Perhaps the most widespread application of palynology is in geochronology, the dating of events in the history of the Earth. Palynomorphs and certain microfossils can be used in geochronology, that is, dating rock units. Fossils can only provide a relative date for strata, that is in relation to other units. Absolute dating relies on other methods to give a specific date (see radiometric). Dating with paly-nomorphs is possible because many change through time or possess unique features that allow them to be distinguished from other types. For example, Upper Cretaceous and lower Paleogene rocks in the Northern Hemisphere contain a unique type of fossil angiosperm pollen termed triprojec-tates (Farabee, 1990). These grains are unusual in that they possess polar and equatorial projections (FIG. 1.73) and a variety of ornamentation patterns that make them especially good biostratigraphic markers. Although the botanical affinities remain problematic, it has been suggested that at least some triprojectates possess sufficient characters to include
them within the modern flowering plant order Apiales (Farabee, 1991).
Closely associated with the dating of rocks based on the presence of certain types of palynomorphs is the correlation of rock units in time and space based on the fossils within them, a discipline termed biostratigraphy (Gray et al., 1985). Various types of fossils can be found in different sedimentary environments, for example terrestrial and marine, and each may have its own biostratigraphic markers. For example, calcareous nannofossils are especially good markers in marine sediments from the Jurassic to the recent; plant spores are useful in terrestrial rocks from the Devonian onwards and diatoms (a type of microfossil) from the Paleogene to the recent. At one time, applied biostratig-raphy was the method of choice in petroleum exploration. Although a number of other methods are used today, paly-nostratigraphic techniques are still used today to correlate strata based on the presence of certain types of microfossils. Today there are a large number of consulting companies that provide services in exploration and interpretation of petroleum and mineral deposits based on biostratigraphy and geo-chronology. The underpinnings of these commercial firms are based on the types and distribution of microfossils.
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