MAGNETOSTRATIGRAPHY AND TEPHROCHRONOLOGY OF AN UPPER PLIOCENE TO HOLOCENE RECORD IN LAKE-SEDIMENTS AT TULELAKE, NORTHERN CALIFORNIA

Combined paleomagnetic and tephra chronologies of one of the most comp lete middle Pliocene through Holocene stratigraphic records yet recove red in western Noth America provide a reference section for much of no rthwestern North America and adjacent Pacific Ocean. Five long drill c ores of lacustrine sediments at Tulelake, northern California, recover ed 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, Old uvai, Reunion(?), and Kaena(?) Subchronozones are present. Six short s tratigraphic intervals exhibit anomalous remanent inclinations that ma y record excursions and brief subchrons within the Brunhes and Matuyam a Chronozones. Age estimates suggest correlation of five of the anomal ous 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 Blak e Reversed-Polarity Subchron at about 114,000 yr B.P., (4) the Kamikat sura Normal-Polarity Subchron at about 850,000 yr B.P., and (5) the Co bb Mountain Normal-Polarity Subchron at about 1.10 Ma. Age of the sixt h interval of anomalous inclination is broadly constrained between 117 ,000 and 180,000 yr B.P. Sixty-three individual tephra layers were cha racterized by electron-microprobe and X-ray fluorescence analyses of v olcanic glass shards. Identified tephra of relatively well known age i nclude (1) the basal airfall pumice at Llao Rock, 7015 yr B.P.; (2) th e Trego Hot Springs Bed, 23,400 yr B.P.; (3) the Olema ash bed, betwee n 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 Rocklan d ash bed, about 410,000 yr B.P.; (6) the Lava Creek-B ash bed, 620,00 0 yr B.P.; (7) the Rio Dell ash bed, about 1.45 Ma; and (8) the Bear G ulch ash bed, about 1.9 Ma. A sedimentation-rate curve based on indepe ndently dated tephra and polarity reversals is used to infer age estim ates of undated or previously unidentified ash beds. Some of these ash beds are found over large areas of the western United States and east ern Pacific Ocean basin and provide widespread horizons for correlatio n. Most of the tephra at Tulelake records eruptions from the nearby so uthern and central Cascade Range of Oregon and northern California, an d the Medicine Lake Highland of northern California. Deposition took p lace during most of the past 3 m.y. within the Tulelake basin; notable periods of slow or sporadic accumulation, or erosion, occurred betwee n 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, betwee n 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.