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Underground ice in permafrost, Mackenzie Delta-Tuktoyaktuk Peninsula, N.W.T. Gell, William Alan


A study was made of the petrology of a variety of underground ice types in permafrost on the Tuktoyaktuk Peninsula and Pelly Island, Mackenzie Delta, N.W.T. Ice bodies of a considerable range of ages occur, including some deformed in the Wisconsin glaciation; also permafrost and ice is growing ab initio beneath recently drained lake bottoms. The spectrum of ice body size is also wide, extending from pore-sized particles to beds 25 m thick. The major objective of the study was an understanding of the growth and deformation of such ice bodies from a petrologic viewpoint. Thus several bodies of known, recent, age -were analyzed in order to enumerate features typical of growth. This was possible for icing mounds, tension cracks and active layer ice which grew in winter 1973-74. Growth conditions were inferred in terms of water supply, freezing directions and rates, solute rejection (bubble formation) and crystal size, shape, lattice and dimensional orientation. On the basis of this knowledge of growth features, older and larger ice bodies were studied, and post-solidification characteristics ware analyzed. Soma near-surface ice gave evidence of thermomigration of bubbles, but the major changes in fabric ware due to thermally and mechanically induced stresses. In the case of wedge ice, progressive changes in crystal size, shape, lattice and dimensional orientation ware recognized from the centre to the boundary of the wedge, due to recrystallization and grain growth associated with wedge development. Segregated ice was studied ia pingos and an involuted hill. A pingo core with steeply-dipping beds showed little evidence of flow while broader pingo with a greater pore ice content had undergone some flow in the segregated ice layers. A range of fabrics was found in the involuted hill, optic axis orientations becoming increasingly concentrated normal to compositional layering while dimensional orientations tended towards parallelism with the layering in anticlines in the ice. The influence of bubbles on deformation is pointed out in that larger crystals occur in clear ice and thus have greater intracrystalline slip than in bubbly ice. Where a wedge penetrated such a fold, the fabric changed along the fold limb in a manner symmetrically related to the wedge. Additionally, several near-surface ices ware studied and showed evidence of multiple growth periods, and multiple freezing directions, indicating that the ice grew in enclosed water in frozen material. Thus the complexity of freezing and melting histories may be recognized petro-graphically while it is not readily apparent in the field.

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