stratigraphy

stratigraphy The definition of stratigraphy is simple: it is the study and description of rock successions. Its more philosophical aspects, especially those concerned with the extent to which the physical nature of the record can be used to mark the passage of geological time, are less easy to grasp and have been much debated. Stratigraphy seeks to interpret rock successions as sequences of events in the history of the Earth. Many different field methods and laboratory techniques contribute to stratigraphical investigations, while, in its turn, stratigraphy provides the basis for basin analysis, palaeogeography, and the geology of petroleum and other resources. Stratigraphy is the heart of much historical geology, because it reveals the order in which past events and processes occurred. The Cryptozoic part of the geological record, with its largely unfossiliferous and commonly highly deformed rocks is much less susceptible to the techniques of stratigraphy than is the Phanerozoic (fossiliferous) part.

Stratigraphy figures large in the early history of the Earth sciences. Between the Renaissance and the Age of Reason the foundations of geology were laid in western and central Europe, where stratified rocks were ubiquitous. Nicolaus Steno's mid-seventeenth century recognition of the law of superposition and other fundamentals of what we might call field relationships was soon followed by classifications of rock types grouped into successions. These ultimately became the ‘stratigraphical column’ in use today. The English civil engineer William Smith (1769–1839), sometimes referred to as the ‘Father of Stratigraphy’ laid the foundations of biostratigraphy and produced a geological map of England and Wales in which rock units correlated by their fossil contents were depicted. This, the first modern geological map, first appeared in 1815. Throughout the nineteenth century many geologists were to investigate their local geology, and the geological map of Europe began to take shape. A primary objective almost everywhere was to establish local stratigraphical successions. This brought many new formation names into the literature. Correlation with the aid of fossils was highly successful and made it possible to begin building the stratigraphical column to the point where it seemed to represent every moment of Phanerozoic time, if not some Proterozoic time too. Charles Lyell in his Principles of geology in 1833 included a columnar diagram in which the main divisions of rocks and the corresponding periods of Phanerozoic time were represented. He was among the first to appreciate—indeed to demonstrate—the magnitude of the time involved. He argued that a vast period of time was necessary on two counts: the uniformity of the laws and processes that affect the accumulation of stratified rocks, and the rates of these processes.

Disputes and disagreements about the relative positions of newly described sections and formations, and the terminology to be employed, abound in the literature of the time. One of the most active and important scientific bodies that sprang up early in the nineteenth century was the Geological Society of London, which provided a forum for the presentation and discussion of new facts and ideas.

Today's stratigraphical column has evolved as knowledge of the rocks has improved, yet it still contains inconsistencies that derive from the divisions employed more than a century ago. The stratigraphy of the more recent geological periods is more accessible than that of very ancient times, and the problems encountered in the higher (i.e. younger) parts of the stratigraphical column tend to be different from those found below. As research continues and our methods of geochronometry improve, some of the problems may be resolved, but it remains a truism that the older rock formations, and especially those that have been severely deformed, are the more difficult to date and correlate. Stratigraphy, then, was originally concerned with rocks, that is, lithostratigraphy; it soon afterwards included the ranges of fossils throughout the successions, biostratigraphy; and the more abstract chronostratigraphy. The techniques of magnetostratigraphy make use of the stratigraphical variations in the magnetic properties of rocks as a basis for correlation. The dating of rocks in years (‘absolute dating’) is the science of geochronometry, which today usually entails isotope analysis. Figure 1 illustrates links between different stratigraphical approaches, some of which are discussed below in more detail.

Lithostratigraphy, concerned with the definition and naming of rock units, relies upon the lithological characteristics and geometry of the rock bodies. The basic unit is the formation; it is commonly regarded as the smallest mappable unit, though there are many exceptions. Formation boundaries may be sharp or gradational, and thicknesses vary from a few metres to several hundreds. Within the formation local variations may distinguish one or more members, each of which may consist of several beds, the bed being the smallest recognizable formal unit of all. Several contiguous formations may be distinguished as a group; for reasons of local convenience or genetic similarity several groups together may be considered as supergroup. Biostratigraphy is, as we have seen, the use of fossils for correlation and the organization of strata into units on the basis of their fossil content. Each basic unit is a biozone, identified by a characteristic taxon (or taxa) of fossil (or fossils). Sub-biozones may be distinguished on the basis of small faunal or floral variations within a biozone. Many biozones may also be called range biozones because they exhibit an aspect of the total range of the index species. Different are assemblage biozones, which are characterized by unique assemblages of taxa, several of which have different stratigraphical ranges from the others. These biozones are commonly of restricted or local occurrence.

Chronostratigraphy is the unifying construct that defines (ideally by international agreement) boundaries for systems, series, and stages . Its ultimate product is the global stratigraphical column in which every moment of Phanerozoic time is represented by strata which by their contained fossils are distinct from those above or below. It is a compilation that was begun two centuries or more ago and is still imperfect, if not incomplete. So much confusion has been generated in the past by incorrect correlations, imprecise definitions of stratigraphical units, and other inconsistencies that a regular procedure has been devised for defining stratigraphical units. This procedure involves the selection and definition of a Global Stratotype Section and Point (GSGP) to fix such a boundary. Because most fossiliferous formations are of marine origin, attention has been focused upon these strata in the belief that the most continuous and fullest record of the evolution of life (and hence of the passage of Phanerozoic time) has been recorded in the seas. Other habitats and environments are prone to many disruptions and variations, and they consequently present difficulties in establishing comparable standards.

