Article « Two Middle Holocene Marker Beds in Vertically Accreted Floodplain Deposits, Lower Fraser River, British Columbia » Harry F. L. Williams et Michael C. Roberts Géographie physique et Quaternaire, vol. 44, n° 1, 1990, p. 27-32. Pour citer cet article, utiliser l'information suivante : URI: http://id.erudit.org/iderudit/032795ar DOI: 10.7202/032795ar Note : les règles d'écriture des références bibliographiques peuvent varier selon les différents domaines du savoir. Ce document est protégé par la loi sur le droit d'auteur. L'utilisation des services d'Érudit (y compris la reproduction) est assujettie à sa politique d'utilisation que vous pouvez consulter à l'URI https://apropos.erudit.org/fr/usagers/politique-dutilisation/ Érudit est un consortium interuniversitaire sans but lucratif composé de l'Université de Montréal, l'Université Laval et l'Université du Québec à Montréal. Il a pour mission la promotion et la valorisation de la recherche. Érudit offre des services d'édition numérique de documents scientifiques depuis 1998. Pour communiquer avec les responsables d'Érudit : [email protected] Document téléchargé le 12 février 2017 05:35 Géographie physique et Quaternaire, 1990, vol. 44, n° 1, p. 27-32, 5 fig., 1 tabl. TWO MIDDLE HOLOCENE MARKER BEDS IN VERTICALLY ACCRETED FLOODPLAIN DEPOSITS, LOWER FRASER RIVER, BRITISH COLUMBIA Harry F. L. WILLIAMS and Michael C. ROBERTS, Department of Geography and Anthropology, University of North Texas, Denton, Texas 76203, U.S.A., and Department of Geography and Institute for Quaternary Research, Simon Fraser University, Burnaby, British Columbia V5A 1S6. ABSTRACT Drill cores obtained from the RÉSUMÉ Deux lits repères de l'Holocène ZUSAMMENFASSUNG Zwei Betten, die eastern Fraser Delta and adjoining Fraser moyen dans les dépôts de crue à croissance als Anhaltspunkt dienen, in vertikal angela- River floodplain reveal two middle Holocene verticale du Fraser inférieur, Colombie- gerten Hochflutablagerungen aus dem mit- marker beds—a peat with an age of about Britannique. Des carottes recueillies dans la tleren Holozân, unterer Fraser-Fluss, British 6000 '4C yr BP and a tephra bed, identified partie est du delta du Fraser et de la plaine Columbia. Bohrkerne von dem ôstlichen as Mazama (6800 yr BP). These marker beds, d'inondation adjacente montrent deux lits re­ Fraser-Delta und der angrenzenden Hoch- 4 in conjunction with ' C dates from the flood- pères datant du milieu de l'Holocène, un lit flutebene lassen zwei Betten aus dem mit- plain sediments, indicate that the Fraser River de tourbe datant d'à peu près 6000 BP et un tleren Holozân erkennen, die als Anhalt floodplain aggraded in response to a rise in lit de tephra, correspondant à l'éruption du dienen: ein Torfbett das auf etwa 6000 "1C sea-level, between about 8000 and 2250 yr mont Mazama (6800 BP). Ces lits repères, Jahre v.u.Z. datiert wird und ein Tephra-Bett, BP, and that aggradation kept pace with sea- ainsi que les datations au radiocarbone ob­ das aus der Zeit des Ausbruchs des Maza- level rise. The aggradational deposits form tenues dans les sédiments de crue, montrent mabergs stammt (6800 Jahre v.u.Z.). Die als a sediment wedge consisting mainly of que la plaine d'inondation du Fraser a pro­ Anhaltspunkt dienenden Betten weisen zu- gressé en réponse à une hausse du niveau organic-rich silts and fine sands of overbank sammen mit den von den Hochflutsedimenten origin. The wedge extends at least 20 km into marin survenue entre environ 8000 et 2250 BP ,4 gewonnenen C-Daten darauf hin, dass die the lower Fraser River Valley. Preservation et que !'aggradation suivait la hausse du ni­ Hochflut des Fraser-Flusses als Reaktion auf of the marker beds indicates considerable veau marin. L'accumulation de sédiments, eine Anhebung des Meeresspiegels zwischen channel stability in the lower reaches of the de forme triangulaire, est composée princi­ etwa 8000 und 2250 Jahren v.u.Z. zugen- Fraser River over about the last 7000 years. palement de limons riches en matière or­ ganique et de sables fins d'origine alluviale. ommen hat. und dass diese Zunahme der Le triangle s'étend jusqu'à au moins 20 km Hebung des Meeresspiegels entsprach. Die dans la vallée inférieure du Fraser. La Akkumulationssedimente bilden einen Se- conservation des lits repères démontre la dimentkeil, der vor allem aus organisch rei- grande stabilité des biefs du Fraser inférieur chem Schlamm und feinem Sand alluvialer depuis environ 7000 ans. Herkunft besteht. Der Keil erstreckt sich min- destens 20 km in das TaI des unteren Fraser. Die Erhaltung der als Anhaltspunkte dienen­ den Betten beweist eine beachtliche Kanal- stabilitât in den unteren Stillen des Fraser wâhrend ungefàhr 7000 Jahren. Manuscrit reçu le 5 mai 1989; manuscrit révisé accepté le 30 octobre 1989 28 H. F. L. WILLIAMS and M. C. ROBERTS INTRODUCTION Delta at New Westminster, where the river emerges