Sedimentary Rocks

Basic Geologic Principles

• Principle of Superposition: Younger sedimentary and volcanic rocks are deposited on top of older rocks.

• Principle of Original Horizontality: Sedimentary and volcanic rocks are laid down in approximately horizontal layers.

• Principle of Lateral Continuity: Sedimentary and volcanic rocks are laid down in layers that are usually much greater in lateral extent than in thickness.

Principle of Superposition

Principle of Original Horizontality

Weathering and Erosion

Rocks may be broken down either mechanically or chemically.Mechanical: The physical breakup or disintegration of rocks without changes in their compositionExamples: Frost Action & Wind AbrasionChemical: The breakdown or decomposition of minerals by chemical reaction with water, with other chemicals dissolved in water , or with gases in the airExamples: Ions (Ca, Na, K, Mg) dissolve in water

Mechanical Weathering produces different sizes of material

Oxidation

• Oxidation takes place when oxygen reacts with earth materials. Oxygen dissolved in water combines with atoms of metallic elements abundant in silicate minerals. Attacking metals in the soil, oxidation causes them to rust leaving the soil a brownish red to red color. When oxygen combines with iron, the reddish iron oxide hematite (Fe2O3) is formed:

• 4Fe+3 + 3O2 -> 2Fe2O3

Transportation

Rock fragments move downhill by gravity. The main agents of transportation include:

• Water• Glaciers• Wind

Lithification

Lithification is the process of becoming a rock

• Cementation – minerals traveling by groundwater or seawater travel through the rock. The mineral crystals cement the clasts and hold them together. Examples of binding minerals: Calcite, iron oxide, and Quartz

• Compaction – the layer of sediment is buried by another layer of sediment. This squishes the clasts filling in some of the empty spaces.

There are 3 basic types of sedimentary rocks:

1.Clastic – formed from mechanical weathering debris

2.Chemical - form when dissolved materials precipitate from solution

3.Organic - form from the accumulation of plant or animal debris.

Examples of sedimentary rocks:

• Shale• Limestone• Sandstone• Rock Gypsum• Conglomerat

e• Chert

• Oil Shale• Siltstone• Coal • Rock Salt• Breccia• Iron Ore

Shale(Clastic)

Shale is sedimentary rock that is formed from clay and compacted together by pressure. They are used to make bricks and other material that is fired in a kiln.

Limestone(Organic)

Limestone rocks are sedimentary rocks that are made from the mineral calcite which came from the beds of evaporated seas and lakes and from sea animal shells.

Sandstone(Clastic)

Sedimentary rock formed from cemented sand-sized clasts. The cement that binds the clasts can vary from clay minerals to calcite, silica or iron oxides.

Rock Gypsum(Chemical)

Sedimentary rock that is formed by the evaporative deposition of gypsum from solution. It is commercially very important for the production of plaster of Paris and gypsum wallboard for house construction.

Conglomerate(Clastic)

Sedimentary rock formed from rounded gravel and boulder sized clasts cemented together in a matrix.

Chert(Clastic)

Chert is a sedimentary rock material composed of SiO2. Early people took advantage of how chert breaks and used it to fashion cutting tools and weapons.

Coal(Organic)

Coal is an organic sedimentary rock that forms mainly from plant debris. The plant debris usually accumulates in a swamp environment. Coal is combustible and is often mined for use as a fuel.

Rock Salt(Chemical)

Rock Salt is a chemical sedimentary rock that forms from the evaporation of ocean or saline lake waters.

Calcite(Chemical)

• Calcite is a chemical sedimentary rock that forms from the evaporation of ocean or saline lake waters.