Exploring the Burro Mesa Locale, Part 2: The Formations Illustrated | Big Bend National Park, Texas, USA
This is the same photo that was shown in Part 1 of this series. Refer to that description to get the necessary background information on Burro Mesa.
Facing northeastward. We're just a little northeast of the Burro Spring Trailhead.
This is another of my nifty, annotated geo-educational photo couplets. Please note that my colored-line boundaries are approximate.
Here's the key to the right-hand version:
1 = Rhyolite Member of the Burro Mesa Formation (Oligocene), providing the mesa's resistant cap.
2 = Wasp Spring Member of the Burro Mesa Formation (Oligocene), consisting of tuffs, surge deposits, and lahar breccias.
3 = Younger part of the Chisos Formation, consisting of volcanically derived conglomerate (Oligocene). In these three spots it sticks out of the blanket of colluvium (unit 5).
4 = Bee Mountain Basalt Member of the Chisos Formation (Oligocene).
5 = Colluvium deposits and colluvial fans (Quaternary).
All this igneous extrusive rock reminds us that in the Tertiary period Burro Mesa was one of many eruptive centers that transformed what is now Trans-Pecos Texas into a hellscape. The region was shaped and repeatedly reshaped by spreading lava flows and billowing pyroclastic surges.
But what caused all this mayhem? Long before this time, the Farallon Plate, a slab of oceanic crust, had subducted under the West Coast of North America, but at an usually shallow angle. As it proceeded eastward, it essentially scraped along the underside of the North American plate and triggered the Laramide Orogeny (ancestral Rocky Mountains).
Then, in the late Eocene and early Oligocene, the Farallon Plate started to plunge down at a steeper angle into upper mantle. As it did so, its rock melted and began to rise. A good deal of this magma reached the surface, creating volcanic eruptions from western Mexico up to Colorado.
- - - - -
My main source of information for this post, and for the other photos in this series, is the exquisitely illustrated and detailed USGS Geologic Map of Big Bend National Park, Texas (Turner et al., 2011). And its accompanying "pamphlet"—at 84 pages, more like a full book—is a model of clearly explicated stratigraphic and structural relationships. Both are available online.
To see the other photos and descriptions in this set and my other Big Bend series, visit my my Exploring the Burro Mesa Locale album.
Exploring the Burro Mesa Locale, Part 2: The Formations Illustrated | Big Bend National Park, Texas, USA
This is the same photo that was shown in Part 1 of this series. Refer to that description to get the necessary background information on Burro Mesa.
Facing northeastward. We're just a little northeast of the Burro Spring Trailhead.
This is another of my nifty, annotated geo-educational photo couplets. Please note that my colored-line boundaries are approximate.
Here's the key to the right-hand version:
1 = Rhyolite Member of the Burro Mesa Formation (Oligocene), providing the mesa's resistant cap.
2 = Wasp Spring Member of the Burro Mesa Formation (Oligocene), consisting of tuffs, surge deposits, and lahar breccias.
3 = Younger part of the Chisos Formation, consisting of volcanically derived conglomerate (Oligocene). In these three spots it sticks out of the blanket of colluvium (unit 5).
4 = Bee Mountain Basalt Member of the Chisos Formation (Oligocene).
5 = Colluvium deposits and colluvial fans (Quaternary).
All this igneous extrusive rock reminds us that in the Tertiary period Burro Mesa was one of many eruptive centers that transformed what is now Trans-Pecos Texas into a hellscape. The region was shaped and repeatedly reshaped by spreading lava flows and billowing pyroclastic surges.
But what caused all this mayhem? Long before this time, the Farallon Plate, a slab of oceanic crust, had subducted under the West Coast of North America, but at an usually shallow angle. As it proceeded eastward, it essentially scraped along the underside of the North American plate and triggered the Laramide Orogeny (ancestral Rocky Mountains).
Then, in the late Eocene and early Oligocene, the Farallon Plate started to plunge down at a steeper angle into upper mantle. As it did so, its rock melted and began to rise. A good deal of this magma reached the surface, creating volcanic eruptions from western Mexico up to Colorado.
- - - - -
My main source of information for this post, and for the other photos in this series, is the exquisitely illustrated and detailed USGS Geologic Map of Big Bend National Park, Texas (Turner et al., 2011). And its accompanying "pamphlet"—at 84 pages, more like a full book—is a model of clearly explicated stratigraphic and structural relationships. Both are available online.
To see the other photos and descriptions in this set and my other Big Bend series, visit my my Exploring the Burro Mesa Locale album.