# Lecture 10 Stratigraphy and Geologic Time

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Lecture 10 Stratigraphy and Geologic Time
Basic principles of relative age dating Unconformities: Markers of missing time Correlation of rock units Absolute dating Geologic Time How old is the Earth? When did various geologic events occur? Interpreting Earth history is a prime goal of geology. Some knowledge of Earth history and geologic time is also required for engineers in order to understand relationships between geologic units and their impact on engineering construction.

Stratigraphy: Stratigraphy is the study of rock layers (strata) and their relationship with each other. Stratigraphy provides simple principles used to interpret geologic events.

Two rock units at a cliff in Missouri. (US Geological Survey)

Basic principles of relative age dating
Relative dating means that rocks are placed in their proper sequence of formation. A formation is a basic unit of rocks. Below are some basic principles for establishing relative age between formations. Principle of original horizontality Principle of superposition Principle of faunal succession Principle of cross-cutting relationships

Principle of original horizontality:
Layers of sediment are generally deposited in a horizontal position. Thus if we observed rock layers that are folded or inclined, they must, with exceptions, have been moved into that position by crustal disturbances sometime after their deposition.

Most layers of sediment are deposited in a nearly horizontal position
Most layers of sediment are deposited in a nearly horizontal position. Thus, when we see inclined rock layers as shown, we can assume that they must have been moved into that position after deposition. Hartland Quay, Devon, England by Tom Bean/DRK Photo.

Principle of superposition:
In an undeformed sequence of sedimentary rocks, each bed is older than the one above and younger than the one below. The rule also applies to other surface-deposited materials such as lava flows and volcanic ashes.

Principle of superposition. (W.W. Norton)

Applying the law of superposition to the layers at the upper portion of the Grand Canyon, the Supai Group is the oldest and the Kaibab Limestone is the youngest. (photo by Tarbuck).

Principle of cross-cutting relationships:
When a fault cuts through rocks, or when magma intrudes and crystallizes, we can assume that the fault or intrusion is younger than the rocks affected.

Cross-cutting relationships: An intrusive rock body is younger than the rocks it intrudes. A fault is younger than the rock layers it cuts. (Tarbuck and Lutgens)

Unconformities: Markers of missing time
When layers of rock formed without interruption, we call them conformable. An unconformity represents a long period during which deposition ceased and erosion removed previously formed rocks before deposition resumed. Angular unconformities Disconformity Nonconformity

Angular unconformities:
An angular unconformity consists of tilted or folded sedimentary rocks that are overlain by younger, more flat-lying strata. It indicates a long period of rock deformation and erosion.

Formation of an angular unconformity
Formation of an angular unconformity. An angular unconformity represents an extended period during which deformation and erosion occurred. (Tarbuck and Lutgents)

Angular unconformity at Siccar Point, southern Scotland, that was first described by James Hutton more than 200 years ago. (Hamblin and Christiansen and W.W. Norton)

Disconformity: A disconformity is a minor irregular surface separating parallel strata on opposite sides of the surface. It indicates a history of uplifting above sea (water) level, undergoing erosion, and lowering below the sea level again.

Formation of disconformity. (W.W. Norton)

Disconformities do not show angular discordance, but an erosion surface separates the two rock bodies. The channel in the central part of this outcrop reveals that the lower shale units were deposited and then eroded before the upper units were deposited. (Hamblin and Christiansen)

Nonconformity A nonconformity is a break surface that developed when igneous or metamorphic rocks were exposed to erosion, and younger sedimentary rocks were subsequently deposited above the erosion surface. (Tarbuck and Lutgens)

A nonconformity at the Grand Canyon
A nonconformity at the Grand Canyon. The metamorphic rocks and the igneous dikes of the inner gorge were formed at great depths and subsequently uplifted and eroded. Younger sedimentary layers were then deposited on the eroded surface of the igneous and metamorphic terrain. (Hamblin and Christiansen)

Types of Unconformity This animation shows the stages in the development of three main types of unconformity in cross-section, and explains how an incomplete succession of strata provides a record of Earth history. View 1 shows a disconformity, View 2 shows a nonconformity and View 3 shows an angular unconformity. [by Stephen Marshak] Play Animation Windows version >> Play Animation Macintosh version >>

Distinguishing nonconformity and intrusive contact
The sedimentary rock is younger. The erosion surface is generally smooth. Dikes may cut through the igneous body but stop at the nonconformity. Intrusive contact: Intrusion is younger than the surrounding sedimentary rocks. The contact surface may be quite irregular. A zone of contact metamorphism may form surrounding the igneous body. Cross-cutting dikes may penetrate both the igneous body and the sedimentary rocks.

Contrasting field conditions for (a) a nonconformity and (b) an igneous intrusion. (West, Fig 9.4)

The three basic types of unconformities illustrated by this cross-section of the Grand Canyon. (Tarbuck and Lutgents)

Geologic History A cross-section through the earth reveals the variety of geologic features. View 1 of this animation identifies a variety of geologic features; View 2 animates the sequence of events that produced these features, and demonstrates how geologists apply established principles to deduce geologic history. [by Stephen Marshak] Play Animation Windows version >> Play Animation Macintosh version >>

Principle of faunal succession:
Groups of fossil animals and plants occur the geologic history in a definite and determinable order and a period of geologic time can be recognized by its characteristic fossils.

