Sedimentary RocksSedimentary Rocks

Land sedimentary environmentsLand sedimentary environments

• Mountains – Formed on bedrock. Steep slopes allow sediment to travel far.

• Desert – wind picks us fine particles

• Glaciers – snow compacts to ice and the ice moves sediment below it.

• Beach – stretches of coastline where sediments has accumulated.

Clasts and weatheringClasts and weathering

• Clasts – pieces of rock• 2 types of weathering• Physical – breaking apart of rocks that

are exposed to the environment.• 4 ways• Near-Surface Fracturing – Processes

near the surface cause rocks to fracture

Physical weathering cont’dPhysical weathering cont’d

• Frost and Mineral Wedging – Rocks are pried apart when water freezes

• Thermal Expansion – Heating during the day may cause rocks to break

• Biological Activity – Roots can grow into fractures

Making Sediment: Physical Making Sediment: Physical WeatheringWeathering

FracturingFracturing

Frost and mineral Frost and mineral wedgingwedging

Roots and Roots and other other biologic biologic activityactivity

Chemical WeatheringChemical Weathering

• Minerals are broken down causing new minerals to form.

• 4 processes

–Dissolution – some minerals are dissolved in water

–Oxidation – some minerals react with oxygen to form oxide minerals

Chemical Weathering Chemical Weathering

• Hydrolysis – Silicate minerals react with water, the hydrogen ions in the water react with the minerals

• Biological reactions – decaying plants produce acids, some bacteria consume rocks.

Making Sediment: Chemical Making Sediment: Chemical WeatheringWeathering

OxidationOxidation

HydrolysisHydrolysis

Biological Biological reactionsreactions

What are the characteristics of What are the characteristics of Clastic Sediments? Clastic Sediments?

• Clastic sediments are classified in 3 ways:

• Size of clasts

–Boulders, cobbles and pebbles

–Sand

–Fine particles

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Moderately sortedModerately sorted

Poorly sortedPoorly sorted

Well sortedWell sorted

Observe the range of clast Observe the range of clast size (sorting) in each size (sorting) in each photographphotograph

Characteristics of Clastic Sediment Characteristics of Clastic Sediment cont’dcont’d

• Shape of Clasts

–Angular, rounded, partially rounded

• Amount of Sorting

–Size and range of clasts

–Poorly to well sorted (poorly has wide range)

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Observe the shape of Observe the shape of these claststhese clasts

RoundedRounded

AngularAngular

Partially roundedPartially rounded

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Controls on Size, Shape, and Controls on Size, Shape, and SortingSorting

Steepness of Steepness of slopeslope

Farther from source means rounder and smallerFarther from source means rounder and smaller

Strength of currentStrength of current Agent of transportAgent of transport

Sedimentary Rock FormationSedimentary Rock Formation

• Clastic Sedimentary rocks form 2 ways

• Compaction- increased pressure from being buried beneath sediment forces clasts together. –Compaction forces out excess

water, sediments lose up to 40% of their volume.

Formation of Sedimentary RocksFormation of Sedimentary Rocks

• Cementation- after sediment is compacted open pores between sediment collects water containing dissolved minerals.

–As water evaporates minerals left behind act as a cement holding the clasts together.

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How Clastic Sediment Becomes How Clastic Sediment Becomes RockRock

Compaction of Compaction of clay particlesclay particles

Compaction of Compaction of sand grainssand grains

CementationCementation

Types of CementTypes of Cement

• Rocks are held together by 3 types of “cement”

• Calcite- CaCO3

– Moderate hold

• Silica- SiO2

– Strong hold

• Iron-oxide Minerals- ex. hematite– Come out of water and give mineral a reddish-

color

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Common Clastic Sedimentary Common Clastic Sedimentary RocksRocks

BrecciaBrecciaConglomerateConglomerate

SandstoneSandstone ShaleShale

Nonclastic RocksNonclastic Rocks

• Form when chemicals dissolved in water, precipitate. (when water evaporates)

• Also, can be biochemical –Extracted by an organism and

reformed into a sedimentary rock

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Rocks Formed by Chemical Rocks Formed by Chemical ReactionsReactions

Rock SaltRock SaltSettingSetting

SettingSetting Travertine (limestone)Travertine (limestone)

07.06.aRocks Formed by OrganismsRocks Formed by Organisms

Limestone with fossilsLimestone with fossilsSettingSetting

SettingSetting CoalCoal

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Other Nonclastic Sedimentary Other Nonclastic Sedimentary RocksRocks

ChalkChalkGypsumGypsum

DolostoneDolostone ChertChert

Difference between Clastic and Difference between Clastic and NonclasticNonclastic

• Clastic rocks are made up of rocks and minerals

• Nonclastic rocks are made by dissolved minerals left behind by evaporated water.

Why do sedimentary rocks have Why do sedimentary rocks have layers?layers?

• Most sediment is deposited in layers.

• Classified by thickness and definition of layers. –Thinner, smaller layers referred to

as beds.

How layers formHow layers form

• Discrete event – a one-time occurrence. Ex. Flood.

• Change in current – Caused by change in strength or direction of current that deposited the sediment.

• Sediment Supply – Change in type or amount of sediment being supplied.

How layers formHow layers form

• Sea-level change – global rise or fall in sea level causes sedimentary environments to move.

• Climate Change – Boundaries between layers reflect regional or global changes in climate.

Types of BedsTypes of Beds

• Graded Beds- form when a current loses velocity and drops sediment. Larger sediment falls first followed by finer sediment.

Types of BedsTypes of Beds

• Cross Beds- form when sediment moves over a dune or underwater ripple. At an angle to other beds.

Types of BedsTypes of Beds

• Parallel Beds- form when one layer piling on another layer. Most common type.

Resources in Sedimentary RocksResources in Sedimentary Rocks

• Groundwater – moves between pores in rocks

• Petroleum – formed by organic-rich sedimentary rocks

• Coal – consolidation of plant remains in wetland environments

Resources in Sedimentary RocksResources in Sedimentary Rocks

• Cement from limestone – produced from limestone free of impurities

• Salt – mined from ancient sedimentary salt layers

• Uranium – Occur in sandstone and other sedimentary rocks but it is not formed there…brought by groundwater

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Resources in Sedimentary Resources in Sedimentary RocksRocks

GroundwaterGroundwater PetroleumPetroleum CoalCoal

Cement from Cement from limestonelimestone

SaltSalt UraniumUranium

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Observe this sequence of sedimentary layersObserve this sequence of sedimentary layers

Medium to dark gray Medium to dark gray shale with fossil marine shale with fossil marine organismsorganisms

Sandstone, mudstone, coalSandstone, mudstone, coal

Sandstone with marine Sandstone with marine shellsshells

Sedimentary units with Sedimentary units with fossilized wood and fossilized wood and dinosaur tracksdinosaur tracks