Vous êtes sur la nouvelle plateforme d’Érudit. Bonne visite! Retour à l’ancien site

Notes

Helmeted Muskox (Bootherium bombifrons) from Near Fort Saskatchewan, Alberta: Dating Evidence for Redeposition in Late Pleistocene Alluvium

  • Leonard V. Hills et
  • Michael C. Wilson

…plus d’informations

  • Leonard V. Hills
    Department of Geology and Geophysics, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4
    lvhills@ucalgary.ca

  • Michael C. Wilson
    Department of Geology, Douglas College, P.O. Box 2503, New Westminster, British Columbia V3L 5B2

Corps de l’article

Introduction

Harington (1975) recorded the first occurrence of helmeted muskoxen from Alberta (Fig. 1), the specimen considered and dated here, referring to it as Symbos cavifrons (Leidy) and assigned it a common name of helmeted muskoxen. This partial cranium was collected by Mr. F. Jacknicki from sands in the Steele Brothers of Canada gravel pit along the North Saskatchewan River (53° 39' 40" N, 113° 17' W), 4.8 km SW of Fort Saskatchewan, northeast of Edmonton (Fig. 2). It is in the collections of the Department of Biological Sciences, University of Alberta. In their taxonomic review of fossil and recent muskoxen, McDonald and Ray (1989) synonymized S. cavifrons with Bootherium bombifrons (Harlan, 1825). This nomenclature is followed here. Bootherium originated in the middle part of the Pleistocene, possibly as early as 1 Ma, but most specimens are from the Late Pleistocene. Few have been dated by the radiocarbon method (Clay and Neusius, 1985; McDonald and Ray, 1989; McDonald and Echols, 1990; McDonald et al., 1991; McDonald et al., 2000)

The Fort Saskatchewan specimen was not observed in situ but was reportedly derived from a heavily iron and manganese oxide-stained sand bed near the base of a 6.1 m unit of quartzite-rich gravels and sands with angular shield erratics and woody detritus (Fig. 3). The character and abundance of quartzite clasts and woody material closely resembles the basal member of the Empress Formation (Saskatchewan Gravels and Sands), the last cycle of alluvial deposition in valley floors prior to the onset of glaciation and well represented in the Edmonton-Fort Saskatchewan area (Westgate, 1969; Rutter et al., 1998). The presence of shield-derived clasts, however, led to the conclusion that the deposits at Fort Saskatchewan were “obviously deposited following the initial glaciation of the region, and it cannot belong to the Saskatchewan Gravels” and that the specimen was probably of Late Pleistocene age (Harington, 1975, based on information from Westgate, 1969). The specimen does, however, show abrasion from transport, being a frontlet (posterior cranium with occipitals, frontals and incomplete horn cores). A quartzite pebble is lodged in the foramen magnum. The gravel and sand unit from which the specimen probably came was itself overlain by up to 4.6 m of silt and sand, the lower part of which displayed mechanical rhythmites and was thought to be in part equivalent to glacial Lake Edmonton sediments (Harington, 1975).

Recent discovery of a paleontological site in southern Alberta also containing the Helmeted muskox (B. bombifrons) and dated to the Late Pleistocene, ca. 11 000 yrs BP (Hills et al., 1999; Kooyman et al., 2001) renewed interest in the Fort Saskatchewan specimen as to its comparative value and hence its radiometric age. Therefore a sample was obtained and submitted for an AMS date, which resulted in a radiometric age of 30 570 ± 250 14C yrs BP (Isotrace TO-8973).

Figure 1

A) Posterior view and B) anterior view of Bootherium bombifrons (photographs: courtesy of C.R. Harington).

A) Vue postérieure et B) vue antérieure de la vertèbre de Bootherium bombifrons (photographies : courtoisie de C.R. Harington).

