Sedimentology & Stratigraphy - MSU Billings 2008/Suits/Easc... · Sedimentology The study of the...
Transcript of Sedimentology & Stratigraphy - MSU Billings 2008/Suits/Easc... · Sedimentology The study of the...
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Sedimentology & Stratigraphy
Thanks to Rob Viens for slides
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SedimentologyThe study of the processes that erode,
transport and deposit sediments
Sedimentary PetrologyThe study of the characteristics and origin of
sedimentary rocks.
StratigraphyThe study of the origin, relationship, and
extent of rock layers (strata).
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Sedimentary Rocks on Earth
Shale Sandstone Siltstone LimestoneConglom.
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What do Sedimentary Rocks Record?
Magma
Igneous Rocks
Sedimentary Rocks
Metamorphic Rocks
Sediment
Crystallization Melting
Metamorphism
Lithification
Weathering
Erosion
Transport
Deposition
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What do Sedimentary Rocks Record?
•Source (Provenance) of sediment•Erosion and Transport Agent•Depositional Environment•Paleogeography/Tectonic Setting•Diagenesis (what happened after deposition)
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Rock Identification is based on:
• Composition
What minerals make up the rock?
• Texture
What is the shape, size and orientation of the mineral grains that make up the rock?
Major Difference:Crystalline vs. Clastic
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Types of Sedimentary RocksDetrital Chemical
OrganicClastic
Texture CrystallineTexture
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Detrital Rock Composition
Detrital Rocks• Lithic Fragments• Quartz• Clay Minerals• Fossil Fragments• Iron Minerals• Orthoclase (K-spar)• Muscovite / Biotite
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Note About Sediment Composition:Weathering
The sediments that make up sedimentary rocks are produced by:
Mechanical & Chemical Weathering
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Mechanical Weathering• Frost action
– Mechanic effect of freezing (and expanding) water on rocks
• Pressure release– Removal of overlying rock allows
expansion and fracturing
• Plant growth– Growing roots widen fractures
• Burrowing animals• Thermal cycling
– Large temperature changes fracture rocks by repeated expansion and contraction
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Mechanical Weathering
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But mostly physical weathering is a matter of things just falling down. So in a sense, gravity, is the primary cause of physical weathering.
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Chemical Weathering• Oxidation
– Chemically active oxygen from atmosphere– Iron oxides are common result
• Soil and sedimentary rocks often stained with iron oxides
• Acid dissolution– Hydrogen cations replace others in minerals– Carbonic acid from atmospheric CO2
dissolved in water– Sulfuric, hydrofluoric acids emitted by
volcanic eruptions– Some minerals, such as calcite, may be
totally dissolved– Human activity, such as mining and
burning of fossil fuels, produces acids
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Chemical Weathering
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Chemical Weathering• Feldspars
– Most common minerals in crust– Slightly acidic rain water
attacks feldspar– Clay minerals produced
• K+, Na+, Ca++ ions released into water
• Other minerals– Ferromagnesian minerals
• Clays, iron oxides, Mg++ ions produced– More complex silicate bonds lead to lower weathering susceptibility
• Olivine most susceptible, quartz least
• Warm, wet climatic conditions maximize weathering
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MineralStability
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Products of Weathering
• Lithic (Rock) Fragments(granite, basalt, schist, etc.)
• Dissolved Ions(Calcium, Potassium, Sodium, etc.)
• Rust Minerals (Hematite, Goethite, etc.)• Clay Minerals
(Bentonite, Montmorillonite, etc.)• Residual Minerals
(Quartz, Orthoclase, Muscovite, etc.)
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Sediment• Sediment - loose, solid particles originating from:
– Weathering and erosion of pre-existing rocks– Chemical precipitation from solution, including
secretion by organisms in water
• Classified by particle size – Boulder - >256 mm– Cobble - 64 to 256 mm– Pebble - 2 to 64 mm– Sand - 1/16 to 2 mm– Silt - 1/256 to 1/16 mm– Clay - <1/256 mm
Gravel
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Distinguishing Characteristics of Clastic Sediments:Grain Size - mud/clay, silt (<0.06mm), fine sand (0.06-0.25mm),
coarse sand (0.25-2.0mm), pebbles (2-64mm), cobbles (64-256mm), boulders (>256mm); particle size reflects energy(velocity) of the transport and depositional system.
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From Sediment to Sedimentary Rock
• Transportation– Movement of sediment away from its source, typically by
water, wind, or ice– Rounding of particles occurs due to abrasion during transport– Sorting occurs as sediment is separated according to grain size
by transport agents, especially running water– Sediment size decreases with increased transport distance
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Distinguishing Characteristics of Clastic Sediments (cont.): Sorting - Well-sorted sediment indicates prolonged reworking by wind or water; poorly sorted sediment may indicate rapid deposition, or deposition by ice or mass movement.
Angularity/Roundness and Shape – Well rounded sediment also indicate prolonged reworking by transporting agent; the shape of grains often indicates the transport system, but also may be related to the type of mineral or rock fragment
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Interpretation of Composition
Detrital Rocks:• Source of Sediment (Provenance)• Exposure to Weathering(Distance of Transport)(Type of Climate)
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Note About What Causes Detrital Rock Color
• Grain SizeSmaller is often darker
• CompositionCarbon - black or brownQuartz - tan, clear, whiteOrthoclase - orange
• Depositional EnvironmentIron on land - redIron in deep, oxygen poor water - green
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Chemical & Organic Sedimentary Rocks
(Names based primarily on composition)
Basic Rock Name• Limestone• Dolostone• Chert• Rock Salt• Rock Gypsum• Peat or Coal
Composition• Calcite• Dolomite• Quartz (silica)• Halite• Gypsum• “Carbon” / Plant
Remains
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Limestone
CrystallineLimestone
Chalk
Fossiliferous Limestone
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Formation of LimestoneCalcite Precipitates inWarm WaterWater Under Low Pressure (Shallow)
Or
It is secreted by biological organismsClams, Mussels, Scallops, etc.CoralsForaminifera
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Limestone Represents:
Shallow Tropical Ocean
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Or relatively ‘shallow’ open ocean, i.e. near shore or mid ocean ridges
http://geosci.uchicago.edu/~archer/classes/GeoSci238/
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Chalk(Coccolithophores)
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Black Sea
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Oolitic Limestone
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Travertine(Limestone)
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Dolostone
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Chert (Flint, Jasper, Agate…)
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Diatom Skeletons
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Chert Represents
Highly productive areas of the
Ocean (diatoms, radiolarians as beds) or near
shore (sponges as nodules)
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Evaporites:Bonneville Salt Flats, Utah
RockGypsum
Rock Salt
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MediterraneanGeology
Isolated,Arid,Salty
Sea/Lake
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Peat and Coal
Swamp, Bog