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Lithostratigraphic Correlation – Simple

November 22, 2010 Comments off
  • Mostly based on using rock types in a series of beds.
  • However there is a greater chance it is a sequence of beds in succession, usually with a marker horizon.
  • A very distict bed that can be followed across all others is ideal to be used.

There are 3 methods of doing this:

  •  Corelation using a sequence of beds- if there are 2 layers of different rock together you can assume they were formed at the same time. If another area has the the same rock you can assume that they formed at the same time.
  • Bed thickness- if different areas have the same thicknesses of different beds in the same pattern if can be asummed they formed at the same time in the same conditions.
  • Rare minerals- if a rare mineral if present in 2 areas you can assume the rocks in the area formed at the same time.

Problems with this:

  • Lateral variation – Sedimentary deposits change in thickness laterally, sediment beds may be thicker but others may be thinner.
  • Diachronous Beds – Where one sediment type is laid down at different times
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Chronostratigraphic Correlation

November 14, 2010 1 comment

Summary:
This involves dating and linking rocks using events which may have been occured over large areas eg. Sea level changes or volcanic rocks.

Wikipedia Defenition:
Chronostratigraphy is the branch of stratigraphy that studies the relative, not absolute, age of rock strata. Chronostratigraphy is based upon deriving geochronological data for rock units, both directly and by inference, so that a sequence of time relative events of rocks within a region can be derived. In essence, chronostratigraphy seeks to understand the geologic history of rocks and regions. The ultimate aim of chronostratigraphy is to arrange the sequence of deposition and the time of deposition of all rocks within a geological region, and eventually, the entire geologic record of the Earth.

 

How volcanic rocks are used:

When a volcano erupts, ash can cover and be laid down over enormous areas. All the ash is lain down at the same time and will eventually from Tuff, the best rock for Chronostratigraphic Correlation because:

  • Chemical analysis will determine whether the Tuff’s are related.
  • It is laid down over huge areas very rapidly.
  • It can be used to obtain a radiometric date.
  • It is useful for both relative and absolute dating.

Glacial Varves:
These form due to the variations in the flow of glacial meltwater between  summer and winter. In summer the heat cause greater melting of glaciers and therefore the meltwater has the energy to carry silt in suspension which deposits in a light coloured layer. In winter there is significantly less melting of the glacier so the run-off does not have as much energy. Therefore material can only be carried in solution, this material in clay and organic matter, when these are deposited a dark coloured layer is formed. A pair of these represents one year, therefore you can count layers to find out how many years are present. It’s usefulness in Chronostratigraphic Correlation lies in the fact that thicker layers of silt are produced by warmer summers. The pattern of the band and their widths will be the same for all the lakes in an area. Therefore rocks can be linked back to the same area by their varves.

Problems:

  • Chronostratigraphic Correlation only tells us if two rocks were formed at the same time by the same event, it does not tell us the precise age. A form of absolute dating is required if a precise age is needed.
  • It assumes that two rocks have a similar age if they are in the same area. An intrusion could occur millions of years after the rock formed and therefore be a lot younger than this method would suggest.

Joey’s addition:

the igneous rocks mentioned above by Harry are used to find a solid date for that certain part of the area of the land, by finding this geologists can then use relative dating to estimate the age of other rocks around them. also in addition to the problems area of Harry’s text. there can be large time gaps between the timing of a bed being lain down and the intrusion occuring. this can mislead the chronostrarigraphist into a

Harry, Joey

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