Paleomagnetic Evidence for Multiple Late Cenozoic Glaciations in the Tintina Trench of west central Yukon, Canada

Paleomagnetic evidence for multiple lateCenozoic glaciations in the Tintina Trench, west-central Yukon, Canada1

Rene W. Barendregt, Randolph J. Enkin, Alejandra Duk-Rodkin, and Judith Baker

Abstract: The Tintina Trench in west-central Yukon has preserved an extensive record of late Cenozoic preglacial, glacial,and interglacial deposits. These deposits comprise multiple sequences of tills, outwash, loesses, and paleosols. The sedi-ments that were laid down directly by ice (tills) are of both local (montane) and regional (Cordilleran) provenance. TheTintina Trench area was impacted repeatedly by montane ice from the southern Ogilvie Mountains to the northwest(2500 m above sea level (asl)), and also repeatedly along its southern extent by Cordilleran ice from the Selwyn Mountainsto the east (2759 m asl), the latter forming the continental divide in this region. We report here the magnetostratigraphy ofthree sections: Rock Creek (64813’N, 139807’W), West Fifteenmile River (64829’N, 139855’W), and East Fifteenmile River(64823’N, 139848’W). The majority of the units identified at these sections record late Pliocene to mid-Pleistocene glacia-tions, although relatively thin surficial sequences of late middle Pleistocene to late Pleistocene loesses and tills are presentas well. Of the 11 units described in the Tintina Trench, seven have normal polarity, three have reversed polarity, and onehas an undefined polarity. These units span about 3.0 million years. It appears that most of the polarity chrons and sub-chrons of the late Cenozoic are present and that the sequence of six reversals record at least 10 glaciations (three in theBrunhes Chron and seven in the Matuyama Chron), and 11 interglaciations (four in the Brunhes Chron and seven in theMatuyama Chron). The interglacials are recorded as either paleosols or unconformities between glacial or loess units havingopposite polarity. While not all Matuyama Chron glacial and interglacial cycles recorded in marine isotopic records areseen on land, the terrestrial records found in the Tintina Trench have thus far proven to be the most complete in terms ofthe polarity record. While no absolute ages were obtained from the sediments in the trench, the extensive polarity sequenceconstrains the timing of glaciations to a considerably greater degree than was previously possible for this region. The mag-netostratigraphy of the trench sites are compared with the glacial, glaciofluvial, and loessic deposits at the nearby KlondikeRiver valley and Fort Selkirk sites, central Yukon, where tephras and basalts provide absolute ages, and stratigraphic unitscontain an extensive late Cenozoic climate proxy for northwestern North America (eastern Beringia). In this study, wepresent new paleomagnetic polarity data and establish a magneto-lithostratigraphy describing preglacial, glacial, and inter-glacial deposits in the Tintina Trench. These deposits are referred to as the West Tintina Trench Allogroup and provide abroad framework for establishing a paleoclimate record for the northern Canadian Cordillera.

Resume : Le sillon de Tintina dans le centre ouest du Yukon a preserve un enregistrement complet de depots preglaciai-res, glaciaires et interglaciaires datant du Cenozoıque. Ces depots comprennent de multiples sequences de tills, de depotsfluvio-glaciaires, de lœss et de paleosols. Les sediments qui ont ete deposes directement par la glace (tills) ont une prove-nance locale (subalpine) et regionale (de la Cordillere). Le sillon de Tintina a subi l’impact de la glace de la Cordillere denombreuses fois; cette glace provenait des monts Ogilvie meridionaux situes au nord-ouest (2500 m au-dessus du niveaude la mer (ASL)). Il a aussi subi l’impact repete le long de son etendue sud par de la glace de la Cordillere provenant desmonts Selwyn a l’est (2759 m ASL); ces monts forment la ligne continentale de partage des eaux dans cette region. Nousrapportons ici la magnetostratigraphie de trois sections : Rock Creek (RC) (64o13’N, 139o07’O), West Fifteenmile River(WFR) (64o29’N, 139o55’O) et East Fifteenmile River (EFR) (64o23’N, 139o48’O). La plupart des unites identifiees a cessections enregistrent des glaciations datant du Pliocene tardif au Pleistocene moyen, bien qu’il subsiste encore en surfacedes sequences relativement minces de lœss et de tills datant du Pleistocene moyen tardif au Pleistocene tardif. Des 11 uni-tes decrites dans le sillon de Tintina, sept ont une polarite normale, trois ont une polarite inversee et la polarite d’une uniteest indefinie. Ces unites couvrent environ 3,0 millions d’annees. Il semble que la plupart des chrons et des sous-chrons po-laires du Cenozoıque tardif sont presents et que la sequence de six inversions documente au moins 10 glaciations (troisdans le chron Brunhes et sept dans le chron Matuyama) et 11 interglaciations (quatre dans le chron Brunhes et sept dansle chron Matuyama). Les interglaciaires sont enregistres en tant que paleosols ou discordances entre les unites glaciaires

Received 26 August 2009. Accepted 1 March 2010. Published on the NRC Research Press Web site at cjes.nrc.ca on 20 July 2010.

Paper handled by Associate Editor T. Fisher.

