Correlation of rocks

If a geologist has studied a stratigraphic unit or system in one location and figured out conditions on the Earth at that point when the rock was deposited, this information can be related to the rest of the planet or simply to nearby areas. In order to accomplish this task, the geologist first needs to determine the relative ages of strata in a column, then estimate the absolute ages relative to a fixed time scale. One can determine correlations between stratigraphic units locally using various physical criteria, such as continuous exposure where a formation is recognizable over large areas. Typically, a group of characteristics for each formation distinguishes it from other formations. These include gross lithology or rock type, mineral content, grain size, grain shape, color, or distinctive sedimentary structures such as cross-laminations. occasionally, key beds with characteristics so distinctive that they are easily recognized are used for correlating rock sections.

Most sedimentary rocks lie buried beneath the surface layer on the Earth, and geologists and oil companies interested in correlating different rock units have to rely on data taken from tiny drill holes. The oil companies in particular have developed many clever ways of correlating rocks with distinctive (oil rich) properties. one common method is to use well logs, where the electrical and physical properties of the rocks on the side of the drill hole are measured, and distinctive patterns between different wells are correlated. This helps the oil companies in relocating specific horizons that may be petroleum-rich.

Index fossils are those that have a wide geographic distribution and occur commonly but have a restricted time interval in which they formed. Because the best index fossils should be found in many environments, most are floating organisms that can travel quickly around the planet. If the index fossil is found at a certain stratigraphic level, often its age is well known, and it can be correlated with other rocks of the same age.

See also basin, sedimentary basin; Milankov-itch cycles; sedimentary rock, sedimentation; sequence stratigraphy.

FURTHER READING

Allen, P. A., and J. R. Allen. Basin Analysis, Principles and Applications. Oxford: Blackwell Scientific Publications, 1990.

Bouma, Arnold H. Sedimentology of Some Flysch Deposits: A Graphic Approach to Facies Interpretation. Amsterdam: Elsevier, 1962. Goldhammer, Robert K., Paul A. Dunn, and Lawrence A. Hardie. "High-Frequency Glacial-Eustatic Sea Level Oscillations with Milankovitch Characteristics Recorded in Middle Triassic Platform Carbonates in Northern Italy." American Journal of Science 287 (1987): 853-892. Grotzinger, John P. "Upward Shallowing Platform Cycles: A Response to 2.2 Billion Years of Low-Amplitude, High-Frequency (Milankovitch Band) Sea Level Oscillations." Paleoceanography 1 (1986): 403-416. Hayes, James D., John Imbrie, and Nicholas J. Shakelton. "Variations in the Earth's Orbit: Pacemaker of the Ice Ages." Science 194 (1976): 2,212-2,232. Imbrie, John. "Astronomical Theory of the Pleistocene Ice Ages: A Brief Historical Review." Icarus 50 (1982): 408-422.

Prothero, Donald, and Robert Dott. Evolution of the Earth.

6th ed. New York: McGraw-Hill, 2002. Stanley, Steven M. Earth and Life Through Time. New York: Freeman, 1986.

structural geology Structural geology is the study of the deformation of the Earth's crust or lithosphere. The surface of the Earth is actively deforming, as demonstrated by evidence such as earthquakes and active volcanism and from rocks at the surface of the Earth that have been uplifted from great depths. The rates of processes (or time scales) of structural geology are very slow compared to ordinary events. For instance, the San Andreas fault moves only about an inch (a couple of centimeters)

a year and is considered relatively fast for a geological process. Even this process is discontinuous near the surface, with major earthquakes happening every 50-150 years. At great depths the movement between the plates may be accommodated by more continuous flowing types of deformation, instead of the stick-slip type of behavior that occurs near the surface. Mountain ranges such as the Alps, Himalayas, or those in the American West are uplifted at rates of a fraction of an inch (a few millimeters) a year, with heights of a mile or two (several kilometers) being reached in a few million years. These types of processes have been happening for billions of years, and structural geology attempts to understand the current activity and this past history of the Earth's crust.

