Unconformities

• An unconformity represents a break in the stratigraphical record and occurs when changes in the palaeogeographical conditions lead to a cessation of deposition for a period of time.
• Such a break may correspond to a relatively short interval of geological time or a very long one. An unconformity normally means that uplift and erosion have taken place, resulting in some previously formed strata being removed
• The beds above and below the surface of unconformity are described as unconformable.

The structural relationship between unconformable units allows four types of unconformity to be distinguished.

• In Figure 4.2.(a), stratified rocks rest upon igneous or metamorphic rocks. This type of feature has been referred to as a nonconformity (it also has been called a Heterolithic unconformity)
• An angular unconformity is shown in Figure 4.2.(b), where an angular discordance separates the two units of stratified rocks.
• In an angular unconformity,the lowest bed in the upper sequence of
  strata usually rests on beds of differing ages. This is referred to
  as overstep.
• In a disconformity, as illustrated in Figure 4.2.(c), the beds lie parallel both above and below the unconformable surface, but the contact between the two units concerned is an uneven surface of erosion.
• When deposition is interrupted for a significant period but there is no apparent erosion of sediments or tilting or folding, then subsequently formed beds are deposited parallel to those already existing.
• In such a case, the interruption in sedimentation may be demonstrable only by the incompleteness of the fossil sequence. This type of unconformity has been termed a Paraconformity (Fig. 4.2.(d).)