distribution, stratigraphy, petrochemistry, and ......weighed into a 1.5 cm x 1.5 cm plastic packet....

Distribution, stratigraphy, petrochemistry, and palaeomagnetism of the late Pleistocene Old Crow tephra in Alaska and the Yukon

J . A. WESTGATE AND R. C. WALTER Department of Geology, Scarborough Campus, The University of Toronto, Scarborough, Ont., Canada MIC l A 4

G. W. PEARCE Department of Earth and Planetary Sciences, Erindale Campus, The University of Toronto,

Mississauga, Ont., Canada L5L IC6

AND

M. P. GORTON Department of Geology, The University of Toronto, Toronto, Ont., Canada M5S 1Al

Received October 25, 1984 Revision accepted January l I, 1985

The late Quaternary Old Crow tephra is a two-pyroxene, calc-alkaline dacite whose known areal extent is broadly delimited by a triangle with apices at the Seward Peninsula, the Wrangell Mountains, and the Old Crow Basin in the northern Yukon. Everywhere the tephra is fine grained (Mdu = 5.33 2 0.32) and moderately to poorly sorted (6u = 1.33 * 0.29). Samples from the extremities of the fall-out zone have similarity coefficients of 0.95.

A palaeomagnetic excursion is recorded in sediments just below Old Crow tephra in the Fairbanks area and at Imuruk Lake on the Seward Peninsula. A short, full reversal of the magnetic field has been preserved at the corresponding stratigraphic level near Old Crow in the northern Yukon. Chronological controls based on I4C and fission-track dates and sedimentation rates show that these palaeornagnetic features are almost certainly partial records of the complex geomagnetic Blake Event, which occurred 100- 120 ka ago.

New petrochemical data demonstrate that the source of Old Crow tephra is in the eastern Aleutian arc.

Le tephra d'Old Crow d'age quaternaire supCrieur est forme d'une dacite calco-alcaline a deux pyroxbnes et I'aire d'Ctendue connue est grossikrement dClimitCe par un triangle dont les sommets co~ncident avec la pCninsule Seward, les monts Wrangell et le bassin d'Old Crow dans le nord du Yukon. En tous lieux le tephra est h grain fin (Mdu = 5,33 * 0,32) et le tri des particules varie de modere a pauvre (6u = 1,33 + 0,29). Les echantillons en provenance des deux extrCmitCs de la zone d'truption prCsentent des coefficients de similitude de 0,95.

Une diffkrence palComagnCtique est observke dans les sediments immediatement sous-jacents au tephra d'Old Crow dans la rkgion de Fairbanks et au lac lmuruk sur la pCninsule de Seward. Une inversion du champ magnCtique, Ctroite mais totale, a Ctt preservCe au m&me niveau stratigraphique prbs d'Old Crow dans le nord du Yukon. Les Ctudes chronologiques de contrale par I4C et traces de fission revblent que ces particularitCs palComagnCtiques ne constituent certainement qu'un rkgistre partiel de I'CvCnement gComagnttique de Blake datant d'il y a 100- 120 ka.

Les donnCes p6trochimiques recentes indiquent que la source du tephra d'Old Crow se situe dans la partie orientale de I'arc des AlCoutiennes.

{Traduit par le journal] Can. J. Earth Sci. 22, 893-906 (1985)

Introduction Distribution and stratigraphy In our earlier studies we (I) established the widespread oc-

currence of the dacitic Old Crow tephra across Alaska and the Yukon; (2) determined its age as being between the limits of 60 and 120 ka; (3) noted that the bulk volume of tephra erupted probably exceeded 50 km", although the source volcano is unknown; and (4) documented its important contribution towards establishing a correlation of late Pleistocene sediments laid down in diverse environments across the breadth of Alaska and into the Yukon (Westgate 1982; Westgate et al. 1983). We present in this paper a detailed documentation of the composition of all the samples that we have attributed to Old Crow tephra. These data will allow an independent assessment of our claim that they are all equivalent and, furthermore, enable other workers readily to identify this volcanic unit. For example, interdisciplinary applications involving reconstruction of palaeovegetation maps and synoptic palaeoclimatic charts (Schweger and Matthews 1984) and climatic history studies (Matthews and Schweger, in press) are already underway. We define the palaeomagnetic properties of the sediments that immediately enclose the tephra and assess their chronological significance, offer a revised distribution, and further speculate on the location of the source vent.

Our present understanding of the distribution of Old Crow tephra is shown in Fig. I , and details on the location and stratigraphical setting of each sample are listed in Table 1. A triangle, whose apices are located at Imuruk Lake on the Sew- ard Peninsula (UT 37 I ) , Old Crow Basin in the northern Yukon (UT 50), and the Wrangell Mountains in southeastern Alaska (UA 338), broadly outlines the known areal extent of this tephra. Thickest occurrences are in the Fairbanks area (UA 739), where values are in the range of 10-30 cm. At the apices of the aforementioned triangle, thickness values are 10, 5, and 1 cm, respectively. Grain-size characteristics vary only slightly over this large area: everywhere the tephra is fine grained and moderately to poorly sorted (Fig. 2).

Old Crow tephra occurs in varied sedimentary contexts. The Imuruk Lake (UT 371), Fairbanks (UA 739, UA 368, UT 388, UA 348, UT 543), and northern Yukon (UT I , UT 50) sites are located within the unglaciated zone and are associated with lacustrine, aeolian, and alluvial deposits, respectively. The Koyukuk Basin occurrences (UT 114, UT 115) are within the region affected by glaciers from the Brooks Range, and the southwestern Yukon sites (UA 338, UA 339) are positioned within the area formerly covered by glaciers from the Saint

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8 94 CAN. J. EARTH SCI. VOL. 22. 1985

GULF OF A L A S K A

0 no Z(K1 300 - RG. 1. Location of samples attributed to Old Crow tephra. Stippled areas are the approximate locations of Quaternary volcanic vents (King

1969). Details on location and stratigraphy are given in Table 1.

