Postglacial marine deposits and marine limit determinations, inner Wager Bay area, Kivalliq region, Nunavut

This paper discusses postglacial marine features, including raised beaches and deltas, marine plains, and wave-erosion trimlines near the marine limit. Marine-limit measurements indicate that postglacial seas regionally inundated the area to an elevation of 110 m in the inner part of Wager Bay, and to about 95 m west of Brown Lake. Sporadic marine-limit elevations as low as 50 m correspond to areas with late-glacial ice. Marine shells at an elevation of 60 m yielded radiocarbon ages of about 5800 years. 1 601 Booth Street, Ottawa, Ontario K1A 0E8 Current Research 2005-B3 1 L.A. Dredge and I. McMartin Résumé : Le présent article discute des formes de terrain associées à la mer postglaciaire, notamment des plages et des deltas soulevés, des plaines marines et des encoches créées par l’érosion des vagues près de la limite maximale de transgression. Les mesures de la limite marine indiquent que les eaux marines ont envahi la région jusqu’à une élévation de 110 mètres dans la zone intérieure de la baie Wager, et de 95 mètres à l’ouest du lac Brown. L’observation de limites marines aussi basses que 50 mètres indique la présence d’une masse glaciaire résiduelle tardive dans les zones en question. Des coquillages marins récoltés à une élévation de 60 mètres ont rendu des âges radiocarbones d’environ 5800 ans. INTRODUCTION This report briefly describes the nature, extent, and age of marine deposits near the head of Wager Bay, which lies in the region of Kivalliq on the west side of Hudson Bay (Fig. 1). The main focus is on marine-limit determinations. The marine limit is a term used to refer to the highest elevation reached by the postglacial sea. The elevation of the marine limit yields information about the extent of inundation by postglacial seas, the amount of postglacial crustal deformation, and for the mineral exploration industry, it indicates the elevation below which the geochemical composition of glacial deposits may have been altered by marine processes or by mixing of glacial materials with marine sediments. The study is part of a GSC regional Quaternary project covering the Wager Bay map area (NTS 56 G), for which field mapping and till sampling were completed in 2004 (see Fig. 1 for location). Marine deposits described in this report are confined to the northern part of the map area, and adjacent areas to the north and east in the Walker Lake (NTS 56 J) and Douglas Harbour (NTS 56 H) map areas (Fig. 2). Elevations within the area shown on Figure 2 range from sea level to a maximum height of about 611 m. The area consists of a lowland (<100 m elevation) that includes the head of Wager Bay, Ford Lake (elevation 1 m), Brown Lake (elevation 36 m), and adjacent plains that fringe these water bodies, particularly on their northern sides. South of Wager Bay, cliffs rise abruptly from the shore to a rolling rocky upland surface lying at an elevation of 400–500 m. North of Wager Bay, the land rises more gently to plateau elevations ranging from 200 m to 300 m. FIELD WORK Marine deposits and landforms were mapped and initially interpreted using 1:60 000 airphotos. Ground observations on marine features were made during the course of the more comprehensive regional mapping project. Elevations of features were recorded to a precision of 1 m using a GarminTM GPS. The elevations determined by this method compared well with recent topographic maps that show 10 m contour intervals. The elevations are considered to be accurate to within ±5 m, or better. RESULTS AND INTERPRETATION Evidence for postglacial marine inundation is confined to the lowlands around Wager Bay, Ford Lake, and Brown Lake. Except for raised gravel beaches (Fig. 3), and deltaic sands forming terraces above present-day streams, marine deposits are generally thin, and are limited to the more gently sloping lowlands north of Wager Bay (Fig. 4) and Brown Lake (Fig. 5). Marine limit positions were determined by recording the elevations of raised deltas (Fig. 6) that indicate where meltwater streams and overflow channels from glacial lakes fed into high-level water bodies; ice-contact deltas, on which kettled surfaces are fronted by beach ridges (Fig. 7); and trimlines where the uppermost limit of wave erosion of glacial deposits is prominent (Fig. 8). It was found that marine limits determined by the limit of intense wave washing such as in Figure 8 were 5–10 m above the upper limit of raised beaches, and may result from storm surges or exceptionally high tides. The present tidal range seems to be about 4 m. Marine-limit elevations across the study area are shown on Figure 2. A succession of meltwater deltas from streams flowing into Wager Bay from the south indicates that marine limits on the south shore stand at elevations of 110–115 m at the eastern part of the map area, and decline to 95 m in the central part of the area shown, and possibly to 80 m or 87 m in the west. North of Wager Bay, near the shores, marine limits are commonly at an elevation of about 95 m. These data suggest that the eastern part of the study area opened up first, and that the marine incursion followed rapidly into the rest of the area. In a few of the glacial troughs Current Research 2005-D1 2 L.A. Dredge and I. McMartin 105°W