SEQUENCING GEOLOGIC EVENTS

2 Questions we want to answer about a set of rocks;
1) How old are the rocks?
2) What events happened to get what we see today?

ABSOLUTE AGE DATING
Uses radioactive isotopes to get a number for an age. i.e. 50 million years old
Is accurate within a large margin of error (+- a few million years).
Doesnt tell us what happened, only how long ago

RELATIVE AGE DATING
Deals with the spatial relationship between different rock beds or features.

SEDIMENTARY ROCK SEQUENCING

Superposition

The oldest beds are on bottom, the youngest beds on top.




Horizontality

Beds are initially horizontal, unless deformed by a process such as folding or tilting.

Lateral Continuity

Beds tend to remain horizontal and superpositioned everywhere they are deposited (usually).


CROSS-CUTTING RELATIONSHIPS

Beds or linear features such as faults, igneous intrusions (dikes, etc), may cut “across” other beds or features, establishing a age relationship.




Fault F cuts “across” Bed B, but not Bed A. Therefore, F is younger than B, but older than A.

Same thing with a dike or igneous body.

Metamorphic rocks are generally younger than associated non-intrusive igneous rocks.





INCLUSIONS

Rock “pieces” incorporated into other rocks are older than the rock in which they are contained.

























UNCONFORMITIES

Whenever rocks are exposed at the surface or are intruded by igneous rocks, the boundary left behind is called an unconformity, becasue the rocks do not “conform” to each other.

There are 4 main types of unconformable boundary.

1) Unconformity - A surface of erosion parallel to bedding in sedimentary rocks.

2) Angular unconformity - A surface of erosion on rocks which were previously tilted through some event. The result is a change in the bedding angle across the boundary.

3) Nonconformity - An erosion surface bounding the edges of an igneous intrusion.

4) *Disconformity - 2 parallel-to-bedding boundaries formed along igneous sills (horizontal intrusions)