Elias Mountains. The single occurrence in west-central Yukon (UT 361) is associated with a terrace sequence that has been correlated with advances and retreats of the Cordilleran Ice Sheet (Table 1). Given this diversity of sedimentary environments across Alaska and the Yukon, the important stratigraphic role of Old Crow tephra is obvious.

Methods Rare-earth and other trace elements were determined by in-

strumental neutron activation analysis (INAA), using high- purity glass separates. Seventeen samples were analyzed from 11 localities, including four replicate samples from the type locality in the Old Crow Basin (UT 1). All samples were irradiated at the University of Toronto low-flux reactor (SLOWPOKE 11) using the procedures outlined by Barnes and Gorton (1984).

Portions of each glass separate were pulverized in an AI20, disc grinder, and approximately 0.25 g of each powder was weighed into a 1.5 cm X 1.5 cm plastic packet. Up to 15 samples and two standards were then placed in a 1 cm x 5.5 cm polythene vial. The vials were irradiated for 16 h with a flux of 2.5 X 10" neutrons cm-2 s-'. Flux gradients within the vials are less than 2%. An in-house basalt standard (UTB-I), which is well calibrated against numerous international standards (Barnes and Gorton 1984), was analyzed with each irradiation vial. All samples were counted for 3 h at 7 and at 40 days after irradiation using a coaxial intrinsic Ge detector. Dead time was less than 5%. Peak stripping was accomplished by an on-line CANBERRA series-80 multi-channel analyzer, and data reduction was done by a microprocessor using a programme that corrects for interfering isotopes.

Analytical precision by these methods has been defined by Barnes and Gorton (1984) using 10 analyses of standard UTB- 1. The percentage errors for Old Crow tephra (Table 2) are slightly higher because of lower values than in UTB-1 and especially because of higher Na,O. Errors for Nd and Tb are somewhat greater as a result of interferences. Eu values for some samples are probably influenced by the presence of feld- spar microlites in the glass shards and pumice. This is certainly true for the White River Ash and Sheep Creek tephra but is less so for Old Crow tephra, which is composed predominantly of clear, bubble-wall shards.

The energy-dispersive microprobe analyses were done on an ETEC Autoprobe and an ARL "EMX microprobe. The operating conditions are specified in Tables 3 and 4. The glass analyses were done on the ETEC probe because, as a result of modifications made by J. Rucklidge of the University of Toronto, the detector is very close to the specimen. This condition permits X-ray data to be collected using very small electron beam currents, which lowers the mobility of sodium to levels that make possible reasonable quantitative estimates of sodium concentration. The close agreement in Na20 values of Old Crow tephra glass, as determined by three different methods, supports this assertion (Table 5). These results are in accord with the earlier findings of Goodhew and Gully (1974).

The good precision of the microprobe analyses is seen in the low standard deviations of the glass analyses (Table 3) and the firm correlation of CaO and K20 contents with differentiation index, despite the small absolute range in concentration levels of these oxides (Figs. 3, 4). The probability that no correlation exists between these variables is less than 10%.

Oriented sediment samples for palaeomagnetic analysis were

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WESTGATE ET AL.

TABLE 1. Physical and stratigraphical attributes of Old Crow tephra samples

Location Sample No. (N lat., W long.)

Thickness Grain size (cm) ( M d 4 4 ) Stratigraphical setting

In cryogenically deformed fluvial deposits just below Lake Kutchin sediments (Westgate et al. 1983; Jopling et al. 198 1)

Same as UT 1

Glass has a fission-track age of < 1 16 000 years (3a) (Naeser et al. 1982)

14 C age of wood -5 m above tephra is 41 100 k 1650 (GSC-2574). Bone -1 m above tephra has a U,-series age of 81 000 5 7500 ( l a ) (Morlan, in press)

In organic silts above similar material with large ice-wedge casts. Sequence is on high terrace of Ogilvie River; it is below general pediment surface in area but above supposed "Reid" terrace (V. Rampton, written communication, 1982)

2 m below surface in silts that overlie Mirror Creek Till (V. Rampton, written communication, 197 1)

In peaty silts immediately below Macauley Till (Rampton 197 1)

In loess that mantles weathered schist (Westgate et al. 1983)

14 C age of associated organics

> 50 000 (GSC- 1925)

Thermoluminescence dates on the enclosing loess (A. Wintle, Cambridge University) and volcanic glass (G. Berger, Simon Fraser University) are pending

Wood in overlying Eva Formation is > 56 900 years old

In upper part of Gold Hill Loess (PCwC 1975)

In Gold Hill Loess - 1 m below contact with overlying Goldstream Formation (PCwC 1975)

Near top of Gold Hill Loess In middle of 30 m thick silt

deposits with several prominent organic-rich horizons; overlies 10 m of fluvial gravels (M. Edwards, written communication, 1983)

In loess-aeolian sand deposit of probable Delta age (PCwC and Reger 1983)

Near base of lacustrine silt of late Itkillik age; overlies glacial drift of Kobuk and earlier glaciations (Westgate et al. 1983)

Similar to UT 114

14 C age of wood -5 m above tephra is > 56 000 (QL- 1282)

14 C of wood -2 m above tephra is 52 800 k 1300 (QL- 1283)

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896 CAN. I. EARTH SCI. VOL. 22, 1985

TABLE I . (Concluded)

Location Thickness Grain size Sample No. (N lat., W long.) (cm) ( M d 4 , ~ 4 ) Stratigraphical setting

UT 37 1 65"21.5', 163'12' LO n.d. Occurs at depth of 525-535 A palaeomagnetic excursion, cm in Lake lmuruk V core interpreted as the Blake within the GIV pollen Event, occurs - I m below subzone (Shackleton 1982) the tephra and suggests an

age of slightly less than 1 10 000 years

NOTES: Grain-size parameters are those of lnman (1952); n.d. = not dete~

99 t

I I I I I I I I 1 2 3 4 5 6 7 8 9 l b

Grain diameter $

FIG. 2. Grain-size distribution of Old Crow tephra samples. Small variability is shown despite the large area from which the samples were recovered. Sample localities are given in Table 1.

collected in plastic cubes with 2 cm long sides. At the Old Crow site (Fig. 1, UT I) the cubes were gently tapped into the alluvial silt and fine sand (Pearce et al. 1982), but at the Halfway House site near Fairbanks (Fig. 1, UA 739) the sediment is a weakly cohesive loess, which had to be carved in situ to the shape of the plastic cube. The latter was then placed over the sediment pedestal and oriented prior to detachment from the exposure. At least two samples were collected at each stratigraphic level.

