Biostratigraphy is the use of fossils in stratigraphy. It relies on the study of in situ fossil distributions to allow recognition of stratigraphically restricted and geographically widespread taxa or populations, which enables subdivision and correlation of lithostratigraphical successions. Such taxa may be selected as index fossils and used as the basis of biostratigraphical correlation – one of the stratigrapher’s most powerful tools for correlating Phanerozoic sequences. The basic unit of biostratigraphy is the biozone, which is formally described in terms of its fossil indices and content. Biozones are then ordered in stratigraphical position ultimately to allow correlation of lithostratigraphical units. Biozones can be of any thickness or duration. They can be local to world-wide in scale.
Most Phanerozoic successions with fossil remains, from the lower Cambrian to the present, have been subdivided biostratigraphically. While some of the Lower Palaeozoic zones may be several millions of years in duration, resolution is usually much finer, with a regional resolution of as little as 600 000 years per biozone in the Mesozoic, and considerably less in the Quaternary. Biozones can be further divided. Sub-biozones are defined by taxa that may be of only local importance within the more regionally extensive hierarchy of biozones. Erection of one or more sub-biozones within a biozone does not mean that the whole of the biozone has to be thus subdivided. Biohorizons represent single, laterally widespread palaeontological events, which are not vertically extensive and often provide palaeontological evidence of flooding surfaces or condensed units as applied in sequence stratigraphy.
Historically, biozones were usually defined in areas of supposed ‘continuous’ sedimentary deposition under marine influences. However, in the Quaternary context, biozones are established for lacustrine and fluvial palaeoenvironments (e.g. using diatoms, spores and pollen, Mollusca, Coleoptera) and terrestrial palaeoenvironments, based on mammalian sequences.
Biozones should normally be named after their most characteristic fossil, using standard Linnaean binomial notation (generic form capitalised, specific name lower case, both names italicised, with `biozone’ capitalised e.g. Quercus-Pinus Biozone. If a binomial name is used for the biozone, it should be based on a validly published taxon and follow the relevant code of nomenclature (e.g. Zoological Code, Ride et al. 1985; Botanical Code, Greuter et al. 1988). If the name of the taxon then changes, so should the zonal name. Some authors also recommend that the type of biozone should be included in the name (e.g. Palaeoloxodon Taxon-range Biozone), but this can be rather unwieldy.
It must be noted that there are biozonation schemes in common use which do not follow this procedure but instead use alphanumeric notation. These include the Neogene (Tertiary)-Quaternary planktonic foraminiferid schemes (Banner & Blow 1965; Berggren & Van Couvering 1974) and many commercially based biozonations, published or otherwise. Such schemes are too well established to be replaced.
A reference section or sections should be presented for newly established zonal schemes, with fossil range charts and ideally, descriptive taxonomy or illustrative plates of biozonal taxa and possibly their associated assemblages.
Suitable fossil indices should be geographically widespread, common, stratigraphically restricted and morphologically distinct enough to enable unambiguous recognition. If possible, the marine taxa selected should be planktonic or nektonic (e.g. dinoflagellates, radiolaria, planktonic foraminifera) to lessen the effects of facies-controlled distribution which limits all living organisms, but which can especially influence the distribution and dispersal of benthic taxa (e.g. benthic foraminifera). Most long-established bizonations have relied on macrofossils (e.g. Mollusca, small and large vertebrates), but drilling activities in both lakes and the sea, as well as in terrestrial settings, have led to the extensive use of microfossil groups. Principal microfossil groups studied extensively over the last 30 years include palynomorphs (spores, pollen, dinoflagellates), foraminifera (planktonic and benthic), nannoplankton, radiolaria, marine diatoms and ostracods.
These microfossil groups possess many of the attributes outlined for biozonal indices, but with two important additions that relate directly to their small size (generally less than 1 mm). Firstly, they are commonly preserved in prodigious numbers (e.g. Globigerina oozes; Radiolarites) and secondly, drilling usually destroys macrofossils but microfossils are normally returned to surface in small rock fragments (ditch cuttings). Conversely, some microfossil taxa are longer ranging than macrofossils so that zonations based on them may be less precise than those based on macrofossils. However, some micropalaeontological, and more typically palynological, schemes for offshore areas are comparable in precision with well established macrofossil biozonation schemes.
Types of biozone
There are several categories of biozone, and the biozonation of a single succession can comprise a mixture of types.
Range Biozones reflect the actual range of taxa. A biozonal index does not have to occur throughout the entire vertical extent of its distribution. Hence there are several types of range biozone:
Total Range Biozone: boundaries are defined on the combined first (evolution) and last (extinction) occurrences and represent the total stratigraphical and geographical range of the index taxon.
Local Range Biozone: in practice, it is difficult to define total ranges and many so-called total range bizones are in fact local range biozones.
Concurrent Range Biozone: defined on overlapping limits of stratigraphical ranges of two taxa (both tops and bases).
Partial Range Biozone: established within the stratigraphical range of a taxon, with the biozone being actually limited by the appearance and/or extinction of other taxa (=Overlap Biozone of Hedberg 1976).
Consecutive Range Biozone (= Lineage Zone of Salvador 1994): based on the range of a taxon within an evolving phylogenetic lineage.
Acme and Assemblage biozones are defined on population characteristics:
Acme Biozone: the boundaries are defined by the maximum extent (abundance/superabundance) of a particular taxon. The subjective nature (i.e. the differing assessments of relative abundance) of acme biozone definition often limits the recognition and regional extent of such units. Additionally, acmes are often related to microhabitat/localized nutrient influxes. Therefore, acme biozones tend not to be applicable over large distances.
Assemblage Biozone: this is also subjective in definition and is characterized by a distinct assemblage or association of three or more species, which can be distinguished by its character from adjacent assemblages. This type of biozone can be strongly influenced by facies/environment of deposition in certain conditions and can be difficult to define (Holland & Bassett 1988). However, the slow evolution of taxa in the Quaternary means that such biozones are widely recognised.
Boundaries of biozones. Biostratigraphy is a very practical application of the fossil record, so it is hardly surprising that there are two different approaches to the definition of biozonal boundaries.
In ‘classical’ biostratigraphy, where the biostratigraphical schemes are usually based on macrofossils collected at outcrop, biozones are normally defined by their base, marked by the first appearance (evolutionary or migratory) of a particular fossil (which may not necessarily be the zonal index). The top is then defined by the base of the next overlying unit. In any given section the first appearance may change if subsequent discoveries extend known fossil ranges, so that one cannot apply a ‘Golden Spike’ approach to biozonal boundaries (e.g. Salvador 1994).
Ranges of individual taxa are often indicated by reference to first (evolutionary) occurrences (FO) and last (evolutionary) occurrences (LO). However, in the commercial world, drilling practices usually do not permit the confident selection of first occurrences (evolutionary inceptions), as downhole contamination of younger sediments (cavings) obscures these events. Therefore, the principal methodology, without recourse to expensive core or sidewall core retrieval, is to delimit biozones based on first downhole occurrences (usually extinction events) rather than evolutionary inceptions.
Regional correlation of a biostratigraphical event may only approximate to a time-line as a consequence of diachronous taxa distribution. This is particularly evident with respect to the distribution and dispersal of benthic populations in marine sequences, which are often facies dependent. It is equally so when organisms respond to climate or environmental change by migration.
*This guide is based on that produced by Rawson et al. (2002) for the Stratigraphy Commission of the Geological Society of London.