Environmental Engineering Reference
In-Depth Information
TABLE 5.8
Broad Classification of Sedimentary Rocks
Rock Type
Material
a
Diameter Composition
Depositional Environment
(mm)
Detrital
Conglomerate
Boulders
256
Same as source rock
Along stream bottoms. Seldom found
in rock masses
Cobbles
256-64
Same as source rock
Along stream bottoms. Deposited
as alluvial fans and in river channels
Pebbles
64-4
As for cobbles or sand
As for cobbles: also deposited in
beaches
Granule
4-2
As for cobbles or sand
As for pebbles and sand
Sandstone
Sand
2-0.02
Primarily quartz: also feldspar,
All alluvial deposits: stream
garnet, magnetite. Some
channels, fans, floodplains,
locales: hornblende, pyroxene,
beaches deltas. Occasionally
shell fragments
aeolian
Siltstone
Silt
0.02-0.002
As for sand: often some clay
Deltas and floodplains
particles
Shales
Clay
0.002
Colloidal sizes of the end
Quiet water. Salt water: clay
result of decomposition of
particles curdle into lumps and
unstable minerals yielding
settle quickly to the bottom. Show
complex hydrous silicates
no graded beds. Freshwater: Settle
(see
Section 5.3.3)
slowly; are laminated and well-
stratified, showing graded
bedding
Nondetrital
Limestone
Calcareous
Massive calcite
Deep, quiet water
precipitate
(CaCO
3
)
Coquina
Calcareous
Cemented shells
Along beaches, warm water
precipitates
Chalk
Calcareous
Microscopic remains
Clear, warm, shallow seas
precipitates
of organisms
Dolomite
Calcareous
Dolomite —
Seawater precipitation or alteration
precipitates
CaMg(CO
3
)
2
of limestone
Gypsum
b
Calcareous
Gypsum —
Saline water
precipitates
CaSO
4
⋅
2H
2
O
Detrital
Anhydrite
b
Calcareous
Anhydrite — CaSO
4
Saline water
precipitate
Halite
b
Saline
Sodium chloride
Saline water
precipitates
Coal
Organic
Carbonaceous matter
Swamps and marshes
Chert
Silicate
Silica, opal
Precipitation
a
The Wentworth scale.
b
Evaporites.
Organics
Beds of decayed vegetation remain in place to form eventually coal when buried beneath
thick sediments.
Depositional Characteristics
Horizontal Bedding
Under relatively uniform conditions, the initial deposition is often in horizontal beds.
