Sedimentary RocksSedimentary Rocks

What is a sedimentary rock?What is a sedimentary rock?• Sedimentary rocks result from mechanical

and chemical weathering• Comprise ~ 5% of Earth’s upper crust• About 75% of exposed rocks• Contain evidence of past environments

• Record how sediment is transported• Often contain fossils

What is the economic importance of What is the economic importance of sedimentary rocks?sedimentary rocks?

• They are important for economic reasons because they contain

• Coal• Petroleum and natural gas• Iron, aluminum, uranium and manganese• Geologists use them to read Earth’s history

Remember this when we talk about correlation. Note how beds pinch out or are offset by faults

How does sediment become rock?How does sediment become rock?• Diagenesis – chemical and physical changes that take

place after sediments are deposited

• Diagenesis varies with composition

Chesapeake Bay from SkylabFalse color image

www.nasa.gov

Turning sediment into rockTurning sediment into rock– Diagenesis includes:

–Recrystallization – growth of stable minerals from less stable ones

–Lithification – loose sediment is transformed into solid rock by compaction and cementation

Natural cements: calcite, silica, and iron oxide. Formed from ions in solution in water.

LithificationLithification

• Compaction: As more sediments are piled on top, compaction drives out the excess water.

• Cementation: Precipitation of chemicals dissolved in water binds grains of a sediment together.

• Remember where the dissolved chemicals come from?

Types of sedimentary rocksTypes of sedimentary rocks

•Chemical rocks – sediment from ions that were once in solution

•Detrital rocks –sediment transported as solid particles

Detrital sedimentary rocksDetrital sedimentary rocks• Rocks made of grains• Constituents of detrital rocks can include

• Clay minerals

• Quartz

• Feldspars

• Micas

• Particle size is used to distinguish among the various types of detrital rocks

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Ions weather out of rock,are transported bygroundwater to sediment layers below

Ion-richground-water

Ions transportedto lake or ocean

Dissolved ions precipitateto form cement betweensediment grains

Water enters pore spaces between sediment grains

Sediment grains moved to ocean by streams

Detrital sedimentary rocksDetrital sedimentary rocks

• Mudrocks: less than .06 mm–1. Mud: small particles easily kept in

suspension– Settles in quiet water– Includes Shale: mud-sized particles <.004 mm

deposited in thin bedding layers called laminae

Most common sedimentary rock

2. Larger mudrock grains called siltssilt-sized particles .004-.06 mm

Gritty grains can be felt

Detrital sedimentary rocksDetrital sedimentary rocks

• SandstoneSandstone–Made of sand-sized particles .064 – 2 mmMade of sand-sized particles .064 – 2 mm

–Forms in a variety of environmentsForms in a variety of environments

–Sorting, angularity and composition of grains Sorting, angularity and composition of grains can be used to interpret the rock’s historycan be used to interpret the rock’s history

–Quartz is the predominant mineral (due to its Quartz is the predominant mineral (due to its durable nature)durable nature)

Photomicrograph of quartz rich sandstone (Arenite)

Grains subangular to subrounded, sandstone is poorly sorted

Plagioclase grainClassifying Sandstones

Making thin sections

Types of SandstoneTypes of Sandstone

• Quartz Arenite >90% quartz grains– Beach and dune deposits

• Arkoses >25% feldspar, angular, poor sort.– Transform boundaries; exposed granites– Any felsic rock eroded, not transported far

• Graywackes Quartz, feldspar, volcanics– Port sorted, angular– Erosion of Island Arcs– Rift Valley Sediments

Detrital Detrital sedimentary sedimentary

rocksrocks

• Conglomerate and breccia

–Both composed of particles > 2mm in diameter

–Conglomerate consists largely of rounded clasts. Rounded pebbles in high velocity areas

–Breccia is composed of large angular particles Breccia is made of shattered rock that accumulates at the base of a cliff

EnergyEnergy

• Coarse sediments are deposited in high energy (fast water) environments such as under breaking waves at the beach, or in the beds of fast streams.

• Fine sediments are deposited in low energy environments, e.g. the slow water of deep lagoons, the abyssal plain, etc.

Outcrop of conglomeratewith cobble-sized clasts interbedded with sandstone

Conglomerates are fast-water sediments“High Energy”K.E. = 1/2mv2

In fast water, smaller sizes swept away

ChemicalChemical sedimentary rocks sedimentary rocks

• Precipitated material once in solution

• Precipitation of material occurs two ways:

• Inorganic processes: the minerals precipitate out of water

• Organic processes: animals and plants precipitate the minerals to use as shells or skeletons

http://www.ucmp.berkeley.edu/collections/micro.html

• Common chemical sedimentary rocks

•Limestone–Most abundant chemical rock

–Made of the mineral calcite CaCO3

–Marine biochemical limestones form as coral reefs, coquina (broken shells), and chalk (microscopic organisms)

–Inorganic limestones include travertine (caves) and oolitic limestone (Bahamas)

• Common chemical sedimentary rocks

•Evaporites–Evaporation triggers deposition of chemical precipitates

–Examples include rock salt and rock gypsum

http://www.ndsu.nodak.edu/instruct/ashworth/coursework/g410/evaporites/saltbeds.jpg