Remanent magnetic moment measurements were made using a cryogenic magnetometer built by Develco Corporation. This magnetometer has a 9 cm access hole, so the loose magnetic coupling for small samples limits its sensitivity to about 1-2 X lo-'' A m2. However, this is not a difficulty, since the samples generally had 100- 1000 times this moment. Alternating field demagnetization was done with a Schonstedt instrument, which did not introduce appreciable spurious moments with the peak alternating fields normally used.

Petrochemistry Bulk samples of Old Crow tephra are calc-alkaline dacites

(Westgate et al. 1983), although the glass shards range in composition from rhyolite to dacite, using the classification of Irvine and Baragar (1971) (Figs. 3, 4; Table 3). This compositional range is not as evident on ternary diagrams (Fig. 5) .

mined because of inadequate sample size. Location of samples shown in Fig. 1 .

TABLE 2. Analytical precision of instrumental neutron activation analyses

Old Crow tephra Old Crow tephra (all samples) (UT 1)

SD Error SD Error - X ( 1 0 ) (%I X ( 1 4 (%I

La 25.3 2 0 . 9 3.6 25.5 20 .8 3.1 Ce 51 2 3 5.9 51 2 2 3.9 Nd 24 2 3 12.5 23.5 21 .5 6.4 Sm 5.8 2 0 . 4 6.9 5.76 50.28 4.9 Eu 0.9 20.10 11.1 0.95 20.09 9.5 Tb 0.86 k0.12 13.9 0.85 20.11 12.9 Y b 3.7 2 0 . 2 5.4 3.80 20.12 3.2 Lu 0.65 20.06 9.2 0.62 20.01 1.6

n 17 4

NOTES: Element concentrations in parts per million; n = number of analyses.

The glass shards are mostly clear, bubble-wall fragments. Hypersthene, augite, plagioclase, titanomagnetite, and ilmenite are the major mineral constituents; hornblende, sanidine, biotite, apatite, and zircon occur in minor amounts. The plagioclase is ubiquitous and its composition is highly variable (An,-,,). On the other hand, sanidine has been observed only in the very distal samples from the northern Yukon (Fig. 6; Table 4). The average composition of the hypersthene is W O , E ~ ~ ~ F S ~ ~ and the corresponding composition of the augite is W O ~ , E ~ ~ ~ F S , ~ (Fig. 7; Table 6). Two magnetite phases have been observed. The phase with lower titanium has been seen only in the northern Yukon samples. The other phase shows small compositional variability across the fallout zone, as does the ilmenite (Fig. 8; Table 7). Crystallization of the Fe-Ti oxides took place at temperatures close to 860°C and oxygen fugacity (log foZ) values near -12, as determined by the method of Buddington and Lindsley (1964) with the modifications of Anderson (1968).

The concentrations of trace elements, including the rare- earth elements (REE), are given in Table 8, and the REE pattern is shown in Fig. 9. Total REE concentrations are quite high: for example, they are noticeably greater than in some well known calc-alkaline dacites of western North America (West- gate and Gorton 1981; Westgate et al. 1983). The REE profile has a gentle slope (La/Yb = 7), the light rare earths (LREE) show a moderate enrichment, and a distinct, negative Eu anom- aly (Eu/Eu* = 0.5) exists.

The close agreement in trace-element composition of the glass from the several samples attributed to Old Crow tephra is demonstrated in the similarity coefficient matrix (Table 9).

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WESTGATE ET AL.

I 1 I I I I 86 87 88 89 90 91

Differentiation lndex

FIG. 3. CaO content plotted against the corresponding differentiation index for 47 analyses of glass shards from Old Crow tephra. The regression analysis gives r = -0.62.

1 I I 1 I I I 86 87 88 89 90 91

Differentiation lndex

FIG. 4. K20 content plotted against the corresponding differentiation index for 47 analyses of glass shards from Old Crow tephra. The regression analysis gives r = 0.64.

These coefficients were determined according to the method of Borchardt et al. (1971) and are based on a string of 13 trace elements. Samples with identical compositions have a similarity coefficient of 1 .O. In this study, coefficient values range from 0.85 to 0.96. Samples from the extremities of the fallout zone (UT 1, UT 371, UA 339) have values as high as 0.95. Furthermore, it should be noted that the compositional variability at one site (UA 739, UT 502, UT 506, UT 510) is as great as that exhibited in samples from the known limits of the distribution. Thus, these geochemical data argue strongly for equivalence of all samples.

The REE composition of other felsic tephra deposits across Alaska and the Yukon is compared with that of the Old Crow tephra in Fig. 10 and Table 10. Two very distinctive types of pattern can be discerned. One group possesses high REE abun- dances with relatively flat profiles (La/Yb = 4-9) and distinct, negative Eu anomalies; the other shows steep profiles (La/Yb = 20-30) with small Eu anomalies and low levels of heavy rare-earth elements (HREE).

Tephra in the former group is rich in pyroxenes and has glass shards that are predominantly bubble-wall fragments. Vents in the eastern Aleutian arc produce tephra with these characteristics, as is demonstrated by the Katmai tephra (Fig. 10). The relatively high, flat REE patterns of this group, with its pronounced Eu anomalies, are characteristic of magmas produced by fractionation in high-level, crustal magma chambers. Old Crow tephra is remarkably similar to the 19 12 Katmai tephra,

m - m - W ~ * N - 0 - 0 0 0 0

2 d d d d d d d d +I + I +I tl +I + I t l + I +I * - o r - w m m w w ~ i 2 2 2 2 4 2 1 I-

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898 CAN. I . EARTH SCI. VOL. 22, 1985

