Carbonate platforms

A number of different morphologies of carbonate platform are recognised, the most widely documented being carbonate ramps, which are gently sloping platforms, and rimmed shelves, which are flat-topped platforms bordered by a rim formed by a reef or carbonate sand shoal. The tectonic setting influences the characteristics of carbonate platforms, with the largest occurring on passive continental margins while smaller platforms form on localised submarine highs such as fault blocks in extensional settings and on salt diapirs. The different types of carbonate platform can sometimes occur associated with each other: an isolated platform may be a carbonate ramp on one side and a rimmed shelf on the other and one form may evolve into another, for example, a ramp may evolve into a rimmed shelf as a fringing reef develops.

Carbonate ramps

The bathymetric profile of a carbonate ramp and the physical processes within the sea and on the sea floor are very similar to an open shelf with clastic deposition. The term ‘ramp’ may give the impression of a significant slope but in fact the slope is a gentle one of less than a degree in most instances, in contrast to slope environments associated with rimmed shelves, which are much steeper. Modern ramps are in places where reefs are not developed, such as regions of cooler waters, increased salinity or relatively high input of terrigenous clastic material. However, in the past carbonate ramps formed in a wider range of climatic and environmental settings, especially during periods when reef development was not so widespread. In macro- to mesotidal regimes tidal currents distribute carbonate sediment and strongly influence the coastal facies. Wave and storm processes are dominant in microtidal shelves and seas. The effects of tides, waves and storms are all depth-dependent and ramps can be divided into three depth-related zones: inner, mid- and outer ramp.

Distribution of facies on a carbonate ramp

The inner ramp is the shallow zone that is most affected by wave and/or tidal action. Coastal facies along tidally influenced shorelines are characterised by deposition of coarser material in channels and carbonate muds on tidal flats. Wave-dominated shorelines may have a beach ridge that confines a lagoon or a linear strand plain attached to the coastal plain. Ramps with mesotidal regimes will show a mixture of beach barrier, tidal inlet, lagoon and tidal-flat deposition. Agitation of carbonate sediment in shallow nearshore water results in a shoreface facies of carbonate sand bodies. Skeletal debris and ooids formed in the shallow water form bioclastic and oolitic carbonate sand shoals. Benthic foraminifers are the principal components of some Tertiary carbonate ramp successions. The mid-ramp area lies below fair-weather wave base and the extent of reworking by shallow-marine processes is reduced. Storm processes transport bioclastic debris out on to the shelf to form deposits of wackestone and packstone, which may include hummocky and swaley cross-stratification. In deeper water below storm wave base the outer ramp deposits are principally redeposited carbonate mudstone and wackestone, often with the characteristics of turbidites. Redeposition of carbonate sediments is common in situations where the outer edge of the ramp merges into a steeper slope at a continental margin as a distally steepened ramp. Homoclinal ramps have a consistent gentle slope on which little reworking of material by mass-flow processes occurs. In contrast to rimmed shelves reefal build-ups are relatively rare in ramp settings. Isolated patch reefs may occur in the more proximal parts of a ramp and mud mounds are known from Palaeozoic ramp environments.

Carbonate ramp succession

A succession built up by the progradation of a carbonate ramp is characterised by an overall coarseningup from carbonate mudstone and wackestone deposited in the outer ramp environment to wackestones and packstones of the mid-ramp to packstone and grainstone beds of the inner ramp. The degree of sorting typically increases upwards, reflecting the higher energy conditions in shallow water. Inner ramp carbonate sand deposits are typically oolitic and bioclastic grainstone beds that exhibit decimetre to metre-scale cross-bedding and horizontal stratification. The top of the succession may include fine-grained tidal flat and lagoonal sediments. Ooids, broken shelly debris, algal material and benthic foraminifers may all be components of ramp carbonates. Locally mud mounds and patch reefs may occur within carbonate ramp successions. On shelves and epicontinental seas where there are fluctuations in relative sea level, cycles of carbonate deposits are formed on a carbonate ramp. A sea-level rise results in a shallowing-up cycle a few metres to tens of metres thick that coarsens up from beds of mudstone and wackestone to grainstone and packstone. A fall in sea level may expose the inner ramp deposits to dissolution in karstic subaerial weathering.

Non-rimmed carbonate shelves

Non-rimmed carbonate shelves are flat-topped shallow marine platforms that are more-or-less horizontal, in contrast to the gently dipping morphology of a carbonate ramp. They lack any barrier at the outer margin of the shelf (rimmed shelves) and as a consequence the shallow waters are exposed to the full force of oceanic conditions. These are therefore high-energy environments where carbonate sediments are repeatedly reworked by wave action in the inner part of the shelf and where redeposition by storms affects the outer shelf area. They therefore resemble storm-dominated clastic shelves, but the deposits are predominantly carbonate grains. Extensive reworking in shallow waters may result in grainstones and packstones, whereas wackestones and mudstones are likely to occur in the outer shelf area. Coastal facies are typically low energy tidal-flat deposits but a beach barrier may develop if the wave energy is high enough.

Rimmed carbonate shelves

A rimmed carbonate shelf is a flat-topped platform that has a rim of reefs or carbonate sand shoals along the seaward margin. The reef or shoal forms a barrier that absorbs most of the wave energy from the open ocean. Modern examples of rimmed shelves all have a coral reef barrier because of the relative abundance of hermatypic scleractinian corals in the modern oceans. Landward of the barrier lies a low-energy shallow platform or shelf lagoon that is sheltered from the open ocean and may be from a few kilometres to hundreds of kilometres wide and vary in depth from a few metres to several tens of metres deep.

Distribution of facies on a carbonate rimmed shelf

In cases where the barrier is a reef, the edge of the shelf is made up of an association of reef-core, fore-reef and back-reef facies: the reef itself forms a bioherm hundreds of metres to kilometres across. Sand shoals may be of similar extent where they form the shelf-rim barrier. Progradation of a barrier results in steepening of the slope at the edge of the shelf and the slope facies are dominated by redeposited material in the form of debris flows in the upper part and turbidites on the lower part of the slope. These pass laterally into pelagic deposits of the deep basin. The back-reef facies near to the barrier may experience relatively high wave energy resulting in the formation of grainstones of carbonate sand and skeletal debris reworked from the reef. Further inshore the energy is lower and the deposits are mainly wackestones and mudstones. However, ooidal and peloidal complexes may also occur in the shelf lagoon and patch reefs can also form. In inner shelf areas with very limited circulation and under conditions of raised salinities the fauna tends to be very restricted. In arid regions evaporite precipitation may become prominent in the shelf lagoon if the barrier provides an effective restriction to the circulation of seawater.

