An Introduction to Sedimentary Rocks

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An Introduction to

Sedimentary Rocks


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What is a sediment

Sedimentis any particulate matter that can be transported by fluid flow,and which eventually is deposited.

Sediments are most often transported by water fluvial processes

transported by wind (aeolian processes ) and glaciers. Beach sands and

river channel deposits are examples of fluvial transport and deposition,

though sediment also often settles out of slow-moving or standing water in

lakes and oceans. Desert sand dunes and loess are examples of aeolian

transport and deposition. Glacial, moraine deposits and till are ice

transported sediments.


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Sedimentation & Sedimentology

Sedimentation: The term is typically used in geology, to describe the

deposition of sediment which results in the formation of sedimentary

rock and in various chemical and environmental fields to describe the

motions of often-smaller particles and molecules.

Sedimentology is the study of modern sediments such as sand, mud

(silt),and clay, and the processes that result in their deposition.

Sedimentologists apply their understanding of modern processes to

interpret geologic history through observations of sedimentary rocks

and sedimentary structures.

Sedimentary rocks cover most of the Earth's surface, record much of

the Earths history , and harbor the fossil record. Sedimentology is

closely linked to stratigraphy , the study of the physical and temporal

relationships between rock layers or strata.


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Sedimentary Rocks

Sedimentary rocks are those rocks which form at or near the

earth's surface primarily through:

Deposition of weathered silicate material by water, wind, or ice

(detrital, clastic, terrigenous)

Direct inorganic chemical precipitation from water

Precipitation by organic processes


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Conglomerate Breccia (Bretcha)

Sandstone Coquina


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Formation of Sedimentary Rocks

When rocks are broken down into fragments, either through themechanical means of weathering, or through chemical reactions, thefragments are called sediment. When that sediment is compacted orcemented together, it forms a sedimentary rock.

Sedimentary rocks are the most common rocks that we encounter,

because the Earth has so many efficient weathering processes thatconstantly break down rock and create sediment.

Common sedimentary rocks include sandstone (made up, notsurprisingly, of sand) and limestone (made up of calcium carbonateprecipitated out of solution).

Sediments become sedimentary rock through compaction (squeezingsediments together and forcing out any fluids) and cementation(introduction of a cementing agent).

Sediments are either clastic or chemical. That is, rocks are broken downthrough either mechanical or chemical means.


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Rock Cycle


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Importance of sedimentary Rocks

~ 75% of the Earth's Surface materials sedimentary earthmaterials

Sedimentary rocks contain the majority of Earth Resources

Oil, Natural gas, fossils fuels

minerals

water

building materials

Subsurface fluid storage


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erosion

weathering

transportation

Erosion includes BOTH weathering and transportation


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Sedimentaryrocks make up only 7.9% of the Earths crust.


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Types of Sediments

Clast ic sediment

Clastic sediment is what one usually thinks of when speaking ofsediment. From the Greek word klastos (broken), it refers to the brokenremains of rocks of all types, broken and altered by weatheringprocesses such as wind, water and ice. Clastic sediment is also known

as detrital sediment.

Sandstone, conglomerate, shale etc are the examples of clasticsedimentary rocks.

Clastic rocks (sedimentary) differ from igneous rocks not by the mineral

composition but by the grain texture that shows its history of mechanicalweathering, the cementation material and the possible presence offossils (which cannot survive the high temperatures required to meltigneous rocks).


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Types of sediments Chemical sedim ent ( Non- Clast ic)

Chemical sedimentary rocks may contain fossils and other sedimentarycharacteristics, but their components were not broken up mechanically.Rather, rocks were dissolved in solution (as salt can dissolve in water)and transported, then precipitated chemically (as salt can precipitate outof a saturated solution).

Limestone, dolomite etc are the examples of ( Non-clastic sedimentaryrocks)

Precipitation can occur biochemically. Example: tiny plants living inseawater can decrease the acidity of the surrounding water and so causecalcium carbonate to precipitate.

Precipitation can also occur through inorganic means. Example: Asseawater evaporates, it often leaves behind salts which have precipitatedout. This is how halite (salt) is formed.

