unconformity

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unconformity An unconformity is a large gap in the geological record that is formed when deposition of sediment ceases for a considerable time. The most spectacular and best-known examples are called angular unconformities (Fig. 1a): they illustrate the folding and faulting of an older sedimentary record, its planing down by erosion, and the deposition of a younger sequence that truncates the old one. Wherever sedimentary sequences have been studied they are interrupted by such gaps. The lost intervals range in magnitude from extremely long time periods to geologically very short ones. Thus the time gap represented by a given unconformity varies laterally. It commonly decreases away from an angular unconformity towards the main or central part of the sedimentary basin which received the material eroded during the development of the unconformity. In a situation of this kind the angular unconformity passes down dip, that is, basinwards, into a correlative conformity somewhere in the middle part of an uninterrupted sedimentary sequence.

Before discussing the concept of unconformity in more detail it is important to realize that the prime concern of geologists on first entering a new area, in addition to examining and describing the types of rock present, is to establish the geological succession or sequence. The geologist begins by mapping the rock exposures and recording the structural disposition of the rock layers (strata), and their angle of inclination (dip). From observations like this it is usually possible to elucidate the relationships between adjacent layers of rock and their arrangement in vertical sequence, thus giving a first approximation to the three-dimensional arrangement of the succession of rocks.

In addition, the geologist studies the nature of the stratification. For example, some bedding planes (surfaces that separate strata) are caused by vertical changes in sediment type. Many bedding planes, however, represent pauses in sedimentation, possibly accompanied by some local erosion; such breaks are commonly described as diastems or non-sequences. These relatively short interruptions in sedimentation, representing only a brief interval of time with little or no erosion before the resumption of deposition, are, of course, also gaps in the sequence; they may include the down-dip, basinward, conformable equivalents of unconformities. These very short breaks are those that would occur within a particular sedimentary environment, unlike those related to an unconformity, which may be associated with a major change in environment.

Thus, an early task of the geologist is to distinguish the individual types of rock exposed in an area and to work out their chronological succession: that is, their sequence in time. This elucidation of the stratigraphy begins by separating the succession into its different rock-types or formations; for example, limestone, overlain by shale, overlain in turn by sandstone. This type of stratigraphy is known as rock-stratigraphy or lithostratigraphy. Additional stratigraphical study may work out the distribution of fossils (biostratigraphy) present in the succession so as to facilitate or further refine correlations with strata in adjacent areas.

The present-day concept of stratigraphical sequence was developed from principles first applied in the continental interior of the United States of America using information from surface outcrops. There, rock-stratigraphical units of higher rank than Group (i. e. approximately at System level) are traceable over large areas of the continent and are bounded by unconformities of interregional scope. It is now apparent that the description and mapping of unconformities are the first steps in understanding the regional and super-regional geological history and development of geological provinces and basins.

In addition to the observation of unconformities at outcrop, present-day high-resolution subsurface data now make it possible to recognize unconformities in seismic reflection profiles and also in geophysical well-log data. Most importantly of all, these unconformities (and indeed their correlative conformities) are used as the boundaries of stratigraphical units in the establishment of what is now termed sequence stratigraphy. The methodology of sequence stratigraphy has greatly augmented stratigraphical practice. It is found that such stratigraphical units and their bounding, regional unconformities provide an excellent means of mapping the distribution in time and space of local and regional stratigraphical and tectonic events, and of elucidating and describing the development of geological provinces and basins through time.

The unconformity-bounded units known as sequences are defined formally as relatively conformable successions of genetically related strata bounded at top and bottom by unconformities or their correlative conformities. The sequences are interpreted as depositional sequences which are described from seismic, borehole, and outcrop data. As a depositional sequence is determined by the single objective criterion of the physical relations of the strata themselves, it is useful in establishing a comprehensive stratigraphical framework rather than being primarily dependent on the more subjective criteria used in determination of rock-type, fossils, depositional processes, etc. that vary within a given sequence.