R.W. Barendregt.2 Faculty of Arts and Science, The University of Lethbridge, Lethbridge, AB T1K 3M4, Canada.R.J. Enkin and J. Baker. Geological Survey of Canada - Pacific, Sidney, BC V8L 4B2, Canada.A. Duk-Rodkin. Geological Survey of Canada, Calgary, AB T2L 2A7, Canada.

1This article is a companion paper to Duk-Rodkin et al., also in this issue.2Corresponding author (e-mail: [email protected]).

987

Can. J. Earth Sci. 47: 987–1002 (2010) doi:10.1139/E10-021 Published by NRC Research Press

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ou de lœss a polarite opposee. Bien que les cycles glaciaires et interglaciaires du chron Matuyama enregistres dans lesdonnees isotopiques marines n’aient pas ete tous vus sur le continent, les enregistrements terrestres trouves dans le sillonde Tintina se sont averes etre les plus complets en termes d’enregistrement de polarite. Bien qu’aucun age absolu n’ait eteobtenu des sediments du sillon, la sequence extensive de polarites encadre beaucoup plus le moment des glaciations qu’iln’etait possible auparavant pour cette region. La magnetostratigraphie des sites du sillon est comparee a celle des depotsglaciaires, fluvio-glaciaires et lœssiques des sites avoisinants de la vallee de la riviere Klondike et de Fort Selkirk, dans lecentre du Yukon, ou des tephras et des basaltes fournissent des ages absolus et les unites stratigraphiques contiennentbeaucoup de donnees indirectes du climat au Cenozoıque tardif pour le nord-ouest de l’Amerique du Nord (Beringie orien-tale). Dans cette etude, nous presentons de nouvelles donnees de polarites paleomagnetiques et nous etablissons une lithos-tratigraphie magnetique decrivant les depots glaciaires et interglaciaires dans le sillon. Ces depots sont nommesl’Allogroupe du sillon de Tintina occidental et ils fournissent un cadre general pour etablir des donnees paleoclimatiquespour le nord de la Cordillere canadienne.

[Traduit par la Redaction]

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Introduction

The terrestrial glacial record of northwestern North Amer-ica is one of the most extensive of any preserved worldwide.This record was left by Cordilleran, montane, continental, andplateau glaciers. The oldest glacial sediments deposited dur-ing tidewater glaciations are found near the Yukon–Alaskaborder and are late Miocene in age (Denton and Armstrong1969; Eyles and Eyles 1989; Lagoe et al. 1993). Regional-scale glaciations (Cordilleran and continental) commenced innorthwestern Canada and east-central Alaska in the late Plio-cene, between 2.90 and 2.58 million (Ma) years ago, immedi-ately preceding the Gauss–Matuyama boundary (2.58 Ma),and increased in frequency with the advance of time (seefigs. 6, 11, 12 in Duk-Rodkin et al. 2010). Proceeding backin time from the mid-Pleistocene transition (approximatelyMarine Isotope Stage (MIS) 36–26), periods of ice-free con-ditions were more extensive and appear to correspond to peri-ods of tectonic stability and regional denudation.

The moisture source for the Cordilleran ice was predomi-nantly from the Pacific Ocean, which may have cooled inpart owing to the closure of the Panama seaway (Lear et al.2003) and perhaps also by the (northward) flow of watersthrough the Bering Strait (Marincovich 2000). Most prob-ably, summers were cooler and winters milder as a result ofthe high-amplitude 41 ka obliquity cycles (Haug et al.2005). A total of at least 11 Cordilleran and montane glacia-tions (Duk-Rodkin et al. 2004) have been recorded in themountains of northwestern Canada.

Successive Cordilleran glaciations diminished in size,while those in continental areas increased. The role of tec-tonics in the development of two major physiographic bar-riers (Wrangell – Saint Elias Mountains and the Mackenzie– Selwyn Mountains forming the continental divide) appearsto have been an important controlling variable for moisturedistribution in northwest Canada and east-central Alaska.The timing and interplay of uplift and erosion has likelycontrolled the extent and thickness of ice masses in the inte-rior of the Yukon and in valleys east of the continental di-vide (Duk-Rodkin et al. 2004).

During the late Cenozoic, climates changed from warm andrelatively humid (preglacial climates) to conditions that weresufficiently cold to build local montane ice caps, and as cli-mates cooled further, broad regional (Cordilleran) ice sheetsformed. Extensive thicknesses of outwash deposits were laid

down during deglaciation, and most loess deposits in this regionof North America are thought to have been deposited undercold climates transitional between glacial and interglacial con-ditions. Conditions suitable for the formation of luvisols andbrunisols suggest that interglacial climates were similar to todayand at times were somewhat warmer and more humid.