Rimmed carbonate shelf successions

As deposition occurs on the rimmed shelf under conditions of static or slowly rising sea level the whole complex progrades. The reef core builds out over the fore reef and back-reef to lagoon facies overlie the reef bioherm. Distally the slope deposits of the fore reef prograde over deeper water facies comprising pelagic carbonate mud and calcareous turbidite deposits. The steep depositional slope of the fore reef creates a clinoform bedding geometry, which may be seen in exposures of rimmed shelf carbonates. This distinctive geometry can also be recognised in seismic reflection profiles of the subsurface. The association of reef-core boundstone facies overlying forereef rudstone deposits and overlain by finer grained sediments of the shelf lagoon forms a distinctive facies association. Under conditions of sea-level fall the reef core may be subaerially exposed and develop karstic weathering, and a distinctive surface showing evidence of erosion and solution may be preserved in the stratigraphic succession if subsequent sea-level rise results in further carbonate deposition on top.

Epicontinental (epeiric) platforms

There are no modern examples of large epicontinental seas dominated by carbonate sedimentation but facies distributions in limestones in the stratigraphic record indicate that such conditions have existed in the past, particularly during the Jurassic and Cretaceous when large parts of the continents were covered by shallow seas. The water depth across an epicontinental platform would be expected to be variable up to a few tens to hundreds of metres. Both tidal and storm processes may be expected, with the latter more significant on platforms with small tidal ranges. Currents in broad shallow seas would build shoals of oolitic and bioclastic debris that may become stabilised into low-relief islands. Deposition in intertidal zones around these islands and the margins of the sea would result in the progradation of tidal flats. The facies successions developed in these settings would therefore be cycles displaying a shallowing-up trend, which may be traceable over large areas of the platform.

Carbonate banks and atolls

Isolated platforms in areas of shallow sea surrounded on all sides by deeper water are commonly sites of carbonate sedimentation because there is no source of terrigenous detritus. They are found in a number of different settings ranging from small atolls above extinct volcanoes to horst blocks in extensional basins and within larger areas of shallow seas. All sides are exposed to open seas and the distribution of facies on an isolated platform is controlled by the direction of the prevailing wind. The characteristics of the deposits resemble those of a rimmed shelf and result in similar facies associations. The best developed marginal reef facies occurs on the windward side of the platform, which experiences the highest energy waves. Carbonate sand bodies may also form part of the rim of the platform. The platform interior is a region of low energy where islands of carbonate sand may develop and deposition occurs on tidal flats.

Applications of lithostratigraphy

Lithostratigraphy and geological maps

Lithostratigraphy and geological maps as Part of the definition of a formation is that it should be a ‘mappable unit’, and in practice this usually means that the unit can be represented on a map of a scale of 1:50,000, or 1:100,000. Maps at this scale therefore show the distribution of formations and may also show where members and named beds occur. The stratigraphic order and, where appropriate, lateral relationships between the different lithostratigraphic units are normally shown in a stratigraphic key at the side of the map. In regions of metamorphic, intrusive igneous and highly deformed rocks the mapped units are lithodemes. There are no established rules for the colours used for different lithostratigraphic and lithodemic units on these maps, but each national geological survey usually has its own scheme. Geological maps that cover larger areas, such as a whole country or a continent, are different: they usually show the distribution of rocks in terms of chronostratigraphic units, that is, on the basis of their age, not lithology.

Lithostratigraphy and environments

It is clear from the earlier chapters on the processes and products of sedimentation that the environment of deposition has a fundamental control on the lithological characteristics of a rock unit. A formation, defined by its lithological characteristics, is therefore likely to be composed of strata deposited in a particular sedimentary environment. This has two important consequences for any correlation of formations in any chronostratigraphic (time) framework. First, in any modern environment it is obvious that fluvial sedimentation can be occurring on land at the same time as deposition is happening on a beach, on a shelf and in deeper water. In each environment the characteristics of the sediments will be different and hence they would be considered to be different formations if they are preserved as sedimentary rocks. It inevitably follows that formations have a limited lateral extent, determined by the area of the depositional environment in which they formed and that two or more different formations can be deposited at the same time. Second, depositional environments do not remain fixed in position through time. Consider a coastline where a sandy beach (foreshore) lies between a vegetated coastal plain and a shoreface succession of mudstones coarsening up to sandstones. The foreshore is a spatially restricted depositional environment: it may extend for long distances along a coast, but seawards it passes into the shallow marine, shoreface environment and landwards into continental conditions. The width of deposit produced in a beach and foreshore environment may therefore be only a few tens or hundreds of metres. However, a foreshore deposit will end up covering a much larger area if there is a gradual rise or fall of sea level relative to the land. If sea level slowly rises the shoreline will move landwards and through time the place where sands are being deposited on a beach would have moved several hundreds of metres. These depositional environments (the coastal plain, the sandy foreshore and the shoreface) will each have distinct lithological characteristics that would allow them to be distinguished as mappable formations. The foreshore deposits could therefore constitute a formation, but it is also clear that the beach deposits were formed earlier in one place (at the seaward extent) than another (at the landward extent). The same would be true of formations representing the deposits of the coastal plain and shoreface environments: through time the positions of the depositional environments migrate in space. From this example, it is evident that the body of rock that constitutes a formation would be diachronous and both the upper and lower boundaries of the formation are diachronous surfaces. There is also a relationship between environments of deposition and the hierarchy of lithostratigraphic units. In the case of a desert environment there may be three main types of deposits: aeolian sands, alluvial fan gravels and muddy evaporites deposited in an ephemeral lake. Each type of deposit would have distinctive lithological characteristics that would allow them to be distinguished as three separate formations, but the association of the three could usefully be placed into a group. A distinct change in environment, caused, perhaps, by sealevel rise and marine flooding of the desert area, would lead to a different association of deposits, which in lithostratigraphic terms would form a separate group. Subdivision of the formations formed in this desert environment may be possible if scree deposits around the edge of the basin occur as small patches amongst the other facies. When lithified the scree would form a sedimentary breccia, recognisable as a separate member within the other formations, but not sufficiently widespread to be considered a separate formation.

Lithostratigraphy and correlation

Correlation in stratigraphy is usually concerned with considering rocks in a temporal framework, that is, we want to know the time relationships between different rock units – which ones are older, which are younger and which are the same age. Correlation on the basis of lithostratigraphy alone is difficult because, as discussed in the previous section, lithostratigraphic units are likely to be diachronous. In the example of the lithofacies deposited in a beach environment during a period of rising sea level the lithofacies has different ages in different places. Therefore the upper and lower boundaries of this lithofacies will cross time-lines (imaginary lines drawn across and between bodies of rock which represent a moment in time). If we can draw a time-line across our rock units, or, more usefully, a time-plane through an area of different strata, we would be able to reconstruct the distribution of palaeoenvironments at that time across that area. To carry out this exercise of making a palaeogeographic reconstruction we need to have some means of chronostratigraphic correlation, a means of determining the relative age of rock units which is not dependent on their lithostratigraphic characteristics. Radiometric dating techniques provide an absolute time scale but are not easy to apply because only certain rock types can be usefully dated. Biostratigraphy provides the most widely used time framework, a relative dating technique that can be related to an absolute time scale, but it often lacks the precision required for reconstructing environments and in some depositional settings appropriate fossils may be partly or totally absent (in deserts, for example). Palaeomagnetic reversal stratigraphy provides timelines, events when the Earth’s magnetism changed polarity, and may be applied in certain circumstances. The concept of sequence stratigraphy provides an approach to analysing successions of sedimentary rocks in a temporal framework. 

