Processes controlling the composition of first-cycle sediments deposited in an arid-climate, with implications for provenance reconstruction studies

Petrologic analysis of first-cycle clastic sediments derived from a single source in an arid environment provides a means to determine how well they resemble the petrology and geochemistry of their source. The Stepladder Mountains, located in the Mojave Desert of southeastern California, represents a well-controlled location (i.e., arid environment; single, known source; short transport distance) to examine how naturally formed sediments acquire their compositions. Compositional modifications associated with sediment production were resolved through direct examination of the weathered components (regolith, grus, and sediments). Sediment compositions strongly vary by grain size, indicating that, after the source itself, hydrodynamic sorting played the dominant role in shaping the composition of Stepladder sediment.

Traditional petrographic and geochemical provenance-seeking indicators were also tested to determine their efficacy. Plots employing major element abundances, including Al₂O₃–CaO*+Na₂O–K₂O (A–CN–K) and Al₂O₃–CaO*+Na₂O+K₂O–Fe₂O₃+MgO (A–CNK–FM) ternary plots, proved to be the most accurate at identifying the lack of chemical weathering and the importance of sediment sorting in t­he production of Stepladder sediments. Plots using trace and rare earth element concentrations reveal the significant modifications that sorting can have on sediment composition. All of the Stepladder sediments are enriched in immobile transition metals, most notably Sc, Cr, Ni, and Co, and even though the sediment sourced from a single pluton, compositions are consistent with the mixing of 10 - 20 % of a basaltic component. Sediment trace element ratios, including Th/Sc, Cr/Th, La/Sc, Th/Co, and Zr/Sc, can differ from average bedrock by up to a factor of 10. It must be known that moderate variations in composition and elemental ratios can be the result of sorting during sediment transport and do not necessarily indicate the mixing between sources.