Six days in the crater, day two

Day 1 | Day 2 | Day 3 | Day 4 | Day 5 | Day 6

This post is part of a slowly unfolding saga of my experience at the Meteor Crater Field Camp that was held from October 17-23, 2010. The field camp was run under the NASA Lunar Science Institute and headed by Dr. David Kring of the Lunar and Planetary Institute.

Everyone is surprisingly awake at 7AM, considering the hard sun yesterday. Maybe it’s the brisk 50°F October air, or perhaps everyone had a long sleep (one pro of a dry campsite). My hunch, however, is on the catered breakfast of fruit, eggs, coffee, juice, oatmeal, and scones that awaits us. A local Flagstaff catering company will be bringing us breakfast and dinner each day, and no one wants to miss out after our first taste yesterday. Even without the extra incentive, the 21 other field camp attendees are highly motivated, intelligent and capable researchers from around the world. And then there’s me, just writing accidental haiku in my field notes…

base of mining slope;
two tear faults up wall expose
best Coconino.

The Coconino is an eolian quartz sandstone; white, fine-grained, occasionally massive but often with cross-beds. If you’ve never heard of it, perhaps you’ve seen the Coconino cliffs of the Grand Canyon or Zion National Park. At Meteor Crater, the Coconino is the lowest unit excavated by impact, extending from 90-300 meters below the surface. The crater center is buried under ~100 meters of lake sediment, and a mine shaft is the only portal to the original crater floor. Remnant mine talus piles on the crater floor hint at the intense shock buried 100 meters below, where some sandstone was altered to vesiculated glass; It floats! The major occurrences of Coconino ejecta still present around the rim are generally not shocked to glass, but are no less interesting. The Coconino in the photo below is ‘fuzzy’ because it has been pulverized to rock flour, though relict bedding is preserved.

Overturned Coconino SS ‘rock flour’ with relict cross-bedding in northwest wall of Meteor Crater. This coherent ejecta block was originally 90+ meters beneath the surface and now rests ~15 m above the surrounding terrain.

Over on the south side of the crater, heterogeneous shock distribution resulted in relatively unshocked Coconino in contact with the rock flour variety. A useful reminder on the importance of context!

Into the breach

NASA used to run field training exercises (1963-1970) at Meteor Crater to prepare Apollo astronauts for the rugged terrain and crater geology they would encounter on the Moon. Forty years later we follow in their footsteps along Astronaut Trail to reach the crater floor.

Click to view large and see the group spread out along Astronaut Trail. The path curves toward left-center of image from the guy on the right.
Partial view of north wall of Meteor Crater. The Visitors Museum is visible on the rim. And hey, let’s go check out that gully over there…

In the crater wall below the Visitors Center, a gully has eroded through 3m of overlying talus to expose two friable breccia lenses. The lower of these is a 2m thick breccia of Kaibab Limestone and Coconino, formed in place (authigenic) during impact. Above the authigenic breccia lies a 1m thick Moenkopi siltstone and Coconino breccia. Scientists observe the highest shock pressures in this high energy fall-back breccia. The name says it all, really. Force from the impact brecciated and ejected rocks that subsequently fell back to the surface. In the fall-back breccia we are also treated to a rare sight: an in-situ projectile fragment!

Can you spot the projectile fragment among the breccia units? Top 3m: alluvium, up to boulder size; Middle 1m: fall-back breccia, red Moenkopi & white Coconino; Bottom 2m: authigenic breccia, v. light brown Kaibab & white Coconino. Large boulders at bottom are gully floor. My annotated version below.
Annotated version of the above image, highlighting the three breccia units and projectile fragment (circled in green)

Now it is a shale ball of oxidized, deteriorated projectile material, but until it weathers out it will remain an excellent visual aid in telling the story of the crater.

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