Unconformities (earth science)

Unconformities are surfaces of erosion, nondeposition, or both that have been covered by younger rock through renewed sedimentation. Where they occur, they represent significant portions of geologic time not recorded by rock and, as such, serve to divide a region's geologic history into chapters of more or less continuous deposition separated by depositional breaks.

Rock Record Boundaries

The units that compose the rock record of a region are separated by two fundamental types of boundaries: conformable and unconformable. Rock layers are considered conformable if the deposition of the layer above follows the one below without an appreciable lapse of time. For example, periodic delivery, as during storms, of similar sediment to a depositional site will produce a sequence of homogeneous layers separated from one another by simple bedding planes that represent the short, geologically unmeasurable periods between storms. Such layers are considered conformable. Rocks of contrasting types can also exhibit a conformable relationship. In present oceans, an appreciable area of the sea bottom exhibits different types of sediment collecting simultaneously at adjacent sites. The type of sediment that accumulates at a particular site depends on the type of grains being supplied and the wave and current energy operating there. Adjacent environments of relatively quiet water, such as a lagoon, and agitated water, such as a beach, will collect fine and coarse sediment, respectively. The fine and coarse sediments are said to represent sedimentary facies (facies is the Latin word for the general appearance or aspect of something). Should the agitated conditions spread to operate in the region formerly occupied by quiet-water conditions, the coarser grains will be delivered to the site where only finer grains accumulated previously. At the same time, because of the increase in energy, the finer grains would no longer be allowed to settle where they formerly did but would be moved on to another site. This change in energy within the area would result in the deposition of a layer of coarse material over fine material with essentially no break in sediment accumulation between the two layers.

Most conformable contacts between unlike rock layers in the sedimentary record are interpreted as resulting from such shifts in sedimentary facies through time. These shifts often accompany a rise or fall in sea level. A shift of sedimentary facies toward land during a rise in sea level is termed transgression, and one in which facies are displaced away from land during a lowering of sea level is termed regression. A conformable sequence of strata will generally exhibit an interlayering of grain types or a gradation of grain types between layers.

The rock record exhibits many examples of widespread marine regression that, for a period of time, exposed previously formed rock to subaerial weathering and erosion. In these examples, subsequent transgression of the sea initiates renewed sedimentation, covering the erosional surface with younger layers. Because the time involved allows for broad changes in the distribution of geographic elements and life forms, these new sediments characteristically differ in grain type and fossil content from those below. The rocks of the younger series above the erosional surface exhibit an unconformable relationship to those below, and the buried erosional surface separating the two series is termed an unconformity. The erosional surface, which is to say the unconformity, records an interval of geologic time not represented by the rock succession of the region.

Unconformities divide a region's history into conformable rock groups that are separated from one another by depositional breaks. They are the divisions between the significant chapters of geologic history. The conformable units between a pair of unconformities compose a related group of sedimentary facies that have shifted through time and become intimately interbedded with one another. Because Earth’s history is understood and narrated in terms of the broad divisions outlined by unconformities, the identification and analysis of depositional breaks constitute one of the most important pursuits undertaken by geologists who specialize in studying the stratigraphic record.

Development of Unconformity Studies

Although unconformable relationships were observed and sketched by naturalists in the sixteenth and early seventeenth centuries, the credit for understanding their significance to Earth’s history is generally given to James Hutton, the Scottish geologist whose basic observations are often considered the beginning of modern geological investigations. As early as 1786, Hutton observed stratal relations in which the overlying series of beds rested upon the tilted, eroded edges of the underlying series. In the succeeding two years, Hutton found other examples of angular relations between strata in northern Scotland. He interpreted the sequence of events necessary to develop such a relationship as beginning with marine deposition of the underlying beds, followed by their lithification, deformation (tilting), uplift, and exposure to subaerial erosion, and concluding with submergence by the oceans and deposition of the overlying series of horizontal beds. Hutton was particularly impressed by the time he perceived necessary to produce the angular relationship. Such observations strengthened his belief that the Earth was of great antiquity, a discovery generally accredited to Hutton by science historians.

