MINERALOGICAL AND GEOCHEMICAL COMPARISON BETWEEN FIRST-CYCLE SEDIMENTS FORMED IN AN ARID SETTING AND THEIR GRANODIORITIC SOURCE ROCKS

Using an optimally controlled setting, we undertook a mineralogical and geochemical investigation of first-cycle clastic sediments in order to determine how closely they reflect the known bedrock composition. The Stepladder Mountains granodiorite (Mojave Desert, CA), which crops out as a N/S-trending spine of low-relief inselbergs in the center of a topographic dome, comprises the unique source (with the possible exception of minor wind-transported debris) for the sediment mantle that blankets the erosionally retreating bedrock. With little mineralogical or geochemical variation in the source rocks, essentially no chemical weathering (CIA < 55), and < 10 km of total transport in active stream channels, sediment compositions should closely resemble that of the bedrock from which they were derived. We differentiated six naturally sorted grain-size groups in the sediments, which range from very fine sand to pebble gravel. Although the mineralogy of the sediments closely resembles that of the bedrock, there are significant variations in modal abundances; finer grained material is enriched in feldspar (particularly plagioclase) and biotite, and coarser grained sediments are quartz-rich. Relative to bedrock, all sediment groups (and grus) show enrichment in Co, Ni, Cr, and Sc, but finer sediments are additionally enriched in Zr, Hf, Nb, and Ti. REE patterns from sediments (and grus) develop important characteristics (not observed in the bedrock), such as distinct negative Eu anomalies and enrichment in HREE. Relative to their source rocks, the Stepladder sediments carry provenance characteristics of both more felsic (negative Eu anomaly, enrichment in Zr, Hf) and mafic components (enrichment in Co, Ni, Cr, Sc, flattening of HREE). This study suggests that sedimentary processes, such as hydrodynamic sorting, may introduce compositional variations of sufficient strength to obscure the provenance of sediments. Results from this study indicate that caution be exercised when using mineralogical and geochemical data to evaluate provenance of siliciclastic sedimentary rocks, especially in cases where much of the source-to-sediment system is not preserved.