Relative dating of strata

The successive deposition of identifiable layers of geologic materials provides a way to ascribe relative age to the contents of those layers. The dating is based on the assumption that the closer a layer is to the surface, the more recently it was deposited. It is also assumed that unconnected layers in different locations are of the same age if they have the same composition and the same place within a pattern of layers. Sedimentation, fluvial processes, and cataclysmic or volcanic processes are the most common depositors of strata. Stratigraphic dating can only be relative to other stratigraphic layers; absolute dating requires a “clock” mechanism such as radioactive decay that effectively counts time back from the present.

Stratigraphic Layers

British geological engineer William Smith determined in 1793 that there is a direct correlation between the fossil content of rock layers and the layers themselves. Although stratigraphic layers of rock and soil always have been apparent to the human eye, as have fossils, no combination of the two observations had ever occurred. Smith came to understand that the fossil content of a layer was characteristic of that layer and, hence, of the age in which the layer was formed. His discovery permitted, for the first time, the mapping of subsurface terrains and has since proved invaluable for mining, engineering, resource exploration, hydrology, construction, geology, paleontology, and every other branch of subterranean endeavor.

Smith's realization came from the deceptively simple observation that the same kinds of fossils were always found in the same kind of rock layers in excavations carried out for the construction of canals, bridges, and other structures. By identifying the fossils that were unearthed, he found he could tell exactly what the stratigraphy would be both above and below the location of the fossils, regardless where they were located. In many cases, the layers were discontinuous, having been incised by erosion or other features. This did not seem to alter the stratigraphic ordering of the layers, however, leading to the conclusion that any particular layer must have been laterally continuous when it was formed. Any variations and interruptions in a particular layer were thus caused by erosive and geological processes that occurred in the time since the layer was formed.

Layer Formation and Rock Types

Earth is a dynamic planet. It has an active core and mantle that exert forces on the relatively egg-shell-thin crust, causing it to split and reorganize, bring new material to the surface through volcanic activity, subduct older material back into the mantle, and deform existing structures through the actions of heat and pressure. Each of these processes leaves its imprint in the geological structure of the crustal material in a variety of ways. In addition, fluid processes caused by the movement of air and water over the surface play significant roles in the development of surface structures and stratigraphic layers.

Chronostratigraphy is a practice that allows geologists to assign the relative ages and general dates at which specific processes occurred according to the characteristics of particular stratigraphic layers. When tied to an absolute dating method such as radiometric dating, the assignment of age according to stratigraphic location can be quite accurate.

Stratigraphic layers are formed by two primary methods: volcanic activity and sedimentation. Through volcanic activity, various rock and mineral formations can be formed as stratigraphic layers. Molten lava spreading and solidifying over existing structures forms layers of igneous rock, while volcanic ash and other ejecta fall to the ground and form a covering layer.

Sedimentary processes, in which particulate mineral matter settles out of a fluid medium, deposit layers of sediment, dust, sand, and larger particles on top of existing surface structures. Sedimentation occurs in order of relative density, with the most dense particles settling out first and the least dense last. Typically this means that light organic materials are deposited at the top of each particular deposition layer, over time forming bedding planes between successive layers. In many sedimentary formations, bedding planes permit the cleavage and separation of layers to reveal physical details of materials and objects that have been captured in the formation. Sedimentary rock formations are found to contain fossil formations, while igneous rock formations do not.

Between igneous and sedimentary rock types are the metamorphic rocks. These may be either igneous or sedimentary in origin, but both have been altered through heat and pressure from their original form. An example is the argillaceous shale known as slate. Shale is formed by the deposition of fine clay particles less than four microns in size. Through subjection to prolonged heating and pressure the shale structure becomes baked and compressed, or tempered, into the much harder and more glass-like structure of slate. Metamorphic rock that is sedimentary in origin may still contain fossils, but the metamorphosis tends to destroy fossil remains, and at the very least makes it much more difficult to release them.

Fossils and Fossilization

Essentially all fossils are formed as the result of fluvial processes. Typically, fossilization occurs after an organism dies and its remains become covered by sediments before they are able to decompose or be ravaged by scavengers. As successive layers of sediment build up over the remains, decomposition slowly occurs, as the surrounding material is compressed by the weight of material above it. The extra weight eventually compresses the sediments sufficiently to bring about chemical alterations that cement together its component particles. At the same time, minerals dissolved in water percolating through the sedimentary layer replace the original mineral content of the remains to produce an exact replica of the remains in stone.

To appreciate the formation and importance of fossil formations, it is necessary to understand the length of time over which such processes occur. While it is possible for the entombment of remains to occur very quickly, as when an animal has perished in a flash flood or the collapse of a sand dune, it is far more common that the deposition of sedimentary layers has occurred slowly over many years. Geologists use present-day rates of sedimentation as model data for past processes.