Hutton did not apply the term “unconformity” to the stratal relationship he elucidated. S. I. Tomkeieff credits the origin of the term to a translation, in the first decade of the nineteenth century, of the German phrase abweichende Lagerung (“deviating bedding”) into the English word “unconformity,” which carried the same meaning as the ecclesiastical term “nonconformity” (dissent from the established church). Throughout the nineteenth century, the term “unconformity” was applied exclusively to the angular relation described by Hutton. However, some, notably Charles Darwin (to explain gaps in the record of the evolution of life), argued that the Earth's strata were replete with subtler, yet equally significant, depositional breaks that lacked discordance.

The first half of the twentieth century was a period of describing criteria for recognizing unconformities, identifying the types of stratal relations represented by unconformities, and proposing terminology to be applied to the different types of relations. Building on a century of study, Eliot Blackwelder, in 1909, clearly outlined the three basic types of relations found at erosional surfaces: angular discordance between beds above and below the erosional surface (the “unconformity” of Hutton), parallel strata above and below the erosional surface, and layered rocks resting on an erosional surface developed on homogeneous metamorphic or plutonic igneous rock, such as granite, which lacks bedding.

Following Blackwelder, various terms were suggested to describe the different types of unconformable relations, with the term “unconformity” itself most commonly retained to designate Hutton's original angular relationship. For American geologists, the question of terminology was essentially settled by Carl Dunbar and John Rodgers in their popular advanced geology textbook Principles of Stratigraphy (1957). In a review of the subject, these authors accepted the term “unconformity” in a generic sense to refer to all types of significant depositional breaks in the geologic record. They identified four types of structural relations that previous geologists had associated with these breaks. For three of these, they recommended retaining previously proposed terms: angular unconformity for the discordant relation of Hutton; nonconformity for situations where layered rock rests on eroded, unlayered igneous or metamorphic rock; and disconformity for an erosional surface of “appreciable relief” separating parallel strata. They introduced the term “paraconformity” for the fourth structural type, which earlier geologists had considered as a special kind of disconformity.

Identifying Unconformities

Disconformities and paraconformities are similar since both are interpreted as erosional surfaces separating parallel strata. They differ in that a disconformity exhibits physical evidence of erosion, preferably an undulatory surface that truncates beds and features in the rock below. In contrast, a paraconformity manifests as a simple bedding plane that lacks relief. The paraconformity's interpretation as a surface representing a significant portion of geologic time not recorded by rock is based on an abrupt change in fossil types, commonly accompanied by a sharp change in rock composition across the boundary. Because abrupt changes in fossil content and rock composition can be explained by natural processes that do not involve loss of record, paraconformities are the most subjective of unconformities. For this reason, Dunbar and Rodgers recommended that these apparent breaks in sequence, which otherwise exhibit no evidence of erosion, be separately identified from disconformities.

Angular unconformities are the most vivid and unequivocal of depositional breaks. The special significance of angular unconformities is that they record crustal movement before the development of the unconformity. Care must be taken not to confuse angular beds resulting from crustal movement with beds naturally deposited at an angle, the so-termed cross beds characteristic of subaqueous and subaerial sand dunes.

Nonconformities are also reasonably simple to identify, but they must be distinguished from intrusive contacts in which igneous rock has been emplaced as molten bodies (magma) beneath the layered rock. Although both nonconformable and intrusive contacts produce similar results—layered rock above unlayered plutonic rock—the two differ fundamentally in their history. The former qualifies as an unconformity because the older igneous rock was exposed to weathering and erosion at the Earth's surface for some time before the deposition of the layered rock. In the intrusive relationship, which is not an unconformity, the younger igneous rock was intruded into previously formed layered rock. Geologists separate the two by examining the evidence at the contact. Indications of baking the sedimentary rock would suggest an intrusive relation, whereas a weathered rind along the margin of the igneous rock would point toward a nonconformable contact.

Evidence for unconformities within parallel strata is more subjective than that for angular unconformities and nonconformities. The criteria developed fall into three broad categories: sedimentological, structural, and paleontologic. Sedimentological criteria revolve around phenomena that develop when a land surface that has undergone weathering and erosion for some time is transgressed by the seas. For example, portions of the soil horizon developed on the layers below the unconformity can be preserved, generally significantly modified in appearance. Suppose the underlying rock contains quartz sand or chert nodules, both relatively insoluble by the weak organic acids found in nature. In that case, these grains will, in the weathering process, become scattered through the soil. The energy of the advancing sea will remove the finer materials of the soil, and the coarser insoluble grains will become concentrated in the base of the unit overlying the erosional surface, generally in sharp contact with the underlying unit.

