ABSTRACT: Laurentide and local ice at the northeastern margin of Beringia: Quaternary investigations of Prince Patrick Island, NWT.

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Abstract

During the past fifteen years, the maximum extent of the Late Wisconsinan Laurentide Ice Sheet (LIS) has been redefined at several locations along its margin. In northwestern North America, this extent has not been reassessed since reconnaissance work was completed two decades ago. Prince Patrick Island (PPI), in the remote western Queen Elizabeth Islands, NWT, is underlain mainly by the late Tertiary Beaufort Frm. (unconsolidated sands and gravels), and is low in relief and elevation (<280 m asl). Unequivocal geologic evidence of the regional glaciation of PPI remains to be demonstrated; however, far-travelled granitic erratics, many of them unweathered, are commonly found on the surface. The lack of dating control or a clear relationship to dated glaciomarine sediments has previously led to two interpretations regarding their deposition: early Quaternary glaciations or ice-rafting during times of much higher relative sea level (>190 m asl).In 2004, investigations of erratics and raised marine deposits were undertaken on the east-central and northwest coasts of PPI. The highest probable raised marine deposits occur at ~40 m in the east and ~20 m in the northwest. In each case, these can be tied to lateral meltwater channels outlining the retreat of an ice cap towards the centre of the island. The record of meltwater channels is not straightforward: the largest valleys themselves contain several sets of paired, arcuate, tributary channels. One possibility is that the smaller channels simply record the development of a dendritic fluvial network on an unconsolidated substrate; however, the repeated nesting of arcuate channels at several scales, many of them perched on valley sides, suggests repeated cold-based glacierization of the landscape with minimal subsequent modification. If so, the PPI landscape is palimpsest, and possibly of great antiquity.Arguing against the model of multiple local glaciations is the preservation of only one cycle of Holocene forced regression from marine limit, followed by recent and ongoing transgression. If PPI has been repeatedly glacierized as suggested by the meltwater channels, either the overriding ice lacked sufficient mass to isostatically depress the crust enough to force sea level above its modern position, or the stratigraphic evidence of this sea level cycle, aside from the presumably sea-ice-rafted erratics, was subsequently destroyed. A second possibility involves an initial advance of granite-bearing Laurentide ice across PPI that created a network of diverted drainage channels ahead of the advancing ice front, superimposed on an older Tertiary landscape previously modified by the advance and retreat of local ice caps. The same Laurentide ice may have remained as a local, residual ice cap during deglaciation. The fact that net emergence on PPI ranges from 20 – 40 m, and has been dated to ~12 ka BP, indicates substantial unloading (~40 m + ~80 m eustatic sea level rise = ~120 m total rebound), consistent with the inundation of PPI by the Late Wisconsinan LIS.Five mollusc samples, three driftwood samples and eight erratic samples were collected in 2004 and submitted for radiocarbon and cosmogenic exposure dating to constrain the age of emplacement of the far-travelled erratic boulders, the age of retreat of the youngest local/remnant ice cap to occupy PPI, and if these events occurred during the same glaciation of PPI. Two field camps are planned in 2005 to expand this study to the southeast and southwest coasts of PPI.

Bibliography of Canadian Geomorphology