biostratigraphy

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biostratigraphy Biostratigraphy is that aspect of stratigraphy which is concerned with the sequential subdivision of rock strata on the basis of their fossil content. Biostratigraphy had its origins in the late eighteenth century, when William Smith, a civil engineer, deduced that the same strata invariably occurred in consistent sequences and included unique assemblages of fossils. Its establishment as a discrete field of study is considered to have followed from the mid-nineteenth century work of Albert Oppel on Jurassic strata. The term ‘biostratigraphy’ was used first by Dollo in 1910. Most of its current concepts had been formulated by the start of the twentieth century and have subsequently become more subtle.

By the 1950s confusion had arisen over the use of geological terms referring to strata and to time. As a result, an international commission chaired by Hollis Hedberg was established to formulate principles and to agree practices for stratigraphy. The work of this commission resulted in the publication of the International Stratigraphic Guide in 1976. This states that biostratigraphical classification aims to organize rock strata systematically into named units based upon the content and distribution of fossils. All rocks possess lithostratigraphical and chronostratigraphical features. However, as considerable tracts of the Earth's rocks are devoid of fossils, they lack biostratigraphical character. Furthermore, biostratigraphical units are different from other stratigraphical units because they are based on varied, separated remains scattered in uneven quantities within rocks.

The fundamental biostratigraphical unit is a biozone—the general term for any kind of unit used in calibration and correlation. Fossils that are diagnostic of biozones do not occur everywhere that relevant rocks are found. The areas of distribution of fossils vary; some are limited, others extensive. Widely distributed, abundant, and easily recognizable remains of former living organisms that occur over a narrow span of geological time are called index fossils. These fossils are normally of species, the narrowest major classificatory grouping of organisms, and basic to biostratigraphical investigations. However, other members of the taxonomic hierarchy (genera and families in particular) are also used for biostratigraphical purposes. A taxon (plural taxa) refers to any biological category, whatever its classificatory rank, and is a widely used biostratigraphical term. The lateral limits of a biozone are determined by the distribution of the taxa by which it is defined; its vertical limits by the persistence of these taxa over time. Surfaces of biostratigraphical change or distinctive character (zone boundaries) are called biohorizons. Horizons (biohorizons) are increasingly recognized in modern stratigraphical practice. If a number of biozones possess common features they may be grouped into superzones. Biozones may be divided into subzones to demonstrate finer detail. Division of subzones into zonules is the ultimate expression of such patterns.

Four main formal types of biozone can be designated (Fig. 1). An assemblage-zone is characterized by certain association of fossil taxa. Its biohorizons delimit this characteristic assemblage. Boundaries can be defined either by eye, or by computerized numerical methods (such as cluster analysis, ordination, and principal components analysis) in order to avoid human bias. A constituent taxon may be distributed beyond the boundaries of an assemblage-zone. The written definition of a zone is by the scientific (Latin) names of two or more of the principal taxa of which it is composed; for example, Betula–Pinus–Corylus pollen assemblage-zone. A stratotype (i.e. reference stratal section) should also be designated in order that the zone can be identified at other sites. Assemblage-zones are of most use for local correlation, and their constituent taxa can provide important information on past environmental conditions. A range-zone is a body of strata that represents the horizontal and vertical range of a specified taxon. An acme-zone is characterized by the maximum development, abundance or frequency of occurrence of a taxon but not its entire range. An interval-zone occurs between two clearly defined biostratigraphical horizons. It may include nothing diagnostic biostratigraphically, and is normally named after a characteristic of one of its boundaries, such as the appearance or disappearance of a taxon.

A biozone should also be formally described when it has been defined. Important in this description are its name and type locality (stratotype), its kind and rank, diagnostic fossils, associated lithological and chronostratigraphical units, boundaries, thickness and lateral extent, age and correlation, and facies (individual characteristics) and climatic significance.

Informal biostratigraphical units may also be defined; for example, the ‘reindeer stratum’. Also informal is the use of letters or numbers, or both, for biozones in place of formal names. Unlike names, letters and numbers express position in a sequence. They also facilitate communication between geologists, some of whom may be unfamiliar with Latin nomenclature.

R. L. Jones

Bibliography

Blatt,, H. Berry,, W. B. N.,, and and Brande, S. (1991) Principles of stratigraphic analysis. Blackwell Scientific Publications, Boston, Mass.
Hancock, J. M. (1977) The historic development of concepts of biostratigraphic correlation. In Kauffman, E. G. and Hazel, J. E. (eds) Concepts and methods of biostratigraphy, pp. 3–22. Dowden, Hutchinson and Ross, Stroudsburg.
Hedberg, H. D. (ed.) (1976) International stratigraphic guide: a guide to stratigraphic classification, terminology and procedure. John Wiley and Sons, New York.

biostratigraphy The characterization of rock strata on the basis of the fossils they contain. This involves identifying and establishing the distribution and succession of various fossil groups in order to define biozones, containing particular fossils or fossil assemblages that can generally be correlated with rock strata of a particular type in different locations. Ideally, a fossil used in biostratigraphical zoning has a limited range over geological time, so its occurrence is restricted to rock strata of a fairly narrow vertical range in the sequence. For example, the succession of numerous different ammonite species provides an important means of zoning rocks of the Mesozoic era throughout the world. Biozones thus form the basic biostratigraphy units. There are several types: for example, an assemblage zone is defined by the coincident and overlapping ranges of a particular group of fossil taxa, whereas an acme zone is defined by the exceptional abundance of one group or species.

biostratigraphy The branch of stratigraphy that involves the use of fossil plants and animals in the dating and correlation of the stratigraphic sequences of rock in which they are discovered. A zone is the fundamental division recognized by biostratigraphers.

biostratigraphy The branch of stratigraphy that involves the use of fossil plants and animals in the dating and correlation of the stratigraphic sequences of rock in which they are discovered. A zone is the fundamental division recognized by biostratigraphers.

biostratigraphy A branch of stratigraphy that involves the use of fossil plants and animals in the dating and correlation of the stratigraphic sequences of rock in which they are discovered. A zone is the fundamental division recognized by biostratigraphers.

biostratigraphy Branch of stratigraphy that involves the use of fossil plants and animals in the dating and correlation of the stratigraphic sequences of rock in which they are discovered. A zone is the fundamental division recognized by biostratigraphers.