TABLE 4. Average composition of feldspars in Old Crow tephra

Sanidine Plagioclase

UT1 UT50 U T I UT50 UT388 UT114 UT115 UA739 UT371 UA339 UA348 UA338

SiOz 65.24 65.35 56.70 56.86 56.99 53.46 56.21 52.57 55.72 56.87 57.35 53.56 A1203 18.56 18.37 26.99 26.72 26.78 29.05 27.65 29.65 27.64 26.46 26.28 28.56 FeO - - - 0.39 0.35 0.52 - 0.63 0.41 0.33 0.37 0.47 CaO - 9.89 9.71 9.50 12.35 10.15 13.19 10.64 9.98 9.32 12.32 -

Na20 0.47 0.47 5.95 5.65 5.54 4.22 5.79 3.96 5.19 5.40 5.50 4.21 K20 15.27 16.12 0.31 0.24 0.25 0.28 0.20 0.16 0.27 0.18 0.25 0.15

Total 99.54 100.31 99.84 99.57 99.41 99.88 100.00 100.16 99.87 99.22 99.07 99.27 (n) (5) (2) (3) (4) (5) (8) (3) (5) (10) (5) (5) (5)

Mol. % Or 95.5 95.8 1.8 1.4 1.5 1.7 1.1 0.9 1.6 1.1 1.5 0.9 Ab 4.5 4.2 51.2 50.6 50.6 38.5 50.2 34.9 46.3 48.9 50.9 37.9 An - - 47.0 48.0 47.9 59.8 48.7 64.2 52.1 50.0 47.6 61.2

NOTES: All determinations made on an ARL "EMX" microprobe fitted with an Ortec Si(Li) detector. Microprobe operated at 20 kV with beam current set to give 3000 counts/s on willemite; sample current -20 nA; counting time 100 s. Data reduction by a modified version of PESTRIPS. Standards used: albite (CCNM 157) and Hohenfels sanidine (CCNM 158); n = number of analyses, each on a separate grain. Most grains have an andesine to labradorite composition. Errors (2u) due to analytical factors are within 10%.

COO

FIG. 5. Ternary plots of glass composition based on 49 analyses of glass shards from Old Crow tephra.

TABLE 5. Comparison of NazO values determined in glass of Old Crow tephra

Flame photometry 3.54k0.18 4 IN AA 3.6850.12 14 Microprobe 3.7450.10 10

NOTE: n = number of analyses.

a condition that has important implications for its source. Tephra in the group with steep REE profiles is flooded with

hornblende and is much more pumiceous. These steep, linear REE patterns lack Eu anomalies and cannot have experienced significant fractionation in the crust. Relatively rapid ascent from a deeper source is the most likely origin for this group. Vents in the Wrangell Mountains produce tephra of this type, for the White River Ash comes from Mount Bona (Lerbekmo et al. 1975). It is very likely that the Sheep Creek tephra and Fort Selkirk tephra are also derived from vents in the Wrangells, given their distribution, bed thickness, and grain size (Westgate, in preparation).

These relationships between slope of the REE profile and

FIG. 6. Or- Ab- An ternary plot based on seven sanidine and 53 plagioclase analyses. Most plagioclase analyses plot in the andesine and labradorite fields.

mineralogy may be explained, at least in part, by the large distribution coefficients of hornblende for the middle and heavy REE, whereas the presence of pyroxenes may lead to only a slight enrichment of the LREE over the HREE in the melt (Arth and Barker 1976; Hanson 1980).

We conclude that the geochemical data in toto offer irre- futable evidence that all samples attributed to Old Crow tephra are comagmatic and are derived from one or possibly several closely spaced eruptions.

Palaeomagnetism The palaeomagnetic properties of tephra beds and their

associated sediments provide additional controls for correlation and offer the possibility of indirectly dating the sedimentary sequence. Conversely, distinctive tephra layers offer an un- paralleled opportunity to evaluate the areal consistency of the palaeomagnetic record preserved in sediments.

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~ 1 ~ 0 , TiO, FeO MnO MgO CaO

Total (n)

Si02 A1203 TiO, FeO MnO MgO CaO

0.99 0.89 1.13 0.87 0.23 - 0.26 0.21

21.66 21.84 20.58 22.33 1.37 1.22 0.65 1.34

21.64 21.71 23.06 21.72 1.11 1.13 1.34 1.19

100.03 100.29 100.56 100.92 (4) (2) (5) (5)

Augite 53.07 50.40 52.18 52.69

1.64 3.63 2.73 1.23 0.31 0.63 0.48 0.27 9.26 12.93 9.97 10.32 0.35 0.31 0.20 0.51

15.01 15.21 15.42 13.60 20.09 16.26 18.93 20.73

TABLE 6. Average composition of pyroxenes in Old Crow tephra

UT 1 UT 50 UT 388 UT 114 UT 115 UA 739 UT 371 UA 338

Hypersthene SiO, 53.03 53.50 53.54 52.26 53.29 53.12 52.77 53.16

1.01 1.06 0.64 0.15 0.20 0.17

21.63 21.44 22.16 1.17 1.12 1.34

22.13 22.17 21.59 1 .07 I .06 1.01

100.28 99.82 100.07 (6) (8) (5)

Total 99.73 99.37 99.91 99.35 100.26 100.01 99.93 99.52 (n) (3) (1) (3) (4) (5) (4) (3) (5)

NOTES: All determinations made on an ARL "EMX" microprobe. The operating conditions are specified in Table 4 . Standards used: Wakefield diopside (CCNM 151), albite (CCNM 157), and pyrite (CCNM 168). Average mole percent for augite is wollastonite, 41.1; enstatite, 43.2; ferrosilite, 15.6; and for hypersthene is wollastonite, 0.8; enstatite, 64.4; ferrosilite 35.1.

CoSi03

v v v v v MgSiOj 10 20 30 40 50 60 70 00 90 F ~ S ~

FIG. 7. Composition of pyroxenes in Old Crow tephra. Analyses plot in the augite (n = 34) and hypersthene (n = 43) fields.