Magnetostratigraphy has been used successfully over thepast 50 years to assign glacial and interglacial terrestrialsediments to the global Geomagnetic Polarity Time Scale.In particular, it has provided timelines in glacial–interglacialsedimentary sequences that contain proxy records of pastclimate (e.g., Easterbrook and Boellstorff 1984; Barendregtand Vincent 1990; Cioppa et al. 1995; Jackson et al. 1996;Froese et al. 2000; Roy et al. 2004; Barendregt et al. 1996;Barendregt and Duk-Rodkin 2004; Duk-Rodkin et al. 1996,2004). Glacial deposits for the most part offer few opportu-nities for absolute dating. Where suitable fine-grained sedi-ments exist, reliable polarity determinations can generallybe made, and, in combination with other stratigraphic infor-mation or absolute dates, these sediments can be assigned tothe Geomagnetic Polarity Time Scale. Such deposits may beassigned to the late Pliocene (late Gauss Normal Chron:3.05–2.58 Ma), the early Pleistocene (Matuyama ReversedChron: 2.58–0.78 Ma), the mid and late Pleistocene(Brunhes Normal Chron: <0.78 Ma), or to one of the polar-ity subchrons within the Matuyama Chron (i.e., Olduvai,Gilsa, and Jaramillo).

The late Cenozoic sedimentary record in the northernCanadian Cordillera is extensive and contains evidence ofmultiple glacial and interglacial events. The ages of theseevents are based on surface exposure dates obtained fromglacial erratics, as well as tephrochronology, and magneto-stratigraphy of multiple till, outwash, loess, and paleosol se-quences. Within nonglacial deposits (both preglacial andinterglacial), fission-track ages have been obtained fromtephras, and pollen assemblages have served as useful indica-tors of relative age and paleoclimate. To date, the most reli-able chronostratigraphy has been developed frommagnetostratigraphy and tephras. Here, we report the polarityrecord for the late Cenozoic sediments exposed in the TintinaTrench, and compare these with glacial and interglacial re-cords from the Klondike and Fort Selkirk areas (Fig. 1).Such polarity records are useful in regional correlation andallow comparisons with global paleoclimate archives.

988 Can. J. Earth Sci. Vol. 47, 2010

Published by NRC Research Press

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Stratigraphy and sampling

Three sections in the Tintina Trench (Figs. 1, 2) that ex-posed the most complete stratigraphic record were selectedfor detailed study. The Rock Creek (RC), East FifteenmileRiver (EFR), and West Fifteenmile River (WFR) sectionsare located near the southern margin of the glaciated portionof the Ogilvie Mountains in west-central Yukon and are ex-posed in recent landslide scars within the Trench. The WestTintina Trench Allogroup is defined for the type section atRC (Duk-Rodkin et al. 2010). At all three sections, tiltedMiocene deposits are overlain unconformably by horizon-tally bedded late Pliocene (preglacial) deposits. At the RC andWFR sections, pollen assemblages contain an arboreal compo-nent dominated by the Pinaceae, and with well representedshrubs and herbs, consistent with Pliocene–Pleistoceneages. In particular, the herbaceous species, Polemonium

sp., is characteristic of the Pliocene–Pleistocene Poaceaeand Artemisia zones in interior Yukon and Alaska (Duk-Rodkin et al. 2001; 2010). Polemonium has not been ob-served in the Miocene strata. The absence of Artemisiapollen in the assemblages above the unconformity suggestsan age older than Pleistocene (White et al. 1999). Theselate Pliocene deposits are overlain by multiple diamicts(mostly tills), outwash, and loess units of glacial origin,and many of these units have well-developed paleosols attheir surface. These sequences are capped by a thin loesscover laid down after the last glaciation (McConnell glaci-ation).

Samples collected over three field seasons were selectedfrom sorted, fine-grained sediments (Figs. 3, 4, 5). Wherefine-grained sediments were not present, collections weremade from sorted coarser deposits that contained a fine-grained matrix or from interstitial fine-grained pockets

Fig. 1. Location map of Tintina Trench sites and other stratigraphic sections in Yukon Territory correlated with the West Tintina TrenchAllogroup. Colour relief highlights relationship of study sites to major topographic features (mountain ranges) and proximity to oceans(moisture sources for glaciations).

Barendregt et al. 989


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Fig. 2. Location map of Rock Creek, East Fifteenmile River, and West Fifteenmile River sites in Tintina Trench. Position of local (mon-tane) and regional (Cordilleran) ice limits for the first glaciation (late Pliocene, Gauss Chron) are shown (light and dark shaded areas, re-spectively). Inset map outlines Tintina Trench in west-central Yukon and marks ice limits for late Pliocene – early Pleistocene, middlePleistocene, and late Pleistocene glaciations.



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Fig. 3. Stratigraphy and paleomagnetic sampling localities for Rock Creek site (type section of West Tintina Trench Allogroup).



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Fig. 4. Stratigraphy and paleomagnetic sampling localities of East Fifteenmile River site. See Fig. 3 for legend.



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within till (Fig. 5, unit 5, middle photo), outwash (Fig. 3,units 8 and 9, top photo), and paleosols (Fig. 3, unit 3, lowerphoto). As far as possible, sediments containing pebbleswere avoided. In this way, it was possible to sample mostlithologic units at each of the three sites. In particular,many of the till and outwash units reveal a well-developedpaleosol at their upper contacts, and where suitable, both pa-rent material (till or outwash) and paleosols were sampled.This contrasts with traditional magnetostratigraphic studieswhere collections are usually made in quasi-continuous ver-tical sequences. Glacial deposits represent a ‘‘snapshot’’ intime and, therefore, extensive vertical sampling of unitsfrom bottom to top is unnecessary. Paleosols represent alonger period of time and, depending on soil type and de-gree of soil development, may span *10 to 40 ka (Tarnocai1987, Tarnocai and Schweger 1991, Smith et al. 1986). Inall cases, individual till and outwash units reveal only a sin-gle polarity, and in no case did associated paleosols reveal apolarity opposite to that recorded in the parent material.

