Unconformities: Recognition, Interpretation, and Classification
Introduction.Erosion is an integral part of the sequence of processes by which surface rock bodies become recycled into sediment and eventually into sedimentary rock. It is fundamentally a process of removal, whereby the end-products of weathering are transported by fluid agents from their place of origin to their place of deposition. Erosion acts to reduce the topographic relief of land surfaces to a minimum, that is, to a uniform, planar surface. Topographic irregularities, such as hills and valleys, are therefore in disequilibrium with respect to the agents and forces of erosion. Since the erosional process acts to remove rather than preserve the rock record, a planar discontinuity transcending a body of rock, either parallel or oblique to bedding (while indicating neither displacement nor movement of the adjoining rock masses), is most likely a former erosional surface. That is, it is a surface that has been created by erosive agents, the rocks formerly above which have been broken down and transported elsewhere. These erosional surfaces, which are extremely common in the geologic record, are collectively referred to as unconformities. Bear in mind that the vast majority of the rock material of which the geologic record is comprised consists, in and of itself, as recycled rock material. It is therefore prudent to emphasize the fact that the geologic record, by its very nature, is quite incomplete. The portion of actual, uninterrupted geologic time preserved in a succession of strata, therefore, is probably quite small compared to the time difference between the oldest and youngest identifiable rock strata. In other words, if a layered rock succession is bounded by a 200 million year old rock layer on the bottom and a 100 million year old rock layer on the top, the intervening strata may only preserve a 1 million year total time interval! The record representing the missing 99 million years has either been removed by erosion, or, it was never deposited in the first place. The notion of the general incompleteness of the geologic record is an important one to keep in mind while interpreting Earth history from the rock record.
Types of Unconformities. There are three principal types of unconformity, namely, the disconformity, the nonconformity, and the angular unconformity [Fig. 1].
The disconformity (alternately referred to as the paraconformity) is an erosional surface which exists between and parallel to layers of parallel strata. Disconformities represent simple time gaps in the local geologic record, produced either by the erosion of a previously overlying rock layer or, more likely, by nondeposition. The parallelism of the bounding strata suggest a period of tectonic quiescence, that is, the absence of any dynamic geologic phenomena which would have deformed the older strata prior to the deposition of the younger strata, resulting in a nonparallel (or discordant) relationship between the stratal layers.
The nonconformity is an irregular erosional surface which exists between an older mass of either igneous or metamorphic rock and the layers of younger sedimentary rock which have been deposited on top of it [Figs. 1, 3 and 4]. A nonconformity is usually oblique to the bedding surfaces of the sedimentary rocks with which it is in contact. A nonconformity not only clearly indicates a gap in the geologic record that is probably of significant duration. It also suggests a period of pronounced uplift which preceeded both erosion and deposition due to the surface exposure of rocks which originated at great depth.
The angular unconformity (alternately referred to as the angular discordance) is an erosional surface which separates rock masses that possess differing (or discordant) bedding geometries. The erosional surface is usually parallel to the bedding of the younger overlying layers and is therefore oblique to the older, underlying layers [Figs. 1 and 2]. As with the nonconformity, the angular unconformity clearly indicates a gap in the geologic record that is probably of significant
duration [Figs. 3 through 5]. It also suggests a period of deformation and uplift (perhaps orogenesis) which post-dates the deposition of the older strata yet pre-dates the period of erosion which in turn pre-dates the deposition of the younger strata. Angular unconformities are of great value in helping refine the timing and extent of major deformational events in a particular area by providing both an upper and a lower age limit of the event.
Erosional versus Deformational Surfaces. It is also important to distinguish between unconformities and low angle faults since both possess many features in common. Both faults and unconformities are planar surfaces which transcend rock masses. Additionally, both may be laterally consistent over long distances. However, evidence of relative rock displacement within the zone on contact between adjoining rock masses, such as mechanically shattered, scoured, gouged, or polished rock, for example, is indicative of a fault surface and not an erosional surface. Occasionally, unconformities become may even become activated as fault surfaces in later deformational events in response to stress, although this appears to be rare.
1. The processes of erosion work to produce a topographic expression that has which appearance?
2. Briefly define the unconformity in terms of its genesis and appearance in the geologic record.
3. Define and distinguish among each of the three basic types of unconformity.
4. Which type of unconformity is the most prevalent in the geologic record yet is the most difficult to identify? Explain.
5. Which type of unconformity exists between a mass of 1 billion year old granite and overlying layers of 50 million year old sandstone? Explain.
6. How can we distinguish between a low angle fault surface and an unconformity?
Figure 2. An Angular Unconformity. Photograph taken at Siccar Point, Berwickshire, Scotland. It is the type locality of the angular unconformity,
so named in 1788 by James Hutton. Coin placed at unconformity which
separates steeply inclined Ordovician shales below from gently tilted,
Figure 3. Unconformities within the stratigraphic succession
comprising the wall rocks of the Grand Canyon, Arizona.
[Diagram courtesy of the Department of Geoscience, University of Iowa.]
Figure 4. The Great Unconformity at Frenchman Mountain, east of Las Vegas, Nevada.
The boundary between the overlying cross-bedded (Tapeats) sandstone and
the underlying mixed schist and granite (Vishnu Group) metamorphic suite
is equivalent to the nonconformity at the location labeled X in Figure 3.
[Photographed by Michael A. Klimetz]
Figure 5. Two angular unconformities visible in a vertical succession of sedimentary rocks and unconsolidated sediments (soils) exposed in a quarry wall at Lulsgate, United Kingdom. The lowermost unconformity (at A) separates tilted Carboniferous limestones (underlying)
from gently folded Jurassic shales (overlying). The uppermost unconformity (at B)
separates gently folded Jurassic shales (underlying) from unconsolidated
sediments, weathering products and soils (overlying).
[Photograph Courtesy of the British Geological Survey]