Magnetites from a new unidentified tephra source, Banff National Park, Alberta

NOTE

Magnetites from a new unidentified tephra source, Banff National Park, Alberta

G. R. BREWSTER Dc~partment qf'(~c$ogrrzphy, Untrlersity ofWestc.rn Ontario, Loncion, Ont., C'trntrdn N6A 5C2

.4ND

R. L. BAKNET~I'

Depczrtment o$Geology, Unizwsity of Western Ontczrio, London, Ont.. Cclnclda N6A 5R7

Received January 3, 1979

Revision accepted February 6. 1979

Electron microprobe examination of glass-encased magnetites present within surficial volcanic ash deposits located in Banff and Jasper National Parks revealed five distinct magnetite populations. Three of the magnetite populations represented the Mazama, St. Helens Y, and Bridge River volcanic units previously identificd in this area of the Canadian Kocky Mountains. The reinaining two magnetite groups are characterized by glass-encased magnetites which have titanium oxide concentrations of 11.59 and lQ.33%, values considerably higher than thosc characteristic of either Mazama, St. Helens Y, or Bridge River volcanic units. The high-titanium magnetites are of unknown provenance, and although the section provided no means for dating these volcanic groups, their distribution within the section suggests that they are older than Bridge River, and one group may predate Mazama.

I.'examen B la microsonde electronique des grains de magnitite enrobks de ven-e qu'on retrouve dans les depbts de cendres volcaniques superficiels dans les parcs nationaux de Banff et de Jasper a revkle cinq populations distinctes de magnetite. Trois dcs populations de magnetite representent les unites volcaniques de Mazama, de St. Helens Y et de Bridge Kiver dkj2 reconnues dans cette partie des montagnes Kocheuses canadiennes. Les deux autres groupes de magnetite sont caracterises par des magnetites enrobees de verre qui ont des concentrations en oxyde de titane de 11.59 et de 10.33%, valeurs qui sont considerablement plus ilevees que celles des unites de Mazama, de St. Helens Yet de Bridge River. On ne connait pas la provenance de ces magnetites riches en titane et bien que la coupe ne permette pas de dater ces groupes volcaniques, leur distribution ii I'interieur de la coupe suggere yu'ils sont plus anciens que I'unite de Bridge - -- - River et un des groupes pourrait &re plus ancien que I'unite de Mazama.

['Traduit par le journal] Can. J . Earth Sci., 16, 1294-1297 (1979)

Previous tephrostratigraphical investigations within the Banff and Jasper National Park areas of the Canadian Rocky Mountains have revealed the presence of volcanic ash from three major sources. The determination of the source area, compositional characteristics, and the distribution of these major volcanic ash units, as well as dating materials adjacent to the ash layers, has been carried out by Westgate and Breimanis (1967) and by Westgate et al. (1970). The oldest and thickest deposit is termed Mazama ash, from Mount Mazama, Oregon, and is dated at approximately 6600 years BP (Fryxell 1965). A more recent ash identified in this area of the Canadian Rocky Mountains, St. Helens Y, emanated from an eruption at Mount St. Helens in southwestern Washington approximately 3350 years BP (Crandell et al. 1962). The most recent ash, Bridge River, is thought to have originated from the Meager Mountain volcanic complex in the

Coast Mountains north of Vancouver and west of Lillooet, British Columbia (Read 1977). Volcanic ashes from this eruptioil have been dated at approximately 2450 years BP (Nasmith et ul. 1967).

Although the three volcanic ash deposits present within Banff and Jasper National Parks are each derived from individual source volcanoes, consid- erable compositional variability within each ash unit reflects a somewhat irregular eruptive history. There is evidence for three Mazama ash units, two of which may be present within this area of the Canadian Rocky Mountains (J. A. Westgate, personal communication, 1978). Also, two St. Helens Y volcanic ash units may be present; one, St. Helens Yn ash, has been identified near Entwistle, Alberta, and is dated at 3550 +_ 65 radiocarbon years BP (WIS-343, Westgate et al. 1969) and the other- St. Helens Y ash, recovered from a core in the Kootenai Plains in Alberta, has a projected age of

0008-4077/79/06 1294-Q4$01 .00/0 01979 National Research Council of CanadalConseil national de recherches du Canada

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approximately 4400 radiocarbon years and is prob- ably the oldest bed of set Y tephra (Westgate 1977). Two separate Bridge River volcanic ash units have also been identified, the oldest dated at approximately 2600 years BP, and the youngest 2200-2180 years BP (Westgate 1977). 'Phe Bridge River coup- let has been tentatively identified at Sunwapta Pass, near the boundary between Banff and Jasper National Parks (R. H. King, personal communication, 1978).