A

-> Voir la liste des figures

B

-> Voir la liste des figures

Discussion and conclusions

The new date indicates that the Fort Saskatchewan specimen is Middle Wisconsinan (pre-Last Glacial Maximum) rather than latest Pleistocene in age, corresponding to other dates on vertebrate fossils from pre-Late Glacial Maximum (LGM) Empress Foundation gravels and sands in the Edmonton area (Burns and Young, 1994). It also supports the contention of the late Dr. L.A. Bayrock (personal communication to L.V. Hills, 1963) that some of the gravels along the North Saskatchewan River were Saskatchewan Gravels that have been reworked during incision of the North Saskatchewan River.

Redeposition of vertebrate specimens is a significant concern on the Canadian plains because of the presence of faunas of immediately pre-Late Glacial Maximum and post-Late Glacial Maximum ages, separated by a hiatus of about 8 millennia representing presence or influence of glacial ice. Detailed maps of preglacial channels show that the pre-Late Glacial Maximum and post-Late Glacial Maximum drainage patterns were different, with earlier rivers integrated into a dendritic pattern trending northeastward whereas post-Late Glacial Maximum rivers trend northeastward to eastward but often “jump” laterally (southeastward) as a result of damming along former ice-front positions (Stalker, 1961; Farvolden, 1963). The last pre-Late Glacial Maximum fluvial cycle gave rise to widespread valley fills assigned to the Empress Formation (Saskatchewan Gravels and Sands), dating before about 20 14C ka (Rutherford, 1937; Stalker, 1968a; Burns and Young, 1994; Rutter et al., 1998). Widespread terminal Pleistocene valley fills are assigned to the Bighill Creek Formation and equivalents, dated between about 12 000 and 10 000 14C yrs BP (Churcher, 1968, 1972, 1975; Stalker, 1968b; Jackson et al., 1982; Wilson, 1983; Wilson and Churcher, 1984; Hills and Harington, 2003).

Upstream from eastern Edmonton, the North Saskatchewan Valley is narrow and steep-walled with Cretaceous bedrock exposures, marking postglacial incision through a preglacial upland. Downstream from Edmonton, through the Fort Saskatchewan area, the valley widens abruptly and has less steep walls as it follows the trend of the preglacial Beverly Valley (Farvolden, 1963; Rutter et al., 1998). Farther downstream, near Waskatenau, the river turns southeastward and leaves the Beverly Valley. In the Beverly reach of the river valley, postglacial gravel fills are inset into and superimposed upon preglacial deposits. Empress Formation deposits are dominated by clasts of quartzite, or quartzose sandstone, and black chert, with lesser amounts of arkosic sandstone, jasper, and locally derived coal fragments, wood, and clay ironstone. Their relatively high compositional maturity likely reflects multiple cycling of resistant materials from the Rocky Mountains through Tertiary upland conglomerates to the Quaternary fills (Westgate, 1969). Early postglacial North Saskatchewan River alluvium, on the other hand, is poorly sorted and compositionally less mature, with abundant Shield-derived clasts reworked from till. Vertebrate fossils are known from both deposits.

A diverse Pleistocene fauna was reported from pits in the Beverly (east Edmonton) area by Fuller and Bayrock (1965), who argued that it dated from post-Late Glacial Maximum times. Yet it included not only bison referred to Bison occidentalis and B. crassicornis, but also a horn core of B. latifrons, understood on the basis of finds from the Canadian plains to Mexico to be of earlier Pleistocene (pre-Late Glacial Maximum) age (Kurtén and Anderson, 1980). From photographs in Fuller and Bayrock (1965) and Wilson’s observations of the B. latifrons specimen, it appears that the B. latifrons horn core and at least one of the B. crassicornis crania exhibit more abrasion and damage than does the B. occidentalis material; therefore, redeposition of the larger forms is a strong possibility (Wilson, 1975). A similar conclusion was reached by Guthrie (1970) and Harington (1978). Burns and Young (1994, p. 393) stated that terrace gravels at the Beverly Pits “contain both postglacial and mid-Wisconsinan fossils, having been reworked and left as lag deposits during postglacial incision of the North Saskatchewan River”. These findings remain in need of further testing through direct dating of specimens.

