Sedimentary Rocks Flashcards
What are the steps to deposition?
- Created by physical and chemical weathering of other rocks.
- Eroded by wind, water or ice.
- Transported by these and later deposited when the energy in the environment is lowered and the fluid carrying them slows down.
How do deposition and energy relate?
Larger particles need faster moving fluid to pick them up or carry them than finer particles. Larger particles will settle out of a slowing fluid before finer particles.
What is a deposition sequence?
Sediments are deposited in flat layers as the settle to the bottom. Oldest sediments are at the base. Younger at the top. Each layer can be interpreted to indicate the local conditions at the time of deposition. A geologist can “read” the history from bottom to top!
What is sorting?
Flowing water “sorts” sediments by particle size. Usually, the further sediments travel from the source, the more sorted they are.
What is rounding?
When sediments are moved, they bump around and collide with each other.
This wears grains down, and causes them to become more rounded.
What are the different types of deposited rocks?
- Breccia
- Conglomerate
- Sandstone
- Mudstone
- Shale
- Limestone
What is the least sorted type of deposited rock?
Breccia
What is the most sorted type of deposited rock?
Shale
What type of deposited rock requires the most amount of energy to be deposited?
Breccia
What type of deposited rock requires the least amount of energy to be deposited?
Shale
What type of deposited rock is the most angular?
Breccia
What is a sedimentary structure?
Refers to the way in which sediments are organised or positioned in sedimentary rocks. Does not directly relate to the minerals in the sediment. Usually a shape or textural feature.
What are the different types of sedimentary structure?
- Stratification
- Beds and Laminations
- Graded bedding
- cross-bedding
- ripples
- mud cracks
What is stratification?
This refers to the layering seen in sedimentary rocks.
Layers can be:
-Grain size
-Colouring
-Composition/mineralogy
What does strata mean?
Individual layers of stratification are called strata.
What are differences between beds an laminations?
Beds are thicker strata,
generally greater than 1 mm thick.
can be up to meters thick.
Usually coarse grained sand and gravel.
Laminations are thin strata,
less than 1 mm.
Usually finer grained sediments like silt and clay.
What is graded bedding?
Formed when a huge event occurs, such as a flood, and a large amount of sediment is rapidly sorted. The cobbles and gravel drop, forming breccia or conglomerate. Then sands deposit, forming sandstone.
Then silts and clays finally settle out, forming mudstone and shales.
What is cross bedding?
These are non-horizontal laminations within a horizontal bed. They are caused be the movement of sediment over large ripples in streams and sand dunes.
What are ripples?
Ripples are formed by moving currents of water pushing sediment grains. The shape of the ripple can indicate flow direction.
What is a symmetrical ripple?
Symmetrical ripples are formed when water moves back and forth, such as in tidal locations.
What are mud cracks?
Some clays shrink when they dry out. This causes clay layers to form mud cracks. If a new layer of sediment is then deposited, the cracks can get filled in with new sediment. This causes the mud cracks to be fossilised.
What did James Hutton discover?
“the same natural laws and processes that operate in our present-day scientific observations have always operated in the universe in the past and apply everywhere in the universe”
He noticed that modern processes seemed to occur in a predictable fashion, e.g. when it rained, soil would move from the tops and slopes of hills, into the valleys/ sedimentary basins.
He then noticed that rocks on land showed evidence of these processes having occurred in the past, therefore concluding that the modern land/rock formed in scenarios similar to those occurring on the sea floor / sedimentary basins today.
“The present is the key to the past” so to speak.
