Stratigraphy

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Date: May 1, 2008
From: Earth Sciences for Students(Online ed.)
Publisher: Gale
Document Type: Topic overview
Length: 2,353 words
Content Level: (Level 5)
Lexile Measure: 1340L

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Stratigraphy is the study of layers, or strata, of rock and the interpretation of those strata's features to develop an understanding of Earth's history. By examining the historical record preserved in rock strata (including the fossil record), scientists can reconstruct past climates, environments, oceans, atmospheres, and life forms over geologic time. Stratigraphy involves both the physical and biological characteristics of strata and an understanding of the processes that form strata. The field of stratigraphy is a cornerstone of modern geology.

Principles of Stratigraphy

Strata are formed when sediments such as soil, volcanic ash and dust, particles of rocks and minerals, and the shells of organisms build up in one place over time. Sediments are commonly transported by the action of both wind and water and settle in layers on almost any land surface, as well as on the beds of lakes, rivers, and oceans.

Steno's Principles

In 1669, Danish naturalist Niels Stensen, known as Steno, published a work that put forth several principles of modern stratigraphy that developed from his study of hills in northern Italy. Steno based his principles on examination of layers of sedimentary rock and the traces of ancient life he found within them. The first principle is that layered rocks form from sediments and may contain the remains of past life, or fossils. The second states that the bottom of each layer conforms to the shape of the surface on which it was deposited. The third principle is that in any sequence of layers, the bottom layer is the oldest and the top layer is the youngest. Steno's third principle is that each layer was originally continuous, throughout the area in which it formed. For example, similar strata found on both sides of a river or other geographical feature once formed an unbroken layer that was later separated by that feature.

Bedding Planes

The layers in sedimentary rock are separated by surfaces called bedding planes. Most bedding planes result from a pause in the buildup of sediment in a particular area. During such a pause, some sediment may be removed from the layer by weathering or erosion. Bedding planes reveal that the deposition of sediment does not occur constantly, but rather in distinct episodes that may happen quite randomly. In places where wind or rainfall patterns vary with the seasons, the transport of sediment by wind and water results in deposition only during certain times of the year. In areas of frequent drought, rains heavy enough to transport sediments may occur years apart. Layers of shells on the floors of lakes and oceans may reflect blooms, that is, episodes of superior conditions for growth, accompanied by the life and death of large numbers of marine organisms. The periodic nature of sediment buildup means that new sediment may not be added to some bedding surfaces for long periods of time, and in some cases, sediment may be lost to the action of wind and water during these periods.

Sedimentary deposits may form in environments containing enough oxygen and nutrients to permit organisms to breathe and feed at the site. When organisms live in a sedimentary stratum, they may churn up the sediment in the layer, a process called bioturbation. The sediments become homogenized, or mixed so thoroughly that the layer appears to be completely uniform in composition. When all the oxygen is consumed, or when nutrient-rich sediments stop arriving at the site, the organisms will leave, and the sediment layers will again gather without being disturbed. Bedding planes at such layers may bear features such as tracks, trails, or burrows. Other bedding planes may bear ripples, shrinkage cracks, or other evidence of physical activity such as currents or exposure and evaporation. Sediment layers and the bedding planes that border them can reveal much about the environmental conditions at the site, the origin of the sediment, and the climate and environment of the area from which the sediment originated.

Geologic Time

The concepts of geologic time and the relative ages of Earth's features were among the major ideas that arose from Steno's principles of stratigraphy. As geologists worked to expand their understanding of geologic time, they developed a timescale that divided geologic history into a system of eras, periods, and epochs.

Fossils and Strata

For hundreds of years prior to the eighteenth century, Europeans collected fossils from local strata. Many collectors had described the strata in which the fossils were found and noted the position of the fossils within the strata. However, no one had understood that these fossils and their strata could be linked together to form a broader picture of Earth's history. Steno's recognition that strata were once continuous over large areas and that their relative ages could be determined by their relative positions was key to making sense of the scattered fossil evidence.

The first people to put these ideas together were a Scottish naturalist James Hutton (1726-1797), who studied stratigraphic relations in Scotland in the late 1700s, and an English engineer and canal builder named William Smith (1769-1839), who worked in the late 1700s and early 1800s. Hutton gave stratigraphy its beginning by rediscovering some of Steno basic principles. While surveying England and Wales for suitable canal sites, Smith noted that certain fossils always appeared in the same strata in different areas. By examining the fossils he found in one area, and comparing them to similar fossils he had found in other areas, Smith saw that he could establish links between the strata containing the fossils. Each stratum was characterized by specific rock types and certain fossils, and strata occurred in a definite sequence. Using this knowledge, Smith prepared a geologic map of England and Wales that indicated which fossils were most useful in tracing certain strata from place to place. Through Steno's principle that lower strata are older than higher strata, Smith's map gave a sense of the relative ages of the types of ancient life he found in the stratigraphic record.

Although ridiculed by many geologists, others found Smith's ideas and map quite useful. For example, farmers consulted his map for the types of rock that lay under their fields in order to guide their choices of crops to plant. If, for instance, the map indicated that acidic rocks lay under the fields, the farmers would plant crops that grew well in acidic soil.

Creating the Geologic Timescale

Some geologists, led by Charles Lyell, embraced Smith's work and applied his own ideas to develop a rudimentary geologic time scale based on the types of fossils found in each layer. Lyell argued that geologists must study the mineral and fossil content of each layer, noting where they encounter new types of organisms or minerals. Such changes in composition indicate the beginning of a new layer that is older than the one above it. Each layer is characterized by a unique set of fossils that marks it as a major unit in a geologic timescale. The relative age of each layer can be determined from the fossils it contains as well as the characteristics of the layers below and above it.

