Fossils Series: Understanding Stratigraphy and Biostratigraphy

This is the second article in the Introduction to Fossils and Paleontology series. It introduces the geological context of fossil deposits and how geologists and paleontologists utilize it to track earth history. 

The views expressed in this article reflect those of the author mentioned, and not necessarily those of New Creation.

Stratigraphy: The Study of Rock Layers

In the previous article, we briefly mentioned the important contributions of Nicolas Steno. This seventeenth century Danish scientist changed the study of fossils. Before Steno, many thought that fossils naturally grew in place in the ground. By comparing fossil shark teeth with the teeth of recently-living sharks, he demonstrated convincingly that fossils are really the remains of once-living organisms that were covered by soft sediment that then hardened. This insight not only laid the foundations of paleontology, but also of stratigraphy. Stratigraphy is the study of sedimentary rock layers (strata, singular stratum).

From his premise that sedimentary rocks formed from originally-soft sediment deposited by water or air, Steno argued for several principles that form the cornerstone for how geologists today interpret strata. ¹His principles of stratigraphy are as follows:

Principle 1: Superposition

Since strata are deposited from water or air under the influence of gravity, they accumulate from bottom to top. Thus, when we find a series of strata stacked one on top of another, we can infer that the one at the bottom was laid down first, then the next stratum, and so on.

Principle 2: Original Horizontality

Strata are laid down horizontally. Thus, whenever we find a sequence of strata that are not horizontal, we can infer that they were originally horizontal.

Principle 3: Lateral Continuity

Strata will generally continue as far laterally as their containing depression allows. Because of this, stratigraphers can trace strata from one outcrop to another. They can also identify rocks from the opposite walls of a canyon as being part of the same stratum.

Principle 4: Cross-cutting Relationships

When we see strata that are disrupted (such as by a fault or erosion) or tilted at an angle, we can infer that the events that disturbed or uplifted them took place after the strata were laid down.

You can use these four principles at any outcrop to “rewind the clock” and retrace the events that formed the rocks before you. For practice, take the most famous outcrop of strata in the world: Grand Canyon National Park.

Stratigraphy in Real Life: Grand Canyon

From the rim of the Grand Canyon (picture), you can see that most of the exposed strata are horizontal. However, in certain parts of the canyon, you can see older sedimentary strata and basalts (volcanic rocks) underneath them. They tilt so that their bedding planes dip at an angle. You can see this only faintly in the image above, but see the model to the right. These rocks belong to a sequence called the Grand Canyon Supergroup (GCS). 

Between the GCS and the horizontal strata above them lays a sharp erosional surface called an angular unconformity. An unconformity represents a break in deposition and/or the presence of erosion in a sequence of strata. In an angular unconformity, the strata below the unconformity are at an angle relative to the strata above it. This specific unconformity in the Grand Canyon is the Great Unconformity. It separates the Precambrian rocks from the Paleozoic (Cambrian to Permian) rocks above them. Beneath the GCS rests another erosional surface (not pictured above) called a disconformity. It separates the GCS from the crystalline basement (granite and metamorphic rocks) below.²

From these few observations, we can quickly sketch an outline of the events that formed the Grand Canyon. First, the crystalline basement rocks formed. We know this because all the sedimentary strata are superposed on them. These rocks were eroded and then the GCS was deposited on top of them. The sedimentary strata and basalts of the GCS were originally horizontal, so we infer that tectonic events caused them to be tilted. They also continued laterally, but erosion has cut across them, leaving behind the Great Unconformity. The horizontal strata were then deposited one by one above the unconformity, explaining why they are superposed on the GCS strata. Lastly, the canyon itself formed as erosion cut across all the rocks.

Why the Geological Context Matters

While fossils are interesting by themselves, they aren’t that useful for reconstructing earth history without their geological context. If we want to use fossils as historical artifacts, we need to know where they belong in geologic history. We can use the principles of stratigraphy to tell the relative ages of fossils. Geologists call this relative dating. 