Many of the recent studies analyzing tephl-a and volcanic ash distributions within the Cordillera have been undertaken with the primary objective of identifying and characterizing discrete volcanic ash units (for example, Westgate et ul. 1970). By and large. tephrostratlgraphical studies in the Canadian Rocky Mountains are still generally involved with the solution of primary location-identification- characterization problems. As these studies have progressed, the spatial variability and complexity of the volcanic ash units are found to be increasing, with several volcanic ash units now identified as having originated from a single source (Westgate and Fulton 1975; Westgate 1977). Characterization of an individual volcanic ash unit is obtained through the analysis of individual glass shards or glass-encased magnetite grains present within geologically discrete samples. The resulting diag- nostic compositional character of the volcanic ash unit is, however, based on a mean value (Westgate st nl. 1970; Westgate 1977) from which individual determinations may deviate. The composite value then provides the basis for the identification of similar deposits. In this manner Mazama, St. Helens Y, and Bridge River volcanic ash units have been identified and characterized in the Canadian Cordillera by electron microprobe analysis of glass shards and glass-encased magnetite grains.

During an examination of soil development within Banff and Jasper National Parks, sur-ficial volcanic ash deposits were examined in order to identify and characterize the soil parent materials. The volcanic deposits forming the upper sola of these soils were mixed. and invariably contained subsoil contaminants. Glass-encased magnetite grains from the representative pedogenic horizons were found within the upper volcanic ash-charged sola, but magnetites were found to be totally lack- ing in the underlying subsoil materials composed of locally derived till. Characterization of the glass- encased magnetite grains was accomplished by de- termining 81, Mg, Fe, Ti, and Mn oxide concentrations by means of electron microprobe techniques. Although the concentrations of several oxides were

Mosquito Creek A

St. Helens Y A

Mosquito

:I4 OIBridge River

+x+ mean and 1 IJ Mosquito A Wrstgate a l 1970 Ir Creek A

T

Mazama ( 1 Mosquito T4 1-4 Creek B 1

cr-l lBridge River

St. Helens Y

1 4 5 6 7 8 9 1 0 1 1 1 2

TiO,

FIG. 1. Binary plot of TiO, versus AI,O, and MgO in glass- encased magnetites collected from Mosquito Creek (Banff National Park) and Mount Edith Cavell (Jasper National Park).

determined for each sample, volcanic ash identification is possible by the examination of A1203, TiO,. and MgO concentrations in magnetite (Westgate et al. 1970; Duford 1976).

Examination of the glass-encased magnetite grains within pedons located in Mosquito Creek meadows of Banff National Park (51°40'N, 1 16°15'05"W), and the Mount Edith Cavell meadows of Jasper National Park (52"42'N, 118"02'W) revealed the presence of five distinct populations of glass-encased magnetite (Fig. 1). When compared to published data on magnetite composition and volcanic ash identification (Westgate et al. 19709, three of these volcanic ash groups were found to correspond to Mazama, St. Helens Y, and Bridge River volcanic units. The two remaining groups (Fig. 1) are characterized by

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CAN. J . EARTH SCI. VOL. 16, 1979

TABLE 1. Glass-encased magnetite composition determined by electron microprobe

Mazama St. Helens Y Bridge River Mosquito Creek A Mosquito Creek R -.

l o l o l o l o l o

A1203 2.09 0 .17 MgO 2.40 0.23 FeO 30.03 0 .32 TiO, 8.66 0.36 Fe203* 55.68 0 .64 MnO 0.44 0 .09

Total 99.30 4. 19

*Fez03 determined assuming stoichiometry. tr2 = sample size. NOTE: All determinations were made on a Materials Analysis Corporation 400-S electron ~nicroprobeht 15 kV utilizing a beam diameter of 2 pm and a

beam current of 0.05 PA.

magnetites having a much higher TiO, concentration than the magnetites representing either the Mazama, St. Helens Y, or Bridge River volcanic units. One of these high-titanium magnetite groups, Mosquito Creek A, is characterized by its 11.59% Ti02 concentration and A1,0, and MgO concentrations which are greater than those of the other four groups (Table 1). The second high-titanium magnetite group, Mosquito Creek B, is characterized by a somewhat lower TiO, concentration of 10.33%. The Al,O, concentration of the Mosquito Creek B group is greater than either the Mazama or Bridge River groups but less than the St. Helens Y group, while its MgO concentration is similar to the Mazama volcanic ash group and greater than either the St. Helens Y or Bridge River volcanic units (Table 1).

Volcanic ashes containing glass-encased magnetites with high titanium concentrations have not been reported in the American or Canadian Cor- dillera. High-titanium magnetites were found to be

Bf horizon. The distribution of high-titanium magnetites suggests that both the Mosquito Creek A group and the Mosquito Creek B group are older than Bridge River, and that the Mosquito Creek A group might predate Mazama.

