COMPOSITION AND FELDSPAR ALTERATION IN PLEISTOCENE GLACIAL SANDS, WIND RIVER RANGE, WYOMING

To test the hypothesis that chemical weathering in glacial environments is limited, we analyzed thin sections of glacial sands derived from granitic rocks in the Wind River Range, Wyoming. Sand composition is Q=32%; F=49%; L=15% (lithic arkose; Folk, 1968), with plagioclase more abundant than K-feldspar. Each feldspar grain was assigned to one of four alteration categories. Category 1 feldspars are unaltered to lightly altered with sharply defined cleavage and twin planes and edges. In Category 2 feldspars, although some alteration has occurred along cleavage and twin planes, the structures remain readily identifiable. Category 3 feldspars have cleavage and twin planes so altered that they are difficult to discern. Highly altered Category 4 feldspars lack evidence of any internal structure and are characterized by diffuse, irregular margins. Most of the feldspar grains observed fall into Categories 1 and 2 with the majority in Category 2; very few are in the more altered Categories 3 and 4. This substantiates that chemical weathering of these sediments has not been extensive. However, the abundance of feldspar grains in Category 2 demonstrates that some alteration has occurred and suggests that chemical weathering in glacial regimes of sediments derived from granitic source rocks is not negligible.