Sedimentary Rocks Earth Science

Sedimentary Rocks Sediments – pieces of solid material that have been deposited on Earth’s surface by wind, water, ice, gravity, or chemical precipitation. When sediments become cemented together, they form sedimentary rocks The formation of sedimentary rocks begins when weathering and erosion produce sediments

Sedimentary Rocks Weathering Chemical weathering occurs when the minerals in a rock are dissolved or otherwise chemically changed. Physical weathering occurs when fragments simply break off the solid rock along fractures or grain boundaries. During physical weathering, minerals remain chemically unchanged

Sedimentary Rocks Weathering Weathering produces rock and mineral fragments known as clastic sediments. Clastic sediments range in size from huge boulders to microscopic particles. Clastic sediments usually have worn surfaces and rounded corners caused by physical abrasion during erosion and transport.

Sedimentary Rocks Particle Size Sediment Rock Conglomerate 2-0.062 mm Classification of Clastic Sediments Particle Size Sediment Rock >256 mm 256-64 mm 64-2 mm Gravel } Conglomerate 2-0.062 mm Sand Sandstone 0.062-0.0039 mm Silt Siltstone <0.0039 mm Clay Mudstone or shale Boulder Cobble Pebble

Sedimentary Rocks Erosion and Transport Erosion - the removal and movement of surface materials from one location to another. Four main agents of erosion: wind, moving water, gravity, and glaciers. Eroded materials are almost always carried downhill

Sedimentary Rocks Erosion and Transport Deposition – when sediments are laid down on the ground or sink to the bottoms of bodies of water Sediments in wind and water tend to form layers when deposited – largest particles at the bottom Glaciers and landslides, however, move all materials with equal ease

Sedimentary Rocks Burial Most sediments are ultimately deposited on Earth in depressions called sedimentary basins Some sedimentary basins can contain layers of sediment over 8 km thick As more sediment is deposited in that area, the bottom layers are subject to increasing pressure and temperature

Sedimentary Rocks Lithification The increasing temperature and pressure cause lithification. Lithification is the physical and chemical processes that form sediments into sedimentary rocks.

Sedimentary Rocks Lithification Begins with compaction. The weight of overlying sediments forces the sediment grains closer together. For example: water is squeezed out of layers of compacted mud. Some sediments, such as sand, resist lithification due to it being well-compacted during deposition

Sedimentary Rocks Cementation Sediments that are buried 3-4 km deep experience temperatures that are high enough to start the chemical changes that cause cementation. Cementation occurs when mineral growth cements sediments together into solid rock.

Sedimentary Rocks Cementation Two common types of cementation: First type occurs when a new mineral grows between sediment grains as dissolved minerals precipitate out of groundwater. (Calcite growing between quartz grains) Second type occurs when existing mineral grains grow larger as more of the same mineral precipitates from groundwater and crystallizes around them. (quartz crystallizing around quartz grains)

Sedimentary Rocks Features Bedding – the horizontal layering of sedimentary rocks Bedding can range from a millimeter-thick layer of shale to a sandstone deposit several meters thick The type of bedding depends on the method of transport.

Sedimentary Rocks Features Graded Bedding Bedding in which the particle sizes become progressively heavier and coarser towards the bottom layers Often observed in marine sedimentary rocks that were deposited by underwater landslides

Sedimentary Rocks Features Cross-bedding Formed as inclined layers of sediment move forward across a horizontal surface. Small-scale cross-bedding can be observed at sandy beaches and along sandbars in streams and rivers. Most large-scale cross-bedding is formed by migrating sand dunes

Sedimentary Rocks Evidence of past life Fossils are the preserved remains, impressions, or any other evidence of once-living organisms. When an organism dies, it may be buried before it decomposes If its remains are further buried without being disturbed, it might be preserved as a fossil.

Types of Sedimentary Rocks Classification of sedimentary rocks depends on how they were formed. Three main types: clastic, organic, and chemical

Types of Sedimentary Rocks Clastic sedimentary rocks The most common type of sedimentary rock Formed from the abundant deposits of loose sediments found on Earth’s surface Clastic sedimentary rocks are further classified according to the sizes of their particles.

Types of Sedimentary Rocks Coarse-grained clastics Sedimentary rocks consisting of gravel-sized rock and mineral fragments Conglomerates coarse-grained sedimentary rocks that have rounded particles Gravel is transported by high-energy flows of water. During transport, gravel becomes abraded and rounded as the particles scrape against one another

Types of Sedimentary Rocks Breccias Coarse-grained sedimentary rocks that have angular particles The sediments from which they formed did not have time to become rounded. This suggests that the particles were transported only a short distance and deposited close to their source.

Types of Sedimentary Rocks Medium-grained clastics Sedimentary rocks that contain sand-sized rock and mineral fragments When medium-sized sediments are buried and lithified, sandstone is formed. Sandstone has relatively high porosity – the percentage of open spaces between grains in a rock Sandstone often holds underground oil, natural gas, or groundwater deposits due to its high porosity.

Types of Sedimentary Rocks Fine-grained clastics Sedimentary rocks consisting of silt and mud Siltstone, mudstone, and shale Siltstone is mostly composed of silt-sized grains Shale is mostly composed of silt and clay-sized particles Shale has very low porosity – it usually hinders the movement of underwater liquids.

Types of Sedimentary Rocks Chemical sedimentary rocks During chemical weathering, minerals can be dissolved and carried into lakes and oceans. As water evaporates, the minerals are left behind.

Types of Sedimentary Rocks Also, when the concentration of dissolved minerals in a body of water reaches saturation, crystal grains precipitate out of solution and settle to the bottom The layers of chemical sedimentary rocks that form as a result of this process are called evaporites The three most common evaporites are calcite, halite, and gypsum

Types of Sedimentary Rocks Organic sedimentary rocks Formed from the remains of once-living things The most abundant organic sedimentary rock is limestone. Formed from the shells of ancient shell fish Coal is also an organic sedimentary rock Formed from the remains of ancient plants