Sedimentary Petrology
GEO 333
Lab (3)
Clastic Sedimentary Rocks
(Classification & Description)
2009
Mansour Al-Hashim
Preview of Lab 2
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Classification in general
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Classification of sedimentary rocks
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Clastic sedimentary rocks
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Chemical and biochemical sedimentary rocks
Fundamental constituents of sedimentary rocks
Objectives of Lab 3
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Textures of sedimentary rocks
Grain size
Roundness and sorting
Maturity
Color
Cohesiveness (firmness)
Mineral composition and sedimentary structures
Framework and matrix
Cements in sedimentary rocks
Textures of Sedimentary Rocks
Sedimentary rocks may have
1. Clastic (fragmental) texture
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Grains are stuck together.
Characteristic of clastic sedimentary rocks.
Examples: sandstone and conglomerate.
2. Non-clastic (crystalline) texture
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Interlocking crystals.
Characteristic of chemical sedimentary rocks.
Examples: limestone, dolomite, and chert.
Clastic (fragmental) Texture
From eos.ubc.ca
Non-clastic (crystalline) Texture
From eos.ubc.ca
Grain Size (1)
Grain sizes can be determined by:
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Direct measurement with calipers or meter sticks.
For particles larger than several centimeters.
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Screening and petrographic microscope.
For particles from 2 mm to about 1/16 mm.
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Pipette or hydrometer (settling rates in water)
For particles less than 1/16 mm.
Grain Size (2)
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Results of grain-size analysis can be demonstrated as
histograms, cumulative curves, or frequency curves.
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Many statistical parameters can be calculated (e.g., median,
mean, skewness, kurtosis, etc.).
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Used to characterize depositional processes and
depositional environments.
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Udden-Wentworth grain-size scale is the most widely used.
Wentworth grade scale
Modified
UddenWentworth
grain-size Scale
Simplified grain-size scale
Roundness and Sorting
Roundness (angularity)
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Indicates how smooth or sharp the edges and corners
of particles are.
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Usually measured with reference to a comparative
chart (the commonest is Powers’ , 1953).
Sorting
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The degree of uniformity of grain sizes.
Categories of roundness
From Tucker (2001)
Degrees of sorting
From Compton (1962)
* Note the standard deviation values between classes
Visual estimation of sorting
From Tucker (2001)
Maturity
Depends on how many cycles of erosion and
redeposition the components of a rock have undergone.
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There are two types of maturity:
1. Compositional maturity
Compositional maturity = quartz + chert / feldspars + rock
fragments.
2. Textural maturity
Textural (structural) maturity = sorting and roundness of
sand-sized grains.
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Textural Maturity
From Folk (1951)
Color
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Organic matter, sulfides, and some iron oxides give
rocks a dark color.
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Indicating deposition in a reducing environment.
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Some iron oxides give rocks red or yellowish color.
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Indicating deposition in an oxidizing environment.
Color Indications
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Black - organic matter FeS
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Yellowish - pyrite, markasite FeS2
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Yellowish or bluish - sulphates, carbonates, and
chlorides
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Yellowish to brownish - limonite, goethite
FeO(OH)
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Reddish - iron oxides FeO
From eos.ubc.ca
Cohesiveness (firmness)
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1.
2.
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The degree to which the particles stick
together.
Cohesiveness can be described as:
Fragile
Soft
Intermediate
Hard
Very hard
Mineral Composition
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The most common minerals in sedimentary
rocks are:
Quartz
Clay minerals
Feldspars
Carbonates (calcite and dolomite)
Rock fragments
Small amounts of amphibole, pyroxene, and
mica.
Sedimentary Structures
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Some common sedimentary structures include:
Bedding (stratification)
Lamination
Graded Bedding
Cross-Bedding
Ripple Marks
Mud Cracks
Raindrop Impressions
Nodules, concretions, geodes, and Stromatolites.
Framework and Matrix
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The grains in a sand-sized or coarser grained
sedimentary rock are known as the framework.
These grains are either:
1.
In contact with each other
2.
Surrounded by empty spaces (pore spaces)
3.
Surrounded by finer grained sediment (matrix)
4.
Cemented together by a bonding material (cement)
Framework and Matrix
From Immenhauser (2002)
Cements in Sedimentary Rocks
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The most common cements in sedimentary
rocks are:
Silica (quartz, opal, or chalcedony)
Carbonates (calcite, dolomite, or siderite)
Iron oxides (hematite)
Recognition of matrix and cement is not
always possible in hand specimens.
Assignment 3
References
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Compton, 1962. Manual of field geology.
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Folk, 1974. Petrology of sedimentary rocks.
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Folk, 1951. Stages of textural maturity in sedimentary rocks:
Journal of Sedimentary Petrology, v. 21.
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Immenhauser, 2002. Sedimentology for master students.
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Tucker, 2001. Sedimentary petrology: an introduction to the
origin of sedimentary rocks. Blackwell Science, London, UK.
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eos.ubc.ca
The End