From Lajitas to the Cuevas Amarillas Locale, Part 7: Appreciating the Santana Mesa's Peralkaline Portion | Big Bend Ranch State Park, Texas, USA

Taken along FM 170, just north of the Rio Grande, at the entrance to La Cuesta Campground. Facing northward.

Note that this is the same spot from which I took one of my all-time favorite eye-candy scenic shots, of the Rio Grande wending its way through the Santana Bolson.

In this region, one of the most impressive landforms on the American side of the Rio Grande is the lofty Santana Mesa. It rises over the road like a gigantic. long-hulled warship as one approaches from the east. It’s built not of steel and armor plate, of course, but of stacked units of lava and pyroclastic material. These offer vivid testimony of the intense and immense volcanic activity that occurred here in the Eocene and Oligocene epochs.

By the time I’d reached this spot along FM 170, I was athwart the middle section of the mesa’s southern flank. I remember looking up at this massive pile of rock and photographing it so I could later better understand what its various components are.

And so now, with Christopher D. Henry's Geologic Map of Big Bend Ranch State Park in hand, I can delineate the sequence, from bottom to top:

- The low slope closest to the road is Quaternary landslide material. No surprise there.

- The more-or-less vertical cliffs in the middle are the rhyolite lava section of the Fresno Formation’s Campo Javelina Basalt Member. The rhyolite has been dated to 30.85 Ma, which makes it Lower Oligocene in age. Note that It exhibits a good deal of columnar jointing.

- Much higher up is the mesa’s capping unit, the Santana Tuff. Radiometrically pegged at 27.8 Ma, it is Middle Oligocene.

One interesting tidbit about the rhyolite is that it’s described as peralkaline. That means it’s unusually deficient in aluminum. This lack is compensated for by an abundance of potassium and sodium. Apparently, this geochemical makeup results in this variety’s lava having a lower viscosity than that of most rhyolites, which is typically gloppy, doughlike, and more conducive to explosive eruptions.

To see the other photos and descriptions in this series, visit my From Lajitas to the Cuevas Amarillas Locale album.

Taken in: Mexico / Chihuahua / El Sauz (show map)

Taken on: March 12, 2002

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Camera: MAKER NAME 96660, ISO 50 (more info) (hide info)

Camera:	MAKER NAME 96660
ISO:	50
Flash:	Auto, Did not fire
Exposure Program:	Program AE
Metering Mode:	Average
White Balance:	Auto
Color Space:	Uncalibrated
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