The deposition of silt in a slow-moving stream or river is as little as one millimeter, essentially no more than the layer of clay that coats rocks in slow-moving waters. Such a layer compresses to a considerably smaller thickness, indicating that a stratigraphic layer one meter in thickness and consisting of a single type of sediment could have taken tens of thousands of years to accumulate. In early marine environments that held only simple mollusks and crustaceans with calciferous remains, the life cycle of those organisms would have produced an almost constant deposition of shells and other remains to the sea floor over long periods of time. As the remains, consisting mainly of calcium carbonate, accumulated, they eventually compressed to form thick, uniform deposits of chalk. Such deposits, such as the white cliffs of Dover in England and extending into western Europe, are known to have thicknesses measured in hundreds of meters. It was the consistency of such layers occurring in recognizable patterns, along with their characteristic fossil content, which provided the basis for Smith's observations.

The Suppositions of Chronostratigraphy

Relative dating according to position within stratigraphic layers, or chronostratigraphy, is based on three very basic, but important, suppositions. The first of these is the principle of superposition. This is the principle by which every stratigraphic layer that forms must do so on top of existing stratigraphic layers. That is to say, each layer is correspondingly older than the layer that is immediately above it. The basic premise here is that layers form according to the succession of time; a layer of sediment that forms in any given year can never become situated below the layer that formed in any previous year. This provides the basis for the vertical ordering of layers.

The second supposition is the principle of lateral continuity. According to this principle, stratigraphic layers having identical mineralogical content and structure, containing identical distributions of fossil or other extraneous content, must have formed at the same time and as a single continuous layer. Various erosive and geological processes that operate in the time between the formation of the layer and some more recent date function to form discontinuities in the layer that were not present when it was formed. These may be such actions as water erosion, seismic, tectonic and orogenic activity, and wind erosion.

Each of these processes has the potential to remove over time some portion of a laterally continuous stratigraphic layer, or to induce some form of discontinuity in the layers of a specific region in which the activity takes place. Orogenic processes raise and fold rock formations, producing angled beds that, when coupled with the results of erosive actions, mask the innate continuity of the stratigraphic layers within the affected formation. This is the feature that makes Smith's deductions the stroke of genius rather than mere observation.

The third basic principle is horizontality, in which identical layers are tied to the same period or absolute date regardless of their relative elevations and orientations. For example, a layer formation that has been elevated by orogenic activity is nevertheless the product of the same period as the identical layer at a lower elevation. The dating of a layer is therefore transferable from one location to another only if the two layers are otherwise identical in composition, content, and position within the pattern of the encompassing layers.

Given these three basic principles, the age of any particular layer relative to other layers can be ascribed with a high degree of accuracy. The pattern of stratigraphic layers, however, forms a self-consistent internal system with the condition that such a system might reflect a span of time that could have occurred at any time within the age of the planet. That is to say, it is not possible to know precisely when the span of time reflected in the pattern of stratigraphic layers actually came to pass. This requires the correlation of some point within that span of time to a known point in time, or absolute date.

Absolute or Radiometric Dating

To know precisely when a particular stratigraphic layer in a geological formation came into being, it must be possible to relate that unknown point in time to some known point in time. The most obvious point in time to which the past can be tied is the present. To achieve this, some kind of “clock” process that can be used to count time back to some specific starting point is required. Nature provides just such a clock mechanism in the structure of atoms.

Chemical elements are known to occur in isotopic forms in which certain atoms containing the same number of protons in the nucleus, by which they are atoms of the same element, have different numbers of neutrons. Some isotopes are radioactive. That is, they decay into atoms of different elements by ejecting specific portions of their nuclear mass. The process has a specific starting point and continues through exact steps until a stable atom is formed. Accordingly, a specific radioactive parent element produces only one specific stable daughter element.

Equally important is the rate at which the exponential radioactive decay process takes place. The rate is directly dependent on the amount of radioactive material that is present; this rate is described by the mathematical equation A = A_o e^−Kt, where A is the amount of the element at time t, A_o is the original amount of the element, t is the time that has elapsed form the starting condition, and K is a constant. A special relationship exists for such a system when the amount remaining is exactly one-half of the original amount. According to the mathematical relationship, it takes exactly the same amount of time for any quantity of the material to break down to one-half of that quantity. Thus it requires the same amount of time for one kilogram of a radioactive element to decay to 500 grams as it does for one gram to decay to 500 milligrams. This amount of time is called the half-life of the process. By determining the amounts of starting radioactive element and stable product element in a sample, it is then easy to determine the number of half-lives that have elapsed since the initial condition of the system was established. Essentially, the equation can determine how long ago the initial condition was established.

