Geological history exercise
Using seismic to deduce the
geological history of an area
300m
Horizon A
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.
300m
S
450
Interpret the horizon at CMP 520 TWT 450msec
300m
S
Add faults
300m
CMP
390
S
Unconformity surface
At CMP 390 the horizon stops against another horizon which dips to the north
The red horizon is unconformable with the one above
300m
S
Follow a second horizon to help see the rest of the faults
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”.
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.
Extension Activity
Can other sedimentary features
be seen on seismic?
300m
S
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.
300m
S
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.
300m
S
How wide is the feature?
It is approximately 1km wide
N
London Bridge
Canvey Island
10km
In comparison, the Thames at London Bridge is approx. 265m wide, and 5km wide near Canvey
Island
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.