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Sea Level Curves: Simples

November 22, 2010 Comments off

SEA LEVEL CURVES

1977 – Introduced by Dr Peter Vail and his team on behalf of ESSO Oil Company as a method for interpreting seismic cross sections. This means that they were taking a section of the sea bed, in this case a section used for drilling for petroleum, firing in seismic waves and measuring where they are reflected in the rock by unconformities. In the seas beds, this represents a change in sea level. This could be isostatic or eustatic. Read more…

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Categories: Group Work

Lithostratigraphic Correlation

November 16, 2010 Comments off

LITHOSTRATIGRAPHIC CORRELATION

By Ed, Annabel and Callum

This is mostly based on using rock types in a series of beds, or more likely a sequence of beds in succession, usually with a marker horizon, a very distict bed that can be followed across all others, like a red oxide bed in a sequence of dark rocks.

METHODS

  1. correlation using a sequence of beds – For example if there was a layer of sandstone followed by conglomerate in two bore samples, you could link that area of the bed as forming at the same time. However, fossil evidence must also be used so as the dating can be made certain, as bed sequencing is quite weak evidence. The fossil type is not so much as important as the amount of fossil found in the rock  and they are used mainly in sequencing rocks containing coal and opposite rocks.
  2. bed thicknesses – Similar to reading a barcode, if the same sequence and relative size of beds comes up as a pattern in a number of bore samples, it is safe to assume that the beds below will follow the same order more or less. This is important when drilling for coal, because if in one bore sample you find a correlating sequence from another previously drilled area, you can work out if it is economically beneficial to drill below. For example, in the incredibly accurate image below, we can see repetition in the layers of bands, with which we can assert they are the same beds that have been squeezed.
  3. Rare minerals – The cretacious/tertiary border is always found by correlating the prescence of the mineral iridium, from an asteroid inpact

Coal seams that can be correlated through bed sequence. ©Ed 2010

  1. PROBLEMS WITH LITHOSTRATIGRAPHIC CORRELATION:

1.  Lateral Variation – where sediments change type horizontally.

  • Sedimentary deposits change in thickness laterally e.g. closer to the source of the sediment beds may be thicker but further away they may be thinner.
  • In a fluvial environment, point bar sands are being deposited at the same time as flood plain clays. In a shallow sea environment there will be sandstones being laid down close to the beach while at the same time clays are ebing deposited offshore in low energy environments.
  • Deltaic sediments have different sediments forming in the delta top, fron and offshore at the same time.

2. Diachronous Beds – Where one sediment type is laid down at different times.

  • The result is beds which cut “across time”.
  • e.g. a delta building out into sea. Over the years a continuous layer of sands is left behind. These sands get younger from land to sea – they are NOT the same age. The same is true for the other rock types in a delta cycle.

 

In summary this shows that it is very hard to tell of any correlation in the rock. Although it is helpful for overall sequencing when used with other methods.

Categories: Group Work