Sedimentary Rocks

What is a sedimentary rock?

Sedimentary rocks are formed from the compaction and cementation of sediment.The processes of weathering and erosion break down rocks to produce inorganic sediment (pieces of preexisting rocks).These rock fragments can become a sedimentary rock if sufficient burial and compaction of the sediment occurs.All sedimentary rocks are made from pieces of other rocks.

How is sediment transported?

Sediment is transported by a transporting agent. Transporting agents include:

Water

Wind

Glaciers

Mass Wasting (gravity)

How does sediment become a sedimentary rock?

Sediment can become sedimentary rock through one (or more) of the following processes:CompressionCementationChemical ProcessesBiological Processes

**The scientific term for sedimentary rock formation is lithification.

Rock Formation by Compression

Since most sediment is deposited in basins (areas of lower elevation), it is very common for sediment to get buried below other sediment. This puts tremendous amounts of pressure on the buried rock fragments.

Over time the sediment may be exposed to compression (squeezing) due to pressure of overlaying sediment or water. If sufficient pressure is applied to the sediment it can form a sedimentary rock.

An example of a rock formed by compression is sandstone. How does sandstone differ from siltstone?

Rock Formation by Compression

Since most sediment is deposited in basins (areas of lower elevation), it is very common for sediment to get buried below other sediment. This puts tremendous amounts of pressure on the buried rock fragments.Over time the sediment may be exposed to compression (squeezing) due to pressure of overlaying sediment or water. If sufficient pressure is applied to the sediment it can form a sedimentary rock. An example of a rock formed by compression is sandstone. How does sandstone differ from siltstone?

Answer: Sandstone has a larger grain size than siltstone.

Rock Formation by Cementation

Some minerals that are soluble in water can eventually become cementing agents in a process called cementation. You can think of these minerals as glue that holds the sediment together.Cementation occurs when sediments are combined with mineral cements that precipitate out of solution (ground water) and “glue” clasts of rock together. The cementing agent will often determine the color of the rock. The processes of compression and cementation often work together to form a sedimentary rock.

Rock Formation by Chemical Processes

Some sediment is created through chemical weathering when soluble minerals are dissolved by water to form a solution. Sea water is a common example of this type of solution.Often, evaporation of water causes the mineral precipitation that results in the formation of a chemically formed sedimentary rock. When water evaporates the soluble materials are left behind as chemical deposits. The rocks that form from this process are called evaporites.

If an evaporite is made of only one type of mineral it is referred to as a monomineralic rock.

Rock Formation by Biologic Processes

The terms biologic and organic refer to living things. Therefore, rocks formed from biologic processes must involve things that were once living.

Organic sediments are the remains of any living thing (plants or animals). These sediments form bioclastic sedimentary rocks.

The most common examples of bioclastic rocks are limestone and coal. Briefly describe how these two sedimentary rocks are formed. (See p. 7 of your ESRT).

How are sedimentary rocks classified?

Sedimentary rocks are classified as clastic (fragmental), chemical (crystalline), or organic (bioclastic) depending on how they were formed.

Clastic sedimentary rocks are classified on the basis of grain size.

Organic or chemically formed sedimentary rocks are identified through composition and texture.

Fossils and Sedimentary Rocks

**Fossils are found exclusively in sedimentary rocks.

Give one reason why you would not expect to find fossils in:Metamorphic rocksIgneous rocks

Identification of Sedimentary Rocks

Turn to page 7 of your reference table.