3 April 2014

Cribratina, an index fossil for the Albian age

These four slabs, collected at “Fossil Hill,” north of the Cristo Rey laccolith at the Chihuahua (Mexico) / Texas / New Mexico triple point, bear positively-weathering fossils of the benthic foraminiferid called Cribratina, an index fossil for the Albian age / stage:

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My field notebook serves as an imperfect sense of scale.

Here’s the Albian in the context of the middle of the Cretaceous period of geologic time, from the International Commission on Stratigraphy’s geologic time scale:
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Zooming in closer, you can see they are elongate, with multiple “bubble”-like body chambers:

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I plan to image these specimens with our macro GigaPan rig soon, so that you can explore them on your own.

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2 April 2014

The wonder of cracking open a xenobomb

On “Border to Beltway”‘s visit to Kilbourne Hole, after we whet our appetite with Hunt’s Hole, Michael finds a xenobomb. Ernie and Boris look on with envy:

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A “xenobomb” is a xenolith (in this case, of mantle peridotite), slathered in a coating of lava and tossed out of a volcano in the middle of a liquid droplet (a bomb). Here’s what they look like in cross-section:

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You can experience some of the wonder of these extraordinary structures by exploring this macro GigaPan…
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1 April 2014

Lost in My Own Backyard, by Tim Cahill

cahlostOn Saturday of this past weekend, I led a field trip to Sideling Hill and Paw Paw Tunnel, and on the (1.5+.75+1.5=) 3.75 total hours of driving, I listened to the audio book version of Tim Cahill’s short book on traveling around in Yellowstone National Park. It’s a fun little book, but I wished it was longer. I was able to consume the whole book in a single day’s driving.

Cahill is terrific. I love his writing. I first read his book Jaguars Ripped My Flesh when I was in my senior year of high school, and I’ve read at least a half dozen of his other adventure travelogues since then. He has a terrific self-deprecating, humorous style that is a pleasure to read, or listen to. Cahill has built himself a funny little niche: the “every man” adventure traveler. He does the most amazing, hard-core adventurous stuff, but the angle is nearly always “I just wanted to see if I could pull it off, since I’m just like you. Son of a gun, it turns out we’re just as capable of thrilling adventure as those tough guys.” His humor pops up at least once a paragraph, and it really resonates with me – there’s a great balance between an economy of words, a bit of hyperbole, and then a legitimate sounding distraction before he unleashes a zinger of a punchline.

This volume is exclusively about Yellowstone, which is a departure from the majority of his books, which are compilations of various essays and articles that Cahill has penned for Outside, Esquire, Rolling Stone, and National Geographic Traveler. I liked having a common theme for the whole book – it works well, and I hope Cahill writes more volumes like this in the future.

His subject, of course, is wondrous. Yellowstone is one of the most amazing places I’ve ever been to, and it’s not that I actually learned anything from “reading” this book, but it was a lovely little reminiscence that brought me back to this special spot on the Earth. Highly recommended.

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31 March 2014

Hunt’s Hole: Climbing ripples in volcanic surge deposits

Besides Aden Crater, another volcano we visited in the Potrillo Volcanic Field was Hunt’s Hole, a moderate-sized maar crater south of the more famous Kilbourne Hole.

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Here’s our “Border to Beltway” crew on the lip of the crater:

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In the distance, you can see a cap of basalt (which acted as the “lid” on this hydrothermal “pressure cooker”). In the foreground, you can see patterns resulting from the erosion of the pyroclastic surge deposits from the eruption.

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On the far side of a little “cove” in the crater, I saw a lovely exposure of climbing ripples. The next 3 shots zoom progressively in on this primary volcanic/sedimentary structure:

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Josh and Jonathan make field observations on the pyroclastics:

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This was just a taster volcano for the day…

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…our next stop was Kilbourne Hole.

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28 March 2014

Friday fauxld: the “Not a Box” at Confusion Hill

It’s Friday… that means it’s time for a fold. Let’s try this outcrop at “Confusion Hill” in the Franklin Mountains of West Texas:

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See it? Let’s zoom in…

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From the shadows to the left of the hammer, trace out the dark green layer… it may make you think of a box fold:

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…but it’s not. This is actually a completely planar sedimentary bed, tilted to more or less vertical, and then cut by three differently-oriented outcrop planes:

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The intersection of each (differently-oriented) outcrop plane with the (uniform) plane of the dark green bed produces a line of “apparent dip” – these apparent dips, when viewed en masse, produce an impression of a folded layer – a totally false impression.

Beware faux folds!

(And Happy Friday!)

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27 March 2014

Aden Crater

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That’s Aden Crater, a Pleistocene shield volcano in southern New Mexico.

