SEDIMENTS

 

Sediments are defined as naturally-occurring, loose grains of any composition or origin at the Earth’s surface.  Most sediments have a lithogenous origin - they form by weathering and erosion of any type of rock.  Some sediments have a biogenous/biogenic origin - they are the remains, or fragmentary remains, of once-living organisms (animals, plants, microbes, etc.).

 


 

SEDIMENT  SIZES

Sediments range in size from incredibly small to immensely huge.  In sedimentology, categories of grain sizes are given specific names - see the list below, arranged from the largest sediment grain sizes at the top to the smallest grain sizes at the bottom.

 

Megaliths - 32 to 1000 km

Monoliths - 1 to 32 km

Slabs - 64 m to 1 km

Blocks - 4 to 64 m

Boulders - 256 mm to 4 m

Cobbles - 64 to 256 mm

Pebbles - 4 to 64 mm

Granules (a.k.a. grit) - 2 to 4 mm

Sand - 2 to 1/16 mm

Silt - 1/16 to 1/256 mm

Clay - <1/256 mm

 

Almost all of the clastic-textured sedimentary rock record consists of grains that are boulder-sized or smaller.  The large end of the sediment size scale is reserved for describing the size of clasts in some impact breccias (info. from Jared Morrow’s Geological Society of America presentation, “Impact breccias - a many-splendored thing”, Philadelphia, 2006).

 

Sand- and silt-sized grains are so important and common in the sedimentary record that these categories have been subdivided:

    - very coarse sand - 2 to 1 mm

    - coarse sand - 1 to 1/2 mm

    - medium sand - 1/2 to 1/4 mm

    - fine sand - 1/4 to 1/8 mm

    - very fine sand - 1/8 to 1/16 mm

    - coarse silt - 1/16 to 1/32 mm

    - medium silt - 1/32 to 1/64 mm

    - fine silt - 1/64 to 1/128 mm

    - very fine silt - 1/128 to 1/256 mm

 


 

FLUVIAL SEDIMENTS (river/stream channel sediments)

 

EOLIAN SEDIMENTS (wind-blown sediments)

 

LACUSTRINE SEDIMENTS (lake sediments)

 

LITHOGENOUS MARINE BEACH SEDIMENTS

 

CARBONATE PLATFORM BEACH SEDIMENTS

 


 

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