Palaeomagnetic data on sediments containing Old Crow tephra are now available from three localities: Old Crow in the northern Yukon (UT I ) , Halfway House near Fairbanks (UA 739), and Imuruk Lake (UT 371). The distance between the first and last of these localities is over 1000 km (Fig. I). At its type locality, Old Crow tephra is preserved in fine-grained alluvial sediments that are exposed in high bluffs along the Porcupine River near the village of Old Crow. A complete section was sampled earlier (Pearce et al. 1982) but did not include the tephra. This site is about 1 km upstream from the section described herein. The same tephra occurs in loess at Halfway House and in lacustrine silts at lmuruk Lake. The palaeomagnetic study at the latter site was done by Marino (1977).

The natural remanent moments of samples from Old Crow and Halfway House were measured and then subjected to a normal sequence of alternating magnetic field demagnetization steps up to at least 24 kA/m (300 Oe). Some moments were too weak to be measured accurately after this treatment. A few typical behaviours are illustrated in Fig. 11. The moments were

FIG. 8. TiOz-MgO plot for ilmenite (n = 55) and titanomagnetite (n = 84).

generally stable, with only small present field components being perceivable in the reversed samples of the Old Crow site. These components were always removed by demagnetization with 12 kA/m, and the results after this cleaning were univer- sally used in further analysis. The median destructive field of 16-24 kA/m suggests that the natural moment in all samples is held by magnetite and is of depositional or post-depositional origin.

The cleaned moments were arranged according to stratigraphic position, and the average of samples at the same level

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900 CAN, J . EARTH SCI. VOL. 22, 1985

FIG. 9. Chondrite-normalized REE pattern of Old Crow tephra based on 17 analyses of glass separates.

TABLE 7. Average composition of Fe-Ti oxides in Old Crow tephra

UT 1 UT 1 UT 114 UT 115 UA 739 UT 371 UA 339 UA 338

Magnetite Ti02 5.14 9.46 9.69 9.85 9.37 9.02 9.36 9.71 FeO 34.47 38.25 38.63 38.57 38.21 37.97 36.59 36.56 Fe203 54.22 47.33 46.23 46.27 47.85 48.23 48.31 47.33 MgO 1.36 1.22 1.26 1.51 1.26 1.23 1.64 1.86 MnO 0.33 0.50 0.46 0.46 0.48 0.54 0.77 0.56 ' 4 1 2 0 3 3.12 1.99 1.97 2.13 1.90 2.14 2.02 2.02 SiOz 0.59 0.52 0.41 0.42 0.48 0.53 0.13 0.11

Total 99.23 99.27 98.65 99.21 99.55 99.66 98.82 98.15 (n) (4) (11) (26) (21) (17) (5) (10) (10)

Mol.% Usp. 16.7 28.7 29.3 29.5 28.3 27.4 27.0 28.9

llmenite Ti02 43.69 43.94 44.20 43.43 42.22 42.34 FeO 34.56 35.03 35.20 34.98 32.58 32.27 FezO, 16.82 17.55 17.30 17.96 20.04 20.01 MgO 2.56 2.41 2.45 2.21 2.52 2.97 MnO 0.71 0.67 0.63 0.61 0.88 0.58 A1203 0.20 0.24 0.26 0.00 0.23 0.27 SiOz 0.45 0.39 0.38 0.44 0.00 0.06

Total 98.99 100.23 100.42 99.63 98.47 98.49 (n) (2) (15) (16) (13) (10) (10)

Mol.% Hem. 16.2 16.8 16.5 17.0 19.5 19.4

NOTES: Instrumental conditions are the same as those specified in Table 4. Standards used: 0dergarden ilmenite and Elba hematite (CCNM 166). No ilmenite was found in UT 371. Errors (2u) are within 5% for Ti, Fe, and Mg and within 25% for other elements.

was determined. Then a simple five-point running average was made of the moment directions as a function of stratigraphic level, and palaeopole positions were calculated from these. The pole positions are plotted as latitude (Fig. 12) and longitude (Fig. 13) against stratigraphic level and on equal-area nets (Fig. 14).

A striking feature of the palaeomagnetic record exists below the tephra at both sites. At Old Crow there is a reversed direction episode, which had been hinted at in the earlier collection (Westgate et al. 1983). It commences at the -425 cm level, where the inclination is 80". The inclination becomes shallow and then negative just below the -400 cm level and then steepens to -80' at the -270 cm level, after which it rapidly reverts to a positive value of 75". Thus, a full reversal of the

magnetic field occurs in a stratigraphic interval of 160 cm. At Halfway House there is an event during which inclination becomes distinctly shallow but does not reverse, whereas de- clination changes by about 70" in a pattern similar to that at Old Crow.

The similar stratigraphic position of these features and the fact that no other sizeable feature exists in either record suggest that they might be a local record of the same palaeomagnetic event. However, a more likely interpretation is that the palaeomagnetic excursion at Halfway House is slightly younger than the short, full reversal seen at Old Crow. In support of the latter view, it is unlikely that the closer position ofthe excursion to the tephra at Halfway House is due to a slower sedimentation rate because it has recently been demonstrated that loess is

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WESTGATE ET AL. 90 1

deposited rapidly (Wintle et al. 1984). Although the loess here was sampled at every 10 cm interval between the tephra and underlying bedrock, no reversed event was detected. This probably means that loess deposition at the Halfway House site began after the reversed event. On the other hand, the absence of the younger excursion at Old Crow could be explained by the slower and more discontinuous sedimentation of the fine- grained alluvium. More importantly, the pole positions determined from all the samples at Halfway House are very similar to those determined from the younger samples in the Old Crow sequence (Fig. 14).

Several years ago, Marino (1977) completed a palaeomagnetic study of a sedimentary core from Imuruk Lake. He recognized a tephra at the 525-535 cm level and documented a palaeomagnetic excursion at the 601 -624 cm level, which is characterized by two major inclination swings involving a shal- lowing of 55". The extrapolated age estimate for this event, based on I4C dates in the upper part of the core, is in the range 106- 136 ka BP and suggested to Marino a correlation with the Blake Event (Smith and Foster 1969). Later, doubt was cast on the reality of this excursion because of the anomalous magnetic fabric, as determined by the anisotropy of magnetic sus- ceptibility method (Marino and Ellwood 1978). We have shown in this paper that the tephra in this core is Old Crow tephra (UT 37 l), and, given the presence of a palaeomagnetic excursion just below this tephra at the Old Crow and Halfway House sites, the reality of the excursion at Imuruk Lake must now be considered as confirmed.

