Biostratigraphy
Biostratigraphy
Introduction
Biostratigraphy, which may also be properly called pale-ontological stratigraphy, is the study of the distribution of fossils with the sedimentary rock record of Earth. In the practice of biostratigraphy, emphasis is placed on the vertical and lateral distribution of fossil taxa (meaning fossil species or other groups of fossil organisms) and not on the different types of rock within sedimentary strata. This distinguishes biostratigraphy from physical stratigraphy, which emphasizes changes in rock type alone.
There are several kinds of biostratigraphy. Formal biostratigraphy is concerned with the delineation of biostratigraphic zones, which are bodies of rock defined by the presence of selected nominal taxa (fossil species or groups whose name is attached to the biostratigraphic zone). A special kind of formal biostratigraphy is called biochronostratigraphy, which requires nominal taxa that are short-lived and thus their existence defines well a short interval of geological time. Informal biostratigraphy is concerned with using fossil taxa to help define ancient environments, a type of study called paleoecology (the study of ancient ecology preserved in sedimentary rocks).
Historical Background and Scientific Foundations
The study of biostratigraphy goes back to the late eighteenth and early nineteen centuries when the need for geological mapping and correlation of geological strata was being driven by the search for coal and other natural resources used in the English Industrial Revolution. Early geologists who undertook this search for resources and related geological mapping found that fossils embedded in sedimentary strata could be quite useful for mapping and correlating sedimentary formations across many parts of England. Some of this earliest work was done in the Coal Measures of northern England and in southern England and Wales as well. Certain fossils were readily identified with specific geological formations in the stratigraphic sequence of England, and later similar formations in western Europe.
These fossils were found to be useful even where the sedimentological characteristics of the formations differed over distances. For example, the same fossils persisted from where a formation was sandstone to where the same formation or its equivalent was a shale or limestone. For this reason, fossils emerged as components of sedimentary rock that were worthy of separate study and focus. Eventually, more comprehensive studies of fossils showed that fossil taxa occur in the stratigraphic record in a definite and determinable order. This was the fossil basis for studies of the fossil record with regard to organic evolution.
Geological discoveries about biostratigraphy were cited in early works on evolution including Charles Darwin’s Origin of Species (1859). Through biostratigraphy, and the correlation of geological formations that it promoted, the modern geological timescale was developed. Each of the geological periods in the modern geological timescale was originally defined as groups of formations that could be correlated from place to place (known as the geological systems). Biostratigraphy originally defined all these systems, which ended with gaps in the biostratigraphic record. Today, more narrow subdivision of the geological timescale are defined based on international biochronostratigraphic study.
Impacts and Issues
Today, biostratigraphy is a highly evolved subdiscipline within the field of geology, which is strongly involved in many aspects of economic geology, including the search for petroleum and mineral deposits. For example, when
WORDS TO KNOW
GEOLOGICAL TIME: The period of time extending from the formation of Earth to the present.
SEDIMENTARY ROCK: Rock formed from compressed and solidified layers of organic or inorganic matter.
STRATA: A bed or layer of sedimentary rock in which composition is usually the same throughout.
IN CONTEXT: ETHELDRED BENETT
British geologist Etheldred Benett (1776–1845) was one of the first female geologists. Her work and writing made substantial contributions to the founding of biostratigraphy.
Benett’s understanding of the context of fossils put her in touch with many of the famous geologists of the day. She corresponded with and met many—from Professor William Buckland at Oxford and the famous Sussex paleontologist Gideon Mantell to Charles Lyell, founder of the principle of uniformitarianism, and William Smith, a pioneer of stratigraphy.
Benett was at the forefront of paleontology and biostratigraphy at a time when many people still assumed that fossils were deposited from catastrophic acts of religious significance (such as Noah’s flood), and that scientific investigation should be left solely to men.
She advanced the emerging applications of biostratigraphy in her book, Organic Remains of the County of Wiltshire with extensive drawings, which she herself produced.
drilling petroleum exploration wells, formal biostratigraphy is employed to understand the ages and correlations of the layers penetrated by the drill bit. Small chips of rock called cuttings are recovered from the drill hole and micro-fossils (fossil plankton, such as foraminifera) are separated from the cuttings. These are examined microscopically to determine the biochronostratigraphy of the drilled strata. This information helps exploration geologists determine the age or target rocks for finding oil and gas. Informal biostratigraphy is helpful in this regard as well. Informal biostratigraphy’s delineation of ancient sedimentary environments helps exploration geologists determine the ancient environments of sedimentary rock being drilled.
In economic mineral exploration work, biostratigraphy is used to help correlate sedimentary formations that may be hosts to economic mineral deposits and to help locate and define sedimentary environments, such as reefs and marine sand bars, which may be potential host areas for economic deposits.
See Also Extinction and Extirpation; Geologic History
BIBLIOGRAPHY
Books
McGowran, Brian. Biostratigraphy: Microfossils and Geological Time. New York, Cambridge University Press, 2005.
Web Sites
International Commission on Stratigraphy. “International Commission on Stratigraphy.” January 2008. http://www.stratigraphy.org/ (accessed March 26, 2008).
University of California - Berkeley. “Biostratigraphy: William Smith.” 2006. http://evolution.berkeley.edu/evolibrary/article/_0/history_11 (accessed March 26, 2008).
David T. King Jr.