Microfossils in stratigraphy

The stratigraphical column

The succession of rocks exposed at the surface of the Earth can be arranged into a stratigraphical column, with the oldest rocks at the base and the youngest ones at the top (Fig. 3.1). Although the absolute ages have been determined from studies of radioactive isotopes, it is customary to use the names of stratigraphical units, mostly distinguished on the basis of differences in their included fossils. These units are arranged into a number of hierarchies relating to rock-based stratigraphy (lithostratigraphy), fossil-based stratigraphy (biostratigraphy) and time-based stratigraphy (chronostratigraphy).

Lithostratigraphical units, such as beds, members and formations, are widely used in geological mapping but will not concern us further here. The biozone is the fundamental biostratigraphical unit and comprises those rocks that are characterized by the occurrence of one or more specified kinds of fossil known as zone fossils.

Formal chronostratigraphical time units are also important and include, in ascending order of importance, the age, epoch, period and era. For example we may cite the Messinian Age, of the Miocene Epoch, of the Neogene Period, of the Cenozoic Era. Rock units laid down during these times are properly referred to as stages, series, systems and erathems (i.e. the Messinian Stage, of the Miocene Series, etc.). Less formal divisions are also widely used so that we may talk of the lower Neogene rocks laid down during Early Neogene times. In the following text, these informal subdivisions are abbreviated as follows: lower (L.), middle (M.) and upper (U.) and their equivalents for chronostratigraphy early (E.), middle and late.

Microfossils and biostratigraphy

Biostratigraphy is the grouping of strata into units based on their fossil content with the aim of zonation and correlation. As such biostratigraphy is concerned primarily with the identification of taxa, tracing their lateral and vertical extent and dividing the geological column into units defined on their fossil content.

Microfossils are among the best fossils for biostrati-graphical analysis because they can be extremely abundant in rocks (a particular consideration when dealing with drill cuttings) and they can be extracted by relatively simple bulk processing methods. Many groups are geographically widespread and relatively free from facies control (e.g. plankton, airborne spores and pollen). Many of the groups evolved rapidly, allowing a high level of subdivision of the rock record and a high level of stratigraphical resolution. It should also be emphasized that spores, pollen, diatoms and ostracods are indispensable for the biostratigraphy of terrestrial and lacustrine successions, where macrofossils can be scarce.

Detailed biostratigraphical zonations, using the groups mentioned in this book, have been developed for the entire Phanerozoic. Some areas of the column are better subdivided than others, for example the Cretaceous to Recent can be subdivided into approximately 70 biozones, based on calcareous nannoplankton and planktonic foraminifers, with an average duration of 2 million years per biozone. In comparison the Lower Palaeozoic has only been divided into 39 con-odont biozones at an average duration of 3 million years. Detailed biostratigraphical zonations for the Mesozoic and Cenozoic are to be found in the two volumes of Plankton Stratigraphy (Bolli et al. 1985).

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Neoproterozoic

Mesoproterozoic

Palaeopnoterozoic

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Fig. 3.1 The stratigraphical column (modified from the IUGS correlation chart). British stage/age names have been retained for the Ordovician and Cambrian systems/periods as these have to be internationally agreed. Whittaker et al. (1991) gives further information on stratigraphical terminology.

INTERVAL BIOZONES

ASSEMBLAGE BIOZONES

Local range blozone Concurrent range blozone

Assemblage blozone C Assemblage blozone B

Assemblage blozone A

Contiguous assemblage biozones

Consecutive range blozone 4type1)

Consecutive range blozone H j__'II ) Assemblage biozone Z

'Successive last appearance zone

ABUNDANCE BIOZONE

Barren Interval Non-contiguous assemblage bizones

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Fig. 3.2 Categories of biozones. (After Bassett in Briggs & Crowther 1987 with permission.)

The biostratigraphy of selected microfossil groups can be found in the 'Stratigraphic Index' series published by The Micropalaeontological Society and a host of specialist papers in scientific journals. The additional reading lists in this book provide an entry into this literature.

