麻豆传媒

The Ravn River Valley in Northern Baffin Island, Nunavut, contained a proglacial ice damned lake during the Holocene recession of the Paleo Barnes Ice Cap. The style of deglaciation was complicated. A series of sublacustrine cross valley moraines and both lateral meltwater channels and delta foresets that dip up valley, indicate that initial ice marginal retreat was southward across the west east trending valley. Raised deltas, kame deltas, spillways and paleo lake shorelines in the Ravn River Valley indicate that this ice damned lake had two stillstand lake levels; ~268 m and ~300 320 m throughout its duration. Lake level elevations were controlled by a prominent spillway at the valley head and by contemporaneous glacial ice that was retreating downvalley to the south southwest. The intimate relationship between lake levels and the receding ice permits a chronology of raised deltas to yield both the lake history and the timing of systematic retreat of the Paleo Barnes Ice Cap. The rate of 46 m of incision into the bedrock spillway will also be able to be calculated. Cosmogenic 10Be as measured to date the two lake levels and one tributary lake by sampling eight raised deltas. A boulder was sampled on one of the surfaces. The remaining samples were topset sands collected from 30 cm depth, below a zone of post depositional mixing due to cryoturbation and bioturbation. The results indicate that the tributary valley near the valley head spillway was deglaciated at 8.3 " 0.3 ka (uncertainty is 1s precision of measurement). Exposure ages from the other 7 samples, including the boulder, range from 37.1 " 1.0 ka to 21.7 " 0.7 ka. Based on the preservation of the glacial and glaciolacustrine features, the relative lack of soil development, and chronology of deglacial events elsewhere on Baffin Island, these 7 ages are considered to be too old. The probable explanations for this disparity are (1) that there was a systematic error in the chemistry during sample preparation, or (2) that the samples all contained a concentration of 10Be that was inherited from exposure prior to final deposition in the deltas. If the latter case is correct, the inheritance would indicate that the long term average steady state erosion rate of the upland plateaus (the source of the deltaic sands) ranged from 38 to 91 m/Myr. This erosion rate is reasonable for bedrock that is affected by zones of wet based (erosive) and cold based (non erosive) ice as has been well established for Baffin Island. Using the tributary delta exposure age, the minimum rate of spillway incision was 5.5 m/ka.

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Pages: 135
Supervisor: John Gosse