6 Lab 6—Erosion, sedimentation, and sedimentary rocks

Figure 6.1. Close-up view of the sedimentary rock portion of the rock cycle.

Sedimentary rocks rock from the deposition, burial, cementation, and compaction or precipitation of sediments (Fig. 6.1). Referring back to the rock cycle as illustrated in Figure 3.1, these sediments may originate from any of the three types of rocks—igneous, sedimentary, or metamorphic. When the rocks are exposed at or near Earth’s surface, they experience weathering.

Weathering takes place when Earth surface materials such as rock, soil, and minerals are broken down, chemically or mechanically, into smaller pieces. Some examples of chemical weathering include dissolution, hydrolysis, and oxidation. Examples of mechanical weathering include frost wedging, root wedging, salt wedging, thermal expansion and contraction, exfoliation, and abrasion.

The products of weathering are removed from their source rock in a process called erosion and are then transported to their sink. This is the place where they are deposited. Sedimentary rocks are influenced by the composition of their source rock, the amount of erosion they undergo, and their depositional environment. For a schematic landscape diagram illustrating a variety of depositional environments, see Figure 6.2.

When sediments are buried and compacted, they lithify (or become rocks, from the Greek root word litho, meaning stone or rock). The processes of burial, compaction, and cementation that lead to lithification are collectively known as diagenesis. Diagenesis can occur deep below Earth’s surface and at high temperatures, but not so deep and hot as to cause metamorphosis.

Sedimentary rocks are classified into three main categories: siliciclastic, chemical, and biogenic (Table 6.1). Siliciclastic sedimentary rocks are formed from the fragments of mechanical erosion of other rocks. Siliciclastic rocks are then further classified according to the sizes of the grains that comprise the rock, and how rounded those grains are. Chemical rocks are initially classified by their chemical composition—carbonates are composed of calcite or dolomite; rock gypsum is made of gypsum; and rock salt is made

of halite. Carbonates are further divided according to grain size and presence or lack of visible fossils or ooids. The final category, biogenic sedimentary rocks, is formed as a result of the accumulation of organic matter. The two rocks we are concerned with in this category are bituminous coal and biogenic chert.

Figure 6.2. Sedimentary depositional environments (Mikenorton, 2018).

Clastic rocks: how do we measure the grain size of the sediments? Chester K. Wentworth devised a classification scheme to name the grains of sediments based on the size range of the diameters of the grains (Table 6.1). Grains are further described by how much they have been eroded. Those grains that were recently removed from their source rock and have not been transported are angular. They then receive labels of sub-angular, sub-rounded, or rounded, with increasing erosion and transport distance (Fig. 6.3).

Table 6.1. Wentworth (1922) grain-size classification.

Figure 6.3. Illustration of terms describing clastic grain rounding.

Table 6.2. Sedimentary rock identification table

REFERENCES

Ralph L. Dawes, Ph.D. and Cheryl D. Dawes, 2013, “Basics—Sedimentary Rocks,” in Geology 101—Introduction to Physical Geology, https://commons.wvc.edu/rdawes/g101ocl/basics/sedimentary.html; last access: 2022-07-07.

Mikenorton, 2018, “SedimentaryEnvironment.jpg,” derived from: “Principales medios sedimentarios.svg”, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=68836539; last access: 2022-07- 06.

Wentworth, Chester K. “A Scale of Grade and Class Terms for Clastic Sediments.” The Journal of Geology 30, no. 5 (1922): 377–92.