The basic unit of biostratigraphy is the biozone and fossils that characterize and give their names to a particular biozone are called zone or index fossils, for example the Orbulina universa Biozone of the Miocene. There are three basic types of biozone: the assemblage, abundance and interval biozones (Fig. 3.2). An assemblage biozone is based on the association between three or more species (though this concept is often more loosely applied) with little regard to the strati-graphical range of each. As species associations are strongly dependent upon local ecology, this type of biozone is most suitable for local or intra-basinal applications. The majority of defined biozones are interval biozones based upon the first appearance datum (FAD) and last appearance datum (LAD) of the named species. There are five types of interval biozone (Fig. 3.2), the most commonly used being the local range zone and the concurrent range zone. The latter comprises that interval which lies above the FAD of one species and below the LAD of a second species. The interval between two successive LADs is called a successive last appearance zone and is the most commonly used zone in commercial biostratigraphy where most of the samples are from borehole cores or cuttings and the FAD of a species cannot always be determined due to down-hole contamination ('caving').

Quantitative biostratigraphy

Because microfossils can occur in large numbers they are ideal for use in quantitative methods of biostratigraphy. Over the past 20 years a large number of techniques have become available for measuring biostratigraphical utility, defining and testing the error on a biozone and developing and testing correlations (Armstrong 1999). Typically quantitative methods are best applied to planktonic groups from continuous sections where FADs and LADs can be accurately determined. The most commonly used methods are semi-quantitative methods such as the graphical correlation technique developed by Shaw (1964). Details of this technique can be found in Armstrong (1999).

Graphical correlation uses a two-axis graph to compare the FADs and LADs of taxa found in common between two sections (Fig. 3.3). The heights of the first and last appearances of species are plotted as o <D

120 Ma

Fig. 3.3 Example of a graphical correlation. Shows the correlation of a new section with the composite standard reference section (CSRS). Sections have been correlated using the 25 and 30 !c d q.

standard time unit (stu) datum lines 2

via a line of correlation (LOC) which S

exhibits changes in sedimentation rate and an unconformity plateau. The changing slope of the LOC Curve shows an increased rate of deposits above the unconformity, relative to the CSRS. Once the correlation has been made, other data, for example radiometric dates (85 Ma, 120 Ma) or isotope excursions, can be transferred into the CSRS via the LOC. Open circles, base of range; crosses, top of range.

85 Ma

120 Ma

85 Ma

Microfossils Stratigraphy
CSRS

coordinates in the field of the graph. A line of correlation (LOC) is then drawn through the scatter of points either by hand or using a variety of statistical techniques (e.g. least squares, linear regression or principal components analysis). The LOC is then used to transfer species range data from one section to the other. The latter becomes the composite standard reference section (CSRS). Additional sections are similarly correlated with the CSRS and included range data is also transferred to the composite, so that species ranges are progressively extended with the addition of new sections. When all the data from all available sections have been added, further rounds of correlation are undertaken to refine and stabilize the position of the LOCs. If only a small number of sections are to be correlated then the graphical correlation can be carried out by hand; computer packages are available for correlating large numbers of sections.

Species ranges within the CSRS should span the maximum within the included sections. Where sections are included that cover a wide range of geographical and palaeoecological settings, then these ranges should approach the full temporal span of that species. Lithological, geochemical and palaeomagnetic data can also be included in the CSRS and help strengthen the correlations.

The CSRS can be divided into units of equal length (standard time units-stu). The resultant chronometric timescale can then be transferred into the original sections using the LOCs. Standard time unit datum planes can be matched to provide a high resolution correlation of all the sections. This method of correlation is particularly useful for illustrating diachron-ous lithostratigraphical events: those that appear to be the same but occur at different times in different localities, between sections (e.g. progradation of sedimentary strata or facies or the diachronous nature of an unconformity).

The high resolution available using graphical correlation (limited only by the accuracy in placing the LOC) provides the only means by which the predictions of sequence stratigraphical correlation models can be independently tested (see below). The slope and geometry of the LOC is taken to reflect the relative rates of sedimentation between the two sections being compared. Strata that are missing, owing to faulting or a hiatus, or a highly condensed sequence, will appear as a plateau in the LOC (Fig. 3.3).

