Stratigraphy - Real-life applications



Correlation

The geologist studying the stratigraphic record is a sort of detective, looking for clues. Just as detectives have their methods for solving crimes, geologists rely on correlation, or methods of establishing age relationships between various strata. There are two basic types of correlation: physical correlation, which requires comparison of the physical characteristics of the strata, and fossil correlation, the comparison of fossil types.

A POLARIZED-LIGHT MICROGRAPH SHOWS FOSSILS IN LIMESTONE DATING TO THE EOCENE AND OLIGOCENE EPOCHS. (© A. Pasieka/Photo Researchers. Reproduced by permission.)
A POLARIZED - LIGHT MICROGRAPH SHOWS FOSSILS IN LIMESTONE DATING TO THE E OCENE AND O LIGOCENE EPOCHS . (
© A. Pasieka/Photo Researchers
. Reproduced by permission. )

Actually, chronostratigraphic work is very similar some of the toughest cases confronted by police detectives, because more often than not the geologic detective has little evidence on which to operate. First of all, as noted earlier, only sedimentary rock can be used in making such determinations: for instance, igneous rock in its molten form, as when it is expelled from a volcano, could force itself underneath a rock stratum, thus confusing the stratigraphic record.

POTENTIAL PITFALLS.

Even when the rock is sedimentary, there is still plenty of room for error. The layers may be many feet or less than an inch deep, and it is up to the geologist to determine whether the stratum has been affected by such geologic forces as erosion. If erosion has occurred, it can cause a disturbance, or unconformity (discussed later), which tends to render inaccurate any reading of the stratigraphic record.

Another possible source of disturbance is an earthquake, which could cause one part of Earth's crust to shift over an adjacent section, making the stratigraphic record difficult, if not impossible, to read. Under the best of conditions, after all, the strata are hardly neat, easily defined lines. If one observes a horizontal section, there is likely to be a change in thickness, because as the stratum extends outward, it merges with the edges of adjacent deposits.

Yet another potential pitfall in stratigraphic correlation involves one of the most useful tools available to a geologist attempting to find an absolute age for the materials he or she is studying: radiometric dating. Though this method can provide accurate absolute dates, it is quite possible that the age thus determined will be the age of the parent rock from which a sample is taken, not the age of the sample itself. The grains of sand in a piece of sandstone, for instance, are much older than the larger unit of sandstone, and for this reason, radiometric dating is useful only in specific circumstances.

PHYSICAL AND FOSSIL CORRELATION.

Given all these challenges, it is a wonder that geologists manage to correlate strata successfully, yet they do. Physical correlations are achieved on the basis of several criteria, including color, the size of grains, and the varieties of minerals found within a stratum. By such means, it is sometimes possible to correlate widely separated strata.

Particularly impressive feats of correlation can result from the study of fossils, whose stratigraphic implications, as we have noted, were first discovered by William Smith. Smith hit upon the idea of biostratigraphy while excavating land for a set of canals near London. As he discovered, any given stratum contains the same types of fossils, and strata in two different areas thus can be correlated.

Long before his countryman Charles Darwin (1809-1882) developed the theory of evolution, Smith conceived his own law of faunal succession, which hints at the idea that species developed and disappeared over given phases in Earth's past. According to the law of faunal succession, all samples of any given fossil species were deposited on Earth, regardless of location, at more or less the same time. As a result, if a geologist finds a stratum in one area that contains a particular fossil and another in a distant area containing the same fossil, it is possible to conclude that the strata are the same.

Unconformities

In discussing the many challenges facing a geologist studying stratigraphic data, the role of erosion was noted. Let us return to that subject, because erosion is a source of what are known as unconformities, or gaps in the rock record. Unconformities are of three types: angular unconformities, disconformities, and nonconformities.

Angular unconformities involve a tilting of the layers, such that an upper layer does not lie perfectly parallel to a lower one. Disconformities are more deceptive, because the layers are parallel, yet there is still an unconformity between them, and only a study of the fossil record can reveal the unconformity. Finally, a nonconformity arises when sedimentary rocks are divided from a type of igneous rock known as intrusive (meaning "cooled within Earth").

ANGULAR UNCONFORMITIES.

Angular unconformities emerge as a by-product of the dramatic shifts and collisions that take place in plate tectonics (see Plate Tectonics). Sediment accumulates and then, as a result of plate movement, is moved about and eventually experiences weathering and erosion. Layers are tilted and then flattened by more erosion, and as the solid earth rises or sinks, they are shifted further. Such is the case, for instance, along the Colorado River at the Grand Canyon, where angular unconformities reveal a series of movements over the years.

Another famous angular unconformity can be found at Siccar Point in Scotland, where nearly horizontal deposits of sandstone rest atop nearly vertical ones of graywacke, another sedimentary rock. Observations of this unconformity led the great geologist James Hutton (1726-1797) to the realization that Earth is much, much older than the 6,000 years claimed by theologians in his day (see Historical Geology).

WHERE TO LEARN MORE

Bishop, A. C., A. Woolley, and A. Hamilton. Cambridge Guide to Minerals, Rocks, and Fossils. New York: Cambridge University Press, 1992.

Boggy's Links to Stratigraphy and Geochronology (Web site). <http://geologylinks.freeyellow.com/stratigraphy.html> .

Harris, Nicholas, Alessandro Rabatti, and Andrea Ricciardi. The Incredible Journey to the Beginning of Time. New York: Peter Bedrick Books, 1998.

Lamb, Simon, and David Sington. Earth Story: The Shaping of Our World. Princeton, NJ: Princeton University Press, 1998.

MacRae, Andrew. Radiometric Dating and the Geological Time Scale (Web site). <http://www.talkorigins.org/faqs/dating.html> .

Reeves, Hubert. Origins: Cosmos, Earth, and Mankind. New York: Arcade, 1998.

Spickert, Diane Nelson, and Marianne D. Wallace. Earth-steps: A Rock's Journey Through Time. Golden, CO: Fulcrum Kids, 2000.

Stratigraphy and Earth History—West's Geology Directory (Web site). <http://www.soton.ac.uk/~imw/stratig.htm> .

University of Georgia Stratigraphy Lab (Web site). <http://www.uga.edu/~strata/home.html> .

Web Time Machine. UCMP (University of California, Berkeley, Museum of Paleontology) (Web site). <http://www.ucmp.berkeley.edu/help/timeform.html> .

Also read article about Stratigraphy from Wikipedia

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