Once the local stratigraphy has been determined and mapped so that geological events have been organized into a reasonable geological history, the next step is to compare and correlate with adjacent areas so that geological understanding of the wider region is facilitated. To achieve this, the rocks of the local area must be equated with those of other areas by correlating laterally equivalent rock-types and establishing their ages in terms of the geological timescale. The time framework is based upon the geological systems (e.g. Carboniferous System, Jurassic System), which, in their type areas at least, are well developed in terms of thickness, etc. Each system lies between older strata below and younger strata above, and the systems are contiguous; that is, they do not overlap in time. Thus any geological system was deposited during a discrete portion of geological time. The boundaries of many of the systems were originally chosen at important unconformities, although as research proceeded and more geological data accumulated it became clear that these gaps could be represented by complete stratigraphical successions in other places. The time equivalents of the geological systems are the geological periods, which constitute the standard of time measurement in the geological sciences.

Returning to the idea of gaps in the sequence, it is possible to define an unconformity as a surface of erosion or non-deposition separating younger strata from older rocks and representing a significant hiatus or gap in the geological succession. The dip-discrepancy type of unconformity now known as an angular unconformity (Fig. 1) was the first type discovered in the late eighteenth century. By the early twentieth century, however, several types of unconformity other than the angular discordance type were being recognized as a result of further accumulation of data. One example is what is now known as a disconformity (Fig. 1b), which is characterized by a break between two sets of parallel strata recognized on a regional scale (that is, there is no local discrepancy in dip). Also recognized is what was originally known as a heterolithic unconformity, which is a contact between two rock-types of wholly unlike origin, such as sedimentary rocks overlying granite. This type of unconformity is now sometimes known as a nonconformity. A fourth major category is the local unconformity, that is, a depositional break on either side of which the beds might be parallel to each other, and where the unconformity appears to be only of local extent or significance. From the above it will be seen that unconformities are classified on the basis of the structural relationship between the underlying and overlying rocks and thus are of tectonic or structural significance as well as of time significance.

It will also be apparent that in an area of low dip or horizontal strata it is possible to project the known surface geology into the deep subsurface only to a limited extent: at most to the top of the next lower (usually concealed), underlying unconformity. In some areas boreholes may provide additional constraining information on the thickness of the topmost (i.e. surface-mapped) conformable sequence, or perhaps even by penetration of an underlying and concealed unconformity. However, the fact that borehole information provides data only for the close proximity of the borehole section itself usually precludes too wide or regional an application of the results unless many boreholes are available in a small area.

Primarily as a result of extensive oil exploration during the past two or three decades, geological science has seen the advent of high-quality seismic reflection profiles, which can be accurately calibrated geologically by downhole geophysical logs. These techniques provide a much wider and more accurate image of the regional concealed deep subsurface geology than hitherto. In addition, the new methods have facilitated a much fuller understanding of unconformities, and especially of the associated genetically related sequences that are present between them.

The expense of modern drilling is such that continuous coring is impractical. The geologist thus turns to downhole geophysical logging to provide the data in addition to cuttings samples that are required for sequence-stratigraphical studies. These so-called ‘electric’ logs, which are acquired via an instrument called a sonde, measure various physical properties of the subsurface rocks and formations. Many of the log suites acquired are of great use and relevance to the interpretational methods discussed here. For example, the sonic or acoustic log measures the speed of sound in the concealed rock formations and enables detailed geological calibration of seismic reflection data in terms of depth below the surface and, under the right conditions, rock-type. Microresistivity measurements are used to prepare what are known as dipmeter data and allow the determination of structural dip (and thus the location of unconformities in the concealed subsurface) as well as the location of geological faults and the presence of some sedimentary features such as cross-bedding.

Other types of electric log data allow the detailed determination of a ‘high-fidelity’ record of the rock-stratigraphical section penetrated by the borehole. Furthermore, the log data allow accurate recognition of patterns (sometimes known as electrofacies) which are related to fining-upwards or coarsening-upwards sequences and thus present information of direct use and relevance to sequence interpretation. In fact the geophysical log data make possible the direct recognition of individual unconformities in the uncored borehole section itself (via dipmeter data), or provide evidence of local or regional unconformity by means of correlation and comparison of log-derived stratigraphical and sequence information between individual, widely spaced boreholes.

