Sedimentary Materials & Texture
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Transcript of 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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Types of Sedimentary Material
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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Types of Sedimentary Material
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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Types of Sedimentary Material
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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Describing Siliciclastics:
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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Describing Siliciclastics:
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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Describing Siliciclastics:
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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Describing Siliciclastics:
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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Describing Siliciclastics:
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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Describing Siliciclastics:
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