Structural geology and tectonics are both concerned with reconstructing the motions of the outer layers of the Earth. The terms have similar roots— structure comes from the Latin struere, meaning to build, whereas tectonics comes from the Greek tek-tos, meaning builder.

Rigid body rotations are one type of motion of the surface of the Earth in which a unit of rock is transported from one place to another without a change in size and shape. These types of motions fall under the scope of tectonics. In contrast, deformations are motions involving a change in the shape and size of a unit of rock, something that falls under the realm of structural geology.

When motions occur at faults or when mountains are uplifted, rocks break at shallow levels of the crust and flow like soft plastic at deeper levels of the crust. These processes occur at all scales, ranging from the scale of plates, continents, and regional maps to what is observable only using electron microscopes.

structural geology and tectonics have changed dramatically since the 1960s. Before 1960, structural geology was a purely descriptive science, and since then has become an increasingly quantitative discipline, especially applying principles of continuum mechanics, with increasing use of laboratory experiments and the microscope to understand the mechanisms of deformation.

Tectonics has also undergone a recent revolution (since the understanding of plate tectonics in the 1960s) that provided a framework for understanding the large-scale deformation of the crust and upper mantle. Both structural geology and tectonics have made extensive use of new tools since the 1960s, including geophysical data (e.g., seismic lines), paleo-magnetism, electron microscopes, petrology, and geochemistry.

Most studies in structural geology rely on field observations of deformed rocks at the Earth's surface and proceed either downscale to microscopic obser vations or upscale to regional observations. None of these observations alone provides a complete view of structural and tectonic processes, so structural geologists must integrate observations at all scales and use the results of laboratory experiments and mathematical calculations to interpret observations better.

To work out the structural or tectonic history of an area, the geologist will usually proceed in a logical order. First, the geologist systematically observes and records structures (folds, fractures, contacts) in the rock, usually in the field. This consists of determining the geometry of the structures, including their geographical location, orientation, and characteristics. Additionally, the structural geologist is concerned with determining the number of times a rock has been deformed and which structures belong to which deformation episode.

The term attitude means the orientation of a plane or line in space. Attitude is measured using two angles—one measured from geographic north and the other from a horizontal plane. The attitude of a plane is represented by a strike and a dip, whereas the attitude of a line is represented by a trend and plunge. strike is the horizontal angle, measured relative to geographic north of the horizontal line in a given planar structure. The horizontal line is referred to as the strike line, and is the intersection of a horizontal plane with the planar structure. One can easily measure strike in the field with a compass, by holding the compass against the plane and keeping the compass horizontal. Dip is the slope of the plane defined by the dip angle and the dip direction, which must be specified. It is the acute angle between a horizontal plane and the planar structure, measured in a vertical plane perpendicular to the strike line.

To understand the processes that occurred in the Earth, structural geologists must also examine the kinematics of formation of the structures; that is, the motions that occurred in producing them. This will lead to a better understanding of the mechanics of formation, including the forces that were applied, how they were applied, and how the rocks reacted to the forces to form the structures.

To improve understanding of these aspects of structural geology, geologists make conceptual models of how the structures form and test predictions of these models against observations. Kinematic models describe a specific history of motion that could have carried the system from one configuration to another (typically from an undeformed to a deformed state). such models are not concerned with why or how motion occurred or the physical properties of the system (plate tectonics is a kinematic model).

Mechanical models are based on continuum mechanics (conservation of mass, momentum, angular momentum, and energy) and an understanding of how rocks respond to applied forces (based on laboratory experiments). With mechanical models geologists can calculate the theoretical deformation of a body subjected to a given set of physical conditions of forces, temperatures, and pressures (an example of this is the driving forces of tectonics based on convection in the mantle). Mechanical models represent a deeper level of analysis than kinematic models, constrained by geometry, physical conditions of deformation, and the mechanical properties of rocks.

One must remember, however, that models are only models, and they only approximate the true Earth. Models are built through observations and allow one to make predictions that can be tested to draw conclusions concerning the model's relevance to the real Earth. New observations can support or refute a model. If new observations contradict predictions, models must be modified or abandoned.

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