Sedimentary structures

Sedimentary structures

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Key Structures

Beds/Bedding Planes

Ripple Marks

Sole and Flute marks

Laminations

Graded Bedding

Cross Bedding

Slump and Flame Structures

Dessication Cracks

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Beds/Bedding Planes and Laminations

  • Description: Layers of sediment of homogenous/similar characteristics, normally formed during one main episode of deposition. Beds are over 10mm thick, laminations under 10mm
  • Used for: Separating depositional events, measuring dip and strike, other features are commonly found within the beds such as graded bedding as these are relatively large scale features. 
  • Formation: Deposition is caused by a loss of energy within a sediment system, beds can represent a years deposition, a single flood or a longer period. 
  • Notes: Beds can vary from several mm thick to many meters, and can be formed through airfall, gravity, iceflow or water deposits. They are separated by bedding planes which are preferentially eroded and often easy to identify. If there has been no significant variation in energy or depositional process, the bed will be massive, and even if dunes existed, without any variation in grain size, you would not be able to identify the dune remains.
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Ripple Marks

  • Description: Regular undulations at the top of the bed, or a series of gently cross laminae, with the angle of dip pointing towards the direction of flow. 
  • Used for: Paleo-current indicators and way-up criteria if the basal apron can be identified. Can also be used to estimate current intensity. 
  • Formation: Unidirectional flow produces cross laminae, gentle flow regimes normally in the lower stages of a river, bimodal flow produces. Most commonly found in Aeolian and Fluvial environments. 
  • Notes: Asymmetrical indicates unidirectional flow, Symmetrical indicates bimodal flow. 
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Sole and Flute marks

  • Description: Sole marks are groves and indentations that form on the bottom of the beds. They normally form as a result of pebbles and other material being dragged across or bounced over the top of a prexisiting bed, and these indentations then being infilled rapidly by the next sedimentation event. Flute marks are flute shaped indentations produced as sediment laden currents flow over soft sediment layers
  • Used for: Way up criteria as they are always found on the base of a bed, paleoenvironment indicator if you assess the nature of the rocks they are found in.
  • Formation: Normally associated with turbidity currents.
  • Notes: They can be difficult to identofy and classify as you need to see the base of the bed or a very clear cross section, paleocurrent can be identified from marks that open out in the direction of flow
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Beds/Bedding Planes and Laminations

(http://t3.gstatic.com/images?q=tbn:ANd9GcTlJn73bhy7fo-8EjawnZlGLaBKEFz1mj82OmSu-O0zr0EBUt08zL4iO-93) (http://t2.gstatic.com/images?q=tbn:ANd9GcTMUAku_JK6mxqULE-D1x0QlaDI4pdL7Ooo7z6FaCVOMAIGl-Z2oA) (http://t0.gstatic.com/images?q=tbn:ANd9GcQqhqIbgiGLCdnnr8EQ4iQJn6qMX6CUcTTQiLmGmkAOAO8_VrFTGw)

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What We use structures for

Way Up Criteria

Paleoenvironmental Indicators

Paleocurrent Indicators

Energy of the Environment

(http://higheredbcs.wiley.com/legacy/college/levin/0471697435/chap_tut/images/nw0040-nnc.jpg)

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Sole and Flute marks

(http://t1.gstatic.com/images?q=tbn:ANd9GcRpBENzOJbOC6enJZU17Hmqj39BTHENOtIA9Av_wIGFoVB0rjR9)(http://serc.carleton.edu/images/NAGTWorkshops/sedimentary/images/sole_marks4_250.jpg)

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Ripple Marks

Wave Ripples

(http://www.depauw.edu/acad/geosciences/tcope/SedStruct/WaveRip.jpg)(http://higheredbcs.wiley.com/legacy/college/levin/0471697435/chap_tut/images/nw0039-nnc.jpg)

