Sedimentary Environments

Between the source area where sediments are formed and the sedimentary basin where they are buried and converted to sedimentary rocks, sediments travel through many sedimentary environments. A sedimentary environment is an area of sediment deposition characterized by a particular combination of climate conditions and physical, chemical, and biological processes (Figure 5.9). Important characteristics of sedimentary environments include the following:

• The type and amount of water (ocean, lake, river, arid land)
• The type and strength of transport agents (water, wind, ice)
• The topography (lowland, mountain, coastal plain, shallow sea, deep sea)
• Biological activity (precipitation of shells, growth of coral reefs, churning of sediments by burrowing organisms)
• The plate tectonic settings of sediment source areas (volcanic mountain belt, continent-continent collision zone) and sedimentary basins (rift, thermal subsidence, flexural)
• The climate (cold climates may form glaciers; arid climates may form deserts where minerals precipitate by evaporation)

Figure 5.9: Multiple factors interact to create sedimentary environments.

Consider the beaches of Hawaii, famous for their unusual green sands, which are a result of their distinctive sedimentary environment. The volcanic island of Hawaii is composed of olivine-bearing basalt, from which the olivine is released during weathering. Rivers transport the olivine to the beach, where waves and wave-produced currents concentrate the olivine and remove fragments of basalt to form olivine-rich sand deposits.

Sedimentary environments are often grouped by location: on continents, near shorelines, or in the ocean. This very general subdivision highlights the processes that give sedimentary environments their distinct identities.

Continental Sedimentary Environments

Sedimentary environments on continents are diverse due to the wide variation in temperature and rainfall over the land surface. These environments are built around lakes, rivers, deserts, and glaciers (see Figure 5.9).

• Lake environments include inland bodies of fresh or saline water in which the transport agents are relatively small waves and moderate currents. Chemical sedimentation of organic matter and carbonate minerals may occur in freshwater lakes. Saline lakes such as those found in deserts evaporate and precipitate a variety of evaporite minerals, such as halite. The Great Salt Lake in Utah is an example.
• Alluvial environments include the channel of a river, its borders and associated wetlands, and the flat valley floor on either side of the channel that is covered by water when the river floods (the floodplain). Rivers are present on all the continents except Antarctica, so alluvial deposits are widespread. Organisms are abundant in muddy flood deposits and produce organic sediments that accumulate in wetlands adjacent to river channels. Climates vary from arid to humid. An example is the Mississippi River and its floodplains.
• Desert environments are arid. Wind and the rivers that flow intermittently through deserts transport sand and dust. The dry climate inhibits abundant organic growth, so organisms have little effect on the sediments. Desert dune fields are an example of such an environment.
• Glacial environments are dominated by the dynamics of moving masses of ice and are characterized by a cold climate. Vegetation is present, but has little effect on sediments. At the melting border of a glacier, meltwater streams form a transitional alluvial environment.

Shoreline Sedimentary Environments

The dynamics of waves, tides, and river currents on sandy shores dominate shoreline sedimentary environments (see Figure 5.9):

• Deltas, where rivers enter lakes or oceans
• Tidal flats, where extensive areas exposed at low tide are dominated by tidal currents
• Beaches, where the strong waves approaching and breaking on the shore distribute sediments on the beach, depositing strips of sand or gravel

In most cases, the sediments that accumulate in shoreline environments are siliciclastic. Organisms affect these sediments mainly by burrowing into and mixing them. However, in some tropical and subtropical settings, sediment particles, particularly carbonate sediments, may be of biological origin. These biological sediments are also subject to transportation by waves and tidal currents.

Marine Sedimentary Environments

Marine sedimentary environments are usually classified by water depth, which determines the kinds of currents that are present (see Figure 5.9). Alternatively, they can be classified by distance from land.

