Sedimentology

Sedimentology is the study of modern sediments, such as sand and clay, and the processes that create them, such as erosion, and the study of ancient sedimentary rocks and how they formed. Most of Earth’s rocks are sedimentary. These are the rocks in which fossils are found. These are also the rocks in which petroleum is typically found, which makes sedimentology an important field of study for the petroleum industry.

Interest in sedimentary rock is as old as the study of geology, although study of how sediment forms and acts is more modern. Study of sediment and sedimentary rock was called “sedimentary geology” in the past, while the term sedimentology was first used in 1925.

Background

Many geologists have studied sedimentary rock over the centuries. During the nineteenth century, a great deal of study focused on these rocks and sedimentary facies. Facies refers to the total characteristics of rock, including appearance and chemical and biological structure, as well as changes in the rock, such as erosion, that take place in a geographic area. Sedimentary facies refers to descriptive types of sedimentary deposits; obvious differences indicate sediment was deposited in different ways or under different conditions. For example, layers of sedimentary rock may vary in thickness, color, or texture, and indicate changes over time in the same location. Areas of a single sedimentary layer may erode at different speeds, for example, even if these areas are all exposed to the same conditions.

Swiss geologist Amanz Gressly was an expert on sedimentary rock and coined the term sedimentary facies in 1838. Most geologists at that time were focused on the lateral layers of rock and fossil beds. Gressly was studying these layers, hard beds called strata, in the Jura Mountains of the Western Alps and was intent on the horizontal characteristics of strata. He realized some layers had different characteristics in different areas. He realized that the environments in which the sediment was deposited must have been variable in the same layers. Gressly later worked as an engineer on tunnels for the alpine railroads, and used this experience to further study horizontal differences in strata. This knowledge allowed him to correctly predict the type of sedimentary strata that would be found in areas he had not visited.

Geology remained largely an academic pursuit until the mid-twentieth century. Geologist A.C. Trowbridge coined the word sedimentology in 1925, but it was not commonly used until the 1950s. At that time, the study of sedimentary rock became commercial and research increased. Major oil companies recognized that sedimentology was crucial to identifying likely deposits of petroleum, which was found in sedimentary rock; better-informed drilling was a cost savings to the industry. Previously, oil and gas exploration looked for structural traps that enclosed these substances. Sedimentology identified subsurface stratigraphic traps, or areas in which porous and permeable sedimentary rock was in contact with impermeable sedimentary rock in the same lateral strata. Such layers indicated two paleoenvironments that were side by side, such as a sand bar next to lagoonal mud, and indicated oil and gas might be trapped within this layer. This launched large research projects, often funded by major oil companies, into subsurface stratigraphic traps in the late 1950s.

Until the 1980s, sedimentology was defined in broad terms and covered many areas of geology. Later in the twentieth century, many sedimentologists focused on sequence stratigraphy, or establishing the order in which layers were laid down. This could be difficult in areas of seismic disturbances.

Sedimentary rock only forms where sediment builds up and lies undisturbed long enough to become compacted. The sediment then becomes a hard bed, or strata. The sediment typically needs many years to become strata. The process takes place in sedimentary basins, which may be a few yards wide or thousands of square miles in size. Large sedimentary basins typically contain multiple environments, which means the strata formed in these basins may vary considerably.

Sediment may be of several types. For example, hydrogenous sediment is formed by reactions in the water, as with iron hydroxides. Biogenous sediment is composed of matter from living organisms, such as calcite and silica from shells and bones.

Overview

Sedimentology often focuses on trying to understand how strata formed. Researchers begin with the geologic record and work backward. This is not as simple as finding a pattern or texture in the sedimentary rock, however, and recognizing its history. Some sedimentary deposits can look almost identical but have very different origins. For example, millions of years after they were created, river-deposited sand bars may look like desert sand dunes. River sand bars and marine sand bars are completely different environments and form by different means.

Sedimentologists must also consider the effects of various conditions sediment as it is transformed into rock, a process called diagenesis. This term comes from the Greek words dia, meaning “passing through,” and genesis, or “origin.” Diagenesis refers to all the processes that are involved in changing a sediment or sedimentary rock after it is deposited in a sedimentary basin, not including metamorphism (the effects of high temperatures and pressure) and weathering (this occurs after uplift, when the rock is exposed to the atmosphere). Diagenesis includes many processes, including chemical reactions, compaction, decomposition of organic matter, dissolution of solids, and microbial activity. Bioturbation, which refers to organisms such as worms and mollusks moving through the sediment, is another factor that affects sedimentary layers. Diagenesis is essential to the creation of petroleum deposits and therefore is studied by petroleum geologists in the oil and gas industry.

Sedimentologists may have to examine strata and try to see how conditions changed it hundreds of millions of years ago. They may have to consult paleontological findings—for example, to determine if fossils of marine, land, or freshwater species are found in the strata. Strata formation can also indicate the presence of deep sedimentary basins in the past; finer sediment tends to drift to the deepest areas, while heavy, coarse sediment remains closer to the shore. Sedimentologists may have to conduct experiments. For example, they may observe the movement of sand in a wave pool to see how changes influence the shape and size of sandbars.

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