A recently developed field area, designated “ND112”, exhibits a fascinating assemblage of geologic environments reflected in a myriad of deskcrops. We have identified 14 unique geologic horizons occurring over an area of 4.5 by 6 meters. In this report we briefly describe, map, and give preliminary interpretations on the history of the ND112 region. Relative ages of deskcrops are based on historical records and oral histories of the indigenous peoples of the area.
ND112 is a geographically isolated region, only accessible via a single restricted access route. No other environments are visible from the confines of ND112, which has resulted in a significant loss of inhabitants (50%) in recent months.The population reduction has allowed the remaining locals, or Grads as they call themselves, to flourish and has the added bonus of making deskcrop access easier. However, we learned in the course of our investigation that mass wasting during the exodus caused the loss of several deskcrops in the SW, including coal, olivine sand, vesicular basalt, slag, and cobbles of quartz sandstone and agate. The loss of these invaluable deskcrops severely limits our investigation of ND112’s southwest.
What rolls down stairs, alone or in pairs? That’s right kids, it’s Log! This half-meter diameter log was buried some 200 Myr ago and its organic content was replaced by silica (a process called permineralization). One thing I’ve always wondered: If we split wood parallel to the long axis, why is it so common for petrified wood to break along such nice planes perpendicular to the grain (as it is here)?
Enjoy the rest of the great photos that are sure to come on the last day of Geology Photo Week! Visit the Georneys post that started it all, or view my previous photos from this week (click thumbnails to view post + description):
Today’s contribution to the geology photo week geomeme is Antelope Island. This shot was taken on approach into Salt Lake City. Antelope Island is one of the ranges that makes up the Basin and Range Province of the western United States. The island is particularly photogenic as it is surrounded by the Great Salt Lake, which makes for fantastic plays of contrasting colors. Turns out it’s a state park as well!
Round three of this week’s geomeme of a picture-a-day (previous entries here and here) features halite, a.k.a. rock salt! Pink (or pinky orange) halite, to be more specific. The mine, run by American Rock Salt, is the largest operating salt mine in the United States (3+ million tons annual production). This pink halite is located in the mine wall ~1,400 feet underground, where it remains minus the football-sized chunk we removed for display in our department collection!
Continuing the geology photo week started with yesterday’s dikes and pillows, today’s picture is from the borough of Centralia, PA, where an underground coal fire has been burning since 1962. The heat and volume change caused roads to crack and warp and left the ground unstable in some areas. As a result, the borough has been mostly deserted for almost twenty years and the 2010 census reported 10 residents. And that smoke isn’t just for show. The pavement was hot to the touch, and air escaping from cracks was too hot to hold your hand over for more than a second or two.
Evelyn at Georneys initiated a geology photo week, which gives me the opportunity to share a photo for which I didn’t yet have a place. Ophiolites are oceanic crustal sequences that have been obducted and raised above sea level. To paint with a broad brush, the sequence (from bottom) is one of ultramafic rock, gabbro intrusions, sheeted dykes, pillow lavas, and sometimes topped by pelagic sediment. The Troodos Ophiolite on Cyprus is a fantastic example because it was sort of smeared over the island and at least a little bit of everything can be found at/near the surface. Here we see the sheeted dykes and pillow basalts at a grand scale!
This post is based on field notes and memories supplemented by background reading material from 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.
Twenty-two of us are spread out between three Chevy Suburbans, and it’s strange having legroom on a geology expedition. Not that there’s far to go. We are camped out in an RV park a mere 5.5 miles from our field site: Barringer Meteorite Crater. This is the first day of the first ever Meteor Crater Field Camp, and we are making the first trip first thing in the morning to my first visit to any crater, ever. Everyone’s ready to get started, and we don’t have long to wait. Our first stop is approximately fifteen feet outside the gates of our campsite, and we step out of the vehicles after realizing the stop wasn’t because of a forgotten water bottle or notebook.
At 5.5 miles out there’s not much of the crater to see and so we huddle around Dr. Kring, our fearless leader. He asks us to imagine standing at this exact spot 49,000 years ago* as Meteor Crater formed. “What would you see?” he asks us. Slowly we find our voices: A streak of light and a flash. A fireball. Wind whipping across the plains. A massive volume of ejected material rushing toward you. Then nothing because you are dead.
We learn we wouldn’t even last that long. At this distance the shock wave would reach you in less than a second, turning you inside out before the wind and heat simultaneously flash-cooked your body as it tumbled away in pieces with the desert sage. Oh well. Someone luckier and further away would have seen quite the show.
49,000 years later, as we pile back into the vans, it’s mostly sunny and warming past 50°F on the high desert plains of Arizona.
[*On the basis of thermoluminescence, 26Al, 10Be, and 36Cl studies, it’s generally agreed that Meteor Crater formed 48- to 49-thousand years ago. More recently, updated 36Cl reference material argues for an older age of 60- to 65-thousand years.]
On the docket for our first day at the crater is a 3.7 kilometer (2.3 mile) hike along the rim, ~1.8 km clockwise before lunch, then retracing our steps counterclockwise. There’s House Rock (a.k.a. Monument Rock), the largest (intact) boulder on the crater rim at ~10 meters tall. Pile on two more House Rocks and you’d have a reasonable estimate of the meteorite diameter. And maybe at one time there really was another House Rock or two on top. Surface exposure age dates from the top of House Rock are younger than the 49,000 year formation age.
It wouldn’t be surprising if House Rock had initially been covered by ejecta. Erosion and weathering is evident everywhere at the crater. A gully running downslope beneath the crater museum rapidly cut through three layers of authigenic breccia, both exposing and halfway eroding a projectile fragment over a two year period. The breccias are so friable that when we later head into the crater, we aren’t allowed to touch or even go near the left side of the gully where the projectile is exposed.
On our CCW hike back we generate a little erosion of our own by scrambling down the crater wall a bit from House Rock to see something (else) awesome. We start out on Kaibab on the crater rim and walk down through Moenkopi, and end up back at Kaibab. The top slice of bread in the Kaibab-Moenkopi-Kaibab sandwich is ejected, overturned Kaibab. The bottom slice is in-situ Kaibab. And the meaty Moenkopi center is where it gets interesting. Look at the photo below. Both members of the Moenkopi are visible here. The pale reddish brown Wupatki (at left) is a massive sandstone underlying the dark, reddish brown fissile siltstone called the Moqui. The surrounding pale/tan blocks are dominantly Kaibab (limestone/dolostone), though most shown here are loose boulders.
Originally horizontal beds were uplifted during impact and now dip away from the crater. In the center of the above image, the Moqui member – the surface unit at the time – takes a sharp turn and winds up parallel to the crater wall. This is an exposed portion of a fold hinge, where a flap of target material was overturned to create an inverted stratigraphic sequence (see diagram).
There’s so much more to see at the crater. Sand fields, tear faults, shocked quartz…and we’re still just getting started!