UNCONFORMITIES 541 UNCONFORMITIES A R Wyatt, Sidmouth, UK ß 2005, Elsevier Ltd All Rights Reserved Introduction An unconformity is a surface that separates rocks of significantly different ages This was at one time an exposed part of the Earth’s land surface or the rock surface below a body of water (for example, a lake or the sea), and the younger rocks were deposited on this surface Juxtaposition of rocks of different ages caused by faulting does not give rise to an unconformity An unconformity represents a substantial break or gap in the local or regional depositional record In modern usage this break or gap may have been caused by the erosion of previously deposited rocks or by a long period of non-deposition of sediments (that is, a long enough period that the absence of sediments of the relevant age can be recognized) Early workers confined the use of the term unconformity to places where the older rocks had been deformed and eroded, so that the unconformity cut across the truncated beds of the lower deposits The idea that structural discordance is an essential feature of an unconformity continued for much longer in the UK than in many other parts of the world, such as, for example, the USA Other terms were introduced for breaks where there was no structural discordance In the Phanerozoic these would normally be identified by gaps in the expected fossil sequence For the simple case of non-deposition, the terms diastem and nonsequence were used Although these terms have often been considered to be synonymous, some workers have suggested that a diastem is a break of shorter duration than a non-sequence Where the break can be shown to be associated with erosion but the upper beds are still parallel to the lower beds, the term disconformity was used The disconformity surface is often parallel to the bedding surfaces, but it may also show major relief Terms such as unconformity and disconformity refer to the surface (and, by implication, the time) that separates the older from the younger rocks Terms have also been introduced to refer to the relationship between the bedding of the upper (younger) rocks and that of the lower (older) rocks Where there is no structural discordance, so that the attitude of the upper beds is the same as that of the lower beds, the upper beds are said to be conformable When there is structural discordance, the upper beds are unconformable Where there is structural discordance, as we follow the base of the overlying bed we find that it moves from one to another member of the lower, truncated, series This is known as overstep (see Figure 1A, side face of block) The term is chiefly used when the angular nature of the unconformity is not obvious but is made evident by detailed mapping One of the earliest recorded examples was the overstep of the base of the Cretaceous across the underlying Jurassic Figure Types of unconformity (A) Angular unconformity The front face of the cube shows overlap, caused by onlap from the right The side face shows overstep of the base of the overlying beds over the dipping lower beds (B) Disconformity (C) Heterolithic unconformity