Biostratigraphy : past evolution and future challenges
|Category||Paleontology and Stratigraphy|
|Location||International Geological Congress,oslo 2008|
|Author||Thierry, Jacques; Cita, Maria-Bianca|
|Holding Date||21 September 2008|
The term «biostratigraphie» was introduced by Louis Dollo in 1909 to replace «paléontologie stratigraphique» previously used by pioneers like William Smith since 1815, who recognized the usefulness and practical application of fossils in the dating and correlation of sedimentary rocks. From its birth to the mid 20th century, biostratigraphy had focused on the survey of macrofossils ditribution and their leading part as fossil-index. One of the major episode of this period was the publication of Oppel’s classic Jurassic ammonite study in 1860, where for the first time is explained the species-based biozone concept. Several time improved and next giving rise to numerous kinds of biozones, this concept remained firmed for decades until now, serving for all the Phanerozoic time and using numerous fossil groups.
The second half of the 20th century was time of the publication of the «guides» and the codification of biostratigraphy, the construction of numerous zonal schemes, especially with the rise of micropaleontology and the role of microfossils in biostratigraphy. During the last two decades, biostratigraphy mainly concerns correlations between the various zonal schemes and other approaches in stratigraphy (integrated biostratigraphy and integrated stratigraphy). To day, as respect to a «classical» biostratigraphy, the majority of zonal schemes still follow the guides advises, using the several kinds of biozones as strictly descriptive basic units. But, at the same time, a «quantitative» biostratigraphy has developed using various techniques (unitary associations, graphic correlation, multivariate and probabilistic methods, constrained optimisation, etc ...), each one producing its proper biostratigraphic units and its account to the stratigraphic functions (correlation, ordination, subdivision, etc ...).
At the beginning of the 21th century biostratigraphy has to issue several challenges and this can be possible with integrated stratigraphy. Usually, the classical biostratigraphic units defined in the guides are supposed without time implications; but, if time relationships of rocks bodies may be approximated or defined on biostratigraphy, it is necessary to explain when and why the threshold between biostratigraphy and chronostratigraphy would be crossed. Because the distribution and abundance of fossils is geographically and stratigraphicaly limited, it is necessary to corroborate if biozones may be diachronous or not as claimed by someones. The remarkable surfaces defined in the concept of sequence stratigraphy can be dated by biostratigraphy and eustatic sea level change can be demonstrated and calibrated. Radiometric dating is enough uncommon and has inherent statistic error; magnetostratigraphy is unreliable especially before the Mesozoic. Than, these approaches cannot be performed on most of the stratigraphic record. But, when such physical data exist, they must be considered as «time anchoring points» and used to calibrate fossil zonal schemes.