Tephrochronology, magnetostratigraphy and mammalian faunas of Middle and Early Pleistocene sediments at two sites on the Old Crow River, northern Yukon Territory, Canada

Quaternary Research 79 (2013) 75–85

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Tephrochronology, magnetostratigraphy and mammalian faunas of Middle and Early Pleistocene sediments at two sites on the Old Crow River, northern Yukon Territory, Canada John A. Westgate a,⁎, G. William Pearce b, Shari J. Preece a, Charles E. Schweger c, Richard E. Morlan d, 1, Nicholas J.G. Pearce e, T. William Perkins e a

Department of Geology, University of Toronto, Toronto, ON, Canada M5S 3B1 152 Indian Road, Kingston, ON, Canada K7M 1T4 c Department of Anthropology, University of Alberta, Edmonton, AB, Canada T6G 2H4 d Archaeological Survey of Canada, Canadian Museum of Civilization, Hull, Quebec, Canada J8X 4H2 e Institute of Geography and Earth Science, Aberystwyth University, Wales, SY23 3DB, UK b

a r t i c l e

i n f o

Article history: Received 28 March 2012 Available online 6 October 2012 Keywords: Pleistocene Tephrochronology Magnetostratigraphy Mammalian fossils Old Crow Basin Yukon Alaska

a b s t r a c t Alluvial and lacustrine sediments exposed beneath late Pleistocene glaciolacustrine silt and clay at two sites along the Old Crow River, northern Yukon Territory, are rich in fossils and contain tephra beds. Surprise Creek tephra (SZt) occurs in the lower part of the alluvial sequence at CRH47 and Little Timber tephra (LTt) is present near the base of the exposure at CRH94. Surprise Creek tephra has a glass fission-track age of 0.17± 0.07 Ma and Little Timber tephra is 1.37± 0.12 Ma. All sediments at CRH47 have a normal remanent magnetic polarity and those near LTt at CRH94 have a reversed polarity — in agreement with the geomagnetic time scale. Small mammal remains from sediments near LTt support an Early Pleistocene age but the chronology is not so clear at CRH47 because of the large error associated with the SZt age determination. Tephrochronological and paleomagnetic considerations point to an MIS 7 age for the interglacial beds just below SZt at CRH47 and at Chester Bluffs in east-central Alaska, but mammalian fossils recovered from sediments close to SZt suggest a late Irvingtonian age, therefore older than MIS 7. Further studies are needed to resolve this problem. © 2012 University of Washington. Published by Elsevier Inc. All rights reserved.

Introduction Old Crow Basin in northern Yukon Territory (Fig. 1) escaped glacierization during the late Cenozoic; its thick sedimentary fill, composed mostly of fluviatile and lacustrine sediments, did not suffer the severe erosive effects of glaciers. However, the character of these sediments was controlled at times by the nearby presence of the Laurentide ice sheet to the east and this was certainly true during the late Pleistocene (Hughes, 1972, 1989; Vincent, 1989). A rich archive of environmental change during the Pliocene and Pleistocene is preserved in these sediments (Schweger, 1989) as indicated by their highly fossiliferous nature and chiefly demonstrated by the many bone-covered sand bars along the Old Crow River (Harington, 1989, 2011). Importantly, distal tephra beds, derived from volcanoes of the eastern Aleutian arc and Wrangell volcanic field in Alaska

⁎ Corresponding author. E-mail address: [email protected] (J.A. Westgate). 1 Deceased.

(Westgate et al., 1983; Westgate et al., 1995) offer the prospect of good chronological control and reliable correlations. During the early 1980s, J.V. Matthews, Jr. (Geological Survey of Canada), Charles Schweger, and Richard Morlan conducted reconnaissance studies on Quaternary deposits exposed in the bluffs of Old Crow River. Among the numerous sites they examined, two were of special note because of their highly fossiliferous nature and presence of tephra beds: localities CRH47 and CRH94 (Fig. 1), hereafter for brevity referenced as sites 47 and 94. These site designations were made by C.R. Harington (Canadian Museum of Nature, Ottawa) more than 40 years ago and continue to be used by scientists working in the Old Crow Basin. In 1985, Westgate was invited to participate in stratigraphic studies of these two sites with the aim of throwing light on the age of these sediments by means of tephrochronological and paleomagnetic investigations. This report contains the results of that study. Following a description of the lithostratigraphy, we give the salient characteristics of the tephra beds, noting their glass fission-track ages, and provide information supporting recognition of correlative beds in Yukon and Alaska. The detailed magnetostratigraphic study provides corroborative chronological controls and enables local correlation of the defined lithostratigraphic units. Finally, we present

0033-5894/$ – see front matter © 2012 University of Washington. Published by Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.yqres.2012.09.003

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Figure 1. A: Location of the study area in northern Yukon and the two main sources of late Cenozoic tephra beds across eastern Beringia (shaded). WVF is the Wrangell volcanic field. Open circle in Alaska represents approximate position of the Chester Bluff tephra sample and the one near Dawson represents site D72 on the Sixtymile Creek, Yukon (Preece et al., 2011b). B: Location of sites 47 and 94 along the Old Crow River in the Old Crow Flats, which contains many thaw lakes but only the larger ones are shown in this figure.

