Sedimentary Rocks and Resources
adapted to HTML from lecture notes of Prof. Stephen A. Nelson Tulane
Introduction to sedimentary rocks
, and rain runoff all have the
ability to carry the particles washed off of eroding rocks. Such material,
called detritus, consists of fragments of rocks and minerals. When the
energy of the transporting current is not strong enough to carry these
particles, the particles drop out in the process of sedimentation. This
type of sedimentary deposition is referred to as clastic sedimentation.
Another type of sedimentary deposition occurs when material is dissolved
in water, and chemically precipitates from the water. This type of
sedimentation is referred to as chemical sedimentation. A third process
can occur, wherein living organisms extract ions dissolved in water
to make such things as shells and bones. This type of sedimentation
is called biogenic sedimentation. Thus, there are three major types
of sedimentary rocks: Clastic
, chemical sedimentary rocks, and biogenic
Classification - Clastic sedimentary particles are classified in terms of
| Name of Particle
|| Size Range
|| Loose Sediment
|| Consolidated Rock
|| >256 mm
|| Conglomerate or Breccia (depends on rounding)
|| 64 - 256 mm
|| 2 - 64 mm
|| 1/16 - 2mm
|| 1/256 - 1/16 mm
|| <1/256 mm
|| Claystone, mudstone, and shale
The formation of a clastic sedimentary rock involves three processes:
Transportation- Sediment can be transported by sliding down slopes, being
picked up by the wind, or by being carried by running water in streams,
rivers, or ocean currents. The distance the sediment is transported and
the energy of the transporting medium all leave clues in the final
sediment of the mode of transportation.
Deposition - Sediment is deposited when the energy of the transporting
medium becomes too low to continue the transport process. In other words,
if the velocity of the transporting medium becomes to low to transport
sediment, the sediment will fall out and become deposited. The final
sediment thus reflects the energy of the transporting medium. Diagenesis -
Diagenesis is the process that turns sediment into rock. The first stage
of the process is compaction. Compaction occurs as the weight of the
overlying material increases. Compaction forces the grains closer
together, reducing pore space and eliminating some of the contained water.
Some of this water may carry mineral components in solution, and these
constituents may later precipitate as new minerals in the pore spaces.
This causes cementation, which will then start to bind the individual
particles together. Further compaction and burial may cause
recrystallization of the minerals to make the rock even harder. Other
conditions present during diagenesis, such as the presence of absence of
free oxygen may cause other alterations to the original sediment. In an
environment where there is excess oxygen (Oxidizing Environment) organic
remains will be converted to carbon dioxide and water. Iron will change
from Fe2+ to Fe3+, and will change the color of the sediment to a deep red
(rust) color. In an environment where there is a depletion of oxygen
(Reducing Environment), organic material may be transformed to solid
carbon in the form of coal, or may be converted to hydrocarbons, the
source of petroleum.
Textures of Clastic Sedimentary Rocks
When sediment is transported and deposited, it leaves clues to the mode of
transport and deposition. For example, if the mode of transport is by
sliding down a slope, the deposits that result are generally chaotic in
nature, and show a wide variety of particle sizes. Grain size and the
interrelation ship between grains gives the resulting sediment texture.
Thus, we can use the texture of the resulting deposits to give us clues to
the mode of transport and
Sorting - The degree of uniformity of grain size. Particles become sorted
on the basis of density, because of the energy of the transporting medium.
High energy currents can carry larger fragments. As the energy decreases,
heavier particles are deposited and lighter fragments continue to be
transported. This results in sorting due to density.
If the particles have the same density, then the heavier particles will
also be larger, so the sorting will take place on the basis of size. We
can classify this size sorting on a relative basis - well sorted to poorly
sorted. Sorting gives clues to the energy conditions of the transporting
medium from which the sediment was deposited. Examples
- Beach deposits and wind blown deposits generally show good sorting
because the energy of the transporting medium is usually constant.
- Stream deposits are usually poorly sorted because the energy
(velocity) in a stream varies with position in the stream.
Rounding- During the transportation process, grains may be reduced in size
due to abrasion. Random abrasion results in the eventual rounding off of
the sharp corners and edges of grains. Thus, rounding of grains gives us
clues to the amount of time a sediment has been in the transportation
cycle. Rounding is classified on relative terms as well.
Chemical Sediments and
Sedimentary Rocks Cherts
- chemically precipitated SiO2
Evaporites - formed by evaporation of sea water or lake water. Produces
halite (salt) and gypsum deposits by chemical precipitation as
concentration of solids increases due to water loss by evaporation.
Biogenic Sediments and Sedimentary Rocks
Limestone - calcite (CaCO3) is precipitated by organisms usually to form a
shell or other skeletal structure. Accumulation of these skeletal remains
results in a limestone. Diatomite - Siliceous ooze consisting of the
remains of radiolarian or diatoms can form a light colored soft rock
called diatomite. Coal - accumulation of dead plant matter in large
abundance in a reducing environment (lack of oxygen). Oil Shale - actually
a clastic sedimentary rock that contains a high abundance of organic
material that is converted to petroleum during diagenesis.
Features of Sedimentary Rocks That Give Clues to the
Environment of Deposition
Stratification and Bedding
Rhythmic Layering - Alternating parallel layers having different
properties. Sometimes caused by seasonal changes in deposition (Varves).
i.e. lake deposits wherein coarse sediment is deposited in summer months
and fine sediment is deposited in the winter when the surface of the lake
Cross Bedding - Sets of beds that are inclined relative to one another.
The beds are inclined in the direction that the wind or water was moving
at the time of deposition. Boundaries between sets of cross beds usually
represent an erosional surface. Very common in beach deposits, sand dunes,
and river deposited sediment.
Graded Bedding - As current velocity decreases, first the larger or more
dense particles are deposited followed by smaller particles. This results
in bedding showing a decrease in grain size from the bottom of the bed to
the top of the bed.
Non-sorted Sediment - Sediment showing a mixture of grain sizes results
from such things as rockfalls, debris flows, mudflows, and deposition from
Ripple Marks - Characteristic of shallow water deposition. Caused by waves
Mudcracks - result from the drying out of wet sediment at the surface of
the Earth. The cracks form due to shrinkage of the sediment as it dries.
Raindrop Marks- pits (or tiny craters) created by falling rain. If
present, this suggests that the sediment was exposed to the surface of the
Fossils - Remains of once living organisms. Probably the most important
indicator of the environment of deposition.
- Different species usually inhabit specific environments.
- Because life has evolved - fossils give clues to relative age of
- Can also be important indicators of past climates.
- Iron oxides and sulfides along with buried organic matter give rocks
a dark color. Indicates deposition in a reducing environment.
- Deposition in oxidizing environment produces red colored iron
A sedimentary facies is a group of characteristics which reflect a
sedimentary environment different from those elsewhere in the same
deposit. Thus, facies may change vertically through a sequence as a result
of changing environments through time. Also, facies may change laterally
through a deposit as a result of changing environments with distance at
the same time.
Common Sedimentary Environments
- Non-marine environments
- Continental Shelf sediments
- Deltaic sediments
- Beach sediments
- Carbonate shelf sediments
- Marine evaporite sediments
- Deep Sea Fans
- Sediment drifts
- Land-derived sediments