To illustrate, let’s consider the fossils found at Grand Canyon National Park. Near the bottom of the canyon, we find a greenish-gray layer called the Bright Angel Formation. It contains fossils of trilobites, an extinct group of marine arthropods. In stratigraphic terminology, a formation is a package of strata that is distinct, thick, and extensive enough to be traced on a geologic map.³

Higher up in the canyon, we find a tall, red-stained limestone cliff. This contains abundant fossils of straight-shelled nautiloids, a squid-like animal with a chambered shell. Appropriately, we call this formation the Redwall Limestone. 

Near the top of the canyon is a whitish-tan, cliff-forming formation called the Coconino Sandstone. It contains abundant trackways of tetrapods (four-legged animals).

Because we know the stratigraphic order of these three formations, we can easily make a chronology for when the fossils contained in them were deposited. The trilobites in the Bright Angel Formation were buried first. Later, the nautiloids were covered in limey marine sediment that became the Redwall Limestone. Later still, tetrapods left their traces in soft sand that later lithified (became rock) as the Coconino Sandstone.

Relative dating allows us to establish a sequence of events for the formation of fossil-bearing strata. It does not, however, tell us how much time passed between each stratum. Today, geologists assign absolute ages to rocks and fossils using radiometric dating. Radiometric dating methods are a massive topic that goes way beyond the scope of this article, so I won’t talk about them here. If you want to read more about radiometric dating, begin with this introductory article. 

Returning to fossils, there is one more principle of stratigraphy we need to discuss because of its relevance for earth history.

The Principle of Faunal Succession

In 1815, an English surveyor-turned-geologist named William Smith produced the first geologic map of Great Britain. A geologic map shows the strata, igneous, or metamorphic rocks that make up the bedrock of a particular region. Different colors represent different rocks. An accompanying cross-section along a particular line on the map (called a transect) helps you to visualize the geology of the area in 3D and to decipher the order of strata. 

Smith’s map was a result of years of observations and fossil collecting throughout Britain. As a surveyor, he observed the strata cut by canal-diggers, giving him a picture of what lay beneath the surface. He observed that the fossil content of each stratum was unique, and that regardless of where he went, the same types of rocks contained the same types of fossils. Using this observation, he could correlate strata from different areas across the island.⁴

The observation that we can discern a reliable pattern in fossil-bearing strata formed the basis of Smith’s principle of faunal succession. “Fauna” refers to the animal life found at a particular time or place, and “succession” denotes the pattern of one set of fossil organisms stratigraphically coming after another. Though Smith’s observations originally pertained to Great Britain, later workers showed the principle of faunal succession works everywhere we find fossils. 

A Layer Deeper: Biostratigraphy

Using the principle of faunal succession, geologists and paleontologists have developed a special sub-field of stratigraphy focused entirely on fossils. This field is biostratigraphy (bios is Greek for life). Unlike the related sub-field of lithostratigraphy (lithos is Greek for stone), which seeks to distinguish and correlate strata based on rock type, the goal of biostratigraphy is to distinguish and correlate strata based on the types of fossils they contain.⁵ In lithostratigraphy, formations are the basic units. In biostratigraphy, the basic units are biozones.⁶

Biozones are sections of strata marked by a unique fossil content. To set the boundaries of a biozone, you have to collect many fossils along the vertical section of strata you are studying. You must then carefully identify and record the fossil taxa present in every stratum. You take special note of where taxa appear and disappear, where taxa overlap in their stratigraphic ranges, and where certain taxa are more abundant. After you’ve studied one area, you move on to other outcrops to study the exposed strata just above and below your original site.  

Once you have your data, you can start to develop a biostratigraphic framework for your study region. First, you draw the sequence of strata present at each of your study sites. Next to these sections, you draw the stratigraphic ranges of the fossil taxa you discovered at each site. As you do this, certain patterns start to emerge. You notice that some taxa show up only for short intervals, while others continue all the way through the section. Each time you see a change in fossil content, you mark it with a horizontal line next to your section. As you do this, you are dividing up your section into distinct intervals. These intervals are your biozones. Now that you have your biostratigraphic framework, you can use it to correlate strata across your study area and beyond!   

