Improved age estimates for the White River and Bridge River tephras, western Canada1

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1 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 cal years BP or AD ). 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 14C ages is 2360 cal years BP or 411 BC (20 age range = cal years BP or BC). I 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 annces cal avant le present ou 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 411 A.C. (1'Ccart entre les mesures d'bge 2a = annces cal avant le PrCsent ou 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 incision (Fuller 1986), alluvial-fan growth (Ryder ; Roed and Wasylyk 1973), slope failure (Osborn and Luckman 198 I), glacier fluctuations (Rampton 1970), and vegetation and climate change (Rampton 197 1; Alley 1976; Mathewes 1973; Mathewes and Rouse 1975; Slater 1985). Mazama tephra, the oldest and most widespread of the Holocene tephras, was erupted about C 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, Accepted March 3, 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 Corresponding author ( 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 C years ago, produced a northerly directed lobe of tephra that straddles the Alaska - Yukon border. A, larger eruption, about C years ago, left a layer of volcanic ash a few Can. J. Earth Sci. 32: (1995). Printed in Canada 1 Imprim6 au Canada

2 Clague et al. Fig. 1. Distribution of Holocene tephras in western Canada (references: Bostock 1952; Nasrnith et al. 1967; Westgate et al. 1970; Hughes et al. 1972; Mullineaux et al. 1975; Westgate 1977; Mathewes and Westgate 1980; Bacon 1983; Clague 1992). Numbered sites are 14C localities for the White River and Bridge River tephras (Table 1). Fig. 2. In situ trees buried in White River tephra, Little Boundary Creek, Yukon Territory. The site is about 30 km from the source vent. Geological Survey of Canada photograph The two new 14C ages give statistically equivalent calibrated ages for the death of the tree and, by inference, the time of the eruption: = 1129 and 1144 cal years BP; 20 range = cal years BP (AD 1950 datum). The new ages fall within the range of the two previous 14C ages on stumps buried in White River tephra (Fig. 3). The weighted mean of all four calibrated ages (weighting by inverse square of standard deviation of 14C ages) is 1147 cal years BP or AD 803. millimetres to a few tens of centimetres thick over southern and central Yukon and part of western District of Mackenzie. Two new 14C ages have been obtained on a stump in growth position buried in White River lapilli at Little Boundary Creek, about 30 km from the vent. The dated stump is one of many in this area that are entombed in tephra (Rarnpton 197 I), providing evidence for forest kill during the eruption (Fig. 2). Bark is not preserved on the dated stump, but the wood is exceptionally well preserved and the outermost rings are not truncated, suggesting that there are few or no missing rings. Rings from the perimeter of the stump yielded an age of 1260 f 50 14C years BP, and rings gave an age of 1430 f 70 14C years BP (Table 1). These ages came from a stump that was dated in the late 1960s at 1210 f C years BP. This last age, along with one of 1300 f C years BP on another stump at a nearby site, previously provided the best age control on the eastern lobe of the White River tephra. They are consistent with several limiting 14C ages from sediments above and below the tephra at sites distant from the volcano (Fig. 3; Table 1). However, all of the previous 14C ages have 20 error terms of f 130 or 140 years, far larger than those of the two new ages. Bridge River tephra Bridge River tephra extends eastward from Mount Meager across southern British Columbia into Alberta (Fig. 1). Over most of the fallout area, the tephra is a thin layer of ash, but thicker and coarser pyroclastic materials are present within several tens of kilometres of the vent (Nasmith et al. 1967; Westgate et al. 1970). There are excellent exposures of proximal Bridge River eruptive deposits in the canyon of Lillooet River, about 4 krn east of Mount Meager (Read 1977, 1979, 1990; Stasiuk and Russell 1990; Evans 1992; Stasiuk et al. 1994); it is from these