Lithostratigraphy and time: gaps in the record

One of the most difficult questions to answer in sedimentology and stratigraphy is ‘how long did it take to form that succession of rocks?’. From our observations of sedimentary processes we can sometimes estimate the time taken to deposit a single bed: a debris-flow deposit on an alluvial fan may be formed over a few minutes to hours and a turbidite in deep water may have been accumulated over hours to days. However, we cannot simply add up the time it takes to deposit one bed in a succession and multiply it by the number of beds. We know from records of modern alluvial fans and deep seas that most of the time there is no sediment accumulating and that the time between depositional events is much longer than the duration of each event: in the case of the alluvial fan deposits and turbidites there may be hundreds or thousands of years between events. If we consider a succession of beds in terms of the passage of time, most of the time is represented by the surfaces that separate the beds: for example, if a debris flow event lasting one hour occurs every 100 years the time represented by the surfaces between beds is about a million times longer than the time taken to deposit the conglomerate. This is not a particularly extreme example: in many environments the time periods between events are much longer than events themselves – floods in the overbank areas of rivers and delta tops, storm deposits on shelves, volcanic ash accumulations, and so on. The exceptions are those places where material is gradually accumulating due to biogenic activity, such as a coral reef boundstone. A bedding plane therefore represents a gap in the record, a hiatus in sedimentation, also sometimes referred to as a lacuna (plural lacunae). There are, however, some features that provide us with clues about the relative periods of time represented by the bedding surface. In continental environments, soils form on exposed sediment surfaces and the longer the exposure, the more mature the soil: analysis of palaeosols can therefore provide some clues and we can conclude that a very mature palaeosol profile in a succession would have formed during a long period without sedimentation. In shallow marine environments the sea floor is bioturbated by organisms, and the intensity of the bioturbation on a bedding surface can be used as an indicator of the length of time before the next depositional event. Sediment on the sea floor can also become partly or wholly lithified if left for long enough, and it may be possible to recognise firmgrounds, with associated Glossifungites-type ichnofauna, and hardgrounds with a Trypanites ichnofacies assemblage. Unconformities represent even longer gaps in the depositional record. On continental margins a sealevel fall may expose part of the shelf area, resulting in a period of non-deposition and erosion that will last until the sea level rises again after a period of time lasting tens to hundreds of thousands or millions of years. This results in an unconformity surface within the strata that represents a time period of that order of magnitude. Plate tectonics results in vertical movements of the crust and areas that were once places of sediment accumulation may become uplifted and eroded. Later crustal movements may cause subsidence, and the erosion surface will become preserved as an unconformity as it is overlain by younger sediment. Unconformity surfaces formed in this way may represent anything from less than a million to a billion years or more. The problems of determining how long it takes to deposit a succession of beds and the unknown periods of time represented by any lacunae, from a bedding plane to an unconformity, make it all-but impossible to gauge the passage of time from the physical characteristics of a sedimentary succession. In the 18th and 19th centuries various different estimates of the age of the Earth were made by geologists and these were all wildly different from the 4.5Ga we now know to be the case because they did not have any way of judging the period of time represented by the rocks in the stratigraphic record. Radiometric dating now provides us with a time frame that we can measure in years. This has made it possible to calibrate the stratigraphic chart that had already been developed for the Phanerozoic based on the occurrences of fossils.

Description of Lithostratigraphy

In lithostratigraphy rock units are considered in terms of the lithological characteristics of the strata and their relative stratigraphic positions. The relative stratigraphic positions of rock units can be determined by considering geometric and physical relationships that indicate which beds are older and which ones are younger. The units can be classified into a hierarchical system of members, formations and groups that provide a basis for categorising and describing rocks in lithostratigraphic terms.

Stratigraphic relationships

Superposition

Provided the rocks are the right way up the beds higher in the stratigraphic sequence of deposits will be younger than the lower beds. This rule can be simply applied to a layer-cake stratigraphy but must be applied with care in circumstances where there is a significant depositional topography (e.g. fore-reef deposits may be lower than reef-crest rocks).

Unconformities

An unconformity is a break in sedimentation and where there is erosion of the underlying strata this provides a clear relationship in which the beds below the unconformity are clearly older than those above it. All rocks which lie above the unconformity, or a surface that can be correlated with it, must be younger than those below. In cases where strata have been deformed and partly eroded prior to deposition of the younger beds, an angular unconformity is formed. A disconformity marks a break in sedimentation and some erosion, but without any deformation of the underlying strata.

Cross-cutting relationships

Any unit that has boundaries that cut across other strata must be younger than the rocks it cuts. This is most commonly seen with intrusive bodies such as batholiths on a larger scale and dykes on a smaller scale. This relationship is also seen in fissure fills, sedimentary dykes that form by younger sediments filling a crack or chasm in older rocks.

Included fragments

The fragments in a clastic rock must be made up of a rock that is older than the strata in which they are found. The same relationship holds true for igneous rocks that contain pieces of the surrounding country rock as xenoliths (literally 'foreign rocks'). This relationship can be useful in determining the age relationship between rock units that are some distance apart. Pebbles of a characteristic lithology can provide conclusive evidence that the source rock type was being eroded by the time a later unit was being deposited tens or hundreds of kilometres away.

Way-up indicators in sedimentary rocks

The folding and faulting of strata during mountain building can rotate whole successions of beds (formed as horizontal or nearly horizontal layers) through any angle, resulting in beds that may be vertical or completely overturned. In any analysis of deformed strata, it is essential to know the direction of younging, that is, the direction through the layers towards younger rocks. The direction of younging can be determined by small-scale features that indicate the way-up of the beds or by using other stratigraphic techniques to determine the order of formation.

Lithostratigraphic units

There is a hierarchical framework of terms used for lithostratigraphic units, and from largest to smallest these are: 'Supergroup', 'Group', 'Formation', 'Member' and 'Bed'. The basic unit of lithostratigraphic division of rocks is the formation, which is a body of material that can be identified by its lithological characteristics and by its stratigraphic position. It must be traceable laterally, that is, it must be mappable at the surface or in the subsurface. A formation should have some degree of lithological homogeneity and its defining characteristics may include mineralogical composition, texture, primary sedimentary structures and fossil content in addition to the lithological composition. Note that the material does not necessarily have to be lithified and that all the discussion of terminology and stratigraphic relationships applies equally to unconsolidated sediment. A formation is not defined in terms of its age either by isotopic dating or in terms of biostratigraphy. Information about the fossil content of a mapping unit is useful in the description of a formation but the detailed taxonomy of the fossils that may define the relative age in biostratigraphic terms does not form part of the definition of a lithostratigraphic unit. A formation may be, and often is, a diachronous unit, that is, a deposit with the same lithological properties that was formed at different times in different places. A formation may be divided into smaller units in order to provide more detail of the distribution of lithologies. The term member is used for rock units that have limited lateral extent and are consistently related to a particular formation (or, rarely, more than one formation). An example would be a formation composed mainly of sandstone but which included beds of conglomerate in some parts of the area of outcrop. A number of members may be defined within a formation (or none at all) and the formation does not have to be completely subdivided in this way: some parts of a formation may not have a member status. Individual beds or sets of beds may be named if they are very distinctive by virtue of their lithology or fossil content. These beds may have economic significance or be useful in correlation because of their easily recognisable characteristics across an area. Where two or more formations are found associated with each other and share certain characteristics they are considered to form a group. Groups are commonly bound by unconformities which can be traced basin-wide. Unconformities that can be identified as major divisions in the stratigraphy over the area of a continent are sometimes considered to be the bounding surfaces of associations of two or more groups known as a supergroup.