Age and source Old Crow tephra has an age between the limits of 60 and

120 ka, set by I4C and fission-track measurements, respectively (Table 1) (Westgate et al. 1983). The U-series ages of bones on an unconformity just above the tephra in the Old Crow region range from 49 to 118 ka, but mostly cluster around 60-80 ka BP (Morlan, in press). However, the large dispersion in age points to a reworked origin for most of the bones, so these dates do not provide tight chronological control on the tephra. No other direct age determinations on Old Crow tephra or its associated sediments exist at present, although the thermoluminescence (TL) age of the enveloping loess at Halfway House should be available soon (A. Wintle, written communication, 1984) as well as the TL age of the volcanic glass (G. Berger, personal communication, 1984).

Marino (1977) estimated an age in the interval 80- 100 ka, using information gathered at Imuruk Lake. He based this estimate on I4C dates in the upper part of the sedimentary core, the assumption of uniform sedimentation rate, and the identity of the palaeomagnetic excursion beneath the tephra as the Blake Event. Schweger and Matthews (1984) refined the age limits to 87- 105 ka, using the same assumptions in con- junction with information on the chronology of late Quaternary regional vegetation change.

Certainly, the I4C and fission-track data are compatible with the presence of the Blake Event just below the Old Crow tephra because this event most likely occurred some time between 100 and 120 ka ago (Smith and Foster 1969; Kawai et al. 1972; Denham 1976; Verosub 1982).

The source volcano of Old Crow tephra is unknown, although our earlier view favoured an origin in the Wrangell Mountains, with a dispersal axis directed to the northwest (Westgate 1982). Several factors now militate against this view. Despite the discovery of additional occurrences, there is

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CAN. 1. EARTH SCI. VOL. 22, 1985

1 La Ce Pr Nd Sm EU Gd Tb Dy Ho Er Tm Yb Lu

FIG. 10. Chondrite-normalized REE profiles of glass from Old Crow tephra and other widespread felsic tephra units in Alaska and the Yukon. Units designated in thick lines are: Ester Ash, dash - double dot; Katmai tephra (1912 eruption), dash - single dot; Old Crow tephra, continuous line; Lost Chicken tephra, dashed line. Units designated in thin lines are: Fort Selkirk tephra, dash - double dot; White River Ash, dash - single dot; Sheep Creek tephra, continuous line. See Table 10 for more details.

TABLE 9. Similarity coefficient matrix comparing the trace-element composition of glass from samples attributed to Old Crow tephra

NOTE: Trace-element string is La, Ce, Sm, Eu, Tb, Yb, Lu, U, Th, Hf, Sc, Cs, Ba.

TABLE 10. Trace-element concentrations in glass from some widespread felsic tephra beds in Alaska and the Yukon

Lost Chicken White River Ash Sheep Creek tephra Fort Selkirk Old Crow tephra Katmai tephra Ester Ash tephra tephra

(average) UA 200 UA 743 UT 86 UA 452 UA 459 UA 207 UT 40 UT 366 UT 81 - - -.

La 25.2 19.8 26.5 14.1 20.1 21.6 10.0 15.7 13.7 16.4 Ce 5 1 43 61 30 41 39 18 27 25 29 Nd 26 20 - - 12 11 - - - 10 Sm 5.78 6.32 8.25 2.51 2.32 2.69 1.45 1.27 1.21 1.87 Eu 0.89 0.88 1.76 0.49 0.77 0.63 0.52 0.45 0.42 0.52 Tb 0.87 1.01 1.4 0.41 0.26 0.26 0.17 0.14 0.14 0.21 Y b 3.74 4.84 6.10 1.64 0.67 0.76 0.46 0.42 0.46 0.97 Lu 0.65 0.79 1.03 0.29 0.13 0.14 - - 0.07 0.17

U 4.0 2.5 3.1 2.8 2.2 2.4 1.8 1.4 1.3 1.6 Th 9.7 6.2 7.3 6.0 6.0 6.4 2.5 5.7 5.1 5.7 H f 6.27 5.04 7.43 2.03 3.63 3.49 2.07 2.97 2.60 2.52 Sc 7.10 8.50 14.70 1.80 2.80 2.2 3.45 1.60 1.69 2.17 Ta 0.85 0.40 0.90 0.67 0.51 0.62 0.33 0.50 0.39 0.47 Cs 3.82 2.61 3.80 1.74 1.30 1.34 0.64 1.01 0.94 1.25 Ba 1000 940 930 890 970 840 700 830 780 930

La/Yb 6.7 4.1 4.3 8.6 30.0 28.4 21.7 37.4 29.8 16.9 Th/U 2.4 2.5 2.4 2.1 2.7 2.7 1.4 4.1 3.9 3.6

NOTES: Element concentrations (in parts per million) determined by instrumental neutron activation analysis. Some Nd and Lu values were not determined because of interference problems. See the text for further details. UA 200 is the 1912 Katmai tephra collected near the source; UA 743 is from Ester, Alaska; UT 86 and UT 366 are from Lost Chicken, Alaska; UA 452 and UA 459 come from southeastern Alaska; UA 207 is from Fairbanks, Alaska; UT 40 is from McQuestern, Yukon; and UT 81 is from Fort Selkirk, Yukon.

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WESTGATE ET AL

Demagnetizing field Demagnetizing field (kA/m)

F I G . 1 1. Typical alternating field demagnetization behaviour of specimens collected at Old Crow, Yukon (a) and Halfway House, Alaska (b). The locations of these two sites are shown in Fig. 1 as UT 1 and UA 739, respectively. The sediment at Old Crow is alluvial silt and fine sand, whereas that at Halfway House is loess. The stratigraphic level of each sample is given with respect to the base of Old Crow tephra: (1) 0 cm, (2) -305 cm, (3) - 120 cm, (4) 340 cm, (5) -55 cm, (6) -165 cm, and (7) -85 cm. On the equal-area nets, (+) represents positive inclinations and (0) represents negative inclinations; N = north; and n = NRM direction.

still no discernible trend of bed thickening towards the south- east (Fig. 1; Table 1). Tephra plumes of historic eruptions in Alaska mostly have a pronounced easterly component to their dispersal direction (Kienle and Swanson 1983), and no extensive late Quaternary pyroclastic-flow deposits, which are likely to be associated with a large-magnitude eruption, have so far been located in the Wrangell Mountains.