Samples were collected by inserting plastic cylinders hor-izontally into vertically cleaned faces. The cylinders (2.5 cmdiameter, 2.2 cm length) were made from shatterproof poly-carbonate plastic. They have a sharp tapered cutting edge,and internal raised splines on the base and side that serve asorientation marks and generally prevent movement of sedi-

ment inside the cylinder, should shrinkage occur. Sampleazimuths were measured using a magnetic compass.

At sampling horizons, vertical trenches (1 to 4, median 2)were cut into the unconsolidated deposits using a shovel andpick. There were 2 to 29 samples per stratigraphic unit (me-dian 13). Altogether 304 useable samples were obtained, 85from eight units at RC, 125 from 10 units at the EFR sec-tion, and 94 from eight units at the WFR section. The loca-tions of the sampling horizons within each of thestratigraphic units, as well as the relative thickness of eachunit, are shown in Figs. 3–5.

Paleomagnetism

Remanence measurements were made at the GeologicalSurvey of Canada - Pacific, Sidney, B.C., using an AGICOJR5-A spinner magnetometer. Stepwise alternating field de-magnetization in peak fields up to 100 mT was carried outusing a Schonstedt GSD-5 with a three-axis tumbler. Sam-ples were typically demagnetized at 10, 20, 40, 60, and often80 mT steps. Sample remanence directions were determinedby principle component analysis (PCA; Kirschvink 1980).

Normal and reversed magnetizations occurred at all threesections, and polarities could be assigned to each sampledunit. In comparison with other glacial lithologies studied,

Fig. 5. Stratigraphy and paleomagnetic sampling localities of West Fifteenmile River site. See Fig. 3 for legend.



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Fig. 6. (A) Example of a typical well-behaved normal-polarity sample. (B) Typical well-behaved reverse-polarity sample. (C) Reverse-po-larity sample with strong present field overprint (primary direction determined using great circle path). (D) Pre-Brunhes normal-polaritysample overprinted during a subsequent reverse-polarity chron. For each sample, the orthogonal plots (bottom figure) show horizontal (ver-tical) projections of each step with a closed (open) circle. The stereographic plots (upper figure) show magnetization direction at each stepwhere the open squares (triangles) lie on the lower (upper) hemisphere. NRM, natural remanent magnetization; mA/m, milliamperes permetre. Location of numbered samples given in Figs. 3–5.



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the magnetic stability of these collections was relativelyhigh, accounting for the high proportion (94%) of samplesthat gave satisfactory results. (In our experience, 50% ofsamples are often rejected in paleomagnetic analyses of gla-cial deposits because of weak or unstable magnetizations orincoherent remanence directions held by randomly orientedsand or pebbles within the sample; Barendregt et al. 1991.)Most samples showed magnetizations typical of magnetite.Median destructive fields for magnetite-bearing sedimentstypically ranged widely from 10 to 80 mT.

The preglacial deposits showed red pigmentation, indicat-ing the presence of hematite, and had median destructivefields higher than the maximum demagnetization step (80mT) used in this study. Several of these samples were de-magnetized thermally up to 150 8C to test for the presenceof goethite, which is often a product of recent weathering.No significant demagnetization was observed at these steps.

The demagnetization data almost always revealed a singlehigh coercivity component, so directions were fit using PCAof lines that were forced through the origin of orthogonalplots. Figures 6A and 6B show examples of well-behavednormal- and reverse-polarity demagnetization curves; theseare typical of 85% of the sampled stratigraphic units. Occa-sionally, it is impossible to isolate a primary reverse-polarityremanence from the present field overprint; however, thegreat circle demagnetization paths (e.g., Fig. 6C) permit po-larity assignment and the unit mean direction can be calcu-lated using the intersection of great circles (McFadden andMcElhinny 1988). Rarely, a reverse-polarity overprint is ob-served over a stable normal component (e.g., Fig. 6D), indi-cating that the primary remanence predates a subsequentreverse-polarity chron and proving that the remanence is nota present field overprint.

In Table 1, Fisher-averaged directions of each unit are

Table 1. Summary of directions.