Perhaps more than anything else it has been the introduction of sophisticated seismic reflection interpretation techniques which has allowed further extension and application of the concept of depositional discontinuities or unconformities between stratigraphical sequences. The seismic reflection technique is rather akin to echo-sounding. Energy, usually from an explosive or vibrating source at the surface, is put into the Earth's crust so that where there is a change in rock-type (known as an acoustic impedance contrast) some of the downgoing waves are reflected back upwards towards recording devices (geophones or hydrophones) at the surface. After processing, the data are presented as seismic reflection profiles which represent the stratal layers in the Earth's crust. The representation looks rather like a geological cross-section except that the vertical dimension is given in two-way travel time rather then depth. However, by incorporating relevant velocity information into the section it is possible to transform the vertical axis of the seismic section from time to depth. It is important to remember that the vertical resolution of the method is coarse (say, tens of metres), as compared with exposed rock sections (observable by the naked eye down to millimetre thickness) and geophysical log data (resolvable down to decimetre thickness or less).

The objective of seismic sequence analysis is to interpret depositional sequences on seismic sections by identifying discontinuities (or unconformities) using reflection termination patterns. Two types of patterns (onlap and downlap) occur above the discontinuity; three types of patterns (truncation, toplap, and apparent truncation) occur below the discontinuity. Sequence boundaries are characterized by what geologists call regional onlap and truncation. Such depositional sequences (see Fig. 2a) are defined as stratigraphical units composed of relatively conformable successions of genetically related strata bounded at top and bottom by unconformities and their correlative conformities. Seismic reflection data give a clear and unequivocal demonstration that unconformities at basin margins or in mid-continent situations pass basinwards into correlative conformities. The results provide a useful reminder that the onshore regions (geological ‘highs’) are in fact atypical stratigraphically and are thus not the best places to define type sections.

A depositional sequence is of great interest to geologists because it was deposited during a given interval of geological time limited by the ages of the sequence boundaries where they are conformable, although the age range of the strata within the sequence may differ from place to place where the boundaries are unconformable (see Fig. 2). The hiatus represented by the unconformable part of a sequence boundary generally is variable and may range from about a million years to hundreds of millions of years. In addition, unconformities are of considerable importance in time stratigraphy (chronostratigraphy) because strata above an unconformity are everywhere younger than rocks below it. The conformable part of a sequence boundary is usually thought of as being effectively synchronous because the hiatus is so small as to be virtually immeasurable. It is clear that a depositional sequence may have more significance in geological history and associated stratigraphical studies than a unit bounded only by synchronous surfaces that are chosen arbitrarily. This is because a sequence represents a genetic unit that was deposited during a single episodic event, whereas the arbitrarily chosen unit may span two or more incomplete portions of genetically related units, and may therefore not accurately display the depositional history.

The definition of an unconformity surface as representing a significant gap in the geological succession begs the question of the amount of time involved. Angular unconformities (Fig.3) can represent many millions, even hundreds of millions, of years; some estimate is usually possible from outcrop studies using fossil evidence, or from regional geological studies. With regard to smaller time-spans, of great interest and relevance in this connection is the magnitude of a hiatus along a sequence boundary, which is analogous to the length of time taken to deposit a sequence. Estimates of the order of magnitude depend essentially on the vertical resolving capability of the method used. A sequence determined primarily from seismic reflection data may encompass a geological time-span of a few million years. A well-log sequence may be recognized that is much smaller than that determined from seismic data and may encompass a million years to hundreds of thousands of years. Very small-scale stratification, bedding, or laminae may be used to determine a hiatus of very short duration, but such examples are likely to fall below what is normally considered to be a significant hiatus and therefore not to fall within the definition of an unconformity.

Alfred Whittaker

Bibliography

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This account is published with the approval of the Director, British Geological Survey (NERC).

unconformity Surface of contact between two groups of unconformable strata, which represents a hiatus in the geologic record due to a combination of erosion and a cessation of sedimentation. Compare diastem. See also ANGULAR UNCONFORMITY; and DISCONFORMITY.

unconformity In geology, break in the time sequence of rocks layered one above the other. The gap may be caused by interruptions in the deposition of sediment, ancient erosion, earth movements, or other activity.