New petrochemical data strongly suggest that Old Crow tephra in fact comes from a volcano located in the eastern Aleutian arc. The close similarity of its REE pattern to that of the 1912 Katmai tephra has already been noted (Fig. 10). An examination of additional petrochemical data on volcanics from the eastern Aleutian arc supports this idea; a source in the western Aleutians is excluded because of the basaltic nature of volcanism there (Kay et al. 1982).

Volcanics of the eastern Aleutian arc are calc-alkaline and show little compositional variation along the trace of the arc. Andesites predominate, but those volcanic centres that lie along segment boundaries are characterized by a higher degree of fractionation, resulting in a more diverse assemblage of rock types, including rhyolites (Kienle and Swanson 1983). Plagio- clase phenocrysts commonly show a wide range in compositions, a range of An3,-,, being considered typical (Kienle and Swanson 1983). Hypersthene varies from Wo3En45F~52 to Wo3En8,,Fs17, and reported compositions of augite range from W045En51F~6 to W044En36F~20 (KOSCO 198 1). Fe - Ti oxides are common accessory phases and give temperature and oxygen fugacity (log f,,) values of 780- 940°C and - 10.7 to - 14.8, respectively ( ~ o s c o 198 1 ; Hildreth 1983). These mineral compositions and T-fo, conditions closely match those

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CAN. J . EARTH SCI. VOL. 22. 1985

Palaeolatitude

C OLD CROW 0

TEPHRA (UTI) I

Palaeolatitude

O r -

FIG. 12. Palaeolatitude of the pole as a function of stratigraphic level for Old Crow (a) and Halfway House (b) locations. Palaeolatitude is calculated from a five-point running average of the natural moments after they had been cleaned by alternating fields to 12 kA/m (about 150 Oe).

Palaeolongitude 45 90 135 180 225 270

I 1 I I t

Palaeolongi tude 4' 90 I3;5 Iep 2p 2?"

FIG. 13. Palaeolongitude of the pole as a function of stratigraphic level for Old Crow (a) and Halfway House (b) locations. Details as for Fig. 12.

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FIG. 14. Palaeopole positions for the interval just prior to and following deposition of the Old Crow tephra at the Old Crow (a) and Halfway House (b) localities. Pole positions are plotted on an equal-area Wulff net with (0) indicating the upper and (*) the lower hemisphere.

documented herein for Old Crow tephra. Given the observed thicknesses of Old Crow tephra and the

large distance of the known fallout area from the late Qua- ternary volcanic centres (Fig. I ) , it is most probable that the bulk volume of tephra erupted exceeded 50 km3. Such an eruption would almost certainly have been accompanied by caldera formation. In the eastern Aleutian arc, calderas are associated with volcanoes on the segment boundaries, where structural discontinuities allow ponding of magma at shallow depths in the crust (Kienle and Swanson 1983). Today, these sites are occupied by Aniakchak, Ugashik, Katmai, and Kaguyak volcanoes. Hence, these volcanoes or their immediate ancestors must be considered as prime candidates for the source of Old Crow tephra.

Acknowledgments It is a pleasure to acknowledge the financial support of

the Natural Sciences and Engineering Research Council of Canada. Some of our field expenses were covered by the Northern Yukon Research Project of the University of Toronto and the Alaska Tephrochronology Center of the University of Alaska at Fairbanks. We have been aided in this work by Paul Colinvaux, Ohio State University; Mary Edwards, University of Washington; Tom Hamilton, United States Geological Survey; Owen Hughes and John Matthews, Geological Survey of Canada; W. Irving, University of Toronto; Troy L. Ptwt, Arizona State University; Vern Rampton, Carp, Ontario; and R. Thorson, University of Alaska at Fairbanks.

ANDERSON, A. T. 1968. Oxidation of the La Blache Lake titaniferous magnetite deposit, Quebec. Journal of Geology, 76, pp. 528-547.

ARTH, J. G., and BARKER, F. 1976. Rare-earth partitioning between hornblende and dacitic liquid and implications for the genesis of trondhjemitic-tonalitic magmas. Geology, 4, pp. 534-536.

BARNES, S.-J., and GORTON, M. P. 1984. Trace element analysis by neutron activation with a low flux reactor (SLOWPOKE 11): results for international reference rocks. Geostandards Newsletter, 8, pp. 17-23.

BORCHARDT, G. A., HARWARD, M. E., and SCHMITT, R. A. 1971. Correlation of volcanic ash deposits by activation analysis of glass separates. Quaternary Research, 1, pp. 247-260.

BUDDINGTON, A. F., and LINDSLEY, D. H. 1964. Iron-titanium oxide

minerals and synthetic equivalents. Journal of Petrology, 5, pp. 310-357.

DENHAM, C. R. 1976. Blake polarity episode in two cores from the Greater Antilles Outer Ridge. Earth and Planetary Science Letters, 29, pp. 422-434.

GOODHEW, P. J., and GULLY, J. E. C. 1974. The determination of alkali metals in glasses by electron probe microanalysis. Glass Technology, 15, pp. 123-126.

HANSON, G. N. 1980. Rare earth elements in petrogenetic studies of igneous rocks. Annual Review of Earth and Planetary Sciences, 8 , pp. 371-406.

HILDRETH, W. 1983. The compositionally zoned eruption of 1912 in the Valley of Ten Thousand Smokes, Katmai National Park, Alaska. Journal of Volcanology and Geothermal Research, 17, pp. 1-56.

INMAN, D. L. 1952. Measures of describing the size distribution of sediments. Journal of Sedimentary Petrology, 22, pp. 125- 145.

IRVINE, T. N., and BARAGAR, W. R. A. 1971. A guide to the chemical classification of the common volcanic rocks. Canadian Journal of Earth Sciences, 8, pp. 523-548.

