Relative Dating of Strata. Relative Dating Determining relative ages of rocks or strata compared to another rock or strata. Can say which layer is older.

Relative Dating of Strata

Relative Dating

• Determining relative ages of rocks or strata compared to another rock or strata.

• Can say which layer is older and younger but can’t say absolute age.

• For example, Rock Layer A is older than rock Layer B.

• Cannot say that Rock layer A is 32 million years old and rock layer B is 30 million years old.

Law of Superposition

• Each bed in a sequence of sedimentary rocks (or layered volcanic rocks) is younger than the bed below it and older than the bed above it.

• Based on two assumptions:

• 1.) Principle of Original Horizontality

• 2.) The beds of strata are not overturned

That which cuts through is younger than the Object that is cut

dike cuts through

granite is cut

Cross Cutting Relationships

What happened first in the picture onthe right? Second? Third?

Principle of Faunal Succession

• States that fossils aren’t randomly placed in strata, but rather follow a succession where they succeed each other in a definite and determinable order.

• Organisms live for a certain amount of time and then go extinct, so we can use them to tell us age of rocks.

• Index Fossils: A good index fossil is is widespread but short-lived.

• Certain Trilobites are good index fossils.

Fossil Correlation

• Fossils can be used to correlate strata hundreds of miles apart.

• If an index fossil is found in two different rock layers, we know they must be the same age.

• We can then correlate or connect them together.

Relative Ages of Lava Flows and Sills

Principle of Inclusions• Inclusions (one rock type contained in another rock type) are

older than the rock they are embedded in. That is, the younger rock contains the inclusions

Principle of Inclusions

Example: Storm beds can include rip-up clasts.

Which is younger in each case?

Unconformities

• Gaps in the geologic record, called unconformities, are common where deposition stopped and erosion removed the previously deposited material.

• Drawn as wavy lines.

3 Types of Unconformities

• Angular Unconformity

• Disconformity

• Nonconformity

Angular Unconformity

• Where tilted beds have been eroded and horizontal beds have been deposited on top.

• Gap in time between tilting of beds and deposition of horizontal beds.

Disconformity

• Erosion surface (gap in time) between two horizontal beds of sedimentary rocks.

• For example:

• Limestone is deposited

• Limestone is eroded

• Sandstone deposited on top of Limestone.

Nonconformity

• Unconformities (gap in time) that separate igneous or metamorphic rocks from overlying sedimentary rocks.

• They usually indicate that a long period of erosion occurred prior to deposition of the sediments.

• Example: Sandstone overlying granite.