Effects of Chemical Weathering and Sorting on the Petrogenesis of Siliciclastic Sediments, with Implications for Provenance Studies

The bulk chemical compositions, mineralogy, and mineral proportions of sands and muds of the Mallacoota Basin in southeastern Australia reflect the composition of weathering profiles mantling source rocks, rather than bedrock. Muds contain abundant clay minerals that are virtually absent from the source rocks but abundant in the weathering profiles. Sands are strongly enriched in quartz relative to source rocks, even in the headwaters of the fluvial system, demonstrating that feldspar destruction occurs by in situ chemical weathering within profiles and before detritus enters the fluvial system. K-feldspar is proportionally more abundant in fluvial sands than in source rocks because plagioclase is more rapidly destroyed than either quartz or K-feldspar in weathering profiles. Subsequent erosion and sorting produce sands enriched in quartz, with high K-feldspar:plagioclase ratios relative to source rocks. The composition of plagioclase incorporated into the fluvial sands is also controlled by chemical weat...