Geological history exercise

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Geological history exercise

• Using seismic to deduce the geological history of
  an area

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(No Transcript)
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This seismic profile has horizon A identified at
CMP 520 and TWT 450msec. Interpret this horizon
and mark on any faults with arrows to show the
displacement. What happens to the interpreted
horizon at CMP 390? What is the strong feature
at around 400ms that dips gently to the
north? Use the laws of superposition and
cross-cutting relationships to put the geological
events you can see into the correct order.
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Interpret the horizon at CMP 520 TWT 450msec
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Add faults
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CMP 390
At CMP 390 the horizon stops against another
horizon which dips to the north
The red horizon is unconformable with the one
above
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Follow a second horizon to help see the rest of
the faults
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Geological History

• Start with the oldest first.
• Use the law of superposition If one sedimentary
  rock lies above the other then the upper rock is
  younger than the lower one.
• Use the law of cross-cutting relationships A
  rock or structure must be younger than any rock
  or structure which it cuts across.

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Geological History

• The sedimentary sequence below 400ms would have
  been laid down as horizontal layers. It is now
  dipping to the south and so must have been
  subjected to deformation.
• There is an unconformity at around 400ms which
  dips to the north. This represents a break in the
  geological sequence and an eroded surface,
  therefore deposition stopped and this area became
  a terrestrial environment.
• There are several faults seen below the
  unconformity but not above and so they must have
  occurred before deposition resumed. These are
  normal faults which are caused by tensional
  tectonic forces.
• It is possible that the cause of the normal
  faults was also the cause of the unconformity,
  with subsidence to the south causing a drop in
  sea level and so exposing the area to erosion.
• At 400ms we see a younger sequence of rocks
  deposited on top of the unconformity which
  suggests the area was flooded again.

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Extension Activity

• Can other sedimentary features be seen on seismic?

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Look at the sediments above the unconformity
The horizon has the shape of a depression. This
could possibly be a channel, with the banks to
the N and S.
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The channel could have been flooded and infilled
with sediment.
To be sure we would need more evidence, from
other seismic lines or a borehole.
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How wide is the feature?
It is approximately 1km wide
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Canvey Island
London Bridge
10km
In comparison, the Thames at London Bridge is
approx. 265m wide, and 5km wide near Canvey Island
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Conclusion

• Sedimentary features are visible on seismic data.
• To be visible on seismic data the feature must be
  large such as a river channel.
• Detailed features such as cross-bedding are too
  small to see.