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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Imbricate Structures
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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Salt Pseudomorphs
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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8. Palaeocurrents
Measurements of sedimentary strucutrs can show palaeocurrents:
You can measure the orientation of the structure with a compass
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