Description of lithostratigraphic units

The formation is the fundamental lithostratigraphic unit and it is usual to follow a certain procedure in geological literature when describing a formation to ensure that most of the following issues are considered. Members and groups are usually described in a similar way.

Lithology and characteristics

The field characteristics of the rock, for example, an oolitic grainstone, interbedded coarse siltstone and claystone, a basaltic lithic tuff, and so on form the first part of the description. Although a formation will normally consist mainly of one lithology, combinations of two or more lithologies will often constitute a formation as interbedded or interfingering units. Sedimentary structures (ripple cross-laminations, normal grading, etc.), petrography (often determined from thin-section analysis) and fossil content (both body and trace fossils) should also be noted.

Definition of top and base

These are the criteria that are used to distinguish beds of this unit from those of underlying and overlying units; this is most commonly a change in lithology from, say, calcareous mudstone to coral boundstone. Where the boundary is not a sharp change from one formation to another, but is gradational, an arbitrary boundary must be placed within the transition. As an example, if the lower formation consists of mainly mudstone with thin sandstone beds, and the upper is mainly sandstone with subordinate mudstone, the boundary may be placed at the point where sandstone first makes up more than 50% of beds. A common convention is for only the base of a unit to be defined at the type section: the top is taken as the defined position of the base of the overlying unit. This convention is used because at another location there may be beds at the top of the lower unit that are not present at the type locality: these can be simply added to the top without a need for redefining the formation boundaries.

Type section

A type section is the location where the lithological characteristics are clear and, if possible, where the lower and upper boundaries of the formation can be seen. Sometimes it is necessary for a type section to be composite within a type area, with different sections described from different parts of the area. The type section will normally be presented as a graphic sedimentary log and this will form the strato type. It must be precisely located (grid reference and/or GPS location) to make it possible for any other geologist to visit the type section and see the boundaries and the lithological characteristics described.

Thickness and extent

The thickness is measured in the type section, but variations in the thickness seen at other localities are also noted. The limits of the geographical area over which the unit is recognised should also be determined. There are no formal upper or lower limits to thickness and extent of rock units defined as a formation (or a member or group). The variability of rock types within an area will be the main constraint on the number and thickness of lithostratigraphic units that can be described and defined. Quality and quantity of exposure also play a role, as finer subdivision is possible in areas of good exposure.

Other information

Where the age for the formation can be determined by fossil content, radiometric dating or relationships with other rock units this may be included, but note that this does not form part of the definition of the formation. A formation would not be defined as, for example, 'rocks of Burdigalian age', because an interpretation of the fossil content or isotopic dating information is required to determine the age. Information about the facies and interpretation of the environment of deposition might be included but a formation should not be defined in terms of depositional environment, for example, 'lagoonal deposits', as this is an interpretation of the lithological characteristics. It is also useful to comment on the terminology and definitions used by previous workers and how they differ from the usage proposed.

Lithostratigraphic nomenclature

It helps to avoid confusion if the definition and naming of stratigraphic units follows a set of rules. Formal codes have been set out in publications such as the 'North American Stratigraphic Code' (North American Commission on Stratigraphic Nomenclature 1983) and the 'International Stratigraphic Guide'. A useful summary of stratigraphic methods, which is rather more user-friendly than the formal documents, is a handbook called 'Stratigraphical Procedure'. The name of the formation, group or member must be taken from a distinct and permanent geographical feature as close as possible to the type section. The lithology is often added to give a complete name such as the Kingston Limestone Formation, but it is not essential, or necessarily desirable if the lithological characteristics are varied. The choice of geographical name should be a feature or place marked on topographic maps such as a river, hill, town or village. The rules for naming members, groups and super groups are essentially the same as for formations, but note that it is not permissible to use a name that is already in use or to use the same name for two different ranks of lithostratigraphic unit. There are some exceptions to these rules of nomenclature that result from historical precedents, and it is less confusing to leave a well established name as it is rather than to dogmatically revise it. Revisions to stratigraphic nomenclature may become necessary when more detailed work is carried out or more information becomes available. New work in an area may allow a formation to be subdivided and the formation may then be elevated to the rank of group and members may become formations in their own right. For the sake of consistency the geographical name is retained when the rank of the unit is changed.

Lithodemic units: non-stratiform rock units

The concepts of division into stratigraphic units were developed for rock bodies that are stratiform, layered units, but many metamorphic, igneous plutonic and structurally deformed rocks are not stratiform and they do not follow the rules of superposition. Nonstratiform bodies of rock are called lithodemic units. The basic unit is the lithodeme and this is equivalent in rank to a formation and is also defined on lithological criteria. The word 'lithodeme' is itself rarely used in the name: the body of rock is normally referred to by its geographical name and lithology, such as the White River Granite or Black Hill Schist. An association of lithodemes that share lithological properties, such as a similar metamorphic grade, is referred to as a suite: the term complex is also used as the equivalent to a group for volcanic or tectonically deformed rocks.

Geophysical and geological logging

There is a wide range of instruments, geophysical logging tools, that are lowered down a borehole to record the physical and chemical properties of the rocks. These instruments are mounted on a device called a sonde that is lowered down the drill hole (on a wireline) once the drill string has been removed. Data from these instruments are recorded at the surface as the sonde passes up through the formations. An alternative technique is to fix a sonde mounted with logging instruments behind the drill bit and record data as drilling proceeds. The tools can be broadly divided into those that are concerned with the petrophysics of the formations, that is, the physical properties of the rocks and the fluids that they contain, and geological tools that provide sedimentological information. The interpretation of all the data is usually referred to as formation evaluation – the determination of the nature and properties of formations in the subsurface. Many of these tools are now used in combinations and provide an integrated output that indicates parameters such as sand:mud ratio, porosity, permeability and hydrocarbon saturation.

Petrophysical logging tools

Caliper log

The width of the borehole is initially determined by the size of the drill bit used, but it can vary depending on the nature of the lithology and the permeability of the formation. The borehole wall may cave in where there are less indurated lithologies such as mudrocks, and this can be seen as an anomalously wide interval of the hole. The caliper log can also detect parts of the borehole where the diameter is reduced by the accumulation of a mud cake on the inside: mud cakes are made up of the solid suspension in the drilling mud and form where there is a porous and permeable bed that allows the drilling fluid to penetrate, leaving the mud filtered out on the borehole wall.

