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Old Alluvium on Bandolier Tuff EarthCache

Hidden : 12/31/2007
Difficulty:
1 out of 5
Terrain:
1 out of 5

Size: Size:   not chosen (not chosen)

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Geocache Description:


The Sediments
When you reach the posted coordinates you will immediately notice, in the wall (roadcut) to the west, the interesting cross section of sedimentary rocks. What you are looking at is alluvium deposited on top of tuff. As of about October of 2008, the tuff is very hard to locate, due to being covered up by erosion off the face of the outcrop. (One of the things you will be required to do to log this cache is to describe the “alluvium” you see, so you may as well look up the definition now if you do not already know it…) The alluvium deposit is formally known as the “Old Alluvium" formation and the tuff (no longer visible here) is known as the upper member of the “Bandolier Tuff” formation.

Pieces of glass within the alluvium were dated by geologists to be about 1.229 (+/- 0.017) Ma (millions of years old). The tuff upon which the alluvium sits was dated to be about 1.25 Ma. So what this tells us is that the tuff was deposited first, and then was overlain within about 20 thousand years by the alluvial deposits.


In the picture to the left, the alluvium occupies the area from approximately the man's knees upward. The area below his knees is where the tuff contacts the alluvium, but can no longer be seen behind the pile of eroded alluvium.

If you look carefully at the alluvium, you will see pieces of obsidian, reworked pumice, and dacite. It is thought that the pumice within the alluvium originated from an eruption of Cerro del Medio, which is a part of the Valles Caldera complex to the west of Los Alamos.



If you could still see the tuff, you would notice it is quite different in texture and color from that of the alluvium. This deposit is thought to be from a large pyroclastic flow that was a result of the massive eruption of what is now known as Valles Caldera.

Logging the EarthCache
In order to log this cache, you must do two simple things:

1) Post a picture (with your logged find) of you and your GPS in front of the roadcut you see at the posted coordinates and,

2) In an e-mail (do NOT post in your log), describe what you see in the “alluvium” layers at the location. Note things like: size of grains, colors, layering, thickness of layers and any other cool thing you notice. Please also include the number of people in your party and please include the cache title "Alluvium" in the subject line.

A bonus question, for you overachievers: You will notice a big crack in the wall towards the south. This is a small, localized fault. Tell me what the vertical displacement distance is between the two sides of the fault and which side you think moved upwards. If you like, you may include your answer in your e-mail when you send me your observations of the alluvium.

Geology is a fun and interesting subject for me, and I enjoy sharing it with others. I hope you enjoyed this opportunity to learn more about this site and will better appreciate the geologic forces that made it possible. As always, please stay on marked trails, take only pictures and memories, and leave only footprints. Practice CITO too!

Acknowledgments
I’d like to thank the organizers of the NMGS Field Conference for bringing us to this location- Barry Kues, Shari Kelley, Jamie Gardner, Fraser Goff, Cathy Goff and Kate Ziegler.

References
Geology of the Jemez Region II (New Mexico Geological Society Fifty-eighth Annual Field Conference Guidebook, September 19-22, 2007).


Geological Society of America

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