Sedimentary Rocks

Sediments Become Rock After transported sediments are deposited, they can be changed into solid rock units. Most sediments are deposited in large bodies of water. Individual rocks tend to be rounded because of water action. The presence of sedimentary rocks suggests the type of environment the rocks formed in.

Identifying Chemically Formed Sedimentary Rock.. is difficult, because their texture is variable – they do not readily show properties like most other sedimentary rocks.

Compression and Cementation Most sedimentary rocks form as a result of compression and sedimentation of broken rock material. Compression is created by water and overlying sediments. Enough force can be generated to force particles close together, drive out water, and compact sediments into rock.

Shale Shale is formed from the compression of very small particles – mostly clay

Cementation The addition of minerals (silica, lime, iron oxide) tends to “glue” rock particles together to form solid rock. Sandstone and conglomerate are composed of sand, pebbles, and small rocks that are cemented together. The color of the rock is often determined by the cementing agent.

Chemical Processes Some sedimentary rocks form by evaporation/precipitation A mass of intergrown crystals results Limestone (calcite), dolostone, rock salt, rock gypsum are examples. These are called evaporites

On the ESRT

Biological Processes Some sedimentary rocks form as a result of processes acting on the remains of flora and fauna. Coal – compression of dead plant material deposited in shallow water. Corals, shellfish produce calcite shells, which are cemented together to make fossilized limestone.

Where to find sedimentary rocks Sedimentary rocks are found as a thin veneer covering much of the continents.

Fossils Fossils are found almost exclusively in sedimentary rocks. As sediments collect, plants and animals that die are buried. The formation of sedimentary rocks helps preserve fossils.

Features of Sedimentary Rocks Sedimentary rocks are similar to the sediments in which they form. There are three kinds of sedimentary rocks: Clastic Crystalline Bioclastic

Clastic Sedimentary Rocks Clastic sedimentary rocks form from the compaction and cementation of fragments of other rocks.

Chemical Sedimentary Rocks Chemical sedimentary rocks form from mineral grains that precipitate out by evaporation or other chemical action.

Organic (Bioclastic) Sedimentary Rocks Organic sedimentary rocks form from the remains of plants and animals

Properties of Sedimentary Rocks The properties of rocks that are similar to sediments are Discrete layers Fragmental particles Organic composition A range of particle sizes, or A predominant particle size Rocks may have one or all of these properties simultaneously.

Discrete Layers Layers formed when one layer of sediment settles on top of another; the two layers harden into rock. The discrete layers remain in the rock. This layering can be seen in shale or sandstone. These layers are called beds, or strata.

Fragmental Particles Fragmental particles are called clastics. Clastic sedimentary rocks are classified by particle size.

Organic Composition Some rocks are composed of plants and animals to a greater or lesser extent. Organic bioclastic and chemically formed sedimentary rocks are identified through composition and texture. Limestone contains the carbonate ion, which can be broken down by acid.

Predominance of one particle size Many rocks are composed of sediment particles of the same size and type. This indicates that the conditions under which the sediments were deposited did not change for an extended period of time.

Inferred Characteristics of Rock Types The environment in which a sedimentary rock forms is often inferred from characteristics of the rock’s Composition Structure Texture

Composition The presence of marine fossils in a rock indicates that it is formed at or near Earth’s surface, deposited in a large body of water. If salt is the primary material in the rock, it was probably formed by evaporation of water. Limestone is also a precipitate; marine animals are responsible for this.

Structure Sedimentary rocks tend to a have a layered look. Rock layers show ripple marks, cross bedding, and fossil shells indicating sediments deposited in a shallow sea.

Texture The texture of sedimentary rocks is an important clue in helping determine the environment in which the sediments were deposited. Rock particles transported by water for a short time will be angular and rough (brecchia) Rock particles transported by water for long distances will be rounded and smooth (conglomerate)

Sedimentary Rock Lab