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EvC Forum: The Grand Canyon Paradox -->
EvC Forum: The Grand Canyon Paradox:
Iceage writes:
What is always ignored is the simple fact that the bottom layer is often metamorphosed. The A/U at the bottom of Grand Canyon is high grade metamorphic rock - requiring high temperature/pressure/time. The temperature of that massive formation would have to be several hundred degrees!.
I took exception to the above statement (in the Admin mode) but I considered further discussion of it there to bee off-topic.
What is always ignored is the simple fact that the bottom layer is often metamorphosed.
The lower rock of an angular unconformity may or may not be metamorphosed. But if it were to be high grade metamorphosed it would no longer be a bedded sediment. By definition, an unconformity with the lower rocks being high grade metamorphics or intrusives would be a nonconformity.
The A/U at the bottom of Grand Canyon is high grade metamorphic rock
There are two unconformities at the bottom of the Grand Canyon. The lowest one indeed is over high grade metamorphics, and is a nonconformity. The higher (the Great Unconformity) is an angular unconformity. The lower ("angular") rocks of that are at most low grade metamorphics, and I suspect, are unmetamorphosed.
See
here for a cross-section.
Moose