27 February 2015
Back at the beginning of January, I asked for help on Twitter for Friday fold fodder. Here are three responses I got:
Thanks for sharing, gentlemen!
Happy Friday to all.
24 February 2015
My student Robin has been busy cranking out great new macro GigaPans. Check out a few of these new examples:
17 February 2015
Today is the 10-year-anniversary of the day I adopted Lola the cat. She’s been a faithful companion for a quarter of my life!
Here’s the day it happened, as recorded in my 2005 calendar:
Look at this historical document – Titanic opens; Malcolm Gladwell giving a talk; I was still doing woodcut block printing – and I was teaching structural geology at GMU then, too. Earlier in the same week, I had a note to check on application procedures for NOVA – applying for the job that I have now!
The little beastie herself:
Physically, she looks pretty much the same; a bit heavier, perhaps. Lola has slowed down in recent years – can’t jump as high, shows no interest in playing with cat toys, etc.
It’s the way of the world; I reckon – a symptom of passing time. Thanks for ten great years, Lolie!
13 February 2015
Pygmatic folds in the Precambrian Irving Formation. I think this is 1.7 Ga deformation, late in the Yavapai orogeny, which added various arcs in Colorado to North America. Good place to think about strain ellipses in progressive deformation.
Zooming in on the best part, and dialing up the contrast a bit:
Seriously strained rocks. What fun!
10 February 2015
Today, let’s harken back 11 months, to when a dozen students and I flew to El Paso for a week of geologizing with Joshua Villalobos and a dozen of his El Paso Community College students.
We visited the Permian Basin of West Texas and southern New Mexico. Here’s a diagram of the situation that existed here during the Permian: it was a high-bathymetric-relief reef system. There was a reef core of sessile, skeletal organisms like corals and brachiopods, and then a deeper forereef basin which received periodic submarine landslides.
What I propose to focus on today, however, is the backreef – that shallow lagoonal environment which saw high rates of evaporation. There, the seawater crystallized, precipitating out CaCO3 that coated any little nugget to make really large free-rolling concretions called pisolites. They’re basically enormous ooids:
(Villalobos thumb for scale)
These structures are part of the backreef Tansil Formation.
This one had a little mite on it for scale:
We explored these wonders in detail at a commercial campground just outside of White’s City, New Mexico, not far from the boundary of Carlsbad Caverns National Park. Here are two macro GigaPan of samples I brought back home from there:
If you explore those images, you’ll find ample evidence that the pisolites accumulated layer after layer, then were broken into chunks, then the chunks accumulated layer after layer, and then some of those were broken again, and recoated (with layer after layer) again. Historical geology writ small!
Students in front of the ridge where the pisolites can be found as float:
We did visit the park itself, too…
Descending into the maw…
There are some fine speleothems inside, as Tansil Formation calcite is removed from location A and redeposited by groundwater at location B:
I won’t be running a spring break field course this year, and while I’m looking forward to using the break as a break (i.e., rest), I’ll certainly miss having adventures and making discoveries of stuff like we saw at Carlsbad.
6 February 2015
Kim Hannula shares a fold today:
The rocks folded here mostly the Devonian Ouray Limestone. There’s a fault through the outcrop, and another fault to the left of the photo. Regionally, the faults are mapped as normal faults, mostly with the east (right in photo) side down. Locally, that’s not what I see in this outcrop, which makes this a funky place to look at a fold with class…
Thanks for sharing, Kim! Happy Friday, all!
27 January 2015
Today, It’s going to get a bit coarser.
There’s a nice roadcut in Miette gritstone (coarse sandstone and mudrock with rounded pebble conglomerate) on the Trans-Canada Highway just east of the British Columbia / Alberta border. Aaron Barth and I GigaPanned it last summer:
Here’s the resulting image:
As you explore it, you will get plenty of good looks at the “grit”:
There is also a nice example or two of graded bedding, a sedimentary structure with a gradational change in grain size, bottom to top through a bed:
Elsewhere, you can see two other striking features:
- mudchip rip-up clasts (dark, angular chunks), and
- arcuate-bottomed contacts between coarser upper layers and finer lower layers:
Let’s consider the mudchips first:
Mud is made out of small particles, mainly clay. As such, it tends to remain in suspension in the water column if the water is agitated (energetic). It only gets deposited if the water is really calm. As such, these mudchips tell us about calm-water deposition.
But they are not mud layers, they are chunks of mud, and they are angular, and they are surrounded by “grit”…
Clay is more-self-cohesive than sand (dominated by quartz, say). When a pre-existing deposit of mud gets blasted by a powerful current (one capable of transporting pebbles, for instance), the mud rips up in flakey chunks. These pliable chunks then become sedimentary particles in their own right, tumbled along with the new stuff (whatever this interloper current dragged in).
When the current wanes in energy, the mudchips and pebbles drop out together, a mixed deposit:
Next, consider how the arcuate (concave-up) shape of the cross-bedding is mimicked along the contact between lower (earlier) finer-grained strata and upper (younger) coarser-grained strata, like right here:
I interpret that as evidence of scouring in the depositional environment. As with the mudchip rip-ups, the story is one of changing water current strength over time. At one point, a moderate current deposited finer sand, and then the energy of the water got jacked up (a storm, an avulsing river) and the water switched from depositing to eroding these easy-to-move smaller grains. As the current energy dropped again, the bigger stuff it was carrying filled in the hole that had been eroded in the older deposit.
In other words, these are channel forms, trough-like scours into the pre-existing sedimentary deposits, then filled with the detritus that the scouring current carried in.
An animated GIF can impart some perspective on how this arcuate contact weathers out in three dimensions:
A closer look at the bottom of the incision:
There is also evidence in the outcrop of directional currents in the depositional environment. Here are several nice examples of cross-bedding:
In this case, the water currents were moving from the right side of the photo toward the left, depositing sand in the slightly-less-energetic waters downstream of a migrating ripple (bedform).
Lastly, the Miette shows in some places the presence of “outsized clasts,” really big chunks mixed in with the relatively fine “grit.” Here’s an example, from about two miles east of the site above, right near the start of the Icefields Parkway:
It’s hard to tell a story of waxing and waning currents of water when something like that is staring you in the face.
These outsized clasts (boulders, essentially) are so much bigger than the rest of the deposit that it’s hard to imagine how they could co-exist without multiple factors influencing the deposition. They could potentially be interpreted as “dropstones,” which are iceberg-rafted chunks. Glacial ice is capable of supporting much larger particles than liquid water, and when a glacier calves, its icebergs float away with an internal sedimentary load that could include big boulders. When the iceberg melts, the boulder drops into the (water-deposited) finer sediment below. Because of its different mode of transport, it fails to “match” the character of the local sedimentary deposit.
These putative dropstones could be evidence of the Snowball Earth glaciations, which froze a significant portion of Earth’s surface during the Neoproterozoic.
23 January 2015
Elizabeth Kosters contributed this week’s Friday fold:
It’s from Rainy Cove, Nova Scotia, Canada. Click through to read Elizabeth’s post on the site.
22 January 2015
One last post from my September trip to Greece. Here’s a look north along the inside wall of the central caldera of Santorini, taken from the deck of the Santos Winery.
It’s not hard to imagine the volcanic edifice that filled the space to the left (west) prior to The Big One.