Stratigraphic correlation

Stratigraphic correlation is the process of determining the equivalence of age or stratigraphic position of layered rocks in different areas. It is critical to understanding the Earth's history because stratigraphic correlation is one of the principal methods by which the succession and synchrony of geological events are established.

Methods of Stratigraphic Correlation

Stratigraphic correlation is the process of determining the equivalence of age or stratigraphic position (position in a vertical sequence of rock layers) of strata in different areas. Strata thus determined to be of the same age or in the same stratigraphic position are called “correlative” or “correlated.”

The most apparent stratigraphic correlation method is to demonstrate that strata are continuous over a given area. This is achieved by tracing the layers (often by walking along them) throughout the area to demonstrate their physical continuity. This direct method is best in areas where strata are continuously exposed (the Grand Canyon in Arizona is a good example). Over much of the Earth's surface, however, the strata that scientists wish to correlate are covered by younger rocks or vegetation, or erosion has removed them over some portion of their geographic extent, making this method impractical.

A commonly used method of stratigraphic correlation that is not subject to this drawback is correlation by lithologic similarity or similarity in rock composition and texture. Thus, strata of the same lithology (rock type) in different areas may be considered correlative. In effect, this method attempts to identify the same layer in different regions without demonstrating the continuity of (that is, without tracing) the layer. However, the major problem with this method is that many layers in the Earth's crust are of similar lithology. For example, it is challenging in eastern Colorado to tell a 90-million-year-old sandstone layer from a 70-million-year-old sandstone from lithology alone. Thus, mistakes in determining age equivalence are easy to make when correlating only by lithologic similarity.

A third stratigraphic correlation method relies on the idea that two layers in different areas occupying the same stratigraphic position are correlative. This method can be referred to as correlating by position in the stratigraphic sequence. For example, if a limestone layer overlies sandstone A in one region and a shale layer overlies sandstone A elsewhere, scientists may conclude that the limestone and shale are correlative. This method, however, has its pitfalls. In the example, the limestone layer at the first location was deposited right after sandstone A was deposited. In contrast, at the second location, the shale layer was deposited many millions of years after sandstone A. Correlation between limestone and shale, in this case, is done to recognize their equivalence of stratigraphic position but not to demonstrate their age equivalence.

Age equivalence in stratigraphic correlation is almost always based on fossils. Stratigraphic correlation by fossils depends on biostratigraphy, which recognizes strata by their fossil content. If a fossil represents an organism characteristic of a particular interval of geologic time, it is called an index fossil. Thus, if an index fossil is collected from a stratum, that stratum is the same age as strata elsewhere that contain the index fossil.

Besides fossils, there are some other, less often used methods of determining the age equivalence of strata. The most common of these methods is to obtain a numerical estimate (usually in millions of years) of the age of the strata. This can only be undertaken if the strata contain volcanic material containing chemical elements needed to obtain a numerical age. These elements are generally not present in sedimentary rocks; volcanic ash beds are not nearly as common in sedimentary rocks as are fossils. That is why fossils have been and continue to be the primary means for determining the age equivalence of strata. To undertake stratigraphic correlation, it is thus necessary to define the lateral extent of strata in a given region, characterize their lithology, determine their stratigraphic sequence, and collect and establish the stratigraphic ranges of fossils. Once these data have been collected in one or more regions, a geologist can attempt to correlate the strata within an area or between disparate areas.

Development of Stratigraphic Correlation

The development of stratigraphic correlation began in 1669 when Nicolaus Steno, a Danish naturalist, recognized that the sequence of strata is directly related to their relative ages. Steno's principle of superposition thus identified the oldest layers as those at the bottom and the youngest strata as those at the top of a sequence of strata. Steno's principle and recognizing fossils as the remains of past life allowed William Smith in England and Georges Cuvier and Alexandre Brongniart in France to undertake the first stratigraphic correlations based on fossils during the early nineteenth century. Smith's correlations resulted in the first geologic map of England (1815). Cuvier and Brongniart were able to correlate stratigraphically the rock layers in the vicinity of Paris and thereby reconstruct the geological and biological history of this area. Previously, during the late eighteenth century, German mining engineer Abraham Werner and his students laid the basis for stratigraphic correlation by lateral continuity and lithologic similarity by arguing for the continuity of strata of a particular lithology over a broad area. Thus, by the early nineteenth century, stratigraphic correlation by lateral continuity, lithologic similarity, position in the stratigraphic sequence, and fossils were already being practiced by European geologists.

