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70 Million Years Ago, a River Ran Through It: Tracing the “Paleo Rio Grande” in Big Bend

Cullen Kortyna, a PhD student, analyzes ancient river deposits from Big Bend at the UTChron Laboratories in Austin, as part of a research group led by Dr. Danny Stockli.

In its jagged confusion of peaks and ranges, its canyons, hoodoos and badlands, Big Bend National Park is a complex landscape. Unsurprisingly, it’s a seductive place for geologists. Aspects of the Earth that elsewhere lie concealed and buried are here raised, broken and exposed. And for scientists, these diverse strata tell stories: of an ocean’s advance and retreat, of tectonic upheavals and volcanic eruptions, and of vanished creatures – from giant marine reptiles and flying pterosaurs to extinct mammals.

Now, geologists are reading another story in the park’s rugged terrain: of an ancient river. The “paleo Rio Grande” flowed through Big Bend for some 25 million years. Its traces shed light not only on our region’s past, but on the continent’s.

Cullen Kortyna is a PhD student at UT-Austin, studying the paleo Rio Grande in Big Bend's Tornillo Basin.

“It's fun to reimagine these old landscapes,” Kortyna said, “and to figure out where we are today, what it looked like millions and millions of years ago and what was causing it to look that way.”

In episodic flooding, across millions of years, the ancient river laid down sandstones and mudstones. Those “fluvial” deposits are exposed today in Tornillo Flats – in the northeast part of the park – and farther west, near the entrance from Terlingua.  

The oldest of these river deposits date to the late Cretaceous Period – the very end of the Age of Dinosaurs – about 70 million years ago. The youngest are from the middle Eocene, about 45 million years ago.

They speak of a continent in transformation.

For long eons, rivers here flowed westward, to the Pacific. But in the late Cretaceous, mountain-building events, including the Laramide Orogeny, raised the Rockies and other ranges in western North America. 

“And that essentially causes a total reorganization of paleo-drainage in the U.S.,” Kortyna said. “You start seeing sand and mud deposition in the Gulf of Mexico for the first time, and that is recording these rivers starting to reroute themselves into the Gulf of Mexico and create a framework that we're pretty familiar with today, with the Mississippi River and the Rio Grande River. So the Tornillo Basin records the earliest parts of that drainage reversal.”

Today’s Rio Grande rises in the San Juan Mountains of Colorado. Its major tributary, the Rio Conchos, begins in Mexico’s Sierra Madre. What were the headwaters of our river's ancient precursor? 

To answer that question, Kortyna is relying on a rare mineral, with a very special chemistry.

Zircon forms in molten rock, of the elements zirconium, silicon and oxygen – with trace amounts of uranium. Uranium – a radioactive element – decays into lead. That decay takes millions of years – but the rate at which it occurs is reliable, and well-known. By pinpointing where the uranium is in its “decay chain,” a geologist can determine precisely when a zircon crystal formed.

And that, in turn, allows the geologist to identify the volcanic event – the eruption or igneous intrusion – in which the zircon crystal was created.

At the UTChron Laboratories in Austin, Kortyna starts with a sample of rock from Big Bend – weighing perhaps 5 lbs. Isolating the zircon takes work – the rock is crushed, run through chemical solutions, magnetized. Eventually, a chunk of rock is reduced to a vial of sand.

“Zircon's rare,” Kortyna said. “It takes a lot of steps to separate it out so you can see it. And amazingly, despite all that, you'll still get probably hundreds of thousands if not millions of zircon out of that sample. There are a lot of sand grains in just a handful of rock – it's pretty impressive.”

Next, it's a process called laser ablation mass spectrometry. Zircon crystals are blasted to dust with a laser – the ratio of uranium to lead is measured, and the crystal is dated.

The zircon is probed for other secrets. Additional devices decode the zircon's “thermal history” – revealing when tectonic forces raised its host rock to the Earth's surface.

The results of Kortyna's analysis are fascinating. The paleo Rio Grande followed a similar course to today's river along what's now the Texas-Mexico border. But its headwaters, he's found, were entirely different.

Zircons from Big Bend originated from points as far as a thousand miles away – and to the west.  The paleo Rio Grande flowed out of mountains in what's now southern California, Arizona and Sonora, Mexico.

Geologists have traced a “paleo Colorado River,” that flowed from the Rockies through Texas, on a course similar to our present-day Colorado River. By comparison, the paleo Rio Grande was thought to be a smaller stream, with local headwaters. But Kortyna's work shows that wasn't the case.

“What I'm able to do is actually start making estimates of the amount of sediment, the amount of rock, that came through that system,” Kortyna said, “and when I do that it looks to be on par with the paleo Colorado. The paleo catchment was much bigger than people thought. It captured much more of the hinterland than people thought.”

The river changed over time. The deposits indicate that, early in its history, the Big Bend stretch of the paleo Rio Grande flowed slow and meandering through swampy terrain – something like the present-day Brazos River, though larger. Then, some 15 million years into its history, its flow altered – it becoming a swifter stream. The tectonic forces that created the Rockies were raising mountains here in Big Bend – and the changing topography may have transformed the river. But it was also a time of dramatic climate change, and whether the river's shift was a product of climate or tectonics is still debated.

The river ran on for epochs – alongside a changing cast of Big Bend animals, from dinosaurs to primates and caimans. Then, about 45 million years ago, the paleo Rio Grande vanished from Big Bend. The end of its journey corresponds with the powerful volcanism that produced the Chisos, Davis and Chinati mountains. Kortyna suspects these rising volcanic landscapes forced the river on another, as-yet-unknown path. 

But its traces remain – evoking another world, and reminding us that Big Bend National Park has many more stories to tell.

Drew Stuart is the producer for the Marfa Public Radio series Nature Notes.