| The oldest preserved terrestrial sedimentary rocks—the Moodies Group—provide a unique opportunity to study Earth's earliest surface environment and how potentially unique conditions may have influenced weathering and the generation of siliciclastic sediments,; A study of modern plutoniclastic sediment generation in different climates was used to understand the effects of climate on weathering. Sediment and bedrock samples were collected from the and southern Sierra Nevada Mountains and Mojave Desert and from the temperate northern Sierra Nevada and Klamath Mountains, California. With increasing precipitation, feldspar is destabilized due to weathering compared to quartz; potassium feldspar is more resistant to weathering than plagioclase but does show destabilization in temperate climates. Temperate-climate sands show a loss of CaO and Na2O compared to K2O and Al2O3. With the exception of biotite and hornblende, the mineralogy of medium-grained stream sand reflects that of medium-grained weathering profile material in both climates.; Archean weathering rinds on pebbles from the Moodies alluvial-fluvial conglomerate record geochemical interactions between the atmosphere and early Earth surface. Rinds developed during weathering and transport based on the truncation of rinds at pre-depositional cracks. Microprobe analysis shows evidence of hydrolysis: potassium feldspar and sericite altered to kaolinite during weathering. Rinds contain higher concentrations of Fe-Mg carbonate, indicative of reducing conditions and high pCO2. The co-existence of the two processes—both hydrolysis and reduction—in one micro-weathering system implies anoxic and elevated pCO 2 conditions in a well-aerated, fluvial system, and an anoxic and CO 2-rich early atmosphere.; Alluvial and braided fluvial deposits north of the Inyoka Fault were analyzed petrographically and geochemically to define their provenance and to then assess their degree of weathering. Conglomerate, sandstone, and shale data give varying pictures of provenance due to differential decomposition during weathering. Shale geochemistry gives the most accurate provenance, although rare-earth elements and Th, Sc, Zr, Cr, and Ti together give the best results. The Moodies Group is remnant of aggressive weathering: labile source materials decomposed to clays and the CIA for Moodies shale is 85, above global average. A combination high temperature, rainfall, and pCO2 likely offset the inhibitory weathering effects of a plant-free environment. |