paleontological information and interpret its significance with respect to paleoenvironmental reconstruction and stratigraphic age. Lithostratigraphy Site 47 is located in the northern part of the Old Crow Basin on the right bank of the Old Crow River at 68° 15.630′ north latitude, 140° 23.021′ west longitude (Fig. 1). The exposure is north-facing on a tight hairpin bend, nearly 2 km long and about 24 m high. In 1985, it was dissected by three major gullies that separated four sublocalities lettered A–D from upstream to downstream (Fig. 2 A, B). The exposed sediments are divided into four lithostratigraphic units, presented from river level to the top of the exposure. Unit A is massive dark gray lacustrine silt to clayey silt that is at least 4 m

thick, extending below river level (Fig. 3). It is unfossiliferous and contains dropstones, which must have been ice-rafted to their present positions, but there is no evidence suggesting a glaciolacustrine origin. Contact with the overlying unit B is conformable along most of the exposure but at 47D a channel was cut into the top of unit A and infilled with sediments of unit B. Unit B is about 12 m thick and consists of alluvial silt, clayey silt and minor sand. Its sediments are locally deformed, probably as a result of cryoturbation, and very fossiliferous. Autochthonous peat, peaty sediments, wood fragments, and molluscs are abundant throughout, and ice-wedge casts are also present. Numerous disconformities undoubtedly exist in this alluvial sequence. Surprise Creek tephra, which is thin, discontinuous, and has a maximum thickness of 3 cm, is just over 4 m above the base of unit B at 47A (Fig. S1A; the “S” designation to figures and tables

J.A. Westgate et al. / Quaternary Research 79 (2013) 75–85

Figure 2. A: Location of studied sections (A, B, C and D) along the Old Crow River at site 47. The black dot refers to section 47A; B: North-facing bluff of the Old Crow River at site 47A showing the three trenches where paleomagnetic samples were collected and the stratigraphic position of Surprise Creek tephra (arrow). Photograph taken in 1985. See Fig. 1 for location of this site.

indicates those illustrations that are available in the Supplementary Folder). The massive clay (unit C2) and underlying laminated silt (C1) with a total thickness of ~6 m is thought to represent late Wisconsinan inundation by glacial meltwater originating from the Laurentide ice sheet in the Richardson Mountains to the east (Hughes, 1972, 1989; Zazula et al., 2004; Kennedy et al., 2010). Unit D consists of ~0.5 m of peat and soil at the modern surface (Fig. 3). The same lithostratigraphic sequence exists at 47C, but Surprise Creek tephra is absent (Fig. 3). Site 94 (68° 03.727′ north latitude; 139° 46.028′ west longitude) is on the right bank of the Old Crow River about 65 km by air to the southeast of site 47 (Figs. 1 and S2). The exposure is slightly over 26 m high and is made up predominantly of alluvial silt, clayey silt, and sand that exhibit shallow channel structures, and, in places, contain peat, molluscs, and wood fragments (Fig. 3). Lacustrine or pond deposits of clayey silt to silty clay occur near the base of the section and enclose pods of tephra up to 25 cm thick. This is the Little Timber tephra, shown in Figure S1B. It was separated into pods as a result of loading into the underlying sediments which must have been saturated and of low bulk density relative to the tephra when the latter was deposited. A brief visit to the site with Duane Froese (University of Alberta) 19 yr later, in 2004, revealed no tephra bed except for a small inclusion (UT2048) in sediments just above river level. This observation suggests that Little Timber tephra has limited lateral extent, and together with underlying organic-rich clayey silt, supports the view that it was deposited in a pond (oxbow lake) in a floodplain environment rather than a large lake. The section is capped by glaciolacustrine silt and clay, which strongly resemble unit C at site 47. Tephrochronology Surprise Creek (SZt) and Little Timber tephra (LTt) beds – first mentioned by Westgate et al. (1995) – have very similar petrographic

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characteristics. Both consist mostly of pumiceous glass with sparse amounts of bubble-wall shards. Hornblende, hypersthene (i.e. an intermediate member of the enstatitie–ferrosilite series), and plagioclase are all abundant whereas FeTi oxides and a reddish amphibole (oxyhornblende) are present but not as conspicuous. Hornblende in SZt has a greenish-brown color but is generally darker in LTt. Hypersthene is much more common in LTt with ratio values of hornblende to hypersthene being 3:1 for SZt and 1:1 for LTt. Glass shards and pumice of both tephra beds have a calc–alkaline rhyolitic to dacitic composition that is broadly very similar (Table 1), but each bed occupies distinctive compositional space on scattergrams using FeOt, CaO, and Cl concentrations (Fig. 4A, B). In addition, LTt shows much greater compositional dispersion. Distinction can also be made on the basis of trace-element concentrations (Fig. 4C, D; Table 2); Nb and the rare-earth elements have higher concentrations in the glass of LTt (Figs. 4C, D, S3). It should be noted that whereas the average glass composition of SZt and LTt is given in Tables 1 and 2, all individual analyses of glass shards are presented in Tables S1 and S2. It has been demonstrated earlier that FeTi oxides are useful for both identification and correlation of tephra beds (Lerbekmo et al., 1975; Westgate et al., 1977; Shane, 1998). Here, we show that this is true for SZt and LTt. Ilmenites in LTt are higher in Ti, Mg, and Mn but lower in FeOt than those of SZt, which are ferrian ilmenites. Magnetites from LTt are higher in Ti but lower in Mg (Tables 3, S3, S4). In terms of the classificatory scheme described by Preece et al. (2011b) in which a Sr/Y–La/Yb plot is used, LTt and SZt belong to the “transitional” group, that is, transitional between adakites and typical arc volcanics (Fig. 5). Their high Sr and low Y concentrations (Table 2) plot close to adakites and suggest that they contain an adakitic end member in their petrogenetic history. With respect to possible source areas, only a few adakitic sources are presently known in the eastern Aleutian arc (EAA) and Wrangell volcanic field (WVF) — namely, Mount Churchill and Mount Drum in the WVF and Hayes volcano at the northeastern end of the EAA. A comprehensive compositional dataset on glass and FeTi oxides shows that SZt is the same bed as Chester Bluff tephra (CBt) in east-central Alaska, first described by Jensen et al. (2008). Their average major-element glass composition is identical, taking into account the associated errors (Table 1), and both cluster tightly together on bivariate scattergrams (Fig. 4A, B). The trace-element composition of the glass tells the same story (Table 2), but is especially well demonstrated in Table S2, which shows LA-ICP-MS analyses of their individual glass shards, both SZt and CBt being analyzed in the same November 2011 run. Ilmenites from both tephra beds are also the same, the compositional space occupied by SZt ilmenites coinciding closely to that of CBt (Fig. S4, Tables 3, S4). No correlative bed of LTt is known. Temperature and oxygen fugacity of the parental magma can be estimated from the composition of coexisting Fe–Ti oxide pairs (magnetite and ilmenite) (Ghiorso and Evans, 2008) providing useful data for tephra discrimination purposes but also for petrogenetic inquiries. The estimated temperature and oxygen fugacity of LTt (based on FeTi oxides) is 805°C and 1.08 Δ NNO (Table 3), but corresponding values for SZt could not be determined because no magnetite was found in this sample. However, CBt contained both ilmenite and magnetite, which gave values of 838°C and 1.48 Δ NNO (Table 3). Glass fission-track ages were determined for both SZt and LTt (Table 4). Initially, the isothermal plateau method was used (Westgate, 1989), but their glass shards were too small for a successful result. For example, the grain-size distribution by weight of LTt is 12% sand, 83% silt, and 5% clay-sized grains. The thermal treatment required to correct for partial track fading resulted in longer etch times to reveal the fission tracks, thereby reducing the glass surface area for counting tracks to the point where it was insufficient to get a statistically meaningful age. The diameter-corrected fission-track method (Sandhu and Westgate, 1995) does not require any thermal pretreatment and is more suited for dating fine-grained tephra. The

