Sedimentary Processes and Rocks Flashcards
How do sedimentary rocks form?
At/near the Earth’s surface as a result of:
Weathering of pre-existing rocks -> specific minerals, ions
Erosion & transportation of sediment -> rounded clasts
Deposition -> layers (beds) of sediment reflect different depositional events and environments
Burial
Compaction & cementation turn sediment into rock
What minerals occur in sedimentary rocks?
Minerals that survive weathering (residual) e.g. quartz
Minerals produced in weathering reactions (secondary) e.g. clay minerals, iron oxides
Minerals that precipitate from dissolved products of weathering e.g. calcite, dolomite, gypsum and halite
Where do sediments come from?
Plate Tectonics produces: Uplift
- > topographic relief for erosion & transport
- > rock sources of sediment
Where do sediments collect?
Plate Tectonics produces: Subsidence
-> depressions, basins for deposition
how are sediments transported?
Rivers, streams Mass wasting Glaciers Wind Ocean currents
what are the different types of sedimentary rock?
Clastic
Chemical
Organic
what are clastic rocks formed from?
formed from fragments of rocks or minerals
e.g. mudstone, sandstone, conglomerate
what are chemical rocks formed from?
formed when dissolved ions carried in solution (usually seawater) are precipitated
e.g. limestone, chert
what are organic rocks formed from?
formed from the dead remains of plants or animals
e.g. coal
what are the steps to forming clastic sedimentary rocks?
1) Weathering and erosion -> sedimentary detritus (clasts = grains)
2) Transportation of sediment grains
3) Deposition of sediments
4) Lithification: unconsolidated sediment -> rock
how are terrigenous sediments transported?
by rivers to continental margins
what are turbidity currents? what do they do?
underwater mass flows
transport sediment down continental slope to floor of ocean basin
Grain sizes deposited vary with…
velocity/energy and become sorted
larger grains are deposited first
which clasts would be more mature?
more rounded ones (as opposed to more angular ones)
what is lithification?
loose sediments -> rock
what are the stages of lithification?
(1) Compaction:
As the pile of sediment grows, water is squeezed out from sediment at the base of the pile -> grains forced closer together (compacted)
(2) Cementation:
Fluids in the pile precipitate minerals -> forms a cement - binds the grains together
what is cementation?
minerals (carbonates/silica/clays) precipitate and fill pore space between the clasts (grains)
what are the steps to forming chemical sedimentary rocks?
1) Chemical weathering -> ions in solution
2) Transportation of dissolved ions
3) Precipitation of minerals from ions in solution (usually seawater) – due to changing physical conditions or due to actions of living organisms (biogenic)
4) Lithification
Chemical sedimentary rocks: classified as non-biogenic or biogenic
what are non-biogenic chemical sedimentary rocks?
(no organisms involved)
Travertine (precipitation of calcite)
Tufa (calcite)
Evaporites (halite; gypsum)
what are biogenic chemical sedimentary rocks?
Corals, colonial invertebrates, precipitate CaCO3 from sea water to build their skeletons/shells
Accumulation of dead corals and other fragments, compaction, cementation -> limestone
where do most coral reefs form? why?
Most coral reefs form in the “photic” zone since they need light
They are restricted to warm, shallow sea
how do atolls form?
Reef forms at edges of newly formed volcano
As volcano subsides, reef grows upward
As volcano continues to subside, reef continues to grow upward, forming an atoll
what are deep-sea biogenics?
slow accumulation of silica (quartz=SiO2) skeletons of micro-organisms: diatoms, dinoflagellates, and radiolaria
e.g. siliceous ooze -> chert
Chert can also be made of glass sponges (silica skeletons)
what is an example of an organic sedimentary rock?
Carbon-rich rock composed of plant and animal remains: coal.
Buried, decaying organic matter
Maturity increases with depth/ duration of burial
Depositional environment - Swamp
Why are sedimentary rocks important?
Resources
–Host rocks for oil, gas, water
–Source of potash, phosphate, iron, aluminium, salt…
–Building stone, ceramic clays, aggregates
Provide a record of past conditions & processes on the Earth’s surface
They reflect energy of environment and type of depositional environment
rocks record changes in sea level such as…
Transgression (sea level rises)
Regression (sea level falls)
Uplift/Erosion
how are sediments deposited?
in layers (beds or strata)
Provide a record of Earth processes over geological time: climate, biologic evolution, environment, plate movement
Sedimentary rocks preserve fossils and thus…
a record of prehistoric life & conditions