Geologists assigned the major sets of strata to broad units of geologic time and analyzed the patterns of occurrence of fossils in many parts of the world. By comparing the fossils from strata around the world, geologists divided the major timescale units into smaller units. A layer can be dated by comparing the fossils it contains to fossils considered characteristic of a particular time on the geologic timescale. Layers with similar fossils can be considered about the same age, even if found in very different places and environments around the world. For example, if one finds fossils in marine sediments that match fossils from terrestrial rocks found in mountain ranges, one can conclude that they came from sediment that was deposited at the same time in both places. One may also conclude that the environments of the two areas, although very different now, were similar at the time the sediment layer formed. A geologist might next consider what processes could have caused the two regions to reach their present conditions.

Sea-Level Changes and Seismic Stratigraphy

Changes in sea level over time have had a major influence on sedimentary strata, affecting their locations, dimensions, and composition. Seismic stratigraphy is an important modern method for studying strata that has proven especially useful for learning about strata buried deep beneath the surface.

Sea-Level Changes

The ancient Greeks found shells of marine creatures on hilltops far from the sea and realized that sea levels must have changed over time. Movements of the sea onto land are called transgressions; retreats of the sea to lower levels are called regressions. Such movements leave traces in the stratigraphic record that help geologists reconstruct ancient sea-level changes and environmental changes that accompanied them.

Sea levels may change when ice sheets freeze and grow, using massive amounts of water and lowering the oceans. When ice sheets melt and return water to the oceans, sea levels rise. Heating or cooling beneath tectonic plates can also cause sea-level changes by raising or lowering continents. For example, during the Ordovician Period (about 488 to 443 million years ago), the middle of the North American plate was pushed upward by heating beneath the crust. This caused the contingent to move upward, and marine environments in the region became much shallower. When the heating lessened, the middle of the plate relaxed and flattened, and the waters returned to their former levels.

Seismic Stratigraphy

Changes in sea level and location are easiest to study in the strata of shallow continental margins such as the Gulf of Mexico. However, the most complete records of such changes lie in strata hidden under deep waters. In the 1920s, companies looking for fossil fuels devised a way to study strata hidden underground (certain types of strata are commonly associated with oil and gas deposits) by sending seismic waves through the strata, a method called seismic stratigraphy. The waves bounce off layers of different density, and the reflections are captured by detectors that record and display the returned signals. Bedding planes and surfaces between layers of different density often appear clearly on seismic recordings. However, most bedding planes are so closely spaced that they cannot be distinguished by seismic waves alone: geologists must also examine core samples recovered by drilling through rocks beneath the seafloor.

Analyses of seismic reflections can tell much about the formation of a sediment layer. Parallel layers usually suggest that sediment arrived and settled at about the same rate across an area. A change in the space between two boundaries indicates a change in the rate of deposition or settling in part of the layer. A succession of related beds that are separated by well-marked boundary layers is called a parasequence. The boundaries between parasequences often indicate past changes in sea level. The beginning of a period of transgression is usually marked by a set of strata in which the rate of settling exceeded the rates of sediment delivery and buildup. Strata created as transgression continues often show that the rates of delivery and settling were balanced. The beginning of a period of regression by the oceans is marked by strata in which the rate of delivery exceeded the rates of settling and buildup.

Sets, or tracts, of parasequences that indicate major phases in a transgression-regression cycle are the largest units recognized by seismic stratigraphy, and each is bounded by surfaces that reflect seismic waves quite well. These surfaces may be traced from strata that were originally deposited on land or in shallow marine environments into deeper sea basins. This pattern reveals that the continental shelf underwent tectonic activity and phases of heating and cooling. Environmental changes presumed to have occurred during parasequence set formation can be confirmed by drilling a core sample and studying the fossils contained within it. By comparing the fossils to those found in other strata whose ages are known, geologists can determine when sea-level changes occurred and what the environment was like at the time. With such techniques, geologists use stratigraphy to understand past life and the history of Earth's crust.

Key Terms

sediment
soils, rock particles, and other materials that are deposited over time and make up the ground, whether on dry land or at the bottom of a body of water
geologist
scientist who studies rocks and Earth
sedimentary rock
rock formed from deposits of sediments (soils, rock particles, and other materials) over long periods of time
weathering
physical and chemical breakdown of rock that is stationary on Earth's surface
erosion
wearing away of land by wind and water
nutrient
substance that promotes growth and health in living things
terrestrial
pertaining to land or Earth
tectonic plates
large segment of Earth's crust and uppermost mantle (region just below the crust) that moves as a unit over Earth's surface, floating on a partially molten layer of rock below
continental margin
region where continental crust meets oceanic crust
fossil fuel
substance such as coal, oil, or natural gas, found underground in deposits formed from the remains of organisms that lived millions of years ago
seismic waves
vibrations associated with earthquakes, explosions, and impacts of solid bodies that fall to Earth from space--these vibrations travel through Earth
density
amount of mass (matter that causes an object to have weight) in a unit of volume
continental shelf
underwater plain buried under relatively shallow waters along the edge of a continent
tectonic
having to do with shaping the crust of a planet or moon

ANALYZING OCEAN CORES

Drilling and coring materials from the ocean floor has given geologists new insights into the development of life-forms and the history of ocean currents. Some deep-ocean cores are almost completely made up of shells of marine organisms, which provide a great deal of data about how the organisms changed over time. The cores are also valuable because their shell makeup helps geologists track the positions of the masses of water that could support such organisms, as these masses moved, as currents, through ancient oceans. Although such cores provide only a glimpse of the deep-ocean sediment record, they contain significant clues to marine history.

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Gale Document Number: GALE|CV2640550213