Table 1. Chronostratigraphic divisions and time divisions

Chronostratigraphic	Corresponding divisions
divisions	of geological time
Eonothem	Eon
Erathem	Era
System	Period
Series	Epoch
Stage	Age

The procedure used for establishing a GSGP entails collection of all possible biological and other information about the rocks in an unbroken stratigraphical section and the selection of a point in the rock sequence that can be defined by the fossils around it. Correlation elsewhere with this point has to be achieved as accurately as possible, using all the available data. GSGPs are used to define the base of a standard chronostratigraphic unit. In effect this also defines the top of the underlying unit. The section containing the point is known as the stratotype section, and the point is said to be designated by an imaginary ‘golden spike’. All this is achieved by investigation and agreement under the aegis of the Commission on Stratigraphy (and its Subcommissions), an offspring of the International Union of Geological Sciences.

The chronostratigraphic hierarchy of divisions corresponds to divisions of time, as listed in Table 1. Most of the names in use for the series and stages refer to the place where the type section is located. Ultimately, it is hoped, a global stratotype section and point will be designated for every stage.

When the detailed biostratigraphic division of a set of rocks is undertaken, zones may be established at the outset which eventually prove to be of great value in very wide geographical, if not global, correlation. These zones have been called chronozones, the smallest chronostratigraphic unit. They are exemplified by the ammonite biozone stratigraphy of the Mesozoic, but so far few have been established for the Palaeozoic erathem.

Geochronometry is the application of techniques which determine the ages of rocks in years. Many of these techniques have been concerned with assessing the rates of geological processes and using these values to obtain the age of their products at specific sites. The study and analysis of sections of annual clay and silt laminae (varves) from Quaternary lakes has had some success, and the measurement of the growth rings of long-lived trees has led to the establishment of dendrochronology as useful in archaeological and Holocene studies. Radiometric or isotopic dates, however, are the most favoured dating techniques; radiocarbon dating refers to isotope analysis of carbon from biological materials not older than about 140 000 years.

High-resolution stratigraphy is the term applied to the kinds of precise stratigraphical resolution that may be possible on the basis of microfossil biostratigraphy, magnetostratigraphy, carbonate analysis, and oxygen or other stable isotope stratigraphy. It has great value in deciphering the sedimentary record where it reflects specific geological events (event stratigraphy), and it has been much influenced by fine-scale stratigraphical studies of ocean sediment sequences. These approaches have been singularly successful where Recent and Cenozoic sediments are concerned, and they are also increasingly applied to Mesozoic and Palaeozoic successions. Some of the studies have included attempts to establish rates of geological processes such as erosion, sediment transport, and organic evolution.

Ecostratigraphy, according to the American pioneer in this field, A. J. Boucot, covers ‘the evolutionary, ecologic, biogeographic, stratigraphic correlation and basin analysis consequences of the basic fact of biofacies’.

The study of regional stratigraphy can make it possible to recognize the uneven and geographically varying rates at which sedimentary basins subside and are filled and to reveal the erosional history of upland areas. The constant shift and evolution of drainage, glacial, and other physiographical systems may be indicated, and, most significant of all, the advance and retreat of the shoreline can be measured by stratigraphy. By these means, too, relative rates of geological processes can be demonstrated. During sedimentation certain distinctive beds may be formed time and time again in a regular pattern, and we refer to deposition of this type as cyclic sedimentation (see also cyclicity).

Because the distribution of many important economic raw materials, especially the hydrocarbon fossil fuels, is controlled by stratigraphy, this branch of the Earth sciences has been vigorously pursued by applied geologists. The first petroleum well in the United States was sunk in the mid-nineteenth century, and within a few decades companies were employing geologists in the search for petroleum deposits. Since then the industry has invested immense capital sums in exploration work in many parts of the world. The job of stratigraphers in the petroleum industry is to locate the strata that generate and retain commercial quantities of oil and gas. Coal, evaporites, and many other important materials also occur as stratified deposits in association with specific lithologies and under special stratigraphical conditions. To assist in the exploration for these materials at depth, techniques of rock sampling in boreholes, and the measurement of the geophysical properties of stratigraphical formations have reached a very high degree of sophistication. Some of these techniques have been employed also in purely academic research in stratigraphy, with far-reaching effects of the interpretation of the stratigraphical record.

D. L. Dineley

Bibliography

Blatt, H.,, Berry, W. B. N.,, and and Brande, S. (1990) Principles of stratigraphic analysis. Blackwell Scientific Publications, Oxford.
Briggs, D. E. G. and Crowther, P. C. (eds) (1990) Palaeobiology. Blackwell Scientific Publications, Oxford.
Dineley, D. L. (1992) Stratigraphy and dating. In Encyclopedia of Earth Systems Science. Academic Press, New York.
Prothero, D. M. (1989) Interpreting the stratigraphic record. W. H. Freeman, New York.
Schoch, R. M. (1989) Stratigraphy: principles and methods. Van Nostrand Reinhold, New York.