Quaternary Shelf Sequence Stratigraphy

Relative sea level curve

Shallow water reworking (LmST)

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ALL AGES

Alluvial-

coastal plain Estuarine

TERTIARY AND MESOZOIC

ALLAGES

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Shallow water reworking (LmST)

Fig. 3.4 Palaeoenvironmental distribution of some of the main microfossil groups through time. These are placed in a sequence stratigraphical framework. Insert shows the principle sequence statigraphical terms related to rising and falling sea level. HST, highstand systems tract; LST, lowstand systems tract; mfs, maximum flooding surface; si, sea level; ts, transgressive surface; TST, transgressive systems tract; SB, sequence boundary. (After Hogg in Emery & Myers 1996 with permission.)

Microfossils in sequence stratigraphy

Sequence stratigraphy represents a powerful method for analysing familiar stratigraphical concepts such as transgression, regression and eustatic cycles and microfossils have a key role to play in sequence interpretation. The methods were largely developed as an extension of seismic stratigraphy and the need for correlation in the subsurface, but are equally applicable to outcrop geology where they have proved invaluable in understanding the influence of climate change on sedimentary successions. The reader is directed to Emery & Myers (1998) for a more detailed review of the principles of sequence stratigraphy. The basic philosophies of sequence stratigraphy are, firstly, that sediment accumulation occurs in discrete sequences, which are relatively conformable successions bounded by unconformities (or the correlative conformities in deep water). A sequence is considered to represent all the sediments deposited in an interval of time (0.5-5 Ma) and the sequence boundaries (intervals of no or very slow deposition) are considered effectively synchronous over large areas and can be used for matching sections. Secondly, the interaction of the rates of relative sea-level changes (eustasy), basin subsidence and sediment supply lead to variations in accommodation space, which is the space potentially available for sediment accumulation. The fundamental building blocks of sequences are parasequences, which generally represent shallowing or coarsening upwards cycles of short duration (10-100 kyr).

Every sequence comprises three systems tracts and potentially has a distinctive assemblage of microfossils (Fig. 3.4): a lower one representing periods of rapid but decelerating sea-level fall (LST, lowstand systems tract); a middle one relating to increasing acceleration in sea-level rise (TST, transgressive systems tract); and an upper one relating to a decreasing rate of sea-level rise and initial sea-level fall (HST, highstand systems tract). The base of each systems tract is defined as the sequence boundary, transgressive surface and maximum flooding surface respectively.

The interplay of environmental conditions, biological evolution, preservation potential of the microfossil group and cyclic changes in depositional style control the microfossil content of different sedimentary sequences. In a sequence stratigraphical analysis, it is the primary role of the micropalaeontologist to document changes in biofacies, and hence palaeo-environment, and to provide a high-resolution bio-stratigraphical framework.

In the oil industry benthic foraminifera are commonly used to define marine benthic palaeoenviron-ments, although conodonts, ostracods and benthonic algae have also been used. Palynofacies analysis is most useful in defining fluvio-deltaic subenvironments (e.g. Brent Field, North Sea, Parry et al. 1981; see also Tyson 1995 for a review of palynofacies in sequence stratigraphy). Terrestrial microfossil assemblages can also provide a detailed record of climate changes around the margins of the sedimentary basin. With increasing knowledge of the ecological controls on microfossil groups, the relative abundances of different marine groups can be used to elucidate the changing palaeooceanography.

The transport or reworking of species into the marine environment by wind (e.g. bisaccate pollen) or rivers (e.g. miospores, charophytes, ostracods and woody material) or tides (e.g. foraminifera, dinoflagellates) can be problematic in biostratigraphy and palaeo-environmental analysis. However the abundance gradients and size range of these derived fossils can be used to indicate the proximity of the source, location of palaeo-shorelines and exposure and uplift histories of the hinterland.

Few published studies have integrated the biostrati-graphy, biofacies analysis and sequence stratigraphy. Exceptions include Armentrout (1987), Loutit et al. (1988), McNeil et al. (1990), Allen et al. (1991), Armentrout & Clement (1991), Armentrout et al. (1991), Jones et al. (1993) and Partington et al. (1993).