Unit Lithology Nt Ns D I k a95 PPolarity chron orsubchron

Rock Creek section11 Colluvial cover (loessic) 0 Cryoturbated (not suitable for sampling) late Brunhes (McConnell

glac.)9 Outwash 4 24 18.9 62.6 14.0 8.2 N early–mid Brunhes8 Till 1 2 218.5 –61.5 12.5 180.0 R late Matuyama7 Outwash and till stringers 1 8 48.4 64.0 100.1 5.6 N Jaramillo6 Outwash 2 13 163.5 –71.3 35.1 7.2 R mid Matuyama5 Till and paleosol 0 Too coarse for sampling ?5 Outwash 2 8 4.7 68.8 20.5 12.5 N Olduvai4 Mudflow 0 Too coarse for sampling ?3 Till and paleosol 3 16 184.5 –76.1 13.8 10.4 R early Matuyama2 Outwash 1 3 39.6 61.9 31.8 22.2 N late Gauss1b Preglcial lacustrine (late Pliocene) 1 11 49.2 70.2 10.0 15.2 N late Gauss

East Fifteenmile River section11 Loess (surface) 1 2 72.4 68.1 217.1 17.0 N late Brunhes (McConnell

glac.)10 Till 1 2 71.6 66.3 152.7 20.4 N late Brunhes (Reid glac.)9 Loess 1 14 14.4 77.8 78.5 4.5 N early–mid Brunhes8 Loess 2 13 157.4 –40.6 8.1 15.5 R late Matuyama6 Till and paleosol + minor lacustrine base 4 14 163.3 –75.1 22.0 8.7 R mid Matuyama5 Till and paleosol 3 21 11.9 66.8 34.9 5.5 N Olduvai3 Till and paleosol 2 24 153.2 –67.9 7.9 11.2 R early Matuyama2 Till, outwash, till, paleosol 3 14 230.7 88.1 5.5 18.6 N late Gauss1b Preglacial fluvial sand and gravel (late

Pliocene)3 17 249.0 57.3 4.4 19.3 N late Gauss

0 Tilted Miocene beds 1 4 162.9 –72.9 9.8 32.6 R Gilbert?

West Fifteenmile River Section11 Loess (surface) 2 10 51.1 68.3 32.1 8.7 N late Brunhes (McConnell

glac.)10 Till + minor lacustrine at base 2 9 17.6 70.9 17.7 12.6 N late Brunhes (Reid glac.)9 Loess 1 8 45.8 65.9 66.5 6.8 N early-mid Brunhes8 Loess 3 13 155.3 –77.8 15.4 11 R late Matuyama7 Loess 2 14 2.9 71 15.5 10.4 N Jaramillo3–6 Till, paleosol, and gravel inclusions 0 inaccessible exposure ?2 Till, paleosol, and gravel inclusions 2 6 295 84.1 30.6 12.3 N late Gauss1a Preglacial sands and gravels (late Pliocene) 1 5 236 –42.4 16.9 19.2 R Kaena subchron?0 Tilted Miocene beds 2 29 215.3 –70.7 7.9 10.1 R Gilbert?

Note: Nt, number of sample sites within each unit; Ns, total number of samples used in calculation of mean direction for each unit; D, declination; I,inclination; k, precision parameter; a95, radius of 95% confidence level; P, polarity; N, normal; R, reverse; glac., glaciation.



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listed and polarities defined. The mean directions are plottedin Fig. 7. In most paleomagnetic studies of glacial deposits,the mean directions show very high dispersion; this must re-flect depositional and post-depositional disturbances. In theTintina Trench sections, we note a surprising coherence ofthe unit mean directions, especially for sites with a95 < 158,allowing the computation of mean directions for each site(Table 2). The mean of inclinations (73.18) is only marginallyshallower than the expected geocentric axial dipole (GAD)inclination of 76.68 for the sampling latitude (64.58N). Thenormal- and reverse-polarity means are within 8.8 ± 6.48 ofantipodal, which probably reflects a very minor degree ofpresent field contamination on the reverse-polarity samples.

DiscussionAt all three sites (Fig. 8), preglacial sediments (units 1a,

1b) outcrop at the base and exhibit both normal and reversemagnetic polarities. Where these sediments are sufficientlyfine-grained, they contain pollen (Polemonium) characteris-tic of the Pliocene–Pleistocene in interior Yukon andAlaska. We interpret the normal unit (i.e., unit 1b) to datefrom the Gauss Normal Chron (3.58–2.58 Ma) and the re-versed unit (i.e., unit 1a) from the Kaena subchron withinthe Gauss Chron. The magnetic carriers are predominantlyfine-grained hematite marking extensive oxidation of ironminerals within these sediments. At the EFR site, Miocene(and older) beds are tilted, and the overlying unit 1 fluvialsediments have scattered paleomagnetic directions, indicat-ing the effect of rotational slumping (see unit 1 mean direc-tion stereoplots for EFR and WFR, Fig. 7 and Table 1).

The first direct evidence of glaciation is marked by nor-mally magnetized tills and outwash at all three sites (Fig. 8,unit 2). These deposits mark the first entry of local, as wellas Cordilleran, ice into the Tintina Trench region. We assignthis normal polarity to the late Gauss Chron (3.05–2.58 Ma)based on the number of magnetozones and subzones overly-ing this unit. Above unit 2 at the WFR site, a large cliff face(13 m vertical) precludes sampling, but a reversely magne-tized till and paleosol (unit 3) overlies unit 2 at the othersites. Unit 4, which overlies unit 3, is a glacial mudflowseen only at the RC section, but it is too coarse for paleo-magnetic sampling. It is overlain by an outwash and till(unit 5) of probable Olduvai age (1.78–1.97 Ma), and there-fore unit 4 most likely falls within the early MatuyamaChron. Unit 5 till and till–outwash sampled at EFR and RC,respectively, exhibit normal polarity and are assigned to theOlduvai subchron based on the sequence of polarities thatoccur above this unit. Magnetostratigraphic arguments wereadvanced in the Klondike area (Froese et al. 2000) and inthe Mackenzie Mountains (Duk-Rodkin et al. 1996) for apre-Brunhes normal-polarity glacial event. Based on polaritysequences there and at the Tintina Trench sites, it is very

Fig. 7. Stereographic plot of unit means and 95% ovals of confi-dence, shown for each of the sampled units at the three sites. Nor-mal (reversed) polarity is represented by solid (open) squares andsolid (dashed) ovals of confidence, and they are projected on upperand (lower) hemispheres, respectively. Numbers correspond tounits listed in Table 1 and shown in Figs. 3–5, and 8.