• Continental shelf environments are located in the shallow waters off continental shores, where sedimentation is controlled by relatively gentle currents. Sediments may be composed of either siliciclastic or biological carbonate particles, depending on how much siliciclastic sediment is supplied by rivers and on the abundance of carbonate-producing organisms. Sedimentation may also be chemical if the climate is arid and an arm of the sea becomes isolated and evaporates.
• Organic reefs are carbonate structures, formed by carbonate-secreting organisms, built up on continental shelves or on oceanic volcanic islands.
• Continental margin and slope environments are found in the deeper waters at and off the edges of continents, where sediments are deposited by turbidity currents. A turbidity current is a turbulent submarine avalanche of sediment and water that moves downslope. Most sediments deposited by turbidity currents are siliciclastic, but at sites where organisms produce abundant carbonate sediments, continental slope sediments may be rich in carbonates.
• Deep-sea environments are found far from continents, where the waters are much deeper than the reach of waves and tidal currents. These environments include the lower portion of the continental slope, which is built up by turbidity currents traveling far from continental margins; the abyssal plain of the deep sea, which accumulates carbonate sediments provided mostly by the shells of plankton; and the mid-ocean ridges.

Siliciclastic versus Chemical and Biological Sedimentary Environments

Sedimentary environments can be grouped not only by their location, but also according to the kinds of sediments found in them or according to the dominant sediment formation process. Grouping of sedimentary environments in this manner produces two broad classes: siliciclastic sedimentary environments and chemical and biological sedimentary environments.

Siliciclastic sedimentary environments are those dominated by siliciclastic sediments. They include all of the continental sedimentary environments as well as those shoreline environments that serve as transitional zones between continental and marine environments. They also include those marine environments of the continental shelf, continental margin and slope, and deep seafloor where siliciclastic sands and muds are deposited (Figure 5.10). The sediments of these siliciclastic environments are often called terrigenous sediments to indicate their origin on land.

Figure 5.10: These sedimentary rocks exposed at El Capitan, in the Guadalupe Mountains of West Texas, were formed in an ancient ocean about 260 million years ago. The lower slopes of the mountains contain siliciclastic sedimentary rocks formed in deep-sea environments. The overlying cliffs of El Capitan are limestone and dolostone, formed from sediments deposited in a shallow sea when carbonate-secreting organisms died, leaving their shells in the form of a reef.

Chemical and biological sedimentary environments are characterized principally by chemical and biological precipitation (Table 5.2).

Carbonate environments are marine settings where calcium carbonate, mostly secreted by organisms, is the main sediment. They are by far the most abundant chemical and biological sedimentary environments. Hundreds of species of mollusks and other invertebrate animals, as well as calcareous (calcium-containing) algae and microorganisms, secrete carbonate shells or skeletons. Various populations of these organisms live at different depths, both in quiet areas and in places where waves and currents are strong. As they die, their shells and skeletons accumulate to form carbonate sediments.

Except for those of the deep sea, carbonate environments are found mostly in the warmer tropical and subtropical regions of the oceans, where carbonate-secreting organisms flourish. These environments include organic reefs, carbonate sand beaches, tidal flats, and shallow carbonate platforms. In a few places, carbonate sediments form in cooler waters that are supersaturated with carbonate ions—waters that are generally below 20°C, such as in some regions of the Southern Ocean south of Australia. These carbonate sediments are formed by a very limited group of organisms, most of which secrete calcite shells.

Siliceous environments are unique deep-sea sedimentary environments named for the silica shells deposited in them. The planktonic organisms that secrete these silica shells grow in surface waters where nutrients are abundant. When they die, their shells settle to the deep seafloor and accumulate as layers of siliceous sediments.

An evaporite environment is created when the warm seawater of an arid inlet or arm of the sea evaporates more rapidly than it can mix with seawater from the open ocean. The degree of evaporation and the length of time it has proceeded control the salinity of the evaporating seawater and thus the kinds of chemical sediments formed. Evaporite environments also form in lakes lacking river outlets. Such lakes may produce sediments of halite, borate, nitrates, and other salts.