Abstract

Combined paleomagnetic and tephra chronologies of one of the most complete middle Pliocene through Holocene stratigraphic records yet recovered in western North America provide a reference section for much of northwestern North America and adjacent Pacific Ocean. Five long drill cores of lacustrine sediments at Tulelake, northern California, recovered a nearly continuous 331-m-thick record spanning the past 3 m.y. The Brunhes Normal-Polarity, Matuyama Reversed-Polarity, and Gauss Normal-Polarity Chronozones are recognized; within these, the Jaramillo, Olduvai, Reunion(?), and Kaena(?) Subchronozones are present. Six short stratigraphic intervals exhibit anomalous remanent inclinations that may record excursions and brief subchrons within the Brunhes and Matuyama Chronozones. Age estimates suggest correlation of five of the anomalous intervals with (1) one of the Biwa excursions at about 18,000 yr B.P., (2) the Mono Lake excursion at about 27,000 yr B.P., (3) the Blake Reversed-Polarity Subchron at about 114,000 yr B.P., (4) the Kamikatsura Normal-Polarity Subchron at about 850,000 yr B.P., and (5) the Cobb Mountain Normal-Polarity Subchron at about 1.10 Ma. Age of the sixth interval of anomalous inclination is broadly constrained between 117,000 and 180,000 yr B.P.

Sixty-three individual tephra layers were characterized by electron-microprobe and X-ray fluorescence analyses of volcanic glass shards. Identified tephra of relatively well known age include (1) the basal airfall pumice at Llao Rock, 7015 yr B.P.; (2) the Trego Hot Springs Bed, 23,400 yr B.P.; (3) the Olema ash bed, between 55,000 and 75,000 yr B.P.; (4) the airfall pumice at Cloudcap Road ("Pumice Castle-like tephra 2"), about 120,000 yr B.P.; (5) the Rockland ash bed, about 410,000 yr B.P.; (6) the Lava Creek-B ash bed, 620,000 yr B.P.; (7) the Rio Dell ash bed, about 1.45 Ma; and (8) the Bear Gulch ash bed, about 1.9 Ma. A sedimentation-rate curve based on independently dated tephra and polarity reversals is used to infer age estimates of undated or previously unidentified ash beds. Some of these ash beds are found over large areas of the western United States and eastern Pacific Ocean basin and provide widespread horizons for correlation. Most of the tephra at Tulelake records eruptions from the nearby southern and central Cascade Range of Oregon and northern California, and the Medicine Lake Highland of northern California. Deposition took place during most of the past 3 m.y. within the Tulelake basin; notable periods of slow or sporadic accumulation, or erosion, occurred between about 620,000 and 200,000 yr B.P. and between about 2.5 and 2.1 Ma. Rapid deposition occurred during marine oxygen-isotope stage 6, between about 170 and 125 ka. Regional volcanism during the past 3 m.y. was markedly episodic, with notable volcanic activity from about 2.1 to 1.9 Ma and from 0.4 Ma to the present.

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