- Horozontal layers form when sediment is deposited in beds.
- Each bed is separated by a bedding plane.
- Sedimentary structres are features found on bedding planes.
- Sedimentary structures are produced by a variety of processes in a range of evironements.
- They provide evidence for the environment in which they form and are 'way-up- indicators.
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1. Cross Bedding
- Sand grains are moved by the wind, river or sea currents.
- The currents flow in one direction over time.
- The results ia gentle slope on the upstream and a steep slope on the downstream sides (grains avalanche down).
- Grains setlle at a max angle of 37 degrees (angle of rest)
- Dunes migrate down current
- Only the steep slope is preserved
- A new dune migrates over an priginal dune and cuts it off. This produces new cross bedding layers.
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2. Graded Bedding
- Largest particles at the bottom and finer particles at the top.
- Abrupt change and then bed is repeated
- Useful way up structure
- Made by turbidity currents onto the abyssal plain or rivers floing into calm lakes.
- Found in sandstone, greywacke, conglomerates.
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- Pebbles rolled along river bed and piled up next to one another.
- Flat pebbles stack up with their long axes parallel (max resistance to movement) pebbles are inclines in an upstream direction with the tops of the pebbles pointing downstream.
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- Cubic halite (salt) crystals grow at the surface of a bed, due to evaporation of salt water in aris environments.
- The crystals become embedded in the sediment being deposited, which dries them out.
- The lake refills and the halite dissolves, leaving cubic shaped moulds.
- these are infilled with sediment - the name psuedomorphs means 'imitate form'.
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5. Ripple Marks
- Formed as sands is transported by saltation in high energy conditions.
- Used in palaeocurrent indicators
- Symmetrical ripple marks = oscillating currents moving sediment up and down a beach.
- Aysmmetrical ripple marks = one direction current e.g. river.
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6. Flute Casts
- Found at the base of the bed
- The result of erosion caused by turbulent flow
- Often associated with turbidity currents
- Form in an environment where water flows with high energy of soft mud
- Form parallel to the current
- Deeped and pointed at the upstream end
- Infilled by overlying sediment
- Good palaeocurrent indicators
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7. Desiccation Cracks
- Form in clay rich sediment
- Loss of water due to evaporation by the sun causes mud to contract
- Polygona shaped blocks are fomred
- Each crack has a v-shaped cross section that is wider at the top than the bottom (more evaporation at the top so wide crack).
- Good way up indicator
- Infilled by sediment of a different type of colour
- Good palaeoenvironment indicator - arid with high evaporation rates.
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- Measurements of sedimentary strucutrs can show palaeocurrents:
- You can measure the orientation of the structure with a compass
- Then plot the result in a rose diagram.
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