View
230
Download
0
Tags:
Embed Size (px)
Citation preview
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
6_116_11
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
6_196_19
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
6_56_5
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
6_276_27
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