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Figure 3. Generalized lithostratigraphy at sites 47A, 47C, and 94. Lithologic units A–D refer only to site 47. Magnetozones refer to 47A and the vertical dashed lines indicate the stratigraphic interval sampled for paleomagnetic analysis. Note ice-wedge cast in magnetozone 3a. The white silt bed in the alluvial deposits at 47C is shown because it serves as a marker bed for the paleomagnetic study.

glass fission-track age of SZt is 0.17 ± 0.07 Ma, based on a single age determination. The high standard error is due to the small sample, even though all the exposed tephra was collected, and fine grain size, which limited the number of tracks counted. LTt has a glass fission-track age of 1.37 ± 0.12 Ma, the weighted mean age of four separate determinations. The older ages for this tephra bed, mentioned in Westgate et al. (1995), are due to poor estimates of the diameter correction factor, the uncorrected age estimates being the same as those given in Table 4.

Examination of the lithostratigraphy at Chester Bluff along the Yukon River in east-central Alaska (Jensen et al., 2008) shows that CBt (= SZt) is below several tephra beds but none is of known age. Old Crow tephra (OCt), which was deposited during the latest phase of MIS 6 and has a glass fission-track age of 0.12 ± 0.01 Ma (Reyes et al., 2010; Preece et al., 2011a), occurs nearby but its stratigraphic position with respect to SZt is uncertain, although Jensen et al. (2008) interpret it as being stratigraphically above SZt. This interpretation has led to the view that the organic-rich silt directly below SZt

Table 1 Average major-element composition of glass shards of Little Timber, Surprise Creek, and Chester Bluff tephra beds, eastern Beringia. Little Timber tephra wt%

UT398

SiO2 TiO2 Al2O3 FeOt MnO CaO MgO Na2O K2O Cl H2Od n

75.95 0.17 13.59 1.35 0.04 1.51 0.28 3.83 3.07 0.21 4.37 8

UT625 (1.04) (0.05) (0.53) (0.26) (0.02) (0.20) (0.07) (0.21) (0.18) (0.02) (0.59)

74.67 0.22 14.34 1.62 0.06 1.75 0.38 4.01 2.75 0.20 5.71 16

UT2048 (1.35) (0.10) (0.57) (0.32) (0.05) (0.36) (0.12) (0.18) (0.23) (0.04) (0.54)

75.49 0.19 14.02 1.42 0.04 1.56 0.30 3.95 2.83 0.20 6.05 16

(0.96) (0.07) (0.44) (0.25) (0.04) (0.35) (0.09) (0.16) (0.25) (0.05) (0.59)

Surprise Creek tephra

Chester Bluff tephra

UT589

UT1660

74.73 0.20 14.69 1.29 0.05 1.75 0.43 4.01 2.84 0.04 5.82 19

(0.29) (0.07) (0.12) (0.06) 0.03 (0.08) (0.04) (0.24) (0.11) (0.03) (0.70)

74.48 0.27 14.73 1.35 0.07 1.78 0.42 4.05 2.82 0.04 5.01 17

(0.39) (0.06) (0.20) (0.07) 0.03 (0.09) (0.04) (0.27) (0.10) (0.02) (1.21)

Notes: All analyses done on Cameca SX-50 wavelength dispersive microprobe operating at 15 kV accelerating voltage, 10 μm beam diameter, and 5 nA beam current. Standardization achieved by use of mineral and glass standards. Analyses recast to 100% on a water-free basis. Standard deviation is given in parentheses; n, number of analyses; FeOt , total iron as FeO; H2Od , water by difference.

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Figure 4. Major-element concentrations in glass shards (A, B) show that Chester Bluff tephra is equivalent to Surprise Creek tephra but different from Little Timber tephra. Trace-element concentrations (C, D) in glass shards likewise show the distinctive composition of Surprise Creek tephra and Little Timber tephra.

represents the Marine Oxygen Isotope Stage (MIS) 7 interglaciation because its abundant plant macrofossils suggest a vegetation cover that was not significantly different from that of today (Bigelow, 2003).

