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Lec 8 b - sedimentary rocks - Geology for 1th year | Zagazig university.

Ahmed Alkawaliny
Ahmed Alkawaliny

Lec 8 b - sedimentary rocks - Geology for 1th year | Zagazig university. Outlines: _ origin. Diagenesis. Grain size. Naming and Classifying Sedimentary Rocks Texture and Weathering

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Geology_______________________________________________________sedimentary Rocks Page | 1 Geology Sedimentary Rocks ____________________ Outlines: _ origin. Diagenesis. Grain size. Naming and Classifying Sedimentary Rocks Texture and Weathering Origin: 1- Mechanical physical = Clastic. 2- Chemical = Non clastic. a) Organic b) Inorganic
Geology_______________________________________________________sedimentary Rocks Page | 2 Diagenesis: Diagenesis collectively refers to the physical, chemical, and biological changes which may occur during the formation of sedimentary rocks. Recrystallization, compaction, cementation, and lithification, are all examples of diagenetic changes. Recrystallization occurs when unstable minerals recrystallize to form more stable minerals. Recrystallization most often occurs during the formation of chemical sedimentary limestone rocks that previously contained aragonite a chemically unstable form of calcium carbonate (CaCO3). Compaction occurs when sediments are progressively deposited on top of one another, and over time the weight of the accumulated sediments increases and compresses the buried sediments. Continued compression of buried
Geology_______________________________________________________sedimentary Rocks Page | 3 sediments reduces pore-spaces and removes excess water, as a result the closely packed individual grains begin to slowly compact into a solid rock. Cementation involves a chemical change whereby individual grains are cemented together as minerals are precipitated out of saturated solution that is percolating as a matrix between individual sediments. The accumulation of the precipitated minerals causes the grains to cement together. Cementation can occur in combination with the presence of other minerals, rock fragments, or organic constituents such as fossilized organisms. Lithification occurs when unconsolidated sediments are cohesively bound to form a solid sedimentary rock. Compaction and/or cementation are generally the precursor to the lithification process.
Geology_______________________________________________________sedimentary Rocks Page | 4 Grain size: Grain size is used to describe the size of the individual mineral grains, rock fragments, or organic material that are cemented together to form a clastic or chemical sedimentary rock Grain Size Categories Grain Size Divisions Particle size Pebbles, cobbles, boulders Gravel- > 2mm- 0. Sand- 2mm 063mm Silt - 0.063mm - 0.004mm Clay- < 0.004
Geology_______________________________________________________sedimentary Rocks Page | 5
Geology_______________________________________________________sedimentary Rocks Page | 6 Naming and Classifying Sedimentary Rocks Geologists name and classify sedimentary rocks based on their mineral composition and texture Mineral composition refers to the specific minerals in the rock. For example sandstone will contain predominantly quartz, while limestone will contain mainly calcite (calcium carbonate). Texture includes the grain size and shape, sorting, and rounding of the sediments that form the rock. very coarse-grained > 16 mm coarse grained > 2 mm < 16 mm medium grained > 0.25 mm < 2 mm fine grained > 0.032 mm < 0.25 mm very fine-grained > 0.0004 mm < 0.032 mm
Geology_______________________________________________________sedimentary Rocks Page | 7 Texture and Weathering Sorting Sorting is used to describe the grain size distribution or range of grain sizes in a rock. a) Poorly sorted rocks contain a variety of different sized grains. Poorly sorted rocks contain a wide range of grain sizes including fine, medium, b)and coarse. c) Well sorted rocks contain almost all grains of the same size. d) Moderately sorted rocks contain particles of relatively similar grain sizes. Moderately sorted rocks may contain fine and medium grains, or medium and coarse grains.
Geology_______________________________________________________sedimentary Rocks Page | 8 Rounding Rounding is used to describe the relative shape of the grains. Classifications are describe as deviations from rounded or spheroidal grain shapes. a) Well rounded grains are smooth with rounded edges. b) Moderately rounded grains are in-between the sharp, angular edges of a poorly rounded grain and the smooth, roundness of a well-rounded grain. c) Poorly rounded grains may be sharp or angular.

