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