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SGES 1302INTRODUCTION TO EARTH SYSTEM

LECTURE 16: Sedimentary Rocks

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Sedimentary Rocks – weathering & transportation Sedimentary rocks begin with the process of weathering. Wheathering, which is a set of physical, chemical and biological processes

that breaks rock into smaller particles while some dissolved into solution. Gravity and erosion agents remove the products of weathering and carry

them into a new location where they are deposited. These small pieces of rock that are moved and deposited by water, wind,

and gravity are known as sediments. During transportion, the particles are further broken down and abraded. During transportation and deposition, the particles are also sorted, both in

terms of size and density. Most sediments are deposited by water in the sea, river, swamp or lake. Following deposition, the sediments may transform into rocks (litification) by

compaction and/or cementation. Sediments produced by weathering and erosion form sedimentary rocks

through the process of lithification.

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Sedimentary Rocks - deposition Sediments are deposited when transport stops, eg. when river

enters a quiet lake or sea, or when the stream velocity decreases. Fast moving water can transport larger particles better than slow

moving water. As water slows down, the largest particles settle out first, then the

next largest, and so on, so that different-sized particles are sorted into layers.

Such deposits are characteristic of sediment transported by water and wind.

Wind, however, can move only small grains. Glaciers move all materials with equal ease. Large boulders, sand,

and mud are all carried along by the ice and dumped in an unsorted pile as the glacier melts.

Landslides create similar deposits when sediment moves downhill in a jumbled mass.

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Sedimentary Rocks - lithification The weathering of bedrock and the transport and deposition of

sediments are continuous. As piles of sediment accumulate, the materials near the bottom are

compacted by the weight of the overlying layers. Water flowing through spaces between sediment grains may carry

dissolved mineral matter, and over time or changes in physico-chemical conditions, the sediments can be cemented by precipitation of minerals.

About 75% of the land surface is covered by sedimentary rocks, however it make up only about 15% of the volume of the upper crust.

A large part of the Earth’s history has been reconstructed based on information obtained from sedimentary rocks.

Sedimentary rocks are also important economically (coal, hydrocarbon, Fe, Al, Mn, etc).

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Features of Sedimentary Rocks Sedimentary rocks provide important evidence of Earth’s long history They are formed at Earth’s surface, accumulate layer upon layer, each

records the nature of the environment at that time. These layers are called strata or beds, are the most characteristic feature

of sedimentary rocks. The beds are separated by bedding planes, each bedding plane marks

the end of one episode of deposition and the beginning of another. The thickness of the beds varies from microscopic to tens of meter. Bedding in which the particle sizes become progressively coarser toward

the bottom layers is called graded bedding. When inclined layers of sediment are deposited across a horizontal

surface, crossed beddings are formed. Other important features found in sedimentary rocks include fossils and

sedimentary structures such as ripple marks, mud cracks etc.

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Sedimentary Rocks – classification Sedimentary rocks are classified by their mode of formation. There are 2 main categories of sedimentary rocks:

Detrital sedimentary rocks: deposition of solid particles Chemical sedimentary rocks: precipitation of dissolved substance

Detrital sedimentary rocks are distinguished based on particle size.

Although a wide variety of minerals and rock fragments may occur in the sedimentary rocks, clay minerals and quartz dominates.

From particle size and other features in the rock, we can interprete the environment of deposition.

Particle Size Sediment Name Rock Name

Coarse (> 2 mm) Gravel Conglomerate /

Breccia

Medium (1/16 – 2 mm) Sand Sandstone

Fine (1/256 – 1/16 mm) Mud/Silt Siltstone

Very Fine < 1/256 mm Mud/Clay Shale / Mudstone

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Conglomerates have rounded, gravel-sized particles. Because of its relatively large mass, gravel is transported by high-energy flows of water, such as those generated by mountain streams, flooding rivers, strong ocean waves, and glacial meltwater. During transport, gravel becomes abraded and rounded as the particles scrape against one another.

Conglomerate

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Breccias are composed of angular, gravel-sized particles. The angularity indicates that 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.

Breccia

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Stream and river channels, beaches,and deserts often contain abundant sand-sized sediments that lithified to form sandstone.Sandstone can have a porosity of up to 30%, which makes them valuable as undergroundreservoirs of oil, natural gas, and groundwater.

Sandstone

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These rocks represent environments which have still or slow-moving waters (lakes, lagoons, deep ocean basins), are the most abundant sedimentary rocks. In the absence of strong currents and wave action, these fine grained sediments settle to the bottom where they accumulate in thin horizontal layers.

Shale / Mudstone

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Chemical Sedimentary Rocks During weathering, dissolved materials are carried into lakes and oceans. Some organisms that live in the ocean use the calcium carbonate or silica

that is dissolved in seawater to make their shells. When these organisms die, their shells settle to the bottom of the ocean

and can form thick layers of biogenic chemical sediment. Chemical sedimentary rocks are distinguished based on composition.

Composition Texture Rock Name

Calcite (CaCO3) Fine to coarse crystalline

Limestone

Quartz (SiO2) Very fine crystalline Chert (light coloured)

Flint (dark coloured)

Gypsum (CaSO42H2O) Fine to coarse crystalline

Rock Gypsum

Halite (NaCl) Fine to coarse crystalline

Rock Salt

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Chemical Sedimentary Rocks Limestone is the most abundant chemical sedimentary rock Limestone commonly forms in shallow water environments where

coral reefs thrive. Many types of limestone contain evidence of their biological origin

in the form of abundant fossils. Not all limestone contains fossils. Some limestone has a

crystalline texture, some consists of tiny spheres of carbonate sand, and some is composed of fine-grained carbonate mud.

Dissolved silica precipitates to form various types of microcrystalline to amorphous rocks (chert, opal, etc). Could also formed by biochemical accumulations of siliceous shells fragments.

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Chemical Sedimentary Rocks Limestone & chert requires precipitation of a relatively insoluble

substance from aqeous solution or from biogenic accumulations. Evaporite minerals such as gypsum and halite precipitated from a

saline solution that has been concentrated by evaporation. Evaporites do no precipitate until evaporation has removed most

of the water. Evaporites are forming today in desert basins and locally on

shallow sea margins in hot climates (sabkha). If a column of sea water 1000 m thick is evaporated to dryness,

the precipitated salt deposit would be about 15 m thick. Of this, 0.5 m would be gypsum, 11.8 m would be halite, and the rest, 2.7 m, would be mainly salts of potassium and magnesium.

The evaporites are important economically.

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Chemical Sedimentary Rocks Coal is an unique chemical sedimentary rock. It is not precipitated

from dissolved substances. It is formed by direct accumulation of organic matter, mainly plant

material. Plant structures such as leaves, bark and wood may still be

identified in coal. Coal is produced by the burial of large amounts of plant material

over extended periods. Special conditions are required for the accumulation of large

quantities of plant material because plant debris normally decompose when exposed to the atmosphere.

What is the ideal place for the accumulation of plant material?