Sedimentary Environments

The setting in which a sedimentary rock forms is called the sedimentary environment. Every environment has a characteristic combination of geologic processes and circumstances. The type of sediment that is deposited is not only dependent on the sediment that is transported to a place, but also on the environment itself.

Figure Above: Different sedimentary environments are associated with a stream moving in the ocean. They include a delta where the river fans out (shallow marine), a tidal flat (shallow marine), and the deep ocean.

Marine environment means the rock was formed in a sea or ocean. Often, a distinction is made between deep and shallow marine environments.

• Deep marine usually refers to environments more than 200 m below the water surface. The water current over the sea bottom is small in deep marine environments. Only fine particles can be transported to such places. Typically sediments depositing on the ocean floor are fine clay or small skeletons of micro-organisms.
• Shallow marine environments exist adjacent to coastlines and can extend out to the boundaries of the continental shelf. The water in such environments has generally higher energy than that in deep environments because of wave activity. This means coarser sediment particles can be transported, and the deposited sediment can be coarser than in deep environments. When the available sediment is transported from the continent, an alternation of sand, clay, and silt is deposited. When the continent is far away, the amount of such sediment may be small, and biochemical processes dominate the type of rock that forms. Especially in warm climates, shallow marine environments far offshore mainly see the deposition of carbonate rocks. The shallow, warm water is an ideal habitat for many small organisms that build carbonate skeletons. When these organisms die, their skeletons sink to the bottom, forming a thick layer of calcareous mud that may lithify into limestone. Warm shallow marine environments are also ideal for coral reefs, where the sediment consists mainly of the calcareous skeletons of larger organisms.

Figure Above: More sedimentary environments are depicted. First, there is a gentle, meandering river. Second a river delta, third is a lake, and fourth is wetlands.

Meandering River

• Rocks created: mudstone (shale)

Delta

• Where along a coast (either the coast of a sea or a lake) rivers enter the body of water, deltas can form. Large accumulations of sediment transported from the continent to places in front of the mouth of the river. Deltas are dominantly composed of clastic sediment.
• Rocks created: sandstone, mudstone (shale)

Wetlands, Lake, & Lagoons

• Fine sediment is deposited in the quiet water of swamps, lakes, and lagoons, mingled with organic material from dead plants and animals.
• Rocks created: mudstone (shale), coal.

Figure Above: More shallow marine environments are shown. They include a sandy beach, sandy coastal dunes, a calm lagoon, and a coral reef.

Beach

• The coast is an environment dominated by wave action. At the beach, predominantly coarse sediment like sand or gravel is deposited, often mingled with shell fragments.
• Rocks created: Sandstone, conglomerate.

Coral Reef

• Rocks created: Limestone

Figure Above: More high-energy sedimentary environments are depicted. They include mountain slopes with rocky, angular sediment, glaciers with angular sediment, steep streams or rivers with large stones, and dunes.

Mountains & Glaciers

• The sediment tends to be angular because it has not traveled far. Sediment transported by a glacier is called glacial and is characterized by very poor sorting.
• Rocks created: Breccia

Rivers

• In rivers, the energy of the water is much higher, and the transported material consists of clastic sediment that is rounded in shape with a large range of clast sizes.
• Rocks created: Conglomerate

Coastal Dunes & Dessert Dunes

• Sediment transported by wind is called Aeolian and is sand-sized and always very well sorted.
• Rocks created: Sandstone.