2005 lecture 4 texture and grains

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 1/43

102GLY202

Textural Analysis of 

Clastic SedimentaryRocks

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 2/43

102GLY202

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

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 3/43

102GLY202

Grain Size

• Defined by Udden-WentworthScale (handout)

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 4/43

102GLY202

Udden-Wentworth ScaleGrain SizeAnalysis

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 5/43

102GLY202

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

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 6/43

102GLY202

Assessing Grain Size

Distributions• Plot histogram of number of grains of a given

size range against all size ranges

Grain SizeAnalysis

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 7/43

102GLY202

• Histogram may be:

 – Unimodal

 – Bimodal

 – Polymodal

Grain SizeAnalysis

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 8/43

102GLY202

Sorting:

Measure of the standard deviation

of the distribution

Grain SizeAnalysis Sorting

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 9/43

102GLY202

Grain SizeAnalysis Sorting gauge

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 10/43

102GLY202

• 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

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 11/43

102GLY202

Good sorting

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 12/43

102GLY202

Poor Sorting

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 13/43

102GLY202

Skewness• Symmetry about the mean grain

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

negative: waves winnow the fine particles

Grain SizeAnalysis

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 14/43

102GLY202

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

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 15/43

102GLY202

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

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 16/43

102GLY202

Grain SizeAnalysis Graded bedding

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 17/43

102GLY202

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 

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 18/43

102GLY202

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

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 19/43

102GLY202

Form & Sphericity

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 20/43

102GLY202

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.

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 21/43

102GLY202

Well rounded, well sorted

Poorly rounded, poorly sorted

Roundness

& Angularity

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 22/43

102GLY202

• 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

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 23/43

102GLY202

Angularity Classification

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 24/43

102GLY202

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.

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 25/43

102GLY202

Sedimentary Fabric• Preferred orientation of grains:

usually aligned parallel to

water flow direction :

Palaeocurrent Indicator.

PREFERRED ORIENTATION

OF GRAINS

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 26/43

102GLY202

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

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 27/43

102GLY202

Grain Contacts/ Fabrics• Concavo-convex.

 – Well packed.

• Sutured.

 – Point contacts: dissolution.Increased burial causes further 

penetration of point contacts.

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 28/43

102GLY202

POINT CONTACTS CONCAVO-CONVEXCONTACTS

SUTURED CONTACTS

Types of grain contacts

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 29/43

102GLY202

Support

GRAIN-SUPPORTEDFABRIC

MATRIX-SUPPORTEDFABRIC

Support

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 30/43

102GLY202

Matrix and Cement• Very important to distinguish these:

 – Cement: chemically precipitated

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

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 31/43

102GLY202

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

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 32/43

102GLY202

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

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 33/43

102GLY202

Matrix &Cement

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 34/43

102GLY202

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

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 35/43

102GLY202

• 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)

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 36/43

102GLY202

Composition• Also indicates textural maturity:

• TRANSPORT BREAKS SOFTER GRAINSDOWN SO:

 – Mature: high quartz content, low lithics,

feldspar, micas etc.

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 37/43

102GLY202

Provenancing: Quartz

different quartz grains presentcan give clues as to its origin:

common (plutonic)

volcanic

vein

recrystallized metamorphic

schistose metamorphic

stretched metamorphic

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 38/43

102GLY202

•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

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 39/43

102GLY202

•elongate, composite with straight borders

•mica incusions

•straight to slightly undulose extinction

Schistosequartz

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 40/43

102GLY202

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.

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 41/43

102GLY202

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)

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 42/43

102GLY202

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

8/8/2019 2005 Lecture 4 Texture and Grains

http://slidepdf.com/reader/full/2005-lecture-4-texture-and-grains 43/43

102GLY202

Feldspar Sources