Sequence boundaries (SB) and correlative conformities

A sequence boundary is produced by a fall in relative sea level and may be associated with considerable erosion of the underlying sequence. It can be recognized by discrepancies in age and palaeoenvironment across the SB. The scale of these differences reflects the magnitude of the sea-level fall and location within the basin (McNeil et al. 1990). For example a SB can be characterized by a marked hiatus in nearshore sections or by subtle changes in biofacies across the correlative conformity within deep basinal settings. Our ability to resolve sequence boundaries biostratigraphically is limited by the biozonal resolution of the index fossils, commonly ~1 Ma or less if graphical correlation is used. Absence of preserved microfauna may mark the period of maximum regression. Reworking of specimens associated with erosion is commonplace above sequence boundaries.

Lowstand systems tract

This comprises two components, the lowstand wedge and fan. Both are produced by gravity sliding as sediment provided by rivers bypasses the shelf and upper slope through incised valleys and canyons which cut the continental shelf. Consequently both wedge and fan deposits will contain reworked terrestrially derived material and older, often polycyclic, marine microfossil assemblages when compared with adjacent shales with indigenous microfossil assemblages. Lowstand fan deposits in the Palaeogene of the North Sea, for example, only contained an impoverished microfauna comprising long-ranging agglutinated foraminifera.

The lowstand wedge is initiated as sea level begins to rise and can be progradational (sediment supply is greater than the rate of relative sea-level rise; facies belts migrate basinwards) or aggradational (sediment supply and relative sea-level rise are roughly balanced; facies belts thus stack vertically). In a complete vertical section through a prograding wedge microfossils will tend to indicate a shallowing upward signature from deep marine through to non-marine biofacies. Aggradational wedges will typically comprise a thick accumulation of the same biofacies. In nutrient-starved basins increased sediment supply to the basin during the lowstand will tend to bring additional nutrients which can result in increased plankton productivity and blooms. Thus, the distal parts of low-stand wedges may be represented by interbedded hemipelagic shales rich in marine palynomorphs that are similar to assemblages in the underlying highstand sediments.

Transgressive surfaces

The transgressive surface separates the lowstand and transgressive systems tracts. It is characterized by local winnowing and reworking of sediment. Glauconite-and/or phosphate-rich hardgrounds may also develop. The processes associated with the deposition and diagenesis at the transgressive surface may therefore result in poor preservation and selective removal of microfossils. The transgressive surface represents a retro-gradational (i.e. sediment supply is less than the rate of sea-level rise and facies belts migrate landwards) diachronous boundary between terrestrial and marine biofacies. The presence of this surface can be inferred by the abrupt superposition of marine above terrestrial biofacies.

Transgressive systems tract (TST)

The TST contains retrogradational sequences which show an overall deepening upwards in the fossil biofacies. Transgression creates new shelf habitats that are rapidly colonized by opportunistic species. In addition they generate large areas of new wetland and saltmarsh habitat. The former may develop thick accumulations of peat and ultimately of coal. Diachronous shoreface deposits will contain shallowing marine biofacies.

As sediment supply becomes progressively reduced during the transgression, water turbidity decreases and clear water microfaunas (e.g. larger benthic foraminifera and seagrass species) will become more abundant (e.g. Van Gorsel 1988). Further sediment starvation will result in condensed sequences rich in well-preserved marine microfossils. These condensed sections will progressively onlap onto younger marine deposits up to the maximum flooding surface.

In deep basinal settings marine microfossil assemblages in pelagic condensed sections will contain abundant and diverse, typically cosmopolitan, planktonic species. The development of submarine fans formed by the regrading of the continental slope during transgression can be recognized by the presence of reworked shelf and upper slope microfossils within deep basinal condensed deposits (Galloway 1989). Shaffer (1987) used the presence and abundances of warm-water nannofossil assemblages to plot the progress of a transgression across an existing shelf.

Maximum flooding surface (mfs)

This surface separates the transgressive and highstand systems tracts and reflects the maximum landward development of marine conditions. Widespread condensed sections may occur across the shelf and basin due to sediment starvation. The mfs may also record a biostratigraphically distinctive event, usually with abundant planktonic fossils, and thus has the greatest potential for regional and global correlation. Partington et al. (1993) used palynomorph and microfossil assemblages at successive maximum flooding surfaces to produce a biochronostratigraphic framework for the Jurassic and Cretaceous of the North Sea.