JOPLING, A. V., IRVING, W. N., and BEEBE, B. F. 1981. Srrati- graphic, sedimentological and faunal evidence for the occurrence of pre-Sangamonian artefacts in northern Yukon. Arctic, 34, pp. 3-33.

KAWAI, N., YASKAWA, K., NAKAJIMA, T., TORI, M., and HORIE, S. 1972. Oscillating geomagnetic field with a recurring reversal discovered from Lake Biwa. Proceedings of the Japan Academy, 48, pp. 186-190.

KAY, S. M., KAY, R. W., and CITRON, G. P. 1982. Tectonic controls on tholeiitic and calc-alkaline magmatism in the Aleutian arc. Journal of Geophysical Research, 87(B5), pp. 4051 -4072.

KIENLE, J . , and SWANSON, S. E. 1983. Volcanism in the eastern Aleutian arc: late Quaternary and Holocene centers, tectonic setting and petrology. Journal of Volcanology and Geothermal Research, 17, pp. 393-432.

KING, P. B. 1969. Tectonic map of North America. United States Geological Survey, scale 1 : 5 000000.

Kosco, D. G. 1981. Part I : The Edgecumbe volcanic field, Alaska: an example of tholeiitic and calc-alkaline volcanism. Part 2: Char- acteristics of andesitic to dacitic volcanism at Katmai National Park, Alaska. Ph.D. thesis, University of California, Berkeley, CA, 249 p.

LERBEKMO, J. F., WESTGATE, J. A., SMITH, D. G. W., and DENTON, G. H. 1975. New data on the character and history of the White River volcanic eruption, Alaska. In Quaternary studies. Edited by

Can

. J. E

arth

Sci

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

Stan

ford

Uni

vers

ity -

Gre

en L

ibra

ry o

n 07

/26/

13Fo

r pe

rson

al u

se o

nly.

906 CAN. J. EARTH SCI. VOL. 22. 1985

R. P. Suggate and M. M. Cresswell. The Royal Society of New Zealand, Bulletin 13, pp. 203-209.

MARINO, R. J. 1977. Paleomagnetism of two lake sediment cores from Seward Peninsula, Alaska. M.Sc. thesis, Ohio State University, Columbus, OH, 183 p.

MARINO, R. J., and ELLWOOD, B. B. 1978. Anomalous magnetic fabric in sediments which record an apparent geomagnetic field excursion. Nature (London), 274, pp. 581-582.

MAITHEWS, J. V. , JR., and SCHWEGER, C. E. In press. Old Crow tephra: its significance for understanding the early and middle Wis- consinan climate history of East Beringia. Syllogeus.

MORLAN, R. E. In press. Pleistocene archaeology in Old Crow Basin: a critical reappraisal. In Proceedings of the loth Congress of the International Union of Prehistoric and Protohistoric Sciences, Mexico City, 1981. Edited by A. L. Bryan. University of Alberta, Edmonton, Alta.

NAESER, N. D., WESTGATE, J. A,, HUGHES, 0 . L., and PEWE, T. L. 1982. Fission-track ages of late Cenozoic distal tephra beds in the Yukon Territory and Alaska. Canadian Journal of Earth Sciences, 19, pp. 2167-2178.

PEARCE, G. W., WESTGATE, J. A., and ROBERTSON, S. 1982. Magnetic reversal history of Pleistocene sediments at Old Crow, northwestern Yukon Territory. Canadian Journal of Earth Sciences, 19, pp. 919-929.

WwE, T. L. 1975. Quaternary geology of Alaska. United States Geological Survey, Professional Paper 835, 145 p.

PEWE, T. L., and REGER, R. D. 1983. Middle Tanana River valley. In Richardson and Glenn highways, Alaska: guidebook to permafrost and Quaternary geology. Edited by T. L. PCwC and R. D. Reger. 4th International Conference of Permafrost, Fairbanks, AK, Guidebook I, pp. 5-45.

RAMFTON, V. 1971. Late Pleistocene glaciations of the Snag-Klutlan area, Yukon Territory. Arctic, 24, pp. 277-300.

SCHWEGER, C. E., and MATTHEWS, J. V . , JR. 1984. Mid- Wisconsinan climatic history for Alaska-Yukon as revealed by Old Crow tephra. American Quaternary Association, 8th Biennial Meeting, Boulder, CO, Program and Abstracts, p. 1 13.

SHACKLETON, J. 1982. Paleoenvironmental histories of Whitefish and Imuruk lakes, Seward Peninsula, Alaska. Institute of Polar Studies, Columbus, OH, Report 76, 50 p.

SMITH, J. D., and FOSTER, J. H. 1969. Geomagnetic reversal in Brunhes normal polarity epoch. Science, 163, pp. 565-567.

VEROSUB, K. L. 1982. Geomagnetic excursions: a critical assessment of the evidence as recorded in sediments of the Bruhnes Epoch. Philosophical Transactions of the Royal Society of London, A306, pp. 161-168.

WESTGATE, J . A. 1982. Discovery of a large-magnitude, late Pleis- tocene volcanic eruption in Alaska. Science, 218, pp. 789-790.

In preparation. Composition of some extensive Quaternary tephra beds in the Yukon Territory.

WESTGATE, J. A., and GORTON, M. P. 1981. Correlation techniques in tephra studies. In Tephra studies. Edited by S. Self and R. S. J. Sparks, NATO Advanced Studies Institute Series, Reidel, Dor- drecht, The Netherlands, pp. 73-94.

WESTGATE, J. A, , HAMILTON, T. D., and GORTON, M. P. 1983. Old Crow tephra: a new late Pleistocene stratigraphic marker across north-central Alaska and western Yukon Territory. Quaternary Research, 19, pp. 38-54.

WINTLE, A. G., SHACKLETON, N. J . , and LAUTRIWU, J . P. 1984. Thermoluminescence dating of periods of loess deposition and soil formation in Normandy. Nature (London), 310, pp. 491 -493.

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distribution, stratigraphy, petrochemistry, and ......weighed into a 1.5 cm x 1.5 cm plastic packet....

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