Stratigraphy — Introduction to Paleontology (Part III)

Stratigraphy

In the seventeenth century, the Danish anatomist and geologist Nicolaus Steno proposed the concept of the principle of superposition. According to this principle, if rock layers have not been overturned, older strata will be overlain successively by younger strata above them. Steno observed that sediments accumulate horizontally and that the sides of rock layers reveal the stacked results of different deposits, with deeper layers representing more ancient geological time.

Steno also recognized that geological features change through time. This idea later influenced the Scottish geologist and physician James Hutton, who proposed the concept of uniformitarianism and introduced the idea of geological time. His book Theory of the Earth laid the foundations of modern geology. Subsequently, geology developed techniques such as radioisotope dating, which allow scientists to determine the formation age of rock layers. Fossils discovered within those layers can therefore reveal the time periods during which ancient organisms lived.

Stratigraphy can be further subdivided into several branches, including lithostratigraphy, biostratigraphy, chronostratigraphy, tectonostratigraphy, magnetostratigraphy, and chemostratigraphy. Among these, the three most essential for the study of paleontology are lithostratigraphy, biostratigraphy, and chronostratigraphy.

Lithostratigraphy

Rock layers can be classified into different units based on characteristics such as their lithology, structure, and composition. Lithostratigraphic units are organized hierarchically from large to small as supergroup, group, formation, member, and bed (Figure 1).

（Figure 1）Stratigraphic units（水也佑製作）

Among these categories, the formation serves as the fundamental unit of lithostratigraphic classification. Rock layers with similar lithological characteristics and structural features are grouped into formations. For example, the rock sequence of the Colorado Plateau (Figure 2) shows five formations from bottom to top: the Moenkopi Formation, Chinle Formation, Wingate Sandstone, Kayenta Formation, and Navajo Sandstone.

The Moenkopi Formation dates to the Early Triassic, whereas the Navajo Sandstone dates to the Early Jurassic. Differences in the structural appearance of these layers can be observed directly. However, the precise boundaries and definitions of formations must ultimately be determined through detailed field investigation.

（Figure 2）Colorado Plateau（圖片來源：Matt Affolter，採用 CC BY-SA 3.0 授權）

Biostratigraphy

The relationship between fossils and rock strata was recognized during the eighteenth century by the English geologist William Smith and the French naturalist Georges Cuvier.

Smith worked as a canal engineer and, while surveying terrain between Wales and London, recorded fossils of trilobites, ammonites, and other mollusks found in different rock layers. Meanwhile, Cuvier studied vertebrate fossil assemblages from Tertiary strata in the Paris Basin and recognized the chronological continuity between rock layers and biological communities. He also suggested that abrupt breaks in fossil species might be associated with catastrophic events.

Combined with discoveries made by other researchers of the same period, it became clear that earlier strata predominantly contain fossils such as brachiopods, trilobites, and graptolites. Higher layers contain ammonites, belemnites, marine reptiles, and dinosaurs, while still higher layers reveal mammalian fossils. Based on these fossil patterns, the British geologist John Phillips defined the Paleozoic, Mesozoic, and Cenozoic eras according to changes in fossil assemblages.

Fossils are the most powerful tool for defining biostratigraphy. Known fossil species found within a rock layer can be used to infer its geological age without requiring radioisotope dating. To connect fossils with stratigraphic layers, biostratigraphy defines several types of biostratigraphic units based on taxonomic groups of fossils:

• Assemblage zone – a biostratigraphic zone defined by the coexistence of three or more taxonomic groups (Figure 3A).

• Abundance zone (acme zone) – a zone defined by the interval where a particular taxon reaches its greatest abundance (Figure 3B).

• Taxon-range zone – the stratigraphic interval from the first appearance datum (FAD) of a taxon to its last appearance datum (LAD) (Figure 3C).

• Concurrent-range zone – the stratigraphic interval in which the ranges of two or more taxa overlap (Figure 3D).

• Interval zone – a stratigraphic interval between the upper boundary of an earlier biozone and the lower boundary of a later one; fossils may or may not be present in this interval (Figure 3E).

• Lineage zone (consecutive-range zone) – a biozone defined by morphological changes within an evolutionary lineage of a taxon (Figure 3F).

However, most fossils preserved in rock layers are incomplete, and many organisms never fossilize at all. As a result, it is often difficult to determine the exact first and last appearance of a taxon. Consequently, the actual range of a biostratigraphic zone is often broader than expected. This phenomenon is known as the Signor–Lipps effect.

（Figure 3）Biostratigraphic units（水也佑製作）

Chronostratigraphy

The division of geological time through stratigraphy was gradually developed by geologists during the eighteenth and nineteenth centuries. Early classifications often used unconformities in rock layers as boundaries. However, because identical strata around the world do not always contain such unconformities, the International Union of Geological Sciences redefined chronostratigraphic boundaries according to globally recognizable fossil assemblages.

Today, internationally accepted standards are defined by Global Boundary Stratotype Sections and Points (GSSP), commonly referred to as "golden spikes" (Figure 4).

（Figure 4）The GSSP of the Ediacaran Period（圖片來源：Bahudhara，採用 CC BY-SA 3.0 授權）

As an example, consider the Wenlock Series of the Silurian Period (Figure 5). Lithostratigraphy divides the rocks into formations and members, while biostratigraphy subdivides them according to different species of graptolites. Chronostratigraphy then divides this interval into two stages: the Sheinwoodian and the Homerian.

（Figure 5）The Wenlock Series: chronostratigraphy, lithostratigraphy, and biostratigraphy（水也佑製作）

The difference between chronostratigraphy and geochronology lies in their reference frameworks. Chronostratigraphy uses rock layers as its units, whereas geochronology uses units of time. The two systems correspond to each other directly (Table 1).

（Table 1）（水也佑製作）

Chronostratigraphic units describe the positions of rock layers, while geochronological units refer to time intervals. Rock strata can be described as lower or upper, corresponding respectively to earlier and later periods in time. For example, one might state that the pterosaur Quetzalcoatlus northropi lived during the Maastrichtian age of the Late Cretaceous, and that its fossils were discovered in rock layers belonging to the Upper Cretaceous Maastrichtian stage.

Author: Shui-Ye You