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 weathering. In weather-ing profiles the anorthite component of plagioclase weathers more rapidly than the albite component so that vestigal plagioclase in the profiles, and in sands derived therefrom, is more albitic than in the source rocks. Traditional point-counting techniques to obtain modal estimates of sediments and sedimentary rocks are not widely applicable because (1) most sedimentary rocks-shales and equivalent mud-grade materials-are too fine-grained for petrographic examination, and (2) rock fragments are rare to non-existant in most "basement" source terranes. We use a method that involves identification of mineral species present, information on their compositions, and bulk chemical compositions to calculate modal compositions by an albegraic method (CAM). The technique is equally applicable to source rocks, sediments, and sedimentary rocks of all size grades, so that meaningful mass balances and provenance studies can be carried out. Results of this study show that, where substantial chemical weathering has occurred, Q:F ratios and P:K ratios more closely reflect those of weathering profiles than bedrock from which they were ultimately derived.