World of Earth Science | Stratigraphy

Stratigraphy is that subarea of geology that treats the description, correlation, and interpretation of stratified Earth materials. Typically, geologists consider stratified Earth materials as layers of sediment or sedimentary rock. This definition, however, clearly encompasses other materials such as volcanic lava, ash flows, ash-fall layers, meteoritic impact ejecta layers, and soils. In fact, using this definition, any material that obeys the law of superposition during its formation could be placed in the domain of stratigraphy. Generally, internal layers within Earth (crust, mantle, and core) are not considered the type of layers studied by stratigraphers because they formed by Earth's internal differentiation processes.

Some geologists give a broader definition to the term stratigraphy. Planetary geologists sometimes view stratigraphy as if it were the study of the sequence of events on a planet or moon's surface. In addition, stratigraphy has been broadly used by some geologists who study mountain building and plate tectonics to mean the study of order of emplacement of rock units of various types, including igneous and metamorphic rocks, to which the law of superposition does not apply. In some cases, stratigraphy is used to define the study of geologic history of an area or country, but it is more correct to say that stratigraphy is the practical foundation for historical geology. In this article, the concept of stratigraphy expressed in the first paragraph is viewed as best and most correct.

Stratigraphy had its origins in the Renaissance writings of Nicholas Steno (1638–1687), who was the first to write lucidly about sedimentary strata. He observed strata exposed in the Arno River valley of Italy, and noted three axiomatic ideas, which became known as the first three "laws" of stratigraphy (Prodromus, 1669). These laws are known today as superposition, lateral continuity, and original horizontality. Superposition holds that layers are deposited so that the older layer is on the bottom. Unless strata are disturbed, this is always true. Lateral continuity holds that sedimentary layers extend laterally until they become so thin that they end at a "feather edge," abut against an obstruction, or grade laterally into other layers. Original horizontality holds that sedimentary layers are originally formed horizontally and remain so unless deformed by subsequent processes.

Steno's writings were full of common sense. In super-position, he noted the most important criterion for relative age dating. In lateral continuity, he wrote about how correlation of sedimentary layers would be possible. In original horizontality, he noted the criterion necessary for any sort of analysis of later deformation, that is, the original state of a sedimentary layer can be assumed to be horizontal.

As insightful as Steno's writings were, there is no strong evidence that they were influential beyond the Renaissance era in which he lived. Later on, during the Enlightenment, naturalists like James Hutton (1726–1797), John Playfair (1748–1819), and Charles Lyell (1797–1875) apparently independently "re-discovered" the importance of these common-sense concepts and used them in their influential writings about geology and stratigraphy. Hutton, Playfair, Lyell, and others of their time wrote books and papers, which established the foundations of modern thought about stratigraphy. Their most important contributions included promoting the concepts of actualism (understanding the past by studying modern processes) and demonstrating such key concepts as stratigraphic correlation, predictable fossil succession, and the great antiquity of Earth.

The advancement of these key concepts were given a great boost by the pioneering work of the English field engineer William Smith (1769–1839), who compiled and published the first large-scale geologic map (Wales and southern England; 1815) employing modern concepts of stratigraphic correlation and fossil succession. Smith's success inspired others to this kind of work, and was particularly important in influencing the Geological Society of London (the first geological

organization; founded 1807) to embark upon its "stratigraphical enterprise" of research in the United Kingdom. The Society and the British Geological Survey (the first geological survey, founded 1835) were important promoters of early stratigraphic studies and venues for presentation of early research. Based upon these efforts, it is fair to assert that modern stratigraphy was born in the United Kingdom during this period.

In the nineteenth century, major efforts were made by British stratigraphers and their colleagues on the European continent to develop a unified stratigraphic succession (or "geological column") for rocks in their areas. Cambridge Professor Adam Sedgwick (1785–1873) and Scottish naturalist Roderick Murchison (1792–1871) became quite famous as the preeminent "system builders" of their time. Sedgwick studied and named the Cambrian System himself and with Murchison, the Devonian. Murchison studied and named the Silurian and Permian Systems by himself. There were others who did the same during the nineteenth century, thus establishing the basis of our modern geological time scale (which has periods of the same names as those given to "systems" of rock during an era when exact ages of rock strata were unknown). This was the birth of modern chronostratigraphy, which emphasizes subdivision of geological time by studying Earth's stratigraphic record.

