Unconformities

An unconformity is a stratigraphic feature that is formed by broad erosion of an area causing a significant gap to occur in the stratigraphic record. Generally, development of an unconformity is accompanied by or preceded by uplift of the rock units or strata that will be eroded, and/or subsidence or fall of sea level, thus exposing rock units or strata to erosion.

An unconformity can consist of two hypothetical parts: the hiatus and the erosional vacuity. The hiatus is the amount of stratigraphic record removed during erosion and the erosional vacuity is the amount of stratigraphic record that might have been deposited during the time erosion was occurring. The sum of these parts is referred to as the lacuna, or total missing stratigraphic record. An unconformity is commonly referred to as a significant gap in the chronostratigraphic record as well.

Unconformities can be classified according to their scale or scope. For example, a cartographic unconformity is one that can be mapped regionally or seen in a regional cross section. A macrographic unconformity may be seen at outcrop scale where truncation may be evident. A petrographic unconformity is seen under a microscope (usually in a glass-mounted thin section or on the face of a cut slab of rock) and is very small.

Unconformities can also be classified according to their physical appearance. Angular unconformities are those which show truncation of upturned or tilted layers below the unconformity surface. Disconformities are unconformities showing erosional relief between otherwise parallel layers of sedimentary strata. A nonconformity is an unconformity developed between igneous or metamorphic rock (below) and sedimentary layers (above). Finally, a paraconformity is an enigmatic type of unconformity that appears as a hardly distinguishable plane between two parallel sedimentary rock layers.

Unconformities of large scale (i.e., cartographic unconformities) have been used by some geologists to delineate large groups of sedimentary formations that are bounded by unconformities. These unconformity-bounded units are called synthems, and their origin is related to grand cycles of sea-level change and mountain-building in Earth history. Either regional or inter-regional unconformities bound these large synthems.

Unconformities are recognized using three types of criteria: physical, structural, and paleontologic. Physical criteria include erosional relief between sedimentary rock layers, mineralized zones in rocks, basal conglomerates, iron-stained zones in rocks, zones of truncation and encrustation of fossils , small and large-scale solution (karst) features, and ancient soil horizons. Structural criteria include: dikes and faults truncated by erosion, discordance of structural dip between layers, and tectonic deformation. Paleontologic criteria include: abrupt change in fossil assemblage, gaps in evolutionary lineage among fossils, missing fossil zones, bone and tooth conglomerates, and reworked or corroded fossils. In study of unconformities, it is preferred that evidence from at least two of the three categories be used to recognize an unconformity and criteria from all three categories, if possible, be used to assess the scope of an unconformity.

Assessment of the scope of an unconformity, that is the amount of geological history or chronostratigraphic record missing at an unconformity, requires careful study. The scope of an unconformity is commonly situational, i.e., related to the type of sediment and the depositional environment of that sediment. For example, erosional removal of 4 in (10 cm) of deep-sea sediment, wherein rates of sediment accumulation are very low (<0.1 cm/1,000 years), would likely result in a significant gap in the chronostratigraphic record. An unconformity formed in this way might be a paraconformity or petrographic unconformity, which could be quite significant in terms of lost record. However, the erosional removal of 4 in (10 cm) of sediment formed in a depositional environment wherein sedimentation rates were much higher would not result in a significant gap in the rock record.

Many erosional surfaces that look superficially like unconformities are in reality usually only minor erosional or scour surfaces. These features are not unconformities, but are instead diastems. Diastems do not rise to the level of unconformity because the amount of missing record is not significant. As noted above, the assessment of such significance is situational, and it may not be known immediately after discovery if an erosional surface is an unconformity or a diastem.

The term stratigraphic break is commonly used instead of unconformity or diastem, particularly where the significance of a stratigraphic break in question is not known or is ambiguous. In other words, an unconformity may be thought of as a stratigraphic break that is significant in terms of lost record and a diastem as an insignificant break by the same measure. It is well established that unconformities and diastems both contribute to the overall incompleteness of the stratigraphic record.

See also Bedding; Chronostratigraphy; Depositional environments; Stratigraphy