A new Holocene relative-sea-level (RSL) curve is proposed for the central west coast of Vancouver Island based on topo- graphic surveys of coastal landforms ...
Holocene sea-level change on the central west coast of Vancouver Island, British Columbia PIERREA. FRIELEAND IAN HUTCHINSON Department of Geography and The Institute for Quaternary Research, Simon Fraser University, Bumaby, B. C., Canada V5A 1S6
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Received April 22, 1992 Revision accepted January 28, 1993 A new Holocene relative-sea-level (RSL) curve is proposed for the central west coast of Vancouver Island based on topographic surveys of coastal landforms and the stratigraphy of coastal deposits. Six study sites are described on Vargas Island and Esowista and Ucluth peninsulas and 23 previously unpublished radiocarbon ages that constrain mean-sea-level (MSL) position are presented. A long-term rise in RSL from an early Holocene ( >7000 BP) lowstand below -3 m as1 is documented from dates on tree stumps in the intertidal zone and salt-marsh peats and forest-floor deposits buried by marine sands and gravels. Submergence culminated in a stillstand from 6000 to 4800 BP, marked by the development of a strandline at 6 m as1 (MSL = 3.2 m asl). A fall in RSL is documented by a strandline at 4.0-4.5 m asl, which dates from 2700 to 2000 BP (MSL = 2.0 m asl). Little is known of the dynamics of emergence between 4800 and 2700 BP or from 2000 BP to the present, although the absence of strandlines below 4 m as1 suggests continuous emergence during the last two millenia. This submergence-emergence cycle is unique in southern British Columbia. A simple simulation model suggests that late Holocene emergence was induced by tectonic uplift of the edge of the America plate margin. Une nouvelle courbe du niveau marin relatif durant 1'Holocbne est sugCrCe pour la rCgion centrale de la cBte occidentale de l'ile de Vancouver; elle a CtC dressCe 2 partir des lev& topographiques des formes de terrain de la cBte et de la stratigraphie des dCpBts cBtiers. Les six sites d'Ctudes de l'ile de Vargas et des pkninsules d'Esowista et d'Ucluth sont dCcrits et les 23 nouveaux ages radiocarbone present& permettent de dClimiter l'altitude du niveau marin moyen. Une Cltvation long terme du niveau marin relatif, a partir d'une terre basse d'lge holocbne prCcoce (>7000 Av.P.) sous -3 m au-dessus du niveau marin, est documentCe par les datations sur souches d'arbres dans la zone intertidale, sur tourbe de marais salants et sur dtpBts de sols forestiers recouverts par des sables et des graviers. De 6000 a 4800 Av.P., la submersion marine maximum est demeurCe stable; elle est marquCe par le dCveloppement d'une ligne de rivage situte B 6 m au-dessus du niveau marin (niveau marin moyen = 3,2 m au-dessus du niveau marin). Une baisse du niveau marin relatif est indiquCe par une ligne de rivage i 4,O-4,s m au-dessus du niveau marin et datCe de 2700 2 2000 Av.P. (niveau marin moyen = 2,O m au-dessus du niveau marin). La dynamique de l'tmersion est mCconnue pour les pCriodes de 4800-2700 Av.P. et de 2000 Av.P. a aujourd'hui, quoique l'absence de ligne de rivage sous 4 m au-dessus du niveau marin suggbre une phase d'Cmergence continue durant les deux derniers milliers d'annCes. Ce cycle de submersion-Cmergence est unique en Colombie-Britannique mkridionale. Un modble simple de simulation suggbre que 1'Cmergence 2 1'Holockne tardif a CtC causCe par un soulkement tectonique de la frange de la marge de la plaque de 1'AmCrique. [Traduit par la rkdaction] Can. I. Earth Sci. 30, 832-840 (1993)
Introduction The general pattern of relative-sea-level (RSL) change for the last 4000 years along the central west coast of Vancouver Island was established by Clague et al. (1982), primarily from dated archaeological sites. They concluded that long-term emergence on the order of 1-2 m m . ax1 had taken place over the late Holocene, which is equivalent to the contemporary uplift rate for the area (Wigen and Stephenson 1980; Holdahl et al. 1989). In this paper we describe littoral depositional sequences at a number of sites on the central west coast of Vancouver Island and date a variety of sea-level indicators, thereby constraining and extending the previously published sea-level curve, and discuss possible causes for the displacement of Holocene shorelines in the study area.