One of the most-used relationships for determining geological time is the ⁴⁰K–⁴⁰Ar decay system, in which radioactive potassium atoms of mass 40 decay into stable argon atoms of mass 40. Because this occurs within the structure of potassium-containing rocks, the argon that is produced is trapped within the same rocks. Mass spectrometric analysis of the potassium and argon recovered from a particular rock formation can determine the relative amounts of ⁴⁰Ar and ⁴⁰K that remain. Measurement of the rate of decay for the process can determine the value of K. These values provide the data needed to determine the time t that has elapsed.

There are, of course, practical limits based on the error limits of detection of the isotopes. The half-life of the K-Ar transition is 1.28 × 10⁹ years. An error of just 0.01 percent on this determination translates to a temporal error of 128,000 years, an error range of 256,000 years altogether. More precise measurement enables a more precise determination of absolute age, but it must be remembered that other sources of error also are involved. Thus, geological ages are always rounded off or expressed as a range.

An excellent example of these limits is the so-called K-T boundary event, which terminated the Cretaceous period and ended the age of dinosaurs. In that event, a very large meteorite collided with Earth near the Yucatan Peninsula, initiating a globe-spanning firestorm and depositing a recognizable layer of iridium-rich material over the entire planet. This layer delineates the Cretaceous-Tertiary boundary and is dated radiometrically to 65 million years ago. A more precise date is not yet knowable, even though the actual event occurred at one specific instant in time.

Relating Ages

With an appropriate absolute-dating method, the absolute age of specific contents (such as a fossil) of a stratigraphic layer can be determined. From that point on, the presence of fossils identified as the same species in a stratigraphic layer at any other location also identifies the age of that layer. Similarly, the presence of a specific mineral type in one layer can be used to assign an age to another layer in which that same mineral type appears. This is somewhat less reliable method, however, because Earth is a dynamic planet and because the chemical processes that create specific minerals are constantly in operation, permitting the formation of the same mineral type at different times. In unique cases, such as that of the K-T boundary, however, the identification of relative age is absolute, and it is certain that that stratigraphic layer identifies the same point in time wherever it is found.

Principal Terms

arenaceous: rocks or sediments having a sandy composition, composed of grains of sand

argillaceous: rocks or sediments formed principally from clay or clay-mineral particles

dip: the angle between the horizontal and the apparent plane between stratigraphic layers

facies: the combination of characteristic lithology and paleontology of a sediment from which the environmental conditions at the time the sediment was deposited can usually be inferred

fluvial: having to do with or being the result of flowing water or other liquids

mudstone: sedimentary rock type formed from mixtures of particles ranging from fine clay to coarse sand grains

orogeny: the process of mountain building by tectonic movement

sandstone: sedimentary rock type formed from sand-sized silicate grains agglomerated and consolidated usually with carbonate minerals

seat earth: a fossil soil layer generally found directly beneath coal beds, often with plant rootlets still in place

siltstone: sedimentary rock type, typically shale, formed from fine clay and silt particles less than four microns in size

strata: defined layers in sedimentary rock, typically separated from each other by identifiable bedding planes

Bibliography

Bennett, Sean J., and Andrew Simon, eds. Riparian Vegetation and Fluvial Geomorphology. Washington, D.C.: American Geophysical Union, 2004. This book closely examines the various processes that take place along flowing water, such as how erosion and sedimentation are related and how they are affected by vegetation and geological structures.

Koutsoukos, Eduardo A. M., ed. Applied Stratigraphy. New York: Springer, 2007. This book presents the historical background of stratigraphy, and builds on that foundation to present the theory and practice of chronostratigraphy as a research tool.

Petersen, James F., Dorothy Sack, and Robert Gabler. Fundamentals of Physical Geography. Belmont, Calif.: Cengage Learning, 2011. Chapter 14 is dedicated to fluvial processes and how they relate to various landforms, an important factor in the formation of sedimentary stratigraphic layers.

Rey, Jacques, and Simone Galeotti, eds. Stratigraphy Terminology and Practice Paris: Technips, 2008. This book focuses on the essential theory and practices of the five major fields of chronology based stratigraphic methods, relating them to each other and to the fields of study in which each is best employed.

Wicander, Reed, and James S. Monroe. The Changing Earth:. Exploring Geology and Evolution. 5th ed. Belmont, Calif.: Cengage Learning, 2009. This basic college-level geology textbook provides a thorough discussion of geological processes before discussing how they provide the stratigraphic basis for relative dating.

Winchester, Simon. The Map That Changed the World:. William Smith and the Birth of Modern Geology. New York: HarperCollins, 2001. This well-researched book details the history of William Smith's realization of stratigraphy and the lateral continuity of those layers.