Sedimentary Rocks - Clastic Rocks

Name ______Block ______Sedimentary Rocks - Clastic Rocks

All sedimentary rocks come from sediments. Some sedimentary rocks are called clastic because they are made of rock fragments. These fragments can be as small as microscopic clay particles or as large as boulders. The type of rock that forms is the result of the sorting of sediments that occurs when sediments are eroded and deposited elsewhere.

A river moves sediments into a lake. Particles are sorted by size. The largest gravels are first to be deposited, followed by sands, and then silts and clays. Over time, the sediments are buried and compacted. The particles may become cemented together.

Conglomerates form from the larger particles, sandstones from sands, and shale from silt and clay. When minerals fill the spaces between sand and grains, they bind the fragments together in a process called cementation.

When carried by a stream or river, the fragments become smooth and rounded from rubbing against one another and the stream bed. The farther the particles are carried, the more rounded they become.

Sedimentary Rocks - Chemical Rocks

The water in seas, lakes, swamps, and underground reservoirs often contains dissolved minerals. Chemical sediments form when these minerals precipitate, or fall out of solution. Precipitation can occur through evaporation or through chemical action. Common chemical sedimentary rocks are rock salt, rock gypsum and some limestones. Rock salt, or halite, occurs in thick layers in many parts of the world.

A thin layer of pooled, mineral-laden water bathes and slowly sculpts a portion of the Bonneville Salt Flats, east of the twin desert towns of Wendover, Utah, and West Wendover, Nevada. The sheen slightly reflects the vault of the blue sky, as shown in the photograph above, taken on April 23, 2013.

Winter and springtime snow and rainfall replenish and revive the surface minerals, including sodium chloride (table salt), potassium and magnesium that compose the Bonneville Salt Flats; a 159 sq mi (412 sq km) pan left behind by the prehistoric waters of vanished Lake Bonneville. The ancient lake covered much of western Utah and eastern Nevada during the Pleistocene ice ages. Due to climate change and geologic factors, the great lake dried up thousands of years ago, leaving behind only encrusted basins like this and a notable remnant, the Great Salt Lake.

Sedimentary Rocks - Organic Rocks

An organic sedimentary rock forms from sediments consisting of the remains of plants and animals. Common organic sedimentary rocks are limestone and coal. Limestone organic sedimentary rocks are formed from seashell sediments.

Coquina ("co-KEEN-a") is a limestone composed chiefly of shell fragments.

Limestone is a rock composed primarily of calcium carbonate. It can form organically from the accumulation of shell and coral fragments. It can also form chemically from the precipitation of calcium carbonate from lake or ocean water.

The single most characteristic feature of sedimentary rocks is stratification, the arrangement of visible layers. A change in the type of sediment being laid down in one place results in the formation of a new rock layer. For example, when sand is deposited on top of clay, a layer of sandstone may form on top of a layer of shale. In this way, sedimentary rocks become stratified. This can be caused by a river that brings sediment to an ocean or lake that may break off and pick up new types of rock.

Sequences of sedimentary rock in the Grand Canyon show the characteristic stratification or layering of this rock type (A). The cross section in (B) emphasizes the layering and shows the difference between the sedimentary rocks and the older igneous and deformed metamorphic rocks near the floor of the canyon.