Sedimentary cheat sheet

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Sedimentary Rocks
Characteristics of Sedimentary Rocks
Formed at or very close to the earth's surface
Deposited in layers or beds often horizontal
Frequently contain fossils
Classification of Sedimentary Rocks
3 Groups recognised according to mode of origin
Chemical Precipitates
Clastic rocks represent the accumulation of weathered and eroded fragments of older,
preexisting rocks of all types
Organic Sedimentary Rocks. Formed from the remains of onceliving organisms
Chemical Sedimentary Rocks. Rocks precipitated directly from solution
Clastic Rock Groups
Rudaceous Coarse >2mm
Arenaceous Medium 1­ 2mm
Argillaceous ­ Fine <1mm
Rudaceous Rocks
Over 50% of the clasts (particles) are over 2mm in diameter
Primarily consist of rock fragments
If particles rounded = Conglomerate
If particles angular = Breccia
Arenaceous Rocks
Over 50% of the particles are 1mm to 2mm in diameter
Comprise a high percentage of quartz grains
These rocks are generally called sandstones
However, a wide variety of sandstones occur
Desert Sandstone, Arkose, Greywacke
Micaceous Sandstone, Orthoquartzite, Grit
Clastic Rock Terminology
Phenoclast ­A large clast/rock fragment
Matrix ­the finer material often sand, silt and clay surrounding the phenoclasts
Cement ­material precipitated from solution to stick the sediment together. This is often quartz,
calcite or haematite.
Well Sorted ­ all of the clasts are very similar in size (unimodal)
Poorly Sorted ­ clasts show a wide range of particle sizes (polymodal)
Oligomict ­ all clasts are of the same type
Polymict ­ clasts are of a variety of types

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Typical deposit of a highenergy shallow marine environmentbeach
Silica cement
Clasts range in size 1mm ­ 3cm, poorly sorted, polymodal
Clasts all well rounded texturally mature.
Over 50% of the clasts (particles) are over 2mm in diameter and Rounded.…read more

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Most grains between 1 and 4mm, but polymodal. Well cemented together by a Haematite
cement. Texturally and mineralogically immature. Comprises sub angular to sub rounded grains.
Formed by buried sand, ground water percolates through with silica impurities. Silica binds rock
90% Quartz
10% Feldspar
Desert Sandstone
Windblown deposit Formed in a desert . Red/brown haematite cement
Grains well rounded and texturally mature. Grains have frosted surfaces
due to constant abrasion. All grains are quartz mineralogically mature.
Poorly consolidated grains rub off in the fingers.…read more

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Deposited in a low energy environment such as a river estuary or marine harbour . Grain size
really small. Structureless with little evidence of laminations. Represents a clay that has been
consolidated and the water content reduced. Comprises entirely clay minerals. Feels smooth when
rubbed on the teeth and implies absence of quartz. Represents a clay that has been consolidated
and the water content reduced. Long transportation. Green indicates oxygen, no energy.…read more

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Thin layers of the same rock each layer must be less than 2 cm
Bedding is large layers of different rock greater than 2cm
Symmetrical ripple marks
Water oscillates (rotates). This oscillates the water underneath rotating it moving the sand grains
up and down (two directions of flow) If there are rounded peaks then it means a high energy level
of waves whiles if they are angular then it is low energy.…read more

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Graded bedding
After an earthquake or volcanic eruption the loose sediment at
the top of an incline is moved down the slope into turbidity
current and as the turbidity current loses energy it deposits
bigger sediment (not enough energy to carry) keeps losing
energy and so drops the finer sediment off until we get this
picture occurring.…read more


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