RUI: Paleomagnetism and Tectonic Rotation of the Makah Formation

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EAR-0309538ProtheroRUI: Paleomagnetism and Tectonic Rotation of the Makah FormationDonald Prothero (EAR-0309538)Occidental CollegeABSTRACTThe Makah Formation in the northwest Olympic Peninsula is one of the thickest and most complete deep-marine Eocene-Oligocene sequences in coastal Washington, consisting of about 2800 meters of thin-bedded siltstones and turbidite sandstones that lie in a northwest-trending linear belt along the northwestern corner of the Olympic Peninsula. It yields Refugian (late Eocene) benthic foraminifera in the lower half, and Zemorrian (Oligocene) foraminifera in the upper half. Based on PI's magnetostratigraphic calibration of these benthic foraminiferal stages in the Lincoln Creek Formation of Washington and the Keasey and Alsea Formations of Oregon, he is confident that magnetic sections in the Makah Formation can be confidently correlated to the global time scale. Such correlations will aid in our understanding of the major climatic changes occurring during the Eocene-Oligocene transition. In addition, the Makah Formation yields some of the oldest fossil baleen whales known from the Pacific Rim, and dating of these specimens is important to hypotheses about whale evolution. Over 57 paleomagnetic sites from the type section of the Makah Formation at Baada Point were analyzed and yield a stable primary remanence with both normal and reversed polarities, which passed a reversal test. Finally, PI's preliminary sampling of the Makah Formation shows that the rocks have undergone about 30 degrees of post-Oligocene counterclockwise rotation. This conflict with earlier paleomagnetic results showing that many of the Eocene-Oligocene sedimentary units in the Olympics have clockwise tectonic rotations; however, these results are consistent with recent work nearby showing counterclockwise rotations. Further paleomagnetic analyses of the Makah Formation would improve our understanding of the complex tectonics of this region.