Geologic Time and Stratigraphic Correlation

Geology 200

Geology for Environmental Scientists

Relative Time

Geologists first worked out the sequence of events recorded in the rock record using the principles of relative time:
original horizontality
original lateral continuity
superposition
fossil succession
cross-cutting and intrusive relationships
unconformities

Unconformities: They are significant in that they are indicators of missing time in the rock record.

These various principles were used to construct the Geologic Time Scale, which was done without any knowledge of the number of years involved.

Time Units of the Geologic Time Scale

Time can be separated into "pure" time and "rock" time. Rock time is divided into time stratigraphic units. Pure time is divided into geochronologic units.
Time stratigraphic units sometimes parallel formation boundaries, but often they cross formation boundaries.

Time Units

Geochronologic
Era
Period
- Epoch
- Age
Devonian Period
Time Stratigraphic
(Erathem)
System
- Series
- Stage
Devonian System

Rock Units, not a part of the Geologic Time Scale

Sedimentary rocks are divided into formations.
Formations can be divided into members.
Formations can be combined into groups.
Formation name consists of two parts:
Geographic name
Lithology or simply Formation
Examples:
Burlington Limestone
Waynesburg Sandstone
Juniata Formation (no dominant lithology)
Example of rock unit divisions:
Conemaugh Group: 2 formations
Casselman Formation
Glenshaw Fm.: several members
- Ames Member
- Harlem Coal Mbr.
- Pittsburgh Red Shale Mbr.

Absolute Time: Geochronology

Radiometric Dating: the source of the dates on the Geologic Time Scale

Radiometric Dating

Actually a simple technique.
Only two measurements are needed:
1. The parent:daughter ratio measured with a mass spectrometer.
2. The decay constant measured by a scintillometer.

Basis of the Technique

Radioactive elements "decay." Decay occurs as an element changes to another element, e.g. uranium to lead.
The parent element is radioactive, the daughter element is stable.
The decay rate is constant.

What is Radioactivity?

Radioactivity occurs when certain elements literally fall apart.
Usually protons and neutrons are emitted by the nucleus.
Sometimes an electron is emitted by the nucleus, which changes a neutron to a proton.
Sometimes an electron is captured.

What causes radioactivity?

Carbon-14 is produced by cosmic ray bombardment of Nitrogen-14 in the atmosphere.
All other radioactive elements were produced by supernova explosions before our solar system formed. This is called explosive nucleosynthesis.

Common Radioactive Elements, Parents and Daughters

Carbon-14, C¹⁴ Nitrogen-14, N¹⁴
Uranium-235, U²³⁵ Lead-207, Pb²⁰⁷
Potassium-40, K⁴⁰ Argon-40, Ar⁴⁰
Uranium-238, U²³⁸ Lead-206, Pb²⁰⁶
Rubidium-87, Rb⁸⁷ Strontium-87, Sr⁸⁷

Basis of the Technique

As the parent element decays, its amount decreases while the amount of the daughter element increases. This gives us a ratio of parent:daughter elements.
The decay rate is geometric rather than linear. Unaffected by heat or pressure.

Key Term

Half-Life: the amount of time for half the atoms of a radioactive element to decay. Doesn’t matter how many atoms started, half will decay.

Half-Lives

Counting half-lives:
Half-lives: 1 2 3 4
Parent :1/2, 1/4, 1/8, 1/16, etc.
Daughter :1/2, 3/4, 7/8, 15/16, etc.
P:D Ratio: 1:1, 1:3, 1:7, 1:15

Measuring Half-Lives

Ratios of 1:3, 1:7, 1:15, etc. are for whole half lives, but any ratios can be measured; e.g. 1:4.2, or 8.6:1

The Decay Constant, l

The rate of decay is called the decay constant. It determines the half-life of a radioactive element.
The decay constant is unique for each radioactive element.
Measured with a scintillometer.

The Decay Constant, l

Some values of the decay constant:
C¹⁴: 1.21x10^-4atoms per year
U²³⁵: 9.72x10^-10atoms per year
K⁴⁰: 5.34x10^-10atoms per year

Calculating a Radiometric Date

t = ln (P+D)/P

What is the half life of Carbon-14?
t = (ln ((1+1)/1))/1.21x10^-4
t = (ln 2)/1.21x10^-4
t = 5,728 years

Some Half Lives

Carbon-14: 5,728 years
Uranium-235: 713 MY
Potassium-40: 1.3 BY
Uranium-238: 4.5 BY
Rubidium-87: 48.8 BY

Setting the Radiometric Clock

When an igneous melt crystallizes, parent and daughter elements are chemically separated into different crystals.
Further radioactive decay keeps the parent and daughter elements in the same crystal.
Individual crystals of the same mineral are dated to give the age of crystallization or cooling. Examples include zircon, muscovite, and biotite.
Note that whole rock analysis would not give the age of cooling.
Carbon-14 is different in that it occurs in organic remains rather than in rocks.
Clock is set when an organism dies.
Carbon-14 is absorbed by all living organisms from the atmosphere or the food they eat.
Useful for about 10 half lives, or only about 57,000 years.

Calibrating the Geologic Time Scale

Radiometric dates from igneous rocks can be used to indirectly date sedimentary rocks and their fossils. Principles such as superposition and cross-cutting relationships come into play.
Thousands of radiometric dates have been obtained.

Age of the Earth: 4.6 BY

The oldest rocks found on earth are 4.0 BY from NW Canada.
4.3 BY detrital zircons have been found in younger sandstones in Australia.
Meteorites and moon rocks are 4.6 BY.
Rocks older than 4.0 BY on earth have apparently been destroyed by weathering and plate tectonics.

Stratigraphic Correlation

Correlation

Determination of the equivalence of bodies of rock at different locations. There are two kinds of correlation:
Lithostratigraphic - matching up continuous formations.
Chronostratigraphic - matching up rocks of the same age. Usually done with fossils using biostratigraphy.

Correlation

Over short distances lithostratigraphic correlation is the same as chronostratigraphic correlation.
Over medium distances they are not the same.
Over long distances only chronostratigraphic correlation can be used.