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7 April 2014

Contact between Muleros Andesite and Mesilla Valley Formation shale at Mt. Cristo Rey

There are two rock units in this photo. One is igneous, one is sedimentary. Can you find the contact between them? It’s somewhere along this dashed line… The Mesilla Valley Formation is Cretaceous shale with some sandstone. The Muleros Andesite (pretty much identical to the Campus Andesite you find at UTEP) is Eocene. Here’s a closer, more precisely-constrained, look at it: …but that one is in the shade. It’s bolder …

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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: 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 …

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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: 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: You can experience some …

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31 March 2014

Hunt’s Hole: Climbing ripples in volcanic surge deposits

Check out the immense climbing ripples preserved in surge deposits at Hunt’s Hole (a maar volcanic crater in southern New Mexico) and imagine the strong currents couples with an extraordinary amount of entrained pyroclastic material.

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27 March 2014

Aden Crater

That’s Aden Crater, a Pleistocene shield volcano in southern New Mexico. Here’s what it looks like from above (Google Maps view): 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 …

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20 December 2013

Friday fold: Another from Cristo Rey

The laccolith of Cristo Rey, at the Chihuahua / Texas / New Mexico triple point, is host to some cool geology. It’s cored by the Campus Andesite (47 Ma, Eocene) but surrounding the intrusion are a slew of sedimentary rocks, include the Turitella-bearing limestones of the Buda Formation and the shales and sandstones of the Mesilla Valley Formation. My colleague and friend Joshua Villalobos of El Paso Community College in …

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16 May 2013

GigaPanning Kilbourne Hole

Kilbourne Hole is the crater of a maar volcano in southern New Mexico, just across the state line from El Paso, Texas. I went there the weekend before last with a team from El Paso Community College, led by Joshua Villalobos. This is the place where xenobombs come from! If you go to the right area, you can find dozens of these mantle xenoliths sheathed in fine-grained basalt, like chocolate-coated …

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8 March 2013

Friday fold: Fault propagation fold from Cristo Rey

The Friday fold comes from the Texas – New Mexico – Chihuahua triple point, on the flanks of Cristo Rey mountain.

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22 February 2012

Mudcracks worth writing home about

Some sweet “columnar jointing” style mudcracks from an abandoned quarry in about the farthest west corner of Texas that you can get to, or maybe the southeasternmost corner of New Mexico. One of them, anyhow… Did you spot the imposter?

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18 February 2012

The xenobomb

Callan shows off a new sample from Texas, a peridotite xenolith launched into the air from a maar volcanic eruption, slathered in a layer of basalt. With full intent to coin a neologism, he dubs it a “xenobomb.”

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