Magnetic Stratigraphy of the Eocene-Oligocene Transition in Northwestern Oregon

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The Keasey Formation in Northwestern Oregon contains one of the most complete geologic records of the Eocene-Oligocene transition in the Pacific Northwest. Samples from this sequence of sedimentary rocks were collected in Columbia County, Oregon, and analyzed in the Paleomagnetics Lab at the California Institute of Technology. The magnetic polarity pattern obtained from these analyses together with previously collected biostratigraphic data allows these rocks to be directly correlated with the global geologic timescale and dated to within 100,000 years. It also reveals important clues about the tectonic history of this area. The Keasey Formation spans magnetic Chrons C16n to C12r, making it approximately 35 million to 38.2 million years old, or late Eocene to early Oligocene in age. The paleomagnetic directions of these samples show that this region experienced approximately 77 degrees of tectonic rotation. These results enable scientists to further understand this important transitional period in the history of the earth.


D. R. Prothero



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