Sedimentary Petrology GEO 333 Lab 2 Basic Classification

Sedimentary Petrology GEO 333 Lab (2) Basic Classification 2009 Mansour Al-Hashim

Preview of Lab 1 n Introduction n Main minerals of sedimentary rocks

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

Introduction to Classification 1. A classification of rocks represents a theory of how these rocks were formed, and how they relate to each other. 2. Many classifications have been proposed for sedimentary rocks. 3. Since 1950, more than 50 classifications of sandstones alone have been proposed. 4. No one classification is fully satisfactory. 5. The type of classification depends on the purpose of the study being conducted.

A Good Classification 1. Should be as inclusive as possible. 2. Includes as much variety as possible. 3. Leaves only few or no orphan rocks. 4. Dose not contain any internal contradictions. n We will see several classification systems, ranging from basic to sophisticated.

Sedimentary Rock Classification n a) b) c) Sedimentary rocks are generally grouped into three main categories: Siliciclastic (clastic) rocks Chemical rocks Biochemical rocks

1. Simple Classification of Sedimentary Rocks

Modified from Boggs (1992)

Clastic (terrigenous/detrital) Sedimentary Rocks 1. Derived from pre-existing rocks. 2. Composed of physically derived components. 3. Sometimes referred to as extrabasinal. 4. Grains are mechanically durable and chemically stable. 5. Clasts are made of quartz, feldspar, muscovite, clay minerals, and rock fragments. 6. Silica (Si. O 2) is the major component. 7. Classified based on texture (grain size). 8. Examples: breccia, conglomerate, sandstone, siltstone, and shale.

Basic Classification of Clastic Sedimentary Rocks

Breccia from geology. com Conglomerate

Chemical and Biochemical Sedimentary Rocks 1. Derived from precipitation of dissolved minerals in the solution. a) Direct precipitation from seawater. The result is Chemical Rocks. b) Organism extraction of the dissolved minerals from the sea. The result is Biochemical Rocks. 2. Classified based on the chemical composition. 3. Examples: limestone, chert, and chalk.

Basic Classification of Chemical and Biochemical Sedimentary Rocks

Chert from geology. com Coquina

Fundamental Constituents of Sedimentary Rocks Sediments consist of three components: A. Terrigenous components B. Allochemical components C. Orthochemical components n They may be mixed in almost all proportions.

From Folk (1974)

(A) Terrigenous components n Derived from erosion of areas outside the depositional basin. n Brought to the basin as solids. n Examples: quartz and feldspar sand, heavy minerals, clay minerals, and chert and limestone clasts.

(B) Allochemical components n Greek: “allo” meaning different from normal. n Precipitated directly from solution within the depositional basin. n Termed “allo” because they have been moved as solids within the basin. n Examples: broken or intact shells, oolites, calcareous pellets, or fragments of carbonate sediments.

(C) Orthochemical components n Greek: “ortho” meaning true. n Termed “ortho” because they are produced chemically within the basin. n Examples: microcrystalline calcite or dolomite ooze, some evaporites, and cements in sandstones.

Notes n Classes (B) and (C) are referred to as “chemical” constituents. n Classes (A) and (B) may be termed “fragmental”.

Basic Classification of Sedimentary Rocks (1) n Sedimentary rocks are divided into five classes based on proportions of the fundamental components: 1. T (Terrigenous Rocks) 2. IA (Impure Allochemical Rocks) 3. IO (Impure Orthochemical Rocks) 4. A (Allochemical Rocks) 5. O (Orthochemical Rocks)

From Folk (1974)

Basic Classification of Sedimentary Rocks (2) n Terrigenous Rocks (T) Examples: mudrocks, sandstones, and conglomerates. n Impure Allochemical Rocks (IA) Examples: very fossiliferous shales; sandy fossiliferous or oolitic limestones. n Impure Orthochemical Rocks (IO) Examples: clayey microcrystalline limestones.

Basic Classification of Sedimentary Rocks (3) n Allochemical Rocks (A) Examples: fossiliferous, oolitic limestones and dolomites. n Orthochemical Rocks (O) Examples: limestone, dolomite, anhydrite, and chert.

Assignment (2) No. Rock Name Origin (clastic, chemical, biochemical)

References n Journal of Geological Education, 1991, v. 39, p. 382. n Petrology of Sedimentary Rocks, Boggs, 1992. n Petrology of Sedimentary Rocks, Folk, 1974. n csmres. jmu. edu n geology. com

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