Structural criteria involve demonstrating truncation (erosion) of features in the rock underlying the postulated erosional surface. Commonly, that involves channels cutting through whole beds in the underlying sequence at the base of the overlying unit demonstrating the presence of topography—in some examples, with relief of several hundred feet—on the erosional surface. On a smaller scale, fossil shells and other relatively coarse grains and structures can be cut off at the erosional surface.

Paleontologic evidence for an unconformity is constituted by a sharp change in fossil species, commonly associated with missing forms known to be present in the established sequence of fossils in nearby regions. The thought is that if sedimentation had been continuous, a complete paleontologic sequence would be present. Paleontologic evidence is almost always the only indication of the time value of an unconformity. If it is the only evidence for an unconformity, then the postulated erosional surface is most properly called a paraconformity.

Evaluation of Disconformities and Paraconformities

Because most evidence for unconformities in parallel strata can also be explained by natural processes not involving loss of record, some postulated disconformities and many paraconformities are debated among geologists. For example, a sandy layer with a sharp basal contact could result from a rapid shift in depositional facies rather than grains concentrated through the weathering of the rocks below. A stream can break out of its channel and cut a new channel in the adjacent floodplain. Truncation of the bedding of the floodplain sediments would be pronounced. Yet the contact between the channel deposits and those of the floodplain into which the channel scoured would not be considered unconformable because the age difference between the two would at most be only a few years, a geologically insignificant value.

Paraconformable surfaces are the most difficult to evaluate. Those involving large gaps in the fossil record, expressible in terms of units of the geologic time scale, generally exhibit some sedimentological or structural evidence when examined closely over a large geographic extent. They are nevertheless enigmatic since the indication is that large areas lay exposed to subaerial erosion for considerable lengths of time without developing distinct criteria for weathering and rock removal. As the fossil gap represented at paraconformities decreases, interpretation becomes more contentious. The problem is that fossils were once life forms, all of which are controlled, at least to some extent, by environmental conditions. The absence of a few fossil forms at a boundary could result from the absence of the environments to which the forms, when living, were adapted rather than from the loss of record through erosion. Norman Newell suggests that some paraconformities might develop during a prolonged standstill of sea level on a shallow shelf adjacent to a low-lying land area. Under such conditions, sedimentation on the shelf would eventually raise the sediment surface into water so shallow that the available wave energy would not allow more sediment to accumulate. Instead, it would move it across the shelf to deeper water. If such conditions of nonaccumulation persisted long enough, ultimately, surfaces that represented a significant extent of geologic time, which exhibited no physical evidence of erosion, could develop. These surfaces would qualify as paraconformities.

No one criterion serves as unequivocal proof of an unconformity in parallel strata. In attempting to establish the position of an unconformity in undisturbed beds, geologists pursue several lines of sedimentological, structural, and paleontologic evidence. Geologists have become increasingly aware that the stratigraphic record contains a spectrum of sedimentological breaks ranging from a few days of nondeposition to several hundred million years involving uplift, subaerial exposure, and erosion. In general, they agree that the term “unconformity” should apply only to those surfaces that involve a “significant” amount of time. However, just how much time qualifies as significant is undefined. It is apparent that if an area is undergoing erosion, the sediment produced must be deposited somewhere. Therefore, all unconformities diminish laterally and eventually pass into conformable surfaces. In practice, geologists apply the term “unconformity” to those surfaces where the missing record can be expressed in terms of the regional stratigraphic column or in units of the international geologic time scale. Sedimentological breaks too small to be expressed in local or international terms are commonly referred to as diastems.

Principal Terms

angular unconformity: an unconformity in which the beds below are at an angle to the beds above

diastem: a short depositional break of geologically unmeasurable duration caused by a limited period of nondeposition

disconformity: an unconformity separating parallel strata and exhibiting physical evidence of erosion

intrusive contact: contact resulting from injection of molten igneous rock into previously formed rock

nonconformity: an unconformity in which rock below is homogeneous igneous or metamorphic rock and rock above is layered

paraconformity: an unconformity for which the only evidence is a gap in the fossil record

regression: a shift of sedimentary facies away from land, commonly caused by a drop in sea level

sedimentary facies: different types of sediment that collected simultaneously in adjacent areas of the seafloor

transgression: a shift of sedimentary facies toward land, commonly caused by a rise in sea level

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