into the lowland of the Strait of Georgia through a narrow gap in the The Fraser River Delta (Fig. 1) has been growing into the Pleistocene uplands (Fig. 1). Strait of Georgia for about the last 9000 years (Williams, 1988). An investigation of the evolution of the delta has dem­ The floodplain of the lower Fraser River occupies a former onstrated that it aggraded in response to a 13 m rise in relative arm of the sea, which presumably was infilled by Fraser River sea-level between about 8000 and 2250 yr BP. Vertical ac­ sediments between about 11 000 to 9000 yr BP, when the cretion on the delta kept pace with rising sea-level as the river established its present course following déglaciation delta continued to prograde into the Strait (Williams and (Armstrong, 1981 ; Clague ef a/., 1983). Roberts, 1989). The river below Hope is divided according to planform into Extension of subsurface investigations from the delta into a straight reach, a wandering gravel-bed reach (Morningstar, the adjoining western Fraser River floodplain has revealed 1987). and a meandering reach (Fig. 1). The wandering gravel- two widespread marker beds within the aggradational deposits. bed reach is characterized by multiple channels separated The upper marker bed is a hitherto undiscovered peat layer by large vegetated islands, large width to depth ratios and at a mean depth of about 4.5 m; the lower marker bed is a gravelly bed material. The meandering reach has, for the Mazama tephra layer at a mean depth of about 7.1 m, pre­ most part, a single sinuous channel containing a few large, viously recognized in a number of locations in the area well-vegetated islands, smaller width to depth ratios and sandy (Blunden, 1973, 1975; Clague et al., 1983). These marker bed material (McLean and Church, 1986). beds provide valuable chronologic control for paleogeomorphic reconstructions in the lower Fraser River Valley. The objectives LATE QUATERNARY SEA-LEVEL CHANGES of this paper are: 1 ) to describe the nature and presently During and immediately following retreat of the Late Wis- known extent of the marker beds; and, 2) to assess the sig­ consinan Cordilleran Ice Sheet about 13 000 yr BP, the sea nificance of the marker beds for paleogeomorphic reconstruc­ occupied the Fraser Lowland up to an elevation of at least tions in lower Fraser River Valley. 200 m above present sea-level (Armstrong, 1981). Isostatic rebound proceeded rapidly following removal of the ice load SETTING (Fig. 2). Raised glacio-deltaic deposits in the Fraser Lowland suggest that relative sea-level fell to about 40 m elevation by GEOMORPHOLOGY around 11 500 yr BP and reached its present level between Lower Fraser River Valley extends from Hope, at the 11 000 and 10 000 yr BP (Clague, 1981 ). Emergence of the downstream end of Fraser canyon, to the apex of the Fraser Fraser Lowland continued, causing the sea to drop about FIGURE 1. Location map of Fraser Delta and lower Fraser River Localisation du delta du Fraser et de la vallée inférieure du Fraser Valley, showing characteristic channel cross-sections in the wandering et coupes caractéristiques dans les biefs à méandres et à lits de gravel-bed and meandering reaches. gravier divagants. Géographie physique ef Quaternaire. 44(1). 1990 TWO MIDDLE HOLOCENE MARKER BEDS 29 overbank sedimentation in a relatively low-energy, vegetated, floodplain environment on the basis of their fine grain size and abundant incorporated organic matter. These deposits contain freshwater pollen and spores, including arrowhead (Sagittaria sp.), horsetail (Equisetum sp.), skunk cabbage (Lysichiton amehcanum) and buckbean (Menyanthes trifoliata) (Williams and Roberts, 1989). The peat and tephra marker beds are found within these floodplain deposits, and are clearly traceable from the delta into the adjoining floodplain of the lower Fraser River (Fig. 3). 1110 9876 54 3 2 10 3 PEAT BED RADIOCARBON AGE (x 10 YEARS BP) FIGURE 2. Postglacial sea-level curve for Fraser Lowland (from Clagueef a/., 1982). The peat bed, averaging 250-300 mm thick, in almost all Courbe du niveau marin postglaciaire des basses terres du Fraser cases grades into the enclosing floodplain deposits; the one (de Clague et al.. 1982) exception is site D68 (Fig. 3), where the top of the peat is marked by a sharp contact overlain by a 10 cm thick bed of 13 m below its present level (Clague et al., 1982; Williams fine sand presumably of flood origin. Palynological analysis and Roberts, 1989). of the peat bed from the delta has shown that it represents The early Holocene low stand of sea-level, resulting from the initial stages of a raised peat bog. The pollen spectrum isostatic rebound and possible forebulge migration (Clague, is dominated by shrubs of the Rosaceae family and has rel­ 1983), was followed by a rise in relative sea-level until about atively low arboreal and herbaceous pollen components.