Fortunately, this uncertainty can be resolved by reference to another site, recently described by Preece et al. (2011b) — a sign of the growing maturity of tephrochronological studies in eastern Beringia. At site D72

Table 2 Average trace-element composition of glass shards of Little Timber, Surprise Creek, and Chester Bluff tephra beds, eastern Beringia. Little Timber tephra ppm

UT622

Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Th U n

61.0 435.7 14.9 241 12.4 1.34 1137 32.1 55 6.1 23.0 4.08 0.75 3.21 0.51 2.51 0.57 1.74 0.21 1.74 0.34 5.98 1.06 7.2 2.57 9

Surprise Creek tephra UT398

(6.9) (83.9) (2.6) (54) (2.2) (0.60) (115) (4.1) (7) (0.9) (4.4) (1.22) (0.33) (1.69) (0.16) (0.71) (0.20) (0.50) (0.08) (0.48) (0.12) (1.18) (0.31) (1.4) (0.44)

63.2 350.9 13.0 199 12.2 1.54 1181 33.3 53 6.0 19.9 3.31 0.74 4.61 0.47 2.62 0.4 1.51 0.37 1.61 0.37 5.84 0.91 8.4 3.05 10

UT589 (7.3) (62.7) (2.3) (55) (1.6) (0.26) (106) (3.5) (4) (0.8) (3.6) (1.18) (0.24) (1.42) (0.27) (1.05) (0.29) (0.67) (0.31) (0.61) (0.12) (1.21) (0.27) (0.7) (0.39)

62.5 417.1 10.0 233 7.0 1.4 1085 22.7 35 4.1 14.6 2.35 0.61 2.45 0.32 1.96 0.47 1.39 0.15 1.29 0.24 7.2 0.64 8.7 2.07 10

Chester Bluff tephra UT617

(4.5) (14.0) (1.1) (10) (1.0) (0.27) (51) (1.4) (2) (0.4) (2.0) (1.01) (0.27) (0.83) (0.14) (0.56) (0.22) (0.68) (0.10) (0.53) (0.13) (0.94) (0.23) (0.9) (0.27)

61.6 398.9 11.2 206 6.9 1.08 1082 22.1 38 4.2 14.2 2.5 0.61 2.87 0.43 2.43 0.28 1.1 0.27 1.25 0.25 5.91 0.52 8.2 2.11 11

UT1660 (3.7) (48.6) (6.0) (27) (1.6) (0.63) (114) (4.3) (10) (0.9) (2.7) (1.30) (0.39) (1.53) (0.20) (1.50) (0.16) (0.41) (0.23) (0.67) (0.12) (1.23) (0.20) (1.5) (0.49)

64.2 337.8 8.4 187 6.0 1.31 1116 18.9 33 3.6 12.8 2.3 0.77 2.3 0.27 1.87 0.44 0.94 0.14 1.06 0.17 5.34 0.62 6.9 2.19 24

(6.68) (69.0) (1.6) (29) (0.8) (0.37) (112) (1.9) (3) (0.6) (2.8) (1.19) (0.32) (0.93) (0.17) (0.84) (0.23) (0.55) (0.08) (0.41) (0.09) (1.21) (0.26) (1.0) (0.37)

Notes: Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) trace-element analyses were performed at Aberystwyth University, Wales, using a Coherent GeoLas 193 nm Excimer laser system coupled to a Thermo Finnegan Element 2 sector field ICP-MS. Details of analytical methods, calibration strategies, and typical detection limits are given in Pearce et al. (2004, 2007) with calibration against the NIST 61× reference glasses using 29Si as an internal standard (Pearce et al., 1997). Analyses on single glass shards; standard deviation in brackets; n is number of analyses. Fractionation factor used is based on Pearce et al. (2011).

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Table 3 Average composition of FeTi oxides and geothermometry estimates in Little Timber, Surprise Creek, and Chester Bluff tephra beds, eastern Beringia. Ilmenites wt%

Little Timber

SiO2 TiO2 Al2O3 Cr2O3 V2O3 Fe2O3 FeO MgO MnO CaO NiO Total FeOt n

0.04 41.33 0.27 0.07 0.31 22.37 32.66 2.13 0.51 0.04 0.04 99.77 52.79 10

Geothermometry T°C ΔNNO n

estimates 805 1.08 4

Magnetites Surprise Creek (0.03) (1.87) (0.08) (0.04) (0.05) (4.11) (2.02) (0.51) (0.13) (0.02) (0.04) (0.95) (1.83)

0.09 30.61 0.45 0.10 0.43 40.02 24.50 1.57 0.23 0.05 0.04 98.10 60.51 8

Chester Bluff (0.01) (0.72) (0.02) (0.05) (0.03) (1.36) (0.45) (0.10) (0.03) (0.03) (0.02) (0.33) (0.82)

(61) (0.10)

Little Timber

0.04 30.71 0.47 0.13 0.41 41.08 24.62 1.55 0.21 0.03 0.04 99.30 61.58 9

0.02 (0.74) (0.02) (0.03) (0.05) (1.46) (0.48) (0.13) (0.03) (0.07) (0.04) (0.64) (0.98)

838 1.48 3

(10) (0.02)

0.10 7.01 2.36 0.12 0.38 52.54 35.55 1.19 0.38 0.21 0.05 99.89 82.82 9

Chester Bluff (0.02) (0.86) (0.23) (0.04) (0.03) (1.64) (0.87) (0.34) (0.04) (0.45) (0.03) (0.63) (1.40)

0.09 5.99 2.63 0.22 0.43 53.72 33.87 1.65 0.41 0.02 0.08 99.09 82.21 10

(0.02) (0.23) (0.16) (0.04) (0.05) (0.56) (0.28) (0.05) (0.03) (0.02) (0.02) (0.54) (0.48)

Notes: Analyses obtained by wavelength dispersive spectrometry (WDS) using a Cameca SX50 microprobe. Operating conditions as follows: 15 kV accelerating voltage, 25 nA beam current, and 1 μm beam diameter. Standardization was achieved by the use of synthetic oxides and mineral standards. Primary FeTi oxides were recognized by their attached glass, which was analyzed by energy dispersive spectrometry (EDS) prior to analysis of the FeTi oxides. Total Fe split into FeO and Fe2O3 using the methods of Carmichael (1967). Geothermometry estimates are based on coexisting FeTi oxide pairs, which meet the criteria of Bacon and Hirschmann (1988). Estimates are calculated using the method of Ghiorso and Evans (2008). T°C is temperature in degree Celsius, ΔNNO is oxygen fugacity log10fO2 relative to the NNO buffer, n is number of analyses.