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Lec 8 b - sedimentary rocks - Geology for 1th year | Zagazig university.

  • 1. Geology_______________________________________________________sedimentary Rocks Page | 1 Geology Sedimentary Rocks ____________________ Outlines: _ origin. Diagenesis. Grain size. Naming and Classifying Sedimentary Rocks Texture and Weathering Origin: 1- Mechanical physical = Clastic. 2- Chemical = Non clastic. a) Organic b) Inorganic
  • 2. Geology_______________________________________________________sedimentary Rocks Page | 2 Diagenesis: Diagenesis collectively refers to the physical, chemical, and biological changes which may occur during the formation of sedimentary rocks. Recrystallization, compaction, cementation, and lithification, are all examples of diagenetic changes. Recrystallization occurs when unstable minerals recrystallize to form more stable minerals. Recrystallization most often occurs during the formation of chemical sedimentary limestone rocks that previously contained aragonite a chemically unstable form of calcium carbonate (CaCO3). Compaction occurs when sediments are progressively deposited on top of one another, and over time the weight of the accumulated sediments increases and compresses the buried sediments. Continued compression of buried
  • 3. Geology_______________________________________________________sedimentary Rocks Page | 3 sediments reduces pore-spaces and removes excess water, as a result the closely packed individual grains begin to slowly compact into a solid rock. Cementation involves a chemical change whereby individual grains are cemented together as minerals are precipitated out of saturated solution that is percolating as a matrix between individual sediments. The accumulation of the precipitated minerals causes the grains to cement together. Cementation can occur in combination with the presence of other minerals, rock fragments, or organic constituents such as fossilized organisms. Lithification occurs when unconsolidated sediments are cohesively bound to form a solid sedimentary rock. Compaction and/or cementation are generally the precursor to the lithification process.
  • 4. Geology_______________________________________________________sedimentary Rocks Page | 4 Grain size: Grain size is used to describe the size of the individual mineral grains, rock fragments, or organic material that are cemented together to form a clastic or chemical sedimentary rock Grain Size Categories Grain Size Divisions Particle size Pebbles, cobbles, boulders Gravel- > 2mm- 0. Sand- 2mm 063mm Silt - 0.063mm - 0.004mm Clay- < 0.004
  • 6. Geology_______________________________________________________sedimentary Rocks Page | 6 Naming and Classifying Sedimentary Rocks Geologists name and classify sedimentary rocks based on their mineral composition and texture Mineral composition refers to the specific minerals in the rock. For example sandstone will contain predominantly quartz, while limestone will contain mainly calcite (calcium carbonate). Texture includes the grain size and shape, sorting, and rounding of the sediments that form the rock. very coarse-grained > 16 mm coarse grained > 2 mm < 16 mm medium grained > 0.25 mm < 2 mm fine grained > 0.032 mm < 0.25 mm very fine-grained > 0.0004 mm < 0.032 mm
  • 7. Geology_______________________________________________________sedimentary Rocks Page | 7 Texture and Weathering Sorting Sorting is used to describe the grain size distribution or range of grain sizes in a rock. a) Poorly sorted rocks contain a variety of different sized grains. Poorly sorted rocks contain a wide range of grain sizes including fine, medium, b)and coarse. c) Well sorted rocks contain almost all grains of the same size. d) Moderately sorted rocks contain particles of relatively similar grain sizes. Moderately sorted rocks may contain fine and medium grains, or medium and coarse grains.
  • 8. Geology_______________________________________________________sedimentary Rocks Page | 8 Rounding Rounding is used to describe the relative shape of the grains. Classifications are describe as deviations from rounded or spheroidal grain shapes. a) Well rounded grains are smooth with rounded edges. b) Moderately rounded grains are in-between the sharp, angular edges of a poorly rounded grain and the smooth, roundness of a well-rounded grain. c) Poorly rounded grains may be sharp or angular.