Sedimentary Structures


Sedimentary Structures

  • 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
    • Then plot the result in a rose diagram. 

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