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6.3: Depositional Environments and Sedimentary Basins

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    Sediments accumulate in a wide variety of environments, both on the continents and in the oceans. Some of the more important of these environments are illustrated in Figure \(\PageIndex{1}\).

    Figure \(\PageIndex{1}\) Some of the important depositional environments for sediments and sedimentary rocks.

    Table 6.4 provides a summary of the processes and sediment types that pertain to the various depositional environments illustrated in Figure \(\PageIndex{1}\). We’ll look more closely at the types of sediments that accumulate in these environments in the last section of this chapter. The characteristics of these various environments, and the processes that take place within them, are also discussed in later chapters on glaciation, mass wasting, streams, coasts, and the sea floor.

    Table 6.4 The important terrestrial depositional environments and their characteristics
    Environment Important transport processes Depositional environments Typical sediment types
    Glacial gravity, moving ice, moving water valleys, plains, streams, lakes glacial till, gravel, sand, silt, and clay
    Alluvial gravity steep-sided valleys coarse angular fragments
    Fluvial moving water streams gravel, sand, silt, and organic matter (in swampy parts only)
    Aeolian wind deserts and coastal regions sand, silt
    Lacustrine moving water (flowing into a lake) lakes sand (near the edges only), silt, clay, and organic matter
    Evaporite moving water (flowing into a lake) lakes in arid regions salts, clay
    Table 6.5 The important marine depositional environments and their characteristics
    Environment Important Transport Processes Depositional Environments Typical Sediment Types
    Deltaic moving water deltas sand, silt, clay, and organic matter (in swampy parts only)
    Beach waves, longshore currents beaches, spits, sand bars gravel, sand
    Tidal tidal currents tidal flats silt, clay
    Reefs waves and tidal currents reefs and adjacent basins carbonates
    Shallow water marine waves and tidal currents shelves and slopes, lagoons carbonates in tropical climates, sand/silt/clay elsewhere
    Lagoonal little transportation lagoon bottom carbonates in tropical climates
    Submarine fan underwater gravity flows continental slopes and abyssal plains gravel, sand, mud
    Deep water marine ocean currents deep-ocean abyssal plains clay, carbonate mud, silica mud

    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 because they have only been deposited relatively recently—perhaps a few centuries or millennia ago—and are likely to 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 that will last that long. Most such basins are formed by plate tectonic processes, and some of the more important examples are shown in Figure \(\PageIndex{2}\).

    Figure \(\PageIndex{2}\) Some of the more important types of tectonically produced basins: (a) trench basin, (b) forearc basin, (c) foreland basin, and (d) rift basin.

    Trench basins form where a subducting oceanic plate dips beneath the overriding continental or oceanic crust. They can be several kilometers deep, and in many cases, host thick sequences of sediments from 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 is a forearc basin. A foreland basin is caused by the mass of the volcanic range depressing the crust on either side. Foreland basins are not only related to volcanic ranges, but can form adjacent to fold belt mountains like the Canadian Rockies. A rift basin forms where continental crust is being pulled apart, and the crust on both sides of the rift subsides. As rifting continues this eventually becomes a narrow sea, and then an ocean basin. The East African rift basin represents an early stage in this process.

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