Geologic Time

Prof. Gorring
GEOS 112 PHYSICAL GEOLOGY
Oct. 5, 1998 Geologic Time

1. Intro
The rock record is the only means of deciphering Earth's history. Two fundamental techniques are used: (1) Relative Dating; (2) Absolute Dating

2. Relative Dating

The Stratigraphic Record: principle of horizontality; principle of superposition.

3 main problems:

rates of deposition are not constant; they are episodic.
there are "gaps" in the record (ie. non-deposition, erosion). Unconformities
can't determine relative age from widely separated outcrops. Correlation problems

*Fossils are used as time-markers; (ie. we can solve some of these problems).

Faunal Succesion; correlation.

Unconformities (represents significant gap in time; also implies tectonic uplift and erosion). Angular unconformities.
Cross-Cutting Relationships: igneous intrusions and faults must be younger than the youngest rocks that they cut.

3. Geologic Time Scale

Constructed using the 3 Relative Dating techniques above. Time intervals are based on distinct sets of rock and fossil types found around the world.
Divided into four major time units:

Eon: Archean, Proterozoic, Phanerozoic (largest time division; based on major physical and biological changes on Earth).
Era: Paleozoic, Mesozoic, Cenozoic (major evolutionary changes, extinctions).
Period: (fossil assemblages; names based on geography or rock characteristics).
Epoch: (smallest time divisions of the Cenozoic Era).

Numerical boundaries have been determined by absolute dating techniques

4. Absolute Dating and Calibration of the Geologic Time Scale

Radiometric Dating: radioactive decay: (Unstable nucleus gives of particles; e- or 4He); concept of half-life: (avg. decay rate is fixed, basis for clock).
Different Radiometric Clocks: ⁸⁷Rb, ²³⁸U, ⁴⁰K, ¹⁴C. They all have different half-lives and thus, have various applications depending on how old the material is.
Limitations of radiometric dating (limited to igneous and metamorphic rocks; concept of a "closed-system clock").
Calibrating the Geologic Time Scale:

Look for igneous rock interlayered with fossil-rich sed rocks that are very close to the boundary between time periods. Use recent modifications to the base of the Cambrian as a good example.