Chalk Outcrops in SE USA

Chalk Hand Specimen

Oolitic Limestone - Bahama Shoals

Oolitic Limestone - Hand Specimen

Ooids under microscopeInorganic Chemical Sediments

• Other chemical sedimentary rocks• Chert

– Made of microcrystalline quartz

– Usually deposited as siliceous ooze in deep oceans (can be diatomaceous)

chert Diatomaceous chert

• Other chemical sedimentary rocks– Dolostone (made of mineral Dolomite)– Like Calcite, but some Ca is replaced by Mg

The Dolomites, sediments thrust up when the Alps formed

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Evaporation

Seawater enriched in Mg2

Limestone

Dolostone

Mg2 replaces some of theCa2 in limestone

Mg2 -rich seawater circulatesthrough porous limestone

How to make Dolomite

Chemical Sediments: CoalChemical Sediments: Coal

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Particles are large andirregular, and consist ofa variety of lithologies,including the leastresistant.

Particles are mid-sizedand of intermediatesphericity, and includeresistant and nonresistantlithologies.

Particles are small andnearly spherical, andconsist mainly of themost resistant lithologies,such as quartz.

Character of detrital sediments depends on time, distance, and energy. For example, in streams:

HIGHLANDS LOWLANDS NEAR-COASTAL

Sedimentary environment determines roundness sorting, mineral diversity

• Sedimentary Facies•Different sediments accumulate next to each other at same time

•Each unit (called a facies) possesses a distinctive characteristics reflecting the conditions in a particular environment

•The merging of adjacent facies tends to be a gradual transition

Nearshore sandsStillwater mudsAbyssal Ooze

Some Facies in an oversimplified drawing

A picture glossary of sedimentary environments

Turbidite: underwater landslide = graded bedding

Sedimentary structuresSedimentary structures

• Tell us something about past environments

• Types of sedimentary structures•Strata, or beds (most characteristic of sedimentary rocks)-bedding planes that separate strata caused by variation in deposition

Strata- Bedding PlanesStrata- Bedding Planes

Fine Scale Bedding- LamellaeFine Scale Bedding- Lamellae

6_6Fine-grained sediment

On floodplain

Flood water

Erosion of uppermostfine-grained sediment

Older sediment

1 Pre-flood

Flood stage2

Post-flood3

Coarse-grained below

Bedding plane

Floods change the local conditions

Waning flow

Bounders on bottom, sands and muds suspended

Graded bedding

Fine-grained above

Graded Beds – grains fine upward

Note: Beds were tilted from horizontal after deposition

Recognizable Sedimentary StructuresRecognizable Sedimentary Structures• Ripples

• Irregularities in bottom sediment lead to ripples

• Asymmetric types indicate flow direction.

• Symmetric types formed in tidal areas

Slabs of eroding sandstone with ripple marks

Cross Beds areCross Beds are ripples in cross sectionripples in cross section

• Irregularities lead to ripples, dunes, sand bars.

• In cross section these look like lines at an angle to the horizontal – “cross beds”

• Ripples can indicate direction of air or water flow if asymmetrical, a tidal environment if symmetrical. Size and shape indicate fluid velocity.

Cross bedding in Sand Dune deposits

Sandstone deposited

in ancient sand dunes

Frosted Grains, well sorted

Navaho Sandstone

Mud Cracks: clay layer shrinks during drying, curls upward; cracks fill next flood. Useful for right-side up

Sedimentary EnvironmentsSedimentary Environments

• Sediments are formed in many different environments

• Each have characteristic appearance today, features that allow them to be recognized in the geologic record

• Streams (includes big Rivers), with floodplains and levees, called fluviatile. The Point Bar Sequence is typical for meandering streams. Cutoffs generate Oxbow deposits.

High gradient streams with high sediment load are Braided.

• Lake deposits called lacustrine, generally still waters, often varved deposits if winters cold

Fresh Water Facies

http://hays.outcrop.org/gallery/rivers/arid_meander?full=1

Floodplain

MeanderingStream

OxBow

http://hays.outcrop.org/gallery/rivers/arid_meander?full=1

Deposits Associated with Meandering Streams

Point-bar Sequence:

Point Bar SequencePoint Bar Sequence

Erosion

Gravel of bed

Crossbeds of Bar

Fines of Floodplain

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Continentalshelf

Continentalslope

Shallowmarine

Deep marine

Submarinevolcanoes

Terms for Marine (i.e. Ocean) Environmentsand some characteristic sediment facies

AbyssalPlain

6_29River Direction of migration

of shoreline, and landwardshift of sedimentary facies

Shoreline attime B

Shoreline attime A

Time B

Time ASea levelrising

Depositedat time A

Depositedat time B

Shallowmarine

BeachRiver

Deepmarine

Deepmarine

Shallowmarine

Beach

Shallowmarine

Comparison of sediments deposited

Facies changes due to rising sea level - water getting deeper everywhere

REMEMBER: the facies follow the shoreline

Fossils are traces of prehistoric life generally Fossils are traces of prehistoric life generally

preserved in sedimentary rockpreserved in sedimentary rock

Dinosaur footprint in mudstoneDinosaur footprint in mudstone

End of Sedimentary End of Sedimentary RocksRocks