sedimentary rocks earth science. sedimentary rocks sediments – pieces of solid material that have...

Sedimentary RocksSedimentary Rocks

Earth ScienceEarth Science


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


• 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


• 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.


Particle Size Sediment Rock

>256 mm

256-64 mm

64-2 mmGravel } Conglomerate

2-0.062 mm Sand Sandstone

0.062-0.0039 mm Silt Siltstone

<0.0039 mm ClayMudstone or

shale

BoulderCobblePebble

Classification of Clastic Sediments


• 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


• 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


• 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


• Lithification– The increasing temperature and pressure

cause lithification.– Lithification is the physical and chemical

processes that form sediments into 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


• 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.


• 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)


• 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.


• 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


• 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


• 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 RocksTypes of Sedimentary Rocks

• Classification of sedimentary rocks depends on how they were formed.

• Three main types: clastic, organic, and chemical


• 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.


• 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


• 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.


• 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.


• 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.


• Chemical sedimentary rocks– During chemical weathering, minerals can be

dissolved and carried into lakes and oceans.– As water evaporates, the minerals are left

behind.


– 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


• 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

sedimentary rocks earth science. sedimentary rocks sediments – pieces of solid material that have...

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clastic sediments

chemical weathering

physical weathering

solid rock

unchanged slide

chemical precipitation

physical abrasion

earths surface