At the time of writing the presence of high- titanium glass-encased magnetites in volcanic ash deposits had not been reported from the American or Canadian Cordillera. Because these results may indicate the presence of a new stratigraphic marker in the Cordillera they are important, but because they apparently do not occur in a geologically discrete layer of ash, regional correlation and characterization has not been possible. Possibly the high-titanium magnetites may indicate an individual Mazama eruptive phase, or perhaps even a volcanic deposit which is discrete and older than the Mazama ash. Unfortunately, no stratigraphic means were found for dating the volcanic ash with the hitherto unreported high-titanium magnetite grains.

present only within the ~ o s q u i t o Creek pedon, Acknowledgements while both of these magnetite groups and magnetites characteristic of Mazama volcanic ash were The authors would like to express their sincere

absent from the volcanic materials examined from appreciation to Dr. J. A. Westgate and Dr. G. M. Mount Edith Cavell. Although the volcanic ash- Ruthefiord for their assistance with earlier forms of

charged Ae, Bhf, and Bf horizons of the Mosquito this manuscript. The manuscript also benefitted

creek pedon contained magnetites representative from the comments of two Journal reviewers.

of ~ridrze River and Mazama volcanic ashes, their CRANDELL, D. R., MULLINEALJX, D. R., MILLER, K. D., and - distribution was Magnetites RUBIN, M. 1962. @roclastic deposits of Recent age at Mount

teristic of id^^ ~i~~~ ash dominated within the Rainier, Washington. In Short papers in geology, hydrology, and topography. United States Geological Survey, Rofes-

Ae and Bhf horizons, while magnetites charac- sional paper 450-D, pp. DM-WX. teristic of Mazama ash dominated within the DUFOKD, J. M. 1976. Late ~leistocene and Holocene cirque lowermost Bf horizon. Magnetites re~resentative glaciations in the Shushwap highland area. British Columbia. -

M.Sc. thesis, University of Calgary, Calgary, 41ta. 100 p. of the Mosquito Creek group were identified FRYYELL, R. 1965. Mazamil and C;laciel- Peak volcanic within the Bhf and Bf horizons, while magnetites layers: relative ages. Science, 147, pp. 1288-1290. representative of the Mosquito Creek A group were NASMITH, H.. MATHEWS. w. H., and ROUSE. G. E. 1967. only identified within the Mazama ash dominated Bridge Kiver ash and some other Recent ash deds in British

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Columbia. Canadian Journal of Earth Science\, 4, pp. 163-170.

READ, P. B. 1977. Meager Creek volcanic complex, southwestern British Columbia. Report of Activities, Part A. Gco- logical Survey of Canada, Paper 77- 1 A, pp. 277-28 1 .

WI-STGAI F , J . A . 1977. Identification and significance of late Holocene tephra from Otter Creek, wuthern British Colum- bia. and localitic\ in west-central Alberta. Canadian Journal of Earth Sciences, 14, pp. 2593-2600.

W E S I G ~ T E ~ , J . A., and DRL IMANIS, A. 1967. Volcanic ash laycrs of Recent age at Banff National Park. Alberta, Canada. Canadian Journal of Earth Sc~ences, 4, pp. 155-161.

WESTGATE, J . A., and FLJL I O N , R. J . 1975. Tephro\tratigraphy of Olympia interglacial sediments in south-central British

Columbia. Canada. Canadian Journal of Earth Sciences, 12. pp. 489-502.

W ~ S T G A T E , J. A., SMI TH. 1). W. (;., and Nrc~ior,~ , H . 1969. Late Quaternary pyroclastic layers in the Edmonton area, Alberta. In Pedology and Quaternary research, Editetl by S. Pawluk. University of Alberta Printing Department, Edmonton, A h . , pp. 179-186.

WFSTC;AT~-, J . A., SMII H , D. W. G . , and TOMI INSON, M . 1970. Late Quaternary tephra layers in southwestern Canada. In Early man and environments in northwest North America. Eclited hy R. A. Smith and J. W. Smith. I'roceedings of the Second Annual Paleoe~ivironmental Workshop, University of Calgary Archaeological Association, The Student's Press, Calgary, Alta., pp. 13-34.

Erratum: Skeleton Lake, Ontario-evidence for a Paleozoic impact crater

E. D. WADDINGI ON

L)epl,clrtrrz~nt oj'Geoph.vsic*s anti Astronomy, Uniwr.vity ofBritish Colurnhicz, Vunc.outler, B.C., Cunudca V61' I W5

A N D

M. R. DENCL: Grtltlity and Geodynumicss Dilisic~n, E~zrtll I-' /z~s~c's B Y I I ~ ~ . / z , Depurtment (PfEnf>rgy, Mines und Resources.,

Ort~rwa, Ont., Ctrncidn KIA OY3

Received March 20, 1979 (Ref.: Can. J . Earth Sci. 16,256 (1979))

In the English and French abstracts and in line 4 of the Introduction, the longitude of Skeleton Lake is incorrectly given as 70"27' W. The correct position is 7Y27'W.

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Magnetites from a new unidentified tephra source, Banff National Park, Alberta