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