In the Guadalupes, Geologists Trace Pangaea and the Paleozoic Past
By Andrew Stuart
The mountainous landscapes of West Texas testify to diverse aspects of the Earth’s past – to the tectonic upheavals that raised the Rockies, to episodes of powerful volcanism, to a seaway that covered this land in the Age of Dinosaurs.
But as a window into deep time, the Guadalupe Mountains are unique. The rocks that form the Top of Texas, that cast a celestial glow in the towering colossus of El Capitan, were laid down some 270 million years ago. There were no grasses, no flowering plants, no mammals in that Paleozoic time. But the planet didn’t lack for life, or for drama. In the Guadalupes, geologists are tracing the mysteries of that vanished world – in often stunning detail.
Dr. Lynn Soreghan is a professor at the University of Oklahoma.
“I think it’s one of the big things that attracts many of us to geology,” Soreghan said, “is the storytelling you can do with just grains of sand.”
Soreghan studies the changing landscapes and climate of the late Paleozoic Earth. And though her work takes her around the world, she said the Guadalupes are a singular destination.
“So what you see today is essentially, in large part, how it would have looked 260 million years ago, 270 million years ago,” she said, “with just trying to imagine putting the water back in. When we take the Permian Reef trail, it’s like we’re scuba diving up a reef system.”
The shining rocks that today rise as the Guadalupes are the nearly pristine remains of a reef, that formed at the margin of an inland sea. That sea teemed with life, and an observant hiker today can see its fossil remains. But for geologists, these mountains offer insight not only into ancient West Texas, but into the late Paleozoic Earth as a whole.
And a very different Earth it was.
The planet’s land masses had then fused to form the supercontinent of Pangaea. And this land was at Pangaea’s western edge.
With a technique called detrital-zircon geochronology, Soreghan analyzed sandy sediments here, to identify their sources. Those sands, she found, had largely originated from land that’s now in Central America.
“It turns out the short answer is they came from the eroding Appalachian Mountains,” Soreghan said. “It’s kind of cool – these would have been pretty big mountains, and these sands were coming across the whole continent and ending up in the last hole that was left.”
It’s a long way from Guatemala to West Virginia today. But the Appalachians were formed when Central and South America and Africa collided with this continent, in the creation of Pangaea. Sands found here originated in those rising Appalachians – which likely topped 10,000 feet – and were swept across Pangaea before landing in the “hole” of the Guadalupes’ inland sea.
What’s now West Texas was then near the equator – with an “umbrella-drink climate,” Soreghan said. But that wasn’t the case everywhere.
Great ice sheets advanced and receded across parts of Pangaea. Sea levels fell as glaciers advanced, and rose as they receded. Those cycles are revealed in the Guadalupes, where limestones – formed in marine environments – alternate with sandstones and mudstones.
“There were such big sea level changes,” Soreghan said, “that you went from being underwater to being completely exposed, meaning that you formed a fossil soil. And then you would go back to being underwater – and you can definitely tell when you’re underwater. So it would go back and forth and back and forth.”
Alicia Bonar, a PhD student working with Soreghan, is asking another question in the Guadalupes: how did this sea become so rich in life?
For while we might think of oceans as fecund, their life depends on materials from land. The creatures at the base of the marine food chain need nutrients the sea can’t provide. It’s why ocean life is richest near coastlines and river deltas.
Bonar is hoping to identify the source of one nutrient in particular.
“One of the most limiting micro-nutrients is iron,” Bonar said. “So that’s what we’re really looking at: What is the source of iron in these systems? Because you need iron for these primary producers to grow and live.”
Volcanic ash is a possible source. But the leading candidate is dust – the atmosphere in Pangaea was likely the dustiest in Earth’s history.
Amidst the Texas desert, the high Guadalupes are a place apart. But geologically speaking, they truly express another world.
Nature Notes is supported by the Shield-Ayres Foundation and produced by Marfa Public Radio with the Sibley Nature Center. The program can be heard each Tuesday and Thursday, at 7:45 a.m. and 4:45 p.m., Central time, on KRTS Marfa, 93.5 FM, and KXWT Odessa/Midland, 91.3 FM. This episode was written by Andrew Stuart.