along the Sixtymile River, west of Dawson (Fig. 1), ~3 m of massive, inorganic brown silt covers gravel on a bedrock bench. Old Crow tephra is near the top of this exposure, MF tephra is 90 cm below it, and Coal Creek tephra is 85 cm below MF tephra. Old Crow and MF tephra beds were previously known to be stratigraphically close to one another from their occurrence in the uppermost part of the Gold Hill Loess, near Fairbanks (Preece et al., 1999), and at Chester Bluff, Coal Creek tephra is just above CBt (= SZt) (Jensen et al., 2008). Hence, SZt is below OCt and must be older than late MIS 6 (~130 ka), supporting its glass fissiontrack age of 0.17±0.07 Ma. These facts, combined with the absence of any organic-rich horizon at the D72 site in the Sixtymile area, strongly favor an MIS 7 age for the organics just below SZt. Magnetostratigraphy A paleomagnetic study was carried out on the fine-grained fluvial and lacustrine sediments at sites 47 and 94 in order to provide independent chronological constraints as well as serve to check on the reliability of the glass fission-track age determinations. In addition,

Figure 5. La/Yb–Sr/Y plot from Preece and Hart (2004) showing tephra beds in the adakite (A), transitional (B), and typical arc (C) fields. Both Little Timber and Surprise Creek tephra beds occur in the transitional field. Analyses by LA-ICP-MS on individual glass shards.

detailed sampling was done at site 47 to see if variations in paleomagnetic properties, such as directional and magnitude data, could be used for local correlation. For example, can the stratigraphic position of SZt be recognized elsewhere along the 2-km-long exposure (i.e. where the tephra is absent) from the paleomagnetic signature of its host sediments? Oriented samples were collected in plastic cubes with 2-cm-long sides in the following manner. A horizontal bench was excavated into the face of the exposure and a pedestal of sediment carved out with dimensions corresponding to those of the cube. The latter was then gently pushed down on the pedestal to capture the sediment, air being extruded through a hole in the top face of the cube. Following orientation of the cube, it was carefully removed from the remaining part of the pedestal, its lid attached, and then sealed with glue and labeled. Two and sometimes three oriented samples were collected at each stratigraphic level, which, for the most part, were spaced at 10 cm intervals. Remanent magnetic moment measurements were performed in two laboratories. One was equipped with a cryogenic magnetometer built by Develco Corporation with the alternating field demagnetization being done on an instrument made by Schonstedt Corporation. The other laboratory contained MOLSPIN equipment. Further information on laboratory procedures is given in notes to the figures and tables. All measurements were made in oersteds, but whenever these values are mentioned, the equivalent SI value is provided. All samples were stored in a field-free chamber for 6 yr prior to measurement and all were demagnetized up to peak fields of at least 100 Oe (8 kA/m). Some detailed stepwise demagnetizations were carried out to higher fields, specifically, to 900 Oe (72 kA/m) in steps of 100 Oe (8 kA/m), to determine the stability of the natural moments and the nature of the magnetic carrier mineral grains. The stability is generally good with mean destructive fields of 200– 400 Oe (16–32 kA/m) for sediments at site 47, with the exception of the lacustrine clayey silt of unit A (Fig. 3), which shows a dramatic decrease in magnetic moment at 200 Oe (16 kA/m) (Fig. S5). Sediments at site 94 have higher mean destructive field values of 500– 700 Oe (40–56 kA/m). No significant drift of directions above 100 Oe (8 kA/m) is seen (Fig. S6) until the intensities of the moments become sufficiently low that randomly introduced moments or measurement errors cause large variations. The natural moment in these samples is almost certainly held by magnetite, given the mean

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Table 4 Glass fission-track age of Surprise Creek and Little Timber tephra beds, Old Crow Basin, northern Yukon Territory. Sample number

Date irradiated

Surprise Creek tephra UT617 Jan., 1993

Etching Ds Spontaneous track Corrected spontaneous Induced track Track density on conditions density track density density muscovite detector over dosimeter glass HF:temp: time

Di

(102 t/cm2)

(102 t/cm2)

(105 t/cm2)

(%: °C: s)

(μm)

(μm)

2.83 ± 0.12

6.79 ± 0.05

26: 20: 70

5.27

6.42

0.82

0.14 ± 0.06

0.23 ± 0.10

(523) 2.83 ± 0.12

(23761) 6.79 ± 0.05

[6] 5.27

[197] 6.42

1.22#

0.17 ± 0.07

(6)

(523)

(23761)

[6]

[197]

11.40 ± 0.12

4.90 ± 0.01

(9642) 11.40 ± 0.12

(15640) 4.90 ± 0.01

(9642) 4.72 ± 0.16

(15640) 6.51 ± 0.04

(854) 4.71 ± 0.16

(32050) 6.51 ± 0.04

(854) 23.20 ± 0.75 (964) 23.20 ± 0.75 (964) 14.70 ± 0.28 (2769) 14.70 ± 0.28 (2769)

(32050) 7.13 ± 0.05 (22591) 7.13 ± 0.05 (22591) 7.13 ± 0.05 (22591) 7.13 ± 0.05 (22591)

0.19 ± 0.07

Jan., 1993

Little Timber tephra UT398 June, 2004

7.44 ± 0.63 (141)

UT398

June, 2004

UT398

March, 1992 3.00 ± 0.57

UT398

March, 1992

UT398*

Jan., 1984

UT398*

Jan., 1984

UT398

Jan., 1984

8.92 ± 0.75 (141)

(28) 3.61 ± 0.68 (28)

UT398

Jan., 1984

Age ± 1σ

(104 t/cm2)