Here’s what it looks like from above (Google Maps view):

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I also noted the position of two nearby maar craters: Kilbourne and Hunt Holes.

When you climb up to the edge of Aden and look in, you see the congealed and fractured remnants of a lava lake that once filled this to the brim (and indeed, spilled over the edges many times, as you can see in the satellite view above):

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Hiking across this crust isn’t as profound as hiking across Kilauea Iki, but it’s super neat to get to the other side and see a spatter rim with awesome features.

Here, Mando (EPCC) points at the accumulated blips and bombs that make up the spatter rim.

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An individual spatter bomblet looks like this:

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And here’s what a few dozen of them look like, all welded together:

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As I told the students then, this is a strong visual echo of a Mongolian latrine in mid-winter!

Nearby, there were super cool lava “slicks”:

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Ernie stands atop (the volcanic vent would have been behind him):

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And down in the foreground, you can see the big bomb/blob that slid down and made these striations:

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A little mosey further, and we came to a fumarole vent which became the tomb of an unwitting giant ground sloth.

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The skeleton is long gone, so here’s some students waving from the fumarole lip instead:

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Looking away to the east, we could see the valley created when a lava tube collapsed:

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For many of the NOVA students, this was the first time ever visiting a real volcano. They were psyched!

Rob (EPCC faculty), Sergio (EPCC), Josh (NOVA), Mercer (NOVA), Adriana (EPCC), and Denise (EPCC) enjoy the warm sunlight on the cool lava:

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What a cool spot. Really worth the time to visit if you find yourself in the empty reaches of southern New Mexico.

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26 March 2014

The Great Unconformity in the Franklin Mountains

Good morning! Let’s take a walk up the east side of the Franklin Mountains, north of El Paso, Texas, to walk across the Great Unconformity.

The basement rock exposed here is the Red Bluff Granite, a 1.1 Ga felsic magma that intruded the columnar basalts of the Mundy “Breccia” and the Castner Marble. (It is unknown what substrate the Castner Marble was deposited upon.) This is what the Red Bluff looks like from the parking lot of the Wyler Aerial Tramway, which we rode to the crest of the Franklins on a very cold and windy morning.

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Typical high-potassium granite. Now let’s climb up the trail…

It’s hard to spot where you leave the Red Bluff and enter the overlying conglomerate, which is made of Red Bluff cobbles and pebbles. It’s called the Coronado Hills Formation, and this is what it looks like:

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This isn’t a granite. It’s a conglomerate. Yes, I know: the cobbles are really hard to spot. Let me outline a few that I spotted, and then you can go back to the unannotated original photo and see if you can spot any others.

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Atop this is the Bliss Formation, a Cambrian quartz arenite (quartz sandstone). It has Skolithos trace fossils in it, just like Virginia’s Antietam Formation (which is of the same age):

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Though these aren’t the most impressive Skolithos I’ve ever seen, they are at least diagnostic of vascularized body plans, and therefore of the Cambrian period of geologic time.

Better yet, or at least more impressively expressed, are the cross-beds:

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Notice how in one bed, the cross beds tilt to the left, and in the next, they tilt to the right. This is indicative of currents that shift direction. Sometimes, it’s dubbed “herringbone” cross-stratification.

Here’s another lovely exposure showing these primary sedimentary structures:

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Beds highlighted in black, cross-bedding in white, and the interpreted current flow direction shown with red arrows:

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A great little hike – gaining 500 million years in far less than 500 vertical feet.

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25 March 2014

Root wedging: examples from Maryland and Texas

While I’m showing photos from last week’s Billy Goat Trail field trips (3 in total), let me share a striking example of root wedging from Olmstead Island, on the walkway out to see Great Falls:

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And, since I meant to get back to blogging about west Texas this week, here’s another example of the same process, seen with a different kind of tree in different sorts of rocks, in the Guadalupe Mountains of west Texas:

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Both examples are in national parks – another commonality!

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24 March 2014

Monday migmatite

Here’s a sweet little sample of migmatite (~470 Ma late Ordovician Taconian Orogeny, U/Pb date from zircon), that my students and I spotted last week on the Billy Goat Trail, downstream of Great Falls in Maryland’s metamorphic Piedmont province:

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Note the white translucent quartz, the orangey (partially kaolinitized and rusty stained) opaque potassium feldspar, and the shreds of biotite torn and tangled between them:

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I love the fact that I have such easy access to gorgeous migmatite here in my local stomping grounds. It’s a profoundly satisfying experience to watch the comprehension of “birth of a granite” dawn on my students’ faces when we visit these terrific rocks.

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21 March 2014

Friday fold: a random sample from the campus of UTEP

I know nothing about this sample, other than the fact that it was a thrown-away sample found in the rock pile next to the old geology building at the University of Texas at El Paso.

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Happy Friday!

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