Geology 101 - Lectures 14 and 15

Relative and Absolute Age Dating

I. Relative and Absolute Dating

A. Review: Three principles:

Most sediments are deposited as horizontal beds. (so if see folded or tilted beds, know that they had to have undergone a tilting and uplifting even after their deposition.

Within a sequence of undisturbed sedimentary rocks, layers get younger from bottom to top

A disrupted rock is older than the rock disrupting it.

B. Unconformities

An unconformity is a gap, or break, in the geologic record. The rock above an unconformity is significantly younger than the rock below it.

There are three types of unconformities:

Nonconformity - layered sedimentary rocks overlie igneous or metamorphic rocks
Disconformity - erosional surface where layers above and below are parallel to each other, but the erosional surface itself is irregular and not parallel to bedding
Angular unconformity - Younger layers overlie an erosion surface on tilted or folded layered rock.

C. Correlation

Correlate sedimentary strata from one region to another to produce a more complete representation of the entire sedimentary package (i.e., a more complete representation of geologic time) for a large area

Example (click here): Correlation of sedimentary layers from Zion National Park, Bryce Canyon National Park, and Grand Canyon National Park yield a complete stratigraphic section (sedimentary record) from the Precambrian to the Tertiary (from Tarbuck and Lutgens "Earth: An Introduction to Physical Geology").

D. Absolute Ages

Using radioactive dating methods, the absolute age of a rock can be determined.

The rate of decay of radioactive elements can be estimated. By looking at ratios of parent elements (the decaying elements) to daughter elements (the products of decay) in a rock, and using the known decay rate, an absolute age can be determined.

Class activity - decay by roll of a dice

The results of the activity are plotted up HERE Make sure to look at these results before class Friday

Since sediments can't be dated radioactively, the relationship of these sediments to igneous rocks that *can* be dated yields important information:

i.e., a diorite was radioactively dated at 87 million years old and it crosscuts a package of sediment (Sediment A), but eroded and overlain by a second package of sediment (Sediment B). This means that Sediment A must be older than 87 million years and Sediment B must be younger than 87 million years.