Role of Sorting on the Composition of Siliciclastic Sediment: Implications for Interpreting Provenance after Limited Transport in an Arid Climate

This study tested whether transport distances (< 500 m) have the capacity to shape the geochemistry of sediments across multiple grain-size populations due to sorting derived from a single source. In the Stepladder Mountains, Mojave Desert, CA, a < 1 km² [square kilometers] watershed allows for a controlled study to understand how modern sediments acquire their composition from a single granodioritic source in an arid climate where there is no chemical weathering. Sediments are naturally sorted into distinct grain-size populations, with modes ranging from very fine sand to gravel within a single, alluvial channel. Sediment samples representative of each population were petrographically and geochemically analyzed in order to test the effectiveness of commonly used discrimination diagrams. Sediments became proportionally enriched in plagioclase and biotite and depleted in K-feldspar and quartz with decreasing grain size. Major elements were plotted in Al₂O₃ [aluminum oxide]–CaO^*+Na₂O [sodium oxide]–K₂O [potassium oxide] and Al₂O₃–CaO^*+Na₂O+K₂O–FeO+MgO compositional space and indicate that a negligible degree of chemical weathering was involved in sediment production. Trace-element plots normalized to average granodiorite bedrock show strong enrichments in elements thought to be immobile during sedimentary processing (Cr, Co, and Sc) across nearly all sediment samples. Using any of these elements as ratios (Th/Co, La/Sc, Th/Sc, Cr/Th, Zr/Sc) in provenance discriminating plots reveal that sediment was formed source mixing, as sediments in these plots considerably deviate from bedrock composition due to the control grain size has on geochemistry. Sediment rare-earth element (REE) contents also deviate from source composition and show an increase in total REE content, decrease in fractionation of light REEs and heavy REEs, and an increase in the magnitude of the negative Eu anomaly with decreasing grain size. Variations in sediment composition and thus geochemical ratios result from mineral sorting during transport, no matter how short. Thus, strong caution must be used when using discrimination diagrams to interpret sediment and sedimentary rock provenance.