Applying Stratigraphic Correlation

Stratigraphic correlation plays a central role in understanding geological history. By allowing geologists to determine the synchrony or diachrony of strata in different areas, the geological and biological events recorded in these strata can be ordered in time. This ordering is the basis of the chronology of geological and biological events during the last 3.9 billion years of Earth's history. Stratigraphic correlation is thus one of the methods by which the relative geological time scale of eons, eras, periods, epochs, and ages was constructed.

An excellent example of stratigraphic correlation that helped to decipher geological history and led to the discovery of a giant oil field comes from the Guadalupe Mountains of western Texas and eastern New Mexico. In the years before World War II, geologists and paleontologists used stratigraphic correlation (especially biostratigraphy) to unravel the complex geological history of the strata that form these mountains. This history revealed massive barrier reefs had developed as the sea encroached from the south about 260 million years ago. When it was later learned that hydrocarbons often accumulate around reefs, interest in the petroleum potential of these rocks was aroused. Because drilling the rocks in the rugged Guadalupe Mountains was impractical, an effort was made to trace the reef strata into the nearby lowlands. There, drill cores brought up rocks and fossils from deep beneath the surface. Stratigraphic correlation, by lithologic similarity, position in stratigraphic sequence, and fossils, was used to identify the strata adjacent to the reef in what is now called the Delaware basin, south and east of the Guadalupe Mountains. Successful drilling for petroleum soon turned the Delaware basin into one of the world's giant oil fields, thanks to identifying petroleum source rocks through stratigraphic correlation.

By ordering events in the history of the Earth, stratigraphic correlation has been critical to the development of a global geologic time scale. Such a time scale provides geologists with a shared temporal framework to view their observations. A good example of the use of stratigraphic correlation in the development of the global geological time scale comes from the concept of the Cambrian period. The British geologist Adam Sedgwick coined the term “Cambrian” (from “Cambria,” the ancient Roman name for Wales) in 1835 to refer to rocks in northern England that he believed contained the oldest fossils. Geologists now recognize the Cambrian period (544 to 505 million years ago) worldwide because stratigraphic correlation proved that an interval of geologic time corresponding to Sedgwick's original Cambrian rocks can be identified across the globe. One of the most distinctive aspects of Cambrian strata is their abundance and types of trilobites. Correlation of the Cambrian strata of England with strata elsewhere has been mostly undertaken based on these trilobite fossils. Indeed, Sedgwick himself first conducted such stratigraphic correlation in continental Western Europe.

By the 1880s, when American geologist Charles Doolittle Walcott identified Cambrian trilobites in North America, the Cambrian was well on its way to becoming a geologic time period recognized worldwide. Stratigraphic correlation, especially using fossils (biostratigraphy), thus played a significant role in the recognition of the Cambrian. All geologic periods now recognized worldwide are similarly rooted in stratigraphic correlation. Stratigraphic correlation continues to play an essential role in discoveries regarding the study of Earth. Paleontologists often use it to refine the timeline of life on Earth and better understand evolutionary processes. Stratigraphic correlation is vital in locating oil and natural gas deposits. It has provided a more complete understanding of past cultures and civilizations, aiding in the field of archeology. Finally, stratigraphic correlation allows scientists to study Earth’s past climate, leading to a more complete understanding of climate in the twenty-first century, and allowing researchers to apply these findings to mitigating global climate change. 

Principal Terms

biostratigraphy: the identification and organization of strata based on their fossil content and the use of fossils in stratigraphic correlation

correlation: the determination of the equivalence of age or stratigraphic position of two strata in separate areas; more broadly, the determination of the geological contemporaneity of events in the geological histories of two areas

index fossil: a fossil that can be used to identify and determine the age of the stratum in which it is found

lithology: loosely used by many geologists to refer to the composition and texture of a rock

sedimentary rocks: rocks formed by the accumulation of particles of other rocks, organic skeletons, chemical precipitates, or some combination of these

stratigraphy: the study of layered rocks, especially of their sequence and correlation

stratum (plural strata): a single bed or layer of sedimentary rock

Bibliography

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