Erosion and deposition 3+4+5 (Environements of creation (The size of…
Erosion and deposition 3+4+5
Erosion and transportation by wind/ water
Water is a more effective agent of erosion and transportation than wind: it has 55 times the density of air at 20degreesC.
Failure of steep slopes is often a response tp earthquakes or torrential rainstorms.
Water erodes by
Impact of transported particles (abrasion).
Pressure difference in turbulent water (eddies).
The relationship between water velocity and the size of grains it can erode and transport is non-linear.- expressed in the Hjulstrom diagram.
Greater velocities needed to erode smaller grains due to electrostatic attraction between them.
How is sediment transported?
Sediment grains move via mounds of sediment called bed-forms that are moulded by the flow of wind/water.
Few cm high. Forms on the floors of sandy deserts, streams and oceans. 2 types.
Asymmetrical- formed by unidirectional flow (e.g. stream). Gentle slope on one side, steep slope on the other. few cm high.
Symmetrical- formed by oscillating flow (e.g. waves). Equal slopes a few cm high.- sediment scoured from trough, sediment deposited on lee, sediment scoured from trough, sediment deposited on lee (other lee)
Ripples and dunes have a distinctive internal structure, comprising layers at 30-35degrees to the horizontal. This is cross bedding.
These layers form by the avalanching of sheets of sediment down the steep sides of ripples and dunes.
Cross lamination and cross bedding
Indicate the direction of sediment movement.
Tell if a rock has been overturned.- correct way up- layers truncated by an erosion surface at the top of the bed. Layers flattened out towards the base of the bed.
Environements of creation
Hot temperature- Limestone, sandstone, red sandstone, mudstone.
Humidish climate- Limestone.
Arid climate- Sandstone, red sandstone and mudstone.
Low sea level- Sandstone, red sandstone and mudstone.
High sea level- Limestone.
Sedimentary rocks provide information on; sea level, temperature and humidity, rainfall, wind speed and direction, river flow and ocean currents.
The size of sedimentary rocks indicates the energy of the transporting environment.
The shape and mineralogy reflects the duration of transport.
Sorting increases with duration of transport because grains have more time to adjust to current velocity.- poorly sorted = wide grain size= texturally immature sediment.
Arenite= <15% clay matrix.
Wake= 15-75% clay matrix.
Mud-rock- Composed of clay and fine grained quartz, deposited in low energy environments, lakes, river floodplains, ocean basins, mudflats.
Lithification of clastic sediments
Turns a clastic sediment into a sedimentary rock, by:
Compaction- physical process.
Cementation- chemical process
Clastic sediments are initially loosely packed (cubic packing) with air/ water between grains.
As pressure increases, grains rearrange to occupy a smaller volume.
Clastic sediments can be lithified solely by compaction, but an inter-granular cement is usually required to turn them into a rock.
Common cements are quartz and calcite- quartz forms overgrowths on grains. Calcite forms large crystals enclosing many grains.
Coal an organic sedimentary rock- made from the remains of organic matter.
Plant remains convert to coal by physical and chemical burial changes- 4-10km of burial to make coal.
Volatiles are driven off by heating, increasing the carbon content of the rock.
Coals with the greatest carbon content (anthracite) have the highest heat value and are the most valuable.
Most British coals were deposited in tropical deltas during the Carboniferous period.
A liquid produced from clastic sedimentary rocks with abundant remains of water-living microbes (bacteria, algae).
These organic rich rocks were deposited in marine basins and freshwater lakes whose oxygen-poor (anoxic) bottom waters preserved the microbial remains.
Oxygenated surface water supports seasonal algal blooms.
Algal matter was preserved with oxygen-poor (anoxic) bottom waters.
Oil forms by heating of the organic-rich clastic sedimentary rocks (mud-rocks) to -100-150C= 3-6lm burial depth.
The oil then moves upwards through overlying rocks to be trapped once it meets an impermeable layer (cap rock).
Oil accumulates beneath the cap rock in reservoir rocks that are porous (lots of space between grains) and permeable (interconnected pore spaces).