & Sphericity



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Roundness/ Angularity• defined as the average radius

of curvature of corners (r iin

figure) to that of the largest

inscribing circle (R in figure).

• As you can see, very tricky.



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Well rounded, well sorted

Poorly rounded, poorly sorted

Roundness

& Angularity



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• Roundness influenced by:• Mineralogy

• (e.g. mica breaks down easily…)

• Source Rock:• weathered material often well

rounded before transport begins(e.g. Causeway basalts)

• Degree of abrasion sufferedduring transport

• Post depositional chemicalattack

• Degree of roundness

reflects duration of transport

GrainMorphology



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Angularity Classification



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Grain Surface Texture• Useful in inferring mechanism of transport:

– E.g.• Striations on pebble surface : scratched during ice

transport.

• Impact marks on pebbles: common in beach andriver channel deposits.



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Sedimentary Fabric• Preferred orientation of grains:

usually aligned parallel to

water flow direction :

Palaeocurrent Indicator.

PREFERRED ORIENTATION

OF GRAINS



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Packing: how well the grains are

packed together.affects porosity and permeability.

Well sorted sand: lot of spaces

after packing: good porosity.Poorly sorted sand – smaller particles fill voids – lower porosityand K.

CUBIC PACKING

(48% POROSITY)

RHOMBOHAEDRAL PACKING

(26% POROSITY)

Packing



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Grain Contacts/ Fabrics• Concavo-convex.

– Well packed.

• Sutured.

– Point contacts: dissolution.Increased burial causes further

penetration of point contacts.



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POINT CONTACTS CONCAVO-CONVEXCONTACTS

SUTURED CONTACTS

Types of grain contacts



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Support

GRAIN-SUPPORTEDFABRIC

MATRIX-SUPPORTEDFABRIC

Support



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Matrix and Cement• Very important to distinguish these:

– Cement: chemically precipitated

– Matrix: mechanically deposited (e.g. clayinfills)



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The ability of rocks to store (porosity) and

transmit (permeability) fluids is one of themost important properties of sediments in

economic and engineering terms.

At deposition, sediments are extremely

porous with very high volumes of voids

(space) per unit volume of sediment.

Cementation:

Reduces porosity and permeability.

Common cements: Calcite, silica, iron

oxides .

Matrix &Cement



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Infilling:

Pore space is infilled by finer sediments:

both porosity and permeability arereduced dramatically.

Clean sands free from fines (silt and

clay) make the best aquifers and reservoir

rocks.

Dirty sands have the porosity plugged byfine particles.

Matrix &Cement



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Matrix &Cement



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Estimating Textural

Maturity• Immature

– Poor sorting – Angular grains

– High matrix content

• Mature – No matrix

– Moderate-good sorting

– Degree of rounding

• Supermature – No matrix

– Very good sorting

– High degree of roundness



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• Both porosity and K increasewith textural maturity

• Textural maturity reflects

depositional processes• Little current activity: immature sed

(glaciers, some rivers)

• High current activity: mature(deserts, beaches)



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Composition• Also indicates textural maturity:

• TRANSPORT BREAKS SOFTER GRAINSDOWN SO:

– Mature: high quartz content, low lithics,

feldspar, micas etc.



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Provenancing: Quartz

different quartz grains presentcan give clues as to its origin:

common (plutonic)

volcanic

vein

recrystallized metamorphic

schistose metamorphic

stretched metamorphic



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•Abundant vacuoles; sometimesvermicular chlorite; rarely, zoned

phantom crystals

•semi-composite to straight or

undulose extinction, some may be

badly sheared. Often shows comb

structure.

•Often forms large grains and pebbles.

VeinQuartz



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•elongate, composite with straight borders

•mica incusions

•straight to slightly undulose extinction

Schistosequartz



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Provenancing: Feldspars

• Feldspars: – K-spar: derivation from alkalineplutonics or metamorphics.

– sodic plagioclases: derivation from

alkaline volcanic rocks. – calcic plagioclases: derivation from

basic volcanic rocks.

• Plagioclase zoning: – oscillatory (microscale) zoning:

volcanic or hypabyssal origin.

– progressive (coarse) zoning: igneous

source, undifferentiated.



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Provenancing: stability Relative chemical stability and

degree of weathering also helpsgeologists to interpret climate

and relief of a source area.

Examples:Presence of fresh, large, angular feldspar fragments in a sandstone imply:

a high relief source area (rapid erosion,transport, burial, with little weathering effect

ORa very arid or extremely cold climate(retards chemical weathering)



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Provenancing: stability Small, rounded, highly weathered

feldspars imply: a low relief source area and/or warm, humid

climate (modern-intense weathering

processes)

Absence of feldspars imply:

intense weathering (destroying feldspars) OR

no feldspars in original source



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Feldspar Sources

2005 lecture 4 texture and grains

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