Sedimentary environments

Sedimentary environments can be classified according to the climate in which they occur and/or the geometrical arrangement of the sediments.

In general, we classify ancient sedimentary rocks according to their similarity to current sedimentary environments. When describing sedimentary sequences, we usually do so in terms of facies (the sum total of all sedimentary features) columns in which the sequence is graphically illustrated from the base to the top. Facies columns include all details on sedimentary structures, variations in grain size, contacts between the individual beds and fossils present.

Rivers

In geology, river environments are known as fluvial environments. Within fluvial environments deposition occurs with normal settling of relatively coarse to medium-grained sediments on the river bottom and along bends in the river, and also during times of flooding when finer-grained sediments are deposited along the river banks.

Lakes

When rivers flow into lakes, the coarser-grained sediments are usually deposited at the toe of the deltas while the finer-grained sediments are carried in suspension to be deposited in the centre of the lakes. In many cases, the river breaks up into tributaries when it enters into the lake, and the banks between these tributaries are mainly composed of sand-sized sediments. As it contains a relatively high porosity, detrital organic material (particularly plant material) is deposited into the centre of the lakes along with the clay and silt-sized sediments.

River mouths

All major rivers (along with many minor ones) eventually reach the sea. As they approach the sea, they generally divide into many channels before eventually reaching it. The banks of these channels are generally composed of sand deposits along with minor mud which is deposited in times of flooding. Between the channels, shallow bays are filled by fine-grained sediments such as sands and muds. When the river eventually reaches the sea, the coarser sand-sized particles are deposited first (sometimes forming bars between the river mouth and sea) and the finer-grained silt and clay particles are deposited in deeper water out to sea

Coral reefs

Coral reefs only occur in warm shallow water environments. They are formed by the interaction of biological (i.e. corals) and sedimentary (i.e. silts and muds) processes. Many coral reefs are formed on pre-existing drowned volcanic necks (e.g. many of the islands in the Pacific have formed in this way) while others form on pre-existing masses of rock (e.g. the Great Barrier Reef). Coral reefs grow far more rapidly on the windward side as the currents driven by the wind bring in abundant nutrients.

Coral reefs are very fragile environments. As the coral itself is the shell of a living organism, it needs abundant sunlight (for photosynthesis) and nutrients. If the coral is blanketed in sediments (such as silts produced by rapid erosion caused by intensive irrigation on land), sunlight will not penetrate and the coral (and reef community) will die out. Also, if there is too much nutrient in the water (sometimes produced on land by farmers using fertilisers, e.g. super-phosphate, to increase their crop yield), other predators (such as the crown-of-thorns starfish) will move in, eat the coral and eventually sterilise the reef.

Glaciers

In polar and cold climates, glaciers are produced. As glaciers move downhill, they grind down any underlying rocks into fine powder and carry along with them any pebbles or boulders that have fallen into the glacier from avalanches and rockfalls. When the glacier starts to melt, these sediments are deposited at the foot of the glacier (particularly the coarser-grained sediments) and are called moraine deposits. After glaciation, suspended valleys known as hanging valleys are formed, along with steep waterfalls and alluvial fan deposits.

Arid environments

In arid environments, there is little available water or vegetation to act on the rocks and sediments present. Most weathering and erosion is caused by wind action and frost. Because of the lack of water and vegetation, sedimentary structures produced in these environments last for a relatively long time period. The Namib Desert of Namibia contains the world's largest sand dunes which are believed to be about 30 million years old. Most sediment deposits in arid regions are wind-blown in origin, and hence tend to be very well sorted. Other characteristic sedimentary deposits in arid environments are evaporite deposits that form in shallow lakes. Because of low rainfall the water table in these areas is very low, and soluble minerals occur near the surface and accumulate over time.

Tropical environments

In areas of very high rainfall (i.e. tropical areas) the rainfall is so high and the vegetation is so dense that there are very few visible rock outcrops. The soil profile is very deep because the interaction of rain with the vegetation forms humic acids that strongly leach and rapidly break down any rocks present. Sedimentary deposits in these environments are largely formed by mudslides, particularly if the topography is steep.


Ondine Evans , Web Researcher/Editor
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