Sedimentary layering is also a powerful clue to sedimentary origin.Igneous rocks have minerals which grow in every direction. Sedimentaryrocks, on the other hand, are often laid down in layers that mimic thehorizontal surface of the Earth.


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Sedimentary Rocks

T=Terrigenous Residual and secondary weathering products (siliciclastic)

Allogenic (extra-basinal) origin

A= Allochemical

Chemical or biochemical particles, shell fragments

Authigenic (form within basin) but locally reworked

O= Orthochemical

Primary chemical precipitation from dissolved ions

Authigenic (form within basin of deposition), no reworking


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Sedimentary Rocks: Terrigenous

Terrigenous (clastic,detrital) sediments androcks

Also called siliciclastic

since most particles aresilicate mineral grains

Grains created byweathering

Transported by surfaceprocesses water, wind,ice

Deposited as horizontal, stratifiedlayers in sedimentary basins

Buried and lithified by

Compaction

Cementation
http://www.geoscapesphotography.com/ca-pointreyes-conglomerate-p2987.htm


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Sedimentary Rocks: Allochemical

Allochemical (mainly carbonates) sediments

and rocks

Dominantly biologic origin (shells or bones)

Carbonate system develop where siliciclastic

source lands are low and/or very distant

The water is shallow marine

Climates are tropical to subtropical


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Sedimentary Rocks: Orthochemical

Orthochemical (chemical precipitate) sediments and rocks

Dominated by limestones and dolostones of precipitate origin

Also includes evaporites, chert, and iron formations

Precipitate from marine or non-marine waters due to chemicalchanges


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Environmental interpretation:

The present is the key to the past.

By examining the characteristics of variousenvironments on Earth today we can interpret the

environments in which ancient sediments were

deposited.


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Sedimentary Rock TypesCover most of the land surface and seafloor

Clastic Rocks particles derived from the weathering and erosion of precursor

rocks and consist primarily of fragmental material ( sandstone ,conglomerate)

classified by grain size and composition

Carbonates precipitated by a variety of organic and inorganic processes

(limestone, dolomite)

Evaporites evaporation of water at the Earth's surface (Halite)

Chemical sedimentary rocks (chert, jasperiod)


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Sediment Characteristics

Porosity is the volume of voids within a rock which can containliquids.

Permeability is the ability of water or other liquids (e.g. oil) to

pass freely through a rock.

Roundness refers to the roughness of the surface of thesedimentary grain.

Sorting refers to the range of particle sizes in a sediment orsedimentary rock.

Matrix is the fine-grained material (usually clays or silt) that isdeposited originally with the coarser-grained material


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Classification by GRAIN SIZE

Gravel > 2mm

Sand >1/16 mm < 2 mm

Mud


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The Wentworth scale was published in 1922 by Chester K. Wentworth, modifying an earlier scale

by Johan A. Udden. Wentworth's grades and sizes were later supplemented by William

Krumbein's phi or logarithmic scale, which transforms the millimeter number by taking the

negative of its logarithm in base 2 to yield simple whole numbers.
http://www.augustana.edu/academics/geology/department/uddenbio.htmhttp://www.augustana.edu/academics/geology/department/uddenbio.htm


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Grain Size Distribution


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TEXTURE

Grain shape (attributes which refer to the externalmorphology of particles) surface texture,

roundness

form. is determined by:

Grain shape (Bustin, 1995) internal structure

mineral cleavage

characteristics of source rock such as jointing and

bedding lithology

hardness

fracture

transport


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Sediment transported by

Suspension load is when sediments are carried insuspension (usually fine-grained sediments that can becarried along easily by the flow)

Bed load is when the forward force of the moving currentacts more directly on the larger particles at the bottom as itpushes, rolls, and slides them along

Saltation is more complex and usually affects sand-sizedparticles. Here, the particles are sucked up by eddies intothe flow, travel with the flow for a while, and then fall backto the bottom


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Sedimentary Structures

Asymmetrical ripples are ripples that have a gentle slopeupstream and a steep slope downstream.

Cross-bedding is inclined bedding and commonly forms in

alluvial environments.

Potholes are rounded depressions caused by swirlingcurrents and eddies.

Mud cracks are formed by evaporation on mudflats or inshallow lakes.

An Introduction to Sedimentary Rocks

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