Research area The physiographic elements of western Vancouver Island include the Vancouver Island Ranges and the Estevan Coastal Plain (Holland 1976). The coastal plain extends from the Brooks Peninsula to the entrance of Juan de Fuca Strait and has a maximum width of 13 krn (Fig. 1). Surficial deposits of the coastal plain include till and glaciofluvial sediments Printed in Canada 1 Irnpr~rnCau Canada
associated with the Fraser Glaciation and glaciomarine mud and beach facies deposited during the subsequent marine inundation (Howes 1981). The sites chosen for this study of RSL change include Vargas Island and Esowista Peninsula in Clayoquot Sound and Ucluth Peninsula in Barkley Sound (Fig. 1). Vargas Island displays an extensive sequence of emergent beach ridges that are unique on this section of the coast (Fig. 2a). Although Howes (1981) noted the Vargas Island beach ridges as an obvious indicator of late Holocene emergence, and Dragert and Rogers (1988) suggested that they may have resulted from coseismic uplift, their genesis and relationship to past sea levels have not been determined. Tides on the west coast of Vancouver Island are mixed semidiurnal. The range of the large tide is 4.1 m at Tofino, and the height of the higher high large tide is 2 m above mean sea level (asl) (Canada Department of Fisheries and Oceans 1990). In sheltered lagoons the littoral zone reaches only to this 2 m as1 limit, but on sand beaches it extends to about 3 m asl, and on steep cobble beaches it may extend to 4 m asl. Local variation in the littoral zone limit is also influenced by beach orientation. In winter prevailing winds are from the southeast; in summer the dominant wind is from the northwest (Thomson 1981).
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FRIELE AND HI
a traverse consisted of a series of closed loops, allowing frequent checks of errors along its length. For the Vargas Island and Ucluth Peninsula sites there are two noninstrumental sources of error: (i) the difference between the predicted and observed tidal elevation, and (ii) the bias induced by swash in the formation of log lines and wrack lines. These were evaluated by establishing a tide gauge in a sheltered lagoon and comparing observed water surface elevations with the corresponding wrack-line elevations from a nearby exposed beach. The analysis indicated that the mean-sea-level estimates for the survey profiles tied to wrack-lines have an inherent error of -0.10 to +O. 35 m (Friele 1992). The magnitude of this error reflects wave power and the steepness of the beach face, and will vary through time in response to wave fetch and beach morphology (Andrews 1989). We assume that wood in beach deposits was laid down at the lowest limit of the log line (i.e., 2 m asl; Friele 1992), but its true elevation on the beach may have been 1-3 m higher. Therefore, the -2 m calibration to mean sea level derived from log-line wood represents a maximum position for mean sea level, with the minimum limit being 1-3 m lower.
FIG.1. (a) Plate-tectonic setting of the research area. (b) Regional location. (c) Location of the traverses and study sites.
Methods Data were collected using a five-step procedure: (i) identification of paleolittoral features by airphoto analysis and ground reconnaissance; (ii) topographic surveys of modern and paleolittoral landforms; (iii) stratigraphic description of paleolittoral deposits from exposures, vibracores, and shovel pits; (iv) analysis of fossil assemblages from critical sections; and (v) radiocarbon dating of materials of known stratigraphic association. R e datum estimate The datum used in this research is mean sea level (MSL) at Tofino; all elevations are cited in metres above this datum. Most sea-level indicators (salt marsh, log lines, wrack lines), however, relate to the higher high tide (HHT) level, which is a function of tidal range and varies regionally. Thus, these indicators must be corrected for the local tidal amplitude so that regional correlations of mean sea level can be made (Clague et al. 1982). All the reference ports on the west coast of central Vancouver Island have essentially the same tidal range (Canada Department of Fisheries and Oceans 1990), and it was assumed that the tidal range on Vargas Island is identical to that at the nearest reference port of Tofino (range = 4 m). It was also assumed that the tidal range on this open coastline has remained constant throughout the Holocene. On Esowista Peninsula topographic surveys were tied to benchmarks, whereas on Vargas Island (where no geodetic control exists) and Ucluth Peninsula surveys were tied to the most recently deposited wrack line. On the longest surveys benchmarks were established roughly every 70 m on the leg out from the beach and were closed on the return leg. Thus,
Radiocarbon dating Radiocarbon ages on marine shell material were corrected for the oceanic reservoir effect using the local correction factor of - 800 years (Robinson and Thompson 1978). Radiocarbon ages