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Cross Bedding

  • Description: This is bedding that runs at a different angle from the bedding plane. Usually formed by the migration of a bedform such as a ripple or a dune. It varies greatly in size from mm to many meters.It can be in alternate directions especially in aeolian environments where the wind can blow dunes back over older deposits formed herringbone type bedding.
  • Used for: Paleocurrent indicators - the basal apron of the dune which is normally the shallowest end points towards direction of flow. Way up - identifying the basal apron allows way up to be determined. Paleoenvironment - other aspects of the lithology will determine if it is aeolian, fluvial or marine
  • Formation: Forms in the same way as ripple marks, migration of either aeolian or fluvial dunes produces a series of avalanches down the lee side of the dune which is preserved when a subsequent dune moves over it.
  • Notes: Scale can vary greatly from a few cm to meters in height. The colour of the rocks is normally the best clue to the environment, with red haematite cement being aeolian, and green/yellow/white being either marine or fluvial.
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Graded Bedding

  • Description: A bed that shows  change in grain size from the bottom, normally coarse,to the top which is usually fine grained. This shows a decrease in energy over time.
  • Used for: Way up criteria - normally the finer sediments are at the top
  • Formation: Can represent seasonal or long term change, ie a river over the course of a year, or a short term rapid depositional event such as a turbidite. They can be found in alluvial, fluvial or deep water marine environments.
  • Notes: This can be observed on scales ranging from a few centimeters to several meters thick. It is always more pronounced in water bourne sediments as water allows slow settling and sorting
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Dessication Cracks

  • Description: Roughly polygonal cracks in the surface of muddy beds.
  • Used for: Way up criteria - the cracks between the dessication polygons fill when covered by new sediment and the "V" shape is preserved, with the tip of the V pointing to the oldest beds. Paleoenvironmental indicator is of intense heat and low moisture. There are no paleocurrent indicators associated with this feature
  • Formation: Formed as clay dries out and contracts producing polygonal V shaped cracks which are then fill in by sediment and preserved.
  • Notes: They can vary in size, and the depth of the V, the deeper the V the longer period of dessication, the size of the polygons is determined by a very wide range of factors you dont need to know.
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Slump and Flame

  • Description: These are rounded bottomed structures, visible in cross section with "flames" of mud surrounding them. The flames are forced upwards when wet sand collapses into the mud below.
  • Used for: Way up criteria - the flames always point to the top surface, and the sand layer had to be on top for the structure to form.
  • Formation: Flame structures are sharp wave crests or flame like structures which project up into the overlying sediments. This is usually a process that occurs by mud being injected up into a weakness in the overlying layer, most often sands. These are a post depositional feature, as it is a result of sediments collapsing into layers beneath them. It is only ever formed when sand overlies mud, which helps with way up.
  • Notes: These are normally found associated with graded bedding. They are sometimes called ball and pillow, but use slump and flame in the exam.
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Graded Bedding

(http://www.depauw.edu/acad/geosciences/tcope/SedStruct/GradedFlamed.jpg)(http://higheredbcs.wiley.com/legacy/college/levin/0471697435/chap_tut/images/le03_19.jpg)

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Cross Bedding

(http://t1.gstatic.com/images?q=tbn:ANd9GcTCuyExgwnGatbhTUiRnJPkUVQfC0T0vuX8ehonqnZiQ9TpIE8d) 

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Slump and Flame

(http://farm3.static.flickr.com/2755/4554040525_883f5843d6_o.jpg)

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Dessication Cracks

(http://t2.gstatic.com/images?q=tbn:ANd9GcTiYA_U1jd_f9fZkGqRXRDjF5fpf7TK6tVEqEJbmgSNpU04rl4kYA)(http://t3.gstatic.com/images?q=tbn:ANd9GcSzF9ODrtPg3azwfxuDpGp8YQHPEggTHHwG0VOSL6Nbn56Ix1u-rQ)

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Comments

G Monger

This is a useful resource for WJEC geology

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