A Swiss geologist, Amanz Gressley (1814–1865), studied Jurassic strata in Europe in hopes of understanding what happens to sedimentary layers where they grade into other layers. He recognized that lateral continuity of layers revealed many changes, which reflected different ancient environments. To this concept, he gave the name facies, meaning an aspect of a sedimentary formation. A German stratigrapher, Johannes Walther (1860–1937), took up Gressley's ideas in his own work and became more widely known than Gressley for work with sedimentary facies. To Walther, the facies represented primary characteristics of the rock that would help him understand how and where the rock formed. He used what he called the ontological method in facies stratigraphy, which he described whimsically as "... from being, we explain becoming." This was a direct application of actualism, advocated earlier by Hutton and others, but now applied in a time of enhanced understanding of the natural world. Walter was the first naturalist to spend large amounts of time in the field studying modern environments in order to better interpret the past. His two-volume work, Modern Lithogenesis (1983; 1984), was a watershed for modern research with sedimentary facies. Accordingly, Walter is regarded as the founder of modern facies stratigraphy. Although his work was not accepted well in the United States for many years (due, in part, to anti-German feelings during the early twentieth century), it later was studied extensively for its rich descriptions of modern sedimentary environments and ancient sedimentary facies. In the latter part of the twentieth century, facies stratigraphy became much more than an academic exercise when it was realized that such knowledge could help predict the occurrence of petroleum and certain ore minerals—and facilitate more productive extraction of these materials—in host sedimentary rocks.

At the outset of the twentieth century, Austrian stratigrapher Eduard Suess (1831–1914) became the first advocate of global changes of sea level and how those changes might relate to global stratigraphy. This concept, called eustatsy, holds that global sea level rises and falls during geological history lead to the great marine transgressions and regressions noted in many sedimentary strata from locales around the world. Suess called upon subsidence of the sea floor and displacement of seawater by sediment as reasons for this global effect (today we know that gain and loss of polar ice is another contributor to sea-level change). His work stimulated much research, and strongly influenced the well-known American geologist T.C. Chamberlain (1843–1928), who perpetuated these ideas through his many well-known papers on the subject. These ideas were important in the development of a modern concept in stratigraphy called sequence stratigraphy.

Sequence stratigraphy, which holds that large bodies of sedimentary strata are bounded by interregional unconformities, formed as a result of global eustatsy. In the early 1960s, sequence stratigraphy was put forth by the American stratigrapher L.L. Sloss (1913–1996) in a series of widely read papers. During the 1970s, Sloss's student, Peter Vail (1930–), formerly with Exxon Corporation (now Exxon-Mobil Corporation), further developed these concepts while studying seismic profiles of stratigraphy from the world's continental shelves. Vail's paper's established sequence stratigraphy as one of the main subdivisions of modern stratigraphy. To recognize their contributions, sequence stratigraphy is often referred to as Sloss-Vail sequence stratigraphy in their honor.

Vail's work spawned a huge effort to produce a highly detailed, eustatic sea-level cycle chart of Earth's history based upon the vast data collection at Exxon. His work was published in 1987 in the prestigious journal Science. Sequence stratigraphy and global sea-level cycle charts are concepts used today major petroleum-company exploration laboratories all over the world.

Today, facies stratigraphy and sequence stratigraphy are not the only types of stratigraphy practiced by geologists. Modern stratigraphy includes: lithostratigraphy (naming of formations for purposes of geological mapping); biostratigraphy (correlating rock layers based upon fossil content); chronostratigraphy (correlating rock layers based upon their similar ages); magnetostratigraphy (study and correlation of rock layers based upon their inherent magnetic character); soil stratigraphy (study and mapping of soil layers, modern and ancient); and event stratigraphy (study and correlation of catastrophic events in geological history). The latter may include global or regional layers formed by asteroid or comet impacts, major volcanic events, global climate or ocean-chemistry changes, and effects of slight changes in Earth's orbital parameters (e.g., Milankovitch cycles). Modern procedures and practices in stratigraphy are summarized in two widely read documents: the North American Stratigraphic Code, published by the American Association of Petroleum Geologists, and the International Stratigraphic Guide, 2^nd edition, published by the Geological Society of America.

Because layered Earth materials possess so much information about Earth's past, including the entire fossil record— and a sedimentary record quite sensitive to atmospheric, climatic, and oceanic changes of the past—stratigraphy is the one subarea of geology entirely focused upon retrieving and understanding that record.