Gamma-ray log

This records the natural gamma radioactivity in the rocks that comes from the decay of isotopes of potassium, uranium and thorium. The main use of this tool is to distinguish between mudrocks, which generally have a high potassium content and hence high natural radioactivity, and sandstone and limestone, both of which normally have a lower natural radioactivity. The gamma-ray log is often used to determine the ‘sand: shale ratio’ in a clastic succession (note that for petrophysical purposes, all mudrocks are called 'shales'). However, it should be noted that mica, feldspar, glauconite and some heavy minerals are also radioactive, and sandstones rich in any of these cannot always be distinguished from mudstones using this tool. Organic-rich rocks can also be detected with this tool because uranium is often naturally associated with organic matter. Mudrocks with high organic contents are sometimes referred to as 'hot shales' because of their high natural radioactivity. The spectral gamma-ray log records the radioactivity due to potassium, thorium and uranium separately, allowing the signal due to clay minerals to be separated from radioactivity associated with organic matter.

Resistivity logs

Resistivity logging tools are a range of instruments that are used to measure the electrical conductivity of the rocks and their pore fluids by passing an electrical current from one part of the sonde, through the rocks of the borehole wall measuring the current at another part of the sonde. Most minerals are poor conductors, with the exception of clay minerals that have charged ions in their structures. The resistivity measurements provide information about the composition of the pore fluids because hydrocarbons and fresh water are poor electrical conductors but saline groundwater is a good conductor of electricity. Resistivity logging tools are usually configured so that they are able to measure the resistivity at different distances into the formation away from the borehole wall. A microresistivity tool records the properties at the borehole wall, a ‘shallow’ log measures a short distance into the formation and a 'deep' log records the current that has passed through the formation well away from the borehole (these are sometimes called laterologs). Comparison of readings at different distances from the borehole wall can provide an indication of how far the drilling mud has penetrated into the formation and this gives a measure of the formation permeability. Induction logs are resistivity tools that indirectly generate and measure the electrical properties by the process of induction of a current.

Sonic log

The velocity of sound waves in the formation is determined by using a tool that comprises a pulsing sound source and receiver microphone that records how long it has taken for the sound to pass through the rock near the borehole. The sonic velocity is dependent upon two factors. First, lithologies composed of high-density material transmit sound faster than low-density rocks: for example, coal is a low-density material, basalt is high-density, and sandstones and limestones have intermediate densities. Second, if the rock is porous, the bulk density of the formation will be reduced, and hence the sonic velocity, so if the lithology is known, the porosity can be calculated, or vice versa. The velocities determined by this tool can be used for depth conversion of seismic reflection profiles.

Density logs

These tools operate by emitting gamma radiation and detecting the proportion of the radiation that returns to detectors on the tool. The amount of radiation returned is proportional to the electron density of the material bombarded and this is in turn proportional to the overall density of the formation. If the lithology is known, the porosity can be calculated as density decreases with increased porosity. The application of this tool is therefore very similar to that of the sonic logging tool.

Neutron logs

In this instance the tool has a source that emits neutrons and a detector that measures the energy of returning neutrons. Neutrons lose energy by colliding with a particle of similar mass, a hydrogen nucleus, so this logging tool effectively measures the hydrogen concentration of the formation. Hydrogen is mostly present in the pore spaces in the rock filled by formation fluids, oil or water (which have approximately the same hydrogen ion concentration) so the neutron log provides a measure of the porosity of the formation. However, clay minerals contain hydrogen ions as part of the mineral structure, so this tool does not provide a reliable indicator of the porosity in mudrocks or muddy sandstones or limestones.

Electromagnetic propagation log

The dielectric properties of the formation fluids are measured with this tool. It consists of microwave transmitters that propagate a pulse of electromagnetic energy through the formation and measures the attenuation of the wave with receivers. The measurements are related to the dielectric constant of the formation, which is in turn determined by the amount of water present. The tool therefore can be used to distinguish between oil and water in porous formations.

Nuclear magnetic resonance logs

Conventional porosity determination techniques do not provide information about the size of the pore spaces or how easily the fluid can be removed from those pores. Fluids that are bound to the surface of grains by capillary action cannot easily be removed and are therefore not producible fluids, and if pore spaces are small more fluid will be bound into the formation. The nuclear magnetic resonance (NMR) tool works by producing a strong magnetic field that polarises hydrogen nuclei in water and hydrocarbons. When the field is switched off the hydrogen nuclei relax to their previous state, but the rate at which they do so, the relaxation time, increases if they interact with grain surfaces. Measurement of the electromagnetic 'echo' produced during the relaxation period can thus be used as a measure of how much of the fluid is 'free' and how much of it is close to, and bound on to, grain surfaces. The tool operates by producing a pulsed magnetic field and measuring the echo many times a second.

Geological logging tools

Dipmeter log

The sonde for this tool has four or six separate devices for measuring the resistivity at the borehole wall. They are arranged around the sonde so that if there is a difference in the resistivity on different sides of the borehole, this will be detected. If the layering in the formations is inclined due to a tectonic tilt or crossstratification it is possible to detect the degree and direction of the tilt by comparing the readings of the different, horizontal resistivity devices. Hence this tool has the potential to measure the sedimentary or tectonic dip of layering.

Microimaging tools

These tools, often called borehole scanners, are also resistivity devices and use a large number of small receiving devices to provide an image of the resistivity of the whole borehole wall. If there are fine-scale contrasts in electrical properties, for instance where there are fine alternations of clay and sand, it is possible to image sedimentary structures as well as fractures in the rock. The images generated superficially resemble a photograph of the borehole wall, but is in fact a ‘map’ of variations in the resistivity.

Ultrasonic imaging logs

High-resolution measurements of the acoustic properties of the formations in the borehole walls are made by a rotating transmitter that emits an ultrasonic pulse and then records the reflected pulse with a receiver. The main use of this tool is to detect how uneven the borehole wall is, and this can be related to both lithology and the presence of fractures.

Borehole stratigraphy and sedimentology

The interpretation of seismic reflection profiles provides a model for the stratigraphic and structural relationships that may exist in the subsurface. Data from these sources can provide some indicators of the lithologies in the subsurface, but a full geological picture can be obtained only by the addition of information on lithology and facies. This can be provided by drilling boreholes through the succession and either taking samples of the rocks and/or using geophysical tools to take detailed measurements of the rock properties. When a borehole is drilled there are a number of ways of collecting information from the subsurface, and these are briefly described below.