exposures that the new radiocarbon ages came. In situ charred trees up to 1 m in diameter are buried in pumiceous lapilli at Lillooet River canyon (Figs. 4 and 5) (see Stasiuk and Russell 1990 for a detailed description of the eruptive products in this area), The air-fall deposits are overlain by a matrix-supported diamicton containing charred logs and large pumice blocks (pyroclastic flow deposit), and this, in turn, is overlain by a welded vitroclastic volcanic breccia. The charred trees at Lillooet canyon were killed by fallout of hot lapilli during the earliest stage of the eruption. Soon thereafter, a hot pyroclastic flow swept down the valley and buried the air-fall deposits; the upper parts of the trees were

3 Table 1. Radiocarbon ages pertaining to the White River and Bridge River tephras. Radiocarbon agea 613c Calibrated ageb (I4C years BP) (%o) (cal years BP) Laboratory no.' Location Site Lat. N Long. W (Fig. 1) Dated material Reference Comment White River tephra ( ) Charcoal (Betula ~ p.)~ Lowdon and Blake 1970b Peat' Lowdon and Blake Maximum age of ttephra Wd (Picea sp.)g Blake 1987 Bridge River tephra GSC-577 S-580 W d h Lowdon and Blake 1970a Peati Lowdon and Blake Wood (Picea sp.)j This paper Wood" Lowdon and Blake 1970a Wood (Picea sp.)" Lowdon and Blake 1973 Woodn Lowdon and Blake 1970a Wood (Piceu sp.)" This paper Approximate age of tephra Tephra ca. 30 years Tephra ca. 20 years Tephra ca. 180 years Lowdon and Blake Minimum age of tephra Rutherford et al Minimum age of tephra GSC GSC-5190 GSC-5633 GSC-5690 GSC-5631 GSC-5203 GSC-529 GSC-5366 Peat' Lowdon and Blake 1976 Evans 1992 Minimum age of tephra Charred woodu Approximate age of Wood (Pseudotsuga menziesii)" Charred woodw Wood (Pseudotsuga menzie~ii)~ Charred woody This paper This paper This paper tcphra Tephra ca. 52 years Tephra years Age of tephra Evans 1992 Tephra ca. 25 years Lowdon and Blake This paper Approximate age of tephra Lowdon and Blake 1976

4 Table 1 (concluded). 0!a In Radiocarbon agea (I4C years BP) C Location (D 6I3c Calibrated ageb Laboratory Site Lat. N Long. W (Fig. 1) Dated material Reference Comment (%o) (cal years BP) no.' EL 2 GSC '38.4' 123'25.2' 16 Chad wood This paper (7huja p li~ata)~~ GSC '38' 123'26' 17 Charred wood Lowdon and Blake (Pseudotsugn rnen~iesii)''~ 1978 GSC ' 123'30.6' 18 Cha"edwod This paper (Abies sp.)" GSC '38.4' 123'25.2' 16 wood1.. This paper Approximate age of tephra GSC '39.7' 123'26.8' 12 Cham& wood Lowdon and Blake (Pseudotsuga rnenzie~ii)~~ 1978 Tephra ca. 150 years GSC ' 123'26.8' 12 Charredwood" This paper Tephra years S '44.4' 121 "55.5' 10 Charcoalr Rutherford et al Minimum age of tephra ( ) GSC "58' 116'43' 9 Charred wood^^ Lowdon and Bhke 2685 & ( ) GX ' 122"45' 19 Gyttjakk 2775 & ( ) S '44.4' 121'55.5' 10 Charcoal" Mathewes and Westgate 1980 Rutherford et al ( ) GX ' 122'45' 19 Gyttjamm Mathewes and Westgate Minimum age of tephra 1980 Bmr terms are 20 for Geological Survey of Canada ages and lo for Geochron Laboratories 'There are three possible calendric ages for a radiocarbon age of 2240 "C years BP. and Saskatchewan Research Council ages. Ages are corrected to 6% = -25.0% PDB. 'Directly above tephra. 'Reference datum is AD Calibrated From bidecadal data of Pearson and Stu~ver (1993) "Outer portion of a charred, in situ tree buried in tephra. and Stuiver and Pearson (1993). The values in parentheses represent the 20 age range calculated "Rings from the surface of an in situ tree buned in tephra. using an error multiplier of 2.0. *Innermost 15 rings of a charred. in situ tree that has about 185 annual rings. 'GSC, Geological Survey of Canada: GX. Gemhron Laboratories (K~eger Enterprises); 'Outermost 3 rings of an in situ tree buried in tephra. S, Saskatchewan Research Council. YOutermost 50 rings of a charred, in situ tree buried in tephra. "earth directly below tephra. :'I'here are three possible cdendric ages for a radrocarboo age of 2440 I4C years BP. 'Directly below tephra. "There are three possible calendric ages for a radiocarbon age of 2460 I4C years BP. *here are three possible calendric ages for a radiacarbon age of 1210 I4C years BP. bb~uter portion of a charred. in S~N ~ree buried in diarnicton. RDetrital wood enclosed in tephra. *There are three possible calendric ages for a radiocarbon age of 2480 I4C yean BP. h~uter portion of an in situ tree butied in tephra. "Detrital charred wood from reworked tephra. 