WEATHERING OF QUARTZITE ON A CRYOPLANATION TERRACE IN NORTHERN YUKON,CANADA

Sub-horizontal cryoplanation terraces are ideal laboratories to study the weathering of rock in Arctic environments. The terraces are develo ped in mechanically isotropic rock, are generally not greatly perturbe d by gravity processes, and show blocks that have been exposed to the weathering elements for varying time. The terrace studied, herein name d the Ptarmigan terrace, is 180 m in length along the slope and occurs in Precambrian chlorite-bearing green quartzite on a mountain to the north of the village of Old Crow (Yukon). The blocks of quartzite on t he terrace tread record two distinct weathering events. Near the talus , the blocks show a breakdown of the chlorite and the dissolution of a ccessory pyrite grains. The results of this first event are the produc tion of a white quartzite with enhanced porosity and the nearly comple te leaching of iron from the rock. The second event involves the reint roduction of iron to form rusty-brown rinds that are composed principa lly of goethite. These rinds increase in thickness with distance from the scarp towards the slope of the mountain. The second event is devel oped when iron is reintroduced into the porous rocks as water-soluble compounds are drawn into the blocks by capillary forces. Occasionally, a reddish-purple front, presumably composed of hematite, is found to encroach more deeply into the blocks. In these blocks we speculate tha t the rock acted as a chromatographic column and separated the differe nt ionic species into two fronts. The first event occurs mainly at the scarp of the terrace where a persistent snowpatch maintains moist and slightly acidic conditions in the ground. In contrast, the second eve nt occurs on the tread where arid conditions prevail. By analogy with other studies in similar rocks, the formation of the terrace may be mi d-Pleistocene. (C) 1997 by John Wiley & Sons, Ltd.