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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
Detrital sedimentary rocks
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
Detrital sedimentary rocks
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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Chemical sedimentary rocks
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
Chemical sedimentary rocks
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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Chemical sedimentary rocks
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
Chemical sedimentary rocks
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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Coal depositional environment ?
Coal depositional environment ?
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Coal depositional environment ?
Classification of sedimentary rocks
Classified according to the type of materialTwo major groups
Detrital (clastic)Chemical
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Classification of sedimentary rocks
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
Classification of sedimentary rocks
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Sedimentary environments
Geographic settings where sediment is accumulatingDetermine the nature of the sediments that accumulate (grain size, shape, etc.)
Sedimentary environments
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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Sedimentary environments
Types of sedimentary environmentsTransitional (shoreline)
Tidal flatsLagoonsDeltas
Sedimentary environments
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Sedimentary environments
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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