Borehole cuttings

In the course of drilling a deep borehole, a fluid is pumped down to the drill bit to lubricate it, remove the rock that has been cut (cuttings) and to counteract formation fluid pressures in the subsurface. Due to the weight of rocks above, fluids (water, oil and gas) trapped in porous and permeable strata will be under pressure, and without something to counteract that pressure they would rush to the surface up the borehole. The drilling fluid is therefore usually a ‘mud’, made up of a mixture of water or oil and powdered material, which gives the fluid a higher density: powdered barite (BaSO4) is often used because this mineral has a density of 4.48. The density of the drilling mud is varied to balance the pressure in the formations in the subsurface. The drilling mud is recirculated by being pumped down the inside of the drill string (pipe) and returning up the outside: because it is a dense, viscous fluid, it will bring the cuttings with it as it reaches the surface. The cuttings are filtered from the mud with a sieve and washed to provide a record of the strata that have been drilled. These cuttings are typically 1–5mm in diameter and are sieved out of the drilling mud at the surface. Recording the lithology of these drill chips (mud-logging) provides information about the rock types of the strata that have been penetrated by the borehole, but details such as sedimentary structures are not preserved. Microfossils such as foraminifera, nanofossils and palynomorphs can be recovered from cuttings and used in biostratigraphic analysis. There is usually a degree of mixing of material from different layers as the fluid returns up the borehole, so it is the depth at which a lithology or fossil first appears that is most significant.

Core

A drill bit can be designed such that it cuts an annulus of rock away leaving a cylinder in the centre, a core, that can be brought up to the surface. Where coring is being carried out the drilling is halted and the section of core is brought up to the surface in a sleeve inside the hollow drill string. As each section of core is brought to the surface it is placed in a box, which is labelled to show the depth interval it was recovered from. Recovery is often incomplete, with only part of the succession drilled preserved, and the core may be broken up during drilling. The core is then usually cut vertically to provide a smooth-surfaced slab of rock that is typically 90 mm to 150 mm across, depending on the width of the borehole being drilled. Cores cut in this way provide a considerable amount of detail of the lithologies present, the small-scale sedimentary structures, body and trace fossils. In exploration for oil and gas and in the development of fields for hydrocarbon production, cores are cut through ‘target horizons’, that is, parts of the succession that have been identified from the interpretation of seismic interpretation as likely source rocks, or, more importantly, reservoir bodies. Core is usually only cut and recovered through these parts of the stratigraphy: the rest of the succession has to be interpreted on the basis of geophysical wireline logs. However, continuous cores may be cut through successions that cannot be interpreted satisfactorily using geophysical information alone, as can occur when the properties of the rock units do not allow differentiation between different lithologies using wireline logging tools. In contrast to oil and gas exploration, coal and mineral exploration normally involves taking a complete core through the section drilled. The width of the core that is cut is smaller, often just 40mm, and the core is not split vertically. The small size and the curved surface of the core may make it more difficult to recognise sedimentary structures than in the conventional, larger, split core used in oil and gas exploration, but the continuous core provides good vertical coverage of the drilled succession.

Core logging

The procedure for recording the details of the sedimentary rocks in a core is very similar to making a graphic sedimentary log of a succession exposed in the field. Core logging sheets are similar in format to field logging sheets, and the same types of information are recorded (lithology, bed thickness, bed boundaries, sedimentary structures, biogenic structures, and so on). The scale is usually 1:20 or 1:50. In some ways recording information about strata from core is easier than field description. If the core recovery is good then there will be an almost complete record of the succession, including the finer grained lithologies. Weathering of mudrocks in the field usually means that they are less well preserved than the coarser beds, but in core this tends to be less of a problem, although weaker, finer grained beds will often break up more during the drilling. The main limitations are those imposed by the width of the core. It is not possible to see the lateral geometry of the beds and recognise features such as channels easily, and only parts of larger scale sedimentary structures are preserved. On the other hand, the details of ripple-scale features may be more easily seen on the smooth, cut surface of a core. Palaeocurrent data can be recorded from sedimentary structures only if the orientation of the core has been recorded during the drilling process, and this is not always possible. The other, not insignificant, difference between core and outcrop is that the geologist can carry out the recording of data in the relative comfort of a core store, although it is unlikely to be such an interesting environment to work in as a field location in an exotic place. Not all cores pass through the strata at right angles to the bedding. If the strata are tilted then a vertical drill core will cut through the beds at an angle, so all bed boundaries and sedimentary structures observed in the core will be inclined. During the development phase of oil and gas extraction, drilling is often directed along pathways (directional drilling) that can be at any angle, including horizontal. Interpretation of inclined and near-horizontal cores therefore requires information about the angle of the well.

Taxa used in biostratigraphy

No single group of organisms fulfils all the criteria for the ideal zone fossil and a number of different groups of taxa have been used for defining biozones through the stratigraphic record. Some, such as the graptolites in the Ordovician and Silurian, are used for worldwide correlation; others are restricted in use to certain facies in a particular succession, for example corals in the Carboniferous of northwest Europe. Some examples of taxonomic groups used in biostratigraphy are outlined below.

Marine macrofossils

The hard parts of invertebrates are common in sedimentary rocks deposited in marine environments throughout the Phanerozoic. These fossils formed the basis for the divisions of the stratigraphic column into Systems, Series and Stages in the 18th and 19th centuries. The fossils of organisms such as molluscs, arthropods, echinoderms, etc., are relatively easy to identify in hand specimen, and provide the field geologist with a means for establishing the age of rocks to the right period or possibly epoch. Expert palaeontological analysis of marine macrofossils provides a division of the rocks into stages based on these fossils.

Trilobites

These Palaeozoic arthropods are the main group used in the zonation of the Cambrian. Most trilobites are thought to have been benthic forms living on and in the sediment of shallow marine waters. They show a wide variety of morphologies and appear to have evolved quite rapidly into taxa with distinct and recognisable characteristics. They are only locally abundant as fossils.

Graptolites

These exotic and somewhat enigmatic organisms are interpreted as being colonial groups of individuals connected by a skeletal structure. They appear to have had a planktonic habit and are widespread in Ordovician and Silurian mudrocks. Preservation is normally as a thin film of flattened organic material on the bedding planes of fine-grained sedimentary rocks. The shapes of the skeletons and the ‘teeth’ where individuals in the colony were located are distinctive when examined with a hand lens or under a microscope. Lineages have been traced which indicate rapid evolution and have allowed a high-resolution biostratigraphy to be developed for the Ordovician and Silurian systems. The main drawback in the use of graptolites is the poor preservation in coarser grained rocks such as sandstones.

Brachiopods

Shelly, sessile organisms such as brachiopods generally make poor zone fossils but in shallow marine, high-energy environments where graptolites were not preserved, brachiopods are used for regional correlation purposes in Silurian rocks and in later Palaeozoic strata. 

Ammonoids 

This taxonomic group of cephalopods (phylum Mollusca) includes goniatites from Palaeozoic rocks as well as the more familiar ammonites of the Mesozoic. The nautiloids are the most closely related living group. The large size and free-swimming habit of these cephalopods made them an excellent group for biostratigraphic purposes. Fossils are widespread, found in many fully marine environments, and they are relatively robust. Morphological changes through time were to the external shape of the organisms and to the ‘suture line’, the relic of the bounding walls between the chambers of the coiled cephalopod. Goniatites have been used in correlation of Devonian and Carboniferous rocks, whereas ammonites and other ammonoids are the main zone fossils in Mesozoic rocks. Ammonoids became extinct at the end of the Cretaceous.

