Abstract:
Cores recovered from Saanich Inlet during Ocean Drilling Program leg 169S have provided important insight into the paleoenvironmental history of southeastern Vancouver Island. We have undertaken a related study in the Saanich Inlet watershed, to examine long-term changes in patterns of ice retreat, sea level, sedimentary processes, hydrology and climate. The focus of investigation is a geomorphically dynamic period during the late Pleistocene and early Holocene, ca. 15 000 to 8000 calender years before present (yBP). At around 15 000 yBP, the area was covered by a south- to southeast-flowing piedmont glacier from montane sources on Vancouver Island and the adjacent British Columbia mainland. The minimum limit to glaciation was 1200 m asl, and ice thicknesses ranged from less than 500 m in uplands to 1500 m over Saanich Inlet. Between 14 000 and 12 000 yBP, Juan de Fuca Strait, Haro Strait and the Strait of Georgia were deglaciated as the sea inundated glacio-isostatically depressed coastal areas. Uplands became ice free as the regional equilibrium line altitude rose, while active glaciers were confined to major valleys. Valley glaciers terminated in a calving embayment that developed in the vicinity of Saanich Inlet and Cowichan Bay. Proglacial outwash was rapidly deposited, and is graded to marine still-stands at approximately 90, 75, 40 and 15 m asl. These elevations probably correspond to periods when rates of glacio-isostatic rebound and eustatic sea-level rise were similar. The flux of meltwater and sediment decreased markedly after 11 000 yBP as deglaciated slopes stabilised, stagnant debris-covered ice disappeared, and glaciers retreated within their Neoglacial limits. Isostatic rebound continued until the early Holocene, and between 10 000 and 8 000 yBP, sea-level was tens of metres lower than present. During this period, Pleistocene valley fill was intensively dissected by postglacial streams. After 8 000 yBP, eustatic sea-level rise was dominant and aggradation of valley floors resumed. An increase in the organic content of lake sediments likely reflects a transition to a wet, cool climate. Modern hydrologic conditions in the watershed were established by the middle Holocene as sea level rose to near the present datum.
Coverage:
West: -128.4500 East: -122.3000 North: 51.0000 South: 40.5600
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