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likely that a glaciation occurred during the Olduvai sub-chron. A large negative (cold) d 18O peak (MIS 70, Fig. 8)falls within the Olduvai subchron and is a possible candidatefor this extensive glaciation.

Unit 6 marks a subsequent glaciation with till exposed atthe EFR site and outwash sands and gravels and intercalatedbeds of fine-grained deposits exposed at RC. The till andfine-grained deposits at these two sites are reversely magne-tized and, based on the polarity sequences above and belowthese units (Fig. 8), can be reliably assigned to the late Ma-tuyama Chron. This unit is overlain by a normally magne-tized loess (unit 7) at the WFR site and normal outwash(unit 7) at the RC site. The outwash that reached RC ap-pears not to have reached the WFR site. We interpret thesenormal units to be coeval and assign them to the Jaramillosubchron (1.05–0.99 Ma). Deposits of Jaramillo age wereapparently not deposited or were not preserved at the EFRsite, as the reversed unit 6 till and paleosol is directly over-lain by a reversely magnetized loess (unit 8).

Reversely magnetized loess and outwash (unit 8) overliethe sediments assigned to the Jaramillo and can, therefore, beassigned to the uppermost Matuyama (0.99–0.78 Ma). Unit 8is overlain by normally magnetized loess and outwash (unit9) at all three sites. Unit 9 is assigned to the early BrunhesChron (<0.78 Ma). Normally magnetized unit 10 (Reid glaci-ation, MIS 6) overlies unit 9 at the WFR and EFR sites and isabsent at RC because Cordilleran ice did not reach RC duringthe late Brunhes (Duk-Rodkin et al. 2010). Surficial depositsat all three sites are composed of normally magnetized lo-esses (< MIS 6) and are most likely coeval with the late Pleis-tocene (late Wisconsinan) McConnell glaciation.

Glaciations in the Tintina Trench were followed by periodsof soil formation. Some paleosols exhibit thick B horizonsand significant clay accumulation, suggesting relatively longperiods of soil development (in the order of 40 ka). Despitethe lengthy period of soil formation, all paleosols have re-tained their primary magnetization. Some paleosols exhibitweak secondary magnetizations (overprints), presumably be-cause of greater alteration of magnetic minerals by weather-ing and leaching. In all cases (Table 1), soil polarities matchthose of the parent materials (till and outwash), suggestingthat the magnetization is held as a detrital remanence.

Comparison of Tintina Trench stratigraphy with othersites in central Yukon

Klondike River valley sitesExposures 15–20 km south-southwest of the Tintina

Trench in Klondike River valley (KRV) near Dawson, Yu-kon, reveal an extensive record of preglacial, glacial, and in-terglacial sediments. A composite magnetostratigraphy(Fig. 9) developed for these KRV sites (Jackson Hill, Aus-tralia Hill, and Midnight Dome; Froese et al. 2000) providesa record that can be compared with that of the TintinaTrench. Like the Tintina Trench, the KRV sites contain pre-glacial sediments (Upper White Channel gravels) at theirbase. Fine-grained units within these gravels are normallymagnetized, with an indication of reverse overprinting atsome localities, suggesting that this normal polarity is pre-Brunhes in age. Based on the presence of the Quartz Creektephra (3.00 ± 0.33 Ma; Sandhu et al. 2000) in similar grav-els at a nearby site, the Upper White Channel gravels appearto be late Pliocene (late Gauss Normal Chron). Near theircontact with the overlying Klondike gravels, the WhiteChannel gravels contain ice-wedge casts marking the onsetof periglacial conditions and may in fact point to an ice cap(glaciation) in the nearby mountains at this time. The WhiteChannel and Klondike gravels interfinger at their contact,indicating that there is not a significant hiatus in deposition.The Klondike gravels are outwash deposits from the first in-cursion of Cordilleran ice into the area, and this same incur-sion also brought the first lobe of Cordilleran ice into theRC site in the Tintina Trench. At both sites, these depositsare normally magnetized and assigned to the late Gauss Nor-mal Chron. The two gravel units (Upper White Channel andKlondike) are coeval with the Polemonium-bearing pregla-cial lacustrine sediments, till, outwash, and paleosol (Fig. 8)of units 1b and 2, respectively (Duk-Rodkin et al. 2001). Itis seen from both Tintina Trench and KRV sites (JacksonHill, Fig. 9) that the first glaciation in this region was Cor-dilleran and occurred *2.7 million years ago (MIS G6).Following the deposition of the Klondike gravels, a long hi-atus occurred (*1 Ma), possibly marked by an extensiveperiod of erosion, after which the Midnight Dome sedimentswere deposited. At the base of the Midnight Dome section,

Table 2. Site means.

