sediments and sedimentary rocks - geology...

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Sediments and Sedimentary Rocks

(Shaping Earth’s Surface, Part 2)

Science 330 Summer 2005

What is a sedimentary rock?

Products of mechanical and chemical weatheringAccount for about 5 percent of Earth’s crust (by volume)Contain evidence of past environments

Provide information about sediment transportOften contain fossils

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What is a sedimentary rock?

Important for economic considerations – may contain:

CoalPetroleum and natural gasSources of iron, aluminum and manganese

Turning sediment into rock

Many changes occur to sediment after depositionDiagenesis

All of the changes that take place after sediments are depositedOccurs within the upper few kilometers of the earth’s crust

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Turning sediment into rock

DiagenesisIncludes:

RecrystallizationDevelopment of more stable minerals from less stable ones

LithificationUnconsolidated sediments are transformed into solid sedimentary rock by compaction and cementationNatural cements include calcite, silica, and iron oxide

Types of sedimentary rock

Sediment originates from mechanical and/or chemical weatheringRock types are based on the source of the material

Detrital rocks ( = “clastic” )Sediment transported as solid particles (clasts)

Chemical rocksSediment that was once in solution

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Detrital sedimentary rocks

Chief constituents include:Clay minerals

QuartzFeldspars

Micas

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

Clastic (detrital) particle size classification

Shale, mudstone, or

siltstoneMud

SiltClay

1/256 - 1/16< 1/256

SandstoneSandSand1/16 – 2

Conglomerate or brecciaGravel

BoulderCobblePebbleGranule

> 25664 - 2564 - 642 – 4

Rock NameCommon Sediment

Name

Particle Name

Size Range(millimeters)

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Detrital sedimentary rocksCommon detrital sedimentary rocks (in order of increasing particle size):

ShaleMud-size particles in thin layers commonly referred to as laminae

fissility

Most common sedimentary rock

MudstoneBreaks into chunks or blocks

SiltstoneNot fissile

Del Mar fm. / Torrey Pines ss.

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Del Mar fm.

Mudstone

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Mudstone

Shale

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Shale / Siltstone


SandstoneComposed of sand-sized particles

Formed in a variety of environmentsSorting, shape, and composition of the grains can be used to interpret the rock’s historyQuartz is the predominant mineral

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Sandstone

Sandstone

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Sandstone


Conglomerate and brecciaBoth are composed of particles greater than 2 mm. in diameterConglomerate consists of rounded gravels

Breccia is composed mainly of large angular particles

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conglomerate

conglomerate

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breccia

Chemical sedimentary rocks

Consist of precipitated material that was once in solutionPrecipitation of material happens in two ways:

Inorganic processes

Organic processes (biochemical origin)

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Common chemical sedimentary rocks

LimestoneMost abundant chemical rockComposed chiefly of the mineral calciteMarine biochemical limestones form as coral reefs, coquina (broken shells), and chalk (microscopic organisms)Inorganic limestones include travertine and oolitic limestones

redwall limestone

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Tyndall Limestone (Manitoba)

Fossils in Tyndall Limestone

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coquina

Chalk – Dover, England

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Travertine – malachite and rhodochrosite


Common chemical sedimentary rocks

DolostoneTypically formed secondarily from limestone

ChertMade of microcrystalline quartzVarieties include:

FlintJasperAgate

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dolostone

Chert (agate)

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Chert (agate)

Chert (“coprolite”)

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Common chemical sedimentary rocksEvaporites

Evaporation triggers deposition of chemical precipitatesExamples:

Rock saltGypsum

Rock salt (halite)

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Gypsum

Gypsum

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Fish Creek Gypsum Mine


Common chemical sedimentary rocksCoal

Different from other rocks because it is composed of organic materialStages in coal formation:

Plant materialPeatLigniteBituminous

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Successive stages in coal formation

Coal depositional environment ?

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Classification of sedimentary rocks

Classified according to the type of materialTwo major groups

Detrital (clastic)Chemical

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Two major textures are used in the classification of sedimentary rocks

ClasticDiscrete fragments and particlesAll detrital rocks have a clastic texture

NonclasticPattern of interlocking crystalsMay resemble an igneous rock


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Sedimentary environments

Geographic settings where sediment is accumulatingDetermine the nature of the sediments that accumulate (grain size, shape, etc.)


Types of sedimentary environmentsContinental

Dominated by erosion and deposition associated with streamsGlacialWind (eolian)

MarineShallow (to about 200 meters)Deep (seaward of continental shelves)

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Types of sedimentary environmentsTransitional (shoreline)

Tidal flatsLagoonsDeltas


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Sedimentary faciesDifferent sediments often accumulate adjacent to one another at the same timeEach unit (called a facies) possesses a distinctive set of characteristics reflecting the conditions in a particular environmentThe merging of adjacent facies tends to be a gradual transition

Sedimentary facies

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Sedimentary structuresProvide information useful in the interpretation of Earth historyTypes of sedimentary structures

Strata, or beds (most characteristic of sedimentary rocks)Bedding planes that separate strataCross-beddingGraded bedsRipple marksMud cracks

Strata, or beds

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Cross-bedding

Graded bedding

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Ripple marks

modern

ancient

Mud cracks

modern

ancient

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Fossils: evidence of past lifeTraces or remains of prehistoric life now preserved in rockGenerally found in sediment or sedimentary rock

Rarely in metamorphic rock

Never in igneous rock

Fossils: evidence of past lifeGeologically important for several reasons

Aid in interpretation of past environments

Serve as important time indicatorsAllow for correlation of rocks in different places

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Natural casts of shelled invertebrates

Natural casts of shelled invertebrates

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Dinosaur footprint in limestone

Dinosaur footprint in limestone

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More trace fossils

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