Gastropods

These also belong to the Mollusca and as marine ‘snails’ they are abundant as fossils in Cenozoic rocks. They are very common in the deposits of almost all shallow marine environments. Distinctive shapes and ornamentation on the calcareous shells make identification relatively straightforward and there are a wide variety of taxa within this group.

Echinoderms

This phylum includes crinoids (sea lilies) and echinoids (sea urchins). Most crinoids probably lived attached to substrate and this sessile characteristic makes them rather poor zone fossils, despite their abundance in some Palaeozoic limestones. Echinoids are benthic, living on or in soft sediment: their relatively robust form and subtle but distinctive changes in their morphology have made them useful for regional and worldwide correlation in parts of the Cretaceous.

Corals

The extensive outcrops of shallow marine limestones in Devonian and Lower Carboniferous (Mississippian) rocks in some parts of the world contain abundant corals. This group is therefore used for zonation and correlation within these strata, despite the fact that they are not generally suitable for biostratigraphic purposes because of the very restricted depositional environments they occur in.

Marine microfossils

Microfossils are taxa that leave fossil remains that are too small to be clearly seen with the naked eye or hand lens. They are normally examined using an optical microscope although some forms can be analysed in detail only using a scanning electron microscope. The three main groups that are used in biostratigraphy are the foraminifers, radiolaria and calcareous algae (nanofossils): other microfossils used in biostratigraphy are ostracods, diatoms and conodonts.

Foraminifera

'Forams' (the common abbreviation of foraminifers) are single-celled marine organisms that belong to the Protozoa Subkingdom. They have been found as fossils in strata as old as the Cambrian, although forms with hard calcareous shells, or ‘tests’, did not become well established until the Devonian. Calcareous forams generally became more abundant through the Phanerozoic and are abundant in many Mesozoic and Cenozoic marine strata. The calcareous tests of planktonic forams are typically a millimetre or less across, although during some periods, particularly the Paleogene, larger benthic forms also occur and can be more than a centimetre in diameter. Planktonic forams make very good zone fossils as they are abundant, widespread in marine strata and appear to have evolved rapidly. Schemes using forams for correlation in the Mesozoic and Cenozoic are widely used in the hydrocarbon industry because microfossils are readily recovered from boreholes and both regional and worldwide zonation schemes are used.

Radiolaria

These organisms form a subclass of planktonic protozoans and are found as fossils in deep marine strata throughout the Phanerozoic. Radiolaria commonly have silica skeletons and are roughly spherical, often spiny organisms less than a millimetre across. They are important in the dating of deep-marine deposits because the skeletons survive in siliceous oozes deposited at depths below the CCD. These deposits are preserved in the stratigraphic record as radiolarian cherts and the fossil assemblages found in them typically contain large numbers of taxa making it possible to use quite high resolution biozonation schemes. Their stratigraphic range is also greater than the forams, making them important for the dating of Palaeozoic strata.

Calcareous nanofossils

Fossils that cannot be seen with the naked eye and are only just discernible using a high-power optical microscope are referred to as nanofossils. They are microns to tens of microns across and are best examined using a scanning electron microscope. The most common nanofossils are coccoliths, the spherical calcareous cysts of marine algae. Coccoliths may occur in huge quantities in some sediments and are the main constituent of some fine-grained limestones such as the Chalk of the Upper Cretaceous in northwest Europe. They are found in fine-grained marine sediments deposited on the shelf or any depths above the CCD below which they are not normally preserved. They are used biostratigraphically in Mesozoic and Cenozoic strata.

Other microfossils

Ostracods are crustaceans with a two-valve calcareous carapace and their closest relatives are crabs and lobsters. They occur in a very wide range of depositional environments, both freshwater and marine, and they have a long history, although their abundance and distribution are sporadic. Zonation using ostracods is applied only locally in both marine and non-marine environments. Diatoms are chrysophyte algae with a siliceous frustule (skeleton) that can occur in large quantities in both shallow-marine and freshwater settings. The diatom frustules are less than a millimetre across and in some lacustrine settings may make up most of the sediment, forming a diatomite deposit. They are only rarely used in biostratigraphy. Conodonts are somewhat enigmatic tooth like structures made of phosphate and they occur in Palaeozoic strata. Despite uncertainty about the origins, they are useful stratigraphic microfossils in the older Phanerozoic rocks, which generally contain few other microfossils. Acritarchs are microscopic spiny structures made of organic material that occur in Proterozoic and Palaeozoic rocks. Their occurrences in Precambrian strata make them useful as a biostratigraphic tool in rocks of this age. They are of uncertain affinity, although are probably the cysts of planktonic algae, and may therefore be related to dinoflagellates, which are primitive organisms found from the Phanerozoic through to the present day and also produce microscopic cysts (dinocysts). Zonation based on dinoflagellates is locally very important, especially in non-calcareous strata of Mesozoic and Cenozoic ages: the schemes used are generally geographically local and have limited stratigraphic ranges.

Terrestrial fossil groups used in biostratigraphy

Correlation in the deposits of continental environments is always more difficult because of the poorer preservation potential of most materials in a subaerial setting. Only the most resistant materials survive to be fossilised in most continental deposits, and these include the organo-phosphates that vertebrate teeth are made of and the coatings of pollen, spores and seeds of plants. Stratigraphic schemes have been set up using the teeth of small mammals and reptiles for correlation of continental deposits of Neogene age. Pollen, spores and seeds (collectively palynomorphs) are much more commonly used. They are made up of organic material that is highly resistant to chemical attack and can be dissolved out of siliceous sedimentary rocks using hydrofluoric acid. Airborne particles such as pollen, spores and some seeds may be widely dispersed and the occurrence of these aeolian palynomorphs within marine strata allows for correlation between marine and continental successions. However, although palynomorphs can be used as zone fossils, they rarely provide such a high resolution as marine fossils. Identification is carried out with an optical microscope or an electron microscope after the palynomorphs have been chemically separated from the host sediment using strong acids.

Biozone and zone fossil in biostratigraphy

A biostratigraphic unit is a body of rock defined by its fossil content. It is therefore fundamentally different from a lithostratigraphic unit that is defined by the lithological properties of the rock. The fundamental unit of biostratigraphy is the biozone. Biozones are units of stratigraphy that are defined by the zone fossils (usually species or subspecies) that they contain. In theory they are independent of lithology, although environmental factors often have to be taken into consideration in the definition and interpretation of biozones. In the same way that formations in lithostratigraphy must be defined from a type section, there must also be a type section designated as a stratotype and described for each biozone. They are named from the characteristic or common taxon (or occasionally taxa) that defines the biozone. There are several different ways in which biozones can be designated in terms of the zone fossils that they contain.

