Page 15 - Drilling Technology in Nontechnical Language

6 Drilling Technology in Nontechnical Language Second Edition

Sometimes the surface rocks are eroded away, and then more
sediments are deposited. This is called an unconformity; part of the

geological sequence is missing entirely. The photo in figure 1–5 shows an
unconformity, visible in a cliff face. The white line marks the unconformity
surface.
This photo tells an interesting story. Sediments were deposited at the
surface or seabed. These became buried, changed into rock, and then
were uplifted back to the surface. During this burial and uplift, the rocks
became tilted. Now surface erosion removed rock but at an angle to the
bedding planes of the rock. Notice the angle between the lower rock beds
and the white line of the unconformity surface, which would have been
horizontal. Now new sediments were deposited, buried, became rock, and
then were uplifted back to the surface and tilted in the opposite direction
of the original tilt, but by a lesser amount. The lower rocks have had two
return trips down into the earth’s crust and have been tilted one way on
one trip and the opposite way on the second.

Fig. 1–5. Unconformity surface shown by the white line

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