New evidence, based on mineral dating, suggests that rocks of the Blue Mountains, the oldest geological formation in Oregon, may have been derived from the Klamath and Sierra Nevada mountain chains, University of Oregon researchers report.
The findings, presented October 29 at the annual meeting of the Geological Society of America, come from zircon grains in Triassic and Jurassic sedimentary rocks (dating from 144 million to 248 million years ago) from Blue Mountain basins in northeastern Oregon. The approximate ages of the zircons, based on isotropic dating, match the ages of rocks to the south in the Klamath Mountains, said Todd LaMaskin, a UO doctoral student in geological sciences.
Detrital zircon dating is a much-used technique that uses isotopes of uranium and lead from zircon grains found in sedimentary rocks, usually sandstones. Such dating allows geologists to look back in time by billions of years.
"Zircons are very stable, can be weathered out of a rock, transported long distances and deposited in sedimentary basins," LaMaskin said. "We start with a block of sandstone, haul it out, crush it and mill it, and use various chemical and magnetic techniques to concentrate the zircons. We might start with 40 pounds of sandstone and end up with 100 to 200 zircons, all of which would fit on the head of a pin."
The findings are part of a much larger project in which LaMaskin is working with Rebecca Dorsey, his UO adviser and professor of geological sciences, and Jeffrey D. Vervoort, professor of geology at Washington State University. Together, they are attempting to reconstruct the sedimentary basins and mountain ranges of northeastern Oregon to clarify major driving forces and plate interactions involved in the tectonic evolution of the western United States. Dorsey was to present related findings during the same session.
"We're looking at zircons that came from many different sources, where they were weathered and eroded and transported down streams and dumped into sedimentary basins," LaMaskin said. "If you dug up a bunch of sand from the Willamette River, for example, it would contain zircons eroded from older rocks of the Cascade Mountains."
The similarities of rock composition and detrital zircon ages, LaMaskin said, suggest that millions of years ago the region's westernmost chain of mountains and basins may have been continuous from California to Oregon -- back when parts of the West Coast were along the Oregon-Idaho border. "So we think at this point that we may have the most definitive evidence to date that during in Jurassic time there was a connection between the rocks of the Blue Mountains and the northern Sierra Nevadas and eastern Klamath ranges."
The findings, LaMaskin said, suggest the possibility that during these tectonic collisions in the ancient Northwest, sediment was transported in a south-to-north direction in a large early to middle marine sedimentary basin. Since the Jurassic, plate tectonics has caused accretion of younger terranes, driving them into existing rocks and shifting the coastline farther west.
"Alternatively, what we've found might suggest that rocks in Oregon were just like the rocks in the eastern Klamath, but they are now covered by the 15 million-year-old Columbia River basalt flows," he said. "It seems more likely that the Triassic and Jurassic sandstones were sourced from the south and shed northward into Oregon, because we don't have evidence that rocks of the correct age exist in eastern Oregon.
"The gist is that as tectonic plates are moving and volcanoes form, there is always rock being eroded and deposited into adjacent sedimentary basins," he said. "These processes are recorded in the basins we are studying. The Blue Mountains provide a window through the younger volcanisms that allows us to view the older rocks and history of western North America."
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