0111 sedimentary geology overview_am
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SEDIMENTARY GEOLOGY OVERVIEW
Andrew S. Madof
Orals Review - 2007
January 12, 2007
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SEDIMENTARY GEOLOGY =Sedimentology (process) + Stratigraphy (response)
Sedimentology = study of PROCESSES (i.e.
production, composition, transport, and deposition ofsediment)
Stratigraphy = study of RESPONSES (i.e.inferring the controls on the spatial and temporal
changes of strata) exact processes that created therocks cant be know because only the rocks are left,not the processes
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Sedimentation And Sedimentary Rocks
Sedimentary Rocks:
Form 75% of the rocks exposed at the Earths
Surface
Are the reservoirs for fossil fuels, iron and
aluminum ores, and groundwater
Record of Earths history
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Sediment
Sediment = loose, solid particles and can be: Terrigenous = fragments from silicates (igneous and/or
metamorphics)
Biogenic = fossils (carbonate - reefs; silicates - forams)
Chemical= precipates (halite, gypsum, anhydrite, etc) - note:with chemical sedimentary rocks, evaporation > precipitation
and/or supersaturation in closed basins (lakes or oceans)
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 -
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Grain size (diameter) and grain-shape depend on:
Transport media: rivers (pebbles bounce on river bottom, sandmoved in traction, and silt/clay suspended in water column); oceans
and lakes (near-shore and deep-water systems); glaciers (sediment
moved on glacier bottom); wind (sand dunes)
Distance from parent rock: the longer the distance traveled,generally the smaller and the more well-rounded the grains (due to
higher kinetic energy)
Mineral hardness: the harder the parent rock, the longer it will
take the sediments to erode (example: silicates are more resistant to
weathering and erosion than feldspars, and this is why beaches areoften comprised of sand, not feldspar-rich sediments)
Consider: sorting (= range of grain sizes) winds sort well
(meaning grain sizes are very similar); glaciers sort poorly (meaning
there is a large spread of grain sizes in glacial deposits)
Grain size
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Classification Of Sedimentary Rocks
DETRITAL (TERRIGENOUS) SEDIMENTARY ROCKS:
Mudstones Sandstones
Conglomerates
Breccias
DETRITAL SEDIMENTARY ROCKS:
Classification Based On Particle Size
a) All detrital rocks are clastic
b) Sand and silt are predominantly quartz
c) Finer-sized particles of clay minerals
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SANDSTONES:a) 25% of all sedimentary rocks
b) Sandstone particles (1/16-2 mm in diameter)
c) Practical uses of sandstones: buildings and reservoir for
fossil fuels and groundwater
CONGLOMERATES AND BRECCIAS:a) Grain diameters larger than 2 mm
b) Conglomerates have rounded grains
c) Breccias have angular grains
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Breccia (from
fault motion?)
Sandstone
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MUDSTONES:
a) More than half of all sedimentary rocks
b) Contain the smallest particles (0.004 mm in diameter)
c) Environments of deposition: lakes, lagoons, deep ocean
basins, river floodplains
d) Color variety of shale represents mineral composition
e) Practical uses of shale: bricks, ceramics, cement, and oilshale
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Chemical Sedimentary Rocks
Inorganic Chemical Sedimentary Rocks
a) LIMESTONE (inorganic):
I) FORMATION
II) Oolitic Limestone
III) Tufa
IV) Travertine
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Lithification = Turning Sediment Into Sedimentary Rock
Diagenesis = Changes in the sediment due toincreased heat, pressure, and circulating
groundwater
Lithification = Compaction + Cementation
Compaction = Diagenetic process by which theweight of overlying materials reduces the
volume of sedimentary body (decreases
porosity)
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Cementation: Precipitation of dissolved ions in the pore
space
a) calcium carbonate - CaCO3b) silica - SiO2c) iron compounds - Fe+2 and Fe+3
Texture of Rock: Formed by compaction and cementationof sediment particles
Recrystallization: recrystallization of certain unstable
minerals into new, more stable minerals (this happensprimarily in carbonates, when you start with carbonate mud
[a.k.a. micrite] heat it up, then cool it to form larger grains
[a.k.a. sparite])
Cementation & Recrystallization
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Types of Sediment
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Common Geological Environments
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Locations of Subsurface Evaporites
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Sedimentary Structures
Bedding (stratification): arrangement of sedimentparticles into distinct layers
A) Changes in sediment change bedding
B) Changes in transport energy change bedding
Normally graded bedding: sediment layer (formed
by a single depositional event) in which particlesize varies gradually with the coarsest particles onthe bottom (note: inversely graded bed = fines onbottom and coarse grains on top )
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Cross-bedding: sedimentary layers deposited atan angle to the underlying set of beds
Surface sedimentary featuresA) Ripple Marks: small surface ridges produced
when water or wind flows over sediment after
it is deposited
B) Mudcracks: occur on the top of a sediment layer
when muddy sediment dries and contracts
Cross-bedding and Mudcracks
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Development of Cross-Bedding
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Asymmetric and Symmetric Ripples
river or wind currents
(uni-directional)tidal currents
(bi-directional)
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Formation of Coal from Swamp Deposits
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Formation of Coal from Swamp Deposits
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Initial Deposits of Flat/Tabular Clay
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Formation of Ooliths
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Origin of Mud Cracks
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Lithification of Sediments
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Sedimentary Facies Formation
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Sedimentary Facies Formation
S di t i St
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Sediment in a Stream
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Marine sedimentary environment
Landward Migration of Shoreline Regression
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Landward Migration of Shoreline = Regression(regression can either form due to 1) lower sea level or 2) shoreline building basinward [a.k.a. progradation])
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Graded Bedding = Vertical Decrease ofSediment Size
Turbidity Current =
PROCESS
Turbidite =RESPONSE