Scientists deepen the understanding of ancient rivers

Scientists deepen the understanding of ancient rivers

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These gravel-covered river ridges in northeastern Colorado were among the remains of “fossil rivers” studied by Nebraska geologists Jesse Korus and Matt Joeckel. The eroded remains of ancient river channels provide important information about ancient hydrology, landscape changes, and the uplift of the Great Plains. Credit: Jesse Korus | Natural sources

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These gravel-covered river ridges in northeastern Colorado were among the remains of “fossil rivers” studied by Nebraska geologists Jesse Korus and Matt Joeckel. The eroded remains of ancient river channels provide important information about ancient hydrology, landscape changes, and the uplift of the Great Plains. Credit: Jesse Korus | Natural sources

Nebraska’s rivers can be thought of as places for quiet contemplation, popular local landmarks, or as precious natural resources to be protected.

To Jesse Korus, the state’s rivers are everything, but they’re also something more: they’re messengers from Nebraska’s deep geologic past. And their messages, discerned through innovative analyzes of remnant river channel deposits preserved in modern topography and rock outcrops, are important in helping Nebraskans understand the ongoing geologic dynamics affecting the state.

Korus, an associate professor with the University of Nebraska–Lincoln’s School of Natural Resources, is deepening knowledge of Nebraska’s geologic past by co-authoring two recent studies of ancient river systems in the Great Plains.

“This is the first time we have directly observed the courses of ancient river channels over a wide area of ​​the Great Plains and linked them to past landscapes and events,” Korus said.

A paper, published in Geosphere, notes that the landscapes of the Great Plains “are the products of a system of long-lived continental sediment flow” via rivers, “and yet remarkably little is known about these ancient rivers.” Journal by Korus and Matt

Joeckel, senior associate director of the School of Natural Resources and Nebraska state geologist, helps fill a knowledge gap about the history of the Platte River’s ancestral river system dating back 33 million years.

“Rivers were the building agents of the Great Plains,” said Korus, who is also a groundwater geologist for the Nebraska Division of Conservation and Research. Joeckel is the director of the CSD.

A second paper by the two scientists, published in The sedimentary recordexplains how fluvial sediment deposition patterns from the Rocky Mountains to the Gulf of Mexico convey key geologic information about upstream conditions.

Large fan-shaped sedimentary deposits on the Nebraska Old Plain stabilized the river’s sediment delivery system, holding sediment for millions of years and preventing climate fluctuations from rapidly changing the landscape in the lower reaches, Korus and Joeckel write.

“These rivers carried not only the sediments and the water itself, but information about the geological past — the past environment, the past events,” Korus said. “For example, uplift in the Rocky Mountains would have created a pulse of sediment that eventually ended up as a thick body of sediment deposited in the sink” far downstream in the Gulf of Mexico.

Giant “megafans” of sedimentation in ancient southwestern Nebraska, as well as parts of Colorado and Wyoming, delayed that transmission of information by acting as a “signal shield,” Korus and Joeckel found.

Vast fan-like structures, up to 56 miles wide, deposited sediment eroded from the Rocky Mountains for perhaps as long as 5 million years before some of it was transported downstream. This geological process delayed the fluvial transmission of uplift and climate change signals affecting the ancient river basin.

The Nebraska scientists’ analysis strengthens understanding of how rivers respond to tectonic movement, geomorphic overturning and climate change over geologic time periods. Their work also sets the stage for Korus and Joeckel to pursue extended geologic analysis of western Nebraska, eastern Wyoming, and northeastern Colorado. Sediments exposed at the surface in those areas are analogous to the subsurface Ogallala Aquifer further east. This future research could advance understanding of the aquifer, Nebraska’s primary natural resource.

Such a study, Korus said, could “improve our understanding of the deposits in the subsurface, the very deposits that are part of the Ogallala Aquifer of the High Plains. So it lends itself to a greater understanding of our large aquifer systems here in Nebraska.”

For both of these projects, Korus and Joeckel used remote sensing technology known as lidar, or light detection and ranging, which enables analysis of terrain in extremely fine detail. For the first paper, Korus and Joeckel identified more than 3,100 river crests, the eroded remnants of ancient river channels that provide important information about ancient hydrology and drainage patterns.

Careful analysis of those “fossil rivers,” spanning from 2 million years ago to about 33 million years in the past, revealed the evolution of the Platte River system. He showed that the streams of the ancient South Platte River system were originally small and varied, following a course quite different from the modern river system.

“The earliest South Plateau was much different than it is today,” Korus said.

“Several rivers flowed from northwest to southeast, perpendicular to the modern South Platte River in eastern Colorado. This suggests that that river system was not yet established. It was still in its earliest stages. Then, the younger ridges tell us that the rivers became larger, leading to the cutting of the modern South Platte Valley. Only after the modern South Platte route was established did the modern North Platte Valley begin to develop.”

Understanding these ancient geologic details depends on discerning the messages found in Nebraska’s geology.

“Sediments and rocks in general can be read like a book and tell us about the geologic past,” Korus said.

More information:
Jesse T. Korus et al, Exhumed Fluvial Landforms Reveal Late Eocene-Pliocene River Evolution in the Central and Northern Great Plains, USA, Geosphere (2023). DOI: 10.1130/GES02587.1

Jesse Korus et al, Telescopic megafans in the High Plains, USA were signal buffers in a large source-to-end system, The sedimentary record (2023). DOI: 10.2110/001c.89096

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