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USGS CMG InfoBank: Angular Unconformity

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Comment: 18:26 - 19:30 (01:04)
Source: Annenberg/CPB Resources - Earth Revealed - 8. Earth's Structures
Keywords: "Dee Trent", "angular unconformity", "geologic structure", unconformity, erosion, uplift, "tilted bed", "sea level change", "tectonic activity", deposition, "Principle of Original Horizontality", sandstone, "sea level change", erosion, "sedimentary rock", contact

Our transcription: The classification of unconformities is less complex than that of folds and faults.
Erosion rather than stress causes them to form.
There's three kinds of unconformities, three major kinds of unconformities.
The most readily recognized is called a angular unconformity.
Where you have sedimentary rocks and those beneath the unconformity have been tilted at an angle, they have been eroded, and then they have been overlain by horizontal rocks.
The implication is, then, that the sedimentary rocks, which were originally deposited horizontally in accordance with the Law of Original Horizontality have been deformed by structural forces, tectonics, uplift, and then erosion has taken over, truncated and cut off the edges.
Later, the seas return and new layers were deposited on top.
So the implications of angular unconformity is that there has been a great time lapse between the original deposition and the subsequent deposition.
So it tells us a lot of history has taken place, and much of it's missing because of the erosion.

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