Researchers Solve Colorado River Mystery

A team led by University of Oregon geologist Rebecca Dorsey published two papers that provide new information on the origins of the Colorado River, using data from ancient sedimentary deposits east of the Fault. San Andreas near the Salton Sea in Southern California.

The papers, edited by former masters student Brennan O’Connell and doctoral student Kevin Gardner respectively, present evidence that the now desert landscape of the lower river valley was submerged about 5 million to 6 million ago. years under shallow seas with strong and fluctuating tides. currents that came and went along the path of the current river.

Layers of sedimentary rocks containing marine fossils, burrows and diagnostic sedimentary structures, the researchers concluded, record a dramatic rise in sea level that has drowned ancient alluvial cones and caused the invasion of tidal waters in the valley before the arrival of the Colorado River and its bulky load of water and sediment.

The studies, both funded by the National Science Foundation, were published online ahead of print in the international journal Sedimentology.

The first paper, published on October 29 and edited by O’Connell, who is currently pursuing a doctorate at the University of Melbourne in Australia, looked at sediments from the late Miocene to early Pliocene Bouse formation exposed to the east. of the Colorado River, south of Blythe, California.

Previously, O’Connell and a team of UO geologists had found evidence that the Gulf of California once reached north as far as Blythe, as detailed in the journal Geology in 2017.

In the new paper, O’Connell and his co-authors describe a rich mixture of carbonate mudstones, plant debris, and traces of ancient organisms that record brackish water conditions where seawater was diluted by a large influx of fresh water due to heavy annual rainfall, before waters from the Colorado River poured into the area.

Sediments, they found, formed in large mudflats along a former humid climate marine coastline. An abrupt transition to a low-energy subtidal lime mudstone records widespread marine flooding associated with a sustained regional sea-level rise, they concluded. The relative rise in sea level, they added, results from long-term tectonic subsidence.

The combined evidence from palaeontology, ichnology and process sedimentology “provides a clear record of freshwater supply and brackish water conditions due to the mixing of freshwater and water from sea ​​in a humid climate with heavy annual rainfall, ”the O’Connell team wrote.

The second article, published Dec. 5 and edited by Gardner, documents tidal sediments of the same age across the same ancient tidal strait. Sediment accumulated where strong daily reverse currents swept north and south along the axis of the tidal strait, causing the migration of large subtidal dunes and large-scale depositing of cross-littered carbonate sands.

The authors argue that the bottom forms of migrating dunes could not have been formed by tidal processes in a lake, as some scientists have proposed. The team’s results indicate that after the Late Miocene to early Pliocene, sediments were lifted to elevations of up to 330 meters above sea level in the Chocolate Mountains, following long-term crustal deformation linked to movement on the San Andreas fault.

“Taken together, our new papers provide conclusive evidence that the southern Bouse Formation formed in and around the margins of a marine tidal strait that filled the lower Colorado River valley before the arrival of the river system. modern, ”said Dorsey, professor in UO’s Department of Earth Sciences.

A map of the Bouse South Tidal Seaway, reconstructed from data from the two studies, shows that communities in the Blythe Desert in Palo Verde, Calif., Were submerged under shallow seas with strong tidal currents fluctuating from the end of the Miocene to the beginning of the Pliocene.

The Miocene, a geological epoch, lasted from 23 million years to 5.3 million years; the Pliocene occurred between 5.3 million and 2.6 million years ago.

Co-authors with O’Connell and Dorsey on the first article were Stephen T. Hasiotis, geologist at the University of Kansas, and Ashleigh Hood at the University of Melbourne.

In 2018, another article edited by Dorsey in the newspaper Sedimentary geology found that the lower reaches of the Colorado River were influenced by tectonic changes in the underlying bedrock and changes in sea level. The river, this study concludes, has experienced a series of stops and starts of production of sediment during the early stages of the river, about 6.3 to 4.8 million years ago.