Improved age estimates for the White River and Bridge River tephras, western Canada1 J.J. Clague, S.G. Evans, V.N. Rampton, and G.J. Woodsworth Abstract: New I4C ages date the eruptions that produced the White River and Bridge River tephras, two important Holocene marker beds in western Canada. The 14C ages were obtained on trees in growth position buried in coarse tephra and a pyroclastic flow near the source vents. The mean calendric age of the White River eruption, based on four I4C ages, is 1147 cal years BP (calibrated years, approximately equivalent to calendric years) or AD 803 (the 2a age range, obtained from the two most precise 14C ages, is 1014- 1256 cal years BP or AD 694-936). The mean age of the Bridge River eruption, determined both from (i) the single most precise outer-ring I4c age and (ii) the weighted mean of six outer-ring I 14C ages is 2360 cal years BP or 411 BC (20 age range = 2349-2704 cal years BP or 755-400 BC). I RCsumC : De nouveaux bges 14C datent les Cruptions qui ont produit les tephra de White River et de Bridge River, deux lits marqueurs cruciaux de 1'Holoctne dans 1'0uest du Canada. Les bges I4C ont CtC obtenus sur des arbres en position de croissance, enfouis dans un tephra ?i grain grossier et une coulCe pyroclastique prts des orifices Cruptifs. L'bge-calendrier moyen de 1'Cruption de White River, fond6 sur quatre dktenninations au I4C, est 1147 annCes cal avant le PrCsent (annCes calibrks, approximativement Cquivalentes aux ann6es-calendrier) ou 803 A.D. (1'Ccart entre les mesures d'bge 20, dCduit de deux mesures les plus prkcises d'bges au I4C, est 1014-1256 annCes cal avant le present ou 694-936 A.D.). L'2ge moyen de 1'Cruption de Bridge River est dCtermink par les deux donnCes suivantes : (i) la mesure unique la plus prCcise de l'bge I4C pour l'anneau externe du dCp6t de tephra; et (ii) la moyenne pondCrCe de six ages I4C de l'ameau externe du dCp6t de tephra, est de 2360 annCes cal avant le Present ou 41 1 A.C. (1'Ccart entre les mesures d'bge 2a = 2349-2704 annCes cal avant le PrCsent ou 755 -400 A.C.). [Traduit par la r&laction] Introduction Several Holocene tephras are present in the Cordillera of western Canada. They record large explosive eruptions of volcanoes in British Columbia, Oregon, and Alaska, and are important stratigraphic markers that provide age control on a variety of phenomena, such as fluvial aggradation and inci- sion (Fuller 1986), alluvial-fan growth (Ryder 197 1 ; Roed and Wasylyk 1973), slope failure (Osborn and Luckman 198 I), glacier fluctuations (Rampton 1970), and vegetation and climate change (Rampton 1971; Alley 1976; Mathewes 1973; Mathewes and Rouse 1975; Slater 1985). Mazama tephra, the oldest and most widespread of the Holocene tephras, was erupted about 6800 14C years ago from the present site of Crater Lake, Oregon, and is found over large areas of southern British Columbia, Alberta, and Received December 3, 1994. Accepted March 3, 1995. J.J. Clague2 and G.J. Woodsworth. Geological Survey of Canada, 100 West Pender Street, Vancouver, BC V6B 1R8, Canada. S.G. Evans. Geological Survey of Canada, 601 Booth Street, Ottawa, ON KIA OE8, Canada. V.N. Rampton. Terrain Analysis and Mapping Service Ltd., P.O. Box 158, Carp, ON KOA 1L0, Canada. ' Geological Survey of Canada Contribution 48594. Corresponding author (e-mail: jclague@gsc.emr.ca) . Saskatchewan (Fig. 1). Other important tephras include St. Helens Wn and Yn, derived from Mount St. Helens, Washington; White River (two layers), with a source near the Alaska - Yukon boundary; and Bridge River, erupted from Mount Meager in southwestern British Columbia (Fig. 1). In this paper, we present new data on the ages of the White River and Bridge River tephras. Although these tephras have been dated previously, new radiocarbon ages presented here are more precise and are generally more closely linked to the eruptive events than those reported earlier. In addition, with recent improvements in 14C age calibration (e.g., Pearson and Stuiver 1993; Stuiver and Pearson 1993; Stuiver and Reimer 1993), calendric ages for these events can now be determined with greater precision. White River tephra Much of south and central Yukon Territory lies within the fallout area of the White River tephra (Fig. 1). The tephra was deposited during two separate eruptions in the last 2000 years (Bostock 1952; Lerbekmo and Campbell 1969; Hughes et al. 1972; Lerbekmo et al. 1975). The older eruption, sometime between 1900 and 1500 14C years ago, produced a northerly directed lobe of tephra that straddles the Alaska - Yukon border. A younger, larger eruption, about 1200 14C years ago, left a layer of volcanic ash a few Can. J. Earth Sci. 32: 1172- 1179 (1995). Printed in Canada 1 Imprim6 au Canada