Sedimentary Materials & Texture

5/28/2018 Sedimentary Materials & Texture

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Assoc Prof Dr Abdul Hadi Abd Rahman

DGPE, Room:16-03-29

Tel: 605-368 7039

E-mail: [email protected]

QAB2023 Clastic Sedimentology & Petrography


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Types of Sedimentary Material

Terrigenous Clastics (TC) Detrital Particles

Derived from pre-existing rocks

Derived external to the

depositional basin

Transported by surfaceprocesses to the site ofdeposition

Particulate Residues: quartz,

feldspar, rock fragments, etc(unaltered rock formingmineral/rock grains)

Secondary Minerals: mineralsnew-formed in the surface

weathering environment: clayminerals, oxides,amorphous silica, etc
http://www.geoscapesphotography.com/ca-pointreyes-conglomerate-p2987.htm


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Allochemical Particlesformed in situat the site ofdeposition; of chemical/

biochemical origin Carbonates: ooids, fossil

fragments, pellets, lithoclasts

Glauconite, phosphate:in situ

authigenic/particulate minerals Biogenic sediments: pelagic

tests, siliceous and calcareous


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Orthochemical Components Chemical Precipitates

Secondary cement

Primary chemical sediments: halite, etc

Organic Particulate Material(detrital organic matter) terrestrial and particulate

marine pelagic

95% found in mudrocks and indicative oflow Eh and low current strength

Laminated Castile Formation basinalevaporites. Dark laminae are calciteplus organic matter; light laminae

are gypsum (Peter Scholle)

Coal


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Pyroclasts

particles fragmented and transported by volcanicprocesses

Tephra: tuff deposits Volcanic mudflows: lahar and volcanic breccia deposits

Tephra VolcanicAsh
http://volcanoes.usgs.gov/Products/Pglossary/tephra.htmlhttp://volcanoes.usgs.gov/Products/Pglossary/ash.htmlhttp://volcanoes.usgs.gov/Products/Pglossary/ash.htmlhttp://volcanoes.usgs.gov/Products/Pglossary/tephra.htmlhttp://volcanoes.usgs.gov/Products/Pglossary/ash.html


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Sedimentary Analysis Requires description (qualitative, quantitative)

Analysis (graphical, statistical) interpretation


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Describing Siliciclastics Description

Size

Texture

Fabric

Analysis

Maturity Textural

Compositional


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Describing Siliciclastics-Size Size

Gravel and larger (> 2 mm) Sand (1/16 - 2 mm)

Mud (< 63mm = < 1/16 mm)

Silt 4mm >x


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Gravel-Size (Pebbles & Cobbles)

(> 2 mm)


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Sand (1/16 - 2 mm)


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(< 63mm = < 1/16 mm)

Mud


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Describing Siliciclastics I:

Size

The Wentworth scale Udden- Wentworth size scale

Udden, 1914; Wentworth, 1922

Resolves problems with size classification

Cumbersome to discuss size

Limiting to restrict to 3 classes

Four basic groups + modifiers make moreClay (< 4 mm)

Silt (4 mm - 63 mm)

Sand (63mm - 2 mm)

Gravel (> 2 mm)


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Siliciclastic Rock Classification:Texture

Descriptive TexturalClassification Grain Size

Uden-Wentworth grain size scale

Phi ()=-log2(grain diameter in mm)

naturally occurring groups;

Gravel~ rock fragments,

Sand~ individual mineral grains(particulate residues)

Clay~ chemical weathering products

(clay minerals, etc.)Mud~ particulate residues +/-

chemical weathering products


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Describing Siliciclastics II:

Size

The Wentworth scale Subdivided scale by factor of 2

.0039 mm clay

.0078 mm very fine silt

128 mm = cobbles

256 mm = boulders

Logarithmic (base 2) progression!

= -log2(grain diameter in mm)

As grain size increases, phi size decreases


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Describing Siliciclastics:

Sedimentary TextureAspects of texture

Shape

Degree of sorting Surface texture

Result of

Parent rock type (shape) Weathering

Transport history (sorting, shape)

Generally for siliciclastics but can be useful for other types


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Describing Siliciclastics

Roundness Degree of angularity

Function of transport history

Edges chip off as clasts knock into one another (progressive)

Estimate visually or calculate from cross- section

SphericityHow closely clast approximates a sphere (equant)Inherited feature! (function of shape formed in weathering) slab may

become discoidal but stays flat with time

Form/Shape Zingg indices

spherical (equant), oblate (disk or tabular), bladed, prolate (roller)


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ZINGG DIAGRAM

Where:di= intermediate

diameter

dl= long diameter

ds= short diameter


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Angularity & Roundness


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Texture: Sorting & Shape Sorting: measure of the

diversity of grain size

A function of grain originand transport history

Clast Rounding: surfaceirregularity Due to prolonged agitation

during transport and

reworking


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Degree of sorting

Measure of distribution of clast sizes Well sorted

most clasts fall into one class on the Wentworth scale Poorly sorted

wide range of clast sizes

Due to origin and transport history Greater distance (or repeated agitation of sediment), better

separation of sizes

Qualitative (visual) and quantitative methods


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Sorting


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Statistical/Graphic Presentation of Texture:

Grain Size/Sorting Quantitative assessment of the % of different grain

sizes in a clastics

Mean: average particle size Mode: most abundant class size Median: 50thpercentile


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Grain size analysis

Quantitative analysis (granulometric analysis)

Quantitative assessment of % of different grain sizes in clasticsediments

Useful in interpretting depositional history of clasts,especially in modern environments

Technique used varies with grain size Direct

Indirect


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Grain size analysis- techniques

Gravel direct measurement in the field

measure all within a quadrantmeter is used for pebbles, cobbles

Sandpass through a stack of sieves with mesh keyed to

weigh contents of each sieve, get distribution

by wt.


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Describing Siliciclastics

Grain size analysis- graphic analysis Plots

Histogram of weight percentage of size fractions Frequency curve

Cumulative frequency curveWhen plotted, grain size increases from right to left, fines to right, coarse to left

Graphically represent grain size distribution mean grain size standard deviation from a normal distribution (sorting) symmetry (skewness) flatness of curve (kurtosis)


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Grain size analysis- graphic analysis

Different depositional environments exhibitdifferent grain size distributions

Glacial sediments

poorly sorted

River sediments

moderately sorted

Beach sedimentswell sorted


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Statistical/Graphic Presentation of Texture;

Granulometry


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Refraction Index: immersion oils of known refractive index

and comparing the unknown mineral to the oil

Sedimentary Materials & Texture

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