Westgate (1969) described the terrace sequence along the North Saskatchewan in the Edmonton area. Of four postglacial terraces, the second lowest was ~20 m above river level with a fill of sands and poorly sorted cobble gravels. Vertebrate fossils from this unit included postcranial bones of extinct bison and horse (“Camelus sp.”, in error; Westgate (1969), listed Titanotylopus sp., but Harington (1978), cited Camelops hesternus, and mammoth (Mammuthus sp.) were from a buried gravel fillstrath in a terrace 7.5 to 10.5 m above river level, capped by younger fine alluvium. They suggested an age of about 8000 14C yrs BP for these fossiliferous gravels; but Westgate showed that the fill on a terrace of comparable elevation was made up of fine sediments with Mazama tephra (6730 ± 40 14C yrs BP by Hallett et al., 1997; 7627 ± 150 cal yrs BP from Greenland core GISP2, Zdanowicz et al., 1999). Westgate suggested that the gravels of the ~20 m terrace were similar to the Bighill Creek gravels and therefore of terminal Pleistocene age. The pits sampled by Fuller and Bayrock (1965) likely included superimposed or inset gravel deposits of more than one age (Harington, 1978), or the gravels are postglacial but elements of the fauna have been redeposited. Burns and Young (1994) found that in the Consolidated Concrete Pit, in the Beverly Valley at Edmonton, 3-5 m of gravels and sands without shield stones (Empress Formation) were unconformably overlain by another 3 m of gravels and sands “reworked during incision of the postglacial North Saskatchewan”. Most of the nearly 800 bones from that pit were from the lower gravel, but older bones from the early gravels were also found in the younger deposits.

Figure 2

Location (X) of Bootherium bombifrons between Edmonton and Fort Saskatchewan.

Site d'origine du spécimen de Bootherium bombifrons (indiqué par un X), entre Edmonton et Fort Saskatchewan.

Location (X) of Bootherium bombifrons between Edmonton and Fort Saskatchewan.

-> Voir la liste des figures

Deposits and relationships reported by Westgate (1969) allow for three possible pathways of redisposition in these situations: (i) Empress Formation to post-Late Glacial Maximum, Bighill Creek-equivalent gravels; (ii) Bighill Creek-equivalent gravels to Holocene terrace fills; and (iii) Empress Formation directly to Holocene terrace fills. Given that the gravels associated with the Bootherium bombifrons specimen were described as containing shield stones, together with Westgate’s statement to Harington that the gravels were Late Pleistocene, alternative (i) seems indicated in this instance. The same could also be true for the Bison latifrons specimen, if not the B. crassicornis material as well. It is clear from this that direct dating of important vertebrate specimens, together with examination for evidence of redeposition, is advisable for both stratigraphic and paleontological studies in this area. Diagenetic changes during fossilization may also provide chemical means for the differentiation of pre-Late Glacial Maximum and post-Late Glacial Maximum faunas. Intersections of modern drainage with filled preglacial channels emerge as probable “hotspots” for redeposition, much as they are also key areas for sampling of the earlier alluvial fills. In contrast, fossiliferous early post-Late Glacial Maximum gravels in the Cochrane-Calgary area have yielded no redeposited pre-Late Glacial Maximum material (Churcher, 1968, 1975; Wilson, 1983; Wilson and Churcher, 1984).

The specimen from the St. Mary Reservoir now stands as the first terminal Pleistocene occurrence of Bootherium bombifrons in Alberta (Hills et al., 1999; Kooyman et al., 2001). Helmeted muskox was certainly a member of both the pre- and post-Late Glacial Maximum vertebrate faunas on the Alberta plains and proper chronometric control will allow direct morphological comparisons to assess evolutionary changes between ca. 30 000 and 11 000 14C yrs BP. Radiometric control will also aid in better definition of palaeofaunal associations, their distribution and duration.

Figure 3

Schematic cross-section illustrating origin of reworked Bootherium bombifrons (modified from Rutter et al., 1989).

Coupe schématique du site d’origine de Bootherium bombifrons (modifié de Rutter et al., 1989).

Schematic cross-section illustrating origin of reworked Bootherium bombifrons (modified from Rutter et al., 1989).

-> Voir la liste des figures

Parties annexes