strat and geological time

8/8/2019 Strat and Geological Time

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Cross-cutting relationships

• Relationships showing that one geologic event took place after another one

• Example 1: A mass of igneous rock intruded into layers of sedimentary rock

• Example 2: A gully that was eroded into a layer of sediment, then filled with

sediment itself

Unconformity

• Evidence of a "gap" in the record of deposition of layers of rock or sediment.

• The gap represents a pause in deposition, erosion, and/or tectonic activity such

as folding or fault movement.

Types of unconformities

• Angular unconformity

• Disconformity• Nonconformity

Angular Unconformity

• Tilted or folded rocks or sediments are overlain by younger rocks or sediments

that are still in horizontal layers.

• This indicates a long gap in deposition during which tilting or folding and erosion took place.

Disconformity

• Layers of rock or sediment on either side of the unconformity are parallel.

• More difficult to identify, unless the unconformity is highly irregular because

of deep erosion.

Nonconformity

• An unconformity between layers of sedimentary rock and a mass of igneous or

metamorphic rock.

Correlation of rocks and sediments

• "Matching" layers of rock or sediment that are approximately the same age, in

different areas.

Physical correlation

• Correlation based on physical properties like particle size, or distinctive

"marker beds" (example: a layer of volcanic ash).



Correlation using fossils

• Relies on the principle of faunal succession:

• Fossil organisms replaced one another in a definite order, and different time periods had different groups of fossil organisms.

"Absolute Dating"

• Methods for assigning an actual age in years to a rock or sediment layer.

• Most common methods use radiometric dating. Example: radiocarbon dating .

Isotopes

• Many chemical elements have two or more isotopes.• These are different "versions" of the same element, in which the number of

neutrons varies slightly.

Example of isotopes (carbon)

• 12C --> six neutrons + six protons = 12

• 13C --> seven neutrons + six protons = 13

• 14C -- > eight neutrons + six protons = 14

Radioisotopes

• Isotopes that are unstable and decay over time.

• Decay = becomes an isotope of another element.

Examples of radioisotopes

• 14C decays to form

14 N (a stable isotope of nitrogen).

• 40K (a radioisotope of potassium) decays to form

40Ar and

40Ca (stable

isotopes of argon and calcium).

Using radioisotopes to determine the age of rocks or sediments

• Radioisotopes become part of the rock or sediment when it forms (mostly as

elements in minerals, 14C in organic material such as wood).

Using radioisotopes to determine the age of rocks or sediments (cont.)

• We can estimate how much of a radioisotope was originally present in the

rock.

• The amount remaining or the amount of the new isotope produced by decay

can be measured.



• With this information, the age of the rock can be calculated.

Examples of "absolute dating" using radioisotopes

• 14C is used to determine the age of organic material (like wood) that is buried

in sediments.

• 40K is used to determine the age of lava flows.

strat and geological time

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