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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
Clastic
Organic
Chemical Precipitates

Clastic rocks represent the accumulation of weathered and eroded…

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Conglomerate
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.
15% Quartz
80% Lithics
5% Feldspars

Breccia

Produced by a…

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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
together.
90% Quartz
10% Feldspar

Desert Sandstone

Windblown deposit Formed…

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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…

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Thin layers of the same rock each layer must be less than 2 cm



Beds

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…

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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…

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