The use of basic biostratigraphy in different sediment types
Basic biostratigraphy is concerned with the recognition of fossils and the relative position of their occurrences in space and time. Various fossil groups can be found in different sedimentary environments. The two main environments are land (terrestrial) and sea (marine).
In terrestrial sediments, spores and pollen are the fossil group to use. Other fossil groups, such as vertebrates or other larger fossils, can only be used if the sediments samples are sufficiently large. In practice, this limits their use to outcrop samples. The limitation on the use of spore/pollen is the amount of oxidation experienced by the sediments after deposition. Oxygen will remove organic material (spores/pollen) from the sediments. Spores can be used in Devonian (approx. 400 myrs) to recent sediments, pollen in Upper Cretaceous (approx. 80 myrs) to recent sediments. Both can be found blown into marine environments in considerable amounts.
In marine sediments, calcareous nannofossils are the most useful group for Jurassic (approx. 210 myrs) to recent sediments, since their preparation is cheap and the stratigraphic resolution which can be reached with them is high. Occasionally sediments can be leached of calcareous particles, however, which also removes calcareous nannofossils (for instance in deep sea sediments). Their use as environmental indicators is limited, although some species are known to be associated with warm or cold water masses.
Palynomorphs (dinoflagellates, acritarchs and tasmanites) can be used in Permian ( approx. 260 myrs) to recent sediments and in most sediments types (though they tend to be rare in chalky limestone). Since their preparation includes the dissolution of the surrounding sediments, their use is more expensive than nannofossils. The possible stratigraphic resolution can be high. Recently, the use of organic grain size and shape as environmental indicators has been developed and is apparently useful, especially in sub-recent sediments.
Foraminifera can be used in all marine sediments which have not be leached of calcareous material, i.e. from shallow marine to middle bathyal. Their stratigraphic resolution can be high and their association with certain depositional environments makes them good environmental (water depth) indicators. Foraminifera can be found in sediments of Carboniferous (approx. 360 myrs)to recent age. Their preparation cost is lower than that of palynomorphs, unless thin-sections have to be used.
In silica enriched sediments, siliceous microfossils such as diatoms and radiolarians can be of use. The stratigraphic resolution can be high, especially in the Tertiary (approx. 65 myrs), but schemes are still under development for other time frames. Their preparation cost is similar to that of foraminifera.
It is good practice to always analyse at least two of these fossil groups per sample to provide a cross check. This reduces the chance of unexpected sedimentary circumstances leading to erroneous age and environmental estimates.
The age estimates can be numeric if the fossils encountered in a sample can be related to the global zonation schemes which exist today.
The most frequently used fossil groups in biostratigraphy are listed below, together with their use as stratigraphic and environmental indicators.
fossil group environment stratigraphic Time range Time indicator resolution (practical use) x 106 yr spore/pollen poor-medium medium Devonian - recent 400 - 0 nannofossils poor medium - high Jurassic - recent 210 - 0 foraminifera good medium - high Carboniferous - 360 - 0 recent dinoflagellates poor medium - high Permian - recent 260 - 0 diatoms/radiolaria medium medium - high mostly Tertiary 65 - 0
Developments in biostratigraphy
The use of biostratigraphy as a tool in petroleum exploration is well established. Relative age dating is one of the basic elements of the geologic data set available to the explorationists. In addition to this, estimates of the depositional environment of sediments can usually be given.
The most important development in the stratigraphic field is the integration of biostratigraphic dating with observable seismic and geomagnetic cycles, now usually known as sequence stratigraphy . This means that stratigraphers can now provide numeric age estimates for seismic sequences where traditionally only relative ages could be given. This direct link between your seismic data and numeric ages can be a powerful aid in regional correlation .
Biostratigraphic data is also integrated with other disciplines, e.g. sedimentology. Besides being the basic tool in regional correlation of sedimentological units, it allows the position of a sediment unit in a seismic cycle to be pinpointed and also allows more accurate estimates regarding the missing section. The sedimentological sequences can be placed in a regional and global framework.
Other stratigraphic tools, e.g. chemostratigraphy (stable isotope techniques: Oxygen, Strontium) Graphic Correlation or Cyclicity Analysis (duration estimates from electric logs), are now being integrated with biostratigraphic data. These can provide an additional cross-check besides giving useful paleoenvironmental information.
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