All a95 < 158

Groupings N D I k a95 N D I k a95

Rock Creek site 8 26.4 68.5 62.9 7.0 5 14.9 70.0 61.9 9.8East Fifteenmile

River site10 352.7 76.4 13.7 13.5 4 354.7 72.7 94.2 9.5

West FifteenmileRiver site

8 33.5 71.5 25.3 11.2 7 23.0 74.8 53.8 8.3

Normal polarity 16 29.9 74.9 24.4 7.6 10 23.4 71.5 61.4 6.2Reverse polarity 10 185.4 –69.2 18.4 11.6 6 172.4 –74.4 105.3 6.6All 26 18.8 73.1 21.1 6.4 16 13 73.1 61.4 4.7GAD 0.0 76.6PEF 24.0 78.0

Note: Groupings are directional means for all stratigraphic units at each site (lower hemisphere), means of all normaland all reversed units at all sites, and mean of all units at all sites. Directional means are also given for units with a95 <158. N, number of stratigraphic unit;. GAD, geocentric axial dipole direction; PEF, present Earth’s field direction; D, de-clination; I, inclination; k, precision parameter; a95, radius of 95% confidence level.



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Fig. 8. Magnetostratigraphy (polarities) and paleomagnetic sampling sites and sample numbers for Rock Creek (RC), East Fifteenmile River (EFR) and West Fifteenmile River (WFR)sites. Geomagnetic Polarity Time Scale (Cande and Kent 1995) and composite d18O LR04 marine isotopic record (relative paleotemperature) obtained from multiple deep ocean cores(Lisiecki and Raymo 2005. Base of Pleistocene (2.58 Ma) follows recently ratified convention (Gibbard et al. 2010). Black and white areas are normal and reversed polarity, respec-tively. Even numbers to left of curve are cold isotope stages, and odd numbers to the right are warm stages (MIS, Marine Isotope Stages). Letters and numbers used before isotope stage104 (Matuyama–Gauss boundary) follow Shackleton et al. (1995). Arrow at bottom is mean Holocene d18O value. Bold numbers correspond to stratigraphic units in Figs. 3–5 andTable 1 and indicate suggested correlation of glacial deposits to cold stages in marine isotopic record (blue squares). N, normal polarity; R, reversed polarity.

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Fig. 9. Comparison of composite magnetostratigraphy at Tintina Trench, Klondike River valley, and Fort Selkirk sites, west-central Yukon. Fine broken lines highlight location of firstglaciation (Marine Isotope Stage (MIS) G6), Gilsa subchron, and MIS 58 and 56 glaciations, where present. Upper bold line is Matuyama–Brunhes boundary, middle line is Gauss–Matuyama boundary, and lower line is Miocene–Pliocene boundary. For each site, stratigraphic units marking glaciations are shown as coloured squares, and suggested correlation tocold peaks in the marine isotopic record are given. N, normal polarity; R, reversed polarity.

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an interglacial fluvial deposit is overlain by coarse cobbleoutwash gravel (Froese et al. 2000; Westgate et al. 2003).The latter has a proximal glacial source from a pre-Reid gla-ciation to the east (southern Ogilvie Mountains and (or) Cor-dilleran glaciers in the Selwyn Mountains). Overlying thesegravels is an extensive loess sequence composed of at leastfour cycles of sedimentation occurring under glacial condi-tions. Each of these is overlain by pollen-bearing interglacialassemblages (Froese et al. 2000). This loess sequence re-cords a reversed–normal–reversed–normal-polarity sequence,indicating that these deposits were laid down over a consid-erable span of time, perhaps as much as 1.0 Ma. At theirbase, these sediments are interpolated to be *1.4 Ma (Mos-quito Gulch tephra, Sandhu et al. 2000), whereas the upperhalf of the sequence contains the normal Jaramillo subchron(1.05–0.99 Ma) and the Midnight Dome tephra (1.09 ± 0.18 Ma;Froese et al. 2001), and an overlying suite of reversed andnormal units, the latter being Brunhes age (<0.78 Ma).These loess deposits are coeval with units 6, 7, 8, and 9at the Tintina Trench sites. Evidence for glaciations in theearly Brunhes (MIS 18 and (or) 16) are reported fromnearby east-central Alaska (Froese et al. 2003), where gla-cial outburst flood deposits sourced in the Yukon–TananaUpland contain the GI tephra (0.56 ± 0.08 Ma) and arenormally magnetized, constraining the age to earliestBrunhes. Upper (normal) loess deposits at Midnight Domemay be coeval with these glaciations.

