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Unconformities

An Unconformity is a surface on either side of which the dips of the strata are (significantly) different. The two sets of beds do not conform with each other.


Unconformity

For a straightforward example:

  • the beds above the unconformity
    • are unfolded;
    • are younger;
    • may begin with pebble bed or other coarser sedimentary rock .
  • the beds below the unconformity
    • are folded;
    • are sometimes faulted, metamorphosed or changed in some other way;
    • are significantly older.
 Unconformity

The surface of the unconformity itself is reasonably level and shows evidence of being eroded before the upper beds were deposited.

The events leading to the formation of an unconformity are:
  1. deposition of the older beds;
  2. folding and other effects of earth movements; formation of hills/mountains;
  3. a long period of erosion by rivers and the sea to form a level erosion surface;
  4. renewed deposition of the younger beds on top of this erosion surface.
 Unconformity

Unconformities
  • indicate a long period of time which is not represented by any rocks;
  • show strong earth movements and a period of mountain building (refer to Plate Tectonics);
  • may be a convenient dividing line between one geological period and another.
 Unconformity

Geopix Study Topic "Unconformities"
  • gives full images of these, and other, examples to illustrate different types of unconformity;
  • gives images and details about some classic examples;
  • relates these unconformities to periods of earth movements;
  • suggests how to answer exam questions on this topic.
The Structures: "Unconformities" Study Topic is part of the Geopix Structures Study Guide CD.

To order this CD go to the Geopix Online Store

The Study Topic contains the following pages:

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