'From 1 cm above to 1 cm below tephra. "Detrital charred wood at base of tephra. ]Rings from the surface of an in situ tree buried in tephra. This is the same nee that mere are four possible calendric ages for a radiocarbon age of 2490 I4C years BP. yielded GSC-748 (1210 & 130 years BP). "There are seven possible calendric ages for a radiocarbon age of 2500 "C years BP. 'There are three possible calendric ages for a radiocarbon age of 1280 IdC yeas BP. M~entre of a charred, in situ tree buried in tephra; the nee was about 150 years old when it died. 'Detr~taI w d directly below tephra. "Fifteen annual rings, just inside the outermost 0.5 cm of charred wood, of an in situ tree "Detrital wood below tephra. buried in tephra. *Outermost 40 rings of an in situ tree buried in tephra. *Below tephra. "Rings from the surface of an in situ tree buried in tephra. bt~irectly below tephra. Age is possibly too old (not shown in Fig. 6). FThere are three possible calendric ages for a radiocarbon age of 2120 I4C years BP. "Occupation level of a pit house below tephm. qtwo lenss of charcoal above tephra. "Directly above lephra. Age is possibly too old (nor shown in Fig. 6). 'Occupation level of a pit house above tephra.

5 1176 Can. J. Earth Sci. Vol. 32, 1995 Fig. 3. Plot of calibrated I4C ages (Table 1) defining the age of the White River tephra. The vertical line defines the weighted mean calendric age calculated from four I4C ages on stumps buried in tephra. The toned pattern spans the 20 ranges of GSC-5617 and GSC-5619, the two new 14C ages with small error terms. Calendric ages for GSC-1000, GSC-5617, and GSC-5619 were adjusted by the number of tree rings between the dated sample and the outermost ring. Radiocarbon ages were calibrated using the computer program CALIB (version 3.0.3; Stuiver and Reimer 1993) and the,bidecadal data sets of Pearson and Stuiver (1993) and Stuiver and Pearson (1993). Some of the mean 14C ages correspond to three calendric ages; this is indicated by three symbols along horizontal lines. The thick and thin horizontal lines represent, respectively, la and 20 age ranges, calculated using an error multiplier of 2.0 (error multipliers expand laboratory-quoted errors to cover uncertainties in reproducibility and systematic bias; for a discussion, - see Stuiver and Pearson 1993). Approximate. GSC-5619 age v GSC-343 GSC-1000 Maximum age Calendric age (years before AD 1950) Fig. 4. Charred tree in growth position, entombed in Bridge River lapilli tephra and overlain by pyroclastic flow deposits, Lillooet River canyon (site 12, Fig. 1). Two I4C ages ( and I4C years BP; Table 1) were obtained on this tree. The site is 4 km from the source vent. snapped off by the flow and remaining trunks were deeply charred. Read (1977, 1979, 1990) obtained a radiocarbon age of 2500 f 50 14C years BP3 (Table 1) on one of the charred stumps entombed in air-fall and pyroclastic flow deposits. Unfortunately, the dated material was collected from the centre of the trunk, rather than the outside, and thus must be older than the eruption that killed the tree. Read estimated that the dated tree had about 150 rings, and thus concluded that the age of the eruption is about 2350 I4C years BP. This is in general agreement with 14C ages from sedimentary sequences at sites more distant from the volcano that provide maximum and minimum dates for tephra deposition (Table 1). Similar 14C ages also were reported by Evans (1992) on the basis of additional dating of charred trees at Lillooet canyon. Samples of charred wood collected from the outer of two trees entombed in lapilli tephra at Read's site yielded ages of and 2410 f C years BP (Table 1). The older of the two samples comprised the outermost 50 annual rings of the dated tree, whereas the of the two samples included an unknown number of rings. Although these ages are reasonable, uncertainties about the actual number of rings in the dated trees and the possibil- Read quoted an uncorrected age of 2490 f 50 I4C years BP. The age normalized to a 613C of -25.0%0 is 2500 f 50 14C years BP (Lowdon and Blake 1978, p. 10).

