Angular Unconformity: Definition & Formation

Instructor: Amy Lange

Amy has taught university-level earth science courses and has a PhD in Geology.

Angular unconformities are one of the types of unconformities, or missing strata, in the geologic record. In this lesson we will learn how angular unconformities are produced and how to spot them.

Unconformities

If you've ever been to the Grand Canyon, you'll know it's not just famous because it's a giant hole in the ground, but because you can see evidence of millions of years of rock-forming processes in one location. When you walk down into the Grand Canyon, you are walking back into time; the deeper the rock level, the earlier it was formed. Geologic time is considered continuous. However, there are certain events that can cause disruptions in the rock-forming processes, meaning we won't have a geologic record of a certain time period. These breaks in the geologic record are known as unconformities. In this lesson we will examine one specific type of unconformity - the angular unconformity.

The Grand Canyon features many visible rock layers.
Grand Canyon

Angular Unconformities

Unless you knew to look for them, you likely wouldn't know that there are 14 different unconformities in the exposed rock sequences at the Grand Canyon, including one angular unconformity. At an angular unconformity, the older rock layers are deformed, tilted and usually slightly eroded before the deposition of a new rock layer.

You can see that the rocks at the bottom of this image are tilted, while the rocks overlying them are flat. This is an angular unconformity within the Grand Canyon.
Angular Unconformity in the Grand Canyon

According to the principle of original horizontality, sediments are deposited flatly across the landscape due to the force of gravity. Thus, sedimentary rocks are horizontal unless they've been tilted and deformed due to a mountain-building, or tectonic, force.

Therefore, when you observe an angular unconformity, you're actually observing several distinct geologic events. First, original sediments were deposited in a horizontal manner and became compressed to form sedimentary rock. Then tectonic forces deformed the rock, causing it to tilt, and erosion weathered the rock. Finally, a second period of sediment deposition occurred on top of the tilted layer, which also turned into rock, causing the angular unconformity. In order for us to observe an unconformity, the layers must be exhumed. We can observe the unconformity in the Grand Canyon Supergroup because the Colorado river has carved into the earth there.

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