(6) UT617

Ds/Di or Di/Ds#

11.50 ± 2.14 (29) 13.80 ± 2.56 (29) 8.06 ± 0.93 (75) 9.67 ± 1.12 (75)

26: 20: 70

24: 24.5: 70 24: 24.5: 70 26: 22: 85 26: 22: 85 nd nd nd nd

(Ma)

4.52 ± 0.10 5.42 ± 0.06 0.83 ± 0.02

0.96 ± 0.12

[100] [597] 4.52 ± 0.10 5.42 ± 0.06 1.20 ± 0.03# 1.16 ± 0.15 [100] [597] 4.52 ± 0.10 5.42 ± 0.06 0.83 ± 0.02

1.25 ± 0.40

[100] [597] 4.52 ± 0.10 5.42 ± 0.06 1.20 ± 0.03# 1.50 ± 0.48 [100] 4.52 ± 0.10 [100] 4.52 ± 0.10 [100] 4.52 ± 0.10 [100] 4.52 ± 0.10 [100]

[597] 5.42 ± 0.06 [597] 5.42 ± 0.06 [597] 5.42 ± 0.06 [597] 5.42 ± 0.06 [597]

0.83 ± 0.02

1.06 ± 0.20

1.20 ± 0.03# 1.27 ± 0.24 0.83 ± 0.02 1.20 ± 0.03

1.18 ± 0.27 #

1.41 ± 0.32

Notes: The population-subtraction method was used; details are given in Westgate et al. (2007). Ages calculated using the zeta approach and λD =1.551×10−10 yr−1. Zeta value is 301 ± 3 based on 6 irradiations at the McMaster Nuclear Reactor, Hamilton, Ontario, using the NIST SRM 612 glass dosimeter and the Moldavite tektite glass (Lhenice locality) with an 40Ar/39Ar age of 14.34 ± 0.08 Ma (Laurenzi et al., 2003, 2007). Standard error on age estimate is calculated according to Bigazzi and Galbraith (1999). Area estimated using the point-counting method (Naeser et al., 1982) with the exception of UT398⁎ for which an eyepiece graticule was used. Number of tracks counted is given in brackets; number of tracks measured is given in square brackets. Age determinations in bold type are ages corrected for partial track fading; other ages are uncorrected for partial track fading. Weighted mean age of Little Timber tephra is based on the corrected age determinations and is 1.37 ± 0.12 Ma. All age estimates determined by the diameter correction (DCFT) technique (Sandhu and Westgate, 1995). Ds = mean spontaneous track diameter; Di = mean induced track diameter. All samples of Little Timber tephra corrected for partial track fading using the June, 2004 diameter measurements because they all come from the same outcrop. The Di/Ds value for Surprise Creek tephra is based on only 6 spontaneous tracks, but this value is likely a good estimate given that the average Di/Ds of 32 tephra samples that vary in age from late Pleistocene to Eocene is 1.22 ± 0.08. nd = not recorded. The equation used to determine the standard error (se) on age (t) estimate is: se(t) = t √{1/Ys + (1 − Xs / ns) / Xs + 1 /Yi + (1 − Xi / ni) / Xi + 1 / Yd} where Ys = number of spontaneous tracks, Yi = number of induced tracks, Xs = number of points over glass for spontaneous tracks, Xi = number of points over glass for induced tracks, Yd = number of tracks counted on detector over dosimeter glass, ns = number of fields of view for spontaneous tracks, ni = number of fields of view for induced tracks. # simply indicates that the ratio value is Di/Ds, otherwise it is Ds/Di.

destructive field values, and is of depositional or post-depositional origin. The results of the paleomagnetic analyses for site 47A are shown in Figure 6 and 47C in Figure 7. All samples have a normal polarity. On the other hand, sediments close to LTt at site 94 all have southerly declinations and negative inclinations. In other words, they have a reversed polarity (Fig. 7). Hence, the paleomagnetic data and glass fission-track age estimates for SZt and LTt are in agreement based on the geomagnetic polarity time scale (Ogg and Smith, 2004). Site 47A shows considerable variation in the magnetic properties of the sediments permitting them to be conveniently divided into four magnetozones (Figs. 3, 6). Magnetozone 1 broadly coincides with unit A, the lacustrine clayey silt. Magnetic moments are very weak (see also Fig. S5) and mostly preclude determination of directional information. Zone 2 lies within the lower part of the alluvial unit B and can be divided into two parts (2a and 2b) on the basis of directional and magnitude data (Fig. 6). Larger magnetic moments and smaller variations in declination and inclination distinguish 2b from 2a and the boundary between them is probably a disconformity, given the sharp and sudden change in magnetic character of the sediments. The tight consistency of directional data in 2b may well be

due to rapid deposition of the alluvial sediments. The upper part of alluvial unit B includes magnetozone 3, which is distinguished by its cyclical-like fluctuations in declination and inclination, which are most likely the expression of secular variation of the Earth's magnetic field. This zone can also be split into two parts, the upper one (3b) having consistently weaker magnetic moments (Fig. 6). Its boundaries must be demarcated by disconformities because of the very sudden change in magnetic signature across them. Magnetozone 4 is coincident with the glaciolacustrine, laminated silt of unit C1 and is readily recognized by its relatively strong magnetic moments. Surprise Creek tephra is in the upper part of magnetozone 2b, stratigraphically just above the level where inclination values start to decrease in a linear fashion from ~ 80° to ~ 70° (Fig. 6). Paleomagnetic samples were collected in the lower part of alluvial unit B at site 47C (Fig. 3) in the hope that they would help identify the age of these highly fossiliferous sediments because SZt was absent at this locality. Directional trends, especially the stability and similarity of inclination values, strongly suggest that these sediments are positioned in magnetozone 2b, in the upper part of which SZt occurs at 47A (Figs. 6, 7). Another magnetogram was constructed in an effort to locate more precisely the stratigraphic level of SZt at 47C. Changes