To see how biostratigraphy works, let’s return to the Grand Canyon and consider its trilobite fossils in more detail.    

Biostratigraphy in Real Life: Grand Canyon Trilobites

Though trilobites occur throughout Paleozoic rocks, many trilobite taxa have very narrow stratigraphic ranges. Many trilobites appear to have been endemic to certain regions, while others are found on multiple continents. These characteristics make them useful for correlating strata at differing scales (e.g., regional vs. intercontinental scales). This is especially true for the first two systems of the Paleozoic—the Cambrian and Ordovician—where trilobites are the most diverse.⁷

In the Grand Canyon, the Cambrian System is represented by the three lowest horizontal formations: the Tapeats Sandstone, Bright Angel Formation, and Muav Formation. Together, they form a package called the Tonto Group. 

Fossils from the Tonto Group are sparse. However, enough of them remain to allow us to correlate the strata in the canyon to other Cambrian sites in southern Nevada and southeastern California. From these other sites, biostratigraphers have established 12 successive trilobite biozones in the lower-middle part of the Cambrian in southwestern North America.⁸ Each biozone is named after the trilobite species whose stratigraphic first appearance marks the start of that biozone. 

For example, the trilobite species Bristolia mohavensis (see figure below) gives the Bristolia mohavensis Zone its name. B. mohavensis‘s first appearance at Emigrant Pass in southern Nevada marks the lower boundary of the biozone. Within the B. mohavensis Zone, we find fossils of other trilobites as well, including B. bristolensis and Mesonacis fremonti. The ranges of some of these taxa overlap with other trilobite biozones. The upper boundary of the B. mohavensis Zone is marked by the first appearance of the next biostratigraphically-significant trilobite species, Bristolia insolens. This boundary also marks the start of the B. insolens Zone.

Paleontologists have discovered fossils belonging to seven of the twelve trilobite biozones in the Tonto Group at Grand Canyon. However, because fossils there are sparse, they are unable to pin down their exact boundaries in the strata of the canyon.⁹

Applications for Creationist Model-Building

This detailed look at stratigraphy, relative dating, and biostratigraphy provides an important correction to mistakes young-age creationists have often made in discussing the fossil record. In the past, many creationists have expressed skepticism over whether there is truly an order to the fossil record, with some claiming there is no real order. Some also claim that using fossils to date rocks (i.e., by biostratigraphic correlation) while using rocks to date fossils (by relative or absolute dating) is a circular practice. People often make the latter claim to imply that secular scientists give whatever date they want to fossils in accordance with their agenda. Both claims are false. 

Fossils do not occur randomly in earth’s strata. William Smith used the patterns he observed in fossil occurrences to map Great Britain’s geology in the early 1800s. Geologists today use patterns in the occurrences of fossils to trace strata deep beneath the earth’s surface in search of oil and gas. Creationists cannot dismiss these observable patterns without denying reality. 

The way researchers date fossils and rocks isn’t circular, either. As we saw at the Grand Canyon, we can relatively date fossils based on their stratigraphic position (“date fossils with the rocks”). And if strata in one location contain a fossil known to only occur within a narrow stratigraphic window in another location, we can logically correlate the strata across the two locations and assign the same relative age to both. Furthermore, geologists often use a combination of methods (e.g., radiometric dating, correlating chemical signatures, etc.) to correlate and date strata rather than relying exclusively on fossils.

Rather than being hyper-skeptical, we should show the Christian virtue of charity in how we approach the work of our evolutionary colleagues. Most geologists and paleontologists aren’t trying to destroy the faith of Christians or undermine the Bible’s history deliberately. They simply work within the system that trained them from grade-school to grad-school. Most have never met a creation scientist in their life. 