6 Clague et al. Fig. 5. Stratigraphic sections in Lillooet River canyon, showing locations of 14C samples. Elevations are approximate E - C.- - a, o m > W GSC-5631, GSC _?.. (fluvial) ity that a number of the outermost rings were missing due to abrasion and combustion led us to date two additional trees at Lillooet canyon. Two of the new ages came from a Douglas-fir (Pseudotsuga menziesii) stump with preserved bark at a newly discovered site. The stump was rooted in a thin soil lying on granitic bedrock and was buried in air-fall lapilli tephra. The outermost three rings of this stump gave an age of 2410 f 50 I4C years BP (Table 1). Fifteen rings, years in from the bark, were dated at 2360 f 60 I4C years BP. Two ages were obtained on an upright charred tree in lapilli tephra at Read's site: one of 2500 f 60 14C years BP on 15 rings just inside the outermost 0.5 cm of charred wood (thought to be bark); and a second of 2400 * 60 14C years BP on 15 rings, rings in from the outside. All of these ages overlap at the 20 level (Fig. 6). The most precise 14C age (i.e., that with the smallest error term, determined on the sample with the fewest number of annual rings; 2410 f 50 14C years BP) gives a calibrated age for the eruption of 2358 cal years BP; the 20 range for this age is cal years BP. The age of the eruption can also be estimated by calculating the weighted mean of the six outer-ring 14C ages from Lillooet River canyon (GSC-5190, GSC-5203, GSC-5366, GSC-5403, GSC-5631, and GSC- 5675; Fig. 6; Table 1). The weighted mean, 2435 f 26 14C years BP (20 error term), gives calibrated ages of 2373, 2391, and 2436 cal years BP, with a 2a range of cal years BP. The weighted mean, however, may be years too old, because the six samples comprise, on average, a few tens of rings. If the weighted mean is arbitrarily reduced by 15 years, to 2420 f 26 14C years BP, the calibrated age and 20 range are 2362 and cal years BP, respectively. These values are nearly identical to those obtained from the single, most precise 14C age. Discussion and conclusion The mean calendric age of the White River eruptive event, determined from four 14C ages on stumps buried in tephra, is 1147 cal years BP (AD 803). Minimum and maximum ages (20 limits), based on the two most precise 14C ages, are 1014 and 1256 cal years BP (AD 936 and 694). The weighted mean of the six outer-ring 14C ages that approximate the time of the Bridge River eruption is 2435 f 26 14C years BP, similar to the most precise of these six ages (2410 f 50 14C years BP). The outer-ring age data collectively show that the Bridge River eruption probably occurred around 2360 cal years BP (4 1 1 BC). Minimum and maximum ages, determined from the adjusted weighted mean I4C age, are 2349 and 2704 cal years BP (400 and 755 BC). Further refinements in estimates of the ages of the White River and Bridge River tephras could possibly be made, although this is beyond our means at present. High-precision 14C ages with la error terms of years could be obtained by long decay counting of samples in heavily shielded counters. The precision is greater than that obtained by conventional liquid scintillation, gas-proportional, and accelerator mass spectrometry methods, and leads to smaller calibrated age ranges. Another approach might be to correlate ring patterns of stumps buried in the White River tephra with chronologies based on ring patterns of old living trees in Alaska and British Columbia. There are living trees more than 1400 years old in southwestern British Columbia that could be used for this purpose (M.L. Parker, personal communication, 1981). If the fossil material were to correlate well with ring series obtained from living trees, the time of the White River eruption could potentially be determined to a single year. This could not be done for the Bridge River

