SANDSTONE COMPOSITION AND FELDSPAR WEATHERING, WELLER COAL MEASURES (PERMIAN), TRANSANTARCTIC MOUNTAINS, ANTARCTICA

The Weller Coal Measures, exposed in Southern Victoria Land (near Ross Island) were deposited in fluvial and lacustrine environments at very high paleolatitude immediately after Gondwanan glaciation. Fossil wood with growth rings and coal indicate climate conditions warm enough to support forests, but dropstones document seasonal ice. Initial field observations suggested that the sandstones are arkosic and that alteration of feldspar is limited, consistent with minimal chemical weathering in a post-glacial setting. Based on thin section analysis of 47 samples, a typical composition is Q=80%, F=12%, L=8% (subarkose of Folk, 1968), with plagioclase more abundant than K-feldspar. Mica is unusually abundant (up to 15%) and in fact the composition of these sandstones is not accurately described by QFL. To evaluate extent of alteration, feldspar grains were assigned to one of four alteration categories. Category 1 feldspars are unaltered to lightly altered with sharply defined twin and cleavage 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. Analysis of alteration demonstrates that in contrast to initial macroscopic observation, the feldspars are highly altered, with up to 70% of grains falling in categories 3 and 4. This strongly suggests that chemical weathering processes can be significant during post-glacial fluvial-lacustrine transport and deposition of sediment in humid, cool-temperate settings.