Interval biozones These are defined by the occurrences within a succession of one or two taxa. Where the first appearance and the disappearance of a single taxon is used as the definition, this is referred to as a taxon-range biozone. A second type is a concurrent range biozone, which uses two taxa with overlapping ranges, with the base defined by the appearance of one taxon and the top by the disappearance of the second one. A third possibility is a partial range biozone, which is based on two taxa that do not have overlapping ranges: once again, the base is defined by the appearance of one taxon and the top by the disappearance of a second. Where a taxon can be recognised as having followed another and preceding a third as part of a phyletic lineage the biozone defined by this taxon is called a lineage biozone (also called a consecutive range biozone).

Assemblage biozones In this case the biozone is defined by at least three different taxa that may or may not be related. The presence and absence, appearance and disappearance of these taxa are all used to define a stratigraphic interval. Assemblage biozones are used in instances where there are no suitable taxa to define interval biozones and they may represent shorter time periods than those based on one or two taxa. 

Acme biozones The abundance of a particular taxon may vary through time, in which case an interval containing a statistically high proportion of this taxon may be used to define a biozone. This approach can be unreliable because the relative abundance is due to local environmental factors. The ideal zone fossil would be an organism that lived in all depositional environments all overthe world and was abundant; it would have easily preserved hard parts and would be part of an evolutionary lineage that frequently developed new, distinct species. Not surprisingly, no such fossil taxon has ever existed and the choice of fossils used in biostratigraphy has been determined by a number of factors that are considered in the following sections.

Rate of speciation

The frequency with which new species evolve and replace former species in the same lineage determines the resolution that can be applied in biostratigraphy. Some organisms seem to have hardly evolved at all: the brachiopod Lingula seems to look exactly the same today as the fossils found in Lower Palaeozoic rocks and hence is of little biostratigraphic value. The groups that appear to display the highest rates of speciation are vertebrates, with mammals, reptiles and fish developing new species every 1 to 3 million years on average. However, the stratigraphic record of vertebrates is poor compared with marine molluscs, which are much more abundant as fossils, but have slower average speciation rates (around 10 million years). There are some groups that appear to have developed new forms regularly and at frequent intervals: new species of ammonites appear to have evolved every million years or so during the Jurassic and Cretaceous and in parts of the Cambrian some trilobite lineages appear to have developed new species at intervals of about a million years. By using more than one species to define them, biozones can commonly be established for time periods of about a million years, with higher resolution possible in certain parts of the stratigraphic record, especially in younger strata.

Depositional environment controls

The conditions vary so much between different depositional environments that no single species, genus or family can be expected to live in all of them. The adaptations required to live in a desert compared with a swamp, or a sandy coastline compared with a deep ocean, demand that the organisms that live in these environments are different. There is a strong environmental control on the distribution of taxa today and it is reasonable to assume that the nature of the environment strongly influenced the distribution of fossil groups as well. Some environments are more favourable to the preservation of body fossils than others: for example, preservation potential is lower on a high-energy beach than in a low-energy lagoon. There is a fundamental problem with correlation between continental and marine environments because very few animals or plants are found in both settings. In the marine environment the most widespread organisms are those that are planktonic (free floating) or animals that are nektonic (free-swimming lifestyle). Those that live on the sea bed, the benthonic or benthic creatures and plants, are normally found only in a certain water depth range and are hence not quite so useful. The rates of sedimentation in different depositional environments are also a factor in the preservation and distribution of stratigraphically useful fossils. Slow sedimentation rates commonly result in poor preservation because the remains of organism are left exposed on the land surface or sea floor where they are subject to biogenic degradation. On the other hand, with a slower rate of accumulation in a setting where organic material has a higher chance of preservation (e.g. in an anoxic environment), the higher concentration of fossils resulting from the reduced sediment supply can make the collecting of biostratigraphically useful material easier. It is also more likely that a first or last appearance datum will be identifiable in a single outcrop section because if sediment accumulation rates are high, hundreds of metres of strata may lie within a single biozone.

Mobility of organisms

The lifestyle of an organism not only determines its distribution in depositional environments, it also affects the rate at which an organism migrates from one area to another. If a new species evolves in one geographical location its value as a zone fossil in a regional or worldwide sense will depend on how quickly it migrates to occupy ecological niches elsewhere. Again, planktonic and nektonic organisms tend to be most useful in biostratigraphy because they move around relatively quickly. Some benthic organisms have a larval stage that is free-swimming and may therefore be spread around oceans relatively quickly. Organisms that do not move much (a sessile lifestyle) generally make poor fossils for biostratigraphic purposes.

Geographical distribution of organisms

Two environments may be almost identical in terms of physical conditions but if they are on opposite sides of the world they may be inhabited by quite different sets of animals and plants. The contrasts are greatest in continental environments where geographical isolation of communities due to tectonic plate movements has resulted in quite different families and orders. The mammal fauna of Australia are a striking example of geographical isolation resulting in the evolution of a group of animals that are quite distinct from animals living in similar environments in Europe or Asia. This geographical isolation of groups of organisms is called provincialism and it also occurs in marine organisms, particularly benthic forms, which cannot easily travel across oceans. Present or past oceans have been sufficiently separate to develop localised communities even though the depositional environments may have been similar. This faunal provincialism makes it necessary to develop different biostratigraphic schemes in different parts of the world.

Abundance and size of fossils

To be useful as a zone fossil a species must be sufficiently abundant to be found readily in sedimentary rocks. It must be possible for the geologist to be able to find representatives of the appropriate taxon without having to spend an undue amount of time looking. There is also a play-off between size and abundance. In general, smaller organisms are more numerous and hence the fossils of small organisms tend to be the most abundant. The problem with very small fossils is that they may be difficult to find and identify. The need for biostratigraphic schemes to be applicable to subsurface data from boreholes has led to an increased use of microfossils, fossils that are too small to be recognised in hand specimen, but which may be abundant and readily identified under the microscope (or electron microscope in some cases). Schemes based on microfossils have been developed in parallel to macrofossil schemes. Although a scheme based on ammonites may work very well in the field, the chances of finding a whole ammonite in the core of a borehole are remote. Microfossils are the only viable material for use in biostratigraphy where drilling does not recover core but only brings up pieces of the lithologies in the drilling mud.

Preservation potential

It is impossible to determine how many species or individuals have lived on Earth through geological time because very few are ever preserved as fossils. The fossil record represents a very small fraction of the biological history of the planet for a variety of reasons. First, some organisms do not possess the hard parts that can survive burial in sediments: we therefore have no idea how many types of worm may have existed in the past. Sites where there is exceptional preservation of the soft parts of fossils (lagerstatten) provide tantalising clues to the diversity of lifeforms that we know next to nothing about. Second, the depositional environment may not be favourable to the preservation of remains: only the most resistant pieces of bone survive in the dry, oxidising setting of deserts and almost all other material is destroyed. All organisms are part of a food chain and this means that their bodies are normally consumed, either by a predator or a scavenger. Preservation is therefore the exception for most animals and plants. Finally, the stratigraphic record is very incomplete, with only a fraction of the environmental niches that have existed preserved in sedimentary rocks. The low preservation potential severely limits the material available for biostratigraphic purposes, restricting it to those taxa that had hard parts and existed in appropriate depositional environments.