Old Alluvium on Bandolier Tuff EarthCache
Old Alluvium on Bandolier Tuff
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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).
Additional Hints
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