Ice wedge exposed by hydraulic trenching
Location:
Northwest
Arm of Kaminak Lake, Kivalliq, Nunavut, Canada;
NTS 55L
Kaminak Lake;
62º
17.5’ N, 95º 31’W
This photo shows an active ice wedge, exposed by hydraulic washing with a portable fire-fighting pump, of a stream-cut exposure behind the campsite that my crew occupied in 1970, 1971 and 1973. The section comprises till (not shown) overlain by fine-grained marine sediment which grades upward into deltaic sands, reflecting isostatic emergence of this site from the Tyrrell Sea. The deltaic sediments were deposited into the Tyrrell Sea at the mouth of the ancestral version of the present stream, which has built a similar delta into Kaminak Lake. Image 0210 is an aerial view of the campsite as it was in 1970, and the section shown here is at the right side of the image, the modern stream outlet and delta being located at the top of the image. Fine-grained marine sediments do not ordinarily host frost wedges or polygons because of their plastic nature, but when buried in more than 2 m of non-plastic gravel, or 75 cm of non-plastic frozen organic detritus (usually peat), the permafrost table (0º isotherm) is above them, and they crack because they are frozen and non-deformable, as is the case here. In this section, the stream has reactivated erosion of the section in response to a lowering of the level of the northwest arm of Kaminak Lake in the recent past, perhaps by cutting a new outlet through the esker ridge that separates it from the main body of the lake within the last few hundred years. Reactivation of the section has caused the strong winds typical of this region to blow the deltaic sands off the face of the reactivated section and pile the windblown sand over the section and across the frost-cracked delta surface. This process is still going on, and older humus layers are buried at the top of the section. Images 0067, 0211, and 0212 give views of ice wedges and the buried soil layers (humus) at the top of this same exposure. Note that marine fine-grained sediment, which started being deposited here 6600 ± 230 C14 years B.P. (GSC-1434; date on marine shells at the till/marine sediment contact) is oxidized adjacent to the ice wedge, both along its sides and horizontally along some beds within the marine sequence. Since this sediment has been frozen since its emergence from the sea, I am not sure why the oxidation has occurred, particularly along horizontal beds of silty clay, which has a low permeability. For scale, the knife is 25 cm long.
Updated 09/10/2009 CAB