Because radiometric dates are not available for all sequences of rocks in specific geographic regions, it becomes necessary to position a given rock unit accurately relative to its absolute age, a type of relative dating. One way in which a sequence of sedimentary rocks can be grouped according to age is through the use of index fossils. To be effective, an index fossil should (1) distinguishable from other fossils and easily identifiable, (2) have existed during a relatively short period of geologic time, (3) be abundant, (4) be widely distributed geographically, and (5) have lived in different environments, so that it may be preserved in different types of sedimentary rocks. Obviously, not many fossils fulfill all these requirements, and assemblages of several fossil taxa (assemblage zone) are typically more useful than a single species. Generally, the most useful organisms for correlation from one section of rocks to another are those that lived in ancient seas. Pelagic organisms (those that live in the open sea and not on the bottom) provide the best long-range correlations because of their worldwide distribution, at least within certain climatic zones. These organisms include such planktonic forms as diatoms (Chapter 4), foraminifera, sili-coflagellates, and coccoliths (FIG. 4.43). These organisms are especially important because their skeletal remains are so small that a large number can be concentrated in a small sample, such as the cuttings obtained from a well boring. Other organisms, such as those that inhabited the ocean floors (benthic forms), typically have a spatially restricted distribution that enables them to be used effectively in correlations of a more local extent.
In terrestrial rocks, some of the best index fossils are pollen grains and spores (Gonzalez et al., 2006; Souza, 2006).
They can be carried long distances by wind and, consequently, can be deposited in a wide variety of sedimentary environments. Palynostratigraphy has been an especially important tool in providing correlation between marine and non-marine rocks and in determining the various ecological conditions under which plants lived (Dimitrova et al., 2005).
Plant megafossils have also been useful in biostratigraphy, especially when used in the form of assemblage zones. Such studies extend from some of the earliest land plants (Edwards and Richardson, 2004) to measuring Holocene vegetational changes. Noteworthy among plant megafossils used as index fossils are a variety of Carboniferous foliage types (see Chapter 16), which have proven useful in establishing stratigraphic sequences in certain geographic regions (Diaz, 1983, 1985). For example, late Paleozoic foliage types have been useful in delimiting biostratigraphic zones in North America (Read and Mamay, 1964; Gillespie and Pfefferkorn, 1979), southwestern Germany (Germer, 1971), and northern France (Laveine, 1987). In some cases, megafossils have been more reliable than palynology, as palynomorphs are often difficult to extract from the high-rank coals that comprise some of the stratotype sections. In other instances, the identification of particular taxa has been useful in precisely dating tectonic events, such as the Upper Carboniferous folding phases in northwestern Spain (Wagner, 1966), and in documenting climatic changes (Wagner, 2004). There can be little doubt that as fossil plants are better understood they will become increasingly important as stratigraphic markers in biozonation and correlation.
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