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Figure 6. Magnetograms for site 47A. Magnetic moments plotted against stratigraphic position. Directional and magnitude data are for moments after they have been demagnetized by 200 Oe (16 kA/m) peak alternating fields, using a MOLSPIN magnetometer and demagnetizer. Magnitude data in 10−7 emu (10−10 Am2). A dot represents the Fisherian average direction (Fisher, 1953) of two and occasionally three determinations on separate samples from the same horizon and the curves are based on a three-point running mean. Samples measured in 1992 after storage in a field-free chamber since 1985. Magnetozones 1–4 as in Fig. 3.

in the latitude and longitude of the pole position during the time represented by the sediments at 47A and 47C are shown in Figure S7 and a detail of this figure at the relevant stratigraphic level is presented in Figure 8. The pattern of change in pole positions in the neighborhood of the white silt bed at 47C closely resembles that seen at and close to the stratigraphic level of SZt at 47A. The distinctive feature in the longitudinal changes is the linear trend through the white silt bed, commencing at ~ 160° at the 260 cm level and ending at ~ 60° at the 400 cm level. This trend, although less pronounced, is seen just below the level of SZt at 47A (Fig. 8). Corresponding changes in the latitude of the pole during the same stratigraphic interval are also similar. At 47C, latitude increases from ~ 65° to 75° and then decreases back to ~ 60° at the 400 cm level. A matching trend exists in the 150 cm interval below SZt at 47A, although latitude positions are slightly higher ( ~ 75° to ~ 80° and then back to ~ 70° at SZt). Exact duplication of the paleomagnetic signature at both sites, which are about 1 km apart (Fig. 2), would be unlikely because of the fluvial environment, where erosion and deposition can vary laterally over short distances. However, considering all the evidence, the projected position of SZt at 47C is at the 400 cm level, about 80 cm above the white silt bed (Fig. 8). Errors for the paleomagnetic measurements in this part of the stratigraphic record at both sites are relatively small and add to the confidence of this correlation (Figs. S8 to S11). Future stratigraphic studies in this area, therefore, may well profit from detailed paleomagnetic analysis. Paleoenvironments and biochronology Pollen studies were carried out at both sites (47 and 94) on sediments immediately above and below the tephra beds (Schweger et al., 2011). At site 47, pollen records were obtained from the organicrich alluvial silt and fine sand in the lower part of unit B at localities A and B, where SZt is present (Fig. 2). These two pollen records through

SZt are dominated by Betula (63–9%), Alnus (26–3%), and Picea (18– 1%) but Cyperaceae, Gramineae, and Artemisia are all important contributors. This assemblage points to an open boreal woodland or shrub tundra vegetation (Westgate et al., 1995). Lichti-Federovich's (1973) pollen studies were carried out on the middle part of unit B and envisage spruce on the alluvial sites, a rich shrub-herb tundra on the interfluves, willow and alder stands along the water courses, and local heath and dwarf birch tundra on the more stables substrates. Site 47D is a short distance downstream from 47C and the organic-rich alluvium sampled there is also considered to be equivalent to the lower part of unit B. This correlation is based on stratigraphic position, lithologic similarity, and lateral persistence of distinctive magnetozones along more than 1 km of the exposure. These sediments yielded Picea frequencies from 36 to 26%, Alnus from 13 to 2%, and Betula from 40 to 28%, indicating an open spruce forest vegetation occupied the region at this time, which, as discussed earlier, was most probably during MIS 7. A rich assemblage of mammalian fossils has been recovered from the alluvial unit B, especially from the lower part, that is, at and below the level of SZt. These fossils were studied by Richard Morlan and a summary of his findings and identifications is listed in Table S5. The fossils are referenced according to site (A, B, C, D) and stratigraphic position with respect to SZt. As noted by Harington (2011), this in situ fauna includes fish (Pisces), bird (Aves), shrew (Soricidae), rodent (Rodentia, including giant beaver and American beaver), weasel (Mustelidae) and rabbit (Lagomorpha) remains, as well as teeth of steppe mammoth (Mammuthus trogontherii), fox (Alopex), wolf (Canis lupus), horse, caribou, and bison. Richard Morlan (unpublished ms., 1991) correlates the mammalian fossil assemblage from site 47 in the Old Crow Basin with the Akanian horizon of the Olyer Suite of the Kolyma lowland, northeastern Siberia (Sher, 1986) on the basis of faunal similarity and paleomagnetic data: both have a normal magnetic polarity and are interpreted as belonging

J.A. Westgate et al. / Quaternary Research 79 (2013) 75–85

83

Figure 7. Magnetograms for sites 47C and 94. Sampling and measurement conditions at site 47C are the same as those for site 47A (see Fig. 6). Curves based on three-point running mean. Directional and magnitude data for site 94 show moments after demagnetization at 150 Oe (= 12 kA/m) using a MOLSPIN magnetometer and demagnetizer. Samples stored in field-free chamber for 6 years prior to measurement and remanent magnetic moment measurements in 10−7 emu (10−10 Am2). A dot represents the average of two determinations on separate samples from the same horizon and curves based on three-point running mean. The zero level for each of these two sites is shown on Fig. 3, which indicates the stratigraphic intervals sampled for paleomagnetic analysis.

