Sedimentary Rocks : Sedimentary Rocks Form as sediment (rock fragments) that have been eroded (moved) from one location to another are deposited (placed).
Most sediments are deposited in large bodies of water, and tend to be rounded by their erosional agent.
The formation of sedimentary rocks gives us clues about the environment at the time of rock formation.
Sedimentary Rock Formation : Sedimentary Rock Formation Most sedimentary rocks form as a result of compression and cementation of broken rock material, into one solid rock.
Compression:
The pressure created by water and other sediments on top of a layer of sediment, squeezes the water out of the sediment.
The sediment then hardens into solid rock.
Most shale forms this way as clay particles made of mica are compressed.
Shale: : Shale: Shales are made of compacted clay particles.
Do you think you can see individual grains in a shale?
No grains are visible.
Shale is the most common sedimentary rock type on earth.
Sedimentary Rock Formation Cont. : Sedimentary Rock Formation Cont. Cementation
When broken rock fragments are “glued” together by a cementing agent.
Usually silica or calcite acts as a cementing agent and “glue” broken rock fragments together.
What would be a good way to determine if the “glue” is silica or calcite?
Think back to your Identifying Minerals Lab.
Calcite reacts w/ acid.
Sandstones and Conglomerates form in this manner.
Sandstone: : Sandstone:
Conglomerates : Conglomerates A rock containing >50% clasts larger than .2 cm (the limit for coarse sand),
Though conglomerates usually have pebble size grains or larger.
Clasts may be any rock type, and are commonly a mixture in any one outcrop.
Sedimentary Rock Formation Cont. : Sedimentary Rock Formation Cont. Chemical Process:
Some sedimentary rocks form as the result of chemical processes, namely evaporation and precipitation.
Water can dissolve some minerals (halite for example) and carry them in solution.
When the water begins to evaporate the amount of the minerals it can hold decreases.
The mineral then begins to form crystals again.
Rocks that form in this manner are called Evaporites.
Examples: See ESRT pg. 7
Gypsum : Gypsum Rock gypsum: this rather featureless rock is made almost entirely of gypsum, colored with a trace of hematite dust.
Gypsum beds can be extensive, and represent evaporites left from the evaporation of saline water.
Sedimentary Rock Formation Cont. : Sedimentary Rock Formation Cont. Biological Formations:
Are these rocks made out of minerals?
No. They are ORGANIC in origin so are not made of minerals!
Bituminous Coal:
Forms when the remains of plants are deposited in shallow, still water, where they are compressed into rock.
Limestone:
Forms when corals use minerals like calcite to form hard shells, when the corals die they leave the shell behind to be compacted into rock.
Limestone:Remember it Reacts with Acid! : Limestone:Remember it Reacts with Acid!
Bitmuminous Coal: : Bitmuminous Coal: Forms in areas that were once swampy, and rich in plant life
The plant matter was then compacted into solid rock.
Coal is one of the major sources of energy for Electrical Power plants.
Where Sedimentary Rocks are Found. : Where Sedimentary Rocks are Found. Sedimentary rocks are found as a thin layer over 75 % of the Earth.
Sedimentary rocks are not found deep underground because the processes that form them occur at the surface only.
Look at Pg. 3 of the ESRT.
Note the Distribution of Sedimentary Rocks.
Features of Sedimentary Rocks : Features of Sedimentary Rocks Kinds of Sedimentary Rocks
There are 3 main kinds of sedimentary rocks, Clastic, Chemical or Organic (Bioclastic).
Clastic:
Form from fragments of other rocks by compaction and cementation
Chemical:
Form when mineral crystals form as precipitates or other chemical reactions.
Organic (Bioclastic):
Sedimentary Rocks forming from the remains of plants and animals.
Properties/Characteristics of Sedimentary Rocks : Properties/Characteristics of Sedimentary Rocks Discrete Layers:
Layers form as one sediment settles on top of another, and then are compressed into rock.
Fragmental Particles:
Called Clastic sedimentary rocks.
These rocks are made of pieces of rock cemented together.
These rocks are classified based on the size of the grains of sediment.
Look at your ESRT pg. 7
Grain Sizes : Grain Sizes Grain Sizes are recorded on your ESRT pg 6 also.
Your ESRT lists the sizes of each particle in centimeters.
Properties/Characteristics of Sedimentary Rocks : Properties/Characteristics of Sedimentary Rocks Organic Composition
Some rocks are composed partially or entirely out of the remains of once living things.
Such remains are called Fossils.
These rocks are referred to as Bioclastic.
These rocks are identified by composition and texture.
Predominance of One Particle Size:
Gives us clues that the environment the rock formed in was steady over a long period of time.
What sedimentary rocks tell us : What sedimentary rocks tell us The environment in which a sedimentary rock forms is often determined by its characteristics.
Composition:
The presence of marine fossils indicates that the rock formed at the Earth’s Surface in a large body of water (ocean).
If salt is present we know it was probably formed by the evaporation of water, meaning it was shallow water.
If limestone is present we know it was a warm and shallow environment, like those that corals form at today.
What sedimentary rocks tell us (2) : What sedimentary rocks tell us (2) Structure:
The layers in rocks offer clues as to the environment of formation.
Layers with ripple marks, cross bedding, and fossil shells indicate these rocks formed in shallow marine environments.
The size of the clasts in a rock tell us where it formed, if large particles are present only, then the water above was moving enough to carry only small particles of sediment.
What sedimentary rocks tell us (3) : What sedimentary rocks tell us (3) Texture:
The shape of the rock fragments tell us a lot about where the rock formed in relation to the source material (original rock).
Rounded sediments tell us the sediment has been moved a great distance by water.
Angular sediments tell us it hasn’t been moved far, or was not transported by water.
Texture: : Texture: On left, angular on the right rounded sediments