9.4 Depositional Environments and Sedimentary Basins

The setting in which sediments are accumulated is called a depositional environment. (Some of the more important of these environments are illustrated in Figure 9.19.) Thus far you’ve seen that some types of sedimentary rocks—coal, and gypsum, for example—require very specific conditions to form because particular biological processes or chemical reactions are necessary. Different types of clastic sedimentary rocks also form in particular depositional environments. In the case of clastic rocks, the key environmental conditions are related to the amount of energy available to transport sediments, and how far the sediments get from their source before being deposited.

Broadly, depositional environments can be said to be terrestrial, marine, or to reflect a transitional zone between the two. Terrestrial refers to depositional environments on land. These may be depositional environments such as deserts, found on dry land, but they could also be environments such as freshwater lakes or rivers. Marine refers to environments associated with saltwater seas and oceans. Transitional depositional environments include environments such as deltas, where freshwater rivers empty into saltwater seas or oceans.

Figure 9.19 Some of the important depositional environments for sediments and sedimentary rocks. Source: Karla Panchuk (2021) CC BY-SA 4.0. Modified after Mike Norton (2018) CC BY-SA 3.0. Image source.

Tables 9.1 and 9.2 provide a summary of the processes and sediment types that pertain to the various depositional environments illustrated in Figure 9.19. The types of sediments that accumulate in these environments are examined in more detail in the last section of this chapter.

Table 9.1 Terrestrial Depositional Environments. Source: Karla Panchuk (2018), CC BY 4.0. Modified after Steven Earle (2015), CC BY 4.0. Data source.
Environment Key Transport Processes Depositional Settings Typical Sediments
Glacial Gravity, moving ice, moving water Valleys, plains, streams, lakes Glacial till, gravel, sand, silt, clay
Alluvial Gravity, moving water Where steep-sided valleys meet plains Coarse angular fragments
Fluvial Moving water Streams Gravel, sand, silt, organic matter
Aeolian Wind Deserts and coastal regions Sand, silt
Lacustrine Moving Water Lakes Sand, silt, clay, organic matter
Evaporite Still water Lakes in arid regions Salts, clay
Table 9.2 Marine & Transitional Depositional Environments Source: Karla Panchuk (2018), CC BY 4.0. Modified after Steven Earle (2015), CC BY 4.0. Data source
Environment Key Transport Processes Depositional Settings Typical Sediments
Deltaic Moving water Deltas Sand, silt, clay, organic matter
Beach Waves, long-shore currents Beaches, spits, sand bars Gravel, sand
Tidal Tidal currents Tidal flats Fine-grained sand, silt, clay
Reef Waves, tidal currents Reefs and adjacent basins Carbonates
Shallow marine Waves, tidal currents Shelves, slopes, lagoons Carbonates in tropical climates; sand/silt/clay elsewhere.
Lagoonal Little transportation Lagoon bottom Carbonates in tropical climates, silt, clay
Submarine fan Underwater gravity flows Continental slopes, abyssal plains Gravel, sand, silt, clay
Deep water Ocean currents Deep-ocean abyssal plains Clay, carbonate mud, silica mud

Practice with Depositional Environments

Sedimentary Basins Are Needed to Collect Sediment

Most of the sediments that you might see around you, including talus on steep slopes, sand bars in streams, or gravel in road cuts, will never become sedimentary rocks. This is because they’ve only been deposited relatively recently—perhaps a few centuries or millennia ago—and will be re-eroded before they are buried deep enough beneath other sediments to be lithified. In order for sediments to be preserved long enough to be turned into rock (a process that takes millions or tens of millions of years) they need to have been deposited in a basin in which sediments can be preserved for that long. Most such basins are formed by plate tectonic processes (Figure 9.20).

Figure 9.20 Some types of tectonically produced basins: (a) trench basin, (b) forearc basin, (c) foreland basin, and (d) rift basin. Source: Steven Earle (2015), CC BY 4.0. Image source.

Trench basins form where a subducting oceanic plate dips beneath the overriding continental or oceanic lithosphere. They can be several kilometres deep, and in many cases, host thick sequences of sediments from nearby eroding coastal mountains. There is a well-developed trench basin off the west coast of Vancouver Island.

A forearc basin lies between the subduction zone and the volcanic arc, and may be formed in part by friction between the subducting plate and the overriding plate, which pulls part of the overriding plate down. The Strait of Georgia, the channel between Vancouver Island and the BC mainland, is a forearc basin.

A foreland basin is caused by the mass of a mountain range depressing the crust. A rift basin forms where continental crust is being pulled apart, and the crust on both sides the rift subsides. If rifting continues this will eventually becomes a narrow sea, and then an ocean basin. The East African rift basin represents an early stage in this process.

Practice with Types of Sedimentary Basins

Fill in the missing words to complete the descriptions of the different types of sedimentary basins.

One type of sedimentary basin forms when continental crust begins to split apart and sag downward. This is a              basin.

Other types of sedimentary basins are related to subduction zones. The deepest type, a              basin, occurs right where subduction is happening.

If a subducting plate drags down the front edge of the plate with which it’s colliding, a              basin could form between the subduction zone and a chain of volcanic mountains.

Thanks to isostasy, the weight of the mountains themselves can flex the lithosphere downward into a              basin.

To check your answers, navigate to the below link to view the interactive version of this activity.


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