Sedimentary Petrology GEO 333 Lab (3) Clastic Sedimentary Rocks (Classification & Description) 2009 Mansour Al-Hashim

Preview of Lab 2 Classification in general Classification of sedimentary rocks Clastic sedimentary rocks Chemical and biochemical sedimentary rocks Fundamental constituents of sedimentary rocks

Objectives of Lab 3 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 Grains are stuck together. Characteristic of clastic sedimentary rocks. Examples: sandstone and conglomerate. 2. Non-clastic (crystalline) texture 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: Direct measurement with calipers or meter sticks. For particles larger than several centimeters. Screening and petrographic microscope. For particles from 2 mm to about 1/16 mm. Pipette or hydrometer (settling rates in water) For particles less than 1/16 mm.

Grain Size (2) Results of grain-size analysis can be demonstrated as histograms, cumulative curves, or frequency curves. Many statistical parameters can be calculated (e.g., median, mean, skewness, kurtosis, etc.). Used to characterize depositional processes and depositional environments. Udden-Wentworth grain-size scale is the most widely used.

Wentworth grade scale

Modified Udden-Wentworth grain-size Scale

Simplified grain-size scale

Roundness and Sorting Roundness (angularity) Sorting Indicates how smooth or sharp the edges and corners of particles are. Usually measured with reference to a comparative chart (the commonest is Powers’ , 1953). Sorting 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. 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.

Textural Maturity From Folk (1951)

Color Organic matter, sulfides, and some iron oxides give rocks a dark color. Indicating deposition in a reducing environment. Some iron oxides give rocks red or yellowish color. Indicating deposition in an oxidizing environment.

Color Indications Black - organic matter FeS Yellowish - pyrite, markasite FeS2 Yellowish or bluish - sulphates, carbonates, and chlorides Yellowish to brownish - limonite, goethite FeO(OH) Reddish - iron oxides FeO

From eos.ubc.ca

Cohesiveness (firmness) The degree to which the particles stick together. Cohesiveness can be described as: Fragile Soft Intermediate Hard Very hard

Mineral Composition 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 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 The grains in a sand-sized or coarser grained sedimentary rock are known as the framework. These grains are either: In contact with each other Surrounded by empty spaces (pore spaces) Surrounded by finer grained sediment (matrix) Cemented together by a bonding material (cement)

Framework and Matrix From Immenhauser (2002)

Cements in Sedimentary Rocks 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 Compton, 1962. Manual of field geology. Folk, 1974. Petrology of sedimentary rocks. Folk, 1951. Stages of textural maturity in sedimentary rocks: Journal of Sedimentary Petrology, v. 21. Immenhauser, 2002. Sedimentology for master students. Tucker, 2001. Sedimentary petrology: an introduction to the origin of sedimentary rocks. Blackwell Science, London, UK. eos.ubc.ca

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