TEXTURE AND COMPOSITION OF AN ARID-CLIMATE WEATHERING PROFILE AND THE LINK BETWEEN BEDROCK AND DERIVED SEDIMENT COMPOSITION

Multiple compositional aspects of Holocene, terrigenous-clastic sediment differ from its source, the underlying Cretaceous Stepladder granodiorite (Mojave Desert, California). Different processes can cause sediments to change in composition from source, including chemical weathering, hydrodynamic sorting during transport, and diagenesis. Sorting and physical processes have previously suggested. To assess the relative contributions of these processes to understand provenance reconstruction, the textures and compositions of an exposed Holocene saprolite weathering profile, grus, and granodiorite bedrock were studied. Fracture density in the weathering profile decreased from ~10% at the top to ~2% at the bottom of the profile in contact with the bedrock. The entire weathering profile was dominated mineralogically by plagioclase, potassium feldspar, and biotite. All phases showed evidence of chemical weathering through hydrolysis, and the alteration intensity was estimated using a semi-quantitative scoring system for three main minerals, plagioclase, potassium feldspar, and biotite, each showing various degrees of chemical weathering. Samples above the 70 cm depth break had a greater abundance of weathered plagioclase, while potassium feldspar and biotite showed minimal alteration throughout the profile. However, weathering profile samples had chemical index of alteration values that ranged from 53.5 to 56.5, which suggested minimal chemical weathering. Trace element concentrations in the profile showed slight enrichments relative to bedrock for some elements (Rb avg. 1.47 ppm; Sr avg. 1.24 ppm; Cs avg. 1.07 ppm; U avg. 2.10 ppm; Th avg. 1.24 ppm) and slight depletions for others (Ba avg. 0.86 ppm; Zr avg. 0.76 ppm; Y avg. 0.95 ppm; Hf avg. 0.83 ppm). Eu/Eu*_BN was slightly positive (avg. 1.11). There was an overall negative slope (avg. 1.31) for rare earth element (REE) concentrations relative to bedrock, with light REE averaging 1.03 and heavy REE averaging 1.24. Consequently, results from this study indicate that the geochemical data for the saprolite weathering profile do not directly match the source bedrock composition, and the sediment and bedrock compositions differ. Grus, the non-transported material that lies directly on top of the weathering profile, is compositionally the bridge between sediment and bedrock.