Fort Selkirk sitesAn extensive and well-dated sequence of volcanics and

interbedded glacial deposits occurs 200 km south of the Tin-tina Trench sites, and like the KRV deposits, offers an op-portunity to compare evidence for early and middlePleistocene glaciations. The earliest evidence for glaciationin the Fort Selkirk area is at the lower Mushroom (Fig. 9)site, where outwash gravel containing glacially transportedlithologies underlies a reversely magnetized basalt (2.32 ±0.13 Ma; Westgate et al. 2001). The glaciation may consid-erably predate the basalt. Evidence for a second glaciation isfound near the summit of Ne Ch’e Ddhawa volcano (tuya),where exotic glacial pebbles are contained in a reverselymagnetized hyaloclastite (2.08 ± 0.05 Ma; Hunt and Rod-dick 1992). Till and outwash of a third glaciation (also re-versely magnetized) are overlain by reversely magnetizedinterglacial sediments containing the Fort Selkirk tephra(1.48 ± 0.11 Ma Westgate et al. 2001) and a normally mag-netized silt unit, assumed to fall within the Gilsa subchron(1.58–1.60 Ma; Fig. 9). These interglacial sediments areoverlain by a basalt (1.35 ± 0.08 Ma; Westgate 1989). Evi-dence for a fourth glaciation is seen at Ne Ch’e DdhawaNorth, where a reversely magnetized till overlies a basalt(3.05 ± 0.07 Ma; Jackson et al. 2008). Glacial striae andscattered erratics, presumably of this same glaciation arefound at a similar elevation on the upper Pelly sequence(1.33 ± 0.07 Ma; Nelson et al. 2009) at Cave,Mushroom,and Fossil sites. Glacial outwash from a fifth glaciation(Reid, MIS 6) overlies the Black Creek flows (0.311 ±0.032 Ma; Huscroft et al. 2004). The Reid (Ward et al.2007, 2008; Westgate et al. 2008) and McConnell (MIS 2)glaciations approached this area from the east-southeast andleft outwash and loess deposits (Ward and Jackson 2000). In

total, six glaciations are documented for the Fort Selkirkarea. The four earliest glaciations fall within the Matuyamaand the latter two occur within late Brunhes.

Although the glacial record in the KRV and the Fort Sel-kirk area is not a complete match for that of the TintinaTrench, basalt and tephra dates at the KRV and Fort Selkirksites bracket some of the glacial deposits, and where theseare combined with magnetostratigraphy, some correlationsof glaciations can be made with confidence (Fig. 9).

ConclusionsMultiple sequences of tills, outwash, loess, and paleosols in

the Tintina Trench record predominantly late Pliocene to mid-dle Pleistocene glaciations and interglaciations. A relativelythin surficial sequence of late middle Pleistocene to late Pleis-tocene deposits are present as well. Tills are of either local(montane) or regional (Cordilleran) provenance. We suggestthat the Tintina Trench provides a relatively complete recordof the glacial history for the central Yukon region because itis located near the maximum limit of ice advance in the area,precluding extensive glacial erosion of older deposits.

The polarity record established for the Tintina Trenchspans about 3.0 Ma, and sediments, for the most part, havefaithfully recorded the paleofield directions. This is con-firmed by inclination and declination values of unit and sitemeans that are not far removed from the geocentric axial di-pole direction of the sampling latitude (Tables 1, 2). Themagnetostratigraphic record described for the trench (N–R–N–R–N–R–N from bottom to top) indicates that sedimenta-tion occurred intermittently during all polarity chrons andduring most subchrons over the past 3.0 Ma. The polaritysequence that has been identified contains six reversals re-cording at least three glaciations within the Brunhes NormalChron and seven within the Matuyama Reversed Chron, in-cluding at least one glaciation in each of the two major sub-chrons within the Matuyama (Olduvai and Jaramillo).Interglaciations (four in the Brunhes Chron and seven in theMatuyama Chron) are present in the stratigraphic record inthe form of paleosols or unconformities between till, out-wash, and loess units, where the units on either side of theunconformity have opposite polarity.

Preglacial deposits (units 1a, 1b) at the base of the TintinaTrench sequence contain late Pliocene pollen marking the on-set of cooler conditions, while the overlying deposits of nor-mally magnetized tills and outwash (unit 2; late Gauss NormalChron, MIS G6) mark the first glaciation in the region. Thisrecord of late Pliocene cooling, and initiation of major glacia-tion around 2.7 Ma, is also documented at nearby KlondikeRiver valley sites, and elsewhere in North America.

AcknowledgementsWe thank Anne Williams, Duane Froese, Daniel Utting, and

Susan Russell for their painstaking work in exposing each ofthe units in hand-dug excavations, Anastasia Ledwin for labo-ratory assistance, and Ted Irving for helpful advice and dis-cussions of results. The authors acknowledge the valuablecontributions of Paul Sanborn, Charles Tarnocai, and ScotSmith in the field identification of paleosols. Pollen identifica-tion and a discussion of its paleoclimatic significance wereprovided by Charles Schweger and James White. We thank re-



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viewers Julie Brigham-Grette, Maria Cioppa, and the Associ-ate Editor Tim Fisher for helpful comments and suggestions,which certainly improved the paper. The research was sup-ported in part by a Natural Sciences and Engineering ResearchCouncil of Canada grant (#589) to R.W. Barendregt. Fieldsupport was provided by the Geological Survey of Canada.

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Paleomagnetic Evidence for Multiple Late Cenozoic Glaciations in the Tintina Trench of west central Yukon, Canada