THE HUNT FOR THE LGM, NORTHEASTERN BAFFIN ISLAND: ON THE TRAIL OF ELUSIVE COLD-BASED ICE
BRINER, JASON P. INSTAAR, Geological Sciences, University of Colorado.
Miller, Gifford H. INSTAAR, Geological Sciences, University of Colorado.
The extent and timing of Laurentide ice sheet (LIS) advances throughout the eastern Canadian Arctic have been poorly known largely due to a lack of last glacial maximum (LGM)-age radiocarbon ages for glacial deposits. Recent approaches using lacustrine sediment cores and cosmogenic exposure dating have resulted in significant improvements in our understanding of LIS dynamics across southeastern Baffin Island. However, unlike southeastern Baffin Island, where LGM ice left a limited moraine record, the wide coastal forelands of the Clyde region (Fig. 1), in northeastern Baffin Island, afford the opportunity to study a far more extensive glacial record of the last glaciation. Thus far, new glacial mapping and cosmogenic exposure (CE) and radiocarbon dating in the Clyde region point towards a cold-based glacial style during the LGM, followed by warm-based receding ice:
• The oldest distinct ice limits on the Clyde foreland have CE ages that cluster in two groups of ~19 and ~13 ka (n=9). These ice limits are defined not by moraines, but by a pattern of ice-marginal melt-water channels, indicating that cold-based ice was the dominant mode. These melt-water channels grade to a marine limit ~90 m asl, which is far higher than previous studies have shown for the LGM.
• Sediments exposed across the foreland, including in the vast Clyde sea-cliffs, all date to beyond the range of radiocarbon dating. These pre-LGM sediments are within the limit of LGM ice, but reveal no evidence of ever being overridden (no deformation or till cap). Additional evidence of non-erosive LGM ice is the preservation of upland tors (>=65 ka, n=2) beneath the ice sheet as indicated by perched LGM-age erratics (n=3).
• Striated bedrock islands within Clyde fiord, and low-elevation lateral moraines in the outer part of the fiord, indicate that warm-based ice was receding from the fiord mouth by ~10 ka (n=7). Melt-water channels connect some of these moraines to raised marine deltas, marking the marine limit at 40-50 m in the outer part of Clyde fiord.
• Erratic-littered terrain beyond the outermost distinct ice limits on the Clyde foreland suggests that the foreland was once completely glaciated. Two CE ages obtained from this area so far give mixed results: one erratic is ~13 ka and another is ~130 ka. In addition, two CE ages of ~32 ka on moraine boulders at a high lateral location along outer Clyde fiord point toward the possibility of finding a record of pre-LGM glaciation.
Our old tor ages suggest that the highly weathered, high elevation surfaces along northeastern Baffin Island are ancient features unmodified by glaciation, and did not form post-glacially. However, our finding of young erratics on these old landscapes suggests that the use of relative weathering as a criterion for determining ice extent is not valid. Overall, our findings of more extensive ice and higher relative sea levels than previously envisioned are consistent with other recent findings in the Canadian High Arctic and elsewhere.
Figure 1. Digital elevation map (black = high elevation) showing fiords, coastal mountain range, and coastal forelands of northeastern Baffin Island. The inset map shows the outline of the Laurentide ice sheet on North America; Baffin Island is highlighted in dark gray, and the box is the area of figure 2.
Figure 2. Geochronologic information and reconstructed ice margins of the Laurentide ice sheet on the Clyde foreland. The outlet glaciers came from both the Clyde fiord (bottom) and the Ayr Lake valley (middle).
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Copyright © 2001 INSTAAR, Univ. of Colorado