Fossils are the remains of ancient organisms
Fossils are the remains of ancient organisms. There are many types of fossilization. (Top) natural casts of shelled invertebrates. (Middle) Fish impressions. (Bottom) Dinosaur footprint in fine-grained limestone near Tuba, Az.

The principle of fossil succession
The principle of fossil succession. Note that each species has only a limited range in a succession of strata. (W.W. Norton)

Correlation of rock units
The method of relating rock units from one locality to another is called correlation. One way of correlation is to recognize the rock type or rock sequence at two locations. Another way of correlation is to use fossils. A basic understanding of fossils is that fossil organisms succeeded one another in a definite and determinable order, and therefore a time period can be recognized by its fossil content.

The principle of correlation of rock units
The principle of correlation of rock units. The rock columns can be correlated by matching rock types. (W.W. Norton)

William Smith, a civil engineer and surveyor, could piece together the sequence of layers of different ages containing different fossils by correlating outcrops found in southern England about 200 years ago. In this example, Formation II was exposed at both outcrops A and B, thus Formation I and II were younger than Formation III. (Press and Siever).

Correlation of strata at three locations on the Colorado Plateau reveals the total extent of sedimentary rocks in the region.

The geologic column was constructed by determining the relative ages of rock units from around the world. (Next) By correlation, these columns were stacked one on top of the other to give relative ages of rock units (W.W. Norton)

Absolute dating The geologic time based on stratigraphy and fossils is a relative one: we can only say whether one formation is older than the other one. Absolute dating was made possible only after the discovery of radioactivity.

Radioactivity At the turn of the 20th century, nuclear physicists discovered that atoms of uranium, radium, and several other elements are unstable. The nuclei of these atoms spontaneously break apart into other elements and emit radiation in the process known as radioactivity. We call the original atom the parent and its decay product the daughter. For example, a radioactive 92U238 atom decays into a stable nonradioactive 82Pb206 atom.

Alpha decay: an a particle (composed of 2 protons and 2 neutrons) is emitted from a nucleus. The atomic number of the nucleus decreases by 2 and the mass number decreases by 4. Beta decay: a b particle (electron) is emitted from a nucleus. The atomic number of the nucleus increases by 1 but the mass number is unchanged.

Illustration of alpha and beta decays
Illustration of alpha and beta decays. (adapted from Tarbuck and Lutgens)

The decay of U238. After a series of radioactive decays, the stable end product Pb206 is reached. (Tarbuck and Lutgents)

Decay constant The rate of decay of an unstable parent nuclide is proportional to the number of atoms (N) remaining at the time t. dN/dt=-l*N The reason that radioactive decay offers a reliable means of keeping time is that the decay constant l of a particular element does not vary with temperature, pressure, or chemistry of a geologic environment.

Half-life The half-life of an radioactive element is the time required for one-half of the original number of radioactive atoms to decay: T1/2=0.693/l. The half-lives of geologically useful radioactive elements range from thousands to billions of years. The age of the Earth (4.6 billion years) was first obtained using U/Th/Pb radiometric dating. The half-life of U238 is 4.5 billion years.

The radioactive decay is exponential. Half of the radioactive parent remains after one half-life, and one-quarter of the parent remains after the second half-life. (Tarbuck and Lutgens)

The concept of a half-life
The concept of a half-life. The ratio of parent-to-daughter changes with the passage of each successive half-life. (W.W. Norton)

Geologic Time The geologic time scale subdivides the 4.6-billion-year history of the Earth into many different units, which are linked with the events of the geologic past. The time scale is divided into eons: Precambrian and Phanerozoic and eras: Precambrian, Paleozoic ("ancient life"), Mesozoic ("middle life"), and Cenozoic ("recent life"). The eras are bounded by profound worldwide changes in life-forms. The eras are divided into periods. The periods are divided into epochs.

The standard geologic time scale was developed using relative dating techniques. Radiometric dating later provided absolute times for the standard geologic periods. (W.W. Norton)

The awesome span of geologic time
The geologic time represents events of awesome spans of time. If the 4.6-billion-year Earth history is represented by a 24-hour day with the beginning at 12 midnight, the first indication of life would occur at 8:35am. Dinosaurs would appear at 10:48pm and become extinct at 11:40pm. The recorded history of mankind would represent only 0.2 sec before midnight.

The KT extinction At the boundary between Cretaceous (the last period of Mesozoic) and Tertiary (the first period Of Cenozoic) about 66 million years ago, known as KT boundary, more than half of all plant and animal species died in a mass extinction. The boundary marks the end of the era in which dinosaurs and other reptiles dominated and the beginning of the era when mammals became important. The widely held view of the extinction is the impact hypothesis. A large object collided with the Earth, producing a dust cloud that blocked the sunlight from much of the Earth’s surface. Without sunlight for photosynthesis, the food chains collapsed, which affected large animals most severely.

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