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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

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