7 1178 Can. J. Earth Sci. Vol. 32, 1995 Fig. 6. Plot of calibrated 14C ages (Table 1) defining the age of the Bridge River tephra. The toned pattern shows 20 limits calculated from the weighted mean of six outer-ring 14C ages on stumps buried in tephra and pyroclastic flow deposits (GSC-5190, GSC-5203, GSC-5366, GSC-5403, GSC-5631, and GSC-5675). The vertical line defines the mean calendric age adjusted for the number of annual rings that were dated (see text for details). Calendric ages for GSC-2571, GSC-5203, GSC-5633, GSC-5675, and GSC-5690 were adjusted by the number of tree rings between the dated sample and the outermost ring. There are two or more calendric ages for some of the mean 14C ages; this is indicated by multiple symbols along horizontal lines. The thick and thin horizontal lines represent, respectively, la and 20 age ranges, calculated using an error multiplier of 2.0. See Fig. 3 for calibration details. I I GSC-577 Minimum - S-580 age GSC-1520 S-581 Approximate age Maximum age Calendric age (years before AD 1950) eruption, however, without extending the ring record of living trees with ring series from fossil logs and stumps that range in age from about 1000 years old to more than 2500 years old. Acknowledgments Roger McNeely (Geological Survey of Canada) provided the critical new 14C ages reported in this paper. Shane Dennison, Matthew Evans, Olav Lian, and Eugene MacDonald assisted in the field, and Tonia Williams drafted the figures. Gerald Osborn and John Westgate reviewed the paper and contributed to its improvement. References Alley, N.F The palynology and palaeoclimatic significance of a dated core of Holocene peat, Okanagan Valley, southern British Columbia. Canadian Journal of Earth Sciences, 13: Bacon, C.R Eruptive history of Mount Mazama and Crater Lake caldera, Cascade Range, U.S.A. Journal of Volcanology and Geothermal Research, 18: Blake, W., Jr Geological Survey of Canada radiocarbon dates XXVI. Geological Survey of Canada, Paper Bostock, H.S Geology of northwest Shakwak Valley, Yukon Territory. Geological Survey of Canada, Memoir 267. Clague, J.J Natural hazards. In Geology of the Cordilleran Orogen in Canada. Edited by H. Gabrielse and C.J. Yorath. Geological Survey of Canada, Geology of Canada, No. 4, pp (Also Geological Society of America, The Geology of North America, Vol. G-2.) Evans, S Landslides and damming events associated with the Plinth Peak volcanic eruption, southwestern British Columbia. In Geotechnique and natural hazards. BiTech Publishers, Vancouver, pp Fuller, E.A Yukon River evolution since White River Ash. M.Sc. thesis, Simon Fraser University, Burnaby, B.C. Hughes, O.L., Rampton, V.N., and Rutter, N. W Quaternary geology and geomorphology, southern and central Yukon (northern Canada). 24th International Geological Congress (Montreal), Guidebook, Field Excursion A1 1. Lerbekmo, J.F., and Campbell, F.A Distribution, composition, and source of the White River Ash, Yukon Territory. Canadian Journal of Earth Sciences, 6: Lerbekmo, J.F., Westgate, J.A., Smith, D.G.W., and Denton, G.W New data on the character and history of the White River volcanic eruption, Alaska. In Quaternary studies. Edited by R.P. Suggate and M.M. Cresswell. Royal Society of New Zealand, Bulletin 13, pp

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