At the basin margin the mfs is recognized by the sudden influx of low diversity marine plankton inter-bedded with shallower marine or terrestrial microfau-nas. In the deep basin sediment starvation can produce highly fossiliferous deposits while the complete absence of clastic input means that calcareous or siliceous ooze, composed of the remains of diatoms, Radiolaria, planktonic forams and coccoliths, can accumulate.

Highstand systems tracts (HST)

An aggradational HST is characterized by thick, stacked shelf or terrestrial microfossil assemblages whereas a prograding system will exhibit a shallowing upward sequence of biofacies. Shelf assemblages will be strongly influenced by the buildup of deltas associated with rapid sedimentation. In nutrient-rich waters the microbenthos will be characterized by infaunal species with only rare calcareous planktonic species. Dinoflagellate cysts and acritarchs adapted to these conditions will be abundant. If progradation continues long enough to bring deltas to the shelf edge, then the result will be transport of terrestrial and shallow marine microfossils directly into deep basin environments.

The prograding highstand slope will be characterized by gravity flow deposits and considerable microfossil reworking. In vertical sections it may be possible to define highstand slope deposits on the gradual shallowing upward nature of benthonic organisms and the gradual decline in planktonic species (Van Gorsel 1988). In deep basinal settings starvation will result in deep-water species similar to those in the condensed parts of the TST. As the highstand slope migrates towards the basin the change from deeper to shallower marine environments can cause pseudoextinction and diachronous correlation of these strata (Armentrout 1987).

REFERENCES

Allen, S., Coterill, K., Eisner, P., Perez-Cruz, G., Wornardt, W.W. & Vail, P.R. 1991. Micropalaeontology, well log and seismic sequence stratigraphy of the Plio-Pleistocene depositional sequences - offshore Texas. In: Armentrout, J.M. & Perkins, B.F. (eds) Sequence Stratigraphy as an Exploration Tool: concepts and practices. 11th Annual Conference, Gulf Coast Section, SEPM, pp. 11-13. Armentrout, J.M. 1987. Integration of biostratigraphy and seismic stratigraphy: Pliocene-Pleistocene, Gulf of Mexico. In: Innovative Biostratigraphic Approaches to Sequence Analysis: new exploration opportunities. 8th Annual Research Conference, Gulf Coast Section, SEPM, pp. 6-14.

Armentrout, J.M. & Clement, J.F. 1991. Biostratigraphic calibration of depositional cycles: a case study in High Island-Galveston-East Breaks areas, offshore Texas. In: Armentrout, J.M. & Perkins, B.F. (eds) Sequence Stratigraphy as an Exploration Tool: concepts and practices. 11th Annual Conference, Gulf Coast Section, SEPM, pp. 21-51.

Armentrout, J.M., Echols, R.J. & Lee, T.D. 1991. Patterns of foraminiferal abundance and diversity: implications for sequence stratigraphic analysis. In: Armentrout, J.M. & Perkins, B.F. (eds) Sequence Stratigraphy as an Exploration Tool: concepts and practices. 11th Annual Conference, Gulf Coast Section, SEPM, pp. 53-58. Armstrong, H.A. 1999. Quantitative biostratigraphy. In: Harper, D.A.T. (ed.) NumericalPalaeobiology. John Wiley, Chichester, pp. 181-227. Bolli, H.M., Saunders, J.B. & Perch-Nielsen, K. 1985. Plankton Stratigraphy, vols 1, 2. Cambridge University Press, Cambridge. Briggs, D.E.G. & Crowther, P.R. 1987. Palaeobiology - a synthesis. Blackwell Scientific Publications, Oxford. Emery, D. & Myers, K. 1996. (eds) Sequence Stratigraphy. Blackwell Science, Oxford.

Galloway, W.E. 1989. Genetic stratigraphic sequences in basin analysis: architecture and genesis of flooding surface bounded depositional units. Bulletin. American Association of Petroleum Geology 73, 125-142.

Van Gorsel, J.T. 1988. Biostratigraphy in Indonesia: methods and pitfalls and new directions. In: Proceedings. Indonesian Petroleum Association 17th Annual Convention, pp. 275300.