Unless we have good reasons for doubt, we should assume our evolutionary colleagues are reporting their findings honestly. Therefore, we can take the patterns they have observed in the fossil record, strip away the evolutionary and long-age interpretations, and begin asking ourselves, “How do we build a scientifically-rigorous model that explains these patterns while also taking the historical data in the Bible seriously?”

Creationist scientists have already begun this task by investigating large-scale biostratigraphic patterns within the context of the pre-Flood world, the global Flood, and the recovery of life after the Flood.^10,11,12 I will return to their research in a later article in this series. To hold you over until then, check out our summary of the recent Origins 2025 conference, which included several talks dealing with biostratigraphy.

Though we have made some progress, biostratigraphy remains one of many areas of science creationists have yet to fully explore and explain. We need researchers who will work on the minute patterns like trilobite biozones in addition to large-scale patterns. We also need researchers who will help illuminate the processes of sedimentation that formed the stratigraphic record before, during, and after the Flood. In a word, we as creationist model builders have a lot of work ahead of us!

Footnotes

1. Winter, J. G. (1916). The prodromus of Nicolaus Steno’s dissertation concerning a solid body enclosed by process of nature within a solid; an English version with an introduction and explanatory notes. New York: The Macmillan Company. ↩︎
2. Grand Canyon’s Three Sets of Rocks. (March 1, 2024). Retrieved from https://www.nps.gov/articles/000/grcatime-grand-canyon-s-three-sets-of-rocks.htm
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3. Wilkerson, C. (2001). What is a formation? Survey Notes, 33(1). Retrieved from https://geology.utah.gov/map-pub/survey-notes/glad-you-asked/what-is-a-formation/
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4. Scott, M. (2008). William Smith (1769-1839). Retrieved from https://earthobservatory.nasa.gov/features/WilliamSmith/page1.php ↩︎
5. Biostratigraphy. (June 27, 2014). Retrieved from https://www.ga.gov.au/scientific-topics/disciplines/biostratigraphy ↩︎
6. Murphy, M. A., Salvador, A., Piller, W. E., & Aubry, M.-P. Biostratigraphic Units. In International Stratigraphic Guide – An abridged version. Retrieved from https://stratigraphy.org/guide/bio ↩︎
7. Babcock, L. E., Peng, S., & Ahlberg, P. (2017). Cambrian trilobite biostratigraphy and its role in developing an integrated history of the Earth system. Lethaia, 50, 381-399. doi:10.1111/let.12200 ↩︎
8. Webster, M. (2011). Upper Dyeran trilobite biostratigraphy and sequence stratigraphy in the southern Great Basin, U.S.A. Paper presented at the The 16th Field Conference of the Cambrian Stage Subdivision Working Group International Subcommission on Cambrian Stratigraphy, Flagstaff, Arizona and southern Nevada.
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9. Dehler, C., Sundberg, F., Karlstrom, K., Crossey, L., Schmitz, M., Rowland, S., & Hagadorn, J. (2024). The Cambrian of the Grand Canyon: refinement of a classic stratigraphic model. GSA Today, 34(11), 4-11. doi:10.1130/GSATG604A.1
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10. McGuire, K., Southerden, S., Beebe, K., Doran, N., McLain, M., Wood, T. C., & Garner, P. A. (2023). Testing the order of the fossil record: preliminary observations on stratigraphic-clade congruence and its implications for models of evolution and creation. Proceedings of the International Conference on Creationism, 9, 478-486. doi:10.15385/jpicc.2023.9.1.26
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11. Wise, K. P., & Richardson, D. (2023). What biostratigraphic continuity suggests about earth history. Proceedings of the International Conference on Creationism, 9, 611-625. doi:10.15385/jpicc.2023.9.1.31
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12. Clarey, T. L., & Tomkins, J. P. (2023). Developing a comprehensive model of global flood paleontology: integrating the biostratigraphic record with global megasequence deposition. Proceedings of the International Conference on Creationism, 9, 561-587. doi:10.15385/jpicc.2023.9.1.29 ↩︎