Stratigraphic Formations and Their Correlation
We can summarize information about the sequence of sedimentary strata at a location by drawing a stratigraphic column. Typically, we draw columns to scale, so that the relative thicknesses of layers portrayed on the column reﬂect the thicknesses of layers in the outcrop. Then, we divide the sequence of strata represented on a column into stratigraphic formations (“formations,” for short), a sequence of beds of a speciﬁc rock type or group of rock types that can be traced over a fairly broad region. The boundary surface between two formations is a type of geologic contact. (Fault surfaces and the boundary between an igneous intrusion and its wall-rock are also types of contacts.) Typically, a formation has a speciﬁc geologic age.
|The stratigraphic formations and stratigraphic column for the Grand Canyon in Arizona.|
Lets see how the concept of a stratigraphic formation applies to the Grand Canyon. The walls of the canyon look striped, because they expose a variety of rock types that differ in color and in resistance to erosion. Geologists identify major contrasts distinguishing one interval of strata from another, and use them as a basis for dividing the strata into formations, each of which may consist of many beds (figure above a–c). Some formations include a single rock type, whereas others include interlayered beds of two or more rock types. Not all formations have the same thickness, and the thickness of a single formation can vary with location. Commonly, geologists name a formation after a locality where it was ﬁrst identiﬁed or ﬁrst studied. For example, the “Schoharie Formation” was ﬁrst deﬁned based on exposures in Schoharie Creek, in New York.
If a formation consists of only one rock type, we may incorporate that rock type in the name (for example, Kaibab Limestone), but if a formation contains more than one rock type, we use the word “formation” in the name (such as Toroweap Formation). Note that in the formal name of a formation, all words are capitalized. Several adjacent formations in a succession may be lumped together as a stratigraphic group.
Where did the concept of a stratigraphic formation come from? While excavating canals in England, William Smith discovered that formations cropping out at one locality resembled formations cropping out at another, in that their beds looked similar and contained similar fossil assemblages. In other words, Smith was able to deﬁne the stratigraphic relationship between the strata at one locality and the strata at another, a process now called correlation.
|The principles of correlation.|
How does correlation work? Typically, geologists correlate formations between nearby regions based on similarities in rock type. We call this method lithologic correlation (figure above a, b). For example, the sequence of strata on the southern rim of the Grand Canyon clearly correlates with the sequence on the northern rim, because they contain the same rock types in the same order. In some cases, a sequence contains a key bed, or marker bed, which is a particularly unique layer that provides a deﬁnitive basis for correlation.
To correlate rock units over broad areas, we must rely on fossils to deﬁne the relative ages of sedimentary units. We call this method fossil correlation. Geologists use fossil correlation for studies of broad areas because sources of sediments and depositional environments may change from one location to another. The beds deposited at one location during a given time interval may look quite different from the beds deposited at another location during the same time interval. But if fossils of the same relative age occur at both locations, we can say that the strata at the two locations correlate. (Note that the fossils are not necessarily of the same species they won’t be if the depositional environments are different but they are of the same age.) Fossil correlation may also come in handy when rock types are not distinctive enough to allow correlation. For example, imagine that the Santuit Sandstone and Oswaldo Sandstone of figure above a look the same. Only fossils may distinguish one layer from the other, if the intervening Milo Limestone is absent.
|A geologic map depicts the distribution of rock units and structures.|
Once William Smith succeeded in correlating stratigraphic formations throughout central England, he faced the challenge of communicating his ideas to others. One way would be to create a table that compared stratigraphic columns from different locations. But since Smith was a surveyor, and worked with maps, it occurred to him that he could outline and colour in areas on a map to represent areas in which strata of a given relative age occurred. He did this using the data he had collected, and in 1815, produced the ﬁrst modern geologic map. In general, a geologic map portrays the spatial distribution of rock units at the Earth’s surface. Signiﬁcantly, the pattern displayed on a geologic map provides insight into the presence and orientation of geologic structures in the map area (figure above a, b). The inside of this book’s back cover provides a geologic map of North America.
Credits: Stephen Marshak (Essentials of Geology)