Tuesday, May 1, 2012

Angular Unconformkitty

Brian Switek's lovely Margarita takes us back to the origins of modern geology:

Angular Unconformkitty
We have three features here. Long, long ago, the vertical bits at the bottom would have been horizontal, nice sedentary sediments laid down in possibly peaceful waters. Obviously, something rather dramatic happened after they lithified, and they got pushed from horizontal to vertical - much like what happens to me when the alarm goes off in the morning. Time and possibly tide wore away whatever ended up atop the formerly-horizontal vertical bits. Then, another peaceful sea laid down a nice horizontal layer of delight. We can see some minor deformation, some of which may represent soft sediment deformation and some, especially at the left, seeming to represent presently-active tectonic forces rotating some of the horizontal stuff into a different orientation. And to the right, we seem to have a nice example of a mafic dike intruding both layers, which would tell us both sets of sediments were laid down before magma intruded. It's an angular unconformkitty James Hutton would have been proud of!

For sake of comparison, I shall now present Hutton's famous angular unconformities.

Hutton's Unconformity at Jedburgh, Scotland, illustrated by John Clerk in 1787 and photographed in 2003. Image courtesy Wikimedia Commons
And my favorite, Siccar Point:

Hutton's angular unconformity at Siccar Point where 345 million year old Devonian Old Red Sandstone overlies 425 million year old Silurian greywacke. Image courtesy Wikimedia Commons.

Of course, I can't let a mention of Siccar Point go by without linking Chris Rowan's lovely post about it. Enjoy!

3 comments:

  1. That's a beautiful unconformity - although I must point out that the upper sediment is trying very hard to conform to the shape of the lower, tilted layers. Is there much contact metamorphism at the join of the two?

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  2. Unconformities are cool, but I wish they all were unconformkittehs.

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  3. Aw, what an adorable unconformkitteh. :D

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