麻豆淫院

Geology is full of detective stories about the Earth's history, and a new paper in by University of New Mexico Distinguished Professors Emeritus Karl Karlstrom and Laurie Crossey, along with their co鈥恆uthors, links two iconic geologic landmarks of the American Southwest: the Grand Canyon and Meteor Crater.

The article, titled "Grand Canyon landslide鈥恉am and paleolake triggered by the Meteor Crater impact at 56 ka," highlights the striking coincidence in the geologic ages of a meteor impact and a landslide dam that blocked the Colorado River, forming a paleolake in the Grand Canyon about 56,000 years ago.

The study also includes a compelling backstory involving multiple generations of scientists, advances in scientific methods, and an international team of collaborators.

"It would have required a ten鈥恡imes bigger flood level than any flood that has happened in the past several thousand years. Or maybe they are very old deposits left as the river carved down, or maybe they floated in from a paleolake caused by a downstream lava dam or landslide dam? We needed to know the age of the cave deposits," said Karlstrom.

In the mid 1960's, Karlstrom's father, Thor, was in the cave as part of cross鈥恉isciplinary . They found evidence of species that went extinct in the Grand Canyon, like the California condor and Harrington's mountain goat; they also found split twig figurines made 3,000鈥�4,000 years ago by the ancestors of tribes that lived around the Grand Canyon.

The driftwood was dated to be greater than 35,000 years, which was near the limit of the radiocarbon dating method at the time; this age was recalculated in 1984 to 43,500 years. The new paper, using new methods and analyses from two labs in New Zealand and Australia, reports a date of 56,000 years old.

The co鈥恆uthor team involved Chris Bastien, who serendipitously was working on the Stanton's driftwood collections housed at the University of Arizona Tree Ring laboratory, and David Kring, who was science coordinator for Meteor Crater and who had been working to recalculate Meteor Crater ages (53,000鈥�63,000 years old by different methods).

When co鈥恆uthor Jonathan Palmer from the Australian lab was on a road trip to the U.S., he visited both Meteor Crater and Bastien at the UA lab and noted the striking coincidence of ages that set this paper in motion.

"From numerous research trips, Karl and I knew of other highly鈥恆ccessible caves that had both driftwood and sediment that could be dated," said Crossey.

They sent another driftwood sample to the Australia and New Zealand labs, and sediment samples to Tammy Rittenour at Utah State University (USU). Having two dating methods was a key part of this study. Both came back with statistically indistinguishable 55,600鈥恲ear鈥恛ld dates.

In the 1980s, Richard Hereford of the USGS in Flagstaff proposed that there was a rockslide near Nankoweap Canyon at RM 52, about 22 RM downstream of Stanton's Cave, that might have formed a dam and a paleolake that backed up water and that allowed driftwood to float into Stanton's cave.

This hypothesis now came back into the limelight with the additional 56,000鈥恲ear鈥恛ld samples from different caves high above the river. Grand Canyon caver Jason Ballensky had even seen beaver tracks way up and way back in Vasey's Paradise caves, in a place that would be inaccessible to beavers.

Their combined results, of how deep and long the paleolake is thought to have been, are shown in the Colorado River Profile graph. Findings reported in the Geology paper are as follows.

1. Driftwood and sediment are found in many caves as high as 940 m elevation such that the lake backed up water above Lees Ferry, where boating trips launch鈥� the paleolake hypothesis seemed to be "holding water."
2. There are two places where the reddish chaotic dam material at Nankoweap Canyon is overlain by rounded, far鈥恡raveled and identifiable river cobbles, suggesting there was a dam there that got overtopped and deeply eroded, likely within less than 1000 years based on analogies to modern concrete dams are filling up with sediment. so the landslide dam idea was "falling together".

Was the Meteor Crater impact enough to cause such a landslide? Kring and other impacts and has calculated that the impact would have set off a magnitude 5.4 earthquake (or even M6 using different assumptions) and that the shock wave would still have been ~M3.5 after traveling (in seconds) the 100 mile distance to Grand Canyon.

So, the impact could have "shaken loose" the steep cliffs of Grand Canyon that were "waiting and ready to go" as shown by numerous rock falls that happen in Grand Canyon.

"The team put together these arguments without claiming we have final proof; there are other possibilities, such as a random rockfall or local earthquake within a thousand years of the Meteor Crater impact that could have happened independently," explained Karlstrom.

"Nevertheless, the meteorite impact, the massive landslide, the lake deposits, and the driftwood high above river level are all rare and unusual occurrences. The mean of dates from them converge into a narrow window of time at 55,600 卤 1,300 years ago, which gives credence to the hypothesis that they were causally related."