to the Bruhnes Chron. Observations that are particularly relevant to the age of this fossil assemblage at site 47, based on Morlan's work are: (1) Microtus specimens are more highly evolved than the advanced form at Cudahy ash mine in Kansas (Paulson, 1961). The tephra at the latter site is 600 ka (Gansecki et al., 1998). (2) Teeth of Microtus beringianus suggest it is the ancestor of Microtus pennsylvanicus, a species that is very common in sediments of the Rancholabrean mammalian age of North America (Repenning, 1980). (3) The muskrat Ondatra nebracensis is known from late Irvingtonian and early Rancholabrean faunas of southern North America, and (4) Bison, indicative of the Rancholabrean mammalian age of North America, first appears in the upper part of unit B at site 47. Morlan (unpublished ms., 1991) considered that the stage of evolution seen in the microtine assemblages supports an age within the Bruhnes Chron, and that the first samplings of these assemblages, which were recovered from a small bulk sediment sample, “gave a misleading impression of primitiveness.” All things considered, he suggests that the lower part of unit B at site 47 dates to some portion of the interval between the beginning of the Bruhnes Chron (780 ka) and the Irvingtonian–Rancholabrean boundary (~ 470 ka). Harington (2011) tentatively considers this assemblage to be of Middle Pleistocene age. Tephrochronological studies, as noted in this report, point to a late Middle Pleistocene age. A 2 m pollen record completed through LTt at site 94 shows Picea frequencies up to 14%, Alnus with a maximum of 15%, Betula at 30– 50% and Pinus and Abies at ~2% with Cyperaceae, Gramineae, Artemisia

and Ericaceae all being important elements (Schweger et al., 2011). This assemblage points to an open boreal woodland or shrub tundra environment, not unlike that reconstructed for the middle part of unit B at site 47. Sediments associated with LTt have yielded a rich fauna that includes mammoth, horse, the primitive rodent Allophaiomys, a heather vole (Phenacomys deeringensis), red-backed vole (Clethrionomys), primitive brown (Lemmus) and collared (Predicrostonyx hopkinsi) lemmings, as well as giant pika (Ochotona whartoni) (Harington, 2011). All point to an Early Pleistocene age, supporting the glass fission-track age estimate for LTt. Discussion and conclusions New chronostratigraphic information is presented on Quaternary deposits of the northeastern extremity of eastern Beringia, the region of Alaska and Yukon that remained ice-free during Quaternary glaciations. Glass fission-track studies on two tephra beds provide age estimates on associated important and well characterized mammalian faunas, and paleomagnetic data is used to test the accuracy of these ages as well as constrain the age of nearby fossiliferous sediments that lack tephra. A comprehensive set of mineralogical and geochemical information on these tephra beds demonstrates a source in the WVF and will permit recognition of these tephra beds elsewhere in Alaska and Yukon. Although this work will be most relevant to workers in eastern Beringia, the improved chronological control on

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Figure 8. Detail from Fig. S7 showing latitude and longitude of paleopole positions as determined from sediments at sites 47A and 47C (see Fig. 3 for sampled stratigraphic intervals). Oriented samples were stored in a field-free chamber for 6 years prior to measurement in 1992 on a cryogenic magnetometer built by Develco Corporation. Samples demagnetized by 100 Oe (8 kA/m) peak alternating fields with the exception of samples taken in the stratigraphic interval 690–951 cm above the base of 47A, which were demagnetized at 50 Oe (4 kA/m). Curves based on a three-point running mean. Dashed line above white silt bed at site 47C is the projected stratigraphic level of Surprise Creek tephra (SZt).

the Quaternary mammalian fossil assemblages will have a broader relevance. More specifically, alluvial and lacustrine sediments exposed beneath late Pleistocene glaciolacustrine silt and clay at sites CRH47 and CRH94 along the Old Crow River are rich in fossils and contain tephra beds. Surprise Creek tephra occurs in the lower part of the alluvial sequence at site 47 and Little Timber tephra is present near the base of the exposure at site 94. These eruptive units, combined with paleomagnetic analyses, offer an opportunity to determine the age of these fossils, which, in turn, provide an insight into the then existing environmental conditions. Surprise Creek tephra has a glass fissiontrack age of 0.17 ± 0.07 Ma and LTt is 1.37 ± 0.12 Ma. All sediments at site 47 have a normal remanent magnetic polarity and those near LTt at site 94 have a reversed polarity, in agreement with the standard geomagnetic time scale (Ogg and Smith, 2004). Small mammal remains from sediments near LTt support an Early Pleistocene age but the chronology is not so clear at site 47 because of the large error associated with the SZt age determination. SZt is definitely older than Old Crow tephra (OCt), and, therefore, older than MIS 5e, as shown by site D72 in the Sixtymile country of Yukon, where there is no prominent organic-rich horizon between OCt and the underlying Coal Creek tephra (CCt), so that the position of SZt, which is slightly older than CCt, immediately above interglacial deposits at Chester Bluff, Alaska, suggests that the latter represents MIS 7. Even at the + 1σ level, SZt would still fall within MIS 7, which began at 240 ka. Furthermore, SZt at site 47 lies within magnetozone 2b, which exhibits very little change in magnetic directions through 3 m of sediment, which suggests rapid deposition. The interglacial deposits at the base of unit B at 47D are only 2 m below this magnetozone at 47C, arguing that it

is temporally close to SZt. On the other hand, the mammalian fossils from the lower half of alluvial unit B point to a late Irvingtonian land mammal age, according to Richard Morlan, sometime between the beginning of the Bruhnes Chron and the Irvingtonian–Rancholabrean boundary (~ 470 ka). More studies are necessary to resolve this difference of opinion. In particular, new fission-track dating studies are now warranted because of the recent discoveries of more proximal, coarser-grained CBt (= SZt) occurrences. Acknowledgments Funds provided by the Natural Sciences and Engineering Research Council of Canada to JAW and CES are gratefully acknowledged. We benefited from discussions with John Matthews, Jr. (Geological Survey of Canada) in the field and office. He also collected many of the samples at site 47 that were later analyzed for small mammal remains by Richard Morlan. Andrew Westgate assisted JAW in the collection of the many samples for paleomagnetic analysis and Andy Brown (Aberystwyth University, Wales) helped us in our work on trace-element analyses of volcanic glass with his usual care and diligence. The manuscript was improved as a result of thoughtful comments by Alberto Reyes (University of Wisconsin, Madison), John Gosse (Dalhousie University, Halifax), Jim Knox (Associate Editor, QR), and Alan Gillespie (Senior Editor, QR). Appendix A. Supplementary data Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.yqres.2012.09.003.

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