SLO 4 - Geologic Time

Relative Dating:

Relative dating has no actual date, but we generally tell age based on the layers and fossils. We can also tell relative age using three principles:

Principle of Original Horizontality: when layers are first put down, the are put down horizontally

Principle of Superposition: the youngest is on top while the older layer is down below

Principle of Faunal Succession: Fossils occur in specific strata, which are identifiable and in a particular order

With relative dating, we also have unconformities. There are four types of unconformities:

Nonconformity

Disconformity

Angular Unconformity

Paraconformity

(The middle image to the left shows what each of these unconformities may look like)

The lower image is from the Crystal Cove field trip on November 11th, 2022 that I attended. The photo is a great way to show the law of original horizontality as well as the law of superposition... because of the law of original horizontality, we know that these layers (circled in yellow) were originally put down horizontally, but there was a geological event that caused these layers near the surface to fold.

Near the bottom of the image (circled in blue) we can see that these layers were all placed one on top of the other, which shows the law of superposition, so because we know these layers were placed one on top of the other, we know that the lowest layer is the oldest and the layer near the top of that uniform group of rock is the youngest, for that group. We can also see that we have an angular unconformity in this portion of the rock because the layer of rock is tilted at about a 15 to 20 degree angle compared to how it was supposed to be laid down because of the law of original horizontality.

If we were to make a cross section using strike and dip of just the unconformity circled in blue, we would say the strike runs east and the dip is about 15-20 degrees downwards from horizontal from the left.