Unconformity: stages and engineering significance

An unconformity is a surface of erosion or non-deposition that separates younger strata from older rocks and represents a gap (break) in the geological record.

Stages in the formation of an unconformity

1. Deposition of older sedimentary beds in water.
2. Uplift / tilting or folding of the beds by earth movements, raising them above water level.
3. Erosion of the exposed surface so that an uneven surface is formed, representing the time gap.
4. Subsidence of the area below water again.
5. Deposition of new, younger beds over the eroded surface. The surface between old and young beds is the unconformity.

 1 Deposition   2 Uplift/tilt    3 Erosion      4-5 Subsidence +
 ========       ///////         _/\_/\_         new deposition
 ========       ///////        //////        ===============
 ========       ///////                      ~~unconformity~~
                                             //////// old

Engineering significance

• A basal conglomerate or weathered, soft layer at the unconformity is a plane of weakness and can cause sliding.
• The surface is generally uneven and weathered, so foundation depth and rock quality change suddenly.
• Seepage: water can move along the unconformity plane, so dams and tunnels may leak.
• Beds above and below have different strength, attitude and permeability, so differential settlement and stability problems arise.
• Wrong correlation of boreholes and rock depth, since beds are missing.
• Slope instability: sliding of younger beds over older tilted beds.