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USU Professors Discover New Fault and New Answers to the Bonneville Flood

The Great Salt Lake and several smaller lakes are all that remain of the vast, prehistoric inland sea that once covered much of what is today Utah. Lake Bonneville, under which Cache Valley was once submerged, broke its containment at the northern edge of the valley in what came to be known as the Bonneville Flood. The event drained the lake of roughly half of its water, and years of climate change would reduce it to its modern-day ancestors.

Susanne Janecke and Robert Oaks, both geology professors at USU, have published their findings from research at the northern edge of Cache Valley. Their research could provide new insight as to what caused the Bonneville Flood.

Janecke said an earthquake might have caused the event.

"An earthquake on the Riverdale fault could've been the trigger for the flood, and there are lots of different ideas geologists have put forward about the possible trigger for the flood. Nobody had really settled on a preferred interpretation, and earthquake were never a part of that mix of possible triggers. "Our data set is not as robust as we'd like, so right now we can say the evidence is consistent with this possibility but we can't really prove it yet. It would be nice to do some follow-up work and really pin down the timing of that big earthquake, because if it was a couple months after the flood or 100 years before the flood, it wouldn't have been the trigger for the flood. That has to be sorted out still."

The Riverdale fault was discovered after Janecke and Oaks made digital elevation models of Cache Valley. They hypothesized there was an earthquake at the fault around the time of the Bonneville Flood, within the error of their research methods.  

In addition, their findings show that Red Rock Pass in southern Idaho might not have been the northern outlet of Lake Bonneville.

"The outlet of Lake Bonneville when the main Provo shoreline was being formed was about 22 kilometers south of where everybody thinks it is, so if you've ever driven out the north end of the valley and stopped at the sign at Red Rock Pass that says 'This is where the outlet of Lake Bonneville was,' that sign is wrong and should probably be revised because the outlet, the northern edge, the northern shoreline of the lake was never there. "It was two or three kilometers further to the north at a little town called Zenda, but the outlet of the lake was never actually at Red Rock Pass if our hypothesis turns out to be correct."

Their hypothesis still needs testing, but Janecke said their research looks promising.

"There's good evidence that Lake Bonneville flowed out at a stable level for a long period of time, roughly 1,000 years or so. There wasn't anything about all that water rushing out there at Zenda that allowed the water to cut down and incise into the subsurface, so it's a little unusual that the lake was stable for a long time and then abruptly failed and you had a big flood. The earthquake is a good mechanism to explain both the long stability and then the abrupt, catastrophic flood because you have this major event, the earthquake, that could destabilize the dam and destabilize the outlet. "Yes, it is looking promising. It's consistent with the observations, but that doesn't necessarily prove that it's right."