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5: Siliciclastic Sedimentary Rocks

Last updated

Mar 2, 2025
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- 4.4: Erosional and Post-Depositional Structures
- 5.1: Sandstones

Michael Rygel and Page Quinton
SUNY Potsdam

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Siliciclastic sedimentary rocks are made of solid particles that were transported as suspended load, bedload, or under the direct influence of gravity. In all cases, the clastic particles will settle and come to rest when motion ceases.

Grain size is the most important characteristic in the classification of clastic sediments and sedimentary rocks. As discussed in Chapter 3, unconsolidated sediment can be classified as gravel, sand, or mud; these size fractions make up conglomerates, sandstones, and mudrocks, respectively. More specific names for the different size fractions and sedimentary rocks are provided in the table below.

Although dissolved materials are the primary building blocks for carbonates and other types of chemical and biochemical sedimentary rocks, dissolved materials may form mineral cements that bind clastic grains during the transformation from sediment to sedimentary rock.

Table $\PageIndex{1}$ : Grain size classification from Wentworth (1922) and phi scale from Krumbein (1934). Settling velocities from http://www.filtration-and-separation...g/settling.htm and entrainment (erosion) velocities from http://en.Wikipedia.org/wiki/File:We...Size-Chart.pdf; both assume spherical particles of quartz.

Learning Objectives

Identify the main components of sandstones in thin section and use this information to name them.
Use appropriate terminology to described conglomerates and breccias
Use appropriate terminology to described mudrocks and make interpretations based on mudrock color.
Explain the four main types of clay minerals and list some economically important examples.
Explain the possible origins of diamictites and sandstones/mudrocks with outsized clasts.

Topic hierarchy

5.1: Sandstones
Sandstones are made largely of sand-sized grains between 1/16 and 2 mm. They are composed of framework grains and have interstitial matrix, cement, and or pore space between these grains. They are classified based on the relative abundance of quartz, feldspar, and lithic grains and the nature of the interstitial materials.
5.2: Conglomerates and Breccias
Conglomerates and breccias are made mostly of gravel-sized clasts (>2 mm). Conglomerates are made of rounded clasts and breccias are made of angular clasts. They are classified primarily on clast shape, composition and origin. The support mechanism of the clasts and the orientation of clasts can provide important clues about depositional processes.
5.3: Mudrocks
Mudrocks are made of >50% particles in the mud size fraction (<1/16 mm). They can be subdivided into siltstones, mudstones, and claystones based on grain size. The term shale commonly refers to a mudrock that shows fissility. Mudrocks can be described based on grain size, texture, color, mineralogy, fossils, and a variety of other factors. The clay minerals in mudrocks can have physical properties that them economically important.
5.4: Diamictites, Pebbly Sandstones, and Outsized Clasts
Diamictites are mixtures of gravel, sand, and mud. They cannot be explained through traditional hydrodynamic explanations and can form only as the result of debris flows, glacial deposits, or bioturbation. Many pebbly sandstones are simply poorly sorted. However, sandstones and mudrocks with outsized clasts record rafting or volcanic bombs.

Chapter thumbnail shows a sandy, matrix-supported conglomerate (Michael Rygel via Wikimedia Commons; CC BY-SA 3.0).

Learning Objectives

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