Jones, R.W., Ventris, P.A., Wonders, A.A.H., Lowe, S., Rutherford, H.M., Simmons, M.D., Varney, T.D., Athersuch, J., Sturrock, S.J., Boyd, R. & Brenner, W. 1993. Sequence stratigraphy of the Barrow Group (Berriasian-Valanginian) siliciclastics, Northwest Shelf, Australia, with emphasis on the sedimentological and palaeontological characterization of systems tracts. In: Jenkins, D.G. (ed.) Applied Micropalaeontology. Kluwer Academic, Dordecht, pp. 193-223.

Loutit, T.S., Hardenbol, J., Vail, P.R. & Baum, G.R. 1988. Condensed sections: the key to age determination and correlation of continental margin sequences. In: Wilgus, C.K., Hastings, C.G., Kendall, H.W., Posamentier, C.A., R. & Van Wagoner, J.C. (eds) Sea Level Changes - an integrated approach. SEPM, Tulsa 42, special publication, pp. 183-213.

McNeil, D.H., Dietrich, J.R. & Dixon, J. 1990. Foraminiferal biostratigraphy and seismic sequences: examples from the Cenozoic of the Beaufort-Mackenzie Basin, Arctic Canada. In: Hemelben, C., Kaminsji, M.A., Kuhny, W. & Scott, D.B. (eds) Palaeoecology, Biostratigraphy, Palaeo-

oceanography and Taxonomy of Agglutinated Foraminifera. Kluwer Academic Publishers, Dordecht, pp. 859-882.

Parry, C.C., Whitley, P.K.J. & Simpson, R.D.H. 1981. Integration of palynological and sedimentological methods in facies analysis of the Brent formation. In: Illing, L. & Hobson, G.D. (eds) Geology of the Continental Shelf of Northwest Europe. Heydon, London, pp. 205-215.

Partington, M.A., Copestake, P., Mitchener, B.C. & Underhill, J.R. 1993. Biostratigraphic calibration of genetic stratigraphic sequences in the Jurassic-lowermost Cretaceous (Hettangian to Ryazanian) of the North Sea and adjacent areas. In: Parker, J.R. (ed.) Petroleum Geology of Northwest Europe. Geological Society of London, Bath, pp. 71-386.

Shaffer, B.L. 1987. The potential of calcareous nannofos-sils for recognizing Plio-Pleistocene climatic cycles and sequence boundaries on the shelf. In: Innovative Biostratigraphic Approaches to Sequence Analysis: new exploration opportunities. 8th Annual Research Conference, Gulf Coast Section, SEPM, pp. 142-145.

Shaw, A.B. 1964. Time in Stratigraphy. McGraw Hill, New York.

Tyson, R.V. 1995. Sedimentary Organic Matter: facies and palynofacies. Chapman & Hall, London.

Whittaker, A., Cope, J.W.C., Cowie, J.W., Gibbons, W., Hailwood, E.A., House, M.R., Jenkins, D.G., Rawson, P.F., Rushton, A.W.A., Smith, D.G., Thomas, A.T. & Wimbledon, W.A. 1991. A guide to stratigraphical procedure. Journal of the Geological Society 148, 813-824.

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Responses

  • Eija Lilja
    Why are microfossils more useful than macrofossils for age determinations in the petroleum industry?
    8 years ago
  • isaias
    What is MFS, FAD in micropaleontology?
    3 years ago
  • Julie
    What are fossil signature discuss their importance in stratigraphy?
    1 year ago
  • concordio
    What are fossil signatures discuss their importance in stratigraphy?
    1 year ago
  • HELEN
    What role do microfossils play in sequence stratigraphy?
    1 year ago
  • thelma
    How microfossil can be utilized in biostratigrahical analysis?
    1 year ago
  • camryn
    How microfossils can be utilized in biostratigraphical analysis?
    1 year ago
  • asmeret
    How can microfossils be utilized in biostratigraphy analysis?
    1 year ago
  • Silvio
    What Is The Application Of Microfossil In Sequence Stratigraphy?
    8 months ago
  • Lodovico
    How microfossils are used for biozonation?
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